Abstract:

The fault on the northern margin of the Hami Basin is in the eastern segment of the Tianshan tectonic belt and is a deep-seated major fault that offsets the Moho discontinuity. The entire fault lies along the southern piedmont of the Barkol Mountains and the Harlik Mountains. In this study, the section of the fault along the southern piedmont of the Harlik Mountains is referred to as the eastern segment. Previous research on this fault has primarily focused on its western segment, where it has created distinct offset landforms on the surface and displaced Holocene strata, indicating activity during the Holocene. In contrast, the eastern segment of the fault is situated in the piedmont zone where the Harlik Mountains meet the Hami Basin. This area is characterized by a thin overburden, predominantly composed of coarse-grained colluvial deposits. These conditions make fault identification challenging and complicate studies of its activity. Previously, few scholars have conducted research on fault activity in this area, leading to divergent understandings regarding the precise location and activity of this fault segment. Therefore, it is necessary to employ new methods and technologies to carry out further investigation.

This study, integrated with engineering requirements, adopted a multi-technique integrated approach with mutual validation to conduct preliminary research on this fault segment. Detailed interpretation of remote sensing imagery from the Shangmiaoergou to Bamudun Reservoir area revealed that the fault has created several scarps on the surface. However, these scarps are only distributed on older geomorphic surfaces, making it uncertain whether the fault has displaced Late Quaternary landforms. Based on remote sensing interpretation and field geological surveys, microtremor surveys were carried out. The inversion results of the microtremor data reveal a significant low-velocity anomaly zone in the shear wave velocity at the location where the fault passes, exhibiting a certain width. This indicates that the fault traverses this area, and it was observed that the fault has a relatively steep dip at depth. The microtremor inversion results successfully revealed the deep structure of the fault and validated the understanding derived from remote sensing interpretation and field investigations. To address whether the fault extends to the surface and the timing of its most recent activity, two trenches were excavated east of the microtremor survey line, and aeolian loess samples were collected for geochronological analysis to study the fault's activity preliminarily. Trench profiles and geochronological results indicate that the fault has been active since the Late Pleistocene and exhibits characteristics of multiple episodes of activity.

Therefore, this study has obtained important evidence regarding the Late Quaternary activity of the eastern segment of the North Margin Fault in the Hami Basin, leading to the following conclusions: 1)Microtremor surveying offers advantages such as strong anti-interference capability, high efficiency, and minimal site constraints. In this study, the microtremor profiles provided the three-dimensional geometry and sectional characteristics of the fault at depth. This comprehensive multi-method approach, with mutual validation, can be highly effective for active fault detection in similar regions. 2)Geomorphological evidence for the fault's Late Quaternary activity includes the offset of the T3 terrace and alluvial fans formed during the Late Pleistocene. Fault movement has produced discontinuously distributed scarp landforms on the surface, with a total height ranging from 11 to 13m. Geochronological results also indicate that the fault has been active since the Holocene. 3)Microtremor profiles indicate a fault fracture zone width of 100m and a dip angle of 60°. Trenches and an adit were excavated on an alluvial fan, where microtremor surveys detected anomalies that exposed multiple fault planes. These fault planes generally dip northward with dip angles ranging from 35° to 54°; the dip angle is steeper at depth and becomes gentler near the surface. The phenomena revealed by the microtremor profiles are consistent with those observed in the trench and adit profiles. Furthermore, the width of the fault fracture zone measured at the adit entrance is 68m. This discrepancy arises because the microtremor-derived fracture zone includes not only the main boundary faults but also adjacent areas with reduced strength. Therefore, comprehensive analysis suggests that the width of the fault fracture zone is approximately 100m. 4)By sieving and testing loess particles within the colluvial deposits, the vertical slip rate since the Holocene is preliminarily estimated to be approximately 0.09mm/a. Integrated with regional geological data, the vertical slip rate since the mid-Late Pleistocene is inferred to be about 0.2~0.3mm/a.

Key words: eastern segment of the Northern Margin Fault of the Hami Basin, late Pleistocene activity, microtremor, east segment of eastern Tianshan

摘要：

哈密盆地北缘断裂是天山活动构造带东端的一条大型活动断裂。其东段位于哈尔里克山与哈密盆地之间的山麓地带, 该区域覆盖层较薄, 多为砾径粗大的崩坡积物; 需要采用新方法对该断裂的准确位置和活动性开展研究。文中在遥感解译和地质地貌调查的基础上, 采用了微动探测技术, 结合探槽和年代学测试等方法对该断裂的空间展布及其活动性开展了初步研究, 并形成了如下认识: 1)结合微动技术的多手段综合分析、 相互验证的方法在类似地区活断层探测中可以发挥很好的效果; 2)该断裂不仅具有断续分布的断层陡坎等晚第四纪活动的地貌学证据, 年代学结果也显示其在全新世有过活动; 3)微动探测剖面与开挖剖面揭露的现象显示断裂破碎带的宽度约为100m, 断层倾角为35°~60°, 基岩中倾角略大; 4)初步分析断裂在全新世以来的垂直滑动速率约为0.09mm/a, 晚更新世中期以来垂直滑动速率为0.2~0.3mm/a。

关键词: 哈密盆地北缘断裂东段, 晚第四纪活动性, 微动探测, 东天山东段