IMBRICATE STRUCTURE OF THE LUOBUSA OPHIOLITE AND SURROUNDING ROCK UNITS, SOUTHERN TIBET

Abstract

The Cretaceous Luobusa Ophiolite is a tectonic slice less than 1.2 km thick. The structurally underlying unit is the Tertiary Luobusa Formation and the overlying unit is composed of feebly metamorphosed Triassic sedimentary rocks. The top and bottom contacts of the ophiolite dip gently to the south. The major part of the Luobusa Ophiolite is mantle peridotite, including podiform chromitite. This chromitite has received much attention because it contains an ‘unusual mineral assemblage’ which includes diamond and moissanite. A serpentinite mélange zone, including clasts of basaltic and sedimentary rocks, occurs underneath the mantle peridotite. Mesoscopic–microscopic structures were observed in the sheared rocks. Shear-indicating structures (C′-type shear bands and σ-type porphyroclasts) in the rocks near the top and bottom boundaries of the Luobusa Ophiolite show consistent top-to-the-north (or northeast) reverse displacement. The results reveal that the Luobusa Ophiolite was overturned and intercalated into an imbricate structure. The thrust faults on the top and bottom of the ophiolite can be correlated with north-vergent back-thrusting which was associated with crustal shortening along the Main Central Thrust due to the continued northward movement of India, after the welding of India to Asia.