Assessing the geometry of the Main Himalayan thrust in central Nepal: A thermokinematic approach

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.