西秦岭西段光盖山-迭山断裂带坪定-化马断裂的新活动性与滑动速率

位于西秦岭西段的光盖山-迭山断裂带是由3条近平行的断裂所组成,其中坪定-化马断裂是该断裂带的主断层。坪定-化马断裂以宕昌岷江为界分为东、西两段,西段的新活动性明显强于东段。断裂西段线性特征明显,可见清晰的断层崖,不同期次的洪积扇上均有断层陡坎发育,晚第四纪以来有过明显的活动,最新一次活动的离逝时间约为1kaB.P.左右。通过对断错地貌研究得到断裂晚第四纪以来的垂直滑动速率为 0.49±0.08~1.15±0.28mm/a,左旋滑动速率为 0.51±0.13mm/a。而断裂东段由多条斜交或近于平行的断裂所组成,活动性明显减弱,没有发现最新活动的证据。在地貌上多表现为线性沟谷,向东逐渐被褶皱所替代,从基岩断面及相关地貌特征来看断裂活动性质主要表现为逆冲兼具左旋走滑运动。

LATE QUATERNARY ACTIVE CHARACTERISTICS AND SLIP-RATE OF PINGDING-HUAMA FAULT,THE EASTERN SEGMENT OF GUANGGAISHAN-DIESHAN FAULT ZONE(WEST QINLING MOUNTAIN)

Stretching along the West Qinling Mountain in the northeastern Tibet,the Guanggaishan-Dieshan fault composed of three sub-parallel faults,among which the major one is a fault named Pingding-Huama fault.The Pingding-Huama fault can be further defined as a combination of a western segment and an eastern segment separated by Minjiang river at Dangchang.Along the western segment of the Pingding-Huama fault,significant linear characteristics,scars,and fault scarps cutting several alluvial fans can be easily distinguished,indicating that the western segment is active since the Late Quatenary and the elapsed time of the last event should be less than 1kaB.P.We estimated the slip rates of the western segment through geomorphology analysis and dating the age of the top surface of the terraces and deformed strata(OSL). The results show that its reverse slip rate ranges from 0.49±0.08mm/a to 1.15±0.28mm/a and the sinistral slip rate is 0.51±0.13mm/a,indicating fairly low slip rate.In contrast to the simple structure of the western segment,the eastern segment consists of several sub-parallel faults as well as oblique intersected faults.On all faults of the eastern segment,no sign of recent movement was discovered.Along these faults,the tectonic topography characterized as a sequence of linear valleys in the west and dominant folds in the east.Only striations in bedrock and geomorphology show that the eastern segment was reverse dominated with minor sinistral component.In summary,at present the Pingding-Huama fault is active along its western segment while shows very weak deformation along the eastern segment.Combining previous studies support the interpretation that the slip along east Kunlun fault mainly transforms into West Qinling fault by Pingding-Huama fault and others at the region,and is absorbed by these faults and the deformation of basins among mountains.