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- cosmogenic nuclides (3)
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- thermochronology (3)
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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.
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.
The idea that climatically modulated erosion may impact orogenic processes has challenged geoscientists for decades. Although modeling studies and physical calculations have provided a solid theoretical basis supporting this interaction, to date, field-based work has produced inconclusive results. The central-western Alborz Mountains in the northern sectors of the Arabia-Eurasia collision zone constitute a promising area to explore these potential feedbacks. This region is characterized by asymmetric precipitation superimposed on an orogen with a history of spatiotemporal changes in exhumation rates, deformation patterns, and prolonged, km-scale base-level changes. Our analysis suggests that despite the existence of a strong climatic gradient at least since 17.5 Ma, the early orogenic evolution (from similar to 36 to 9-6 Ma) was characterized by decoupled orographic precipitation and tectonics. In particular, faster exhumation and sedimentation along the more arid southern orogenic flank point to a north-directed accretionary flux and underthrusting of Central Iran. Conversely, from 6 to 3 Ma, erosion rates along the northern orogenic flank became higher than those in the south, where they dropped to minimum values. This change occurred during a similar to 3-Myr-long, km-scale base-level lowering event in the Caspian Sea. We speculate that mass redistribution processes along the northern flank of the Alborz and presumably across all mountain belts adjacent to the South Caspian Basin and more stable areas of the Eurasian plate increased the sediment load in the basin and ultimately led to the underthrusting of the Caspian Basin beneath the Alborz Mountains. This underthrusting in turn triggered a new phase of northward orogenic expansion, transformed the wetter northern flank into a new pro-wedge, and led to the establishment of apparent steady-state conditions along the northern orogenic flank (i.e., rock uplift equal to erosion rates). Conversely, the southern mountain front became the retro-wedge and experienced limited tectonic activity. These observations overall raise the possibility that mass-distribution processes during a pronounced erosion phase driven by base-level changes may have contributed to the inferred regional plate-tectonic reorganization of the northern Arabia-Eurasia collision during the last similar to 5 Ma. (C) 2015 Elsevier B.V. All rights reserved.
Alluvial and transport-limited bedrock rivers constitute the majority of fluvial systems on Earth. Their long profiles hold clues to their present state and past evolution. We currently possess first-principles-based governing equations for flow, sediment transport, and channel morphodynamics in these systems, which we lack for detachment-limited bedrock rivers. Here we formally couple these equations for transport-limited gravel-bed river long-profile evolution. The result is a new predictive relationship whose functional form and parameters are grounded in theory and defined through experimental data. From this, we produce a power-law analytical solution and a finite-difference numerical solution to long-profile evolution. Steady-state channel concavity and steepness are diagnostic of external drivers: concavity decreases with increasing uplift rate, and steepness increases with an increasing sediment-to-water supply ratio. Constraining free parameters explains common observations of river form: to match observed channel concavities, gravel-sized sediments must weather and fine - typically rapidly - and valleys typically should widen gradually. To match the empirical square-root width-discharge scaling in equilibrium-width gravel-bed rivers, downstream fining must occur. The ability to assign a cause to such observations is the direct result of a deductive approach to developing equations for landscape evolution.
The potential link between erosion rates at the Earth’s surface and changes in global climate has intrigued geoscientists for decades1,2 because such a coupling has implications for the influence of silicate weathering3,4 and organic-carbon burial5 on climate and for the role of Quaternary glaciations in landscape evolution1,6. A global increase in late-Cenozoic erosion rates in response to a cooling, more variable climate has been proposed on the basis of worldwide sedimentation rates7. Other studies have indicated, however, that global erosion rates may have remained steady, suggesting that the reported increases in sediment-accumulation rates are due to preservation biases, depositional hiatuses and varying measurement intervals8,9,10. More recently, a global compilation of thermochronology data has been used to infer a nearly twofold increase in the erosion rate in mountainous landscapes over late-Cenozoic times6. It has been contended that this result is free of the biases that affect sedimentary records11, although others have argued that it contains biases related to how thermochronological data are averaged12 and to erosion hiatuses in glaciated landscapes13. Here we investigate the 30 locations with reported accelerated erosion during the late Cenozoic6. Our analysis shows that in 23 of these locations, the reported increases are a result of a spatial correlation bias—that is, combining data with disparate exhumation histories, thereby converting spatial erosion-rate variations into temporal increases. In four locations, the increases can be explained by changes in tectonic boundary conditions. In three cases, climatically induced accelerations are recorded, driven by localized glacial valley incision. Our findings suggest that thermochronology data currently have insufficient resolution to assess whether late-Cenozoic climate change affected erosion rates on a global scale. We suggest that a synthesis of local findings that include location-specific information may help to further investigate drivers of global erosion rates.
The uplift of the Shillong Plateau, in northeast India between the Bengal floodplain and the Himalaya Mountains, has had a significant impact on regional precipitation patterns, strain partitioning, and the path of the Brahmaputra River. Today, the plateau receives the highest measured yearly rainfall in the world and is tectonically active, having hosted one of the strongest intra-plate earthquakes ever recorded. Despite the unique tectonic and climatic setting of this prominent landscape feature, its exhumation and surface uplift history are poorly constrained. We collected 14 detrital river sand and 3 bedrock samples from the southern margin of the Shillong Plateau to measure erosion rates using the terrestrial cosmogenic nuclide 10Be. The calculated bedrock erosion rates range from 2.0 to 5.6 m My−1, whereas catchment average erosion rates from detrital river sands range from 48 to 214 m My−1. These rates are surprisingly low in the context of steep, tectonically active slopes and extreme rainfall. Moreover, the highest among these rates, which occur on the low-relief plateau surface, appear to have been affected by anthropogenic land-use change. To determine the onset of surface uplift, we coupled the catchment averaged erosion rates with topographic analyses of the plateau's southern margin. We interpolated an inclined, pre-incision surface from minimally eroded remnants along the valley interfluves and calculated the eroded volume of the valleys carved beneath the surface. The missing volume was then divided by the volume flux derived from the erosion rates to obtain the onset of uplift. The results of this calculation, ranging from 3.0 to 5.0 Ma for individual valleys, are in agreement with several lines of stratigraphic evidence from the Brahmaputra and Bengal basin that constrain the onset of topographic uplift, specifically the onset of flexural loading and the transgression from deltaic to marine deposition. Ultimately, our data corroborate the hypothesis that surface uplift was decoupled from the onset of rapid exhumation, which occurred several millions of years earlier.
Changes in topography on Earth, particularly the growth of major mountain belts like the Central Andes, have a fundamental impact on regional and global atmospheric circulation patterns. These patterns, in turn, affect processes such as precipitation, erosion, and sedimentation. Over the last two decades, various geochemical, geomorphologic, and geologic approaches have helped identify when, where, and how quickly topography has risen in the past. The current spatio-temporal picture of Central Andean growth is now providing insight into which deep-Earth processes have left their imprint on the shape of the Earth's surface.
The central Andes
(2018)
The Central Andes and the Atacama Desert represent a unique geological, climatic, and magmatic setting on our planet. It is the only place on Earth where subduction of an oceanic plate below an active continental margin has led to an extensive mountain chain and an orogenic plateau that is second in size only to the Tibetan Plateau, which resulted from continental collision. In this article, we introduce the history of the Central Andes and the evolution of its landscape. We also discuss links between tectonic forces, magmatism, and the extreme hyperarid climate of this land that, in turn, has led to rich deposits of precious ores and minerals.
An essential, respected, and critical aspect of the modern practice of science and scientific publishing is peer review. The process of peer review facilitates best practices in scientific conduct and communication, ensuring that manuscripts published as accurate, valuable, and clearly communicated. The over 152 papers published in Tectonics in 2017 benefit from the time, effort, and expertise of our reviewers who have provided thoughtfully considered advice on each manuscript. This role is critical to advancing our understanding of the evolution of the continents and their margins, as these reviews lead to even clearer and higher-quality papers. In 2017, the over 423 papers submitted to Tectonics were the beneficiaries of more than 786 reviews provided by 562 members of the tectonics community and related disciplines. To everyone who has volunteered their time and intellect to peer reviewing, thank you for helping Tectonics and all other AGU Publications provide the best science possible.
An essential, respected, and critical aspect of the modern practice of science and scientific publishing is peer review. The process of peer review facilitates best practices in scientific conduct and communication, ensuring that manuscripts published are as accurate, valuable, and clearly communicated. The over 216 papers published in Tectonics in 2018 benefit from the time, effort, and expertise of our reviewers who have provided thoughtfully considered advice on each manuscript. This role is critical to advancing our understanding of the evolution of the continents and their margins, as these reviews lead to even clearer and higher-quality papers. In 2018, the over 443 papers submitted to Tectonics were the beneficiaries of more than 1,010 reviews provided by 668 members of the tectonics community and related disciplines. To everyone who has volunteered their time and intellect to peer reviewing, thank you for helping Tectonics and all other AGU Publications provide the best science possible.
Astronomically tuned cyclic sedimentary successions provide unprecedented insight into the temporal evolution of depositional systems and major geologic events. However, placing astronomically calibrated records into an absolute time frame with confidence requires independent and precise geochronologic constraints. Astronomical tuning of the precessionally modulated sedimentary cycles of the Mediterranean Basin deposited during the Messinian Salinity Crisis (5.96-5.33 Ma) has indicated an similar to 90 k.y. "Messinian gap", corresponding to the evaporative drawdown of the Mediterranean following the closure of the Mediterranean-Atlantic gateway. In the Messinian deposits, a volcanic ash dated by Ar-40/Ar-39 geochronology was used to anchor the sedimentary cycles to the insolation curve. However, the uncertainty of the Ar-40/Ar-39 date introduces a potential two-cycle (similar to 40 k.y.) uncertainty in the tuning. Using high-precision chemical abrasion-thermal ionization mass spectrometry (CA-TIMS) U-Pb geochronology on single zircon grains from two Messinian ash layers in Italy, we obtained dates of 5.5320 +/- 0.0046 Ma and 5.5320 +/- 0.0074 Ma with sub-precessional resolution. Combined with our astronomical tuning of the Messinian Lower Evaporites, the results refine the duration of the "Messinian gap" to at most 28 or 58 +/- 9.6 k.y., which correlates with either the TG12 glacial interval alone, or both TG12 and TG14 glacial intervals, supporting the hypothesis of a glacio-eustatic contribution in fully isolating the Mediterranean from the Atlantic Ocean. Our new U-Pb dates also allow us to infer a precessionally modulated cyclicity for the post-evaporitic deposits, and hence enable us to tune those successions to the insolation curve.
Structural and thermochronologic studies of the western margin of the central Andean Plateau show changing styles of deformation through time that give insights into tectonic evolution. In southwest Peru, uplift of the plateau proceeded in several distinct phases. First, NW striking, NE dipping reverse faults accommodated uplift prior to similar to 14-16 Ma. Subsequent uplift of the plateau relative to the piedmont (between the plateau and the Pacific Ocean) occurred between similar to 14 and 2.2 Ma and was accommodated by NW striking, SW dipping normal faults and subparallel monoclinal folds. The youngest phase of uplift affected the piedmont region and the plateau margin as a coherent block. Although the uplift magnitude associated with phase 1 is unknown, phases 2 and 3 resulted in at least 2.4-3.0 km of uplift. Up to 1 km of this may have occurred during phase 3. Geodynamic processes occurring in both the continental interior and the subduction zone likely contributed to uplift.
Sequences of coral reef terraces characterized by staircase morphologies and a homogeneous lithology make them appropriate to isolate the influence of uplift on drainage morphology. Along the northern coast of Sumba Island, Indonesia, we investigated the correlations between landscape morphology and uplift rates, which range from 0.02 to 0.6 mm.yr(-1). We studied eight morphometric indices at two scales: whole island (similar to 11,000 km(2)) and within sequences of reefal terraces (similar to 3000 km(2)). At the latter scale, we extracted morphometric indices for 15 individual catchments draining mostly the reefal terraces and for 30 areas undergoing specific ranges of uplift rates draining only the reefal terraces. Indices extracted from digital elevation models include residual relief, incision, stream gradient indices (SL and k(sn)), the hypsometric integral, drainage area, mean relief, and the shape factor. We find that SL, the hypsometric integral, mean relief and the shape factor of catchments positively correlate with uplift rates, whereas incision, residual relief, and k(sn) do not. More precisely, we find that only the areas that are uplifting at a rate faster than 03 mm.yr(-1) can yield the extreme values for these indices, implying that these extreme values are indicative of fast uplifting areas. However, the relationship is not bivalent because any uplift rate can be associated with low values of the same indices. For all indices, the transient conditions of the drainage influence the correlation with Pleistocene mean uplift rates, illustrating the necessity to extract morphometric indices with an appropriate choice of catchment scale. This type of analysis helps to identify the morphometric indices that are most useful for tectonic analysis in areas of unknown uplift, allowing for easy identification of short spatial variations of uplift rate and detection of areas with relatively fast uplift rates in unstudied coastal zones. (C) 2015 Elsevier B.V. All rights reserved.
Despite a long history of plate convergence at the western margin of the South American plate that has been ongoing since at least the Early Paleozoic, the southern Peruvian fore-arc displays little to no evidence of shortening. In the light of this observation, we assess the deformation history of the southern Peruvian fore-arc and its geodynamic implications. To accomplish this, we present a new structural and geo-thermochronological data set (zircon U-Pb, mica Ar-40/Ar-39, apatite and zircon fission-track and zircon (U-Th)/He analyses) for samples collected along a 400km long transect parallel to the trench. Our results show that the Mesoproterozoic gneissic basement was mainly at temperatures 350 degrees C since the Neoproterozoic and was later intruded by Jurassic volcanic arc plutons. Along the coast, a peculiar apatite fission-track age pattern, coupled with field observations and a synthesis of available geological maps, allows us to identify crustal-scale tilted blocks that span the coastal Peruvian fore-arc. These blocks, bounded by normal faults that are orthogonal to the trench, suggest post-60Ma trench-parallel extension that potentially accommodated oroclinal bending in this region. Block tilting is consistent with the observed and previously described switch in the location of sedimentary sources in the fore-arc basin. Our data set allows us to estimate the cumulative slip on these faults to be less than 2km and questions the large amount of trench-parallel extension suggested to have accommodated this bending.
Alluvial and transport-limited bedrock rivers constitute the majority of fluvial systems on Earth. Their long profiles hold clues to their present state and past evolution. We currently possess first-principles-based governing equations for flow, sediment transport, and channel morphodynamics in these systems, which we lack for detachment-limited bedrock rivers. Here we formally couple these equations for transport-limited gravel-bed river long-profile evolution. The result is a new predictive relationship whose functional form and parameters are grounded in theory and defined through experimental data. From this, we produce a power-law analytical solution and a finite-difference numerical solution to long-profile evolution. Steady-state channel concavity and steepness are diagnostic of external drivers: concavity decreases with increasing uplift rate, and steepness increases with an increasing sediment-to-water supply ratio. Constraining free parameters explains common observations of river form: to match observed channel concavities, gravel-sized sediments must weather and fine – typically rapidly – and valleys typically should widen gradually. To match the empirical square-root width–discharge scaling in equilibrium-width gravel-bed rivers, downstream fining must occur. The ability to assign a cause to such observations is the direct result of a deductive approach to developing equations for landscape evolution.
The Vicuna Pampa volcanic complex, at the SE edge of the arid Puna Plateau of the Central Andes, records the interplay between volcanic construction and degra-dational processes. The low-sloping Vicuna Pampa volcanic complex, with a 1200-m-deep, southeastward-opening depression, was previously interpreted as a collapse caldera based on morphological considerations. However, characteristic features associated with collapse calderas do not exist, and close inspection instead suggests that the Vicuna Pampa volcanic complex is a strongly eroded, broad, massif-type composite volcano of mainly basaltic to trachyandesitic composition. Construction of the Vicuna Pampa volcanic complex occurred during two distinct cycles separated by the development of the depression. The first and main cycle took place at ca. 12 Ma and was dominated by lava flows and subordinate scoria cones and domes. The second cycle, possibly late Miocene in age, affected the SW portion of the depression with the emplacement of domes. We interpret the central depression as the result of a possible sector collapse and subsequent intense fluvial erosion during middle to late Miocene time, facilitated by faulting, steepened topography, and wetter climate conditions compared to today. We estimate that similar to 65% of the initial edifice of similar to 240 km(3) was degraded. The efficiency of degradation processes for removing mass from the Vicuna Pampa volcanic complex is surprising, considering that today the region is arid, and the stream channels within the complex are predominantly transport limited, forming a series of coalesced, aggraded alluvial fans and eolian infill. Hence, the Vicuna Pampa volcanic complex records the effects of past degradation efficiency that differs substantially from that of today.
Southern Patagonia is a prime example of ongoing oceanic ridge collision and slab-window formation sustained over several million years. The impact of these phenomena on the thermal structure and exhumation of the crust have been mainly assessed with low-temperature thermochronology of bedrock samples. Here, we infer thermal histories from new and existing thermochronological data from the region of most recent ridge collision. In particular, we evaluate the potential far-reaching thermal effects of the evolving slab window, which have previously been considered responsible for patterns of late Miocene reheating associated with back-arc alkaline volcanism. Our model results define protracted cooling since similar to 15 Ma and stepwise exhumation since the late Miocene. The pattern of stepwise exhumation closely matches the onset of Patagonian glaciation at 7 Ma and the successive pulse of glacial incision coeval with neotectonic activity since 3-4 Ma that are also documented by independent geological and geomorphological evidence in the region. Importantly, our findings challenge the recently suggested lack of glacial erosion and incision since 5 Ma in this region. Furthermore, in contrast to previous modelling studies, we find that the available data do not evidence a previously proposed northward-propagating heating event associated with alkaline volcanism. We hypothesize that the anomalous alkaline volcanism in the Patagonian back-arc might be related to trench-orthogonal tears aligned with transform faults in the subducting plate. The substantial differences from the previous modelling procedure on some of the same samples is demonstrated to result from an important lack of convergence in model runs. (C) 2019 Elsevier B.V. All rights reserved.
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.
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.
Apatite and zircon (U-Th)/He ages from Ocona canyon at the western margin of the Central Andean plateau record rock cooling histories induced by a major phase of canyon incision. We quantify the timing and magnitude of incision by integrating previously published ages from the valley bottom with 19 new sample ages from four valley wall transects. Interpretation of the incision history from cooling ages is complicated by a southwest to northeast increase in temperatures at the base of the crust due to subduction and volcanism. Furthermore, the large magnitude of incision leads to additional three-dimensional variations in the thermal field. We address these complications with finite element thermal and thermochronometer age prediction models to quantify the range of topographic evolution scenarios consistent with observed cooling ages. Comparison of 275 model simulations to observed cooling ages and regional heat flow determinations identify a best fit history with <= 0.2 km of incision in the forearc region prior to similar to 14 Ma and up to 3.0 km of incision starting between 7 and 11 Ma. Incision starting at 7 Ma requires incision to end by similar to 5.5 to 6 Ma. However, a 2.2 Ma age on a volcanic flow on the current valley floor and 5 Ma gravels on the uplifted piedmont surface together suggest that incision ended during the time span between 2.2 and 5 Ma. These additional constraints for incision end time lead to a range of best fit incision onset times between 8 and 11 Ma, which must coincide with or postdate surface uplift.
The subduction of bathymetric anomalies at convergent margins can profoundly affect subduction dynamics, magmatism, and the structural and geomorphic evolution of the overriding plate. The Northern Patagonian Icefield (NPI) is located east of the Chile Triple Junction at similar to 47 degrees S, where the Chile Rise spreading center collides with South America. This region is characterized by an abrupt increase in summit elevations and relief that has been controversially debated in the context of geodynamic versus glacial erosion effects on topography. Here we present geomorphic, thermochronological, and structural data that document neotectonic activity along hitherto unrecognized faults along the flanks of the NPI. New apatite (U-Th)/He bedrock cooling ages suggest faulting since 2-3 Ma. We infer the northward translation of an similar to 140 km long fore-arc sliver-the NPI block-results from enhanced partitioning of oblique plate convergence due to the closely spaced collision of three successive segments of the Chile Rise. In this model, greater uplift occurs in the hanging wall of the Exploradores thrust at the northern leading edge of the NPI block, whereas the Cachet and Liquine-Ofqui dextral faults decouple the NPI block along its eastern and western flanks, respectively. Localized extension possibly occurs at its southern trailing edge along normal faults associated with margin-parallel extension, tectonic subsidence, and lower elevations along the Andean crest line. Our neotectonic model provides a novel explanation for the abrupt topographic variations inland of the Chile Triple Junction and emphasizes the fundamental effects of local tectonics on exhumation and topographic patterns in this glaciated landscape.
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.
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.
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.
Deciphering the response of sediment routing systems to climatic forcing is fundamental for understanding the impacts of climate change on landscape evolution. In the Kangra Basin (northwest Sub-Himalaya, India), upper Pleistocene to Holocene alluvial fills and fluvial terraces record periodic fluctuations of sediment supply and transport capacity on timescales of 10(3) to 10(5) yr. To evaluate the potential influence of climate change on these fluctuations, we compare the timing of aggradation and incision phases recorded within remnant alluvial fans and terraces with climate archives. New surface-exposure dating of six terrace levels with in-situ cosmogenic Be-10 indicates the onset of incision phases. Two terrace surfaces from the highest level (T1) sculpted into the oldest preserved alluvial fan (AF1) date back to 53.4 +/- 3.2 ka and 43.0 +/- 2.7 ka (1 sigma). T2 surfaces sculpted into the remnants of AF1 have exposure ages of 18.6 +/- 1.2 ka and 15.3 +/- 0.9 ka, while terraces sculpted into the upper Pleistocene-Holocene fan (AF2) provide ages of 9.3 +/- 0.4 ka (T3), 7.1 +/- 0.4 ka (T4), 5.2 +/- 0.4 ka (T5) and 3.6 +/- 0.2 ka (T6). Together with previously published OSL ages yielding the timing of aggradation, we find a correlation between variations in sediment transport with oxygen-isotope records from regions affected by the Indian Summer Monsoon. During periods of increased monsoon intensity and post-Last Glacial Maximum glacial retreat, aggradation occurred in the Kangra Basin, likely due to high sediment flux, whereas periods of weakened monsoon intensity or lower sediment supply coincide with incision. (C) 2016 Elsevier B.V. All rights reserved.
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.
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.
Emerged marine terraces and paleoshorelines along plate margins are prominent geomorphic markers that can be used to quantify the rates and patterns of crustal deformation. The northern margin of the Central Anatolian Plateau has been interpreted as an actively deforming orogenic wedge between the North Anatolian Fault and the Black Sea. Here we use uplifted marine terraces across principal faults on the Sinop Peninsula at the central northern side of the Pontide orogenic wedge to unravel patterns of Quaternary faulting and orogenic wedge behavior. We leveled the present-day elevations of paleoshorelines and dated marine terrace deposits using optically stimulated luminescence (OSL) to determine coastal uplift. The elevations of the paleoshorelines vary between 4 +/- 0.2 and 67 +/- 1.4 m above sea level and OSL ages suggest terrace formation episodes during interglacial periods at ca 125, 190, 400 and 570 ka, corresponding to marine isotopic stages (MIS) 5e, 7a, 11 and 15. Mean apparent vertical displacement rates (without eustatic correction) deduced from these terraces range between 0.02 and 0.18 mm/a, with intermittent faster rates of up to 0.26 mm/a. We obtained higher rates at the eastern and southern parts of the peninsula, toward the hinterland, indicating non-uniform uplift across the different morphotectonic segments of the peninsula. Our data are consistent with active on- and offshore faulting across the Sinop Peninsula. When integrated with regional tectonic observations, the faulting pattern reflects shortening distributed over a broad region of the northern margin of the Central Anatolian Plateau during the Quaternary.
We document Quaternary fluvial incision driven by fault-controlled surface deformation in the inverted intermontane Gökirmak Basin in the Central Pontide mountains along the northern margin of the Central Anatolian Plateau. In-situ-produced Be-10, Ne-21, and Cl-36 concentrations from gravel-covered fluvial terraces and pediment surfaces along the trunk stream of the basin (the Gökirmak River) yield model exposure ages ranging from 71ka to 34645ka and average fluvial incision rates over the past similar to 350ka of 0.280.01mm a(-1). Similarities between river incision rates and coastal uplift rates at the Black Sea coast suggest that regional uplift is responsible for the river incision. Model exposure ages of deformed pediment surfaces along tributaries of the trunk stream range from 605ka to 110 +/- 10ka, demonstrating that the thrust faults responsible for pediment deformation were active after those times and were likely active earlier as well as explaining the topographic relief of the region. Together, our data demonstrate cumulative incision that is linked to active internal shortening and uplift of similar to 0.3mm a(-1) in the Central Pontide orogenic wedge, which may ultimately contribute to the lateral growth of the northern Anatolian Plateau.
The tectonically driven growth of mountains reflects the characteristics of the underlying fault systems and the applied tectonic forces. Over time, fault networks might be relatively static, but stress conditions could change and result in variations in fault slip orientation. Such a tectonic landscape would transition from a simple to a composite state: the topography of simple landscapes is correlated with a single set of tectonic boundary conditions, while composite landscapes contain inherited topography due to earlier deformation under different boundary conditions. We use fault interaction modeling to compare vertical displacement fields with topographic metrics to differentiate the two types of landscapes. By successively rotating the axis of maximum horizontal stress, we produce a suite of vertical displacement fields for comparison with real landscapes. We apply this model to a transpressional duplex in the south central Alborz Mountains of Iran, where NW oriented compression was superseded by neotectonic NE compression. The consistency between the modeled displacement field and real landforms indicates that the duplex topography is mostly compatible with the modern boundary conditions, but might include a small remnant from the earlier deformation phase. Our approach is applicable for various tectonic settings and represents an approach to identify the changing boundary conditions that produce composite landscapes. It may be particularly useful for identifying changes that occurred in regions where river profiles may no longer record a signal of the change or where the spatial pattern of uplift is complex.
The mechanisms by which climate and vegetation affect erosion rates over various time scales lie at the heart of understanding landscape response to climate change. Plot-scale field experiments show that increased vegetation cover slows erosion, implying that faster erosion should occur under low to moderate vegetation cover. However, demonstrating this concept over long time scales and across landscapes has proven to be difficult, especially in settings complicated by tectonic forcing and variable slopes. We investigate this problem by measuring cosmogenic Be-10-derived catchment-mean denudation rates across a range of climate zones and hillslope gradients in the Kenya Rift, and by comparing our results with those published from the Rwenzori Mountains of Uganda. We find that denudation rates from sparsely vegetated parts of the Kenya Rift are up to 0.13 mm/yr, while those from humid and more densely vegetated parts of the Kenya Rift flanks and the Rwenzori Mountains reach a maximum of 0.08 mm/yr, despite higher median hillslope gradients. While differences in lithology and recent land-use changes likely affect the denudation rates and vegetation cover values in some of our studied catchments, hillslope gradient and vegetation cover appear to explain most of the variation in denudation rates across the study area. Our results support the idea that changing vegetation cover can contribute to complex erosional responses to climate or land-use change and that vegetation cover can play an important role in determining the steady-state slopes of mountain belts through its stabilizing effects on the land surface.
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 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.
Terrestrial cosmogenic nuclide (TCN) concentrations in fluvial sediment, from which denudation rates are commonly inferred, can be affected by hillslope processes. TCN concentrations in gravel and sand may differ if localized, deep-excavation processes (e.g. landslides, debris flows) affect the contributing catchment, whereas the TCN concentrations of sand and gravel tend to be more similar when diffusional processes like soil creep and sheetwash are dominant. To date, however, no study has systematically compared TCN concentrations in different detrital grain-size fractions with a detailed inventory of hillslope processes from the entire catchment. Here we compare concentrations of the TCN Be-10 in 20 detrital sand samples from the Quebrada del Toro (southern Central Andes, Argentina) to a hillslope-process inventory from each contributing catchment. Our comparison reveals a shift from low-slope gullying and scree production in slowly denuding, low-slope areas to steep-slope gullying and landsliding in fast-denuding, steep areas. To investigate whether the nature of hillslope processes (locally excavating or more uniformly denuding) may be reflected in a comparison of the Be-10 concentrations of sand and gravel, we define the normalized sand-gravel index (NSGI) as the Be-10-concentration difference between sand and gravel divided by their summed concentrations. We find a positive, linear relationship between the NSGI and median slope, such that our NSGI values broadly reflect the shift in hillslope processes from low-slope gullying and scree production to steep-slope gullying and landsliding. Higher NSGI values characterize regions affected by steep-slope gullying or landsliding. We relate the large scatter in the relationship, which is exhibited particularly in low-slope areas, to reduced hillslope-channel connectivity and associated transient sediment storage within those catchments. While high NSGI values in well-connected catchments are a reliable signal of deep-excavation processes, hillslope excavation processes may not be reliably recorded by NSGI values where sediment experiences transient storage. (c) 2018 John Wiley & Sons, Ltd.
Fluvial fill terraces in intermontane basins are valuable geomorphic archives that can record tectonically and/or climatically driven changes of the Earth-surface process system. However, often the preservation of fill terrace sequences is incomplete and/or they may form far away from their source areas, complicating the identification of causal links between forcing mechanisms and landscape response, especially over multi-millennial timescales. The intermontane Toro Basin in the southern Central Andes exhibits at least five generations of fluvial terraces that have been sculpted into several-hundred-meter-thick Quaternary valley-fill conglomerates. New surface-exposure dating using nine cosmogenic Be-10 depth profiles reveals the successive abandonment of these terraces with a 100 kyr cyclicity between 75 +/- 7 and 487 +/- 34 ka. Depositional ages of the conglomerates, determined by four Al-26/Be-10 burial samples and U-Pb zircon ages of three intercalated volcanic ash beds, range from 18 +/- 141 to 936 +/- 170 ka, indicating that there were multiple cut-and-fill episodes. Although the initial onset of aggradation at similar to 1 Ma and the overall net incision since ca. 500 ka can be linked to tectonic processes at the narrow basin outlet, the superimposed 100 kyr cycles of aggradation and incision are best explained by eccentricity-driven climate change. Within these cycles, the onset of river incision can be correlated with global cold periods and enhanced humid phases recorded in paleoclimate archives on the adjacent Bolivian Altiplano, whereas deposition occurred mainly during more arid phases on the Altiplano and global interglacial periods. We suggest that enhanced runoff during global cold phases - due to increased regional precipitation rates, reduced evapotranspiration, or both - resulted in an increased sediment-transport capacity in the Toro Basin, which outweighed any possible increases in upstream sediment supply and thus triggered incision. Compared with two nearby basins that record precessional (21-kyr) and long-eccentricity (400-kyr) forcing within sedimentary and geomorphic archives, the recorded cyclicity scales with the square of the drainage basin length. (C) 2017 Elsevier B.V. All rights reserved.
The sensitivity of fluvial systems to tectonic and climatic boundary conditions allows us to use the geomorphic and stratigraphic records as quantitative archives of past climatic and tectonic conditions. Thus, fluvial terraces that form on alluvial fans and floodplains as well as the rate of sediment export to oceanic and continental basins are commonly used to reconstruct paleoenvironments. However, we currently lack a systematic and quantitative understanding of the transient evolution of fluvial systems and their associated sediment storage and release in response to changes in base level, water input, and sediment input. Such knowledge is necessary to quantify past environmental change from terrace records or sedimentary deposits and to disentangle the multiple possible causes for terrace formation and sediment deposition. Here, we use a set of seven physical experiments to explore terrace formation and sediment export from a single, braided channel that is perturbed by changes in upstream water discharge or sediment supply, or through downstream base-level fall. Each perturbation differently affects (1) the geometry of terraces and channels, (2) the timing of terrace cutting, and (3) the transient response of sediment export from the basin. In general, an increase in water discharge leads to near-instantaneous channel incision across the entire fluvial system and consequent local terrace cutting, thus preserving the initial channel slope on terrace surfaces, and it also produces a transient increase in sediment export from the system. In contrast, a decreased upstream sediment-supply rate may result in longer lag times before terrace cutting, leading to terrace slopes that differ from the initial channel slope, and also lagged responses in sediment export. Finally, downstream base-level fall triggers the upstream propagation of a diffuse knickzone, forming terraces with upstream-decreasing ages. The slope of terraces triggered by base-level fall mimics that of the newly adjusted active channel, whereas slopes of terraces triggered by a decrease in upstream sediment discharge or an increase in upstream water discharge are steeper compared to the new equilibrium channel. By combining fillterrace records with constraints on sediment export, we can distinguish among environmental perturbations that would otherwise remain unresolved when using just one of these records.
Climatic controls on debris-flow activity and sediment aggradation: The Del Medio fan, NW Argentina
(2016)
In the Central Andes, several studies on alluvial terraces and valley fills have linked sediment aggradation to periods of enhanced sediment supply. However, debate continues over whether tectonic or climatic factors are most important in triggering the enhanced supply. The Del Medio catchment in the Humahuaca Basin (Eastern Cordillera, NW Argentina) is located within a transition zone between subhumid and arid climates and hosts the only active debris-flow fan within this intermontane valley. By combining Be-10 analyses of boulder and sediment samples within the Del Medio catchment, with regional morphometric measurements of nearby catchments, we identify the surface processes responsible for aggradation in the Del Medio fan and their likely triggers. We find that the fan surface has been shaped by debris flows and channel avulsions during the last 400 years. Among potential tectonic, climatic, and autogenic factors that might influence deposition, our analyses point to a combination of several favorable factors that drive aggradation. These are in particular the impact of occasional abundant rainfall on steep slopes in rock types prone to failure, located in a region characterized by relatively low rainfall amounts and limited transport capacity. These characteristics are primarily associated with the climatic transition zone between the humid foreland and the arid orogen interior, which creates an imbalance between sediment supply and sediment transfer. The conditions and processes that drive aggradation in the Del Medio catchment today may provide a modern analog for the conditions and processes that drove aggradation in other nearby tributaries in the past.
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.
The sensitivity of fluvial systems to tectonic and climatic boundary conditions allows us to use the geomorphic and stratigraphic records as quantitative archives of past climatic and tectonic conditions. Thus, fluvial terraces that form on alluvial fans and floodplains as well as the rate of sediment export to oceanic and continental basins are commonly used to reconstruct paleoenvironments. However, we currently lack a systematic and quantitative understanding of the transient evolution of fluvial systems and their associated sediment storage and release in response to changes in base level, water input, and sediment input. Such knowledge is necessary to quantify past environmental change from terrace records or sedimentary deposits and to disentangle the multiple possible causes for terrace formation and sediment deposition. Here, we use a set of seven physical experiments to explore terrace formation and sediment export from a single, braided channel that is perturbed by changes in upstream water discharge or sediment supply, or through downstream base-level fall. Each perturbation differently affects (1) the geometry of terraces and channels, (2) the timing of terrace cutting, and (3) the transient response of sediment export from the basin. In general, an increase in water discharge leads to near-instantaneous channel incision across the entire fluvial system and consequent local terrace cutting, thus preserving the initial channel slope on terrace surfaces, and it also produces a transient increase in sediment export from the system. In contrast, a decreased upstream sediment-supply rate may result in longer lag times before terrace cutting, leading to terrace slopes that differ from the initial channel slope, and also lagged responses in sediment export. Finally, downstream base-level fall triggers the upstream propagation of a diffuse knickzone, forming terraces with upstream-decreasing ages. The slope of terraces triggered by base-level fall mimics that of the newly adjusted active channel, whereas slopes of terraces triggered by a decrease in upstream sediment discharge or an increase in upstream water discharge are steeper compared to the new equilibrium channel. By combining fillterrace records with constraints on sediment export, we can distinguish among environmental perturbations that would otherwise remain unresolved when using just one of these records.
Drainage-divide migration, controlled by rock-uplift and rainfall patterns, may play a major role in the geomorphic evolution of mountain ranges.
However, divide-migration rates over geologic timescales have only been estimated by theoretical studies and remain empirically poorly constrained.
Geomorphological evidence suggests that the Sierra de Aconquija, on the eastern side of the southern Central Andes, northwest Argentina, is undergoing active westward drainage-divide migration. The mountain range has been subjected to steep rock trajectories and pronounced orographic rainfall for the last several million years, presenting an ideal setting for using low-temperature thermochronometric data to explore its topographic evolution.
We perform three-dimensional thermal-kinematic modeling of previously published thermochronometric data spanning the windward and leeward sides of the range to explore the most likely structural and topographic evolution of the range.
We find that the data can be explained by scenarios involving drainage-divide migration alone, or by scenarios that also involve changes in the structures that have accommodated deformation through time.
By combining new Be-10-derived catchment-average denudation rates with geomorphic constraints on probable fault activity, we conclude that the evolution of the range was likely dominated by west-vergent faulting on a high-angle reverse fault underlying the range, together with westward drainage-divide migration at a rate of several km per million years.
Our findings place new constraints on the magnitudes and rates of drainage-divide migration in real landscapes, quantify the effects of orographic rainfall and erosion on the topographic evolution of a mountain range, and highlight the importance of considering drainage-divide migration when interpreting thermochronometer age patterns.
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.