LATE QUATERNARY DEFORMATION RATE OF THE WENSU FORELAND THRUST BELT IN THE SOUTHERN TIANSHAN MOUNTAINS

doi:10.3969/j.issn.0253-4967.2024.06.004

Abstract

Abstract:

In response to the ongoing India-Eurasia collision in the late Cenozoic, the Tianshan orogenic belt was reactivated and experienced rapid uplift. Strong uplifted topography results in that the mountains propagate from the range front toward the foreland basin to form several fan-shaped foreland thrust belts both on its north and south sides. These foreland thrust belts accommodate the most north-south convergence strain and control the regional deformation pattern. However, in contrast to the well-studied foreland thrust belts, the kinematics and deformation rate of the transition area between different foreland thrust belts have not been well-documented. Therefore, it is still unclear how the crustal shortening in the foreland basins changes along the east-west direction. Further, the deformation characteristics and seismic hazard in this region are poorly understood because quantitative information on active deformation is lacking.

The Wensu foreland thrust belt is located in the Kalpin and Kuqa foreland thrust systems' transition areas. In contrast to the Kuqa and Kalpin foreland thrust belts at its east and west sides, the Wensu foreland thrust belt propagated approximately 20km toward the basin and only developed one row of active thrust fault-anticline belts, namely the North Wensu thrust fault-anticline belt. The North Wensu structural belt shows clear evidence of tectonic solid activity because the late Quaternary sediments and river terraces have been faulted. However, this structural belt's kinematics and late Quaternary deformation rate remain poorly constrained. This study quantifies its deformation mode based on field geological mapping across the anticline. Our results indicate that the North Wensu structural belt is a fault-bending fold. Based on interpretations of detailed high-resolution remote sensing images and field investigations, five levels of river terraces can be identified along the Kekeya River valley. By surveying of the displaced terraces with an unmanned drone, the crustal shortening values of ~20.7m、 ~35.3m and ~46.9m are determined for the T3, T4 and T5 terraces, respectively. Our optically stimulated luminescence(OSL)dating yields a depositional age of(9.02±0.55)ka for the T3 terrace, (24.23±1.58)ka for the T4 terrace, and(40.43±3.07)ka for the T5 terrace. Thus, we estimate a crustal shortening of ~1.31mm/a in the late Quaternary(since approximately 40ka), and approximately 2.29mm/a in the Holocene for the North Wensu structural belt. Our results indicate that the deformation rate of the North Wensu structural belt exhibits an obvious increase in the Holocene. This phenomenon indicates that the strong earthquake activity on the North Wensu thrust belt has increased significantly in the Holocene, implying an irregular activity habit of the strong earthquake recurrence cycle on this tectonic belt. The propagation deformation toward the basin of the Wensu foreland thrust belt is very limited. Therefore, we suggest that the foreland thrust belt is a thick-skinned nappe structure and is dominated by high-angle thrust faulting. The tectonic deformation in the Wensu region seems to be characterized by considered vertical growth. Although the deformation rate is small, the uplift amplitude is significant in this region.

Key words: Southwestern Tianshan, Wensu foreland thrust belt, Late Quaternary, River terrace, Deformation rate

摘要：

受印度-欧亚板块碰撞远程效应的影响, 天山南北发育了多个扇形的前陆冲断带, 它们不仅调节吸收了大部分的SN向会聚应变, 也控制着山体向盆地方向的逆冲推覆。然而, 对于相邻前陆冲断带之间过渡、转换部位的构造特征和变形速率研究较少, 目前对于天山山前地壳缩短沿EW向是如何变化的仍不清楚。温宿前陆冲断带位于柯坪和库车前陆冲断带的过渡转换区, 该前陆冲断带仅发育一排活动逆断裂-背斜带, 即温宿北逆断裂-背斜带。长期以来, 不同学者给出的该构造带的地壳缩短速率结果存在较大差异, 严重影响了对该地区构造变形和强震危险性的认知。文中以温宿北逆断裂-背斜为研究对象, 通过野外地质地貌调查和地质填图, 获得了该构造带的变形特征; 同时, 对背斜中部柯柯亚河谷的河流阶地进行了详细解译与划分, 并利用无人机摄影测量获取的高精度地形数据提取了各级阶地的位错量, 最后根据光释光年代测试结果, 计算得到温宿北逆断裂-背斜带晚第四纪(距今约4万年)以来的SN向地壳缩短速率约为1.31mm/a, 全新世以来的缩短速率约为2.29mm/a。温宿前陆冲断带的缩短速率在全新世以来明显增大, 反映了该构造带强震复发周期并不规律的活动习性。

关键词: 南天山, 温宿前陆冲断带, 晚第四纪, 河流阶地, 形变速率

ZANG Ke-zhi, WU Chuan-yong, ZHANG Jin-shuo, GAO Zhan, YUAN Si-hua, YUAN Hai-yang, YU Xiao-hui, WANG Xue-zhu. LATE QUATERNARY DEFORMATION RATE OF THE WENSU FORELAND THRUST BELT IN THE SOUTHERN TIANSHAN MOUNTAINS[J]. SEISMOLOGY AND GEOLOGY, 2024, 46(6): 1280-1294.

臧柯智, 吴传勇, 张金烁, 高瞻, 袁四化, 袁海洋, 于晓辉, 王雪竹. 南天山温宿前陆冲断带晚第四纪形变速率[J]. 地震地质, 2024, 46(6): 1280-1294.

Figures/Tables 7

Fig. 1 Tectonic map of the western Tianshan region and topographic profile across the Tomur Peak and the North Wensu thrust fault and anticline.

Fig. 2 Geological map of the North Wensu thrust fault-anticline zone and geological profile of the Kekeya River Valley.

Fig. 3 Geomorphological interpretation and longitudinal profile of the Kekeya Valley.

Fig. 4 Interpretation map of faulted landforms and photographs of different levels of terrace sections.

Table1 Optically Stimulated Luminescence(OSL) dating results

阶地序列	样品编号	U /ppm	Th /ppm	K /%	含水量 /%	剂量率 /Gy·ka^-1	等效剂量 /Gy	年龄 /ka
T3	KKY-OSL-01 KKY-OSL-02	5.17±0.08 2.21±0.07	41.9±1.22 18.2±0.49	2.72±0.01 2.27±0.01	1.18 1.21	7.0±0.3 4.24±0.18	49.21±4.05 17.26±1.32	10.6±0.77 9.02±0.55
T4	KKY-OSL-04	1.76±0.02	13.7±0.02	2.64±0.01	1.28	4.21±0.18	69.47±5.12	24.23±1.58
T5	KKY-OSL-05	2.79±0.04	11.7±0.18	2.38±0.02	3.74	3.97±0.16	92.54±7.79	40.43±3.07

Fig. 5 Diagram of the relationship between the surface deformation and underground fault geometry (Fig. 5b model adapted from Yang et al., 2023).

Table2 Crustal shortening amount and crustal shortening rate of the North Wensu Reverse Fault and Anticline during the Late Quaternary

地貌面	褶皱垂直抬升量 H/m	地壳缩短量 S/m	阶地年龄 /ka	隆升速率 /mm·a^-1	地壳缩短速率 /mm·a^-1
T5	30.2±3.1	46.9±4.8	40.43±3.07	0.75±0.13	1.16±0.21
T4	22.7±2.4	35.3±3.7	24.23±1.58	0.94±0.16	1.46±0.25
T3	13.3±1.7	20.7±2.6	9.02±0.55	1.47±0.27	2.29±0.42

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