Abstract Constraining the subsurface structural geometry of the central Himalaya continues to prove difficult, even after the 2015 Gorkha earthquake and the resulting insights into the trajectory of the Main Himalayan thrust (MHT). To this end, we apply a thermokinematic model to evaluate four possible balanced cross section geometries based on three estimates of the MHT in central Nepal. We compare the effect of different décollement and duplex geometries on predicted cooling ages and compare these to new and published ages. We find that the best‐fit geometry able to reproduce the cooling ages at the surface is a hinterland‐dipping duplex, which has been translated over a mid‐crustal ramp located ~110 km north of the Main Frontal thrust. We find that the temporal evolution of the duplex and MHT mid‐crustal ramp both play an integral role in producing the observed cooling ages, implying that the common assumption that the active décollement and ramp geometry solely control the distribution of cooling ages is incorrect. Furthermore, results indicate that the Ramgarh‐Munsiari thrust was emplaced between 17 and ~10 Ma, followed by the Trishuli thrust. Duplex growth occurs between 6.5 and 0.75 Ma, with its constituent thrust sheets moving at variable rates between 10 and 42 mm/yr. Young out‐of‐sequence thrusting (5 km of displacement) in the hinterland produces a slightly improved fit to the cooling ages. Finally, the resulting thermal field modeled from our best‐fit geometry suggests a possible basis for the nucleation and rupture characteristics of the Gorkha earthquake.
Abstract Since the 2015 Gorkha earthquake in Nepal, the relationship between the geometry of megathrusts and the control it exerts over the nucleation and propagation of major earthquakes has become an important topic of debate. In this study, we integrate new geologic mapping, a newly interpreted cross section from the Daraundi valley of central Nepal, two published cross sections from the neighboring Marsyangdi and Budhi Gandaki valleys, and a suite of 270 thermochronometric ages to create an integrated and validated three-dimensional kinematic model for the central Nepal Himalaya. We use this model to investigate the assertion that the westward propagation of the Gorkha rupture was restricted by deep-seated structures in the Main Himalayan thrust. The integrated kinematic model based on these cross sections indicates that the ~30 km southward step in the Main Central thrust system mapped in the Daraundi valley, along with the corresponding step in the distribution of reset muscovite (Ar-Ar) ages, is not the result of a lateral structure in the modern Main Himalayan thrust. Instead, the step in the surface geology is the result of a considerably shorter Trishuli thrust sheet in the Daraundi transect (~30 km compared to between 105 and 120 km in the other transects). The corresponding southward step in the distribution of reset muscovite Ar-Ar ages is the result of the Lesser Himalayan duplex being completely translated over the Main Himalayan thrust ramp, elevating and exposing rocks heated to >400 °C farther south in the Daraundi transect. Our integrated model also highlights the 10–15 km of out-of-sequence thrusting that occurs on the Main Central thrust system across central Nepal. Importantly, these out-of-sequence thrusts sole directly into the modern Main Himalayan thrust ramp, and, together with the distribution of reset zircon (U-Th)/He and apatite fission track ages, show that the modern ramp is distinctly linear from east to west, with no support for a lateral structure at the ramp or to the south.
Topography in compressional mountain ranges represents an interface at which tectonic and climatic forces interact. Understanding the relative contribution of these two components to mountain formation has been at the forefront of research over the last two decades. The theory underlying the mechanics that govern these interactions has been built on Coulomb wedge mechanics, i.e., mechanical failure and rock uplift occur everywhere along the wedge and the orogen. Observed rock displacement along single, discrete fault planes, including the translation of uplifted topography laterally, appears to be counter to such mechanics. However, a critically tapered topography across fold-thrust belts still emerges. If a critically tapered topography along an orogenic wedge can be produced by the sequential evolution of the subsurface fault geometry and the associated motion of bedrock over discrete fault planes, then a mechanical failure everywhere is not required. Here, the geomorphic evolution of the fold-thrust belt in central Nepal since the Miocene is investigated using a numerical surface processes model whereby the structural geometry, location and magnitude of fault motion are prescribed and based on observations. In addition, end-member climatic scenarios are adopted, i.e., uniform precipitation and climatic change over geologic time as predicted by atmospheric general circulation models. The experiments reproduce the first-order topography of central Nepal. Our modelling results indicate a dynamic variability of erosional efficacy that promotes the interplay of two modes of orogenic wedge behaviour and are contrary to a mechanical failure everywhere along the wedge: (mode 1) phases of lateral translation of uplifted topography and in-sequence propagation of deformation fronts, and (mode 2) phases of hinterland incision during out-of-sequence fault activity. The successful replication of first-order geomorphic indices in central Nepal in our experiments confirms an unusually long-lasting Miocene to Pliocene activity of the Main Boundary Thrust in central Nepal. This period is followed by Late Pleistocene hinterland incision coeval with out-of-sequence fault activity prior to the onset of rock displacement along the Main Frontal Thrust during a time of increased precipitation relative to today.               
<p>The 2015 Gorkha earthquake reignited an existing debate about whether geometric barriers on faults play a role in containing the propagation of ruptures. Models suggest that the extent of the Gorkha earthquake rupture, and of other historical earthquakes were controlled by the locations of ramps in the Main Himalayan thrust (MHT), notably on the western edge of the rupture. The existence of such a pronounced lateral boundary to the west of the Gorkha epicenter is supported by an offset in the surface trace of the Main Central thrust (MCT), closely followed by an offset in the distribution of young (<5 Ma) muscovite <sup>40</sup>Ar/<sup>39</sup>Ar (MAr) ages. However, the zircon (U-Th)/He (ZHe) and apatite fission track ages show more linear east-west distributions over the same region, as does Physiographic Transition 2 (PT2). We explore the formation of these relationships by combining forward-modeled balanced cross-sections through the Marsyangdi, Daraundi, and Budhi Gandaki valleys in central Nepal, and investigate the continuity of active structures across the western portion of the Gorkha rupture. The sequential kinematics of each of these sections are combined with a thermokinematic model (PECUBE) to evaluate the exhumation and cooling histories of the rocks exposed at the surface. We gauge the validity of these models by comparing their predicted cooling ages to measured ages, discarding those that do not match the measured distribution of cooling ages.</p><p>Our 3D models show that the offset in the surface geology along the Daraundi is due to a shorter (by 1/3) Trishuli thrust sheet, that has been completely translated to the south of the modern ramp and folded by the Lesser Himalayan duplex. Similarly, the southern extent of the reset MAr ages is also controlled by these relationships requiring observed surface offsets to be the result of changes in the hanging wall rocks translated over the ramp, rather than changes in the geometry of the modern ramp. Notably, the continuity and location of the modern MHT ramp is evidenced by the linear distribution of the youngest ZHe and AFT ages, which are most sensitive to the location of the active ramp. Additionally, the out-of-sequence thrust responsible for PT2 soles directly into the modern ramp during its proposed period of activity at ~1.2 Ma, resulting in the highly linear trace of PT2, running parallel to the location of the ramp. These linear relationships and their reproducibility in thermo-kinematic models argue strongly against any geometric offsets in the modern MHT ramp that have been proposed to limit rupture propagation in central Nepal.</p>
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.
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