TECTONICS OF THE BEND IN A CURVED CRUSTAL-SCALE EARLY NEOPROTEROZOIC ACCRETION ZONE, THE GREATER INDIA PROTEROZOIC FOLD BELT (GIPFOB), INDIA: NEW FINDINGS AND A SPECULATION

Curvatures in collision orogens may be indenter-controlled, develop due to progressive deformation, or form due to overprinting effects of successive deformation episodes. The Greater India Proterozoic Fold Belt (GIPFOB) is a Late Paleoproterozoic to Early Neoproterozoic highly tectonized accretion zone. The NNE-striking Aravalli-Delhi Fold Belt (ADFB) constitutes the western GIPFOB arm, and the E-striking southern arm comprises parts of the Satpura Mobile Belt (SMB) and the Chottanagpur Gneiss Complex (CGC). The two arms converge at Godhra-Chhota Udepur (GC) sector. Structural-metamorphic and LA-ICPMS U-Pb zircon data from the sector are compared with existing data in relevant GIPFOB domains to evolve a tectonic model for the accretion tectonics in the sector and to decipher the origin of the proposed curvature in GIPFOB.

The GC sector comprises a shallow-dipping tectonic mélange formed due to top-to-the-south thrusting (D2 deformation) traversed by steep-dipping E-striking shear zones with sinistral, N-down kinematics (D3). The mélange is composed of nappes of pre-D2 granulite facies gneiss, granite mylonite and an allochthonous low-T supracrustal suite in which high-pressure (≥12 kb) phengite-chlorite-clinozoisite assemblages define the D2 fabric in mica schists. The mélange is intruded by post-D2 granitoids having magmatic fabrics and chessboard quartz microstructure.

The high-grade metamorphism in the gneisses (1.6‒1.7 Ga) and the post-D2 granitoids (~0.95 Ga) are similar to SMB-CGC; on the contrary the pre-D2 Archean granitoids (2.5 Ga) in the north are similar to the ADFB basement rocks. The D2-D3 deformations mark a transition from thrust to wrench dominated tectonics for a similar orientation of orogen-parallel stretching caused by N-S oblique shortening. The E-striking SMB-CGC-GC arm of GIPFOB re-oriented the NNE-striking structures in the ADFB western arm. This and the juxtaposition of the 2.5 Ga granites (lacking the ~1.6 Ga tectonism), and the 1.6‒1.7 Ga gneisses indicate that the D2-D3 shortening structures were induced by Early Neoproterozoic accretion between the two arms of the GIPFOB. In the absence of detachment faults, the voluminous emplacement of 0.95 Ga granites in the GC sector is accounted by invoking subduction polarity reversal starting with northward subduction. [1996 characters; max 2000 characters with no spaces]

Keywords: Greater India Proterozoic Fold Belt (GIPFOB), curved orogens, Early Neoproterozoic accretion, tectonic mélange, Godhra-Chhota Udepur sector