Abstract
Abstract Curvilinear steep shear zones originate in different tectonic environments. In the Chottanagpur Gneiss Complex (CGC), the steeply dipping, left-lateral and transpressive Early Neoproterozoic Hundru Falls Shear Zone (HFSZ) with predominantly north-down kinematics comprises two domains, e.g., an arcuate NW-striking (in the west) to W-striking (in the east) domain with gently plunging stretching lineation that curves into a W-striking straight-walled domain with down-dip lineation. The basement-piercing HFSZ truncates a carapace of flat-lying amphibolite facies paraschist and granitoid mylonites, and recumbently folded anatectic gneisses. The carapace—inferred to be a midcrustal regional-scale low-angle detachment zone—structurally overlies an older basement of Early Mesoproterozoic anatectic gneisses intruded by Mid-Mesoproterozoic/Early Neoproterozoic granitoids unaffected by the Early Neoproterozoic extensional tectonics. The mean kinematic vorticity values in the steep HFSZ-hosted granitoids computed using the porphyroclast aspect ratio method are 0.74–0.83 and 0.51–0.65 in domains with shallow and steep lineations, respectively. The granitoid mylonites show a chessboard subgrain microstructure, but lack evidence for suprasolidus deformation. The timing relationship between the two domains is unclear. If the two HFSZ domains were contemporaneous, the domain of steep lineations with greater coaxial strain relative to the curvilinear domain formed due to strain partitioning induced by variations in mineralogy and/or temperature of the cooling granitoid plutons. Alternately, the domain of gently plunging lineations in the HFSZ was a distinct shear zone that curved into a subsequent straight-walled shear zone with steeply plunging lineation due to a northward shift in the convergence direction during deformation contemporaneous with the Early Neoproterozoic accretion of the CGC and the Singhbhum Craton.
Highlights
Regional scale shallowly dipping foliations produced in the ductile crust are traditionally attributed to large-scale thrusting in compressional regimes [1,2,3,4,5]
Based on detailed structural mapping, kinematic vorticity analysis, and chemical age dating in monazite, this work identifies for the first time two broadly contemporaneous Early Neoproterozoic deformation events in the Chottanagpur Gneiss Complex (CGC) that produced a regional shallowly dipping foliation and a curvilinear steeply dipping shear zone, the Hundru Falls Shear Zone (HFSZ) (Figure 2(a))
This study focuses on the subsequent D3 and D4 deformation events that affected the CGC during the Late Mesoproterozoic-Early Neoproterozoic
Summary
Regional scale shallowly dipping foliations produced in the ductile crust are traditionally attributed to large-scale thrusting in compressional regimes [1,2,3,4,5]. Several occurrences of foreland regions in fold-and-thrust belts experiencing extensional deformation either contemporaneously [12,13,14,15,16,17] or immediately following compressional tectonics at plate margins [11, 18,19,20,21] are known In such cases, the shear sense associated with the shallowly dipping fabric and the types of rocks juxtaposed along the fabric offer significant clues to the origin of the foliation [22]. Based on detailed structural mapping, kinematic vorticity analysis, and chemical age dating in monazite, this work identifies for the first time two broadly contemporaneous Early Neoproterozoic deformation events in the CGC that produced a regional shallowly dipping foliation and a curvilinear steeply dipping shear zone, the Hundru Falls Shear Zone (HFSZ) (Figure 2(a)). This study addresses the structural and kinematic significances of the shallowly dipping foliation and the shear zones/lineaments in the centrally located parts of the CGC and their implications in relation to the accretion history of the CGC with the flanking crustal block comprising the Archean Singhbhum Craton and the Early Mesoproterozoic southern part of the North Singhbhum Mobile Belt ([61]; Figure 1)
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