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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Similarities in rock assemblages and facilitated thrust stacks of juvenile assemblages broad tectonic features between the late Precam-to override the ancient cratonic edge that may now brian to early Paleozoic basement of Arabia, the be buried beneath the SE Desert of Egypt. models that were proposed for Arabia, although at of Egypt and in Arabia, and they supported the least 2 of the Nubian ophiolite belts clearly con-arc accretion model developed for these regions. tinue into the Arabian shield. Subsequently Vail (1976) attempted to integrate The Pan-African structural domain with low-angle the Red Sea Hills into the geology of the entire thrusts and ophiolite m•langes extends at least as Arabian-Nubian shield and suggested that the far W as the River Nile, where the ancient margin "greenschist assemblages" were part of the same of the African craton may be found, The entire Pan-African crustal domain as the Eastern Desert domain farther E is characterized by newly accre-of Egypt and the Arabian basement, while he conted magmatic associations of late Precambrian age sidered the high-grade gneisses at the River Nile that may have evolved in settings similar to those to represent the edge of the ancient African crapresently observed in the Indonesian archipelago. ton. Further work led to a first evolutionary (]976) were the first to recognize the similarity these rocks is revealed by the local presence of of rock types and their tectonic settings in the sillimanite, while primary sedimentary structures NE Red Sea Hills with those in the Eastern Desert were largely destroyed by intense deformation and 236 KRONER ET AL.