QUATERNARY GEOLOGY AND FAULTING IN THE DAMXUNG-YANGBAJAIN BASIN, SOUTHERN TIBET

Detailed geological mapping conducted in the Damxung-Yangbajain basin shows that several sequences of deposits of different origins have been accumulated since the Pliocene or early Pleistocene,of which the most prominent are glacial and fluvioglacial deposits formed in the about 700~500,250~125 and 75~12 ka intervals,indicating that three glacial periods have occurred in the Nyainqêntanglha Mountains since the middle Pleistocene.Study of a fault zone at the southeastern piedmont of the Nyainqêntanglha Mountains indicates that this fault zone consists of three secondary fault zones,which have the features continuous migration toward the interior of the basin and repeated activities.The several important faultings since the middle Pleistocene occurred about 700~500,350~220,140 or so and 70~50 ka.The heights of fault scarps which have offset the sediments formed since the middle Pleistocene decrease with younging stratigraphic ages,suggesting that the rates of vertical movement of the fault change between 0.4 and 2 mm/a,of which the average rate of long-term vertical movement during the Quaternary is 1.1±0.3 mm/a,while that during the Holocene is 1.4±0.6 mm/a.