Naga Hills Ophiolite at the India-Burma Plate Boundary

The India-Burma plate boundary is a convergent plate boundary where the Indian plate had subducted eastward below the Burma microplate between the Early Cretaceous and Early Paleogene. It is comprised of accretionary wedge sediments and ophiolite along the Naga Hills-Chin Hills-Arakan Yoma Hills segment of the Indo-Burman Ranges, and the Andaman-Nicobar Islands.The Naga Hills Ophiolite (NHO), a ~200 km long and ‹=15 km wide, NNE-SSW ophiolite belt is a continuation of the E-W oriented Indus-Yarlung-Tsangpo ophiolite belt in the Himalayas that swerve southward at the eastern Himalayan syntaxis at Namche Barwa and are offset northward by the Sagaing fault, a major N-S, dextral strike slip fault along the boundary between the Burma microplate and the Asian plate.

The NHO is composed of dismembered bodies of serpentinized peridotite tectonite, layered mafic-ultramafics, mafic volcanics, pyroclastics, oceanic sediments, and high-pressure metamorphites such as glaucophane-bearing metabasics and cherts. It has an east-dipping thrust contact with the underlying Cretaceous-Upper Eocene Disang Flysch in the west, and is overthrust from the east by continental metamorphic rocks of the Naga Metamorphics. The mid-Cretaceous, fossil-bearing Nimi Formation occurs at the contact between the ophiolite and the Naga Metamorphics. The presence of ophiolite derived Middle Eocene sediments constrains the minimum age of the ophiolite.

Tectonic slices and lenses comprised of eclogite, garnet-blueschist, glaucophanite and greenschist from core to margin occur within the mafic and ultramafic rocks of NHO. Barroisite, garnet, omphacite, epidote, chlorite, phengite, rutile and quartz constitute the peak metamorphic assemblage. Recent P-T pseudosection analysis indicates that the Naga Hills eclogites followed a clockwise P-T path with a prograde part between ~1.3 GPa/525oC and 1.7-2.0 GPa/580o-610oC, and a retrograde part that ends at ~1.1 GPa/540oC. The peak pressure at Naga Hills is higher than that of the Cretaceous Sapi-Shergol blueschists of the Indus-Tsangpo suture zone (0.9-1.0 GPa). Consequently, the depth of subduction was greater at the eastern margin than at the northern margin of the Indian plate prior to the India-Eurasia collision. However, the peak pressure at Naga Hills is lower than the ultra-high pressures of ~2.8 GPa indicated by coesite occurrence in Kaghan and Tso Morari eclogites of northern Himalayas that are related to the subduction of Indian continental crust. Thus the Early Eocene subduction of the Indian plate that included a continental component was deeper under the Eurasian plate during the collision. Comparison of the P-T conditions at Naga Hills with numerical thermal models of plate subduction indicates that the eclogites probably formed near the top of the subducting crust with convergence rates of approximately 55-100 km/Myr, consistent with high pre-collision convergence rates between India and Eurasia.