STRUCTURE AND GEOCHRONOLOGY OF THE LIUQU CONGLOMERATE ALONG THE YARLUNG-ZANGBO SUTURE ZONE (SOUTHERN TIBET), AND IMPLICATIONS FOR THE MODE AND TIMING OF COLLISION TECTONICS IN THE TIBETAN-HIMALAYAN OROGENIC BELT

The Paleogene-Neogene Liuqu Conglomerate (LQC) along the Yarlung-Zangbo suture zone (YZSZ) in Southern Tibet is a terrestrial deposit providing significant spatial–temporal constraints for the timing and nature of collisional events in the tectonic evolution of the Tibetan-Himalayan orogenic belt. The ~10-km-wide (N-S) LQC is exposed discontinuously for more than 1000 km in an E-W direction, and is tectonically overlain to the north by the Cretaceous Neotethyan oceanic lithosphere along a S-vergent thrust fault and to the south by Triassic-Jurassic metamorphosed sedimentary-volcanic rocks of the Tethyan Himalaya along N-vergent reverse-thrust faults. Internally, the LQC is deformed by E-W-oriented reverse–thrust faults and closed to tight folds, and is segmented by NNE-SSW-striking transtensional–transpressional faults. The LQC is both a matrix- and clast-supported conglomerate, with a poorly to moderately sorted red quartz sandstone and mudstone matrix. In the central and southern parts of the LQC, angular to sub-rounded, pebble to cobble-sized clasts consist of red sandstone, siltstone and mudstone, greyish-green shale, grey phyllite and slate with their provenance in the Triassic Tethyan Himalaya to the south. The clastic material in the northern part includes quartz sandstone, radiolarian chert, minor dolerite, gabbro and peridotite derived from the Cretaceous ophiolite. There is no Gangdese magmatic belt-originated clastic material in the LQC. The LQC was accumulated in an E-W-oriented fluvial-alluvial depocenter, developed within a pull-apart basin system along the YZSZ. In-situ detrital zircon analyses of its sandstones have revealed 163 concordant U-Pb ages ranging from 307±13 Ma (youngest) to 3362 ±51 Ma (oldest). Statistically, this age spectrum displays a prominent peak centered at ~935 Ma, a large peak around ~516 Ma, and two small clusters around ~2429 Ma and ~2772 Ma. Collectively, these U-Pb zircon ages are consistent with the detrital U-Pb zircon ages available from the Tethyan Himalaya lithologies to the south. Our new age and stratigraphic data from the LQC indicate its formation along the suture zone between the northern edge of India and a Jurassic-Cretaceous arc-trench system, long before the collision between Asia and Greater India in the late Oligocene.