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Erosion in the Himalaya is responsible for one of the greatest mass redistributions on Earth and has fueled models of feedback loops between climate and tectonics. Although the general trends of erosion across the Himalaya are reasonably well known, the relative importance of factors controlling erosion is less well constrained. Here we present 25 Be-10-derived catchment-averaged erosion rates from the Yamuna catchment in the Garhwal Himalaya, northern India. Tributary erosion rates range between similar to 0.1 and 0.5mmyr(-1) in the Lesser Himalaya and similar to 1 and 2mmyr(-1) in the High Himalaya, despite uniform hillslope angles. The erosion-rate data correlate with catchment-averaged values of 5 km radius relief, channel steepness indices, and specific stream power but to varying degrees of nonlinearity. Similar nonlinear relationships and coefficients of determination suggest that topographic steepness is the major control on the spatial variability of erosion and that twofold to threefold differences in annual runoff are of minor importance in this area. Instead, the spatial distribution of erosion in the study area is consistent with a tectonic model in which the rock uplift pattern is largely controlled by the shortening rate and the geometry of the Main Himalayan Thrust fault (MHT). Our data support a shallow dip of the MHT underneath the Lesser Himalaya, followed by a midcrustal ramp underneath the High Himalaya, as indicated by geophysical data. Finally, analysis of sample results from larger main stem rivers indicates significant variability of Be-10-derived erosion rates, possibly related to nonproportional sediment supply from different tributaries and incomplete mixing in main stem channels.
Tectonic and climatic control on evolution of rift lakes in the Central Kenya Rift, East Africa
(2009)
The long-term histories of the neighboring Nakuru-Elmenteita and Naivasha lake basins in the Central Kenya Rift illustrate the relative importance of tectonic versus climatic effects on rift-lake evolution and the formation of disparate sedimentary environments. Although modem climate conditions in the Central Kenya Rift are very similar for these basins, hydrology and hydrochemistry of present-day lakes Nakuru, Elmenteita and Naivasha contrast dramatically due to tectonically controlled differences in basin geometries, catchment size, and fluvial processes. In this study, we use eighteen C-14 and Ar-40/Ar-39 dated fluvio-lacustrine sedimentary sections to unravel the spatiotemporal evolution of the lake basins in response to tectonic and climatic influences. We reconstruct paleoclimatic and ecological trends recorded in these basins based on fossil diatom assemblages and geologic field mapping. Our study shows a tendency towards increasing alkalinity and shrinkage of water bodies in both lake basins during the last million years. Ongoing volcano-tectonic segmentation of the lake basins, as well as reorganization of upstream drainage networks have led to contrasting hydrologic regimes with adjacent alkaline and freshwater conditions. During extreme wet periods in the past, such as during the early Holocene climate optimum, lake levels were high and all basins evolved toward freshwater systems. During drier periods some of these lakes revert back to alkaline conditions, while others maintain freshwater characteristics. Our results have important implications for the use and interpretation of lake sediment as climate archives in tectonically active regions and emphasize the need to deconvolve lacustrine records with respect to tectonics versus climatic forcing mechanisms.
Stable-isotopic and sedimentary records from the orogenic Puna Plateau of NW Argentina and adjacent intermontane basins to the east furnish a unique late Cenozoic record of range uplift and ensuing paleoenvironmental change in the south-central Andes. Today, focused precipitation in this region occurs along the eastern, windward flanks of the Eastern Cordillera and Sierras Pampeanas ranges, while the orogen interior constitutes high-elevation regions with increasingly arid conditions in a westward direction. As in many mountain belts, such hydrologic and topographic gradients are commonly mirrored by a systematic relationship between the oxygen and hydrogen stable isotope ratios of meteoric water and elevation. The glass fraction of isotopically datable volcanic ash intercalated in sedimentary sequences constitutes an environmental proxy that retains a signal of the hydrogen-isotopic composition of ancient precipitation. This isotopic composition thus helps to elucidate the combined climatic and tectonic processes associated with topographic growth, which ultimately controls the spatial patterns of precipitation in mountain belts. However, between 25.5 and 27 degrees S present-day river-based hydrogen isotope lapse rates are very low, possibly due to deep-convective seasonal storms that dominate runoff. If not accounted for, the effects of such conditions on moisture availability in the past may lead to misinterpretations of proxy-records of rainfall. Here, we present hydrogen-isotope data of volcanic glass (delta Dg), extracted from 34 volcanic ash layers in different sedimentary basins of the Eastern Cordillera and the Sierras Pampeanas. Combined with previously published delta Dg records and our refined U-Pb and (U-Th)/He zircon geochronology on 17 tuff samples, we demonstrate hydrogen-isotope variations associated with paleoenvironmental change in the Angastaco Basin, which evolved from a contiguous foreland to a fault-bounded intermontane basin during the late Mio-Pliocene. We unravel the environmental impact of Mio-Pliocene topographic growth and associated orographic effects on long-term hydrogen-isotope records of rainfall in the south-central Andes, and potentially identify temporal variations in regional isotopic lapse rates that may also apply to other regions with similar topographic boundary conditions. (C) 2016 Elsevier B.V. All rights reserved.
The southwest margin of the Central Anatolian Plateau has experienced multiple phases of topographic growth, including the formation of localized highs prior to the Late Miocene that were later affected by wholesale uplift of the plateau margin. Our new biostratigraphic data limit the age of uplifted marine sediments at the southwest plateau margin at 1.5 km elevation to <7.17 Ma, and regional lithostratigraphic correlations imply that the age is <6.7 Ma. Single-grain CA-TIMS U-Pb zircon analyses from a reworked ash within the marine sediments yield dates as young as 10.6 Ma, indicating a maximum age that is consistent with the biostratigraphy. Our structural measurements within the uplifted region and fault inversion modeling agree with previous findings in surrounding regions, with early contraction followed by strike-slip and extensional deformation during uplift. Focal mechanisms from shallow earthquakes show that the extensional phase has continued to the present. Broad similarities in the change in the tectonic stress regime (after 8 Ma) and the onset of surface uplift (after 7 Ma) imply that deep-seated process(es) caused post-7 Ma uplift. The geometry of lithospheric slabs beneath the plateau margin, Pliocene to recent alkaline volcanism, and the uplift pattern with accompanying normal faulting point toward slab tearing and localized heating at the base of the lithosphere as a probable mechanism for post-7 Ma uplift of the southwest margin. Considering previous work in the region, there appears to be an important link between slab dynamics and surface uplift throughout the Anatolian Plateau’s southern margin.
This paper is mainly based on field work carried out on the Messinian deposits of the Adana Basin ( southern Turkey), as well as on the interpretation of seismic reflection profiles to understand 3D geometries of the basin fill. Chronostratigraphic constraints for the Messinian deposits are from micropaleontological studies on foraminifera, ostracods, and calcareous nannofossils, recently carried out on the Messinian deposits of the Adana Basin. Our results indicate that this basin developed in a marginal area strictly related to the Mediterranean realm. The Messinian deposits of the Adana Basin record all the main steps of the Messinian Salinity Crisis ( MSC) that affected the Mediterranean area at the end of the Miocene. The new stratigraphic model for the Messinian deposits of the Adana Basin provided in this work gives new insights into both the MSC and the Taurus petroleum system. Despite their complete correspondence with the MSC, the Messinian deposits of the Adana Basin show some differences with respect to the current conceptual model for the MSC. For example, in the current conceptual model for the MSC, only one regional erosional surface ( MES) characterizes the MSC deposits. In the Adana Basin, two regional erosional surfaces, named MES1 and MES2, separate the Messinian deposits related to the MSC in Lower Evaporites, Resedimented Lower Evaporites ( RLE), and upper Messinian continental deposits containing a late Lago-Mare ostracod assemblage ( mainly fluvial coarse-grained and fine-grained sediments). In some places, Brecciated Limestones lie just above the MES1 and beneath the RLE. In addition, the RLE are thought to be related to the same step that brought to the Messinian halite deposition throughout the Mediterranean, pointing to a hyperhaline environment. In contrast, the fine-grained deposits of the RLE of the Adana Basin show the occurrence of Parathetyan brackish ostracod fauna ( early Lago-Mare ostracod assemblages), which defines an oligohaline depositional environment for the RLE. In terms of hydrocarbon prospecting, the Messinian evaporites of the Adana Basin have been considered as a perfect seal for the active Taurus petroleum system. Our results show that due to the complex stratigraphic architecture of the basin fill and the occurrence of two regional erosional surfaces ( MES1 and MES2), the Messinian evaporites are discontinuously present both in surface and in the subsurface of the Adana Basin. However, seal properties in the Adana Basin could be found in the Lower Pliocene deep marine clays of the Avadan Formation. This work leads to suggest a new stratigraphical model for the Messinian deposits of the Adana Basin, allowing us to amend the classical scheme with respect to the Messinian, and to officially define some new formations within the stratigraphy of the Adana Basin.
[1] Orogenic structure appears to be partially controlled by the addition to and removal of material from the mountain belt by tectonic accretion and geomorphic erosion, respectively. We developed a coupled erosion-deformation model for orogenic wedges that are in erosional steady state and deform at their Coulomb failure limit. Erosional steady state is reached when all material introduced into the wedge is removed by erosion that is limited by the rate at which rivers erode through bedrock. We found that the ultimate form of a wedge is controlled by the wedge mechanical properties, sole-out depth of the basal decollement, erosional exponents, basin geometry, and the ratio of the added material flux to the erosional constant. As this latter ratio is increased, wedge width and surface slopes increase. We applied these models to the Taiwan and Himalayan orogenic wedges and found that despite a higher flux of material entering the former, the inferred ratio was larger for the latter. Calculated values for the erodibility of each wedge showed at least an order of magnitude lower value for the Himalaya relative to Taiwan. These values are consistent with the lower precipitation regime in the Himalaya relative to Taiwan and the exposure of crystalline rocks within the Himalayan orogenic wedge. Independently determined rock erodibility estimates are consistent with the accretionary wedge sediments and metasediments and the crystalline and high-grade metamorphic rocks exposed within Taiwan and the Himalaya, respectively. Therefore differences in rock type and climate apparently lead to key differences in the erosion and hence orogenic structure of these two mountain belts
A comparison of deformation rates in active rifts over different temporal scales may help to decipher variations in their structural evolution, controlling mechanisms, and evolution of sedimentary environments through time. Here we use deformed lake shorelines in the Suguta and Turkana basins in northern Kenya as strain markers to estimate deformation rates at the 10(3)-10(4) yr time scale and compare them with rates spanning 10(1)-10(7) yr. Both basins are internally drained today, but until 7 to 5 kyr lake levels were 300 and 100 m higher, respectively, maintained by the elevation of overflow sills connecting them with the Nile drainage. Protracted high lake levels resulted in formation of a maximum highstand shoreline - a distinct geomorphic feature virtually continuous for several tens of kilometers. We surveyed the elevation of this geomorphic marker at 45 sites along >100 km of the rift, and use the overflow sills as vertical datum. Thin-shell elastic and thermomechanical models for this region predict up to similar to 10 m of rapid isostatic rebound associated with lake-level falls lasting until similar to 2 kyr ago. Holocene cumulative throw rates along four rift-normal profiles are 6.8-8.5 mm/yr, or 7.5-9.6 mm/yr if isostatic rebound is considered. Assuming fault dips of 55-65, inferred from seismic reflection profiles, we obtained extension rates of 3.2-6 mm/yr (including uncertainties in field measurements, fault dips, and ages), or 3.5-6.7 mm/yr considering rebound. Our estimates are consistent, within uncertainties, with extension rates of 4-5.1 mm/yr predicted by a modern plate-kinematic model and plate reconstructions since 3.2 Myr. The Holocene strain rate of 10(-15) s(-1) is similar to estimates on the similar to 10(6) yr scale, but over an order of magnitude higher than on the similar to 10(7) yr scale. This is coherent with continuous localization and narrowing of the plate boundary, implying that the lithospheric blocks limiting the Kenya Rift are relatively rigid. Increasing strain rate under steady extension rate suggests that, as the magnitude of extension and crustal thinning increases, the role of regional processes such as weakening by volcanism becomes dominant over far-field plate tectonics controlling the breakup process and the transition from continental rifting to oceanic spreading.
The tectonic and climatic boundary conditions of the broken foreland and the orogen interior of the southern Central Andes of northwestern Argentina cause strong contrasts in elevation, rainfall, and surface-process regimes. The climatic gradient in this region ranges from the wet, windward eastern flanks (similar to 2 m/yr rainfall) to progressively drier western basins and ranges (similar to 0.1 m/yr) bordering the arid Altiplano-Puna Plateau. In this study, we analyze the impact of spatiotemporal climatic gradients on surface erosion: First, we present 41 new catchment-mean erosion rates derived from cosmogenic nuclide inventories to document spatial erosion patterns. Second, we re-evaluate paleoclimatic records from the Calchaquies basin (66 W, 26 S), a large intermontane basin bordered by high (> 4.5 km) mountain ranges, to demonstrate temporal variations in erosion rates associated with changing climatic boundary conditions during the late Pleistocene and Holocene. Three key observations in this region emphasize the importance of climatic parameters on the efficiency of surface processes in space and time: (1) First-order spatial patterns of erosion rates can be explained by a simple specific stream power (SSP) approach. We explicitly account for discharge by routing high-resolution, satellite derived rainfall. This is important as the steep climatic gradient results in a highly non-linear relation between drainage area and discharge. This relation indicates that erosion rates (ER) scale with ER similar to SSP1.4 on cosmogenic-nuclide time scales. (2) We identify an intrinsic channel-slope behavior in different climatic compartments. Channel slopes in dry areas (< 0.25 m/yr rainfall) are slightly steeper than in wet areas (> 0.75 m/yr) with equal drainage areas, thus compensating lower amounts of discharge with steeper slopes. (3) Erosion rates can vary by an order of magnitude between presently dry (similar to 0.05 mm/yr) and well-defined late Pleistocene humid (similar to 0.5 mm/yr) conditions within an intemontane basin. Overall, we document a strong climatic impact on erosion rates and channel slopes. We suggest that rainfall reaching areas with steeper channel slopes in the orogen interior during wetter climate periods results in intensified sediment mass transport, which is primarily responsible for maintaining the balance between surface uplift, erosion, sediment routing and transient storage in the orogen.
The evolution of through-going normal-fault arrays from initial nucleation to growth and subsequent interaction and mechanical linkage is well documented in many extensional provinces. Over time, these processes. lead to predictable spatial and temporal variations in the amount and rate of displacement accumulated along strike of individual fault segments, which should be manifested in the patterns of footwall exhumation. Here, we investigate the along-strike and vertical distribution of low-temperature apatite (U-Th)/He (AHe) cooling ages along the bounding fault system, the Livingstone fault, of the Karonga Basin of the northern Malawi Rift. The fault evolution and linkage from rift initiation to the present day has been previously constrained through investigations of the hanging wall basin fill. The new cooling ages from the footwall of the Livingstone fault can be related to the adjacent depocentre evolution and across a relay zone between two palaeo-fault segments. Our data are complimented by published apatite fission track (AFT) data and reveal significant variation in rock cooling history along-strike: the centre of the footwall yields younger cooling ages than the former tips of earlier fault segments that are now linked. This suggests that low-temperature thermochronology can detect fault interactions along strike. That these former segment boundaries are preserved within exhumed footwall rocks is a function of the relatively recent linkage of the system. Our study highlights that changes in AHe (and potentially AFT) ages associated with the along-strike displacement profile can occur over relatively short horizontal distances (of a few kilometres). This is fundamentally important in the assessment of the vertical cooling history of footwalls in extensional systems: temporal differences in the rate of tectonically driven exhumation at a given location along fault strike may be of greater importance in controlling changes in rates of vertical exhumation than commonly invoked climatic fluctuations. (C) 2016 Elsevier B.V. All rights reserved.
Controversy about the current state and future evolution of Himalayan glaciers has been stirred up by erroneous statements in the fourth report by the Intergovernmental Panel on Climate Change(1,2). Variable retreat rates(3-6) and a paucity of glacial mass-balance data(7,8) make it difficult to develop a coherent picture of regional climate-change impacts in the region. Here, we report remotely-sensed frontal changes and surface velocities from glaciers in the greater Himalaya between 2000 and 2008 that provide evidence for strong spatial variations in glacier behaviour which are linked to topography and climate. More than 65% of the monsoon-influenced glaciers that we observed are retreating, but heavily debris-covered glaciers with stagnant low-gradient terminus regions typically have stable fronts. Debris-covered glaciers are common in the rugged central Himalaya, but they are almost absent in subdued landscapes on the Tibetan Plateau, where retreat rates are higher. In contrast, more than 50% of observed glaciers in the westerlies-influenced Karakoram region in the northwestern Himalaya are advancing or stable. Our study shows that there is no uniform response of Himalayan glaciers to climate change and highlights the importance of debris cover for understanding glacier retreat, an effect that has so far been neglected in predictions of future water availability(9,10) or global sea level(11).
Central Anatolia is a low-relief, high-elevation region where decadal-scale deformation rates estimated from space geodesy suggest low strain rates within a stiff microplate. However, numerous Quaternary faults have been mapped within this low-strain region and estimating their slip rate and seismic potential is important for hazard assessments in an area of increasing infrastructural development. Here we focus on the Sultanhani Fault (SF), which constitutes an integral part of the Eskisehir-Cihanbeyli Fault System, and use deformed maximum highstand shorelines of palaeo-lake Konya to estimate tectonic slip rates at millennial scale. Some of these shorelines were previously interpreted as fault scarps, but we provide conclusive evidence for their erosional origin. We found that shoreline-angle elevations estimated from differential GPS profiles record vertical displacements of 10.2 m across the SF. New radiocarbon ages of lacustrine molluscs suggest 22.4 m of relative lake-level fall between 22.1 +/- 0.3 and 21.7 +/- 0.4 cal. kaBP, constraining the timing of abrupt abandonment of the highstand shoreline. Models of lithospheric rebound associated with regressions of the Tuz Golu and Konya palaeolakes predict only similar to 1 m of regional-scale uplift across the Konya Basin. Dislocation models of displaced shorelines suggest fault-slip rates of 1.5 and 1.8 mm yr(-1) for planar and listric fault geometries, respectively, providing reasonable results for the latter. We found fault scarps in the Nasuhpinar mudflat that likely represent the most recent ground-breaking rupture of the SF, with an average vertical displacement of 1.2 +/- 0.5 m estimated from 54 topographic profiles, equivalent to a M similar to 6.5-6.9 earthquake based on empirical scaling laws. If such events were characteristic during the ultimate 21 ka, a relatively short recurrence time of similar to 800-900 yr would be needed to account for the millennial slip rate. Alternatively, the fault scarp at Nasuhpinar might represent a larger earthquake requiring more frequent smaller events to account for the millennial rate. The relatively fast slip rate of the SF over the past 21 ka is unlikely to have persisted over longer timescales and might reflect spatiotemporal variations in deformation rates within kinematically-linked fault systems within Central Anatolia, or a transient perturbation to the local stress field or fault strength. Such perturbation might have been related to climatically controlled changes in surface and near-surface loads and by interactions among the different tectonic processes that have been proposed to drive the overall slow uplift and associated extension in the Central Anatolian Plateau.
The African Humid Period (AHP) between similar to 15 and 5.5 cal. kyr BP caused major environmental change in East Africa, including filling of the Suguta Valley in the northern Kenya Rift with an extensive (similar to 2150 km(2)), deep (similar to 300 m) lake. Interfingering fluvio-lacustrine deposits of the Baragoi paleo-delta provide insights into the lake-level history and how erosion rates changed during this time, as revealed by delta-volume estimates and the concentration of cosmogenic Be-10 in fluvial sand. Erosion rates derived from delta-volume estimates range from 0.019 to 0.03 mm yr(-1). Be-10-derived paleo-erosion rates at similar to 11.8 cal. kyr BP ranged from 0.035 to 0.086 mm yr(-1), and were 2.7 to 6.6 times faster than at present. In contrast, at similar to 8.7 cal. kyr BP, erosion rates were only 1.8 times faster than at present. Because Be-10-derived erosion rates integrate over several millennia; we modeled the erosion-rate history that best explains the 10Be data using established non-linear equations that describe in situ cosmogenic isotope production and decay. Two models with different temporal constraints (15-6.7 and 12-6.7 kyr) suggest erosion rates that were 25 to 300 times higher than the initial erosion rate (pre-delta formation). That pulse of high erosion rates was short (similar to 4 kyr or less) and must have been followed by a rapid decrease in rates while climate remained humid to reach the modern Be-10-based erosion rate of,similar to 0.013 mm yr(-1). Our simulations also flag the two highest Be-10-derived erosion rates at 11.8 kyr BP related to nonuniform catchment erosion. These changes in erosion rates and processes during the AHP may reflect a strong increase in precipitation, runoff, and erosivity at the arid-to-humid transition either at 15 or similar to 12 cal. kyr BP, before the landscape stabilized again, possibly due to increased soil production and denser vegetation.
Terrigenous sediment supply, marine transport, and depositional processes along tectonically active margins are key to decoding turbidite successions as potential archives of climatic and seismic forcings. Sequence stratigraphic models predict coarse-grained sediment delivery to deep-marine sites mainly during sea-level fall and lowstand. Marine siliciclastic deposition during transgressions and highstands has been attributed to sustained connectivity between terrigenous sources and marine sinks facilitated by narrow shelves. To decipher the controls on Holocene highstand turbidite deposition, we analyzed 12 sediment cores from spatially discrete, coeval turbidite systems along the Chile margin (29 degrees-40 degrees S) with changing climatic and geomorphic characteristics but uniform changes in sea level. Sediment cores from intraslope basins in north-central Chile (29 degrees-33 degrees S) offshore a narrow to absent shelf record a shut-off of turbidite deposition during the Holocene due to postglacial aridification. In contrast, core sites in south-central Chile (36 degrees-40 degrees S) offshore a wide shelf record frequent turbidite deposition during highstand conditions. Two core sites are linked to the Biobio river-canyon system and receive sediment directly from the river mouth. However, intraslope basins are not connected via canyons to fluvial systems but yield even higher turbidite frequencies. High sediment supply combined with a wide shelf and an undercurrent moving sediment toward the shelf edge appear to control Holocene turbidite sedimentation and distribution. Shelf undercurrents may play an important role in lateral sediment transport and supply to the deep sea and need to be accounted for in sediment-mass balances.
The M (w) 6.2 Baladeh earthquake occurred on 28 May 2004 in the Alborz Mountains, northern Iran. This earthquake was the first strong shock in this intracontinental orogen for which digital regional broadband data are available. The Baladeh event provides a rare opportunity to study fault geometry and ongoing deformation processes using modern seismological methods. A joint inversion for hypocentres and a velocity model plus a surface-wave group dispersion curve analysis were used to obtain an adapted velocity model, customised for mid- and long-period waveform modelling. Based on the new velocity model, regional waveform data of the mainshock and larger aftershocks (M (w) a parts per thousand yen3.3) were inverted for moment tensors. For the Baladeh mainshock, this included inversion for kinematic parameters. All analysed earthquakes show dominant thrust mechanisms at depths between 14 and 26 km, with NW-SE striking fault planes. The mainshock ruptured a 28A degrees south-dipping area of 24 x 21 km along a north-easterly direction. The rupture plane of the mainshock does not coincide with the aftershock distribution, neither in map view nor with respect to depth. The considered aftershocks form two main clusters. The eastern cluster is associated with the mainshock. The western cluster does not appear to be connected with the rupture plane of the mainshock but, instead, indicates a second activated fault plane dipping at 85A degrees towards the north.
Despite remarkable tectonostratigraphic similarities along the Himalayan arc, pronounced topographic and exhumational variability exists in different morphotectonic segments. The processes responsible for this segmentation are debated. Of particular interest is a 30- to 40-km-wide orogen-parallel belt of rapid exhumation that extends from central Nepal to the western Himalaya and its possible linkage to a midcrustal ramp in the basal decollement, and the related growth of Lesser Himalayan duplex structures. Here we present 26 new apatite fission track cooling ages from the Beas-Lahul region, at the transition from the Central to the Western Himalaya (77 degrees-78 degrees E) to investigate segmentation in the Himalayan arc from a thermochronologic perspective. Together with previously published data from this part of the orogen, we document significant lateral changes in exhumation between the Dhauladar Range to the west, the Beas-Lahul region, and the Sutlej area to the east of the study area. In contrast to the Himalayan front farther east, exhumation in the far western sectors is focused at the frontal parts of the mountain range and associated with the hanging wall of the Main Boundary Thrust fault ramp. Our results allow us to spatially correlate the termination of the rapid exhumation belt with a midcrustal ramp to the west. We suggest that a plunging anticline at the northwestern edge of the Larji-Kullu-Rampur window represents the termination of the Central Himalayan segment, which is related to the evolution of the Lesser Himalayan duplex. Key Points
The segmentation of major fault systems in subduction zones controls earthquake magnitude and location, but the causes for the existence of segment boundaries and the relationships between long-term deformation and the extent of earthquake rupture, are poorly understood. We compare permanent and seismic-cycle deformation patterns along the rupture zone of the 2010 Maule earthquake (M8.8), which ruptured 500 km of the Chile subduction margin. We analyzed the morphology of MIS-5 marine terraces using LiDAR topography and established their chronology and coeval origin with twelve luminescence ages, stratigraphy and geomorphic correlation, obtaining a virtually continuous distribution of uplift rates along the entire rupture zone. The mean uplift rate for these terraces is 0.5 m/ka. This value is exceeded in three areas, which have experienced rapid emergence of up to 1.6 m/ka; they are located at the northern, central, and southern sectors of the rupture zone, referred to as Topocalma, Carranza and Arauco, respectively. The three sectors correlate with boundaries of eight great earthquakes dating back to 1730. The Topocalma and Arauco sectors, located at the boundaries of the 2010 rupture, consist of broad zones of crustal warping with wavelengths of 60 and 90 km, respectively. These two regions coincide with the axes of oroclinal bending of the entire Andean margin and correlate with changes in curvature of the plate interface. Rapid uplift at Carranza, in turn, is of shorter wavelength and associated with footwall flexure of three crustal-scale normal faults. The uplift rate at Carranza is inversely correlated with plate coupling as well as with coseismic slip, suggesting permanent deformation may accumulate interseismically. We propose that the zones of upwarping at Arauco and Topocalma reflect changes in frictional properties of the megathrust resulting in barriers to the propagation of great earthquakes. Slip during the 1960 (M9.5) and 2010 events overlapped with the similar to 90-km-long zone of rapid uplift at Arauco; similarly, slip in 2010 and 1906 extended across the similar to 60-km-long section of the megathrust at Topocalma, but this area was completely breached by the 1730 (M similar to 9) event, which propagated southward until Carranza. Both Arauco and Topocalma show evidence of sustained rapid uplift since at least the middle Pleistocene. These two sectors might thus constitute discrete seismotectonic boundaries restraining most, but not all great earthquake ruptures. Based on our observations, such barriers might be breached during multi-segment super-cycle events. (C) 2015 Elsevier Ltd. All rights reserved.
This work explores the control of fore-arc structure on segmentation of megathrust earthquake ruptures using coastal geomorphic markers. The Arauco-Nahuelbuta region at the south-central Chile margin constitutes an anomalous fore- arc sector in terms of topography, geology, and exhumation, located within the overlap between the Concepcion and Valdivia megathrust segments. This boundary, however, is only based on similar to 500 years of historical records. We integrate deformed marine terraces dated by cosmogenic nuclides, syntectonic sediments, published fission track data, seismic reflection profiles, and microseismicity to analyze this earthquake boundary over 10(2) -10(6) years. Rapid exhumation of Nahuelbuta's dome-like core started at 4 +/- 1.2 Ma, coeval with inversion of the adjacent Arauco basin resulting in emergence of the Arauco peninsula. Here, similarities between topography, spatiotemporal trends in fission track ages, Pliocene-Pleistocene growth strata, and folded marine terraces suggest that margin-parallel shortening has dominated since Pliocene time. This shortening likely results from translation of a fore-arc sliver or microplate, decoupled from South America by an intra-arc strike-slip fault. Microplate collision against a buttress leads to localized uplift at Arauco accrued by deep-seated reverse faults, as well as incipient oroclinal bending. The extent of the Valdivia segment, which ruptured last in 1960 with an M-w 9.5 event, equals the inferred microplate. We propose that mechanical homogeneity of the fore-arc microplate delimits the Valdivia segment and that a marked discontinuity in the continental basement at Arauco acts as an inhomogeneous barrier controlling nucleation and propagation of 1960-type ruptures. As microplate-related deformation occurs since the Pliocene, we propose that this earthquake boundary and the extent of the Valdivia segment are spatially stable seismotectonic features at million year scale.
The removal, redistribution, and transient storage of sediments in tectonically active mountain belts is thought to exert a first-order control on shallow crustal stresses, fault activity, and hence on the spatiotemporal pattern of regional deformation processes. Accordingly, sediment loading and unloading cycles in intermontane sedimentary basins may inhibit or promote intrabasinal faulting, respectively, but unambiguous evidence for this potential link has been elusive so far. Here we combine 2D numerical experiments that simulate contractional deformation in a broken-foreland setting (i.e., a foreland where shortening is diachronously absorbed by spatially disparate, reverse faults uplifting basement blocks) with field data from intermontane basins in the NW Argentine Andes. Our modeling results suggest that thicker sedimentary fills (>0.7-1.0 km) may suppress basinal faulting processes, while thinner fills (<0.7 km) tend to delay faulting. Conversely, the removal of sedimentary loads via fluvial incision and basin excavation promotes renewed intrabasinal faulting. These results help to better understand the tectono-sedimentary history of intermontane basins that straddle the eastern border of the Andean Plateau in northwestern Argentina. For example, the Santa Maria and the Humahuaca basins record intrabasinal deformation during or after sediment unloading, while the Quebrada del Toro Basin reflects the suppression of intrabasinal faulting due to loading by coarse conglomerates. We conclude that sedimentary loading and unloading cycles may exert a fundamental control on spatiotemporal deformation patterns in intermontane basins of tectonically active broken forelands. (C) 2018 Elsevier B.V. All rights reserved.
Sr isotope records from marginal marine basins track the mixing between seawater and local continental runoff, potentially recording the effects of sea level, tectonic, and climate forcing in marine fossils and sediments. Our 110 new Sr-87/Sr-86 analyses on oyster and foraminifera samples from six late Miocene stratigraphic sections in southern Turkey, Crete, and Sicily show that Sr-87/Sr-86 fell below global seawater values in the basins several million years before the Messinian Salinity Crisis, coinciding with tectonic uplift and basin shallowing. 87Sr/86Sr from more centrally located basins (away from the Mediterranean coast) drop below global seawater values only during the Messinian Salinity Crisis. In addition to this general trend, 55 new Sr-87/Sr-86 analyses from the astronomically tuned Lower Evaporites in the central Apennines (Italy) allow us to explore the effect of glacio-eustatic sea level and precipitation changes on Sr-87/Sr-86. Most variation in our data can be explained by changes in sea level, with greatest negative excursions from global seawater values occurring during relative sea level lowstands, which generally coincided with arid conditions in the Mediterranean realm. We suggest that this greater sensitivity to lowered sea level compared with higher runoff could relate to the inverse relationship between Sr concentration and river discharge. Variations in the residence time of groundwater within the karst terrain of the circum-Mediterranean region during arid and wet phases may help to explain the single (robust) occurrence of a negative excursion during a sea level highstand, but this explanation remains speculative without more detailed paleoclimatic data for the region.
Vegetation has long been hypothesized to influence the nature and rates of surface processes. We test the possible impact of vegetation and climate on denudation rates at orogen scale by taking advantage of a pronounced along-strike gradient in rainfall and vegetation density in the Himalaya. We combine 12 new Be-10 denudation rates from the Sutlej Valley and 123 published denudation rates from fluvially-dominated catchments in the Himalaya with remotely-sensed measures of vegetation density and rainfall metrics, and with tectonic and lithologic constraints. In addition, we perform topographic analyses to assess the contribution of vegetation and climate in modulating denudation rates along strike. We observe variations in denudation rates and the relationship between denudation and topography along strike that are most strongly controlled by local rainfall amount and vegetation density, and cannot be explained by along-strike differences in tectonics or lithology. A W-E along-strike decrease in denudation rate variability positively correlates with the seasonality of vegetation density (R = 0.95, p < 0.05), and negatively correlates with mean vegetation density (R = -0.84, p < 0.05). Vegetation density modulates the topographic response to changing denudation rates, such that the functional relationship between denudation rate and topographic steepness becomes increasingly linear as vegetation density increases. We suggest that while tectonic processes locally control the pattern of denudation rates across strike of the Himalaya (i.e., S-N), along strike of the orogen (i.e., E-W) climate exerts a measurable influence on how denudation rates scatter around long-term, tectonically-controlled erosion, and on the functional relationship between topography and denudation. (C) 2016 Elsevier B.V. All rights reserved.
Recent studies have shown that the 1976-77 global climate shift strongly affected the South American climate. In our study, we observed a link between this climate shift and river-discharge variability in the subtropical Southern Central Andes. We analyzed the daily river-discharge time series between 1940 and 1999 from small to medium mountain drainage basins (10(2)-10(4) km(2) ) across a steep climatic and topographic gradient. We document that the discharge frequency distribution changed significantly, with higher percentiles exhibiting more pronounced trends. A change point between 1971 and 1977 marked an intensification of the hydrological cycle, which resulted in increased river discharge. In the upper Rio Bermejo basin of the northernmost Argentine Andes, the mean annual discharge increased by 40% over 7 years. Our findings are important for flood risk management in areas impacted by the 1976-77 climate shift; discharge frequency distribution analysis provides important insights into the variability of the hydrological cycle in the Andean realm.
The northern Tien Shan of Kyrgyzstan and Kazakhstan has been affected by a series of major earthquakes in the late 19th and early 20th centuries. To assess the significance of such a pulse of strain release in a continental interior, it is important to analyze and quantify strain release over multiple time scales. We have undertaken paleoseismological investigations at two geomorphically distinct sites (Panfilovkoe and Rot Front) near the Kyrgyz capital Bishkek. Although located near the historic epicenters, both sites were not affected by these earthquakes. Trenching was accompanied by dating stratigraphy and offset surfaces using luminescence, radiocarbon, and Be-10 terrestrial cosmogenic nuclide methods. At Rot Front, trenching of a small scarp did not reveal evidence for surface rupture during the last 5000 years. The scarp rather resembles an extensive debris-flow lobe. At Panfilovkoe, we estimate a Late Pleistocene minimum slip rate of 0.2 +/- 0.1 mm/a, averaged over at least two, probably three earthquake cycles. Dip-slip reverse motion along segmented, moderately steep faults resulted in hanging wall collapse scarps during different events. The most recent earthquake occurred around 3.6 +/- 1.3 kyr ago (1 sigma), with dip-slip offsets between 1.2 and 1.4 m. We calculate a probabilistic paleomagnitude to be between 6.7 and 7.2, which is in agreement with regional data from the Kyrgyz range. The morphotectonic signals in the northern Tien Shan are a prime example of deformation in a tectonically active intracontinental mountain belt and as such can help understand the longer-term coevolution of topography and seismogenic processes in similar structural settings worldwide.
The seismicity of the Kenya rift is characterized by high-frequency low-magnitude events concentrated along the rift axis. Its seismic character is typical for magmatically active continental rifts, where igneous material at a shallow depth causes extensive grid faulting and geothermal activity. Thermal overprinting and dike intrusion prohibit the buildup of large elastic strains, therefore prohibiting the generation of large-magnitude earthquakes. On 6 January 1928, the M-S 6.9 Subukia earthquake occurred on the Laikipia-Marmanet fault, the eastern rift-bounding structure of the central Kenya rift. It is the largest instrumentally recorded seismic event in the Kenya rift, standing in contrast to the current model of the rift's seismic character in which large earthquakes are not anticipated. Furthermore, the proximity of the ruptured fault and the rift axis is intriguing: The rift-bounding structure that ruptured in 1928 remains seismically active, capable of generating large-magnitude earthquakes, even though thermally weakened crust and better oriented structures are present along the rift axis nearby, prohibiting any significant buildup of elastic strain. We excavated the surface rupture of the 1928 Subukia earthquake to find evidence for preceding ground-rupturing earthquakes. We also made a total station survey of the site topography and mapped the site geology. We show that the Laikipia-Marmanet fault was repeatedly activated during the late Quaternary. We found evidence for six ground-rupturing earthquakes, including the 1928 earthquake. The topographic survey around the trench site revealed a degraded fault scarp of approximate to 7.5 m in height, offsetting a small debris slide. Using scarp-diffusion modeling, we estimated an uplift rate of U = 0.09-0.15 mm/yr, constraining the scarp age to 50-85 ka. Assuming an average fault dip of 55 degrees-75 degrees, the preferred uplift rate (0.15 mm/yr) accommodates approximately 10%-20% of the recent rate of extension (0.5 mm/yr) across the Kenya rift.
Despite a large number of dated glacial landforms in the Himalaya, the ice extent during the global Last Glacial Maximum (LGM) from 19 to 23 ka is only known to first order. New cosmogenic Be-10 exposure ages from well-preserved glacially polished surfaces, combined with published data, and an improved production rate scaling model allow reconstruction of the LGM ice extent and subsequent deglaciation in the Chandra Valley of NW India. We show that a >1000 m thick valley glacier retreated >150 km within a few thousand years after the onset of LGM deglaciation. By comparing the recession of the Chandra Valley Glacier and other Himalayan glaciers with those of Northern and Southern Hemisphere glaciers, we demonstrate that post-LGM deglaciation was similar and nearly finished prior to the Bolling/Allerod interstadial. Our study supports the view that many Himalayan glaciers advanced during the LGM, likely in response to global variations in temperature.
The eastern flanks of the Central Andes are characterized by deep convection, exposing them to hydrometeorological extreme events, often resulting in floods and a variety of mass movements. We assessed the spatiotemporal pattern of rainfall trends and the changes in the magnitude and frequency of extreme events (ae<yen>95th percentile) along an E-W traverse across the southern Central Andes using rain-gauge and high-resolution gridded datasets (CPC-uni and TRMM 3B42 V7). We generated different climate indices and made three key observations: (1) an increase of the annual rainfall has occurred at the transition between low (< 0.5 km) and intermediate (0.5-3 km) elevations between 1950 and 2014. Also, rainfall increases during the wet season and, to a lesser degree, decreases during the dry season. Increasing trends in annual total amounts characterize the period 1979-2014 in the arid, high-elevation southern Andean Plateau, whereas trend reversals with decreasing annual total amounts were found at low elevations. (2) For all analyzed periods, we observed small or no changes in the median values of the rainfall-frequency distribution, but significant trends with intensification or attenuation in the 95th percentile. (3) In the southern Andean Plateau, extreme rainfall events exhibit trends towards increasing magnitude and, to a lesser degree, frequency during the wet season, at least since 1979. Our analysis revealed that low (< 0.5 km), intermediate (0.5-3 km), and high-elevation (> 3 km) areas respond differently to changing climate conditions, and the transition zone between low and intermediate elevations is characterized by the most significant changes.
We analysed the interplay between coastal uplift, sea level change in the Black Sea, and incision of the Kizilirmak River in northern Turkey. These processes have created multiple co-genetic fluvial and marine terrace sequences that serve as excellent strain markers to assess the ongoing evolution of the Pontide orogenic wedge and the growth of the northern margin of the Central Anatolian Plateau. We used high-resolution topographic data, OSL ages, and published information on past sea levels to analyse the spatiotemporal evolution of these terraces; we derived a regional uplift model for the northward advancing orogenic wedge that supports the notion of laterally variable uplift rates along the flanks of the Pontides. The best-fit uplift model defines a constant long-term uplift rate of 0.28 +/- 0.07 m/ka for the last 545 ka. This model explains the evolution of the terrace sequence in light of active tectonic processes and superposed cycles of climate-controlled sea-level change. Our new data reveal regional uplift characteristics that are comparable to the inner sectors of the Central Pontides; accordingly, the rate of uplift diminishes with increasing distance from the main strand of the restraining bend of the North Anatolian Fault Zone (NAFZ). This spatial relationship between the regional impact of the restraining bend of the NAFZ and uplift of the Pontide wedge thus suggests a strong link between the activity of the NAFZ, deformation and uplift in the Pontide orogenic wedge, and the sustained lateral growth of the Central Anatolian Plateau flank. (C) 2018 Elsevier Ltd. All rights reserved.
Most of the deformation associated with the seismic cycle in subduction zones occurs offshore and has been therefore difficult to quantify with direct observations at millennial timescales. Here we study millennial deformation associated with an active splay-fault system in the Arauco Bay area off south central Chile. We describe hitherto unrecognized drowned shorelines using high-resolution multibeam bathymetry, geomorphic, sedimentologic, and paleontologic observations and quantify uplift rates using a Landscape Evolution Model. Along a margin-normal profile, uplift rates are 1.3m/ka near the edge of the continental shelf, 1.5m/ka at the emerged Santa Maria Island, -0.1m/ka at the center of the Arauco Bay, and 0.3m/ka in the mainland. The bathymetry images a complex pattern of folds and faults representing the surface expression of the crustal-scale Santa Maria splay-fault system. We modeled surface deformation using two different structural scenarios: deep-reaching normal faults and deep-reaching reverse faults with shallow extensional structures. Our preferred model comprises a blind reverse fault extending from 3km depth down to the plate interface at 16km that slips at a rate between 3.0 and 3.7m/ka. If all the splay-fault slip occurs during every great megathrust earthquake, with a recurrence of similar to 150-200years, the fault would slip similar to 0.5m per event, equivalent to a magnitude similar to 6.4 earthquake. However, if the splay-fault slips only with a megathrust earthquake every similar to 1000years, the fault would slip similar to 3.7m per event, equivalent to a magnitude similar to 7.5 earthquake.
The arid Puna plateau of the southern Central Andes is characterized by Cenozoic distributed shortening forming intramontane basins that are disconnected from the humid foreland because of the defeat of orogen-traversing channels. Thick Tertiary and Quaternary sedimentary fills in Puna basins have reduced topographic contrasts between the compressional basins and ranges, leading to a typical low-relief plateau morphology. Structurally identical basins that are still externally drained straddle the eastern border of the Puna and document the eastward propagation of orographic barriers and ensuing aridification. One of them, the Angastaco basin, is transitional between the highly compartmentalized Puna highlands and the undeformed Andean foreland. Sandstone petrography, structural and stratigraphic analysis, combined with detrital apatite fission-track thermochronology from a similar to 6200-m-thick Miocene to Pliocene stratigraphic section in the Angastaco basin, document the late Eocene to late Pliocene exhumation history of source regions along the eastern border of the Puna (Eastern Cordillera (EC)) as well as the construction of orographic barriers along the southeastern flank of the Central Andes. Onset of exhumation of a source in the EC in late Eocene time as well as a rapid exhumation of the Sierra de Luracatao (in the EC) at about 20 Ma are recorded in the detrital sediments of the Angastaco basin. Sediment accumulation in the basin began similar to 15 Ma, a time at which the EC had already built sufficient topography to prevent Puna sourced detritus from reaching the basin. After similar to 13 Ma, shortening shifted eastward, exhuming ranges that preserve an apatite fission-track partial annealing zone recording cooling during the late Cretaceous rifting event. Facies changes and fossil content suggest that after 9 Ma, the EC constituted an effective orographic barrier that prevented moisture penetration into the plateau. Between 3.4 and 2.4 Ma, another orographic barrier was uplifted to the east, leading to further aridification and pronounced precipitation gradients along the mountain front. This study emphasizes the important role of tectonics in the evolution of climate in this part of the Andes
In recent decades, slope instability in high-mountain regions has often been linked to increase in temperature and the associated permafrost degradation and/or the increase in frequency/intensity of rainstorm events. In this context we analyzed the spatiotemporal evolution and potential controlling mechanisms of small- to medium-sized mass movements in a high-elevation catchment of the Italian Alps (Sulden/Solda basin). We found that slope-failure events (mostly in the form of rockfalls) have increased since the 2000s, whereas the occurrence of debris flows has increased only since 2010. The current climate-warming trend registered in the study area apparently increases the elevation of rockfall-detachment areas by approximately 300 m, mostly controlled by the combined effects of frost-cracking and permafrost thawing. In contrast, the occurrence of debris flows does not exhibit such an altitudinal shift, as it is primarily driven by extreme precipitation events exceeding the 75th percentile of the intensity-duration rainfall distribution. Potential debris-flow events in this environment may additionally be influenced by the accumulation of unconsolidated debris over time, which is then released during extreme rainfall events. Overall, there is evidence that the upper Sulden/Solda basin (above ca. 2500 m above sea level [a.s.l.]), and especially the areas in the proximity of glaciers, have experienced a significant decrease in slope stability since the 2000s, and that an increase in rockfalls and debris flows during spring and summer can be inferred. Our study thus confirms that "forward-looking" hazard mapping should be undertaken in these increasingly frequented, high-elevation areas of the Alps, as environmental change has elevated the overall hazard level in these regions.
Intramontane basins may act as important sediment storage areas, serve as recorders of the history of deformation, record syntectonic deposition, and document the evolution of climatic conditions during deposition. We document the timing, cyclicity, and processes that led to the filling and reexcavation of the intramontane Quebrada del Toro basin in NW Argentina. Geomorphic and geologic observations indicate that the basin was filled with sediment that has been subsequently excavated at least two times in the last similar to 8 m.y. The last filling and excavation cycle occurred within the last 0.98 m.y. and has led to the deposition and removal of similar to 61.4 km(3) of material from the basin, leading to a basin-wide averaged minimum denudation rate of 0.16 mm/yr. Aggradation within the basin took place due to channel steepening of the downstream fluvial system that connects the intramontane basin to the foreland. This portion of the fluvial system is actively incising through an uplifting bedrock zone. We use observations within the Toro to test a quasiphysically based model of channel aggradation behind a rising base level that rises due to downstream channel steepening. Our work shows that the bedrock incision rate constant required to reproduce conditions observed within the Toro basin is consistent with values measured independently in similar rock types. Therefore, in intramontane basins that experience similar processes of filling and evacuation, this model may be used to assess the relative importance of tectonic rock uplift, bedrock resistance to fluvial incision, and climate in determining the geomorphic and sedimentologic history of these basins
The intermontane Humahuaca Basin in the Eastern Cordillera of the northwest Argentine Andes lies leeward of an orographic barrier to easterly derived moisture. An average of >2000 mm/yr of rainfall along the eastern flanks of the barrier contrasts with <200 mm/yr in the orogen interior. Paleoenvironmental reconstructions suggest that the basin became disconnected from the foreland during the Miocene-Pliocene by the growth of fault-bounded mountain ranges. Fossil records, sedimentology, and stable isotope data imply that rerouting of the fluvial network by 4.2 Ma and reduced rainfall by ca. 3 Ma were consequences of that range uplift. Here, we present cosmogenic nuclide-derived (Be-10) paleodenudation rates from 6 to 2 Ma fluvial deposits collected from the Humahuaca Basin. Despite increased tectonic activity, our Be-10 data show a tenfold decrease in denudation rates at ca. 3 Ma, documenting a link between uplift-induced semiarid conditions and decreasing hillslope denudation rates. This new data set thus demonstrates the influence of hydrological change on spatiotemporal denudation patterns in tectonically active mountain areas.
Sedimentary basin fills along the windward flanks of orogenic plateaus are valuable archives of paleoenvironmental change with the potential to resolve the history of surface uplift and orographic barrier formation. The intermontane basins of the southern Central Andes contain thick successions of sedimentary material that are commonly interbedded with datable volcanic ashes. We relate variations in the hydrogen isotopic composition of hydrated volcanic glass (delta D-g) of Neogene to Quaternary fills in the semiarid intermontane Humahuaca Basin (Eastern Cordillera, northwest Argentina) to spatiotemporal changes in topography and associated orographic effects. delta D values from volcanic glass in the basin strata (-117 parts per thousand to -98 parts per thousand) show two main trends that accompany observed tectonosedimentary events in the study area. Between 6.0 and 3.5 Ma, delta D-g values decrease by similar to 17 parts per thousand; this is associated with surface uplift in the catchment area. After 3.5 Ma, delta D-g values show abrupt deuterium enrichment, which we associate with (1) the attainment of threshold elevations for blocking moisture transport in the basin-bounding ranges to the east, and (2) the onset of semiarid conditions in the basin. Such orographic barriers throughout the eastern flanks of the Central Andes have impeded moisture transport into the orogen interior; this has likely helped maintain aridity and internal drainage conditions on the adjacent Andean Plateau.
The Tuz Golu Basin is the largest sedimentary depression located at the center of the Central Anatolian Plateau, an extensive, low-relief region with elevations of ca. 1 km located between the Pontide and Tauride mountains. Presently, the basin morphology and sedimentation processes are mainly controlled by the extensional Tuz Golu Fault Zone in the east and the transtensional Inonu-Eskisehir Fault System in the west. The purpose of this study is to contribute to the understanding of the Plio-Quaternary deformation history and to refine the timing of the latest extensional phase of the Tuz Golu Basin. Field observations, kinematic analyses, interpretations of seismic reflection lines, and Ar-40/Ar-39 dating of a key ignimbrite layer suggest that a regional phase of NNW-SSE to NE-SW contraction ended by 6.81 +/- 0.24 Ma and was followed by N-S to NE-SW extension during the Pliocene-Quaternary periods. Based on sedimentological and chronostratigraphic markers, the average vertical displacement rates over the past 5 or 3 Ma with respect to the central part of Tuz Golu Lake are 0.03 to 0.05 mm/year for the fault system at the western flank of the basin and 0.08 to 0.13 mm/year at the eastern flank. Paleo-shorelines of the Tuz Golu Lake, vestiges of higher lake levels related to Quaternary climate change, are important strain markers and were formed during Last Glacial Maximum conditions as indicated by a radiocarbon age of 21.8 +/- 0.4 ka BP obtained from a stromatolitic crust. Geomorphic observations and deformed lacustrine shorelines suggest that the main strand of the Tuz Golu Fault Zone straddling the foothills of the Sereflikochisar-Aksaray range has not been active during the Holocene. Instead, deformation appears to have migrated towards the interior of the basin along an offshore fault that runs immediately west of Sereflikochisar Peninsula. This basinward migration of deformation is probably associated with various processes acting at the lithospheric scale, such as plateau uplift and/or microplate extrusion.
Constraining the pre-Neogene history of the Puna plateau is crucial for establishing the initial conditions that attended the early stage evolution of the southern extent of the Andean plateau. We apply high-to low-temperature thermochronology data from plutonic rocks in northwestern Argentina to quantify the Paleozoic, Mesozoic and early Tertiary cooling history of the Andean crust. U-Pb crystallization ages of zircons indicate that pluton intrusion occurred during the early mid-Ordovician (490-470 Ma) and the late Jurassic (160-150 Ma). Lower-temperature cooling histories from Ar-40/Ar-39 analyses of K-feldspar vary substantially. Basement rocks underlying the western Puna resided at temperatures below 200 degrees C (<6 km depth) since the Devonian (similar to 400 Ma). In contrast, basement rocks underlying the southeastern Puna were hotter (similar to 200-300 degrees C) throughout the Paleozoic and Jurassic and cooled to temperatures of <200 degrees C by similar to 120 Ma. The southeastern Puna basement records a rapid cooling phase coeval with active extension of the Cretaceous Salta rift at similar to 160-100 Ma that we associate with tectonic faulting and lithospheric thinning. The northeastern Puna experienced protracted cooling until the late Cretaceous with temperatures <200 degrees C during the Paleocene. Higher cooling rates between 78 and 55 Ma are associated with thermal subsidence during the postrift stage of the Salta rift and/or shortening-related flexural subsidence. Accelerated cooling and deformation during the Eocene was focused within a narrow zone along the eastern Puna/Eastern Cordillera transition that coincides with Paleozoic/Mesozoic structural and thermal boundaries. Our results constrain regional erosion-induced cooling throughout the Cenozoic to have been less than similar to 150 degrees C, which implies total Cenozoic denudation of <6-4 km.
The northern edge of the western central Tien Shan range is bounded by the Issyk-Ata fault situated south of Bishkek, the capital of Kyrgyzstan. Contraction in this thick-skinned orogen occurs with low-strain accumulation and long earthquake recurrence intervals. In the nineteenth to twentieth centuries, a sequence of large earthquakes with magnitudes between 6.9 and 8 affected the northern Tien Shan but left nearly the entire extent of the Issyk-Ata fault unruptured. Here, the only known historic earthquake ruptured in A.D. 1885 (M6.9) along the western end of the Issyk-Ata fault. Because earthquakes in low-strain regions often tend to cluster in time and may promote failure along nearby structures, the earthquake history of the northern Tien Shan represents an exceptional structural setting for studying fault behavior affected by an intraplate earthquake sequence. We present a paleoseismological study from one site (Belek) along the Issyk-Ata fault located east of the A.D. 1885 epicentral area. Our analysis combines a range of tools, including photogrammetry, differential Global Positioning System, 3D visualization, and age modeling with different dating methods (infrared stimulated luminescence, radiocarbon, U-series) to improve the reliability of an event chronology for the trench stratigraphy and fault geometry. We were able to distinguish three different surfacerupturing paleoearthquakes; these affected the area before 10.5 +/- 1.1 cal ka B.P., at similar to 5.6 +/- 1.0 cal ka B.P., and at similar to 630 +/- 100 cal B.P., respectively. Associated paleomagnitudes for the last two earthquakes range between M6.7 and 7.4, with a cumulative slip rate of 0.7 +/- 0.32 mm/yr. We did not find evidence for the A.D. 1885 event at Belek. Our study yielded two main overall results: first, it extends the regional historic and paleoseismic record; second, the documented rupture events along the Issyk-Ata fault suggest that this fault was not affected in its entirety; instead, these events indicate segmented rupture behavior.
This study analyzes the discharge variability of small to medium drainage basins (10(2)-10(4) km(2)) in the southern Central Andes of NW Argentina. The Hilbert-Huang Transform (HHT) was applied to evaluate non-stationary oscillatory modes of variability and trends, based on four time series of monthly normalized discharge anomaly between 1940 and 2015. Statistically significant trends reveal increasing discharge during the past decades and document an intensification of the hydrological cycle during this period. An Ensemble Empirical Mode Decomposition (EEMD) analysis revealed that discharge variability in this region can be best described by five quasi-periodic statistically significant oscillatory modes, with mean periods varying from 1 to 20 y. Moreover, we show that discharge variability is most likely linked to the phases of the Pacific Decadal Oscillation (PDO) at multi-decadal timescales (similar to 20 y) and, to a lesser degree, to the Tropical South Atlantic SST anomaly (TSA) variability at shorter timescales (similar to 2-5 y). Previous studies highlighted a rapid increase in discharge in the southern Central Andes during the 1970s, inferred to have been associated with the global 1976-77 climate shift. Our results suggest that the rapid discharge increase in the NW Argentine Andes coincides with the periodic enhancement of discharge, which is mainly linked to a negative to positive transition of the PDO phase and TSA variability associated with a long-term increasing trend. We therefore suggest that variations in discharge in this region are largely driven by both natural variability and the effects of global climate change. We furthermore posit that the links between atmospheric and hydrologic processes result from a combination of forcings that operate on different spatiotemporal scales. (C) 2017 Elsevier B.V. All rights reserved.
Foreland basin development in the Andes of central Colombia has been suggested to have started in the Late Cretaceous through tectonic loading of the Central Cordillera. Eastward migration of the Cenozoic orogenic front has also been inferred from the foreland basin record west of the Eastern Cordillera. However, farther east, limited data provided by foreland basin strata and the adjacent Eastern Cordillera complicate any correlation among mountain building, exhumation, and foreland basin sedimentation. In this study, we present new data from the Medina Basin in the eastern foothills of the Eastern Cordillera of Colombia. We report sedimentological data and palynological ages that link an eastward-thinning early Oligocene to early Miocene syntectonic wedge containing rapid facies changes with an episode of fast tectonic subsidence starting at ca. 31 Ma. This record may represent the first evidence of topographic loading generated by slip along the principal basement-bounding thrusts in the Eastern Cordillera to the southwest of the basin. Zircon fission-track ages and paleo-current analysis reveal the location of these thrust loads and illustrate a time lag between the sedimentary signal of topographic loading and the timing of exhumation (ca. 18 Ma). This lag may reflect the period between the onset of range uplift and significant removal of overburden. Vitrinite reflectance data document northward along-strike propagation of the deformation front and folding of the Oligocene syntectonic wedge. This deformation was coupled with a nonuniform incorporation of the basin into the wedge-top depozone. Thus, our data set constitutes unique evidence for the early growth and propagation of the deformation front in the Eastern Cordillera, which may also improve our understanding of spatiotemporal patterns of foreland evolution in other mountain belts.
[1] The Puna-Altiplano plateau in South America is a high-elevation, low internal relief landform that is characterized by internal drainage and hyperaridity. Thermochronologic and sedimentologic observations from the Sierra de Calalaste region in the southwestern Puna plateau, Argentina, place new constraints on early plateau evolution by resolving the timing of uplift of mountain ranges that bound present-day basins and the filling pattern of these basins during late Eocene-Miocene time. Paleocurrent indicators, sedimentary provenance analyses, and apatite fission track thermochronology indicate that the original paleodrainage setting was disrupted by exhumation and uplift of the Sierra de Calalaste range between 24 and 29 Ma. This event was responsible for basin reorganization and the disruption of the regional fluvial system that has ultimately led to the formation of internal drainage conditions, which, in the Salar de Antofalla, were established not later than late Miocene. Upper Eocene-Oligocene sedimentary rocks flanking the range contain features that suggest an arid environment existed prior to and during its uplift. Provenance data indicate a common similar source located to the west for both the southern Puna and the Altiplano of Bolivia during the late Eocene- Oligocene with sporadic local sources. This suggests the existence of an extensive, longitudinally oriented foreland basin along the central Andes during this time
The northern part of the Pamir orogen is the preeminent example of an active intracontinental subduction zone in the early stages of continent-continent collision. Such zones are the least understood type of plate boundaries because modern examples are few and of limited access, and ancient analogs have been extensively overprinted by subsequent tectonic and erosion processes. In the Pamir, it has been assumed that most of the plate convergence was accommodated by overthrusting along the plate-bounding Main Pamir Thrust (MPT), which forms the principal northern mountain and deformation front of the Pamir. However, the synopsis of our new and previously published thermochronologic data from this region shows that the hanging wall of the MPT experienced relatively minor amounts of late Cenozoic exhumation. The Pamir orogen as a whole is an integral part of the overriding plate in a subduction system, while the remnant basin to the north constitutes the downgoing plate, with the bulk of the convergence accommodated by underthrusting. Herein, we demonstrate that the observed deformation of the upper and lower plates within the Pamir-Alai convergence zone resembles highly arcuate oceanic subduction systems characterized by slab rollback, subduction erosion, subduction accretion, and marginal slab-tear faults. We suggest that the curvature of the North Pamir is genetically linked to the short width and rollback of the south-dipping Alai slab; northward motion (indentation) of the Pamir is accommodated by crustal processes related to this rollback. The onset of south-dipping subduction is tentatively linked to intense Pamir contraction following break-off of the north-dipping Indian slab beneath the Karakoram.
Though orogen-parallel shortening and vertical extension have dominated the tectonic evolution of the central Andes, a significant kinematic shift from horizontal contraction to extension appears to have occurred within the high Puna-Altiplano Plateau, with the establishment of extension oblique to the orogen since late Miocene time. We present data from the southern margin of the Puna Plateau, NW Argentina, where new normal faults have been documented in the Fiambala, Punta Negra, and La Quebrada areas. The unifying characteristics of these areas are that young normal faults reactivate or crosscut older thrust and reverse faults. The relationship between the faults and the late Miocene- Pliocene Punaschotter conglomerate suggests that the extensional faulting must be younger than 3.5 to 7 Ma. Existing data are incomplete but indicate that similar horizontal extension has occurred in many regions throughout the Puna- Altiplano Plateau, while shortening continues along the plateau margins. Given the spatial and temporal distribution of this late Miocene to Pliocene kinematic shift, both lithospheric loss in the Puna Plateau and plateau-wide gravitational extensional spreading enhanced by slowing of plate convergence rate could be responsible. The young, disorganized, horizontal extension in the Andes today may be the precursor to more pronounced extension such as observed on the Tibetan Plateau since mid-Miocene time.
The intermontane Quebrada de Humahuaca Basin (Humahuaca Basin) in the Eastern Cordillera of the southern Central Andes of NW Argentina (23 degrees-24 degrees S) records the evolution of a formerly contiguous foreland-basin setting to an intermontane depositional environment during the late stages of Cenozoic Andean mountain building. This basin has been and continues to be subject to shortening and surface uplift, which has resulted in the establishment of an orographic barrier for easterly sourced moisture-bearing winds along its eastern margin, followed by leeward aridification. We present new U-Pb zircon ages and palaeocurrent reconstructions suggesting that from at least 6Ma until 4.2Ma, the Humahuaca Basin was an integral part of a largely contiguous depositional system that became progressively decoupled from the foreland as deformation migrated eastward. The Humahuaca Basin experienced multiple cycles of severed hydrological conditions and subsequent re-captured drainage, fluvial connectivity with the foreland and sediment evacuation. Depositional and structural relationships among faults, regional unconformities and deformed landforms reveal a general pattern of intrabasin deformation that appears to be associated with different cycles of alluviation and basin excavation in which deformation is focused on basin-internal structures during or subsequent to phases of large-scale sediment removal.
Uplift in the broken Andean foreland of the Argentine Santa Bárbara System (SBS) is associated with the contractional reactivation of basement anisotropies, similar to those reported from the thick-skinned Cretaceous-Eocene Laramide province of North America. Fault scarps, deformed Quaternary deposits and landforms, disrupted drainage patterns, and medium-sized earthquakes within the SBS suggest that movement along these structures may be a recurring phenomenon, with yet to be defined repeat intervals and rupture lengths. In contrast to the Subandes thrust belt farther north, where eastward-migrating deformation has generated a well-defined thrust front, the SBS records spatiotemporally disparate deformation along structures that are only known to the first order. We present herein the results of geomorphic desktop analyses, structural field observations, and 2D electrical resistivity tomography and seismic-refraction tomography surveys and an interpretation of seismic reflection profiles across suspected fault scarps in the sedimentary basins adjacent to the Candelaria Range (CR) basement uplift, in the south-central part of the SBS. Our analysis in the CR piedmont areas reveals consistency between the results of near-surface electrical resistivity and seismic-refraction tomography surveys, the locations of prominent fault scarps, and structural geometries at greater depth imaged by seismic reflection data. We suggest that this deformation is driven by deep-seated blind thrusting beneath the CR and associated regional warping, while shortening involving Mesozoic and Cenozoic sedimentary strata in the adjacent basins was accommodated by layer-parallel folding and flexural-slip faults that cut through Quaternary landforms and deposits at the surface.
The northwest Argentine Andes constitute a premier natural laboratory to assess the complex interactions between isolated uplifts, orographic precipitation gradients, and related erosion and sedimentation patterns. Here we present new stratigraphic observations and age information from intermontane basin sediments to elucidate the Neogene to Quaternary shortening history and associated sediment dynamics of the broken Salta foreland. This part of the Andean orogen, which comprises an array of basement-cored range uplifts, is located at similar to 25 degrees S and lies to the east of the arid intraorogenic Altiplano/Puna plateau. In the Salta foreland, spatially and temporally disparate range uplift along steeply dipping inherited faults has resulted in foreland compartmentalization with steep basin-tobasin precipitation gradients. Sediment architecture and facies associations record a three-phase (similar to 10, similar to 5, and <2 Ma), east directed, yet unsystematic evolution of shortening, foreland fragmentation, and ensuing changes in precipitation and sediment transport. The provenance signatures of these deposits reflect the trapping of sediments in the intermontane basins of the Andean hinterland, as well as the evolution of a severed fluvial network. Present-day moisture supply to the hinterland is determined by range relief and basin elevation. The conspiring effects of range uplift and low rainfall help the entrapment and long-term storage of sediments, ultimately raising basin elevation in the hinterland, which may amplify aridification in the orogen interior.
Multiple landslide clusters record quaternary climate changes in the northwestern Argentine andes
(2003)
The chronology of multiple landslide deposits and related lake sediments in the semi-arid eastern Argentine Cordillera suggests that major mass movements cluster in two time periods during the Quaternary, i.e. between 40 and 25 and after 5 14C kyr BP. These clusters may correspond to the Minchin (maximum at around 28-27 14C kyr BP) and Titicaca wet periods (after 3.9 14C kyr BP). The more humid conditions apparently caused enhanced landsliding in this environment. In contrast, no landslide-related damming and associated lake sediments occurred during the Coipasa (11.5- 10 14C yr BP) and Tauca wet periods (14.5-11 14C yr BP). The two clusters at 40-25 and after 5 14C kyr BP may correspond to periods where the El Niño-Southern Oscillation (ENSO) and Tropical Atlantic Sea Surface Temperature Variability (TAV) were active. This, however, was not the case during the Coipasa and Tauca wet periods. Lake-balance modelling of a landslide-dammed lake suggests a 10-15% increase in precipitation and a 3-4 ° C decrease in temperature at ~30 14C kyr BP as compared to the present. In addition, time-series analysis reveals a strong ENSO and TAV during that time. The landslide clusters in northwestern Argentina are therefore best explained by periods of more humid and more variable climates.
The Central Pontides of N Turkey represents a mobile orogenic belt of the southern Eurasian margin that experienced several phases of exhumation associated with the consumption of different branches of the Neo-Tethys Ocean and the amalgamation of continental domains. Our new low-temperature thermochronology data help to constrain the timing of these episodes, providing new insights into associated geodynamic processes. In particular, our data suggest that exhumation occurred at (1) similar to 110 to 90Ma, most likely during tectonic accretion and exhumation of metamorphic rocks from the subduction zone; (2) from similar to 60 to 40Ma, during the collision of the Kirehir and Anatolide-Tauride microcontinental domains with the Eurasian margin; (3) from similar to 0 to 25Ma, either during the early stages of the Arabia-Eurasia collision (soft collision) when the Arabian passive margin reached the trench, implying 70 to 530km of subduction of the Arabian passive margin, or during a phase of trench advance predating hard collision at similar to 20Ma; and (4) similar to 11Ma to the present, during transpression associated with the westward motion of Anatolia. Our findings document the punctuated nature of fault-related exhumation, with episodes of fast cooling followed by periods of slow cooling or subsidence, the role of inverted normal faults in controlling the Paleogene exhumation pattern, and of the North Anatolian Fault in dictating the most recent pattern of exhumation.
Uplifted Neogene marine sediments and Quaternary fluvial terraces in the Mut Basin, southern Turkey, reveal a detailed history of surface uplift along the southern margin of the Central Anatolian plateau from the Late Miocene to the present. New surface exposure ages (Be-10, Al-26, and Ne-21) of gravels capping fluvial strath terraces located between 28 and 135 m above the Goksu River in the Mut Basin yield ages ranging from ca. 25 to 130 ka, corresponding to an average incision rate of 0.52 to 0.67 mm/yr. Published biostratigraphic data combined with new interpretations of the fossil assemblages from uplifted marine sediments reveal average uplift rates of 0.25 to 0.37 mm/yr since Late Miocene time (starting between 8 and 5.45 Ma), and 0.72 to 0.74 mm/yr after 1.66 to 1.62 Ma. Together with the terrace abandonment ages, the data imply 0.6 to 0.7 mm/yr uplift rates from 1.6 Ma to the present. The different post-Late Miocene and post-1.6 Ma uplift rates can imply increasing uplift rates through time, or multi-phased uplift with slow uplift or subsidence in between. Longitudinal profiles of rivers in the upper catchment of the Mut and Ermenek basins show no apparent lithologic or fault control on some knickpoints that occur at 1.2 to 1.5 km elevation, implying a transient response to a change in uplift rates. Projections of graded upper relict channel segments to the modern outlet, together with constraints from uplifted marine sediments, show that a slower incision/uplift rate of 0.1 to 0.2 mm/yr preceded the 0.7 mm/yr uplift rate. The river morphology and profile projections therefore reflect multi-phased uplift of the plateau margin, rather than steadily increasing uplift rates. Multi-phased uplift can be explained by lithospheric slab break-off and possibly also the arrival of the Eratosthenes Seamount at the collision zone south of Cyprus.
The Andean Plateau of NW Argentina is a prominent example of a high-elevation orogenic plateau characterized by internal drainage, arid to hyper-arid climatic conditions and a compressional basin-and-range morphology comprising thick sedimentary basins. However, the development of the plateau as a geomorphic entity is not well understood. Enhanced orographic rainout along the eastern, windward plateau flank causes reduced fluvial run-off and thus subdued surface-process rates in the arid hinterland. Despite this, many Puna basins document a complex history of fluvial processes that have transformed the landscape from aggrading basins with coalescing alluvial fans to the formation of multiple fluvial terraces that are now abandoned. Here, we present data from the San Antonio de los Cobres (SAC) area, a sub-catchment of the Salinas Grandes Basin located on the eastern Puna Plateau bordering the externally drained Eastern Cordillera. Our data include: (a) new radiometric U-Pb zircon data from intercalated volcanic ash layers and detrital zircons from sedimentary key horizons; (b) sedimentary and geochemical provenance indicators; (c) river profile analysis; and (d) palaeo-landscape reconstruction to assess aggradation, incision and basin connectivity. Our results suggest that the eastern Puna margin evolved from a structurally controlled intermontane basin during the Middle Miocene, similar to intermontane basins in the Mio-Pliocene Eastern Cordillera and the broken Andean foreland. Our refined basin stratigraphy implies that sedimentation continued during the Late Mio-Pliocene and the Quaternary, after which the SAC area was subjected to basin incision and excavation of the sedimentary fill. Because this incision is unrelated to baselevel changes and tectonic processes, and is similar in timing to the onset of basin fill and excavation cycles of intermontane basins in the adjacent Eastern Cordillera, we suspect a regional climatic driver, triggered by the Mid-Pleistocene Climate Transition, caused the present-day morphology. Our observations suggest that lateral orogenic growth, aridification of orogenic interiors, and protracted plateau sedimentation are all part of a complex process chain necessary to establish and maintain geomorphic characteristics of orogenic plateaus in tectonically active mountain belts.
Rainfall in the central Andes associated with the South American Monsoon and the South American Low-Level Jet results from orographic effects on atmospheric circulation exerted by the Andean Plateau and the Eastern Cordillera. However, despite its importance for South American climate, no reliable records exist that allow decoding the evolution of thresholds and interactions between Andean topography and atmospheric circulation, especially regarding the onset of humid conditions in the inherently dry southern central Andes. Here, we employ multi-proxy isotope data of lipid biomarkers, pedogenic carbonates and volcanic glass from the Eastern Cordillera of NW Argentina and present the first long-term evapotranspiration record. We find that regional eco-hydrology and vegetation changes are associated with initiation of moisture transport via the South American Low-Level Jet at 7.6 Ma, and subsequent lateral growth of the orogen at 6.5 Ma. Our results highlight that topographically induced changes in atmospheric circulation patterns, not global climate change, were responsible for late Miocene environmental change in this part of the southern hemisphere. This suggests that mountain building over time fundamentally controlled habitat evolution along the central Andes.
Two end-member models have been proposed for the Paleogene Andean foreland: a simple W-E migrating foreland model and a broken-foreland model. We present new stratigraphic, sedimentological and structural data from the Paleogene Quebrada de los Colorados (QLC) Formation, in the Eastern Cordillera, with which to test these two different models. Basin-wide unconformities, growthstrata and changes in provenance indicate deposition of the QLC Formation in a tectonically active basin. Both west- and east-vergent structures, rooted in the basement, controlled the deposition and distribution of the QLC Formation from the Middle Eocene to the Early Miocene. The provenance analysis indicates that the main source areas were basement blocks, like the Paleozoic Oire Eruptive Complex, uplifted during Paleogene shortening, and that delimits the eastern boundary of the present-day intraorogenic Puna plateau. A comparison of the QLC sedimentary basin-fill pattern with those of adjacent Paleogene basins in the Puna plateau and in the Santa Barbara System highlights the presence of discrete depozones. These reflect the early compartmentalization of the foreland, rather than a stepwise advance of the deformation front of a thrust belt. The early Tertiary foreland of the southern central Andes is represented by a ca. 250-km-wide area comprising several deformation zones (Arizaro, Macon, Copalayo and Calchaqui) in which doubly vergent or asymmetric structures, rooted in the basement, were generated. Hence, classical foreland model is difficult to apply in this Paleogene basin; and our data and interpretation agree with a broken-foreland model.
Magmatic continental rifts often constitute nascent plate boundaries, yet long-term extension rates and transient rate changes associated with these early stages of continental breakup remain difficult to determine. Here, we derive a time-averaged minimum extension rate for the inner graben of the Northern Kenya Rift (NKR) of the East African Rift System for the last 0.5 m.y. We use the TanDEM-X science digital elevation model to evaluate fault-scarp geometries and determine fault throws across the volcano-tectonic axis of the inner graben of the NKR. Along rift-perpendicular profiles, amounts of cumulative extension are determined, and by integrating four new Ar-40/Ar-39 radiometric dates for the Silali volcano into the existing geochronology of the faulted volcanic units, time-averaged extension rates are calculated. This study reveals that in the inner graben of the NKR, the long-term extension rate based on mid-Pleistocene to recent brittle deformation has minimum values of 1.0-1.6 mm yr(-1), locally with values up to 2.0 mm yr(-1). A comparison with the decadal, geodetically determined extension rate reveals that at least 65% of the extension must be accommodated within a narrow, 20-km-wide zone of the inner rift. In light of virtually inactive border faults of the NKR, we show that extension is focused in the region of the active volcano-tectonic axis in the inner graben, thus highlighting the maturing of continental rifting in the NKR.
Basement-cored uplift provinces are often characterized by high-angle reverse faulting along preexisting crustal heterogeneities, which may greatly affect the mechanics of deformation and the coupling between erosion and orogenic structure. Herein we construct a coupled deformation-erosion model to understand the mechanics and erosion of mountain belts in which the spatial distribution of deformation is largely influenced by the presence of preexisting high-angle faults. In this case, deformation is accommodated along, and topography is built above, these structures. This topographic loading leads to increasing lithostatic stresses beneath these regions. As a result, active deformation may migrate to frictionally stronger structures in adjacent regions where lithostatic loading is lower. The migration of deformation to such nearby structures depends on the Hubbert-Rubey pore fluid pressure ratio of the crust (lambda), the orientations of the frictionally weaker and stronger preexisting faults (beta(1) and beta(2), respectively), the friction coefficients (mu(b1) and mu(b2)) and Hubbert-Rubey fluid-pressure ratios (lambda(b1) and lambda(b2)) of these faults, and the surface slope of the topography above the frictionally weaker structure (alpha), assuming zero surface slope above the frictionally stronger structure. In general, we found that for a given alpha and beta(1), as mu(b1) increases lambda=lambda(b1)=lambda(b2) increases, and beta(2) decreases, the value of mu(b2) required to force deformation to migrate increases. However, as erosional processes lead to decreasing surface slopes, deformation will be inhibited from migrating to frictionally stronger structures in adjacent regions. Our model results may help to explain some aspects of the deformation observed and the possible correlation between precipitation and the migration of deformation within these tectonic provinces
Tectonically active coasts are dynamic environments characterized by the presence of multiple marine terraces formed by the combined effects of wave erosion, tectonic uplift, and sea-level oscillations at glacialcycle timescales. Well-preserved erosional terraces from the last interglacial sea-level highstand are ideal marker horizons for reconstructing past sea-level positions and calculating vertical displacement rates. We carried out an almost continuous mapping of the last interglacial marine terrace along similar to 5000 km of the western coast of South America between 1 degrees N and 40 degrees S. We used quantitatively replicable approaches constrained by published terrace-age estimates to ultimately compare elevations and patterns of uplifted terraces with tectonic and climatic parameters in order to evaluate the controlling mechanisms for the formation and preservation of marine terraces and crustal deformation. Uncertainties were estimated on the basis of measurement errors and the distance from referencing points. Overall, our results indicate a median elevation of 30.1 m, which would imply a median uplift rate of 0.22 m kyr(-1) averaged over the past similar to 125 kyr. The patterns of terrace elevation and uplift rate display high-amplitude (similar to 100-200 m) and long-wavelength (similar to 10(2) km) structures at the Manta Peninsula (Ecuador), the San Juan de Marcona area (central Peru), and the Arauco Peninsula (south-central Chile). Medium-wavelength structures occur at the Mejillones Peninsula and Topocalma in Chile, while short-wavelength (< 10 km) features are for instance located near Los Vilos, Valparaiso, and Carranza, Chile. We interpret the long-wavelength deformation to be controlled by deep-seated processes at the plate interface such as the subduction of major bathymetric anomalies like the Nazca and Carnegie ridges. In contrast, short-wavelength deformation may be primarily controlled by sources in the upper plate such as crustal faulting, which, however, may also be associated with the subduction of topographically less pronounced bathymetric anomalies. Latitudinal differences in climate additionally control the formation and preservation of marine terraces. Based on our synopsis we propose that increasing wave height and tidal range result in enhanced erosion and morphologically well-defined marine terraces in south-central Chile. Our study emphasizes the importance of using systematic measurements and uniform, quantitative methodologies to characterize and correctly interpret marine terraces at regional scales, especially if they are used to unravel the tectonic and climatic forcing mechanisms of their formation. This database is an integral part of the World Atlas of Last Interglacial Shorelines (WALIS), published online at https://doi.org/10.5281/zenodo.4309748 (Freisleben et al., 2020).
We examined the relationship between the mechanical strength of the lithosphere and the distribution of seismicity within the overriding continental plate of the southern Central Andes (SCA, 29 degrees-39 degrees S), where the oceanic Nazca Plate changes its subduction angle between 33 degrees S and 35 degrees S, from subhorizontal in the north (<5 degrees) to steep in the south (similar to 30 degrees). We computed the long-term lithospheric strength based on an existing 3D model describing variations in thickness, density, and temperature of the main geological units forming the lithosphere of the SCA and adjacent forearc and foreland regions. The comparison between our results and seismicity within the overriding plate (upper-plate seismicity) shows that most of the events occur within the modeled brittle domain of the lithosphere. The depth where the deformation mode switches from brittle frictional to thermally activated ductile creep provides a conservative lower bound to the seismogenic zone in the overriding plate of the study area. We also found that the majority of upper-plate earthquakes occurs within the realm of first-order contrasts in integrated strength (12.7-13.3 log Pam in the Andean orogen vs. 13.5-13.9 log Pam in the forearc and the foreland). Specific conditions characterize the mechanically strong northern foreland of the Andes, where seismicity is likely explained by the effects of slab steepening.
A valley-filling ignimbrite re-exposed through subsequent river incision at the southern margin of the Andean (Puna) plateau preserves pristine geological evidence of pre-late Miocene palaeotopography in the north western Argentine Andes. Our new Ar-40/(39) Ar dating of the Las Papas Ignimbrites yields a plateau age of 9.24 +/- 0.03 Ma, indicating valley-relief and orographic-barrier conditions comparable to the present-day. A later infill of Plio-Pleistocene coarse conglomerates has been linked to wetter conditions, but resulted in no additional net incision of the Las Papas valley, considering that the base of the ignimbrite remains unexposed in the valley bottom. Our observations indicate that at least 550 m of local plateau margin relief (and likely > 2 km) existed by 9 Ma at the southern Puna margin, which likely aided the efficiency of the orographic barrier to rainfall along the eastern and south eastern flanks of the Puna and causes aridity in the plateau interior.
The southern Central Andes (SCA) (between 27 degrees S and 40 degrees S) is bordered to the west by the convergent margin between the continental South American Plate and the oceanic Nazca Plate. The subduction angle along this margin is variable, as is the deformation of the upper plate. Between 33 degrees S and 35 degrees S, the subduction angle of the Nazca plate increases from sub-horizontal (< 5 degrees) in the north to relatively steep (similar to 30 degrees) in the south. The SCA contain inherited lithological and structural heterogeneities within the crust that have been reactivated and overprinted since the onset of subduction and associated Cenozoic deformation within the Andean orogen. The distribution of the deformation within the SCA has often been attributed to the variations in the subduction angle and the reactivation of these inherited heterogeneities. However, the possible influence that the thickness and composition of the continental crust have had on both short-term and long-term deformation of the SCA is yet to be thoroughly investigated. For our investigations, we have derived density distributions and thicknesses for various layers that make up the lithosphere and evaluated their relationships with tectonic events that occurred over the history of the Andean orogeny and, in particular, investigated the short- and long-term nature of the present-day deformation processes. We established a 3D model of lithosphere beneath the orogen and its foreland (29 degrees S-39 degrees S) that is consistent with currently available geological and geophysical data, including the gravity data. The modelled crustal configuration and density distribution reveal spatial relationships with different tectonic domains: the crystalline crust in the orogen (the magmatic arc and the main orogenic wedge) is thicker (similar to 55 km) and less dense (similar to 2900 kg/m(3)) than in the forearc (similar to 35 km, similar to 2975 kg/m(3)) and foreland (similar to 30 km, similar to 3000 kg/m(3)). Crustal thickening in the orogen probably occurred as a result of stacking of low-density domains, while density and thickness variations beneath the forearc and foreland most likely reflect differences in the tectonic evolution of each area following crustal accretion. No clear spatial relationship exists between the density distribution within the lithosphere and previously proposed boundaries of crustal terranes accreted during the early Paleozoic. Areas with ongoing deformation show a spatial correlation with those areas that have the highest topographic gradients and where there are abrupt changes in the average crustal-density contrast. This suggests that the short-term deformation within the interior of the Andean orogen and its foreland is fundamentally influenced by the crustal composition and the relative thickness of different crustal layers. A thicker, denser, and potentially stronger lithosphere beneath the northern part of the SCA foreland is interpreted to have favoured a strong coupling between the Nazca and South American plates, facilitating the development of a sub-horizontal slab.
We analyzed the Lomas de Carabajal area in the intermontane Lerma valley of the Cordillera Oriental to assess the level of neotectonic activity in a densely populated region of northwestern Argentina. In this region, Plio-Pleistocene synorogenic conglomerates are deformed, locally associated with high-angle faults, and NNW-SSE oriented en-echelon folds characterized by wavelengths of < 1 km. The deformed Quaternary units follow the same pattern of deformation as observed in the underlying Neogene deposits; growth-strata geometries are observed near faults. This configuration is compatible with local left-lateral transpressional tectonism driven by ENE-WSW buttressing against the NW-oriented border of a Cretaceous extensional basin (Alemania sub-basin). Optically Stimulated Luminescence analysis of sandy-silty layers interbedded within the folded late Pleistocene conglomeratic sequence helps to determine uplift rates of 0.83-0.87 mm/a during the last 30-40 ka. Nearby the Lomas de Carabajal, a WNW-striking, 3-m-high fault scarp disrupts radiocarbon dated, 10-ka-old loessic deposits providing a Holocene mean uplift rate of 0.30 mm/a. Our data unambiguously show that shallow crustal deformation in the intermontane Lerma valley is ongoing; some of this deformation may be associated with seismicity. Our findings support the notion of temporally and spatially disparate deformation processes in the broken foreland of the northwestern Argentinean Andes.
Late Quaternary intensified monsoon phases control landscape evolution in the northwest Himalaya
(2005)
The intensity of the Asian summer-monsoon circulation varies over decadal to millennial time scales and is reflected in changes in surface processes, terrestrial environments, and marine sediment records. However, the mechanisms of long-lived (2-5 k.y.) intensified monsoon phases, the related changes in precipitation distribution, and their effect on landscape evolution and sedimentation rates are not yet well understood. The and high-elevation sectors of the orogen correspond to a climatically sensitive zone that currently receives rain only during abnormal (i.e., strengthened) monsoon seasons. Analogous to present-day rainfall anomalies, enhanced precipitation during an intensified monsoon phase is expected to have penetrated far into these geomorphic threshold regions where hillslopes are close to the angle of failure. We associate landslide triggering during intensified monsoon phases with enhanced precipitation, discharge, and sediment flux leading to an increase in pore-water pressure, lateral scouring of rivers, and over- steepening of hillslopes, eventually resulting in failure of slopes and exceptionally large mass movements. Here we use lacustrine deposits related to spatially and temporally clustered large landslides (>0.5 km(3)) in the Sutlej Valley region of the northwest Himalaya to calculate sedimentation rates and to infer rainfall patterns during late Pleistocene (29-24 ka) and Holocene (10-4 ka) intensified monsoon phases. Compared to present-day sediment-flux measurements, a fivefold increase in sediment-transport rates recorded by sediments in landslide-dammed lakes characterized these episodes of high climatic variability. These changes thus emphasize the pronounced imprint of millennial-scale climate change on surface processes and landscape evolution
The thick-skinned fold-and-thrust belt on the eastern flank of the Andean Plateau in northwestern Argentina (NWA) is a zone of active contractional deformation characterized by fault-bounded mountain ranges with no systematic spatiotemporal pattern of tectonic activity. In contrast, the thin-skinned Subandean fold-and-thrust belt of northern Argentina and southern Bolivia is characterized primarily by in-sequence (i.e., west to east) fault progression, with a narrow zone of Quaternary deformation focused at the front of the orogenic wedge. To better understand how recent deformation is accommodated across these mountain ranges and the Argentinian portion of the orogen in particular, estimating and comparing deformation rates and patterns across different timescales is essential. We present Late Pleistocene shortening rates for the central Calchaqui intermontane valley in NWA associated with at least three episodes of deformation. Global Positioning System data for the same region reveal a gradual decrease in horizontal surface velocities from the Eastern Cordillera toward the foreland, which contrasts with the rapid velocity gradient associated with a locked decollement in the Subandean Ranges of southern Bolivia. Our new results represent a small view of regional deformation that, when considered in combination with the shallow crustal seismicity and decadal-scale surface velocities, support the notion that strain release in NWA is associated with numerous slowly deforming structures that are distributed throughout the orogen.
We derive a slip rate for a thrust at the central Qilian Shan mountain front by combining structural investigations, satellite imagery, topographic profiling, luminescence dating, and Be-10 exposure dating. The seismically active Zhangye thrust transects late Pleistocene alluvial fan deposits and forms a prominent north facing scarp. The fault consists of two segments that differ in orientation, scarp height, and age. A series of loess-covered terraces records the uplift history of the western thrust segment. Loess accumulation on all terraces started at 8.5 +/- 1.5 kyr and postdates terrace formation. Gravels from the highest terrace yielded a Be-10 exposure age of 90 +/- 11 kyr, which dates the onset of faulting. With a displacement of 55-60 m derived from fault scarp profiles, this yields a vertical slip rate of 0.64 +/- 0.08 mm yr(-1). Along the eastern thrust segment, three Be-10 ages from the uplifted alluvial fan constrain that faulting started at similar to31 +/- 5 kyr. Together with a displacement of 25-30 m this leads to a vertical faulting rate of 0.88 +/- 0.16 mm yr(-1). A dip estimate of 40degrees to 60degrees for the fault plane combined with lower and upper limits of similar to0.6 and similar to0.9 mm yr(-1) for the vertical slip rate gives minimum and maximum horizontal shortening rates of 0.4 and 1.1 mm yr(-1) across the Zhangye thrust. Our results are consistent with geologic and GPS constraints, which suggest that NNE directed shortening across the northeastern Tibetan Plateau is distributed on several active faults with a total shortening rate of 4 to 10 mm yr(-1)
Surface uplift at the northern margin of the Central Anatolian Plateau (CAP) is integrally tied to the evolution of the Central Pontides (CP), between the North Anatolian Fault (NAF) and the Black Sea. Our regional morphometric and plate kinematic analyses reveal topographic anomalies, steep channel gradients, and local high relief areas as indicators of ongoing differential surface uplift, which is higher in the western CP compared to the eastern CP and fault-normal components of geodetic slip vectors and the character of tectonic activity of the NAF suggest that stress is accumulated in its broad restraining bend. Seismic reflection and structural field data show evidence for a deep structural detachment horizon responsible for the formation of an actively northward growing orogenic wedge with a positive flower-structure geometry across the CP and the NAF. Taken together, the tectonic, plate kinematic, and geomorphic observations imply that the NAF is the main driving mechanism for wedge tectonics and uplift in the CP. In addition, the NAF Zone defines the boundary between the extensional CAP and the contractional CP. The syntectonic deposits within inverted intermontane basins and deeply incised gorges suggest that the formation of relief, changes in sedimentary dynamics, and > 1 km fluvial incision resulted from accelerated uplift starting in the early Pliocene. The Central Pontides thus provide an example of an accretionary wedge with surface-breaking faults that play a critical role in mountain building processes, sedimentary basin development, and ensuing lateral growth of a continental plateau since the end of the Miocene.
The timing of the late Cenozoic collision between the Pamir salient and the Tien Shan as well as changes in the relative motion between the Pamir and Tarim are poorly constrained. The northern margin of the Pamir salient indented northward by similar to 300 km during the late Cenozoic, accommodated by south-dipping intracontinental subduction along the Main Pamir Thrust (MPT) coupled to strike-slip faults on the eastern flank of the orogen and both strike-slip and thrust faults on the western margin. The Kashgar-Yecheng transfer system (KYTS) is the main dextral slip shear zone separating Tarim from the Eastern Pamir, with an estimated cumulative offset of similar to 280 km at an average late Cenozoic dextral slip rate of 11-15 mm/a (Cowgill, 2010). In order to better constrain the slip history of the KYTS, we collected thermochronologic samples along the eastward-flowing, deeply incised, antecedent Tashkorgan-Yarkand River, which crosses the fault system on the eastern flank of the orogen. We present 29 new biotite (40)Ar/(39)Ar ages, apatite and zircon (U-Th-Sm)/He ages, and apatite fission track (AFT) analysis, combined with published muscovite and biotite (40)Ar/(39)Ar and AFT data, to create a unique thermochronologic dataset in this poorly studied and remote region. We constrain the timing of four major N-trending faults: the latter three are strands of the KYTS. The westernmost, the Kuke fault, experienced significant dip-slip, west-side-up displacement between > 12 and 6 Ma. To the east, within the KYTS, our new thermochronologic data and geomorphic observations suggest that the Kumtag and Kusilaf dextral slip faults have been inactive since at least 3-5 Ma. Long-term incision rates across the Aertashi dextral slip fault, the easternmost strand of the KYTS, are compatible with slow horizontal slip rates of 1.7-5.3 mm/a over the past 3 to 5 Ma. In summary, these data show that the slip rate of the KYTS decreased substantially during the late Miocene or Pliocene. Furthermore, Miocene-present regional kinematic reconstructions suggest that this deceleration reflects the substantial increase of northward motion of Tarim rather than a significant decrease of the northward velocity of the Pamir. (C) 2011 Elsevier B.V. All rights reserved.
The timing and pattern of surface uplift of Miocene marine sediments capping the southern margin of the Central Anatolian Plateau in southern Turkey provide a first-order constraint on possible mechanisms of regional uplift. Nannofossil, ostracod, and planktic foraminifera biostratigraphy of the Basyayla section (Mut-Ermenek Basin) within the Mut and Kfiselerli Formations suggests a Tortonian age for marine sediments unconformably capping basement rocks at 2 km elevation. The identification of biozone MMi 12a (7.81-8.35 Ma) from planktic foraminifera in the upper part of the section provides the tightest constraint on the age, which is further limited to 8.35-8.108 Ma as a result of the reverse polarity of the collected samples (chron 4r.1 r or 4r.2r). This provides a limiting age for the onset of surface uplift at the margin of one of the world's major orogenic plateaus, from which an average uplift rate of 0.24-0.25 mm/yr can be calculated.
Subhorizontal beds of the uppermost marine sediments exposed throughout the Mut-Ermenek Basin suggest minimal localized deformation, with just minor faulting at the basin margin and broad antiformal deformation across the basin. This implies that the post-8 Ma uplift mechanism must be rooted deep within the crust or in the upper mantle. Published Pn-wave velocity data for the region are compatible with topography compensated by asthenosphere across the southern margin of the plateau, showing a close match to the highest topography when elevations are filtered with a 100-km-wide smoothing window. Uplift along the southern margin of the Central Anatolian Plateau is also reflected by the pattern of Miocene marine sediments capping the margin, which form an asymmetric drape fold over the topography. These observations, together with tomographic evidence for slab steepening and break-off beneath the Eastern Anatolian Plateau, suggest that at least some of the 2 km of post-8 Ma uplift of the southern Central Anatolian Plateau margin is compensated by low-density asthenospheric mantle that upwelled following slab break-off.
Lake sediments in 10 Ethiopian, Kenyan, and Tanzanian rift basins suggest that there were three humid periods at 2.7 to 2.5 million years ago (Ma), 1.9 to 1.7 Ma, and 1.1 to 0.9 Ma, superimposed on the longer-term aridification of East Africa. These humid periods correlate with increased aridity in northwest and northeast Africa and with substantial global climate transitions. These episodes could have had important impacts on the speciation and dispersal of mammals and hominins, because a number of key events, such as the origin of the genus Homo and the evolution of the species Homo erectus, took place in this region during that time.
The northward motion of the Pamir indenter with respect to Eurasia has resulted in coeval thrusting, strike-slip faulting, and normal faulting. The eastern Pamir is currently deformed by east-west oriented extension, accompanied by uplift and exhumation of the Kongur Shan (7719m) and Muztagh Ata (7546m) gneiss domes. Both domes are an integral part of the footwall of the Kongur Shan extensional fault system (KES), a 250 km long, north-south oriented graben. Why active normal faulting within the Pamir is primarily localized along the KES and not distributed more widely throughout the orogen has remained unclear. In addition, relatively little is known about how deformation has evolved throughout the Cenozoic, despite refined estimates on present-day crustal deformation rates and microseismicity, which indicate where crustal deformation is presently being accommodated. To better constrain the spatiotemporal evolution of faulting along the KES, we present 39 new apatite fission track, zircon U-Th-Sm/He, and Ar-40/Ar-39 cooling ages from a series of footwall transects along the KES graben shoulder. Combining these data with present-day topographic relief, 1-D thermokinematic and exhumational modeling documents successive stages, rather than synchronous deformation and gneiss dome exhumation. While the exhumation of the Kongur Shan commenced during the late Miocene, extensional processes in the Muztagh Ata massif began earlier and have slowed down since the late Miocene. We present a new model of synorogenic extension suggesting that thermal and density effects associated with a lithospheric tear fault along the eastern margin of the subducting Alai slab localize extensional upper plate deformation along the KES and decouple crustal motion between the central/western Pamir and eastern Pamir/Tarim basin.
Despite global warming and unlike their Himalayan neighbours, glaciers in the Karakoram mountains do not show signs of significant retreat. Here we report high velocity variations of Biafo Glacier, central Karakoram, which occurred between 2001 and 2009 and which indicate considerable dynamics in its flow behaviour. We have generated a dense time series of glacier surface velocities, based on cross-correlation of optical satellite images, which clearly shows seasonal and interannual velocity variations, reaching 50% in some places. The interannual velocity variations resemble the passing of a broad wave of high velocities, with peak velocities during 2005 and some diffusion down-glacier over a period of at least 4 years. High interannual velocity variations are also observed at other glaciers in the vicinity, suggesting a common cause, although these appear to partly comprise longer acceleration phases. Analysis of weather station data provides some indications of meteorological conditions that could have promoted sustained sliding events during this period, but this does not explain the wave-like nature of the acceleration at Biafo Glacier, and the regular, protracted velocity changes.
Fluvial fill terraces preserve sedimentary archives of landscape responses to climate change, typically over millennial timescales. In the Humahuaca Basin of NW Argentina (Eastern Cordillera, southern Central Andes), our 29 new optically stimulated luminescence ages of late Pleistocene fill terrace sediments demonstrate that the timing of past river aggradation occurred over different intervals on the western and eastern sides of the valley, despite their similar bedrock lithology, mean slopes, and precipitation. In the west, aggradation coincided with periods of increasing precipitation, while in the east, aggradation coincided with decreasing precipitation or more variable conditions. Erosion rates and grain size dependencies in our cosmogenic Be-10 analyses of modern and fill terrace sediments reveal an increased importance of landsliding compared to today on the west side during aggradation, but of similar importance during aggradation on the east side. Differences in the timing of aggradation and the Be-10 data likely result from differences in valley geometry, which causes sediment to be temporarily stored in perched basins on the east side. It appears as if periods of increasing precipitation triggered landslides throughout the region, which induced aggradation in the west, but blockage of the narrow bedrock gorges downstream from the perched basins in the east. As such, basin geometry and fluvial connectivity appear to strongly influence the timing of sediment movement through the system. For larger basins that integrate subbasins with differing geometries or degrees of connectivity (like Humahuaca), sedimentary responses to climate forcing are likely attenuated.
Lake overspill and onset of fluvial incision in the Iranian Plateau: Insights from the Mianeh Basin
(2017)
Orogenic plateaus represent a prime example of the interplay between surface processes, climate, and tectonics. This kind of an interplay is thought to be responsible for the formation, preservation, and, ultimately, the destruction of a typical elevated, low-internal relief plateau landscape. Here, we document the timing of intermontane basin filling associated with the formation of a low-relief plateau morphology, followed by basin opening and plateau-flank incision in the northwestern Iranian Plateau of the Arabia Eurasia collision zone. Our new U-Pb zircon ages from intercalated volcanic ashes in exposed plateau basin-fill sediments from the most external plateau basin (Mianeh Basin) document that the basin was internally drained at least between similar to 7 and 4 Ma, and that from similar to 5 to 4 Ma it was characterized by an similar to 2-km-high and similar to 0.5-km-deep lake (Mianeh paleolake), most likely as a result of wetter climatic conditions. At the same time, the eastern margin of the Mianeh Basin (and, therefore, of the Iranian Plateau) experienced limited tectonic activity, as documented by onlapping sediments and smoothed topography. The combination of high lake level and subdued topography at the plateau margin led to lake overspill, which resulted in the cutting of an similar to 1-km-deep bedrock gorge (Amardos) by the Qezel-Owzan River (QOR) beginning at similar to 4 Ma. This was associated with the incision of the plateau landscape and the establishment of fluvial connectivity with the Caspian Sea. Overall, our study emphasizes the interplay between surface and tectonic processes in forming, maintaining, and destroying orogenic plateau morphology, the transitional nature of orogenic plateau landscapes on timescales of 10(6) yr, and, finally, the role played by overspilling in integrating endorheic basins. (C) 2017 Elsevier B.V. All rights reserved.
The response of surface processes to climatic forcing is fundamental for understanding the impacts of climate change on landscape evolution. In the Himalaya, most large rivers feature prominent fill terraces that record an imbalance between sediment supply and transport capacity, presumably due to past fluctuations in monsoon precipitation and/or effects of glaciation at high elevation. Here, we present volume estimates, chronological constraints, and Be-10-derived paleo-erosion rates from a prominent valley fill in the Yamuna catchment, Garhwal Himalaya, to elucidate the coupled response of rivers and hillslopes to Pleistocene climate change. Although precise age control is complicated due to methodological problems, the new data support formation of the valley fill during the late Pleistocene and its incision during the Holocene. We interpret this timing to indicate that changes in discharge and river-transport capacity were major controls. Compared to the present day, late Pleistocene hillslope erosion rates were higher by a factor of similar to 2-4, but appear to have decreased during valley aggradation. The higher late Pleistocene erosion rates are largely unrelated to glacial erosion and could be explained by enhanced sediment production on steep hillslopes due to increased periglacial activity that declined as temperatures increased. Alternatively, erosion rates that decrease during valley aggradation are also consistent with reduced landsliding from threshold hillslopes as a result of rising base levels. In that case, the similarity of paleo-erosion rates near the end of the aggradation period with modern erosion rates might imply that channels and hillslopes are not yet fully coupled everywhere and that present-day hillslope erosion rates may underrepresent long-term incision rates. (C) 2015 Elsevier B.V. All rights reserved.
A fault scaling law suggests that, over eight orders of magnitude, fault length L is linearly related to maximum displacement D. Individual faults may therefore retain a constant ratio of D/L as they grow. If erosion is minor compared with tectonic uplift, the length and along-strike relief of young mountain ranges should thus reflect fault growth. Topographic profiles along the crests of mountain ranges in the actively deforming foreland of north-east Tibet exhibit a characteristic shape with maximum height near their centre and decreasing elevation toward the tips. We interpret the along-strike relief of these ranges to reflect the slip distribution on high-angle reverse faults. A geometric model illustrates that the lateral propagation rate of such mountain ranges may be deciphered if their length- to-height ratio has remained constant. As an application of the model, we reconstruct the growth of the Heli Shan using a long-term uplift rate of similar to1.3 mm yr(-1) derived from Ne-21 and Be-10 exposure dating
The Cenozoic East African Rift System (EARS) is an exceptional example of active continental extension, providing opportunities for furthering our understanding of hydrocarbon plays within rifts. It is divided into structurally distinct western and eastern branches. The western branch comprises deep rift basins separated by transfer zones, commonly localised onto pre-existing structures, offering good regional scale hydrocarbon traps. At a basin-scale, local discrete inherited structures might also play an important role on fault localisation and hydrocarbon distribution. Here, we consider the evolution of the Central basin of the Malawi Rift, in particular the influence of pre-existing structural fabrics. Integrating basin-scale multichannel 2D, and high resolution seismic datasets we constrain the border, Mlowe-Nkhata, fault system (MNF) to the west of the basin and smaller Mbamba fault (MF) to the east and document their evolution. Intra basin structures define a series of horsts, which initiated as convergent transfers, along the basin axis. The horsts are offset along a NE SW striking transfer fault parallel to and along strike of the onshore Karoo (Permo-Triassic) Ruhuhu graben. Discrete pre-existing structures probably determined its location and, oriented obliquely to the extension orientation it accommodated predominantly strike-slip deformation, with more slowly accrued dip-slip. To the north of this transfer fault, the overall basin architecture is asymmetric, thickening to the west throughout; while to the south, an initially symmetric graben architecture became increasingly asymmetric in sediment distribution as strain localised onto the western MNF. The presence of the axial horst increasingly focussed sediment supply to the west. As the transfer fault increased its displacement, so this axial supply was interrupted, effectively starving the south-east while ponding sediments between the western horst margin and the transfer fault. This asymmetric bathymetry and partitioned sedimentation continues to the present-day, overprinting the early basin symmetry and configuration. Sediments deposited earlier become increasingly dissected and fault juxtapositions changed at a small (10-100 m) scale. The observed influence of basin-scale transfer faults on sediment dispersal and fault compartmentalization due to pre-existing structures oblique to the extension orientation is relevant to analogous exploration settings. (C) 2016 Elsevier Ltd. All rights reserved.
Understanding how Earth-surface processes respond to past climatic perturbations is crucial for making informed predictions about future impacts of climate change on sediment "uxes. Sedimentary records provide the archives for inferring these processes, but their interpretation is compromised by our incomplete understanding of how sediment-routing systems respond to millennial-scale climate cycles. We analyzed seven sediment cores recovered from marine turbidite depositional sites along the Chile continental margin. The sites span a pronounced arid-to-humid gradient with variable relief and related sediment connectivity of terrestrial and marine environments. These sites allowed us to study event related depositional processes in different climatic and geomorphic settings from the Last Glacial Maximum to the present day. The three sites reveal a steep decline of turbidite deposition during deglaciation. High rates of sea-level rise postdate the decline in turbidite deposition. Comparison with paleoclimate proxies documents that the spatio-temporal sedimentary pattern rather mirrors the deglacial humidity decrease and concomitant warming with no resolvable lag times. Our results let us infer that declining deglacial humidity decreased "uvial sediment supply. This signal propagated rapidly through the highly connected systems into the marine sink in north-central Chile. In contrast, in south-central Chile, connectivity between the Andean erosional zone and the "uvial transfer zone probably decreased abruptly by sediment trapping in piedmont lakes related to deglaciation, resulting in a sudden decrease of sediment supply to the ocean. Additionally, reduced moisture supply may have contributed to the rapid decline of turbidite deposition. These different causes result in similar depositional patterns in the marine sinks. We conclude that turbiditic strata may constitute reliable recorders of climate change across a wide range of climatic zones and geomorphic conditions. However, the underlying causes for similar signal manifestations in the sinks may differ, ranging from maintained high system connectivity to abrupt connectivity loss. (C) 2017 Elsevier B.V. All rights reserved.
Resolving Earth's surface at the meter scale is essential for an improved understanding of the dynamics of mass-movement processes. In this study, we explore the applicability and potential of digital elevation models (DEMs) derived from stereophotogrammetry to detect debris-flow channels in the Quebrada del Toro in the northwestern Argentine Andes. Our analysis relies on a high-resolution (3 m) DEM created from SPOT-7 tri-stereo satellite data. We carefully validated DEM quality with ∼6,000 differential GPS points and identified optimal parameters for DEM generation in high-relief terrain. After multiple processing steps, we achieved an accuracy of 0.051 ± 1.915 m (1σ) using n = 3,139 control points with cm precision. Previous studies have used the drainage area and slope framework to identify topographic signatures of debris flows within a catchment. We built upon this and investigated individual river-channel segments using connected-component (CC) analysis on meter-scale topographic data. We define CC as segments of similar slope along the channel profile. Based on seven manually identified debris-flow catchments, we developed a debris-flow similarity index using component length and mean channel-segment slope and identified channel segments that have likely been shaped by debris flows. The presented approach has the potential to resolve intra-catchment variability of transport processes, allows to constrain the extent of debris-flow channels more precisely than slope-area analysis, and highlights the versatility of combined space- and field-based observations for natural-hazard assessments.
Some of the largest and most erosive floods on Earth result from the failure of glacial dams. While potentially cataclysmic ice dams are recognized to have repeatedly formed along ice-sheet margins, much less is known about the frequency and longevity of ice dams caused by mountain glaciers, and their impact on landscape evolution. Here we present field observations and results from cosmogenic nuclide dating that allow reconstructing a > 100-k.y.-long history of glacial damming in the Shyok Valley, eastern Karakoram (South Asia). Our field observations provide evidence that Asia's second-longest glacier, the Siachen, once extended for over 180 km and blocked the Shyok River during the penultimate glacial period, leading to upstream deposition of a more than 400-m-thick fluvio-lacustrine valley fill. Be-10-depth profile modeling indicates that glacial damming ended with the onset of the Eemian interglacial and that the Shyok River subsequently incised the valley fill at an average rate of similar to 4-7 m k.y.(-1). Comparison with contemporary ice-dammed lakes in the Karakoram and elsewhere suggests recurring outburst floods during the aggradation period, while over 25 cycles of fining-upward lake deposits within the valley fill indicate impounding of floods from farther upstream. Despite prolonged damming, the net effect of this and probably earlier damming episodes by the Siachen Glacier is dominated by glacial erosion in excess of fluvial incision, as evidenced by a pronounced overdeepening that follows the glaciated valley reach. Strikingly similar overdeepened valleys at all major confluences of the Shyok and Indus Rivers with Karakoram tributaries indicate that glacial dams and subsequent outburst floods have been widespread and frequent in this region during the Quaternary. Our study suggests that the interaction of Karakoram glaciers with the Shyok and Indus Rivers promoted valley incision and headward erosion into the western margin of the Tibetan Plateau.
The development of rise Cenozoic East African Rift System (EARS) profoundly re-shaped the landscape and significantly increased the amplitude of short-term environmental response to climate variation. In particular, the development of amplifier lakes in rift basins after three million years ago significantly contributed to this exceptional sensitivity of East Africa to climate change compared to elsewhere on the African continent. Amplifier lakes are characterized by tectonically-formed graben morphologies in combination with an extreme contrast between high precipitation in the elevated parts of the catchment and high evaporation in the lake area. Such amplifier lakes respond rapidly to moderate, precessional-forced climate shifts, and as they do so apply dramatic environmental pressure to the biosphere. Rift basins, when either extremely dry or lake-filled, form important barriers for migration, mixing and competition of different populations of animals and hominins. Amplifier lakes link long-term, high-amplitude tectonic processes and short-term environmental fluctuations. East Africa may have become the place where early humans evolved as a consequence of this strong link between different time scales. (C) 2010 Elsevier Ltd. All rights reserved.
Aluminum-26 and beryllium-10 surface exposure dating on cut-and-fill river-terrace surfaces from the lower Sutlej Valley (northwest Himalaya) documents the close link between Indian Summer Monsoon (ISM) oscillations and intervals of enhanced fluvial incision. During the early Holocene ISM optimum, precipitation was enhanced and reached far into the internal parts of the orogen. The amplified sediment flux from these usually dry but glaciated areas caused alluviation of downstream valleys up to 120 m above present grade at ca. 9.9 k.y. B.P. Terrace formation (i.e., incision) in the coarse deposits occurred during century-long weak ISM phases that resulted in reduced moisture availability and most likely in lower sediment flux. Here, we suggest that the lower sediment flux during weak ISM phases allowed rivers to incise episodically into the alluvial fill
The southernmost thrust of the Himalayan orogenic wedge that separates the foreland from the orogen, the Main Frontal Thrust, is thought to accommodate most of the ongoing crustal shortening in the Sub-Himalaya. Steepened longitudinal river profile segments, terrace offsets, and back-tilted fluvial terraces within the Kangra reentrant of the NW Sub-Himalaya suggest Holocene activity of the Jwalamukhi Thrust (JMT) and other thrust faults that may be associated with strain partitioning along the toe of the Himalayan wedge. To assess the shortening accommodated by the JMT, we combine morphometric terrain analyses with in situ Be-10-based surface-exposure dating of the deformed terraces. Incision into upper Pleistocene sediments within the Kangra Basin created two late Pleistocene terrace levels (T1 and T2). Subsequent early Holocene aggradation shortly before similar to 10ka was followed by episodic reincision, which created four cut-and-fill terrace levels, the oldest of which (T3) was formed at 10.10.9ka. A vertical offset of 445m of terrace T3 across the JMT indicates a shortening rate of 5.60.8 to 7.51.1mma(-1) over the last similar to 10ka. This result suggests that thrusting along the JMT accommodates 40-60% of the total Sub-Himalayan shortening in the Kangra reentrant over the Holocene. We speculate that this out-of-sequence shortening may have been triggered or at least enhanced by late Pleistocene and Holocene erosion of sediments from the Kangra Basin.
Intraplate earthquakes pose a significant seismic hazard in densely populated rift systems like the Lower Rhine Graben in Central Europe. While the locations of most faults in this region are well known, constraints on their seismogenic potential and earthquake recurrence are limited. In particular, the Holocene deformation history of active faults remains enigmatic. In an exposure excavated across the Schafberg fault in the southwestern Lower Rhine Graben, south of Untermaubach, in the epicentral region of the 1756 Duren earthquake (M (L) 6.2), we mapped a complex deformation zone in Holocene fluvial sediments. We document evidence for at least one paleoearthquake that resulted in vertical surface displacement of 1.2 +/- 0.2 m. The most recent earthquake is constrained to have occurred after 815 AD, and we have modeled three possible earthquake scenarios constraining the timing of the latest event. Coseismic deformation is characterized by vertical offset of sedimentary contacts distributed over a 10-m-wide central damage zone. Faults were identified where they fracture and offset pebbles in the vertically displaced gravel layers and fracture orientation is consistent with the orientation of the Schafberg fault. This study provides the first constraint on the most recent surface-rupturing earthquake on the Schafberg fault. We cannot rule out that this fault acted as the source of the 1756 Duren earthquake. Our study emphasizes the importance of, and the need for, paleoseismic studies in this and other intracontinental regions, in particular on faults with subtle geomorphic expression that would not typically be recognized as being potentially seismically active. Our study documents textural features in unconsolidated sediment that formed in response to coseismic rupturing of the underlying bedrock fault. We suggest that these features, e.g., abundant oriented transgranular fractures in their context, should be added to the list of criteria used to identify a fault as potentially active. Such information would result in an increase of the number of potentially active faults that contribute to seismic hazards of intracontinental regions.
High Asian glacial landscapes have large variations in topographic relief and the size and steepness of snow accumulation areas. Associated differences in glacial cover and dynamics allow a first-order determination of the dominant processes shaping these landscapes. Here we provide a regional synthesis of the topography and flow characteristics of 287 glaciers across High Asia using digital elevation analysis and remotely sensed glacier surface velocities. Glaciers situated in low-relief areas on the Tibetan Plateau are mainly nourished by direct snowfall, have little or no debris cover, and have a relatively symmetrical distribution of velocities along their length. In contrast, avalanche-fed glaciers with steep accumulation areas, which occur at the deeply incised edges of the Tibetan Plateau, are heavily covered with supraglacial debris, and flow velocities are highest along short segments near their headwalls but greatly reduced along their debris-mantled lower parts. The downstream distribution of flow velocities suggests that the glacial erosion potential is progressively shifted upstream as accumulation areas get steeper and hillslope debris fluxes increase. Our data suggest that the coupling of hillslopes and glacial dynamics increases with topographic steepness and debris cover. The melt-lowering effect of thick debris cover allows the existence of glaciers even when they are located entirely below the snow line. However, slow velocities limit the erosion potential of such glaciers, and their main landscape-shaping contribution may simply be the evacuation of debris from the base of glacial headwalls, which inhibits the formation of scree slopes and thereby allows ongoing headwall retreat by periglacial hillslope processes. We propose a conceptual model in which glacially influenced plateau margins evolve from low-relief to high-relief landscapes with distinctive contributions of hillslope processes and glaciers to relief production and decay.
The late Cenozoic climate of East Africa is punctuated by episodes of short, alternating periods of extreme wetness and aridity, superimposed on a regime of subdued moisture availability exhibiting a long-term drying trend. These periods of extreme climate variability appear to correlate with maxima in the 400-thousand-year (kyr) component of the Earth's eccentricity cycle. Prior to 2.7 Ma the wet phases appear every 400 kyrs, whereas after 2.7 Ma, the wet phases appear every 800 kyrs, with periods of precessional-forced extreme climate variability at 2.7-2.5 Ma, 1.9-1.7 Ma, and 1.1-0.9 Ma before present. The last three major lake phases occur at the times of major global climatic transitions, such as the onset of Northern Hemisphere Glaciation (2.7-2.5 Ma), intensification of the Walker Circulation (1.9-1.7 Ma), and the Mid-Pleistocene Revolution (1.0-0.7 Ma). High-latitude forcing is required to compress the Intertropical Convergence Zone so that East Africa becomes locally sensitive to precessional forcing, resulting in rapid shifts from wet to dry conditions. These periods of extreme climate variability may have provided a catalyst for evolutionary change and driven key speciation and dispersal events amongst mammals and hominins in East Africa. (C) 2007 Elsevier Ltd. All rights reserved.
Megathrust earthquakes are commonly accompanied by increased upper-plate seismicity and occasionally triggered fault slip. In Chile, crustal faults slipped during and after the 2010 Maule (M8.8) earthquake. We studied the El Yolki fault (EYOF), a transtensional structure midways the Maule rupture not triggered in 2010. We mapped a Holocene coastal plain using light detection and ranging, which did not reveal surface ruptures. However, the inner-edge and shoreline angles along the coastal plain as well as 4.3- to 4.0-ka intertidal sediments are back-tilted on the EYOF footwall block, documenting 10 m of vertical displacement. These deformed markers imply similar to 2-mm/year throw rate, and dislocation models a slip rate of 5.6 mm/year for the EYOF. In a 5-m-deep trench, the Holocene intertidal sediments onlap to five erosive steps, interpreted as staircase wave-cut landforms formed by discrete events of relative sea level drop. We tentatively associated these steps with coseismic uplift during EYOF earthquakes between 4.3 and 4.0 ka. The Maule earthquake rupture may be subdivided into three subsegments based on coseismic slip and gravity anomalies. Coulomb stress transfer models predict neutral stress changes at the EYOF during the Maule earthquake but positive changes for a synthetic slip distribution at the central subsegment. If EYOF earthquakes were triggered by megathrust events, their slip distribution was probably focused in the central subsegment. Our study highlights the millennial variability of crustal faulting and the megathrust earthquake cycle in Chile, with global implications for assessing the hazards posed by hidden but potentially seismogenic coastal faults along subduction zones.
[1] The development of topography within and erosional removal of material from an orogen exerts a primary control on its structure. We develop a model that describes the temporal development of a frontally accreting, critically growing Coulomb wedge whose topography is largely limited by bedrock fluvial incision. We present general results for arbitrary initial critical wedge geometries and investigate the temporal development of a critical wedge with no initial topography. Increasing rock erodibility and/or precipitation, decreasing mass flux accreting to the wedge front, increasing wedge sole-out depth, decreasing wedge and basal decollement overpressure, and increasing basal decollement friction lead to narrow wedges. Large power law exponent values cause the wedge geometry to quickly reach a condition in which all material accreted to the front of the wedge is removed by erosion. We apply our model to the Aconcagua fold-and-thrust belt in the central Andes of Argentina where wedge development over time is well constrained. We solve for the erosional coefficient K that is required to recreate the field-constrained wedge growth history, and these values are within the range of independently determined values in analogous rock types. Using qualitative observations of rock erodibilities within the wedge, we speculate that power law exponents of 1/3 less than or equal to m less than or equal to 0.4 and 2/3 less than or equal to n less than or equal to 1 characterize the erosional growth of the Aconcagua fold-and-thrust belt. This general model may be used to understand the development of mountain belts where orogenic wedges grow as they deform at their Coulomb failure limit
Modification of the landscape by glacial erosion reflects the dynamic interplay of climate through temperature, precipitation, and prevailing wind direction, and tectonics through rock uplift and exhumation rate, lithology, and range and fault geometry. We investigate these relationships in the northeast Pamir Mountains using mapping and dating of moraines and terraces to determine the glacial history. We analyze modem glacial morphology to determine glacier area, spacing, headwall relief, debris cover, and equilibrium line altitude (ELA) using the area x altitude balance ratio (AABR), toe-to-headwall altitude ratio (THAR) and toe-to-summit altitude method (TSAM) for 156 glaciers and compare this to lithologic, tectonic, and climatic data We observe a pronounced asymmetry in glacial ELA, area, debris cover, and headwall relief that we interpret to reflect both structural and climatic control: glaciers on the downwind (eastern) side of the range are larger, more debris covered, have steeper headwalls, and tend to erode headward, truncating the smaller glaciers of the upwind, fault-controlled side of the range. We explain this by the transfer of moisture deep into the range as wind-blown or avalanched snow and by limitations imposed on glacial area on the upwind side of the range by the geometry of the Kongur extensional system (KES). The correspondence between rapid exhumation along the KES and maxima in glacier debris cover and headwall relief and minimums in all measures of ELA suggest that taller glacier headwalls develop in a response to more rapid exhumation rates. However, we find that glaciers in the Muji valley did not extend beyond the range front until at least 43 ka, in contrast to extensive glaciation since 300 ka in the south around the high peaks, a pattern which does not clearly reflect uplift rate. Instead, the difference in glacial history and the presence of large peaks (Muztagh Ata and Kongur Shan) with flanking glaciers likely reflects lithologic control (i.e., the location of crustal gneiss domes) and the formation of peaks that rise above the ELA and escape the glacial buzzsaw. (C) 2014 Elsevier B.V. All rights reserved.
Alpine glacial erosion exerts a first-order control on mountain topography and sediment production, but its mechanisms are poorly understood. Observational data capable of testing glacial erosion and transport laws in glacial models are mostly lacking. New insights, however, can be gained from detrital tracer thermochronology. Detrital tracer thermochronology works on the premise that thermochronometer bedrock ages vary systematically with elevation, and that detrital downstream samples can be used to infer the source elevation sectors of sediments. We analyze six new detrital samples of different grain sizes (sand and pebbles) from glacial deposits and the modern river channel integrated with data from 18 previously analyzed bedrock samples from an elevation transect in the Leones Valley, Northern Patagonian Icefield, Chile (46.7 degrees S). We present 622 new detrital zircon (U-Th)/He (ZHe) single-grain analyses and 22 new bedrock ZHe analyses for two of the bedrock samples to determine age reproducibility. Results suggest that glacial erosion was focused at and below the Last Glacial Maximum and neoglacial equilibrium line altitudes, supporting previous modeling studies. Furthermore, grain age distributions from different grain sizes (sand, pebbles) might indicate differences in erosion mechanisms, including mass movements at steep glacial valley walls. Finally, our results highlight complications and opportunities in assessing glacigenic environments, such as dynamics of sediment production, transport, transient storage, and final deposition, that arise from settings with large glacio-fluvial catchments.
The Gofa Province and the Chew Bahir Basin of southern Ethiopia constitute tectonically active regions, where the Southern Main Ethiopian Rift converges with the Northern Kenya Rift through a wide zone of extensional deformation with several north to northeast-trending, left-stepping en-e & PRIME;chelon basins. This sector of the Southern Main Ethiopian Rift is characterized by a semi-arid climate and a largely uniform lithology, and thus provides ideal conditions for studying the different parameters that define the tectonic and geomorphic features of this complex kinematic transfer zone. In this study, the degree of tectonic activity, spatiotemporal variations in extension, and the nature of kinematic linkage between different fault systems of the transfer zone are constrained by detailed quantitative geomorphic analysis of river catchments and focused field work. We analyzed fluvial and landscape morphometric characteristics in combination with structural, seismicity, and climatic data to better evaluate the tectono-geomorphic history of this transfer zone. Our data reveal significant north-south variations in the degree of extension from the Sawula Basin in the north (mature) to the Chew Bahir Basin in the south (juvenile). First, normalized channel-steepness indices and the spatial arrangement of knickpoints in footwall-draining streams suggest a gradual, southward shift in extensional deformation and recent tectonic activity. Second, based on 1-k(m) radius local relief and mean-hillslope maximum values that are consistent with ksn anomalies, we confirm strain localization within zones of fault interaction. Third, morphometric indices such as hypsometry, basin asymmetry factor, and valley floor width to valley height ratio also indicate a north to south gradient in tectonic activity, highlighting the importance of such a wide transfer zone with diffuse extension linking different rift segments during the break-up of continental crust.
Whether variations in the spatial distribution of erosion influence the location, style, and magnitude of deformation within the Himalayan orogen is a matter of debate. We report new Ar-40/Ar-39 white mica and apatite fission- track (AFT) ages that measure the vertical component of exhumation rates along an similar to 120-km-wide NE-SW transect spanning the greater Sutlej region of northwest India. The Ar-40/Ar-39 data indicate that first the High Himalayan Crystalline units cooled below their closing temperature during the early to middle Miocene. Subsequently, Lesser Himalayan Crystalline nappes cooled rapidly, indicating southward propagation of the orogen during late Miocene to Pliocene time. The AFT data, in contrast, imply synchronous exhumation of a NE-SW-oriented similar to 80 x 40 km region spanning both crystalline nappes during the Pliocene-Quaternary. The locus of pronounced exhumation defined by the AFT data correlates with a region of high precipitation, discharge, and sediment flux rates during the Holocene. This correlation suggests that although tectonic processes exerted the dominant control on the denudation pattern before and until the middle Miocene; erosion may have been the most important factor since the Pliocene