Resurfacing of deeply buried oceanic crust in Naga Hills Ophiolite, North-East India: Petrofabric, microstructure and seismic properties

Abstract

We have studied the microstructure, texture and seismic anisotropy of a foliated blueschist-eclogite rock in the Naga Hills Ophiolite (NHO) complex, North-East India. The rock is composed essentially of omphacite, sodic amphibole (glaucophane), sodic-calcic amphibole (barroisite), phengite, and garnet, and represents oceanic crust that experienced Neo-Tethyan subduction and metamorphism before it resurfaced and accreted in the ophiolite. Omphacite and glaucophane show weak shape preferred orientation (SPO), while phengite displays a strong SPO. The Crystallographic Preferred Orientation (CPO) of omphacite is characterized by the [001]-axes gridle within the foliation, and the (010)-poles concentrated sub-perpendicular to the foliation. For glaucophane, the [001] axis aligns parallel to lineation and the [100] axis and (110) pole plunge perpendicular to foliation. These CPOs correspond to SL-type fabrics, related to plane strain deformation geometry. Based on our analysis, we propose that the observed texture had developed from plastic deformation through dislocation creep. Employing the textural data, we have presented the seismic anisotropies of the individual minerals, blueschist-eclogite domains and discussed their contributions in the bulk-rock anisotropy. The calculated bulk-rock seismic anisotropies, AVP and AVS, of the rock are 12.8% and 8.1%, respectively, which correlate with the observed seismic anisotropy of the subducting oceanic crust.