THE LIUQU CONGLOMERATE, SOUTHERN TIBET: AGE AND PALEOCLIMATE

The age and tectonic significance of the terrestrial Liuqu Conglomerate, exposed within the India-Asia suture zone in central-southern Tibet have been much debated. Age estimates range from Paleocene to Miocene, and these deposits have been variously interpreted as representing an island arc collision with India prior to India-Asia collision, exhumation and sedimentation immediately following collision, early paleo-Yarlung River sedimentation, and intra-montane sedimentation related to regional uplift and deformation of southern Tibet.

Here we refine the depositional age of the Liuqu Conglomerate to ca. 20 Ma based on a synthesis of detrital zircon U-Pb, U-Th/He, and fission track analyses; biotite ⁴⁰Ar/³⁹Ar data; soil carbonate stable isotope data; regional structural constrains; and reanalysis of palynologic and plant fossil data.

The Liuqu Conglomerate was deposited in alluvial fans and other coarse-grained fluvial deposystems. Sedimentological analysis shows the unit to be composed of mixed fluvial and sediment-gravity flow lithofacies assemblages, locally punctuated by mature, clay-rich paleosols. Analysis of rare pedogenic carbonates within paleosols yielded δ¹³C (VPDB) values between -12‰ and -8‰ indicating deposition in a well vegetated setting. Major element analyses of profiles across Liuqu paleosols allow calculation of paleoprecipitation and paleoweathering estimates. Major element analyses normalized to paleosol parent material show that Liuqu paleosols experienced an average collapse of ca. 50% and average cation losses of 58% (Na), 27% (Al), 47% (Si), and 42% (Ca). These values are within the same range as modern soils formed in humid, subtropical environments as well as paleosols within the Siwalik Group, formed at low elevation within Indo-Gangetic foreland basin. CALMAG index values suggest that Liuqu paleosols received a minimum of 1500 mm/yr mean annual precipitation. Together, these data suggest that the Liuqu Conglomerate formed in a humid, sub-tropical environment in stark contrast to the modern Tibetan climate. This suggests that at 20 Ma, the India-Asia suture zone was at low to moderate elevation and that some pathway existed that allowed moisture to penetrate deep into southern Tibet.