Tectonic And Environmental Evolution Of The Gyirong Basin, And Its Relationship To The Uplift Of Tibetan Plateau

The Gyirong basins, located in the northwestern slope of Mt. Xixabangma, is an faulted basin system, composed of four parts: the Gyirong basin, a small one between Gyirong and O'ma basins, O'ma basin, and the last one south to O'ma basin.The development of the basins were chiefly controlled by both the south Tibetan detachment system(STDS) and N-S trending normal faults. The basement of the basins is the folded and faulted Tethyn Himalayan sequence composed of limestone and slate of Jurassic. The southern boundaries of the basins are the splays of STDS, the eastern boundary is a N-S trending normal fault of the N-S trending rift system, the northern boundary of the basin system is the Malashan Dome, while the western boundary is a natural sedimentary boundary.The evolution of the Gyirong basin controlling structures can be divided into the following three stages.The first stage on which basin controlling structures and basement landform of Gyirong basin formed(>7.2Ma): Prior to the basin deposition, the magmatic emplacement produced by the melting of mid-upper crust rocks resulted in the formation of the STDS and a series of east west direction listric normal faults which is along the north side of STDS top wall. They formed the east west direction half-graben and landform feature on the early stage of basin development with no high height differcence in the east west direction of basin. In the later stage, the east-west extension induced the development of S-N direction normal fault as the eastern boundary of Gyirong basin, which cut through the prior tilted fault block. The block of top wall was declined and induced drag structures and fault breccia between down wall and top wall. In this case, the landform features,"west high, east low", were started to formed in E-W profile of small basins, the whole pattern of Gyirong basin had changed from E-W trending to S-N comb-like trending. The north boundary fault and sub-slump folds were formed by collapse in the later stage.Basin sedimentary stage(7.2-1.8Ma): The basin had been uplifted in deposited range, as a whole, although there were more uplift events had occurred on Tibet plateau in this period. The whole basin controlling structures tends to be stable, intra-basin and basin boundaries shows no evidences of synsedimentary tectonic activities in this stage, which also created necessary conditions to lake basin sedimentary, development and multiply of paleoecology.Re-movement stage of S-N trending fracture(<1.8Ma): The east boundary of Gyirong basin became activated again as early as the later stage of Quaternary fluvial conglomerate in lower Pleistocene, and cuted through the Quaternary fluvial conglomerate which developed on both side of S-N trending normal fault, according to the latest earthquake data, The fault activity of east boundary of Gyirong basin may lasted until now.Since the boundaries formed, the evolution of the Gyirong basins can be roughly divided into the following stages.The first stage (7.2~5.8Ma): Closed lacustrine basins in the initial stage, mainly controlled by STDS-related E-W trending tilted fault blocks. Coarse fan delta deposits formed on bottoms of the basins under the bushvele-forest type climate environment. From 6.9Ma to 5.8Ma, tectonics and the climate entered an oscillatory period, resulting in several nearshore-swamp sedimentary cycles. Hipparion fauna and its ecosystem occurred around the Gyirong basins during this time.The second stage (5.8~5.4Ma): Warm-humid closed lacustrine basins, accommodating grey nearshore sediments. Later in this stage, with the intensification of evaporation and environment became more oxidative, nearshore deposites turned to be buff-lark.The third stage (5.4~3.9Ma): climate trended to be dry and cold, Gyirong basins began to wither and shrink, and transfered from closed lacustrine basins to open ones, braided-channel sediments and red gossan widely developed.The fourth stage (3.9~1.7Ma): with the rapid uplift of Tibetan plateau since about 3.6Ma, climate in Gyirong basins became further cold. The open lacustrine basins became closed again and received delta-lacustrine sediments, a N-S trending rifting possibly united the small basins as a whole Gyirong basin. Later in this stage, southern rivers cut through the Himalayas, and the Gyirong river cross the lacustrine basin, converged with the southern Himalayan rivers, which resulted in the disappearance of lake and a set of Quaternary fluvial conglomerate on the former lacustrine sediments.The climate and sedimentary environment in the study area were mainly controlled by the uplift of Tibetan plateau. Different tectonics brought different sedimentary facies association and related climate backgrounds, while climatic factors could affect landform, water influx patterns, and these could feed back the relevant tectonics.