关键词: 温泉地堑, 东西向伸展, 正断层, 古地震, 青藏高原

Abstract: The north-south-trending Wenquan graben is located to the north of Tanggula Mountains in central Qinghai-Tibet Plateau,having a length of about 40km and a width of 8～12km. The graben is filled with Quaternary moraine,fluvioglacial deposits and alluvia. An east-dipping boundary normal fault of about 45km length is developed along the western margin of the graben,where the relief is dramatically varied. The fault offsets vertically the bedrock,alluvial fan,river terraces and travertine platform. On the upthrown side of the fault,there are 4 prominent fault facets,which are ～600m,～400m,～200m,and ～80m above the surface of piedmont plain,respectively. At the base of fault facets,sustained faulting has given rise to the formation of east-facing fault scarps and surface ruptures that cut the terraces,alluvial fans and travertine platform. Leveling survey and in-situ measurement with tape measure show that the 6 sets of fault scarps are about 0 3～0.6m,4～5m,8±1m,15±3m,24±4m and 45±5m in height,respectively,and that the higher the scarp,the older the strata cut by the scarp. Among them,the 0.3～0.6m high fault scarp might be associated with M_S6～7 earthquake of middle-late Holocene,the 4～5m and 8±1m fault scarps offset separately the terraces Ⅰ and Ⅱ,while the 15±3m,24±4m and 45±5m high fault scarps offset separately the terrace Ⅲ,Ⅳ,Ⅴ and travertine platform. U-series dating results indicate that the fault scarp that offsets the terracesⅠand Ⅱ was formed 11～7ka BP,the scarp that offsets the terraces Ⅲ was formed 21～25ka BP,and the scarps that offset the terrace Ⅳ and Ⅴ and the correlated travertine platform were formed 45 2～53 6ka BP and 103～127ka BP,respectively. The 80～200m high scarps might be formed since 324～521ka BP. Moreover,the dating results suggest also that the average vertical slip rate along the normal fault on the western side of the Wenquan graben is 0.45mm/a with the maximum value of less than 1.2mm/a. The relief across the normal fault indicates that the minimum cumulative displacement on the fault is about 2.1km. The maximum cumulative throws on the fault is about 8.2km. The trenching across the fault revealed that at least 3 paleoseismic events have occurred along the fault since late Pleistocene. The magnitude of those events can be estimated to be M_S6～7 and the approximate recurrence interval to be from about 400 to 3000yr. The slip rate on normal fault from Wenquan graben implies that the east-west extension rate absorbed by a single graben in central Tibet is significantly lower than that of the grabens in southern Tibet,but this does not imply that the east-west extension rate in central Tibet must be significantly lower than that in southern Tibet. This is because that the east-west extension in central Tibet might be accommodated by a larger number of normal and strike-slip faults distributed in this area. On the basis of the total cumulative throws and the average vertical slip rate on the fault,it is inferred that the initiation time of the normal faulting to be at about 18 2～1 8Ma BP. The broad similarities in the magnitude of slip,the direction of extension and the initiation time of normal faulting in both central and southern Qinghai-Tibet Pateau imply that the east-west extension within the Qinghai-Tibet Plateau has occurred synchronously,and that the crustal thickness in central and southern Tibet Plateau has reached its maximum value since Miocene,while the whole Plateau has reached its present elevation.

Key words: Wenquan graben, east-west extension, normal faulting, paleoearthquake, Qinghai-Tibet Plateau