Transition from shallow to steep foliation in the Early Neoproterozoic Gangpur accretionary orogen (Eastern India): Mechanics, significance of mid-crustal deformation, and case for subduction polarity reversal?

Abstract

The ENE-trending Gangpur Schist Belt (GSB) comprising foreland sediments and distended screens of mafic/ultramafic rocks is sandwiched between the Meso/Neoarchean Singhbhum Craton (SC) in the south, and the Early Neoproterozoic granitoids of the Chottanagpur Gneiss Complex (CGC). These meta-sediments are thrust (D2 deformation) northward along a shallow-dipping decollement in the CGC granitoids. The shallow foliations with orogen-parallel stretching lineation in granitoids are coaxial with axes of D2 recumbent folds in the meta-sediments; locally-developed orogen-normal extensional shears and en echelon quartz vein are associated with this deformation phase. The shallow-D2 structures are dissected by two orogen-parallel south-dipping shear zones (D3) with steep stretching lineations. Sinistral-normal kinematics dominates the shear zones, but the northern margins of the shear zones exhibit dextral and north-down kinematics. Analyses of NCKFMASH P-T pseudosections constructed for the meta-sediments suggest the pre/post-D2 stage was characterized by mid-crustal amphibolite facies near-isothermal loading punctuated by prograde heating. Th-U-Pb (total) ages of metamorphic monazite grains in the schists (1080–940 Ma) and the magmato-metamorphic ages of CGC granitoids (1020–940 Ma from previous studies) overlap with each other, but monazite (>1050 Ma) inclusion trails in the core of pre-S2 garnets predate the magmato-metamorphic ages in the granitoids.The D2 Early Neoproterozoic deformation in the GSB involved top-to-the-north translation of the foreland sediments along a mid-crustal decollement in the CGC, followed by transpressional deformation that caused inclined extrusion and lateral escape along orogen-parallel hinterland vergent shear zones. However, the initiation of SC-CGC accretion (>1050 Ma) possibly involved northward subduction (pre-D2 near isothermal loading) of the GSB foreland sediments that ultimately was linked to the emplacement of granitoids in the over-riding CGC plate. Cessation of the subduction and continued N-S shortening caused CGC to evolve as the overridden basement.