GEOLOGICAL TRANSECT ACROSS THE TSO-MORARI AND SPITI AREAS - THE NAPPESTRUCTURES OF THE TETHYS HIMALAYA

A new geological transect through the Tso Morari gneiss dome and the U pper Precambrian to Cretaceous sediments of the Spiti region has been used to determine the nappe structures of the Tethys Himalaya. The new ly defined SW-directed Tso Morari, Tetraogal and Mata nappes have been formed by shearing off the upper crust of the north Indian continenta l margin during underthrusting below Asia. The NE-directed intra-conti nental Lagudarsi La thrust of the Spiti Valley is older than the front al thrust of the SW-directed Mata nappe. The Mata nappe demonstrates t he validity of the nappe model of Argand (1916), It is a large recumbe nt fold nappe in the ductile rocks of a deep tectonic level to the NE. It changes gradually towards the south-west into an imbricate structu re, a stack of parallel thrust sheets, in the more brittle rocks at a higher tectonic level. The chronology of the Tertiary tectonic events has been established on the basis of structural observations: the init iation of the Mata nappe is related to the continental collision and u nderthrusting of the Indian continental plate below Asia. The NE-direc ted Lagudarsi La thrust - a frontal thrust of the Shikar Beh nappe - h as been formed after the continental collision by intracontinental bre ak off and shearing of the Indian crust. The frontal portion of this s tructure is older than the front of the Mata nappe. The extrusion of t he high-pressure Tso Morari nappe and the later uplift of the Tso Mora ri gneiss dome on the high-angle Ribil fault are responsible for the e xhumation of high pressure eclogite facies rocks of the Tso Morari nap pe. In a later stage of compression, the internal northern part of the Mata nappe and the Tetraogal and Tso Morari nappes are overprinted by NE verging back folds. The formation of these synmetamorphic backfold s and the younger extension of the Dutung-Thaktote faults are probably related to the SE-directed extrusion of the Crystalline nappe and the Main Central Thrust farther south. At present the compression of the Himalayan chain continues as a dextral transpression between India and Asia, developing dome and basin folds at a hundred-kilometer scale, s uch as the Tso Morari dome and the Phirse Valley depression and creati ng the N-striking Tso Morari fault at the western border of a pull-apa rt basin.