New constraints on the India-Asia collision: the Lower Miocene Gangrinboche conglomerates, Yarlung Tsangpo suture zone, SE Tibet

Lower Miocene conglomerates crop out along the length of the Yarlung Tsangpo suture zone on the southern margin of the Lhasa terrane. These conglomerates, known by various local names, are correlated herein as the Gangrinboche conglomerates. All units exhibit broadly similar stratigraphic histories and a basal depositional contact upon an eroded surface of rocks of the Lhasa terrane is ubiquitous. At most localities the tops of sections are either removed by erosion or truncated by north-directed thrusts. These conglomeratic molasse units developed in response to the India/Asia collision and record aspects of its development. In all units initial clast derivation was from the Lhasa terrane on the northern margin of the Yarlung Tsangpo suture zone. Up-section the first appearance of clasts derived from terranes within the suture zone and the northern margin of India, all of which lie to the south of any outcrops of Gangrinboche conglomerates, is observed. Although these units were previously thought to be Eocene, analysis of fossil and structural constraints indicates Early Miocene deposition. As development of the Gangrinboche conglomerates records a significant phase in the evolution of the India-Asia collision understanding of their age and stratigraphic evolution has wider implications for regional tectonic models. (C) 2002 Elsevier Science Ltd. All rights reserved.