MASS ACCUMULATION IN THE CENOZOIC HOH XIL BASIN,NORTHERN TIBET

Sedimentary basins are where the products or erosion of the related reliefs accumulate.The sedimentary records preserved in the basins offer the possibility of quantifying the paleotopography z(x,y,t) at each point( x,y )of a given region at any time t in the past by using methods of mass balanced reconstruction(Hay et al.,1989,Métivier and Gaudemer,1997),The importance of paleotopography is on that atmospheric circulation models and the ocean circulation models which depend on them require knowledge of the topography of the land as an important input boundary condition which strongly affects the model output(Barron and Washington,1984,Ruddiman et al.,1997).Thus,a first step towards establishing paleoelevations is to reconstitute the history of sedimentary basins in terms of mass accumulation(Hay et al.,1989).This work reconstructs the space\|time depositional history and estimates the mass stored in the Hoh Xil basin,northern Tibet.The Hoh Xil basin,with an area of 101000km\+2 and an average elevation of over 5000m,is the largest Cenozoic sedimentary basin in the hinterland of the Tibetan plateau.From the Early Eocene to the Early Miocene,a sediment pile of approximately 5 4km thick of fluvial mudstone,sandstone,and conglomerate and 0 3km thick of limestone was formed in the basin (Liu and Wang,2000;Liu et al.,2000).the mass estimate is derived from 21 measured field cross\|sections,with total thickness of 13479 3m.The results show that the Hoh Xil basin has undergone 7 stages of evolution from the Early Eocene to the Early Oligocene,with a period of no sedimentation during the Late Oligocene.The Fenghuoshan Group was formed from the first four stages as 56 0～52 0,52 0～46 7,46 7～39 7,and 39 7～33 2Ma ago,with the depocenter moving eastward and northward.The Yaxicuo Group was formed from the two next stages as 33 2～32 2 and 33 2～30 2Ma ago.The Wudaoliang Group spread the entire basin during the last stage of ca.23 0～16 0Ma ago,with its depocenter in the northern part.The strata of the Fenghuoshan and Yaxicuo Groups have undergone strong deformation mainly during 30 0 to ca.23 0Ma,whereas only minor tilting has occurred in the Wudaoliang Group later.The space\|time evolution indicates that the Hoh Xil basin could be formed by the collision between Indi and Asia during the Early Eocene,and that its filling processes were controlled by early uplift and crustal shortening of the Tibetan plateau.The sedimentation budget is estimated as 297 15×10 12 t for the Cenozoic sediments deposited in the Hoh Xil basin.The accumulation rate curve shows that the value rises suddenly to around 800t/(m\+2 Ma -1 )during 32 2～30 0Ma ago from around 400t/(m\+2·Ma -1 )during 56～32 2Ma ago.The sudden increase of accumulation rate could be produced by early uplift of the Tibetan plateau.