依据地热井资料探讨北京八宝山断裂北段的基本特点

Characteristics of northern section of Beijing Babaoshan fault based on the stratigraphic data of geothermal drillings

根据地热井的地层资料,绘制了横穿八宝山断裂带北段的地质剖面图,对八宝山断裂带的深部构造特点进行了探讨,并对几个地热钻孔的地层进行了横向对比。研究结果显示:剖面中存在众多倾角不同、规模不一且相互错断的逆断层,反映出八宝山断裂受逆冲作用非常强烈,同时具有多期性;剖面中存在两条较大的断层,一条是八宝山断裂带的主断层,另一条是雾迷山组小断块被顶托至上部后所形成的次断层,在八宝山一带,次断层的上盘被推覆至主断层线之上将其掩盖,地表仅显示一条断层,即平常所说的八宝山断层,由此可见这并非八宝山断裂带的主断层,在研究及监测中应注意加以区分。此外,根据钻孔地层厚度资料估算,本段主断层倾角约为40°,最大切割深度约为5 000 m。根据地层切割与沉积关系,认为八宝山断裂带最终形成于中生代晚期。

Basing on the stratigraphic data of geothermal drillings,a geological profile is plotted across the north segment of Babaoshan fault zone,and deep structure is discussed. According to the lateral stratigraphic alignment,it is found that there are many reverse-faults in different scale and inclination in the section,and these faults are dislocated mutually,which reflects that the thrusting effect of the Babaoshan fault is very strong and is of multiphase. Meanwhile,there are two larger faults in the section. One is the main fault of the Babaoshan fault zone,and the other is the minor fault resulted from the blocking of Wumishan formation put upwards by the main reverse fault. Because the main fault is covered by hanging wall of the minor fault for the thrusting of the minor fault is so strong,therefore in the Babaoshan area,there only one fault line appears on the surface,and the fault we normally refer to as the Babaoshan fault. We can see it is not the main fault line of the fault zone,which should be carefully examined in the research and monitoring of the fault zone. According to the rough estimation basing on the stratigraphic data of geothermal boreholes,it is deduced that the dip angle of the main fault is about 40 degrees,and the maximum cutting depth is about 5 000 m. In additional, the relation between stratigraphic cutting and deposition reveals that the Babaoshan fault was formed approximately in the Late Mesozoic era.