西秦岭北缘断裂带黄香沟段晚第四纪活动表现与滑动速率

LATE-QUATERNARY ACTIVITY AND SLIP RATE OF THE WESTERN QINLING FAULT ZONE AT HUANGXIANGGOU

文章对西秦岭北缘断裂带黄香沟段的沉积建造、构造地貌等开展了野外调查与研究。冲沟位错、断裂剖面、山脊位错、线性槽地、洪积扇变形与断塞塘等详细的构造地貌分析,显示断裂带在黄香沟段晚第四纪具有较强的活动性,以左旋走滑兼有倾滑分量为特征。根据其中两条晚更新世末期以来的冲沟左旋水平位错平均值30.5±3.0m,及其14C限定的冲沟发生位错的年龄为13480±240aB.P.,获得断裂晚第四纪的水平滑动速率为2.3±0.2mm/a。同时,由断裂位错洪积扇形成的1.05±0.30m垂直位移量,与该期洪积扇形成的14C年龄3690±100aB.P.,估算了断裂晚第四纪的垂直滑动速率为0.28±0.08mm/a。

The western Qinling Fault is one of the main left-lateral strike-slip fault zones in northeastern Tibet. The Late- Quaternary behavior of the fault is typically shown at Huangxianggou. Detailed geomorphologic analysis on offset gullies, faulting profiles, offset ridges, linear trough, deformed alluvial fans and sag-ponding, etc. suggests that the fault zone had been intensely active during Late Quaternary at Huangxianggou, characterized by dominant sinistral movement, and partly dip-slip component in Late Pleistocene. Some microgeomorphological features such as periodic sag-ponding deposition and deforming of alluvial fans are major features in the Huangxianggou segment. The sag-ponding deposits disclosed by 3-D excavation show extreme relevance with the faulting. The Ⅰ and Ⅱ fans around the fault were displaced and display 1 -2m high scarps. Displacement of water system is a widespread phenomenon along the fault zone. The young gullies across the fault zone at Huangxianggou are usually sinistrally offset 20 -30m. Gullies A-B are two late Late-Pleistocene streams crossing the fault and they are averagely offset 30.5 ± 3.0m. The radiocarbon age of the earliest recent deposits near gullies A-B is 13480 ±240aB. P. As the incision of these two streams started around that time, this age can thus constrain the date of offset of the streams, then the horizontal slip rate of the fault is determined to be 2.3 ± 0.2mm/a. The fault also displaced the alluvial fans developed at the valley mouth of a gully east to Luanchaigou. The vertical offset of the third alluvial fan is 1.05 ± 0.30m measured by the total station. The formation of the third alluvial fan was almost coeval with the abandonment of the second alluvial fan according to a field observation on the profile. The sample collected from the top of the second fluvial fan was dated by radiocarbon method to be 3690 ±100aB. P. It represents the age of the formation of the third alluvial fan. Based on these, we estimated the late Holocene vertical slip rate of the Huangxianggou segment to be 0.28 ± 0.08mm/a. Although it shows a similar activity in many aspects with another main fault in northeastern Qinghai-Tibet Plateau, our study suggests that, the western Qinling Fault has a lower slip rate than the Haiyuan Fault.