Thermokinematic Modeling Research Articles

Constraining the slip history of the Katschberg normal fault (Eastern Tauern Window) by thermo-kinematic modeling: Implications for the tectonic evolution of the Eastern European Alps in the late Cenozoic

The Katschberg normal fault borders the Tauern Window to the east and played a crucial role during Miocene lateral tectonic extrusion in the Eastern European Alps. In this study, we present new cooling ages from low-temperature thermochronology as well as thermo-kinematic models, which constrain the exhumation history of the Penninic units in the footwall of the Katschberg normal fault and its slip history. Zircon and apatite fission track and apatite (U–Th)/He ages from footwall units range from 16.0 ± 1.9 Ma to 12.8 ± 1.4 Ma, 10.4 ± 1.8 Ma to 7.9 ± 1.3 Ma and 8.2 ± 0.8 Ma to 3.9 ± 0.4 Ma, respectively. Thermo-kinematic modeling indicates that the Katschberg normal fault was active between 21.1 ± 1.8 Ma and 12.2 ± 1.3 Ma and accommodated 27 ± 6 km of crustal extension at a total rate of 3.5 ± 0.3 km/Myr. After the end of normal faulting, exhumation continued with a rate of 0.21 ± 0.06 km/Myr until 2.0 ± 0.5 Ma and with a rate of 0.84 ± 0.08 km/Myr until present. A comparison with another Miocene low-angle normal fault in the Eastern Alps – the Brenner fault – reveals that the amount of extension accommodated by these faults decreases from west to east, which is consistent with an eastward decrease in N-S shortening. Therefore, Miocene deformation is greatest in the western Tauern Window near the Brenner normal fault where shortening in front of the Adriatic Indenter is at its maximum.

Mylonite, cataclasite, and gouge: Reconstruction of mechanical heterogeneity along a low-angle normal fault: Death valley, USA

The spectrum of slip behavior in crustal faults generates various rock types that can inform the mechanics of earthquake genesis. However, a single fault exposure may contain evidence of slip at various depths and temperatures due to progressive fault rock formation and overprinting during exhumation. Here, we unravel the spatiotemporal evolution of mechanical transitions along the Boundary Canyon detachment, a low-angle normal fault northeast of Death Valley, USA. Field, microstructural, and geochemical characterizations of fault rocks are compared to existing laboratory experiments and combined with a thermo-kinematic model of fault evolution. Together, these constrain the depths of mechanical transitions along the fault and reveal the evolution of earthquake nucleation zone thickness. Fault exposures from different initial paleodepths passed through the mechanical transitions during footwall exhumation, resulting in overprinting of mylonite by cataclasite and ubiquitous late formation of foliated, illite-rich gouge within the uppermost crust. We present evidence of coseismic low-angle normal fault slip (e.g., injection veins of cataclasite, laminar and grain-inertial fluidization). Coseismic slip likely nucleated at strength contrasts within the fault zone (i.e., contacts between quartzite breccia and calc-mylonite; quartz ribbons and mylonite matrix; breccia and clay gouge) at ≈ 5–9.5 km depth. Observations including mutually overprinting cataclasite/ultramylonite and exposures of pulverized gouge support that dynamic rupture propagated down-dip through the brittle-ductile transition zone (≈10–11 km depth) and up-dip through velocity-strengthening fault patches (≈0–5 km depth). Rapid fault exhumation increased the geotherm, leading to upward advection of the brittle-ductile transition and shallowing/thinning of the earthquake nucleation zone. This process may explain the rarity of large magnitude earthquakes on low-angle normal faults.

Segmentation of the Tashkurgan normal fault in the eastern Pamir: Insights from geomorphology and thermochronology and implications for fault-slip transfer

At the northwestern end of the India–Asia collision zone, in the eastern Pamir interior, the Kongur Shan extensional system extends for ∼250 km as a composite system of normal faults. As the southernmost segment of the extensional system, the Tashkurgan fault can be divided into a northern and southern segment by the intersection of the Tahman and Tashkurgan faults. To evaluate the tectonic activity along the Tashkurgan fault and explore the extensional characteristics of the southern Kongur Shan extensional system, we integrate geomorphologic analysis with thermochronologic data and modeling. Geomorphic indices, including local relief, normalized channel steepness index (ksn), and river χ-plots, show that the southern Tashkurgan fault is more active than the northern Tashkurgan fault. On the footwall of the northern Tashkurgan fault, most thermochronologic data, which include biotite 40Ar/39Ar (Arbt), zircon (U-Th)/He (ZHe), apatite fission track (AFT), and apatite (U-Th-Sm)/He (AHe) dates, record emplacement-related cooling, and only a few AHe dates possibly record exhumation of <2 km since ∼7 Ma ago. On the footwall of the southern Tashkurgan fault, almost all of the ZHe, AFT, and AHe dates record significant late Miocene to present exhumation; thus, we perform three-dimensional thermokinematic modeling for the normal slip along this fault segment. Modeling results indicate a probably constant slip rate on the southern Tashkurgan fault at a value of 1.4–1.5 mm/a, which corresponds to 6–7 km of both horizontal extension and footwall uplift since 6.5 Ma ago. All together, the geomorphic and thermochronologic evidences imply that the southern Tashkurgan fault is responsible for the ∼E–W extension along the southernmost portion of the Kongur Shan extensional system, while the northern fault segment plays a minor role in the extension. This result, combined with the previously published magnitude of extension along the southern Kongur Shan fault, indicates that the extension at any site along the southern Kongur Shan extensional system can be primarily attributed to normal faulting along a single border fault, which is a typical characteristic of an interbasin transfer zone with a transfer fault.

The Long-Lasting Exhumation History of the Ötztal-Stubai Complex (Eastern European Alps): New Constraints from Zircon (U–Th)/He Age-Elevation Profiles and Thermokinematic Modeling

Abstract The Eastern European Alps formed during two orogenic cycles, which took place in the Cretaceous and Cenozoic, respectively. In the Ötztal-Stubai Complex—a thrust sheet of Variscan basement and Permo-Mesozoic cover rocks—the record of the first (Eoalpine) orogeny is well preserved because during the second (Alpine) orogeny, the complex remained largely undeformed. Here, new zircon (U–Th)/He (ZHe) ages are presented, and thermokinematic modeling is applied to decipher the cooling and exhumation histories of the central part of the Ötztal-Stubai Complex since the Late Cretaceous. The ZHe ages from two elevation profiles increase over a vertical distance of 1500 m from 56 ± 3 to 69 ± 3 Ma (Stubaital) and from 50 ± 2 to 71 ± 4 Ma (Kaunertal), respectively. These ZHe ages and a few published zircon and apatite fission track ages were used for inverse thermokinematic modeling. The modeling results show that the age data are well reproduced with a three-phase exhumation history. The first phase with relatively fast exhumation (~250 m/Myr) during the Late Cretaceous ended at ~70 Ma and is interpreted to reflect the erosion of the Eoalpine mountain belt. As Late Cretaceous normal faults occur at the margins of the Ötztal-Stubai Complex, normal faulting may have also contributed to the exhumation of the study area. Subsequently, a long period with slow exhumation (&amp;lt;10 m/Myr) prevailed until ~16 Ma. This long-lasting phase of slow exhumation suggests a rather low topography with little relief in the Ötztal-Stubai Complex until the mid-Miocene, even though the Alpine orogeny had already begun in the Eocene with the subduction of the European continental margin. Accelerated exhumation since the mid-Miocene (~230 m/Myr) is interpreted to reflect the erosion of the mountain belt due to the development of high topography in front of the Adriatic indenter and repeated glaciations during the Quaternary.

Late-orogenic extension ceases with waning plate convergence: The case of the Simplon normal fault (Swiss Alps)

The Simplon normal fault in the Western Alps caused tens of kilometers of orogen-parallel extension during convergence of the European and Adriatic plates, but the slip rate of the fault and the time when normal faulting ended are still debated. Here, we constrain the slip history of the Simplon fault with low-T thermochronology and thermo-kinematic modeling. Closely spaced samples from an elevation profile in the center of the fault yield zircon (U–Th)/He ages (ZHe) that are nearly invariant over an altitude of 1.4 km and cluster around ∼6 Ma. In contrast, apatite (U–Th)/He ages (AHe) increase with altitude from 3.4 ± 0.3 to 4.6 ± 0.7 Ma, while the AFT ages range from 4.4 ± 0.7 to 5.8 ± 1.5 Ma. In addition, recently published 40Ar/39Ar ages constrain that our samples moved through the brittle-ductile transition (i.e., ∼300 °C) at 8–10 Ma. Our thermo-kinematic inverse modeling shows that these age data can be explained by a single phase of normal faulting, which lasted from 19.8 ± 1.8 to 5.3 ± 0.3 Ma and caused 45 ± 10 km of extension. The slip rate of the 30°-dipping model fault is 3.5 ± 0.3 km/Myr and equivalent to an exhumation rate of ∼1.8 km/Myr. Our modeling reveals that the altitude-dependent difference between ZHe and AHe ages reflects the thermal relaxation after faulting stopped at ∼5.3 Ma. Since then, exhumation by erosion continued at a rate of ∼0.5 km/Myr. Remarkably, the end of slip on the Simplon fault coincides with the cessation of reverse faulting at 6 ± 2 Ma in the external crystalline massifs of the Alps (Aar, Mont Blanc, Aiguilles Rouges) and with a decrease in strain rate by one order of magnitude at 5-4 Ma in the Swiss molasse basin and the Jura mountains. This temporal coincidence suggests that normal faulting in the internal part of the Alps ceased when plate convergence waned and the under-thrusting of European continental lithosphere beneath the Adriatic plate came to an end.

Late Miocene to present synchronous extension and contraction in the eastern Pamir: Insights from inversion of thermochronologic data across the southern Muztaghata dome

Late Cenozoic gneiss domes cover ∼30% of the surface of the Pamir salient in the northwestern end of the India−Asia collision zone. The highest peaks of the Pamir are in the east, where the ∼250-km-long, ∼N−S-trending Kongur Shan extensional system controls the topography. We combined 115 new apatite (U-Th-Sm)/He and zircon (U-Th)/He single-grain dates from 18 samples and previous thermochronologic data with three-dimensional thermokinematic models to constrain the thermo-tectonic history of the southern portion of the Muztaghata dome, one of the largest gneiss domes in the eastern Pamir. The new cooling dates from the western boundary of the southern Muztaghata dome generally increase with distance from the southern Kongur Shan fault and are related to normal faulting along the fault at near-surface levels over the last 6.5 m.y. The new dates across the central−eastern portion of the dome outline the previously recorded U-shaped date pattern at a higher spatial resolution. The modeling indicates that this pattern is most likely the result of uplift and erosion above a flat-ramp-flat thrust fault at depth over the last 7 m.y. Modeling does not resolve how topographic changes may have affected the observed distribution of cooling dates, but it indicates a faster thrust-slip rate associated with an increase in relief and a slower one associated with steady-state topography. Our results suggest that the modern topography along the southern Muztaghata dome, similar to the rest of the eastern Pamir salient, is shaped by normal faulting at shallow depth, but its growth may still be governed by contraction and crustal thickening at depth.

Ongoing India–Asia convergence controlled differential growth of the eastern Tibetan Plateau

The eastern Tibetan Plateau is known for its unique topography and active tectonics, serves as a crucial region for investigating geodynamics and lithosphere–atmosphere interaction. Despite its importance, the formation process and geodynamics of this region remain controversial. We integrated new and reported thermochronological data, and paleo-crustal thickness reconstruction to explore the tectonic deformation, climate–tectonic coupling, and geodynamics of the area. New apatite fission track data, collected from the upstream areas of major rivers in the eastern Tibetan Plateau, revealed multiple rapid river incision phases during the Eocene and Neogene–Quaternary. Apatite (U-Th)/He data obtained from a borehole near the Gaoligong shear zone indicates that the Yingjiang fault experienced two rapid exhumation phases, which initiated at ∼22 Ma and ∼18 Ma, respectively. Thermo-kinematic modeling (Pecube), based on multiple thermochronometers, predicted three phases of thrusting along the Longmenshan fault, which were initiated at ∼34 Ma, ∼17 Ma and ∼14 Ma, respectively. During the late Eocene period, Sr/Y and (La/Yb)n ratios suggest that the eastern Qiangtang and Songpan–Ganzi terranes underwent considerable uplift, followed by outward uplift in the collisional front zone and the southeastern margin of the plateau during the late Oligocene. Paleo-crustal thickness reconstruction also indicate another ∼1 km surface uplift during the Neogene–Quaternary. Our findings, when combined with reported thermochronological data, suggest that during the Neogene–Quaternary, the collisional front zone and the eastern margin of the plateau underwent greater exhumation compared to the eastern Qiangtang and Songpan–Ganzi terranes. The temporally and spatially varied exhumation shaped the landscape of the eastern Tibetan Plateau, driven mainly by precipitation-related erosion and promoted by fold-thrust and strike-slip thrust. This was controlled primarily by eastward lithosphere/asthenosphere extrusion associated with ongoing India–Asia convergence.

Tectono-geomorphological evolution of the Eastern Pyrenees: Insights from thermo-kinematic modeling

Constraining the tectono-geomorphological evolution of the Pyrenees is still a major challenge, especially in the Eastern Pyrenees where late Neogene exhumation history and topographic evolution appear contrasted and have been debated. In this study, we performed thermo-kinematic (Pecube) modeling using a relatively dense spatial compilation of previously-published low-temperature thermochronological data for the Eastern Pyrenees, and quantified the late-Neogene exhumation history based on different topographic evolution models reconstructed from literature constraints. Our modeling outcomes suggest a major decrease in the regional exhumation rate between 37 and 35 Ma, associated to an early-end contractional tectonics for the Eastern Pyrenees. Two main periods of the Têt fault normal activity have been characterized, with the existence of pluri-kilometric (2–3 km) post- Oligocene-Miocene displacement along the western segment of the Têt fault. The different topographic evolution models show relatively similar results for the regional exhumation since ca. 35 Ma and for the most recent (mid to late-Miocene) tectonic event recorded along the Têt fault. Conversely, inversion results obtained for pre-35 Ma regional exhumation rates, and more importantly for the Oligocene-Miocene activity of the Têt fault appear contrasted between the investigated topographic scenarios. Overall, this study confirmed a mid to late Miocene significant extensional tectonic event for the Eastern Pyrenees, which may have had a non-negligible role in late-stage relief evolution for this part of the orogen. However, deciphering the long-term evolution of topographic relief through thermo-kinematic modeling appears still challenging for slowly-exhuming mountain ranges given the limited resolution of low-temperature thermochronological data to topographic changes.

Assessing drivers of high exhumation magnitudes and young cooling ages in the eastern central Andes, southern Peru (13–18°S)

The potential structural controls on exhumation across the southern Peruvian Andes are not well understood, in part due to limited structural studies that co-locate with thermochronometric datasets. We integrate these two datasets and evaluate the relative contribution that fault geometry, magnitude, and shortening rate have on predicted cooling ages. Here we present a balanced cross-section constructed using new structural observations. This section, combined with existing thermochronometer data and a thermokinematic model, investigates the drivers of high exhumation and young canyon thermochronometric ages along the deeply incised Marcapata canyon in southern Peru. Together, these approaches constrain the timing and magnitude of exhumation in this portion of the southern Peruvian Andes and provide a mechanism for documenting how the internal architecture changes along strike.The balanced cross-section (oriented N30E) covers the Subandean Zone to the northeast, the Marcapata canyon on the eastern flank of the southern Peruvian Andes, and the Altiplano-Eastern Cordillera boundary to the southwest (13–18° S). Exhumation is constrained by four low-temperature thermochronometer systems, including apatite and zircon (U-Th)/He (AHe and ZHe, respectively) and fission-track (AFT and ZFT, respectively). The youngest AHe (∼1–3 Ma), AFT (∼3–7 Ma), ZHe (∼4–7 Ma), and ZFT (∼14–17 Ma) ages are located in the center and valley bottom of the Marcapata canyon. The thermokinematically modeled cross-section produces cooling ages determined by fault geometry and kinematics. Reset ZFT ages require burial of Ordovician rocks in excess of 5.5 km above the original 6.5 km depositional depth. We find that the ZFT and ZHe ages in the Eastern Cordillera are sensitive to the history and magnitude of burial, age and location of uplift, and canyon incision. Canyon incision is required to reproduce the youngest canyon thermochronometric ages while slow shortening rates from ∼10 Ma to Present are required to reproduce interfluve thermochronometric ages. Shortening is accommodated by basement faults that feed slip up through three different décollement levels before reaching the surface. The proposed stacked basement geometry sets the first-order cooling signal seen in modeled ages. We determined that the total shortening in this section from the Subandean Zone to the Altiplano is 147.5 km, similar to shortening estimates in an adjacent thermo-kinematically modeled section in the San Gabán canyon 50 km to the southeast. Both the ZHe and ZFT ages in the Marcapata section (4–5 and 14 Ma) are noticeably younger than cooling ages from the San Gabán section (16 and 29 Ma). The Marcapata section's higher magnitude of exhumation is due to a repetition of basement thrusts that continues to elevate the Eastern Cordillera while active deformation occurs in the Subandean Zone. The youngest thermochronometric ages in all four systems are co-located with the overlapping basement thrust geometry. This basement geometry, kinematic sequence of deformation, and canyon incision co-conspire to produce the young cooling ages observed in the Eastern Cordillera.

Timing and drivers of exhumation and sedimentation in the eastern Peruvian Andes: Insights from thermokinematic modelling

This study assesses the impact of fold-thrust belt driven deformation on the topographic evolution, bedrock exhumation and basin formation in the southeastern Peruvian Andes. We do this through a flexural and thermokinematically modelled balanced cross-section. In addition, published thermochronology samples from low-elevation (river canyons) and high-elevation (interfluves) and Cenozoic sedimentary basin datasets along the balanced cross-section were used to evaluate the age, location, and geometry of fault-driven uplift, as well as potential relationships to the timing of ∼2 km of canyon incision. The integrated structural, thermochronologic, and basin data were used to test the sensitivity of model results to various shortening rates and durations, a range of thermophysical parameters, and different magnitudes and timing of canyon incision. Results indicate that young apatite (U-Th)/He (AHe) canyon samples from ∼2 km in elevation or lower are consistent with river incision occurring between ∼8–2 Ma and are independent of the timing of ramp-driven uplift and accompanying erosion. In contrast, replicating the young AHe canyon samples located at >2.7 km elevation requires ongoing ramp-driven uplift. Replicating older interfluve cooling ages concurrent with young canyon ages necessitates slow shortening rates (0.25–0.6 mm/y) from ∼10 Ma to Present, potentially reflecting a decrease in upper plate compression during slab steepening. The best-fit model that reproduces basin ages and depositional contacts requires a background shortening rate of 3–4 mm/y with a marked decrease in rates to ≤0.5 mm/y at ∼10 Ma. Canyon incision occurred during this period of slow shortening, potentially enhanced by Pliocene climate change.

Temperature, Deformation, and Mass Transfer in a Hot Orogen: Insights From Thermokinematic Forward Models for Far Western Nepal

AbstractExhumation and cooling pathways of mid‐crustal metamorphic rocks in the western Nepal Himalaya can be replicated by fold‐thrust belt structures with displacement localized along discrete décollements. New and published muscovite 40Ar/39Ar, zircon U‐Th/He, and apatite fission track cooling ages, peak temperature estimates, geologic mapping, and basin data are integrated with thermokinematic forward models to constrain the geometry, kinematics, and rates of shortening in far western Nepal. The best fit to peak temperatures, cooling ages, and basin accumulation data is achieved with a largely in‐sequence kinematic order, with out‐of‐sequence motion on the Ramgarh‐Munsiari thrust. Fast rates (∼20–40 mm/yr) are required during shortening on early, large displacement faults at ∼23–12 Ma and decrease to ∼10–15 mm/yr during formation of the Lesser Himalayan duplex until ∼1 Ma. Thermokinematic models highlight the relationship between peak temperature, geometry, and shortening on the large displacement Main Central and Ramgarh‐Munsiari thrusts. In the thermokinematic models, we observe a relationship between the location of frontal ramps for the faults that displace lower Lesser Himalayan units and the ∼375°C isotherm, immediately before the ramp becomes active. These correlations suggest that temperature exerts a first‐order control on thrust geometry in a hot orogen. Viable models highlight the position of active ramps, kinematic order of faults, timing of fault motion, and reduction in shortening rates that are required to reproduce the surface geology, basin accumulation, peak temperature conditions, and timing of exhumation. Cooling ages are far more sensitive to the age of fault motion than the rate of fault motion.

Phases of Enhanced Exhumation During the Cretaceous and Cenozoic Orogenies in the Eastern European Alps: New Insights From Thermochronological Data and Thermokinematic Modeling

AbstractAustroalpine nappes in the Eastern European Alps have preserved the record of orogenies in the Cretaceous and Cenozoic but their cooling and exhumation history remains poorly constrained. Here we use low‐temperature thermochronology and thermokinematic modeling to unravel the exhumation history of the Austroalpine nappes in the Gurktal Alps. Our data reveal marked differences between the exhumation of units located at different positions within the nappe stack and relative to the Adriatic indenter. Units located at a high structural level and farther away from the indenter cooled through the zircon fission track closure temperature in the Late Cretaceous and have resided at depths of ≤5–6 km since the Oligocene, as indicated by apatite fission track ages of 35–30 Ma. Thermokinematic modeling constrained that these units experienced enhanced exhumation (∼0.60 km/Ma) between ∼99 and ∼83 Ma due to syn‐ to late‐orogenic Late Cretaceous extension. After a phase of slow exhumation (∼0.02 km/Ma), the exhumation rate increased to ∼0.16 km/Ma at ∼34 Ma due to the onset of the Europe‐Adria collision. In contrast, zircon fission track ages from units at a lower structural level and near the indenter indicate cooling during the Eocene; apatite fission track ages cluster at ∼15 Ma. These units were rapidly exhumed (∼0.76 km/Ma) from ∼44 to ∼39 Ma during an Eocene phase of shortening prior to the Europe‐Adria collision. After slow exhumation (∼0.13 km/Ma) between ∼39 and ∼18 Ma, the exhumation rate increased to ∼0.27 km/Ma in the wake of Miocene escape tectonics in the Eastern Alps.

Incision history of the Mekong River valley revealed by spatially differential exhumation

The evolution of the Mekong River valley offers a unique record of the rock uplift and climate history of southeastern Tibet. Faster erosion rates along the valley than those on the flanks caused valley incision and local incision rates may reflect a mix of driving mechanisms, migrating knickpoints linked to a change in base level, and large-scale surface uplift. The thermochronometric-constrained erosion rates in the 2–3 km deep river channel compared to rates outside of the channels provide unique constraints on the driving mechanisms of the river incision. In particular, the onset of river incision at various points along the Mekong River helps identify whether exhumation signals are the result of knickpoint migration that would generate a younger incision upstream, or the outgrowth of the plateau, which would produce younger incision downstream. We analyze new and published apatite (UTh)/He ages collected along two elevation transects in the upper Mekong River: downstream at Tu'Er and upstream at Deqin. The data and thermokinematic modeling predict that the exhumation rates of the valley near Tu'Er and Deqin increased more rapidly than those of adjacent high-elevation, low-relief surfaces at ~11 Ma and ~ 4 Ma, respectively. Since then, exhumation variations have caused an increase of >2 km in the depth of the valley, indicating that the major incision of the Mekong River valley began at ~11 Ma near Tu'Er and approximately 4 Ma near Deqin. The results indicate that a major phase of the Mekong River valley incision may have developed and progressed upstream since the Miocene.

Testing erosional and kinematic drivers of exhumation in the central Himalaya

This study investigates the influence of fault geometry, kinematics, and displacement on the exhumation history of the central Himalaya using geologic mapping constraints, a new interpreted cross-section, and a suite of 176 thermochronometer ages through the Marsyangdi valley in central Nepal. Guided by the cross-section, we integrate a forward model of fold-thrust belt evolution with a 2D thermokinematic model. Model-predicted and measured thermochronometer ages were compared to evaluate the sensitivity of thermochronometer ages to the geometry and location of structures, and their rates of deformation. Results indicate 84% of the measured data can be reproduced with a largely in-sequence system of faulting where displacement occurs on the Main Central thrust (MCT) from 23-16 Ma, the Ramgarh-Munsiari thrust (RMT) from 16-7.5 Ma, the Trishuli thrust (TT) from 7.5-6 Ma and the Main Boundary thrust (MBT) from 6-3 Ma. Our cross-section solution shows the development of a duplex that initiates at 4 Ma with the TT and MBT as the roof thrust. The duplex is translated over the Main Himalayan thrust (MHT) ramp, concurrent with forward propagation of faulting in the synorogenic Siwaliks from 3 Ma to present. Notably, the 2-0.5 Ma apatite fission-track ages between the MCT and Tethyan strata 45 km north of the MCT are not reproduced by the wide range of in-sequence deformation scenarios we explored. Instead, these data are consistent with simulations that include significant displacement on two out-of-sequence (OOS) faults in the last ∼1 Ma: 10 km of OOS faulting near the MCT followed by 5 km of displacement on a fault 15 km south of the MCT. Modeled OOS thrusting both builds significant topography in the hinterland and flexurally suppresses topography in the foreland. Consequently, our preferred kinematic solution has ∼10 km of final fault motion in the Siwaliks to rebuild topography at the MBT and the active MFT from ∼0.5 Ma to the present.Modeled exhumation rates are highly variable through time, and are highest during translation of rocks over high (∼10 km) ramps and during significant changes in architecture, such as duplex formation and OOS faulting. Notably high exhumation rates include 10.5 mm/yr during the first 4 Myr of MCT motion, 4-6 mm/yr at ∼7 Ma, due to the translation over an ∼10 km high TT ramp, and 7-12 mm/yr from 4 Ma to present driven by the development of the duplex and its translation over the MHT ramp, followed by OOS thrusting. The high (7-12 mm/yr) exhumation rates documented here from 4 Ma, correlate with a time of distinct climate change, including strengthening of the monsoon, and northern hemisphere glaciation, lending support to potential climate-tectonic feedbacks.

Rift propagation in south Tibet controlled by under-thrusting of India: a case study of the Tangra Yumco graben (south Tibet)

Active graben systems in south Tibet and the Himalaya are the surface expression of ongoing east–west extension, although the cause and spatiotemporal evolution of normal faulting is a still a matter of debate. We reconstructed the exhumation history driven by normal faulting in the southern Tangra Yumco graben using new thermochronological data. The Miocene cooling history of the footwall of the main graben-bounding fault is constrained by zircon (U–Th)/He ages (16.7 ± 1.0 to 13.3 ± 0.6 Ma), apatite fission track ages (15.9 ± 2.1 to 13.0 ± 2.1 Ma) and apatite (U–Th)/He ages (7.9 ± 0.4 to 5.3 ± 0.3 Ma). Thermo-kinematic modelling of the data indicates that normal faulting began 19.0 ± 1.1 Ma at a rate ofc.0.2 km myr−1and accelerated toc.0.4 km myr−1atc.5 Ma. In the northern Tangra Yumco rift, remodelling of published data shows that faulting startedc.5 myr later at 13.9 ± 0.8 Ma. The age difference and the distance of 130 km between these two sites indicates that rifting and normal faulting propagated northward at an average rate ofc.25 km myr−1. As this rate is similar to the Miocene convergence rate between India and south Tibet, we argue that the under-thrusting of India beneath Tibet exerted an important control on the propagation of rifts in south Tibet.Supplementary material: Figures S1, S2, and Table S1 with details on apatite fission track analysis are available athttps://doi.org/10.6084/m9.figshare.c.6198584

Why are the Appalachians high? New insights from detrital apatite laser ablation (U-Th-Sm)/He dating

The timing of surface uplift across the Appalachian Mountains, Eastern USA, remains a controversial topic. Thermal history modelling, geomorphological analysis, stratigraphic analysis, and river profile analysis have all suggested the region experienced surface uplift during the late Cenozoic. This is in contrast with many low-temperature thermochronology studies, which consistently present Mesozoic dates and typically infer the landscape to be the remnant of a much older mountain belt that has been slowly exhumed following the end of rifting in the Late Triassic-Early Jurassic. Resolving a regional exhumation history of the Appalachians through traditional low-temperature thermochronology methods is difficult due to the spatial constraints of bedrock sampling, while traditional detrital river sampling has limitations underpinned by analytical issues and grain selection bias. We aim to resolve the exhumation histories of large areas across the Appalachians using detrital laser ablation apatite (U-Th-Sm)/He dating (LA-AHe). LA-AHe is a new dating approach that produces a high number of apatite (U-Th-Sm)/He ages from modern river sands to infer the low-temperature (70–40°C) exhumation history of the bedrock across a catchment. Supplementary apatite and zircon U-Pb dates were obtained to study sediment transport in each catchment and their respective bedrock geological histories. LA-AHe results show most dates to be Cretaceous (69%), with only a small Cenozoic fraction (6%), while thermo-kinematic modelling of data from four catchments can be explained by protracted Mesozoic-Cenozoic exhumation with rates ∼31–20 m/Myr. The lack of Cenozoic dates does imply the magnitude of exhumation has not been significant enough to greatly affect the apatite (U-Th-Sm)/He system, though models incorporating enhanced late Cenozoic exhumation between 2.1–1.4 km are consistent with data from three catchments. These results have implications for the study of elevated topography across passive margins and ancient orogens and highlight how laser ablation apatite (U-Th-Sm)/He dating can be effectively used to study the change of landscapes over geological timescales.

Tectonic segmentation by N-S-trending cona cross structure in the Eastern Himalaya: Evidence from thermochronology and thermokinematic modeling

The Main Himalayan Thrust (MHT), the most prominent tectonic boundary between the subducting Indian crust and the overthrusting Himalaya, occurs as a key structure dominating the active tectonics of the Himalaya. Previous studies indicate that the MHT forms a major buried ramp in the mid-crust, which with varying geometry and kinematics leads to a distinctive pattern of uplift along the Himalaya. However, it is unknown if and how these along-strike variations are correlated with the well-known active N-S-trending normal faults across the Himalaya and Tibetan Plateau. In this study, we report 39 new apatite and zircon fission track ages in the Cona area (~92°E) in the eastern Himalaya, coinciding with the easternmost N-S-trending extensional system in southern Tibet, the Cona Cross Structure (CCS). Spatial patterns of surface cooling ages are consistent with an overthrusting of the Himalayan wedge above a mid-crustal ramp in the MHT. Thermokinematic inverse modeling yields contrasting geometries and kinematics for the mid-crustal ramp on both sides of the CCS. Specifically, to the west of the CCS, the MHT developed a major ramp (with a dip angle of 20.4 ± 2.9°) spanning 50–70 km, which initiated at ~11.7 ± 0.6 Ma. To the east, thermokinematic inversion suggests a much less developed ramp spanning only ~10–15 km, initiated at ~6.8 ± 0.3 Ma. These results imply an ~14-km deeper depth of the MHT to the west of the CCS than to the east. Our modeling also suggests distinctive and evolving ramp-thrusting rates for the MHT on both sides of the CCS. This suggests an independent deformation accommodation by the MHT ramp thrusting in segments divided by the CCS along strike. Our study highlights a genetic correlation between the active orogenic-parallel ramp evolution and the N-S-trending faulting across the Himalaya, which divides the orogenic-parallel tectonics into independent structural segments.

Causes of Variable Shortening and Tectonic Subsidence During Changes in Subduction: Insights From Flexural Thermokinematic Modeling of the Neogene Southern Central Andes (28–30°S)

AbstractThe Andes of western Argentina record spatiotemporal variations in morphology, basin geometry, and structural style that correspond with changes in crustal inheritance and convergent margin dynamics. Above the modern Pampean flat‐slab subduction segment (27–33°S), retroarc shortening generated a fold‐thrust belt and intraforeland basement uplifts that converge north of ∼29°S, providing opportunities to explore the effects of varied deformation and subduction regimes on synorogenic sedimentation. We integrate new detrital zircon U‐Pb and apatite (U‐Th)/He analyses with sequentially restored, flexurally balanced cross sections and thermokinematic models at ∼28.5–30°S to link deformation with resulting uplift, erosion, and basin accumulation histories. Tectonic subsidence, topographic evolution, and thermochronometric cooling records point to (a) shortening and distal foreland basin accumulation at ∼18–16 Ma, (b) thrust belt migration, changes in sediment provenance, and enhanced flexural subsidence from ∼16 to 9 Ma, (c) intraforeland basement deformation, local flexure, and drainage reorganization at ∼12–7 Ma, and (d) out‐of‐sequence shortening and exhumation of foreland basin fill by ∼8–2 Ma. Thrust belt kinematics and the reactivation of basement heterogeneities strongly controlled tectonic load configurations and subsidence patterns. Geo/thermochronological data and model results resolve increased shortening and combined thrust belt and intraforeland basement loading in response to ridge collision and Neogene shallowing of the subducted oceanic slab. Finally, this study demonstrates the utility of integrated flexural thermokinematic and erosion modeling for evaluating the geometries, rates, and potential drivers of retroarc deformation and foreland basin evolution during changes in subduction.

Megathrust Heterogeneity, Crustal Accretion, and a Topographic Embayment in the Western Nepal Himalaya: Insights From the Inversion of Thermochronological Data

AbstractBetween 81°30ʹE and 83°E, the Himalayan range's “perfect” arcuate shape is interrupted by an embayment. We hypothesize that thrust geometry and duplexing along the megathrust at midlower‐crustal depths play a leading role in growth of the embayment as well the southern margin of the Tibetan plateau. To test this hypothesis, we conducted thermokinematic modeling of published thermochronologic data from the topographic and structural embayment in the western Nepal Himalaya to investigate the three‐dimensional geometry and kinematics of the megathrust at midlower‐crustal depths. Models that can best reproduce observed cooling ages suggest that the megathrust in the western Nepal Himalaya is best described as two ramps connected by a long flat that extends further north than in segments to the east and west. These models suggest that the high‐slope zone along the embayment lies above the foreland limb of an antiformal crustal accretion zone on the megathrust with lateral and oblique ramps at midlower‐crustal depths. The lateral and oblique ramps may have initiated by ca. 10 Ma. This process may have controlled along‐strike variation in Himalayan‐plateau growth and therefore development of the topographic embayment. Finally, we analyze geological and morphologic features and propose an evolution model in which landscape and drainage systems across the central‐western Himalaya evolve in response to crustal accretion at depth and the three‐dimensional geometry of the megathrust. Our work highlights the importance of crustal accretion at different depths in orogenic‐wedge growth and that the midlower crustal accretion determines the location of plateau edge.

Variable thermal histories across the Pyrenees orogen recorded in modern river sand detrital geo‐/thermochronology and PECUBE thermokinematic modelling

AbstractThe Pyrenees Mountains are a classic example of a doubly‐verging collisional orogenic system with flanking retro‐ and pro‐foreland basin systems. Previous bedrock and detrital geo‐/thermochronologic studies have observed magmatic and exhumation‐related ages that reflect a complex thermo‐tectonic evolution of the European and Iberian plate margins related to break‐up and assembly of the Gondwana, Pangea and Pyrenean‐Alpine orogenic cycles. This study integrates detrital zircon, rutile and apatite U‐Pb dating and, detrital zircon and apatite (U‐Th)/He dating from modern river sands from the northern and southern Pyrenees, with PECUBE thermokinematic modelling of bedrock cooling ages to simulate detrital age distributions in order to evaluate: (1) regional patterns in long‐term crustal processes associated with pre‐Pyrenean crustal shortening, crustal thinning and magmatism along the Iberian and European plate margin; (2) timing of regional cooling and inferred erosion related to Pyrenean orogenesis; and (3) the exhumation processes associated with post‐orogenic decay and erosion. Modern river multimineral detrital geo‐/thermochronometry results are consistent with previous bedrock thermal history models and records punctuated Variscan and Pyrenean cooling events in the pro‐wedge that contrasts with protracted Permian to Pliocene thermal history preserved in the retro‐wedge of the orogen. Detrital age distributions from PECUBE modelling predict the Pyrenean age component in both detrital apatite and zircon (U‐Th)/He age distributions, indicating the modelled exhumation patterns in the Axial Zone and Northern Pyrenean Zone can predict observed Pyrenean thermochronology ages. The presence of strong Pyrenean age peaks amongst the modern river sand and modelled detrital cooling age distributions suggests retro‐wedge deformation and exhumation remained active during the main phase of pro‐wedge activity and experienced significant orogenic decay. Isolated Miocene apatite He ages from the North Pyrenees modern river record post‐orogenic cooling, due to tectonic mode switch to extension and (or) climate‐driven enhanced exhumation.