Structure and petrology of the Nagaland-Manipur Hill Ophiolitic Mélange zone, NE India: A Fossil Tethyan Subduction Channel at the India – Burma Plate Boundary

Abstract

The Nagaland-Manipur Hill ophiolite belt in NE India represents the southern extension of the Neotethyan Yarlung-Zhangbo suture zone in Southern Tibet, and connects this on-land exposure of the late Mesozoic collision front in the north with a modern trench-arc system in the Andaman Sea region in the south. Ophiolitic subunits in the Nagaland-Manipur Hill area in the Indo-Myanmar Ranges occur as blocks or thrust sheets within a melange with a serpentinite or fine-grained greywacke matrix, and are spatially associated with eclogitic and blueschist rock assemblages. This ophiolitic melange zone is tectonically sandwiched between an older (Triassic - Cretaceous) accretionary prism complex (Nimi Flysch) to the east and a younger (Late Cretaceous -Miocene) accretionary wedge (Disang Flysch) to the west. The Nagaland-Manipur Hill ophiolitic melange is thus part of a progressively westward migrated subduction-accretion complex, and it represents a typical subduction channel melange evolved during the fast subduction of the Neotethyan oceanic lithosphere beneath Asia - Sundaland. The exhumation of this melange was facilitated by return flow within a subduction channel at the plate interface between the downgoing Indian continental margin, subducting at a shallow angle during the Eocene, and the Triassic - Early Cretaceous accretionary prism of the overlying Burma plate. The Upper Eocene - Miocene Pokhpur sedimentary strata uncomformably overlying the ophiolitic melange and the accretionary prism metasedimentary rocks constitute a wedge-top or slope-basin sequence, rather than a postcollisional molasse deposit. The occurrence of ophiolitic subunits in the Nagaland-Manipur Hill area represents an accretionary-type ophiolite, derived mainly from the downgoing plate, and does not characterize a Penrose-type, complete ophiolite sequence.