Deep Structure of the Main Himalayan Thrust and Indian Mantle From Structural Imaging, Geodetic Modeling, and Seismicity
Abstract
We use seismic structural imaging to tie together seismotectonic and geodetic observations of the Himalayan collision, in particular of the deep structure of the Main Himalayan Thrust (MHT) and processes within the downgoing Indian plate underneath it. The MHT appears to be marked by a low-velocity layer in several studies, yet at least part of this signal is likely due to deformation fabric associated with shear along the fault. If a weak, low-velocity layer is present along the MHT, deformation modeling shows that such a layer has the capability to influence co-seismic surface displacements measured following events such as the 2015 Gorkha rupture. The presence of this weak layer generates comparably smaller surface displacement magnitudes than a uniform elastic structure. If the weak layer continues downdip of the rupture, it may influence postseismic slip downdip of the rupture, similar to processes in subduction settings.
While deep small-magnitude seismicity is observed downdip from megathrust rupture depths, the MHT and Indian Moho seem to delineate an aseismic layer in this region. This is in contrast to regions updip from the Gorkha rupture which show seismicity throughout the Indian crust into the mantle. High seismic velocities in the subducted Indian crust below MHT depths of ~ 40 km suggest eclogitization in the crust, which may cause rheological changes as the Indian crust encounters higher pressures and temperatures. Using an approach that directly compares receiver function arrival times to local event S-P times, we show that the deep seismicity in the same area occurs in the mantle. The observation that this deep seismicity occurs only in limited areas along strike suggests that localized embrittlement processes in the mantle may be operating.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T43A..01S
- Keywords:
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- 1242 Seismic cycle related deformations;
- GEODESY AND GRAVITY;
- 7230 Seismicity and tectonics;
- SEISMOLOGY;
- 8004 Dynamics and mechanics of faulting;
- STRUCTURAL GEOLOGY;
- 8123 Dynamics: seismotectonics;
- TECTONOPHYSICS