THE CURVILINEAR HUNDRU FALLS SHEAR ZONE, EASTERN INDIA: IMPLICATIONS FOR GRENVILLIAN-AGE DEFORMATION KINEMATICS IN THE CHOTTANAGPUR GNEISS COMPLEX

A hitherto undocumented intraplate curved shear zone within the Chottanagpur Gneiss Complex (CGC), eastern India, is investigated. The steep-dipping NW-trending sinistral shear zone christened Hundru Falls Shear Zone (HFSZ) curves eastwards in the east and exhibits N-down normal kinematics. Prior to the development of the HFSZ, a pure-shear dominated top-to-the-NE translation of allochthonous supracrustal rocks over a basement of granitoids and anatectic gneisses reminiscent of basement involved thin-skinned deformation produced shallow-dipping foliations within granitoids, duplexing in the supracrustal rocks and recumbent folds in steep-dipping anatectic gneisses. The HFSZ transects all three lithodemic units. Stretching lineations within the shallow-dipping granitoid mylonites follow the curvature of the shear zone.

The HFSZ is associated with gently plunging stretching lineations except in the south of the E-trending arm, where they switch to a down-dip orientation. The gently-plunging stretching lineations are collinear with the NW/E-trending hinges of upright/steeply-inclined asymmetric folds that superpose recumbent folds in the structurally-overlying allochthonous supracrustal rocks. In the HFSZ granitoids, kinematic vorticity values are estimated to be 0.73–0.83 and 0.51–0.65 in domains with shallow and steep stretching lineations respectively. Kinematic modeling suggests the orogen-parallel non-cylindrical folds in supracrustal rocks and gently-plunging lineations in steep HFSZ mylonites resulted from wrench accommodated monoclinic transpression. The curvature of the HFSZ resulted from the reorientation of this early shear zone into the subsequently formed E-trending HFSZ domain with steep lineation originating from increased coaxial strain accommodated triclinic transpression, due to a northward shift in the convergence direction. The Grenvillian-age deformation forming the shallow-dipping foliations and the HFSZ is contemporaneous with the accretion of CGC with the Singhbhum Craton to the south.