Sedimentary Record And Basin Evolution Of The Himalayan Orogen In Xigaze Area, Southern Tibet

Himalayan orogen created by the India-Asia collision and the subsequent crustal thickening along the Indus-Yarlung Zangbo suture zone is the most famous collisional mountain belt on the earth. The knowledge about the evolution of the Himalayas strongly influences our interpretation of the tectonics of other orogenic belts. However, our knowledge about the early history of the Himalayan orogenesis is rather limited. In this study, we investigated the Late Cretaceous to Tertiary sedimentary strata in Xigaze area, southern Tibet. A integrated study of stratigraphy, sedimentology, sandstone petrology, detrital zircon U-Pb ages and Hf isotopes, detrital Cr-spinel geochemistry was carried out on the the Late Cretaceous-Eocene strata in the Sangdanlin Section, the Middle Eocene-Oligocene Liuqu Conglomerate in Liuqu and Xialu localities and the Oligocene-Early Miocene Qiuwu and Qiabulin formations in Xigaze area to provide new constraints to the initial India-Asia collision and the early uplift history of the Himalayan orogen. The studied strata are nearly continuous from Late Cretaceous to Early Miocene, therefore comprise an integrated record for the early Himalayan orogenesis.The Sangdanlin section, located at about10km south of Saga city, consists of a subaerially exposed tectonic block of sedimentary strata embedded within a mud matrix tectonic melange zone. The sedimentary strata outcropping at the Sangdanlin section can be subdivided into three lithologic units. The Upper Cretaceous-Paleocene Denggang Formation is dominated by quartzarenites and silty shales, with red siliceous shales and cherts occurring near the top. The Early Eocene Sangdanlin Formation overlying the Denggang Formation is composed of litharenites, siliceous shales, chert, and minor quartz sandstones. The Zheya Formation, which conformably overlies the Sangdanlin Formation, is characterized by the presence of dark gray shales interbeded with light green litharenites, and minor variegated cherts occurring near the base of the formation. The age of the Zheya Formation is poorly constrained due to a lack of fossils, but might be of early-middle Eocene age based on the stratigraphical relationship and detrital zircon isotopic analyses. The Denggang Formation quartzarenites contain zircons with dominant Proterozoic-Ordovician U-Pb ages, with an additional age peak of Early Cretaceous, which we interpret to be derived from the northern Indian margin. By contrast, the lithic sandstones of the Sangdanlin and Zheya formations are dominated by zircons younger than200Ma, showing one major peak at～80-125Ma and two subdominant peaks at～54-70and～180-196Ma, comparable to those from the Gangdese magmatic arc. Cr-spinels in the Sangdanlin and Zheya formations are abundant and characterized by extremely low TiO2wt%, also indicating material input from the Lhasa terrane. The abrupt sedimentary provenance transition from the north Indian margin to the Lhasa terrane between the Denggang and Sangdanlin formations provides a minimum age constraint for the timing of India-Asia continental collision. The youngest zircon age cluster (～54Ma) combined with the occurrence of RP9(49-50.3Ma) radiolarians at the base of the Sangdanlin Formation suggest the collision happened at least prior to the late Ypresian (～50Ma).The Liuqu Conglomerate, which extents over a distance of150km (from Lhaze to Bainang) immediately south of the Yarlung-Zangbo ophiolite, is comprised by coarsen-grained clastic rocks mainly deposited in alluvial fan environments. As no volcanic clasts derived from the Gangdese magmatic arc were observed within the conglomerate, Davis et al.(2002) suggested that the Liuqu Conglomerate is a molasse record of the India and intra-oceanic arc collision. To extend and test their findings, we undertook detailed petrographic studies and analyses of U-Pb and Hf isotopes of detrital zircons from the Liuqu Conglomerate. Clasts in the conglomerate consist of quartz-arenite, litharenite, slate, radiolarian chert, and basalt, along with minor phyllite, gabbro, and serpentinite. Radiolarian chert, and mafic and ultramafic detritus are clearly derived from the Yarlung-Zangbo ophiolite. Detrital zircon ages from the Liuqu Conglomerate are concentrated in three clusters at80-150,200-400, and～450-1250Ma. Zircons of80-150Ma in age can be subdivided into two groups:a group with positive εHf(t) values shows a Gangdese affinity and is considered to have been recycled from sedimentary strata of the Xigaze forearc basin, while another group with negative εHf(t) values was derived either from Cretaceous strata of the Tethyan Himalaya or from the Xigaze foreac basin (originally derived from the north Lhasa terrane) of the Asian plate. Zircons with ages of200-400Ma and εHf(t) values of-4.3to+9.1were derived from Triassic clastic rocks of the Langjiexue Group, as this is the only possible source, to the best of our knowledge. Zircons older than450Ma may have multiple sources, including the Tethyan Himalayan sequences, the Langjiexue Group, and even the Xigaze forearc sediments. The occurrence of Asian-derived detritus in the Liuqu Conglomerate, deposited above the Indian plate and ophiolite, indicates that the conglomerate deposited after India-Asia collision and recorded the early erosion of the Himalayan-Tibetan orogen. Thus, the results are inconsistent with the proposal that the Liuqu Conglomerate records India and intra-oceanic arc collision.The interior molasse belt in the Xigaze area comprises the Qiuwu Formation and the overlying Qiabulin Formation. The Qiabulin Formation could be further subdivided into three units, which are from bottom to top, the Jiangqingze Member, the Deri Member and the Tunqiong Member. The Qiuwu Formation, comprises a lower part of dark shales and thick-bedded sandstones and an upper part of variegated mudstones and thin to medium-bedded sandstones, was deposited in a lake with intercalated delta sandstones. The Jiangqingze Member consists of intercalated greenish gray sandstones and red mudsotones, deposited in fluvial or braided fluvial environments. The Deri Member is composed mainly by coarse-grained conglomerate, with subordinate intercalated sandstone and mudstone, which was interpreted to have deposited on alluvial fans that were dominated by braided environments. The Tunqiong Member comprises red mudstones or pebbly mudstones with a few sandstone and conglomerate beds, deposited in distal alluvial fan environments. Palynofloras and detrital zircon ages suggest the Qiuwu and Qiabulin formations were deposited at late Oligocene-Early Miocene. Provenance analyses indicate that the Qiuwu Formation and the Jiangqinze Member were mostly derived from the Gangdese arc to the north, with only very limited detritus derived from the Yarlung-Zangbo suture zone to the south. By contract, gravels of radiolarian cherts, mafic and ultramafic rocks and sedimentary rocks are abundant in the Deri and Tunqiong members, which indicate significant influx of material from the Yarlung-Zangbo ophiolites and the Xigaze Forearc basin to the south. Occurrence of Late Oligocene-Early Miocene lake sediments parallel the Yarlung-Zangbo suture zone might record a short period of extension, which was suggested to be related to southward rollback of the hinge line in the subducting/underthrusting Indian continental lithosphere. The influx of detritus from the south in the upper part of the Qiabulin Formation was interpreted as a result of activation of the great counter thrust, which might be caused by the Indian continental slab break-off. Our data provide an alternative interpretation for deposition of the Interior Molasse belt and do not support the notion that these deposits record initial shortening owing to the India-Asia collision.Based on our studies and recent published geological data, a tectonic-depositional model was constructed to illustrate the Late Cretaceous-Tertiary depositional history along the Yarlung-Zangbo suture zone. In the new model, the India-Asia collision was considered to have occurred during the latest Cretaceous-Early Eocene, when the Indian passive margin basin changed to the underfilled Himalaya foreland basin in response to flexural subsidence driven by loading of Asian crust. The Sangdanglin and Zheya formations in Saga are interpreted to have deposited in the foredeep depozone of the foreland basin. A transition of the foreland basin from underfilled stage to overfilled stage occurred at about Middle Eocene, accompanied by the final closing of the Tethyan seaway. It is suggested that Eocene Tethyan slab breakoff, which may generate rapid isostatic uplift along the suture zone, was a deep-level dynamic control for these events. The Liuqu Conglomerate, deposited at this time, was interpreted as having deposited in a wedge-top basin and being a sedimentary record of early Himalayan uplift. During Oligocene-early Miocene, regional extension and subsequent activation of the great counter thrust which might be generated by the rollback and subsequent break off of the subducting Indian continental lithosphere occurred at the south margin of the Gangdese arc, accounting for deposition of the Interior molasse belt.