Photang thrust sheet; an accretionary complex structurally below the Spontang Ophiolite constraining timing and tectonic environment of ophiolite obduction, Ladakh Himalaya, NW India

The Spontang ophiolite is one of the few important remnants of oceanic crust obducted onto the Mesozoic passive margin of the N Indian plate still preserved in the Himalaya. Detailed mapping, with structural, sedimentological and geochemical analysis, shows that a distinct thrust sheet (the Photang thrust sheet) structurally underlies the ophiolite. This thrust sheet comprises distinct thrust slices of alkaline volcanic rocks overlain by pelagic carbonates, ranging from Permian to late Cretaceous in age. Tectonic melanges within the thrust sheet contain blocks of volcanic rocks, serpentinized harzburgites, deep-sea sediments and limestones, also of Permian to Cretaceous age. Trace element geochemistry of the alkaline basalts suggests that they were erupted in a within-plate setting, perhaps as off-axis oceanic islands. The Photang thrust sheet is interpreted as a subduction accretionary complex that formed through thrusting over a considerable period up to the late Cretaceous. It immediately underlies the Spontang ophiolite and restores to a position immediately S of the ophiolite prior to subduction. It follows that the Spontang ophiolite and associated Spong arc must have existed in the hanging wall of the subduction zone at which the accretionary complex formed in the late Cretaceous.