北天山前陆盆地前缘西湖背斜带第四纪褶皱作用

QUATERNARY FOLDING OF THE XIHU ANTICLINE BELT ALONG FORELAND BASIN OF NORTH TIANSHAN

西湖背斜带属于北天山前陆盆地最前缘的第四排活动褶皱-逆断裂带,前人对西湖背斜带的第四纪活动尚无系统研究。文中对5条横穿背斜的地震反射剖面进行解译,结合地表地质地貌的发育特征,对褶皱类型、生长机制、褶皱几何学、运动学特征和变形量等进行了研究。西湖背斜的EW向变形长度>47km,其中隐伏区的变形长度>14km,地表出露区的最大变形宽度约为8.5km。西湖背斜内部连续发育一系列次级断层,在其北部发育一个变形幅度较小的背斜(西湖北背斜)。西湖背斜是以翼旋转方式变形的滑脱褶皱,其缩短量在地形高点附近约为(1070±70)m,向W递减至(130±30)m,向E快速递减至(650±70)m;因外部物质涌入或内部物质流出导致其盈余面积为-0.34~0.56km^2。通过横穿西湖北背斜的2条地震反射剖面(剖面A、B)计算其平均缩短量为(60±10)m、(130±40)m,由外部物质流入导致的盈余面积为0.5km^2、0.74km^2。地震反射剖面在背斜两翼均发育生长地层,生长地层的起始位置在背斜地表出露区的东段位于地下1.9~2.0km处,在背斜西端隐伏区位于地下3.7km处。地震反射剖面成像结果(剖面D、E)显示,在地下3.3km处可见上覆地层超覆在西湖背斜之上,而上覆地层基本未发生变形,表明该段背斜已不再活动。

Tianshan is one of the longest and most active intracontinental orogenic belts in the world.Due to the collision between Indian and Eurasian plates since Cenozoic,the Tianshan has been suffering from intense compression,shortening and uplifting.With the continuous extension of deformation to the foreland direction,a series of active reverse fault fold belts have been formed.The Xihu anticline is the fourth row of active fold reverse fault zone on the leading edge of the north Tianshan foreland basin.For the north Tianshan Mountains,predecessors have carried out a lot of research on the activity of the second and third rows of the active fold-reverse faults,and achieved fruitful results.But there is no systematic study on the Quaternary activities of the Xihu anticline zone.How is the structural belt distributed in space?What are the geometric and kinematic characteristics?What are the fold types and growth mechanism?How does the deformation amount and characteristics of anticline change?In view of these problems,we chose Xihu anticline as the research object.Through the analysis of surface geology,topography and geomorphology and the interpretation of seismic reflection profile across the anticline,we studied the geometry,kinematic characteristics,fold type and growth mechanism of the structural belt,and calculated the shortening,uplift and interlayer strain of the anticline by area depth strain analysis.In this paper,by interpreting the five seismic reflection profiles across the anticline belt,and combining the characteristics of surface geology and geomorphology,we studied the types,growth mechanism,geometry and kinematics characteristics,and deformation amount of the fold.The deformation length of Xihu anticline is more than 47km from west to east,in which the hidden length is more than 14km.The maximum deformation width of the exposed area is 8.5km.The Xihu anticline is characterized by small surface deformation,simple structural style and symmetrical occurrence.The interpretation of seismic reflection profile shows that the deep structural style of the anticline is relatively complex.In addition to the continuous development of a series of secondary faults in the interior of Xihu anticline,an anticline with small deformation amplitude(Xihubei anticline)is continuously developed in the north of Xihu anticline.The terrain high point of Xihu anticline is located about 12km west of Kuitun River.The deformation amplitude decreases rapidly to the east and decreases slowly to the west,which is consistent with the interpretation results of seismic reflection profile and the calculation results of shortening.The Xihu anticline is a detachment fold with the growth type of limb rotation.The deformation of Xihu anticline is calculated by area depth strain analysis method.The shortening of five seismic reflection sections A,B,C,D and E is(650±70)m,(1070±70)m,(780±50)m,(200±40)m and(130±30)m,respectively.The shortening amount is the largest near the seismic reflection profile B of the anticline,and decreases gradually along the strike to the east and west ends of the anticline,with a more rapidly decrease to the east,which indicates that the topographic high point is also a structural high point.The excess area caused by the inflow of external material or outflow of internal matter is between-0.34km^2 to 0.56km^2.The average shortening of the Xihubei anticline is between(60±10)m and(130±40)m,and the excess area caused by the inflow of external material is between 0.50km^2 and 0.74km^2.The initial locations of the growth strata at the east part is about 1.9~2.0km underground,and the initial location of the growth strata at the west part is about 3.7km underground.We can see the strata overlying the Xihu anticline at 3.3km under ground,the strata above are basically not deformed,indicating that this section of the anticline is no longer active.