武汉阳逻王母山断层及地震楔构造研究

Research on Wangmushan Fault and Paleoseismic Wedges in Wuhan

本文研究了武汉阳逻王母山一带新发现的 NWW 走向的张性断层和该区集中分布的古地震楔构造。通过对王母山一带第四系剖面的详细测量，识别出阳逻砾石层（Q1 y）底部的“标志性砾石层”具有结构成熟度低、风化程度高、短距离内厚度稳定和具有弱网纹化的鲜明特征，其夹持于下伏第三系东湖群（E）红色砂岩和上覆阳逻砾石层（Q1 y）之间，层位稳定。经槽探揭露和剖面测量，以“标志层”对比为依据，对王母山断层的特征和性质进行了研究，指出王母山断层是由主干正断层（f1）及其上盘派生两个次级正断层（f2、f3）构成的的张性正断层，主干正断层（f1）走向308°，倾向NE，倾角40°～52°，断距6～8m，同步断错第三系东湖群（E）红色砂岩、下更新统阳逻砾石层（Q1 y）和中更新统王家店组泥砾层（Q2w）。阳逻王母山一带集中分布的古地震楔构造平面可追踪长度3～20m不等，普遍错断阳逻砾石层（Q1 y），向下收敛并尖灭于阳逻砾石层底部，仅发现2处古地震楔尖灭于东湖群（E）红色砂岩顶部并呈风化成白色粘土的节理裂隙。地质分析认为，王母山断层及其近区集中分布的古地震楔构造是在同一构造应力场中发育的，通过对古地震楔和王母山主干断层 f1的应力分析发现，该区主压应力方向为NW335°左右，而主张应力方向为NE65°左右，在此背景下形成的古地震楔的走向主要为NW328°和170°～190°方向，而主干断裂走向则为300°左右。研究认为王母山断层错断下更新统阳逻砾石层及其上覆的中更新统王家店组（Q2 w）泥砾层，时限为中更新世以新，为第四纪活动断层。武汉地区周边为一系列大断裂所环绕，尤其是N W向的襄（樊）-广（济）断裂从阳逻一带穿过，襄广断裂一线历史上曾遭遇过数十次地震的袭击，为湖北省历史中强震活动区之一。而阳逻龙口王母山断层及区内广泛分布的古地震楔构造则可能是同期古地震的反映。阳逻王母山断层及古地震楔研究为武汉地区地震地质构造的研究提供了新的依据。

The NWW tensile fault together with the concentrated distributed paleosesmic wedges which were discovered in Wangmushan of Yangluo area were studied in this paper.Via detailed measurement of Quaternary profiles in this area,the bottom key-bed of Yangluo formation（Q1y）was identified,with such distinct characteristics as low structure maturity,high weathering degree,stable thickness at the short distances and weak reticulate pattern,stably distributed between the underlying tertiary red sandstone of Donghu formation（E）and the upper Yangluo formation（Q1y）.On the base of key-bed contrasting,along with the profile exposing and trenching work,the characteristics of Wangmushan fault was studied.It was proved that the Wangmushan fault is a extensional normal fault,which is composed of the main normal fault （f1）and two secondary normal faults （f2,f3）derived by the hanging wall.The main normal fault（f1） strike 308°,the dip NE∠40°-52°,spacing 6-8m,synchronously dislocated tertiary Donghe group（E）, Yangluo formation （Q1y）,and boulder clay of middle Pleistocene Wangjiadian formation （Q2w）.The traceable length of paleosesmic wedges concentrated distributed in Wangmushan area ranges from 3 to 20m,usually dislocated Yangluo formation,converging downward and pinched out on its bottom.Only respective paleosesmic wedges was found pinched out on the top of tertiary Donghu formation （E）,and the joint fissures emerged were weathered into pryan.It is regarded that Wangmushan faults and the paleosesmic wedges concentrated distributed in nearby were developed in the same tectonic stress field.Via the stress analysis of paleoseismic wedges and Wangmushan fault,it was found that the orientation of the principal compressive stress in this area is about NW335°,and the orientation of the principal extensional stress is about NE65°.On this background,the strike of paleoseismic wedges are mainly NW328°and 170°-190°,and the strike of main faults is about 300°.It reveals that Wangmushan fault is Quaternary active fault whichi cut through the gravel layer of Lower Pleistocene Yangluo formation（Q1y）and the Middle Pleistocene clay gravel layer of Wangj iadian formation（Q2w）,and the time limit is newer than the Middle Pleistocene.Wuhan area is surrounded by a series of major fault,especially the NW Xiangfan-Guangji fault which passing through Yangluo area.It is one of the seismically active zones of moderate earthquakes in Hubei history along Xiangfan-Guangji fault,it was believed that it also the active zone of paleo-earthquakes .The Wangmushan fault in Yangluo area and the paleosesmic wedges widely distributed in this area are possibly the reflection of paleo-earthquake in the corresponding period.Research on Wangmushan faults and paleoseismic wedges provided the new basis for the research of seismic geological structure in Wuhan area.