京西北蔚县-广灵半地堑盆地南缘断裂带的断层生长研究

THE GROWTH OF THE SOUTH MARGIN FAULT OF THE YUXIAN-GUANGLING BASIN IN NORTHWEST BEIJING AREA

通过高分辨率遥感影像解译、高分辨率DEM三维分析、野外地质地貌调查以及探槽开挖等方法,对蔚县-广灵盆地南缘断裂带上多处断层生长发育的现象进行了分析和研究。结果发现,断层的生长发育主要发生在几何结构不规则的区段,亦即断层的生长是由其几何结构不规则性决定的。这种不规则性主要包括几何形态的不平滑性或不连续性。断层的生长发育具有一种"截弯取直"的趋势和效应,消除几何结构的不规则性,使断层变得更加平滑和连续,结果是降低了滑动面的粗糙程度,更加有利于盆地内半地堑地块沿滑动面进行下滑运动。此外,断层的生长发育受不规则形态的空间尺度的影响,"截弯取直"的程度随着不规则形态空间尺度增大而降低。

Yuxian-Guangling Basin is a half-graben basin unit belonging to the basin-ridge structure zone in northwest Beijing area.The southern boundary of this basin is controlled by a normal fault belt called the Yuguang Basin South Margin Fault（YBSMF）.The YBSMF is about 120km long,with a general strike of N70°E,and is an active fault zone.The YBSMF was evolved from the propagation,interaction or linkage of existing isolated segments and the forming of new fault segments,and there are actually many segments and places along the YBSMF where the faults propagate and grow.However,except the study on the fault growth at the Jiugongkou segment by Cheng Shaoping in 1998,which indicated that the fault has propagated several kilometers westwardly in the late Late Pleistocene alluvial fans,the research about the propagation and growth of the faults at other places and segments is quite limited.At these segments and places,in what ways or patterns does the fault propagate,grow,link and evolve？What on earth controls and affects the propagation and growth of the faults？All these questions still remain unanswered yet and deserve further analysis and study.Based on high-resolution remote sensing image interpretation,DEM 3D analysis,field geological investigation,trenching and so on,we made a research on the fault growth of the YBSMF.According to the fault geometry,fault activity and the difference of the faulted landforms,the YBSMF belt can be divided into five segments：Shangbaiyang segment,Tangshankou segment,Beikou segment,Songzhikou segment and Shanghupen segment.The faults grow and evolve both between adjacent segments and within each segment.Besides,some new faults also form in the proluvial fans in front of mountains.After a detailed comparison and analysis of all the sites of fault growth along the YBSMF,we find out several characteristics and rules about the growth of the fault.First,the faults often grow or evolve where the fault geometry is irregular,and the irregularity of fault geometry is a primary factor which determines whether the faults propagate and grow or not.The irregular segments where the faults propagate and grow can be divided into two categories.The first type mainly includes the uneven or unsmooth segments,such as the segments with convex or concave arcs,edges or corners,and so on;the second type mainly consists of two nearly parallel faults with a gap between them,which causes the discontinuity of the fault geometry along the strike.Second,fault growth leads to the ＂cut off＂ and elimination of the irregularity of fault geometry,such as cutting off the uneven or unsmooth segments,and linking the discontinuous segments along the strike.The elimination of the irregularity makes the fault geometry smooth and continuous,and reduces the roughness on the sliding surface,which contributes to the downward slip of the half-graben block inside the basin along the sliding surface.Third,the degree of ＂cut off＂ or elimination may be affected by the spatial scale of the irregular shape.As the scale of the irregularity increases,the fault will propagate a larger distance to overcome the hindrance of the roughness,so it will take more time for the irregular segments to be completely ＂cut off＂ or eliminated,and vice versa.Therefore,after the same period of time,the irregularity with a small scale has been completely ＂cut off＂ or eliminated,while the irregularity with a large scale may be still in the process of segment linkage or cutting off,so the degree of ＂cutting off＂ or elimination is lagging behind and relatively lower.