Ophiolites in the North Himalayan nappes and Indus Suture Zone in Eastern Ladakh (NW Himalaya, India)

Résumé 0

Ophiolites are fragments of ancient oceanic lithosphere, preserved in orogenic belts in a context of plate convergence. They are generated at mid-ocean ridges, in a supra-subduction zone or volcanic arc. Commonly, several magmatic events are recorded, as shown, for instance, in the Oman Ophiolite (Goodenough et al. 2014). The Ophiolitic rocks of Eastern Ladakh are subdivided in two main groups, based on the geodynamic setting during their formation: the supra-subduction zone ophiolite and the ophiolitic “mélanges”, corresonding both to the Indus Suture Zone. Recent detailed studies North-East of the Tso Moriri area revealed a large diversity of ophiolitic rocks and associated sediments. We identified three distinct tectonic units containing ophiolites: The Nidar Ophiolite, the Drakkarpo nappe and the Karzok-Ribil nappe. The Nidar supra-subduction zone Ophiolite represents a complete ophiolitic sequence, from mantle to sediments, which underwent a low greenschist facies metamorphism. This ophiolitic sequence was thrusted towards the South. They record a first magmatic event in a mid-ocean ridge setting, and a second one in a supra-subduction zone at around 130 Ma. The Drakkarpo nappe is a “mélange” unit composed of thick polygenic conglomerates and volcano-sedimentary rocks, mainly composed of tuffs and augite-basalts (OIB), serpentinites, pillow lavas and gabbros. This unit is interpreted as being a part of an accretionary wedge containing slices of oceanic islands arc. This nappe marks the Indus Suture Zone. The Karzok-Ribil nappe is a newly defined tectonic unit involved in the North Himalayan nappe stack. It can be followed at the top of the Tetraogal nappe and around the Tso Morari dome. The Karzok-Ribil nappe is composed of segments of ophiolitic sequence (serpentinites, gabbros, pillow lavas), radiolarites, polygenic conglomerates, agglomeratic slates from the indian margin, augite-basalts (OIB) and limestones. It is interpreted as being originally a seamount, located close to the Indian passive margin in a ocean-continent transition zone. The new lithostratigraphy and structural analyses of the Eastern Ladakh ophiolites and their associated sediments allow us to better constrain the formation and emplacement mechanisms of these tectonic units. It defines or precises the paleogeography and geometry of the north Indian passive margin, prior to the Himalayan collision. REFERENCES Goodenough, Kathryn M., Robert J. Thomas, Michael T. Styles, David I. Schofield, and Christopher J. MacLeod. 2014. “Records of Ocean Growth and Destruction in the Oman–UAE Ophiolite.” Elements 10 (2): 109–114.