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The modern foreland basin straddling the eastern margin of the Andean orogen is the prime example of a retro-arc foreland basin system adjacent to a subduction orogen. While widely studied in the central and southern Andes, the spatial and temporal evolution of the Cenozoic foreland basin system in the northern Andes has received considerably less attention. This is in part due to the complex geodynamic boundary conditions, such as the oblique subduction and accretion of the Caribbean plates to the already complex interaction between the Nazca and the South American plates. In the Colombian Andes, for example, a foreland basin system has been forming since ~80 Ma over an area previously affected by rift tectonics during the Mesozoic. This setting of Cenozoic contractile deformation superposed on continental crust pre-strained by extensional processes thus represents a natural, yet poorly studied experimental set-up, where the role of tectonic inheritance on the development of foreland basin systems can be evaluated. However, a detailed documentation of the early foreland basin evolution in this part of the Andes has thus far only been accomplished in the more internal sectors of the orogen. In this study, I integrate new structural, sedimentological and biostratigraphic data with low-temperature thermochronology from the eastern sector of the Colombian Andes, in order to provide the first comprehensive account of mountain building and related foreland basin sedimentation in this part of the orogen, and to assess as to what extent pre-existent basement anisotropies have conditioned the locus of foreland deformation in space and time. In the Medina Basin, along the eastern flank of the Eastern Cordillera, I integrated detailed structural mapping and new sedimentological data with a new chronostratigraphic framework based on detailed palynology that links an eastward-thinning early Oligocene to early Miocene syntectonic wedge containing rapid facies changes with an episode of fast tectonic subsidence starting at ~30 Ma. This record represents the first evidence of topographic loading generated by slip along the principal basement-bounding thrusts in the Eastern Cordillera to the west of the basin and thus constrains the onset of mountain building in this area. A comprehensive assessment of exhumation patterns based on zircon fission-track (ZFT), apatite fission-track (AFT) analysis and thermal modelling reveals the location of these thrust loads to have been located along the contractionally reactivated Soapaga Fault in the axial sector of the Eastern Cordillera. Farther to the east, AFT and ZFT data also document the onset of thrust-induced exhumation associated with contractional reactivation of the main range-bounding Servita Fault at ~20 Ma. Associated with this episode of orogenic growth, peak burial temperature estimates based on vitrinite reflectance data in the Cenozoic sedimentary record of the adjacent Medina Basin documents earlier incorporation of the western sector of the basin into the advancing fold and thrust belt. I combined these new thermochronological data with published AFT analyses and known chronologic indicators of brittle deformation in order to evaluate the patterns of orogenic-front migration in the Andes of central Colombia. This spatiotemporal analysis of deformation reveals an episodic pattern of eastward migration of the orogenic front at an average rate of 2.5-2.7 mm/yr during the Late Cretaceous-Cenozoic. I identified three major stages of orogen propagation. First, following initiation of mountain building in the Central Cordillera during the Late Cretaceous, the orogenic front propagate eastward at slow rates (0.5-3.1 mm/yr) until early Eocene times. Such slow orogenic advance would have resulted from limited accretionary flux related to slow and oblique (SW-NE-oriented) convergence of the Farallon and South American plates during that time. A second stage of rapid orogenic advance (4.0-18.0 mm/yr) during the middle-late Eocene, and locally of at least 100 mm/yr in the middle Eocene, resulted from initial tectonic inversion of the Eastern Cordillera. I correlate this episode of rapid orogen-front migration with an increase in the accretionary flux triggered by acceleration in convergence and a rotation of the convergence vector to a more orogen-perpendicular direction. Finally, stagnation of the Miocene deformation front along former rift-bounding reactivated faults in the eastern flank of the Eastern Cordillera led to a decrease in the rates of orogenic advance. Post-late Miocene-Pliocene thrusting along the actively deforming front of the Eastern Cordillera at this latitude suggests averaged Miocene-Holocene orogen propagation rates of 1.2-2.1 mm/yr. In addition, ZFT data suggest that exhumation along the eastern flank of the orogen occurred at moderate rates of ~0.3 mm/yr during the Miocene, prior to an acceleration of exhumation since the Pliocene, as suggested by recently published AFT data. In order to evaluate the relations between thrust loading and sedimentary facies evolution in the foreland, I analyzed gravel progradation in the foreland basin system. In particular, I compared one-dimensional Eocene to Pliocene sediment accumulation rates in the Medina basin with a three-dimensional sedimentary budget based on the interpretation of ~1800 km of industry-style seismic reflection profiles and borehole data tied to the new chronostratigraphic framework. The sedimentological data from the Medina Basin reveal rapid accumulation of fluvial and lacustrine sediments at rates of up to ~ 0.5 mm/yr during the Miocene. Provenance data based on gravel petrography and paleocurrents reveal that these Miocene fluvial systems were sourced by Upper Cretaceous and Paleocene sedimentary units exposed to the west, in the Eastern Cordillera. Peak sediment-accumulation rates in the upper Carbonera Formation and the Guayabo Group occur during episodes of gravel progradation in the proximal foredeep in the Early and Late Miocene. I interpreted this positive correlation between sediment accumulation and gravel deposition as the direct consequence of thrust activity in the Servita-Lengupá Fault. This contrasts with current models relating gravel progradation to episodes of tectonic quiescence in more distal portions of foreland basin systems and calls for a re-evaluation of tectonic history interpretations inferred from sedimentary units in other mountain belts. In summary, my results document a late Eocene-early Miocene eastward advance of the topographic loads associated with the leading edge of deformation in the northern Andes of Colombia. Crustal thickening of the Eastern Cordillera associated with initiation of thrusting along the Servitá Fault illustrates that this sector of the Andean orogen acquired ~90% of its present width already by the early Miocene (~20 Ma). My data thus demonstrate that inherited crustal anisotropies, such as the former rift-bounding faults of the Eastern Cordillera, favour a non-systematic progression of foreland basin deformation through time by preferentially concentrating accommodation of slip and thrust-loading. These new chronology of exhumation and deformation associated with specific structures in the Colombian Andes also constitutes an important advance towards the understanding of models for hydrocarbon maturation, migration and trap formation along the prolific petroleum province of the Llanos Basin in the modern foredeep area.
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 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.
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 Himalayan thrust belt comprises three in-sequence foreland-propagating orogen-scale faults, the Main Central thrust, the Main Boundary thrust, and the Main Frontal thrust. Recently, the Munsiari–Ramgarh–Shumar thrust system has been recognized as an additional, potentially orogen-scale shear zone in the proximal footwall of the Main Central thrust. The timing of the Munsiari, Ramgarh, and Shumar thrusts and their role in Himalayan tectonics are disputed. We present 31 new zircon (U–Th)/He ages from a profile across the central Himachal Himalaya in the Beas River area. Within a ∼40 km wide belt northeast of the Kullu–Larji–Rampur window, ages ranging from to constrain a distinct episode of rapid Pliocene to Present exhumation; north and south of this belt, zircon (U–Th)/He ages are older ( to ). We attribute the Pliocene rapid exhumation episode to basal accretion to the Himalayan thrust belt and duplex formation in the Lesser Himalayan sequence including initiation of the Munsiari thrust. Pecube thermokinematic modelling suggests exhumation rates of ∼2–3 mm/yr from 4–7 to 0 Ma above the duplex contrasting with lower (<0.3 mm/yr) middle-late Miocene exhumation rates. The Munsiari thrust terminates laterally in central Himachal Pradesh. In the NW Indian Himalaya, the Main Central thrust zone comprises the sheared basal sections of the Greater Himalayan sequence and the mylonitic ‘Bajaura nappe’ of Lesser Himalayan affinity. We correlate the Bajaura unit with the Ramgarh thrust sheet in Nepal based on similar lithologies and the middle Miocene age of deformation. The Munsiari thrust in the central Himachal Himalaya is several Myr younger than deformation in the Bajaura and Ramgarh thrust sheets. Our results illustrate the complex and segmented nature of the Munsiari–Ramgarh–Shumar thrust system.
The role of feedback between erosional unloading and tectonics controlling the development of the Himalaya is a matter of current debate. The distribution of precipitation is thought to control surface erosion, which in turn results in tectonic exhumation as an isostatic compensation process. Alternatively, subsurface structures can have significant influence in the evolution of this actively growing orogen. Along the southern Himalayan front new 40Ar/39Ar white mica and apatite fission track (AFT) thermochronologic data provide the opportunity to determine the history of rock-uplift and exhumation paths along an approximately 120-km-wide NE-SW transect spanning the greater Sutlej region of the northwest Himalaya, India. 40Ar/39Ar data indicate, consistent with earlier studies that first the High Himalayan Crystalline, and subsequently the Lesser Himalayan Crystalline nappes were exhumed rapidly during Miocene time, while the deformation front propagated to the south. In contrast, new AFT data delineate synchronous exhumation of an elliptically shaped, NE-SW-oriented ~80 x 40 km region spanning both crystalline nappes during Pliocene-Quaternary time. The AFT ages correlate with elevation, but show within the resolution of the method no spatial relationship to preexisting major tectonic structures, such as the Main Central Thrust or the Southern Tibetan Fault System. Assuming constant exhumation rates and geothermal gradient, the rocks of two age vs. elevation transects were exhumed at ~1.4 ±0.2 and ~1.1 ±0.4 mm/a with an average cooling rate of ~50-60 °C/Ma during Pliocene-Quaternary time. The locus of pronounced exhumation defined by the AFT data coincides with a region of enhanced precipitation, high discharge, and sediment flux rates under present conditions. We therefore hypothesize that the distribution of AFT cooling ages might reflect the efficiency of surface processes and fluvial erosion, and thus demonstrate the influence of erosion in localizing rock-uplift and exhumation along southern Himalayan front, rather than encompassing the entire orogen.Despite a possible feedback between erosion and exhumation along the southern Himalayan front, we observe tectonically driven, crustal exhumation within the arid region behind the orographic barrier of the High Himalaya, which might be related to and driven by internal plateau forces. Several metamorphic-igneous gneiss dome complexes have been exhumed between the High Himalaya to the south and Indus-Tsangpo suture zone to the north since the onset of Indian-Eurasian collision ~50 Ma ago. Although the overall tectonic setting is characterized by convergence the exhumation of these domes is accommodated by extensional fault systems.Along the Indian-Tibetan border the poorly described Leo Pargil metamorphic-igneous gneiss dome (31-34°N/77-78°E) is located within the Tethyan Himalaya. New field mapping, structural, and geochronologic data document that the western flank of the Leo Pargil dome was formed by extension along temporally linked normal fault systems. Motion on a major detachment system, referred to as the Leo Pargil detachment zone (LPDZ) has led to the juxtaposition of low-grade metamorphic, sedimentary rocks in the hanging wall and high-grade metamorphic gneisses in the footwall. However, the distribution of new 40Ar/39Ar white mica data indicate a regional cooling event during middle Miocene time. New apatite fission track (AFT) data demonstrate that subsequently more of the footwall was extruded along the LPDZ in a brittle stage between 10 and 2 Ma with a minimum displacement of ~9 km. Additionally, AFT-data indicate a regional accelerated cooling and exhumation episode starting at ~4 Ma. Thus, tectonic processes can affect the entire orogenic system, while potential feedbacks between erosion and tectonics appear to be limited to the windward sides of an orogenic systems.
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.
Continental rifts are excellent regions where the interplay between extension, the build-up of topography, erosion and sedimentation can be evaluated in the context of landscape evolution. Rift basins also constitute important archives that potentially record the evolution and migration of species and the change of sedimentary conditions as a result of climatic change. Finally, rifts have increasingly become targets of resource exploration, such as hydrocarbons or geothermal systems. The study of extensional processes and the factors that further modify the mainly climate-driven surface process regime helps to identify changes in past and present tectonic and geomorphic processes that are ultimately recorded in rift landscapes.
The Cenozoic East African Rift System (EARS) is an exemplary continental rift system and ideal natural laboratory to observe such interactions. The eastern and western branches of the EARS constitute first-order tectonic and topographic features in East Africa, which exert a profound influence on the evolution of topography, the distribution and amount of rainfall, and thus the efficiency of surface processes. The Kenya Rift is an integral part of the eastern branch of the EARS and is characterized by high-relief rift escarpments bounded by normal faults, gently tilted rift shoulders, and volcanic centers along the rift axis.
Considering the Cenozoic tectonic processes in the Kenya Rift, the tectonically controlled cooling history of rift shoulders, the subsidence history of rift basins, and the sedimentation along and across the rift, may help to elucidate the morphotectonic evolution of this extensional province. While tectonic forcing of surface processes may play a minor role in the low-strain rift on centennial to millennial timescales, it may be hypothesized that erosion and sedimentation processes impacted by climate shifts associated with pronounced changes in the availability in moisture may have left important imprints in the landscape.
In this thesis I combined thermochronological, geomorphic field observations, and morphometry of digital elevation models to reconstruct exhumation processes and erosion rates, as well as the effects of climate on the erosion processes in different sectors of the rift. I present three sets of results: (1) new thermochronological data from the northern and central parts of the rift to quantitatively constrain the Tertiary exhumation and thermal evolution of the Kenya Rift. (2) 10Be-derived catchment-wide mean denudation rates from the northern, central and southern rift that characterize erosional processes on millennial to present-day timescales; and (3) paleo-denudation rates in the northern rift to constrain climatically controlled shifts in paleoenvironmental conditions during the early Holocene (African Humid Period).
Taken together, my studies show that time-temperature histories derived from apatite fission track (AFT) analysis, zircon (U-Th)/He dating, and thermal modeling bracket the onset of rifting in the Kenya Rift between 65-50 Ma and about 15 Ma to the present. These two episodes are marked by rapid exhumation and, uplift of the rift shoulders. Between 45 and 15 Ma the margins of the rift experienced very slow erosion/exhumation, with the accommodation of sediments in the rift basin.
In addition, I determined that present-day denudation rates in sparsely vegetated parts of the Kenya Rift amount to 0.13 mm/yr, whereas denudation rates in humid and more densely vegetated sectors of the rift flanks reach a maximum of 0.08 mm/yr, despite steeper hillslopes. I inferred that hillslope gradient and vegetation cover control most of the variation in denudation rates across the Kenya Rift today. Importantly, my results support the notion that vegetation cover plays a fundamental role in determining the voracity of erosion of hillslopes through its stabilizing effects on the land surface.
Finally, in a pilot study I highlighted how paleo-denudation rates in climatic threshold areas changed significantly during times of transient hydrologic conditions and involved a sixfold increase in erosion rates during increased humidity. This assessment is based on cosmogenic nuclide (10Be) dating of quartzitic deltaic sands that were deposited in the northern Kenya Rift during a highstand of Lake Suguta, which was associated with the Holocene African Humid Period. Taken together, my new results document the role of climate variability in erosion processes that impact climatic threshold environments, which may provide a template for potential future impacts of climate-driven changes in surface processes in the course of Global Change.