The ophiolites of northeast India — a subduction zone ophiolite complex of the Indo-Burman orogenic belt

Abstract

The ophiolites of northeast India are rootless blocks of various dimensions, floating in a matrix which belongs to the upper Cretaceous-Lower Tertiary Disang Group. They consist of diverse igneous, sedimentary and metamorphic rocks, of which ultramafics are the main component. They do not constitute a continuous sheet but are made up of units haphazardly juxtaposed along faults or they consist of lensoid slices interbedded with Disang Group rocks. The ultramafics are interpreted as slices of oceanic crust and upper mantle obducted onto the Indian continental margin. Associated blue schist is indicative of subduction zone tectonics. The occurrences of ultramafics showing intrusive contacts and the presence of intermediate — acid volcanics suggest an island arc — continent type of collision along the Benioff Zone coincident with the ophiolite belt. The tectonic history of the Indo-Burman orogenic belt can be tentatively summarised as follows: (1) Introduction of the subduction zone during Cretaceous times; obduction of oceanic crust and upper mantle and intermixing with deep oceanic sediments. (2) Deposition of the Disang sediments. Formation of an island arc separating the sedimentary basin into eastern and western sub-basins. The Barail sediments were deposited in both basins. The formation of the island arc was accompanied by plutonism and volcanism and by deformation and low grade metamorphism of the lower Disang sediments. (3) Gradual shallowing of the basin consequent upon the collision of the island arc with the easterly subducted India plate. (4) Continent-continent collision and rotation of the down-basin normal faults into low angle reverse faults (thrusts) along which the thrust sheets were piled up. The frontal areas of the thrust sheets were asymmetrically folded. This was followed by asymmetrical folding of the rocks of the foreland areas. (5) Formation of gravity faults and conjugate sets of strike-slip faults in the orogen and the foreland areas. The structures associated with the ophiolite belt are attributed first to the collision of the easterly subducting India plate with an island arc and subsequently with the Burma plate.