Research Article Early Neoproterozoic Deformation Kinematics in The
Total Page：16
File Type：pdf, Size：1020Kb
GeoScienceWorld Lithosphere VoVolume 2020, Article ID 8820919, 26 pages https://doi.org/10.2113/2020/8820919 Research Article Early Neoproterozoic Deformation Kinematics in the Chottanagpur Gneiss Complex (Eastern India): Evidence from the Curvilinear Hundru Falls Shear ZoneAnalysis Nicole Sequeira and Abhijit Bhattacharya Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721 302, India Correspondence should be addressed to Nicole Sequeira; [email protected] Received 11 November 2019; Accepted 31 January 2020; Published 27 August 2020 Academic Editor: Damian Nance Copyright © 2020 Nicole Sequeira and Abhijit Bhattacharya. Exclusive Licensee GeoScienceWorld. Distributed under a Creative Commons Attribution License (CC BY 4.0). 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. 1. Introduction ated with the shallowly dipping fabric and the types of rocks juxtaposed along the fabric offer significant clues to the origin Regional scale shallowly dipping foliations produced in the of the foliation [22]. ductile crust are traditionally attributed to large-scale thrust- In contrast, steeply dipping foliations within ductile shear ing in compressional regimes [1–5]. More recently, however, zones are well documented and their origins are explained in extensional processes such as gravity-driven collapse [6, 7], diverse geological settings [23–29], although curvatures in channel flow [8], metamorphic core-complex formation steeply dipping shear zones pose challenges. Plate boundary ([9] and references therein), or midcrustal extension [10, geometry controls the style and kinematics of structures 11] are also considered to be likely mechanisms for nucle- developed along curved shear zones formed at plate margins ation of flat-lying fabrics. Several occurrences of foreland [26, 30–33], while curvatures in intraplate shear zones are regions in fold-and-thrust belts experiencing extensional also influenced by irregularly shaped plate margins and deformation either contemporaneously [12–17] or immedi- indenters [34–38]. Other factors that influence the curvature ately following compressional tectonics at plate margins of intraplate shear zones include the reactivation of preexist- [11, 18–21] are known. In such cases, the shear sense associ- ing rheologically controlled basement dislocation structures Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/doi/10.2113/2020/8820919/5210505/8820919.pdf by guest on 02 October 2021 2 Lithosphere Munger 50 km Rajgir-Munger Belt New Delhi KH Rajgir M RH Gaya Simultala BMB Dumka Daltonganj P Ranchi Saltora T NSMB K Tatanagar N GSB DOB SC R CBT Legend Quarternary alluvium Lameta Formation Mesozoic Basalt Paleozoic Gondwana Supergroup Bijawar sediments Supracrustal rocks Chottanagpur Gneiss Complex (CGC) Proterozoic Older enclaves within CGC Dalma metavolcanics Singhbhum Group Archean Singhbhum Craton Figure 1: Generalized geological map of the Chottanagpur Gneiss Complex, CGC (eastern India) showing the locations of regional-scale shear zones (thick black lines). The belts of supracrustal rocks broadly coincide with the shear zones. Box delineates the area examined in this study. The following acronyms are used: BMB—Bihar Mica Belt; RH—Rajgir Hills; KH—Kharagpur Hills; P—Paresnath Hills; NSMB—North Singhbhum Mobile Belt; GSB—Gangpur Schist Belt; DOB—Dalma Ophiolite Belt; SC—Singhbhum Craton. The Rajgangpur (R)-Tamar (T)-Katra (K) shear zone demarcates the CGC-NSMB accretion zone. The Copper Belt Thrust (CBT) marks the NSMB-SC accretion. Inset map of India shows the location of the CGC (filled area). Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/doi/10.2113/2020/8820919/5210505/8820919.pdf by guest on 02 October 2021 Lithosphere 3 (a) Patratu 2 70 10 km 27 53 6 70 62 25 12 35 60 60 60 47 5 Ramgarh 22 15 64 25 15 62 25 N 20 30 8 11 22 WR 35 17 10 30 12 70 46 55 57 73 21 20 76 59 37 55 40 35 Northern supracrustal 9 15 18 80 31 16 26 belt 25 26 3 50 31 80 RR-6 Domain-II 15 11 75 9 5 29 35 17 3 12 85 4 54 21 50 30 34 13 85 82 10 60 88 85 15 25 17 52 76 80 17 78 55 Pundag 70 24 5 75 13 Domain-I WR Water reservoir 14 65 62 RR-35 85 6 10 5 25 Domain boundary 8 25 15 56 2 65 20 70 65 82 Gola 9 Granitoids lacking 62 18 80 II 18 44 17 24 50 87 30 mesoscale fabrics 65 56 65 20 82 16 64 85 30 31 20 22 Locations of monazite 63 12 68 52 85 79 5 80 79? 68 65 35 dated samples 82 50 29 70 85 78 38 16 46 50 22 85 Ormanjhi 86 35 2 52 Gondwana sediments 55 86 Hundru 80 73 5 85 I 85 84 70 Supracrustal rocks Domain-I 11 70 22 23 Falls WR 80 37 81 13 70 6577 84 75 72 80 75 74 73 80 75 76 76 Domain-I granitoids 70 82 85 78 62 80 71 78 78 84 80 50 60 HFSZ 48 85 86 Mishirhutang 31 47 78 Domain-II granitoids 57 80 31 20 53 57 RR-21 30 26 64 50 Domain-III granitoids Domain-III 30 80 60 85 64 24 85 Southern D3 L D4 L 82 3 4 RR-10 supracrustal belt Granitoids 25 Supracrustal RR-9 Jonha Falls rocks Ranchi Domain-I Domain-II Domain-III Supracrustal unit D3 D4 (b) (c) (d) (e) (f) Poles to foliation Stretching lineation/fold axis lineation Contouring <1% 1%-2% 2%-4% 4%-8% n = 101 n = 113 n = 63 n = 189 n = 68 8%-16% n = 41 n = 56 n = 49 n = 60 n = 63 Figure 2: (a) Geological map of the Ramgarh-Ormanjhi area showing the mesoscale structures in the arcuate Hundru Falls Shear Zone (HFSZ) and adjacent areas. The lithologies were simplified after the District Resource Maps of Koderma-Hazaribagh-Chatra [183] and Ranchi-Gumla-Lohardaga [184] in 1 : 300,000 scale published by the Geological Survey of India. The SW part of the area is poorly exposed due to the Ranchi Township and extensive soil cover. (b–f) Lower hemisphere equal area stereoplots with poles to foliation planes in black squares and lineations in red squares (n = number of measurements). (b) Shallowly dipping D3 foliations (contoured) and LS3 stretching lineations in Domain-I granitoid mylonites. Mean D3 foliation plane shown. (c) D4 steep foliations and LS4A stretching lineations in Domain-II granitoid mylonites. Mean foliation plane shown as great circle. (d) D4 steep foliations and LS4B stretching lineations in Domain-III granitoid mylonites. Mean foliation plane shown as great circle. (e) D3 foliations (contoured) and fold axes in supracrustal rocks. D3 foliation pole girdle shown. (f) D4 planes and fold axis in supracrustal rocks. Mean D4 plane shown. [39, 40], especially those formed at former continental plate [57, 58], and in the north by the Paleoproterozoic (1.7– boundaries [38, 41]; lateral rheological heterogeneities in 1.6 Ga) rocks of the Rajgir hills (Figure 1) [59, 60]. The tec- the lithosphere [42–44]; and the interaction of synchronously tonic relevance of these Paleoproterozoic rocks to the CGC active en echelon faults [45–47] as in releasing and restrain- is unknown. Based on detailed structural mapping, kine- ing bends, pull apart basins, strike slip duplexes, and splay matic vorticity analysis, and chemical age dating in mona- structures [41, 48–50]. Curvatures on steeply dipping shear zite, this work identifies for the first time two broadly zones can also be the result of reorganization of preexisting contemporaneous Early Neoproterozoic deformation events shear zones by later deformation [36, 51–54]. in the CGC that produced a regional shallowly dipping foli- Field studies of natural shear zones primarily use varia- ation and a curvilinear steeply dipping shear zone, the tions in the orientation of stretching lineations/foliations Hundru Falls Shear Zone (HFSZ) (Figure 2(a)). The HFSZ and the strain history of the rocks to understand the mode forms a part of a network of several crustal-scale E/ENE- of formation of the shear zone. Strain modeling based on striking shear zones that transect the CGC, with shorter field observations of changing orientation of stretching line- NW-striking segments curving into the E-striking shear ations along curvilinear shear zones [55, 56] essentially attri- zones (Figure 1).