DISCOVERY OF TWO NEW SEDIMENTARY SUCCESSIONS ALONG THE INDIA-ASIA COLLISION ZONE, TIBET

The Yarlung suture zone in southern Tibet preserves several sedimentary successions that record the transition from oceanic subduction to continent-continent collision. In this study, we report the discovery of two previously undocumented siliciclastic successions that record Early Cenozoic collisional processes. Located east-southeast of the town of Saga, the first succession records the onset of continental collision between India and Asia. The succession is 1 km thick, with the first 463 m comprising 5-20 cm thick beds of fine- to medium-grained quartz-arenitic sandstone with interbedded mudstone. At 463 m, there is a sharp contact with overlying green and red siliceous shale and chert with thickening and coarsening-upward massive to laminated felspatholithic sandstones. Similar to the facies preserved in the Sangdanlin and Zheya Formations, we interpret this succession as a foreland basin assemblage, marking the onset of continent-continent collision at the abrupt transition to red and green shale and chert and feldspatholithic sandstone.

We term the second siliciclastic succession the Raka Conglomerate and interpret it to be equivalent to the Liuqu Conglomerate to the east based on the similarity of facies and structural position. The Raka conglomerate is 400 m thick with the total thickness estimated to be closer to 1 km based on extrapolation across a drainage. The dominant lithofacies are clast-supported, massive pebble-boulder conglomerate interbedded with red massive, mottled mudstones and siltstones. Occasional imbrication and horizontally stratified, clast-supported pebble conglomerates are present. Clasts are sub-rounded and moderately well-sorted, consisting primarily of red and green chert, with sandstone, limestone, basalt and quartzite varying in abundance. These lithofacies are consistent with deposition in an alluvial-fan environment dominated by debris flows and sheetfloods. Based on structural relationships with the Yarlung suture zone ophiolite and the Xigaze forearc basin, this sedimentary succession is likely Early Cenozoic in age. Growth strata and structural relationships are consistent with deposition in the proximal footwall of south-directed thrusts that likely formed as part of the Tethyan Himalaya fold-thrust belt in Paleocene—Eocene time.