湘西沅麻盆地早白垩世红层古地磁学研究及其对川东褶皱带新生代构造变形的指示

Early Cretaceous paleomagnetic characteristics of Yuanma Basin,western Hunan and the indications to the structural deformation of eastern Sichuan fold belt

川东褶皱带作为华南板块中部的“侏罗山式”褶皱,开始形成于晚古生代.自白垩纪晚期开始,受太平洋板块和印度板块对欧亚大陆挤压的影响,这一构造带乃至华南板块中部又叠加了新的构造变形.但是,目前对于川东褶皱带白垩纪以来的构造演化缺乏足够的认识.位于川东褶皱带东侧、雪峰造山带西麓的沅麻盆地形成于早白垩世.晚白垩世以来,沅麻盆地与川东褶皱带处于同一构造应力场中,因此对盆地内早白垩世红层的古地磁研究对于解释川东褶皱带中生代晚期以来的构造演化有着重要的意义.该研究在沅麻盆地早白垩世红层中开展的古地磁学研究获得了可靠的原生剩磁分量:Ds=15.6°,Is=42.9°,k=118.6,α_(95)=2.6°,表明沅麻盆地自早白垩世以来发生了4.1°±3.0°的顺时针构造转动.对川东褶皱带周缘白垩纪古地磁数据所揭示的地壳旋转变形,与断裂和褶皱轴组成的构造线迹变化之间的线性相关性分析,表明川东褶皱带位于齐岳山断裂带东南侧的部分,受印度板块-欧亚大陆、太平洋板块-华南板块间的挤压作用,自晚白垩世以来累积了约50~93 km的右旋错断量.

The East Sichuan Fold Belt(ESFB)in the central part of the South China Block(SCB)was initially formed in the Late Paleozoic.Since the Late Cretaceous,the ESFB was furtherly affected by the combined actions of the Pacific and Neo-Tethyan structural domains since the Late Cretaceous.The tectonic evolution of the ESFB since the Late Cretaceous was still ambiguous until now.The Yuanma Basin between the east margin of the ESFB and the Xuefeng Orogenic Belt was formed in the Cretaceous and was filled by red-beds strata.This basin probably reserved the information of the tectonic stress environment of the ESFB since the Early Cretaceous.The present work conducted paleomagnetic studies in the Early Cretaceous red-beds of the Yuanma Basin,which obtained a primary magnetic component with the site-mean direction of Ds=15.6°,Is=42.9°,k=118.6,α_(95)=2.6°.The result shows that the Yuama Basin experienced 4.1°±3.0°of clockwise rotation relative to the eastern part of the SCB,since the Early Cretaceous.The linear correlation analysis of the rotational characteristics and the tectonic lines being composed by faults and fold axis around the ESFB,indicate that the ESFB at the Qiyueshan fault′s southeast side as the predominant tectonic belt on the western margin of the ESFB,experienced right-lateral shearing,with a total offsets of 50~93 km in the Upper Cretaceous,which was probably induced by the compressions between the Indian Plate and Eurasia,and the Pacific Plate and the SCB.