襄樊—广济断裂带东段构造变形特征及其演化

Deformation Characteristics and Evolution of the Eastern Xiangfan-Guangji Fault Zone

襄樊—广济断裂带是分隔大别造山带和扬子板块北缘前陆褶皱逆冲带的边界断裂,其几何学、运动学及构造演化特征记录了南北两大不同性质的大地构造单元发生碰撞、拼贴及相互作用的地质过程。在野外调查、构造解析和年代学研究基础上,结合区域地质和地球物理资料分析,认为襄樊—广济断裂带东段以深部向南逆冲、浅表向北逆冲的“鳄鱼嘴式”对冲构造为特征,与西段的构造变形样式和次序存在显著差异。中扬子地区东部受控于江南—雪峰造山带和大别造山带南北两大构造体系,深部扬子板块北缘向大别造山带之下俯冲导致造山带自北向南挤出,推覆构造可影响至瑞昌一带,由南向北的浅层逆冲推覆可影响至梅川附近,二者在襄樊—广济断裂带东段的蕲春—武穴—浠水一带对接。襄樊—广济断裂带经历了印支早期同碰撞由北向南的逆冲推覆和深层次的韧性剪切变形(T_(2)末)、燕山早—中期双向对冲构造变形(J_(1-3))、燕山晚期伸展正断层变形(K_(1-2))、喜山早期由北向南小规模逆冲变形(E_(1))阶段。

The Xiangfan-Guangji fault zone(XGF)is a boundary fault separating the Dabie orogen and the foreland fold-thrust belt on the northern Yangtze cratonic margin.Its geometry,kinematics and structural evolution characteristics have recorded the collision,assembly and interaction between two tectonic units in the north and south.Based on field investigation,structural and geochronological analyses,and combined with published regional geological and geophysical data,we considered that the eastern XGF is characterized by a“crocodile mouth”offset structure of deep south-directed thrust and shallow north-directed thrust,which differs significantly from the structural deformation style and sequence of the western XGF.The eastern part of the middle Yang-tze block is controlled by the N-S-directed tectonic system.Subduction of the northern Yangtze cratonic margin beneath Dabie oregen may have extruded the orogen from north to south.The nappe structure may have affected the Ruichang area,and the shallow thrust nappe from south to north may have affected the vicinity of Meichuan.The two structures are likely connected in the Qichun-Wuxue-Xishui area in the eastern XGF.The XGF has undergone various structural deformation stages,including the N-S-directed thrust nappe ductile shearing(end T_(2)),Early-Middle Yanshanian shallow bipolar offset detachment(J_(1-3)),Late Yanshanian normal faulting(K_(1-2)),and minor early Xishanian thrusting(E_(1))from north to south.