It is widely believed that the Tibetan plateau is a late Cenozoic feature produced by the Indo-Asian collision. However, because Tibet was the locus of continental accretion and subduction throughout the Mesozoic, crustal thickening during that time may also have contributed to growth of the plateau. This portion of the geologic history was investigated in a traverse through the central Lhasa block, southern Tibet. Together with earlier studies, our mapping and geochronological results show that the Lhasa block underwent little north-south shortening during the Cenozoic. Rather, our mapping shows that ~60% crustal shortening, perhaps due to the collision between the Lhasa and Qiangtang blocks, occurred during the Early Cretaceous. This observation implies that a significant portion of southern Tibet was raised to perhaps 3-4 km elevation prior to the Indo-Asian collision. INTRODUCTIONAlthough it has long been recognized that Tibet was the locus of continental collision and accretion since the early Mesozoic (Allégre et al., 1984;Sengör, 1984;Searle et al., 1987), the style and intensity of deformation produced by each accretional event remain poorly documented (e.g., Tibetan Bureau of Geology and Mineral Resources [TBGMR], 1982). To address this issue, we conducted systematic mapping of the Coqin area in the northcentral part of the Lhasa block ( Fig. 1), during which three thrust systems were documented (Gugu La, Shibaluo, and Emei La) along a 132-km-long north-south traverse (Fig. 2). None of these thrusts appear to have involved metamorphic basement, and thrusts in their hanging walls are cut by plutons that are in turn overlain by essentially flat-lying tuff deposits. Although the ages of sedimentary sequences in the mapped area have been broadly constrained as Paleozoic and Mesozoic by using index fossils (Tibetan Bureau of Geology and Mineral Resources, 1982), the igneous rocks were not previously dated. In this paper we briefly summarize the structural relationships and age constraints for the three thrusts and, in conjunction with allied data, conclude that the southern Tibetan plateau had begun to form during the Early Cretaceous and remained elevated until the Indo-Asian collision began. GUGU LA THRUST SYSTEMThe north-dipping Gugu La thrust places Cretaceous strata (Gugu La sequence) over Cretaceous (?) conglomerate and volcaniclastic rocks (Burial Hill sequence; Fig. 2a). The thrust has a maximum stratigraphic throw in its central part. Fault slickensides indicate a S10°-20°W transport direction.The lower part of the hanging wall consists of ~1-km-thick volcanic breccias and volcaniclastic sandstones. Paleocurrent measurements of crossbeds in the sandstone indicate a north-directed paleoflow. The top of the lower section (~800 m thick) is marked by a laterally extensive limestone layer, which contains abundant Early Cretaceous rudist bivalves (TBGMR, 1982). Above it is an ~500-m-thick sequence of fluvial sandstone that records a change upsection from north-directed to south-directed paleoflow. We inter...

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