PROVENANCE OF THE INDUS-YARLUNG SUTURE MÉLANGE AND THE ONSET OF TETHYAN HIMALAYA-ASIA COLLISION, SOUTHERN TIBET

The Indus-Yarlung Suture Zone (IYSZ) is the boundary between rocks of Indian and Asian affinity. From north to south in southern Tibet, it exposes the preserved Asian forearc, ophiolitic rocks, tectonic mélanges, and Tethyan Himalayan strata of Indian affinity. Resolving the timing of collision is fundamental to understanding the extent of Greater India and the tectonics of the southern margin of Asia. The mélange formed by subduction accretion associated with the Cordilleran-style southern margin of Asia, prior to India–Asia collision, but its provenance, age, and extent are poorly constrained. We conducted geologic mapping and U/Pb detrital zircon provenance studies of the blocks and matrix of the mélanges and of Tethyan Himalayan units to the south near Lopu Kangri, ~200 km NNW of Kathmandu.

Near Lopu Kangri, the mélange is composed of a mudstone matrix with blocks of basalt, chert, mudstone, metabasite, limestone, and sandstone. The majority of the blocks are deep-water facies or consistent with oceanic crust, supporting the interpretation that the mélange is an accretionary complex formed by oceanic subduction. Over a span of ~10 km to the southwest blocks gradually increase in size to encompass ~2 km x ~10 km areas of measurable section, and the component of matrix gradually decreases. Eventually the matrix is absent and strain is apparently taken up in incompetent layers within the stratigraphic sections. Results from U/Pb dating of detrital zircons suggest that the mélange contains Tethyan affinity blocks with early Cretaceous peak ages of ~130 Ma as well as Gondwanan affinity blocks with peak ages of ~500-550 Ma. Two samples show clear similarity to the Asian forearc and have maximum depositional ages of ~58.5 Ma.

Collectively, our preliminary results suggest that most sandstone blocks in the mélange in this locality are of Tethyan affinity, while two Asian-affinity samples represent an overlap assemblage derived from the forearc and deposited on Tethyan strata at ~59 Ma. We propose a tectonic model in which prior to collision the mélange received no sediment from the upper plate, then at the time of collision Asian forearc material was deposited on Tethyan strata, and as collision proceeded the Tethyan strata were broken up into blocks and imbricated.