论大别山南缘襄樊—广济断裂的两次向南逆冲推覆

A Discussion on Two Times Southward Thrusting of Xiangfan—Guangji Fault in South Dabie Orogen,Central China

位于大别山南缘的襄樊—广济断裂是扬子板块与大别造山带的边界断裂。大体以黄陂为界,断裂东段与西段在几何结构上有所差异：东段在地表浅层表现出向北反向逆冲,深部则以向南大规模逆掩为特征;西段浅层与深部一致,表现为典型的大型低角度叠瓦状逆冲推覆。野外地质数据、40Ar/39Ar定年、锆石U-Pb LA-ICP-MS定年和沉积学资料表明,自中三叠世以来,该断裂带发生了两次明显的向南逆冲：第一次逆冲发生在中三叠世（约240~231M a）,与北界的浒湾拆离断层同时启动,响应于大别山超高压从地幔深度折返到下地壳的挤出过程;第二次逆冲发生在中—晚侏罗世（约160~140M a）,即发生在大别山白垩纪热隆伸展之前,因为伴随热隆伸展侵位的巨量花岗岩体并未卷入逆冲变形。第一次逆冲推覆以超高压折返伴随的中—高角度逆冲为特点,发生在大别造山带内部。第二次逆冲发生在造山带东部南缘,以低角度巨型逆冲推覆为特点,使大别造山带深变质岩大规模覆盖在扬子板块北缘的沉积岩之上,最大覆盖宽度大于60 km。东段由于受江南古陆的准同时向北逆冲改造,在浅层形成向北逆冲的几何学结构。

The Xiangfan—Guangji fault（XGF） which separates the Dabie orogen belt（DOB） from the Yangtze block is the south boundary fault of the DOB.The XGF is divided into two parts by the Jianghan rift basin.The western and eastern parts of the XGF show significantly different structural features.The western XGF is characterized by typical N-dipping listric margin fault that has a thrust displacement of at least 30km,while the eastern part underwent more complex tectonic evolution.After a similar initial southward thrusting with the western part,the eastern DOB made a large-scale southwestward thrust onto the North Yangtze foreland basin along the XGF in late Jurassic.The low angular N-dipping XGF was then rebuilt by succedent northward compression from the Jiangnan uplift while the deep still shown southward thrusting features.New field structural data,40Ar/39Ar ages,zircon LA-ICP-MS ages and sedimentology evidence indicate that the XGF have experienced twice significant southward thrusting since Middle Triassic： 40Ar/39Ar ages indicate that both the XGF and the Huwan detachment,south and north margins of the DOB respectively,were formed contemporaneously as a result of the initial extrusion of UHP from mantle to lower crustal level during the continental—continental collision stage in Early Triassic（about 240~231Ma）;the second apparent thrusting was in Middle—Late Jurassic（about 160~140Ma） before the DOB＇s thermal doming extension for the granitic intrusion during this period wasn＇t involved into deformation.The first thrusting was characterized by inner orogen ductile,Brittle—ductile deformation accompany with exhumation of UHP terrains while the second was characterized by large-scale（thrusting distance60km） low-angle shallow thrusting of the south margin of the DOB against above the North Yangtze foreland.During this period,extension of the north margin,large-scale thrusting of the south margin together with the contemporary sinistral strike-slip of the east margin of the DOB draw a clear tectonic framework and indicate a clockwise rotation of the eastern DOB.