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?

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