Opposite shear senses: Geneses, global occurrences, numerical simulations and a case study from the Indian western Himalaya

Abstract

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. More importantly, the scale of observation may range from micro-to regional. Several terms have been used by previous workers to refer OSS, viz. conflicting shear senses, reverse shear, shear sense reversal/inversion, slip reversal/inversion, kinematic reversal, etc. Although OSS has been reported from several terrains worldwide, a review on the mechanisms and global distribution remained due. Given that shear sense indicators play a crucial role in deciphering the deformation history of a region, the presence of OSS sometimes baffles structural geologists.In this article, we present a catalogue of 59 reports of OSS in ductile and brittle regimes from 56 locations globally, from collisional, extensional and strike-slip tectonic settings besides a few cratons, and provide the possible reasons/mechanisms proposed by the workers for each one of them. Though we primarily discuss OSS with parallel shear planes, six reports of OSS with non-parallel shear planes are also included. 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. Most of the occurrences listed here have arisen due to multiple deformation phases, almost half of which owe their genesis to reorientation of the principal stress axes temporally. Gravitational collapse of over-thickened crust also produces OSS, especially at the orogens. Besides, less than a quarter of all the compiled examples originate during single-stage deformation by coaxial shortening, heterogeneity in strain-rate and viscosity, isoclinal folding, rotation of blocks within shear zones or differences in shortening/stretching rates across the fault/shear zone. The majority of the OSS documented so far and compiled in this study, cluster within or close to few orogens. We speculate two possibilities: (i) complex tectonics, and (ii) richness in the number of research studies undertaken in these terranes. Although orthogonal switch (∼NW–SE → ∼ NE-SW) of the compressive stress axes (σ1) has been discussed for the Zagros fold-thrust belt (ZFTB) in the Paleogene, OSS has not been reported from here so far. The same holds true for the Andes that collapsed gravitationally and switched compression from∼ E-W into ∼ N–S in the Mid-Miocene. Reactivation of pre-Andean extensional faults as thrusts are described by previous workers, but not referred to produce OSS. Positive inversion-reverse reactivation of pre-existing extensional faults-such as those reported from the Andes and the Apennines, can either be beneficial or disadvantageous to the preservation of the hydrocarbon of the region. Identification of inversion structures would be important in hydrocarbon reserve appraisal.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. We also provide an original report of OSS in ductile regime as observed under an optical microscope from the Lesser Himalayan rocks of the Chaura region (Himachal Pradesh state, India). Although a temporal relation between the two shear senses, in this case, could not be established owing to the absence of cross-cut relations of shear fabrics, we speculate dominance of coaxial deformation could have resulted in the near-equal abundance of OSS.