PALEOGENE EROSIONAL RECORD OF THE HIMALAYAN-TIBETAN OROGEN ALONG THE YARLUNG-ZANGBO SUTURE ZONE, SOUTHERN TIBET

An understanding of the history of Himalayan–Tibetan orogen erosion is critical for deciphering crustal deformation processes and for investigation of proposed link between erosion of the orogen and changes in both global climate and ocean geochemistry. The occurrence of orogenic erosion along the Yarlung-Zangbo suture zone in southern Tibet has been lately greatly improved through geological mapping and field investigation. Our studies in southern Tibet document that erosion of Tibetan-Himalayan orogen began as early as Early Paleocene, resulting in formation of six sedimentary successions: 1) the Cuojiangding group in Zhongba, which overlies disconformably the Xigaze forearc deposits and consists of Paleocene-Early Eocene fan-delta deposits; 2) the Early Eocene Zheya Formation in Saga, comprised by deep-water flysch deposits; 3) the Early Eocene Enba Formation in Tingri, which is composed of shallow-water, shelf deposits and includes turbiditic sandstones; 4) the Paleogene Jiachala Formation in Gyangze, which is mainly composed of sandstones with shales deposited in marine fan-delta environment; 5) the outer molasse belt - represented by middle to late Eocene Liuqu Conglomerate deposited in alluvial fan and braided river within the Yarlung-Zangbo suture zone; 6) the interior molasse belt – represented by the Late Oligocene–Miocene Gangrinboche (Dazhuqu and Luobusa) conglomerates deposited in intermountain alluvial fan and braided river. Petrographic study and analyses of U–Pb and Hf isotopes in detrital zircons from the Cuojiangding, Zheya, and Enba units documented occurrence of provincially derived sedimentary material mainly from Gangdese orogen. These sediments were interpreted to be laid down in varies depozones of the underfilled Himalayan foreland basin system. In contrast the Liuqu Conglomerate had multiple sources which include Xigaze forearc deposits, the Yarlung-Zangbo ophiolites, and the Langjiexue Group and Tethyan Himalayan rocks and was most probably deposited in a piggy-back basin. Our data support India–Asia continental collision to occur at least in the early Paleocene, while the Himalayan orogenesis began as early as in middle Eocene.