Strain Signature of The Main Central Thrust: Okhaldunga Region, East-Central Nepal Himalaya

Shreya Roy, University of British Columbia, Okanagan
Kyle Larson, University of British Columbia, Okanagan

Abstract

The ongoing continent-continent collision between India and Asia has accommodated >2500 km of convergence since initial collision ca. 55 Myr ago. Some of that displacement was partitioned into the crustal-scale Main Central thrust (MCT) zone, which comprises a series of thrust-sense structures that facilitated horizontal shortening between the early and middle Miocene. The development of the MCT zone reflects rheological/lithological variation and differential strain partitioning within the exhuming Himalayan mid-crust.

In this study, we present a multi-disciplinary approach to characterizing the deformation in exhumed, mid-crustal rocks of the east-central Nepal Himalayan MCT zone. Quartz microstructures and crystallographic preferred orientation (CPO) data from electron backscatter diffraction analyses record pervasive ductile deformation across the MCT zone with top-south sense at high temperature (550 – 650 ± 50°C). A 48-57% positive correlation between quartz interconnectedness and quartz CPO distribution strength demonstrates that quartz is a dominant strain-bearing phase in our polymineralic samples. Moreover, if quartz CPO distribution strength is assumed to be a proxy for finite strain, it outlines a localized strain gradient within the studied area. The kinematic vorticity number measured using (a) the quartz c-axis fabric and strain ratio method and (b) C prime -c method shows general shearing with increasing pure shear towards higher structural level, and 11-27% shortening perpendicular to the flow plane. Additionally, differential stress estimates calculated from sub-grain piezometry increase towards foreland, and structurally higher (from 7 to 20 MPa), indicating stronger crust where the strain is dominantly partitioning into phyllosilicate layers. Finally, the spatial distribution of new Rb-Sr biotite dates, interpreted to reflect cooling, are consistent with a duplex related thermo-kinematic model of mid-crustal exhumation.

Our study outlines a broad, diffuse zone of ductile shearing toward the orogenic hinterland. In contrast, toward the foreland, deformation becomes more discrete, forming P-T-t-d discontinuities. The observations made herein are consistent with previous kinematic models where both ductile and brittle deformation mechanisms operate within a single orogen at different time and structural depths.

Session

Convergent and transpressional orogens