Late Quaternary Activity of the Northern Section of the Bayanhaote Fault and Its Implications for Regional Seismotectonics
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摘要: 巴彦浩特断裂位于阿拉善地块与鄂尔多斯地块相互作用的边界构造带上，其晚第四纪活动特征和古地震数据对全面理解贺兰山周边区域地震构造和地震危险性具有重要意义，为此在研究相对薄弱的巴彦浩特断裂北段开展了断错地貌和古地震槽探研究。观测显示巴彦浩特断裂阿拉善左旗以北段以右旋走滑活动为主兼具逆倾滑，断层西盘相对抬升，在浅表形成半正花状构造组合。年代（56.28±4.04）~（82.2±5.78）ka的冲洪积地貌面上冲沟断错137 m，并在东侧形成断塞塘地貌，估计断层右旋走滑速率为1.67~2.43 mm/a。探槽揭示了3次具有显著地表逆倾滑破裂的强震事件，时间分别为（56.28±4.04）~（55.33±3.04）、（32.79±2.22）~（13.76±1.1）、（13.76±1.1）~（7.86±0.43）ka，逆倾滑量分别为0.44、0.35、0.29 m。与前人在巴彦浩特断裂南段的古地震研究进行对比，可知这3次古地震可能仅为部分事件记录。结合已有研究成果建立了贺兰山周边区域地震构造模型，贺兰山西侧右旋走滑的巴彦浩特断裂强震发震能力不容忽视，贺兰山两侧盆地不同性质断裂系共同构成了阿拉善地块与鄂尔多斯地块的活动边界构造带。Abstract: As a part of the most frontier of the propagating Tibetan Plateau and the boundary between the Alxa block and the Ordos block, it is of great significance to analyze the late Quaternary activity and paleo-seismicity of the Bayan Haote fault and set a regional seismotectonic model for the Helanshan area. We conducted a study on the offset landforms and paleo-earthquakes along the northern section of the Bayan Haote fault. It shows that the fault deformed dextrally with slight reverse slip to the east. A channel on the alluvial terrace, aged 56.28±4.04 ka–82.2±5.78 ka, was right-laterally offset for 137 m. A sag pond was formed in the east of the fault scarp. We calculated a dextral slip rate of 1.67–2.43 mm/a. Three surface-rupturing paleo-earthquakes were revealed from reversely offset strata in a trench to the south of Sumutu village, which were happened during 56.28±4.04 ka–55.33±3.04 ka, 32.79±2.22 ka–13.76±1.1 ka, and 13.76±1.1 ka–7.86±0.43 ka and had reverse slips of 0.44 m, 0.35 m, and 0.29 m, respectively. By comparing with published paleo-earthquake data along the southern section of the Bayan Haote fault, we infer that these three earthquakes might be a portion of surface-rupturing earthquakes. We set up a regional seismotectonic model around the Helanshan area with our new data and published geological and geophysical data. The seismic risk along the dextral Bayan Haote fault in west of the Helan Mountains is also significant. The dextral Hayan Haote fault in the west of Helan Mountains and the normal fault system in the east constitute the active boundary belt between the Alxa block and the Ordos block.
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Key words:
- Bayanhaote fault /
- Active fault /
- Late quaternary /
- Regional seismotectonics /
- Ordos block /
- Alxa block
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图 4 苏木图南探槽北壁
Figure 4. The north wall of the trench at the site to the south of the Sumutu village
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