Petrofabric and Seismic properties of exhumed subducted oceanic crust
from Naga Hills Ophiolite, North-East India
Abstract
The microstructures, textures and seismic anisotropy of a foliated
blueschist-eclogite rock from Naga Hills Ophiolite (NHO) complex,
North-East India has been investigated to understand the fabric
relationship and deformation mechanism prevailed during the metamorphism
and subduction of the Neo-Tethys oceanic crust. The Naga Hills
blueschist-eclogite rock essentially contains omphacite, phengite,
glaucophane, and garnet, representing an oceanic crust that experienced
P-T stability field of blueschist and further eclogite metamorphism
before it resurfaced accreting in the Naga Hills. Omphacite and
glaucophane show weak shape preferred orientation (SPO), while phengite
displays a strong SPO. The Crystallographic Preferred Orientations (CPO)
of omphacite is characterized by the [001]-axes gridles within the
foliation, and the (010)-poles concentrated sub-perpendicular to the
foliation. For glaucophanes, 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 a
deformation geometry within the plain strain field and they developed
from plastic deformation through dislocation creep. The seismic
anisotropies of the individual minerals, blueschist-eclogite domains and
their contributions in the bulk rock anisotropy has been discussed. The
calculated seismic anisotropies (AVP and AVS) of bulk rock are 12.8%
and 8.1%, respectively. This strong seismic anisotropy is due to the
presence of phengite and glaucophane and can contribute to the observed
seismic anisotropy in the subduction zone. The average low P-wave
velocity of whole rock from NHO compared to blueschist is probably due
to the low P-wave velocity of phengite (avg VP: 6.2 kms-1). Therefore,
the low seismic velocity in the upper layer and the strong seismic
anisotropy of the subducting oceanic crust can be attributed to the
presence of glaucophane and phengite.