Late Miocene-Early Pleistocene sedimentary evolution in Gyirong Basin, southern Tibet

The Gyrong Basin was located in the northern piedmont of the Himalayas. Sedimentary facies in Miocene Danzengzhukang Formation of the basin are composed from the bottom upwards of alluvial fan-braided river. The Orma Formation consists of delta facies-lacustrine facies-delta facies from bottom to top, which represents a progradation-regradation-progradation sequence. The paleocurrent reflected in Orma section was mainly in SEE direction in 10-7.4Ma, and was in SWW direction in 7.4-1.67Ma. These data imply that the paleogeographic framework of Gyrong Basin changed drastically and its topography changed from high in the west and low in the east into high in the east and low in the west after 7.4Ma. Based on the sedimentary, magnetostratigraphic and chronologic data available, the authors hold that Gyrong Basin has experienced three evolutionary stages since 10Ma: 10-7.4Ma was the stage of rift and formation; 7.4-3.6Ma was the stage of expansion and steady development, an 3.6-1.67Ma was the stage of withering and extinction. These three stages represented the three uplift period of the Himalays, which included 10-7.4Ma, 7.4-3.6Ma and 3.6-1.67Ma, and the Himalayas was strongly uplifted during 3.6-1.67Ma.