Structural evolution across the Eastern Ghats Mobile Belt-Bastar craton boundary, India: hot over cold thrusting in an ancient collision zone

摘要

Along the western margin of the Eastern Ghats Mobile Belt (EGMB), ultrahigh-temperature granulites are thrust westward over hornblende granites and sedimentary rocks of the Bastar craton. Multiply deformed migmatitic gneisses, and porphyritic charnockite of the EGMB unit preserve thrust-related shear fabrics (S_(3M)). In the cratonic foreland, undeformed granites show a progressive decrease in grain-size and increase in penetrative foliation to the east, evolving into orthogneiss near the mylonitized contact zone with charnockite. Foreland fabrics with a consistent top-to-the-west shear sense are conformable with S_(3M) in charnockite and correlate with S_(3B) in polydeformed orthogneisses of cratonic windows. S_(3B) in the windows parallels S_(3M) in overlying EGMB migmatitic gneisses. While S_(3M) accompanied granulite metamorphism in the EGMB, S_(3B) temperature estimates vary from T > 700℃ in the windows to T < 550℃ in the west. Decreasing temperature and later fabric formation in the west are explained by an evolving thermal profile in the cold craton, which is caused by thrusting against hot lower crustal EGMB rocks. Based on lithologic, structural and metamorphic variations across the contact, and resemblances between the EGMB and Rayner Complex, the craton-mobile belt boundary is considered a result of Indo-Antarctic collision, leading to the formation of an ancient supercontinent.