EVOLUTION OF SEDIMENTARY BASINS ALONG THE INDUS-YARLUNG SUTURE ZONE, SOUTHERN TIBET: IMPLICATIONS FOR THE TIMING OF THE INITIAL STAGES OF THE INDIA-ASIA COLLISION

The Xigaze forearc basin, preserved along the Indus-Yarlung Suture Zone, preserves the tectonic, erosional and magmatic record of the southern margin of Asia from Cretaceous to Eocene time. This study examines the youngest sedimentary strata preserved in the Xigaze forearc basin in order to (1) reconstruct the paleogeography of the southern margin of Asia prior to collision with India and (2) document and analyze a forearc transition in a continent-continent collision. We report on sedimentological, modal petrographical and U-Pb detrital zircon geochronological data from Upper Cretaceous to Lower Eocene clastic and carbonate sedimentary rocks, preserved in the Xigaze forearc basin northeast of the Lopu Kangri Range in Saga County, and from Upper-Cretaceous to Paleocene siliciclastic sedimentary rocks from the northern Indian passive continental margin, near Sangdanlin.

Our results indicate that the provenance of forearc sediments in this region (88-54 Ma) was the Gangdese magmatic arc to the north. In addition, the forearc transitioned from predominantly deep marine to fluvial deposition between 58-54 Ma, coinciding with a period of high-flux magmatism along the Gangdese magmatic arc. During this same time interval, petrographic data from siliciclastic rocks deposited on the northern margin of India show a gradual change in sediment provenance, whereby the region transitioned compositionally from hypermature quartz-arenites to immature feldspatholithic sandstones. We also document the appearance of Asian-affinity detrital zircons in the Sangdanlin region and a strong similarity between the U-Pb detrital zircon age spectra with the Xigaze forearc at this time. From these results, we propose a model in which initial collision between India and the southern margin of Asia was underway by 58 Ma. In our model, the forearc basin transitioned from underfilled to filled and the accretionary prism formed the southern margin of Asia between 88-59 Ma. Subsequently, between 58-54 Ma, sediments from the forearc basin prograded southward over the accretionary prism onto the northern Indian passive margin. Following this interval, we speculate that the cessation of forearc sedimentation resulted from uplift and erosion in the forearc region as the nascent syn-collisional fold and thrust belt developed to the south.