[1] Two end-member kinematic models of crustal shortening across the Himalaya are currently debated: one assumes localized thrusting along a single major thrust fault, the Main Himalayan Thrust (MHT) with nonuniform underplating due to duplexing, and the other advocates for out-of-sequence (OOS) thrusting in addition to thrusting along the MHT and underplating. We assess these two models based on the modeling of thermochronological, thermometric, and thermobarometric data from the central Nepal Himalaya. We complement a data set compiled from the literature with 114 40 Ar/ 39 Ar, 10 apatite fission track, and 5 zircon (U-Th)/He thermochronological data. The data are predicted using a thermokinematic model (PECUBE), and the model parameters are constrained using an inverse approach based on the Neighborhood Algorithm. The model parameters include geometric characteristics as well as overthrusting rates, radiogenic heat production in the High Himalayan Crystalline (HHC) sequence, the age of initiation of the duplex or of out-of-sequence thrusting. Both models can provide a satisfactory fit to the inverted data. However, the model with out-of-sequence thrusting implies an unrealistic convergence rate ≥30 mm yr −1 . The out-of-sequence thrust model can be adjusted to fit the convergence rate and the thermochronological data if the Main Central Thrust zone is assigned a constant geometry and a dip angle of about 30°and a slip rate of <1 mm yr −1 . In the duplex model, the 20 mm yr −1 convergence rate is partitioned between an overthrusting rate of 5.8 ± 1.4 mm yr −1 and an underthrusting rate of 14.2 ± 1.8 mm yr −1. Modern rock uplift rates are estimated to increase from about 0.9 ± 0.31 mm yr −1 in the Lesser Himalaya to 3.0 ± 0.9 mm yr −1 at the front of the high range, 86 ± 13 km from the Main Frontal Thrust. The effective friction coefficient is estimated to be 0.07 or smaller, and the radiogenic heat production of HHC units is estimated to be 2.2 ± 0.1 mW m −3. The midcrustal duplex initiated at 9.8 ± 1.7 Ma, leading to an increase of uplift rate at front of the High Himalaya from 0.9 ± 0.31 to 3.05 ± 0.9 mm yr −1 . We also run 3-D models by coupling PECUBE with a landscape evolution model (CASCADE). This modeling shows that the effect of the evolving topography can explain a fraction of the scatter observed in the data but not all of it, suggesting that lateral variations of the kinematics of crustal deformation and exhumation are likely. It has been argued that the steep physiographic transition at the foot of the Greater Himalayan Sequence indicates OOS thrusting, but our results demonstrate that the best fit duplex model derived from the thermochronological and thermobarometric data reproduces the present morphology of the Nepal Himalaya equally well.

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International audienc

The paleoelevation of the Sierra Nevada, California, is important to our understanding of the Cenozoic geodynamic evolution of the North America-Pacific plate boundary, and the current debate is fueled by data that argue for conflicting elevation histories. The non-equilibrium or transient landscape of the Sierra Nevada contains information about both past and present controls on the topography of the range. Using geomorphology and thermochronometry, two parts of the landscape of different geodynamic significance and age can be identified: (1) a long-lived, slowly eroding low-relief highland or relict landscape, which we relate to a period of lower relief and elevation from 80-32 Ma; and (2) younger, rapidly-incising river gorges created by at least two stages of elevation and relief increase since 32 Ma. Our data argue for moderate range elevation of ~1500 m at the cessation of arc magmatism in Late Cretaceous time, followed by two events at between 32 and 3.5 Ma and since 3.5 Ma that increased the range elevation to the 4000 m observed elevation today.

The onset of India‐Asia contact can be dated with both biostratigraphic analysis of syn‐collisional sedimentary successions deposited on each side of the Indus Suture zone, and by radiometric dating of Indian crustal rocks which have undergone subduction to great depths in the earliest subduction‐collision stages. These data, together with paleomagnetic data, show that the initial contact of the Indian and Asian continental margins occurred at the Paleocene/Eocene boundary, corresponding to 55 ± 2 Ma. Such dating, which is consistent with all available geological evidence, including the record of magnetic anomalies in the Indian ocean and decrease of magmatic activity related to oceanic subduction can thus be considered as accurate and robust. The sedimentary record of the Tethys Himalaya rules out obduction of oceanic allochtons directly onto the Indian continental margin during the Late Cretaceous. The commonly inferred Late Cretaceous ophiolite obduction events may have thus occurred in intraoceanic setting close to the Asian margin before its final emplacement onto the India margin during the Eocene. Granitoid and sedimentary rocks of the Indian crust, deformed during Permo‐Carboniferous rifting, reached a depth of some 100 km about 1 Myr after the final closure of the Neo‐Tethys, and began to be exhumed between 50 and 45 Ma. At this stage, the foreland basin sediments from Pakistan to India show significant supply from volcanic arcs and ophiolites of the Indus Suture Zone, indicating the absence of significant relief along the proto‐Himalayan belt. Inversion of motion may have occurred within only 5 to 10 Myr after the collision onset, as soon as thicker and buoyant Indian crust chocked the subduction zone. The arrival of thick Indian crust within the convergent zone 50–45 Myr ago led to progressive stabilization of the India/Asia convergent rate and rapid stabilization of the Himalayan shortening rate of about 2 cm yr−1. This first period also corresponds to the onset of terrestrial detrital sedimentation within the Indus Suture zone and to the Barrovian metamorphism on the Indian side of the collision zone. Equilibrium of the Himalayan thrust belt in terms of amount of shortening versus amount of erosion and thermal stabilization less than 10 Myr after the initial India/Asia contact is defined as the collisional regime. In contrast, the first 5 to 10 Myr corresponds to the transition from oceanic subduction to continental collision, characterized by a marked decrease of the shortening rate, onset of aerial topography, and progressive heating of the convergent zone. This period is defined as the continental subduction phase, accommodating more than 30% of the total Himalayan shortening.

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[1] In this study we integrate the apatite (U-Th)/He thermochronometric technique with geomorphic, structural, and stratigraphic studies to pursue the origin and evolution of topographic relief related to extensive late Cenozoic faulting in the southern Sierra Nevada. The geomorphology of this region reflects a transition from a vast region to the north characterized by nonequilibrium fluvial modification of a relict low-relief landscape, little affected by internal deformation, to a more complex landscape affected by numerous faults. Regionally, the relict landscape surface is readily resolved by ageelevation relationships of apatite He ages coupled to geomorphology. These relationships can be extended into the study area and used as a structural datum for the resolution of fault offsets and related tilting. On the basis of 63 new apatite He ages and stratigraphic data from proximal parts of the San Joaquin basin we resolve two sets of normal faults oriented approximately N-S and approximately NW. Quaternary west-side-up normal faulting along the N-S Breckenridge-Kern Canyon zone has resulted in a southwest step over from the Owens Valley system in the controlling structure on the regional west tilt of Sierran basement. This zone has also served as a transfer structure partitioning NeogeneQuaternary extension resulting from normal displacements on the NW fault set. This fault system for the most part nucleated along Late Cretaceous structures with late Cenozoic remobilization representing passive extension by oblate flattening as the region rose and stretched in response to the passage of a slab window and the ensuing delamination of the mantle lithosphere from beneath the region.

International audienceThe Ama Drime range located at the transition between the high Himalayan range and south Tibet is a N-S active horst that offsets the South Tibetan Detachment System (STDS). Within the horst, a paragneissic unit, possibly attributed to the upper Himalayan crystalline series, overly the lower Himalayan crystalline series Ama Drime orthogneissic unit containing large metabasite layers and pods that have experienced pressure >= 1.4 GPa. Combining structural analysis with new and published pressure-temperature (P-T) estimates as well as U-Th/Pb, Ar-39/Ar-40 and (U-Th)/He ages, the P-T-deformation-time (P-T-D-t) paths of the main units within and on both sides of the horst are reconstructed. They imply that N-S normal faults initiated prior to 11 Ma and have accounted for a total exhumation <= 0.6 GPa (22 km) that probably occurred in two phases: the first one until similar to 9 Ma and the second one since 6 to 4 Ma at a rate of similar to 1 mm/yr. In the Ama Drime unit, 1 to 1.3 GPa (37 to 48 km) of exhumation occurred after partial melting since similar to 30 Ma until similar to 13 Ma, above the Main Central Trust (MCT) and below the STDS when these two fault systems were active together. The switch from E-W (STDS) to N-S (Ama Drime horst) normal faulting between 13 and 12 Ma occurs at the time of propagation of thrusting from the MCT to the Main Boundary Thrust. These data are in favor of a wedge extrusion or thrust system rather than a crustal flow model for the building of the Himalaya. We propose that the kinematics of south Tibet Cenozoic extension phases is fundamentally driven by the direction and rate of India underthrusting