OPPOSITE SHEAR SENSES: A COMPREHENSIVE REVIEW OF GENESES, GLOBAL OCCURRENCES, AND A CASE STUDY

Opposite shear sense (OSS) is manifested by the presence of conflicting shear sense indicators in a portion of a rock body, a shear/fault zone or across several parallel shear/fault zones. The scale of observation may range from micro- to regional. Previous workers have also referred to OSS as kinematic reversal, shear sense reversal/inversion, conflicting shear senses, reverse shear, slip reversal/inversion, etc. The origin of OSS can be attributed to either single (either ductile/brittle) or multiple (both ductile/both brittle/ductile followed by brittle) deformation phases. We present a catalog of 59 reports of OSS in ductile and brittle regimes from 56 locations globally, along with the possible mechanisms proposed by the corresponding workers. Majority of these have arisen due to multiple deformation phases. Nearly half of them owe their genesis to the reorientation of the principal stress axes over time, and less than a quarter originated during single-stage deformation. The OSS documented so far and compiled in this study, predominantly cluster in the vicinity of a few orogens. We speculate two possibilities: (i) complex tectonics, and (ii) quantitative richness of the research studies undertaken in these terranes.

Time-dependent numerical simulations using COMSOL Multiphysics v5.4 predict variation in the sense and amount of rotations of the elliptical inclusions across the matrix. However, the competency contrast between the matrix and the inclusions do not seem to affect this variation. Microstructural studies of the Lesser Himalayan mylonites from the Chaura region (Himachal Pradesh state, India) reveal the presence of OSS without any cross-cut relation. We speculate dominance of coaxial deformation could have resulted in the near-equal abundance of the two shear senses.