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 (εHf[t] = -23.5-14.6), 120-135 Ma (εHf[t] = -12.6-13.1), 150-170 Ma (εHf[t] = -14.1-14.7), 500-600 Ma (εHf[t] = -26-3.4), and 1100-1200 Ma (εHf[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.
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