Age and Tectonic Evolution of the Amdo Basement: Implications for Development of the Tibetan Plateau and Gondwana Paleogeography

Abstract

The elucidation of the geologic processes that led to the creation of the Tibetan Plateau, a large area of thick crust and high elevation, is a fundamental question in geology. This study provides new data and insight on the geologic history of central Tibet in the Jurassic and Cretaceous, prior to the Indo-Asian collision, as well as the Gondwanan history of the Lhasa and Qiangtang terranes of the plateau. This investigation is centered on the Bangong suture zone near the town of Amdo and I present new geochronology, thermochronology, thermobarometry and structural data of the Amdo basement, an exposure of high-grade gneisses and intrusive granitoids. Using a range of thermochronometers, I show there were two periods of cooling, one in the Middle-Late Jurassic after high-grade metamorphism and a second in the Early Cretaceous. I attribute Middle-Late Jurassic metamorphism, magmatism, and initial cooling of the Amdo basement to arc related tectonism that resulted in tectonic or sedimentary burial of the magmatic arc. I propose that a second period of cooling, nonmarine, clastic sedimment deposition and thrust faulting in the Early Cretaceous is related to the Lhasa-Qiangtang collision. The thermochronology reveals limited denudation between the Cretaceous and the present, indicating the existence of thickened crust when India collided with Asia in the early Tertiary. U-Pb geochronology of the orthogneisses and detrital zircon geochronology of metasedimentary rocks suggests that the Lhasa and Qiangtang terrane were located farther west along Gondwanan's northern margin than most reconstructions depict.