Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Opposite shear senses: Geneses, global occurrences, numerical simulations and a case study from the Indian western Himalaya Dripta Dutta, Soumyajit Mukherjee ∗ Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai, 400 076, Maharashtra, India ARTICLEINFO Keywords: Shear zone kinematics Deformation mechanism Ductile and brittle deformation of rocks Basin inversion 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 ob- servation 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 re- versal, 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 de- formation 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, hetero- geneity 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 (σ 1 ) 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. https://doi.org/10.1016/j.jsg.2019.05.008 Received 23 April 2019; Received in revised form 28 May 2019; Accepted 28 May 2019 ∗ Corresponding author. E-mail addresses: [email protected] (D. Dutta), [email protected], [email protected] (S. Mukherjee). Journal of Structural Geology 126 (2019) 357–392 Available online 25 June 2019 0191-8141/ © 2019 Elsevier Ltd. All rights reserved. T
Opposite shear senses: Geneses, global occurrences, numerical simulations and a case study from the Indian western Himalaya
Jun 23, 2023
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