EGU General Assembly 2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Assessing the geometry of the Main Himalayan thrust in central Nepal: Insights from thermokinematic modelling

Suryodoy Ghoshal1,4, Nadine McQuarrie1, Delores M. Robinson2, Katherine Huntington3, and Todd A. Ehlers4
Suryodoy Ghoshal et al.
  • 1Department of Geology and Environmental Sciences, University of Pittsburgh, Pittsburgh, USA
  • 2Department of Geological Sciences, The University of Alabama, Tuscaloosa, USA
  • 3Department of Earth & Space Sciences, University of Washington, Seattle, USA
  • 4Department of Geosciences, University of Tübingen, Tübingen, Germany

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 40Ar/39Ar (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.

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.

How to cite: Ghoshal, S., McQuarrie, N., Robinson, D. M., Huntington, K., and Ehlers, T. A.: Assessing the geometry of the Main Himalayan thrust in central Nepal: Insights from thermokinematic modelling, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1319,, 2022.


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