Abstract:

Weihe Basin, which is wide in the east and narrow in the west, deep in the south and shallow in the north, is one of the typical Cenozoic grabens in Asia continent, connecting the Ordos block in the north, Qinling fold belt in the south, adjacent to the arcuate fault belt in the northeast margin of Tibet Plateau in the west and the Shanxi rift zone in the east. The Weihe Basin has experienced strong faulting and sedimentation since early Cenozoic, with many buried active faults developed. The nearly E-W-trending Taochuan-Huxian Fault is one of these faults. The middle-deep depth seismic profiling shows that the buried segment of Taochuan-Huxian Fault in Weihe Basin is located between the Qinling north margin fault and the Weihe Fault and it is a fundamental fault that cuts through the Palaeozoic stratum and divides the Xi'an depression into two parts. To explore and know the location and structural characteristics of the Taochuan-Huxian fault segment hidden in the Weihe Basin and its activity in the Late Quaternary is of important significance for the researches of seismo-tectonic structure and seismic hazard of strong earthquakes in the study region. For this purpose, we deployed 7 profiles for shallow seismic reflection surveys, relied on the “Xingping Active Fault Project”. Based on these surveys, we determined the existence and hidden positions of the Taochuan-Huxian Fault and its branches in the Weihe Basin by combining with the data from some existing seismic reflection profiles of shallow-depths and middle-deep depths. Our research suggests that the Taochuan-Huxian Fault(F₈)is connected to the southern margin fault of the Taibai Basin in the west, and eastward, passes through the northern margin of the Qinling Mountains and enters into the Weihe Basin at the town of Tangyu, Zhouzhi County, and then is concealed under the loose sediment in the Weihe Basin. The strike direction of this fault is northeast when crossing obliquely through the town of Zhouzhi County, then gradually turns to a nearly east-west direction between Zhouzhi and Huxian, showing a northward convex bend in the fault trace buried in the basin. Further eastward, the Taochuan-Huxian Fault(F₈)connects to the Tieluzi Fault near the town of Yinzhen, Huxian County. In addition, a buried antithetic fault(DF₃)(also a secondary branch)of the buried Taochuan-Huxian Fault(F₈)is found between the north of Zhouzhi and the north of Huxian, and it extends roughly parallel to F8 under the loose sediment. This research also reveals that in the central portion of the Weihe Basin, the northern margin fault of the Qinling Mountains, the Weihe Fault and the Taochuan-Huxian Fault, together with their branch faults, constitute a large-scale fault zone with the tectonic feature of negative flower structure, as known from the interpreted cross-sections; among them, the F₈ and DF₃ faults and their secondary strands consist of a relatively small-scale negative flower structure. By combining with relevant information such as that from a composed cross-section using geological logs of multiple boreholes, and so on, we concluded that, within the study region of this research, the fault zone with the buried F₈ fault as its principal fault was active at least in the late Pleistocene, and hence is an active fault zone. Finally, the reason is discussed in this article for the faults, mentioned above, in the Weihe Basin that show the tectonic pattern of negative flower structure, instead of that of stair-stepping or ladder structure, and one possible interpretation is proposed that the dominant motion of these active faults are not normal faulting, but sinistral strike-slip faulting. Since the Cenozoic, the subduction of the Indian plate to the Eurasian plate caused the Tibet Plateau to be pushed out to the northeast and blocked by the Ordos block. Because of obstruction in the north, the material flows eastward along Qinling Mountains in the south, resulting in the extrusion shearing effect on the Weihe Basin in the middle. In addition, recent seismic and geological studies have discovered that many active faults in Weihe Basin and its edges are obviously of sinistral strike-slip, which also proves that the movement of these active faults in the basin is not dominated by normal faulting, but sinistral strike-slipping.

Key words: Taochuan-Huxian Fault, Weihe Basin, shallow seismic surveying, locations of buried faults, structure and distribution of fault zones

摘要：

渭河盆地新生代以来断裂活动和沉积作用强烈, 隐伏活动断裂发育, 近EW向的桃川-户县断裂是其中之一。探明桃川-户县断裂在渭河盆地中的展布位置、 结构特征以及晚第四纪活动性, 对于当地的地震构造及强震危险研究具有重要意义。为此, 文中依托“兴平活断层项目”布设了7条浅层地震测线, 基于探测结果并结合已有的浅层和中深层地震剖面确定了桃川-户县断裂及其次级断裂在渭河盆地内的存在与隐伏位置。结果显示, 桃川-户县断裂(F₈)西与太白盆地南缘断裂相连, 自周至县汤峪镇穿出秦岭北缘进入渭河盆地后隐伏于地表松散层之下; 先呈NE走向斜穿周至县城, 向E逐渐转向近EW走向, 在周至和户县之间呈现局部朝N凸出的弧形展布, 再向E至户县引镇附近与铁炉子断裂相接。另在周至北和户县北之间存在与F₈断裂大致平行展布的反向次级断裂(DF₃)。文中还揭示出在渭河盆地中段, 秦岭北缘断裂、 渭河断裂和桃川-户县断裂连同它们的分支断裂一起构成了1个大型的负花状结构断裂带; 其中F₈和DF₃断裂及其次级断裂组成1个次级的负花状构造带。结合相关的钻孔剖面等信息的分析表明, 在研究区内, 以F₈断裂为主断裂的构造带自晚更新世以来是活动的, 属于活动断裂。文中最后讨论了渭河盆地内的断裂带表现出负花状而非阶梯状结构的可能原因。

关键词: 桃川-户县断裂, 渭河盆地, 浅层地震探测, 隐伏断裂定位, 断裂带结构与展布