ALBERT

Description: 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 north-northwest 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 ( Hf[t] = –23.5–14.6), 120–135 Ma ( Hf[t] = –12.6–13.1), 150–170 Ma ( Hf[t] = –14.1–14.7), 500–600 Ma ( Hf[t] = –26–3.4), and 1100–1200 Ma ( 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 40 Ar/ 39 Ar data, detrital zircon fission-track analyses, and 13 C 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.

Description: 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 north-northwest 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 ( Hf[t] = –23.5–14.6), 120–135 Ma ( Hf[t] = –12.6–13.1), 150–170 Ma ( Hf[t] = –14.1–14.7), 500–600 Ma ( Hf[t] = –26–3.4), and 1100–1200 Ma ( 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 40 Ar/ 39 Ar data, detrital zircon fission-track analyses, and 13 C 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.

Description: The post-collisional uplift history of the India-Asia suture zone in southern Tibet is important for understanding the geodynamic conditions of the India-Asia collision and how it may have modified regional and global climate. Here we use stable isotope and major element analyses of paleosols preserved in the Liuqu Conglomerate to reconstruct India-Asia suture zone paleoclimate and paleoelevation. Paleosol carbonate 13C (Vienna Peedee belemnite) average values of –9.4 ± 1.1 indicate that Liuqu paleosols were well vegetated by C3 plants, unlike thinly vegetated and arid modern Tibet. Major element weathering indices show that these soils experienced significant collapse and loss of cations due to weathering of similar intensity to that in the wet, low elevations of the Neogene Himalayan foreland. Age estimates for the Liuqu vary, but current evidence points to an early Miocene age. Our evidence requires that the India-Asia suture zone experienced wet, well-vegetated conditions during Liuqu Conglomerate deposition. Geodynamically this implies that the suture zone was at relatively low elevation and was topographically open to monsoon moisture as late as 40 m.y. after the start of the India-Asian collision. This challenges the idea that uplift of the India-Asia suture zone was monotonic and tied directly to crustal thickening.