青海热水-日月山断裂带的新活动特征

CHARACTERISTICS OF THE MODERN ACTIVITY OF THE RESHUI-RIYUESHAN FAULT ZONE IN QINGHAI PROVINCE

热水 -日月山断裂带是发育在青藏高原东北缘柴达木 -祁连活动地块内部的 1条重要的NNW向逆 -右旋走滑活动断裂带 ,长约 183km。断裂由 4条不连续的次级断裂段右阶羽列而成 ,阶距 2～ 3km左右 ,在不连续部位形成拉分区。主断裂两端则形成帚状分叉。断裂活动形成了一系列山脊、冲沟和阶地等右旋断错微地貌 ,其中Ⅰ级阶地右旋断错约 8～ 11m ,Ⅱ级阶地右旋断错约 35m。同时沿断裂带还形成了许多断层陡坎 ,Ⅰ级阶地或洪积台地上断层陡坎高约 0 .5～ 1m ,最高达 2 .8m ,Ⅱ级阶地或台地上的断层陡坎高约 2 .5～ 3m ,最高达 4～ 5m。根据相应的阶地年代 ,计算得到断裂带全新世以来的平均水平滑动速率为 3 16mm/a ,垂直滑动速率为 0 .83mm/a。

The Reshui-Riyueshan active fault zone lies within the central part of the Qaidam-Qilianshan active block on the northeastern margin of the Qinghai-Tibet Plateau. It is a NNW-trending right-lateral strike-slip active fault zone with reverse components characterized by clear linear features and strong activities. It initiates to the north of Datong River, and extends southward through the Reshui coal mine, running along the eastern side of the NNW-oriented Datongshan and Riyueshan upheaval area, and obliquely connected with Lajishan active fault zone at the pass of Riyueshan Mountain. The fault zone is generally striking N35°W, having a total length of about 183km and a relatively simple geometrical structure. It consists of 4 discontinuous secondary en echelon faults, which are Datong River (F 1-1), Reshui (F 1-2), Haiyan (F 1-3) and Riyueshan (F 1-4) Faults, respectively. Extensional zones or pull-apart small basins, such as Ketu basin, were formed at the step-over of these secondary faults. The wide of the step-over is about 2～3km. The Reshui-Riyueshan active fault zone was a compressive reverse fault zone in the early period, and caused the strong compressive deformation of late Cenozoic strata on both side of the fault. Since late Quaternary, the fault zone has become a right lateral strike-slip reverse fault, which has given rise to the formation of micro-morphology represented by right-laterally offset ridges, valleys and terraces. The larger offset may reach up to hundreds meters or more, while the smaller offsets are just about several meters. The right-lateral offset of the first level terrace is about 8～11m, while that of the second level terrace is about 35m. At the same time, there are many fault cliffs and fault scarps developed along the fault zone. The height of the fault scarp at the first level terrace is about 0.5～1m, and the highest one is about 2.8m; at the second level terrace and pluvial mesa, the height of the scarp is about 2.5～3m,with the highest value of about 4～5m. According to the 14C age of the first level terrace of about 3 000a, the horizontal slip rate along the Reshui-Riyueshan Fault zone since late Holocene is estimated to be about 3.16 mm/a, and the vertical slip rate is about 0.83 mm/a. According to the TL age of the second level terrace of about 23.8±1.2ka, the horizontal slip rate along the fault zone since late Pleistocene is estimated to be about 1.47mm/a, and the vertical slip rate is about 0.10～0.21mm/a. Comparatively speaking, the higher the terrace, the stronger the erosion in later period, and hence the larger the uncertainty of the measured offsets. On the contrary, the lower the terrace, the smaller the erosion, and hence the smaller the uncertainty of the measured offset. It is believed, therefore, that the Holocene horizontal slip rate of about 3.16 mm/a, and vertical slip rate of about 0 83 mm/a for the fault zone calculated from the first level terrace are more reliable.