33-6 Extensional Fault Dynamics and Strain Migration: Across the Thakkhola Graben, Central Himalayas

Session: Going with the Shear - New Insights into Lithospheric Extensional and Strike-Slip Systems (Posters)

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Abstract:

The Dangardzong Fault (DF) in the Nepal Himalayas is the primary normal fault bounding the Thakkhola graben. It has accommodated orogen-parallel extension since the Miocene. Although active slip rates exist for many faults that accommodate Himalayan orogen-parallel extension, none exist for the Dangardzong Fault despite several decades of research on this prominent rift-bounding structure. In this study, we employed small-baseline subset interferometric synthetic aperture radar time-series analysis to investigate the surface displacement patterns across the Dangardzong Fault. The InSAR data were first processed to extract the east–west velocity component, and three InSAR swath profiles across the fault were then modeled using the simple 2D elastic screw dislocation model (Savage & Burford, 1973), which describes velocity as a function of slip rate, locking depth, and static offset. All three profiles consistently suggest a fault slip rate of 0.0 ± <0.1 mm/yr. The best-fit model estimates a poorly constrained locking depth of 12 km +5.3/-6.9 km. The results show no measurable east-west extensional surface displacement across the Dangardzong Fault indicating a lack of active orogen-parallel deformation in the area.

The absence of measurable present-day fault slip across the Dangardzong Fault is consistent with thermochronologic evidence indicating a progressive decline in footwall cooling rates from ~22 Ma to present, reflecting reduced exhumation and extensional activity (Brubacher et al., 2021). This temporal shift suggests that orogen-parallel strain has progressively migrated away from the Thakkhola Graben, likely accommodated by younger extensional systems across the plateau. Structural analysis further indicates that arc-parallel extension along the graben-bounding faults is minimal (~0.22 mm/yr since 10 Ma), significantly less than estimates for extensional features to the west, supporting the hypothesis that central Nepal represents a transitional zone of reduced orogen-parallel strain due to low convergence obliquity (Baltz et al., 2021). GPS velocities reveal increasing east–west extension toward the west and the MFT (Styron et al., 2011), likely driven by oroclinal bending and enhanced strain accommodation along the outer arc of the Himalaya. These findings point toward a broader geodynamic evolution involving a southward shift in thrusting, underthrusting of Indian lithosphere, and lateral lower crustal flow, which collectively contributed to the spatial redistribution of extensional deformation in the southern Tibetan-Himalayan orogenic system.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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