The Liuqu Conglomerate, southern Tibet: Early miocene basin development related to deformation within the great counter thrust system

The Liuqu basin formed during the early Miocene between ophiolitic mélanges (to the south) and uplifted Cretaceous forearc deposits (to the north) along a central, 150-km-long segment of the India-Asia suture zone in southern Tibet. Sedimentological analysis shows the Liuqu Conglomerate to be composed of mixed fluvial and sediment-gravity flow lithofacies assemblages locally interbedded with mature paleosols. We interpret the Liuqu Conglomerate as coarse-grained fluvial and alluvial-fan deposits. Structural analysis indicates that the Liuqu Conglomerate was deposited in a contractional setting. Paleocurrent and provenance data demonstrate that sediment was transported northnorthwest from the hanging wall of a coeval thrust fault system along the southern limit of Liuqu outcrops. Detrital zircon U-Pb ages (and Hf[t] isotope ratios) cluster around 80-110 Ma (ε_Hf[t] = -23.5-14.6), 120-135 Ma (ε_Hf[t] = -12.6-13.1), 150-170 Ma (ε_Hf[t] = -14.1-14.7), 500-600 Ma (ε_Hf[t] = -26-3.4), and 1100-1200 Ma (ε_Hf[t] = -27.6-2.9), requiring input from both Gangdese and mélange sources. Asian zircons were recycled northward after being incorporated into accretionary mélanges along the southern Asian margin prior to India-Asia collision. The age of the Liuqu Conglomerate is still somewhat uncertain, but new chronologic data, including biotite ⁴⁰Ar/³⁹Ar data, detrital zircon fission-track analyses, and δ¹³C compositions of soil carbonates, all converge on ca. 20-19 Ma as the most probable age. Together, these results indicate that part of the north-to-south sediment transport system that existed prior to India-Asia collision and into the Eocene was reversed by ca. 20 Ma. The Liuqu Conglomerate may represent deposits associated with the paleo-Yarlung River.