The Liuqu Conglomerate, southern Tibet: Early Miocene basin development related to deformation within the Great Counter Thrust system

Abstract

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 40 Ar/ 39 Ar data, detrital zircon fission-track analyses, and d 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.

India‐Asia convergence: Insights from burial and exhumation of the Xigaze fore‐arc basin, south Tibet

“…Based on strain analysis and restored sections, Einsele et al [] proposed that the preserved basin segment had been shortened 65% of the original width from at least 65 km. Two upper Oligocene‐Miocene conglomerates lie on the northern and southern boundaries of the Xigaze basin; the Kailas Group, specifically the Qiuwu‐Dazhuka Formation to the north [ Wang et al , ; Li et al , ]; and the coeval Liuqu Formation to the south [ Davis et al ., ; Li et al , ; Leary et al , ] (Figures and ).…”

Gangdese culmination model: Oligocene–Miocene duplexing along the India-Asia suture zone, Lazi region, southern Tibet

“…We correlate this unit to the Liuqu Formation (Yin et al, 1980), which was deposited in a contractional setting as part of a fluvial and alluvial-fan depositional system (Leary et al, 2016a). The preponderance of geochronological and thermochronological data from the Liuqu Formation suggest that it was deposited during a short interval between 20 and 19 Ma (Li et al, 2015;Leary et al, 2016a). However, Ding et al (2017) argued that the Liuqu Formation was deposited during latest Paleocene time based on U-Pb geochronology of interbedded tuffs.…”

“…4D). The maximum preserved thickness of the Liuqu Formation is ~2 km, near the town of Liuxiang (Li et al, 2015), whereas the maximum thickness of the Liuqu Formation in the Lazi region is ~200 m (Leary et al, 2016a).…”

“…Some workers have interpreted the Kailas Formation as the product of contractional deformation, associated with a lithospheric flexure during a late stage of India-Asia collision (Aitchison et al, 2007), flexural foreland basin deposition related to the Great Counter thrust system (Wang et al, 2015), or wedgetop sedimentation related to out-of-sequence Great Counter thrust system activity (Yin et al, 1999). However, recent investigations of the Kailas Formation sedimentology, fossil assemblages, and basin architecture indicate that the Kailas Formation was deposited in an extensional basin bounded by a north-dipping normal fault, perhaps related to Oligocene-Miocene rollback or peeling-back of the subducted Great Indian slab (DeCelles et al, , 2016Wang et al, 2013;Leary et al, 2016a).…”

“…110 Ma (Huang et al, 2015;Orme and Laskowski, 2016). Another conglomerate unit-the Liuqu Formation-is locally exposed to the south of the Xigaze forearc basin, where it was deposited mainly atop serpentinite-matrix mélange during early Miocene time (Li et al, 2015;Leary et al, 2016a) or late Paleocene time . The Yarlung suture zone ophiolites and serpentinite-matrix mélanges structurally overlie a belt of subduction-accretion, shale-and sandstone-matrix mélange with (meta-) sedimentary and (meta-) basalt blocks (e.g., Cai et al, 2012).…”