川东-鄂西郁江逆冲断层轨迹结构及扩展过程

野外地质调查和剖面测量揭示:郁江断层带的轨迹结构整体上表现为南北分段、东西分带的后展式叠瓦状逆冲断层结构;其中,底板缓倾角逆冲断层多层次(区域尺度、露头尺度和微观尺度)的断坪、断坡轨迹,特别是沿断坡方向的破裂分解,是分支断层呈上叠后展式扩展增殖的基础。根据运动学标志,郁江逆冲断层从北西向南东逆冲,最大断距小于1km,一般断距为10～100m级;后缘高倾角正断层的累计断距与前缘逆冲断距基本对应。结合区域构造分析,郁江逆冲断层轨迹结构定型于燕山期,轨迹扩展过程表现为:上叠分支断层沿底板逆冲断层的断坡方向逐次从前缘向后缘扩展增殖,属于典型的上叠后展式逆冲扩展结构;其中,后缘正断层的形成是断层轨迹结构反馈、运动调整和断层自组织行为的必然结果;而喜马拉雅主期区域应力场的激发,只是强化和加速了后缘正断层的扩展规模和进程。

TRACE PATTERN AND GROWTH PROCESS OF YUJIANG THRUST BORDERING EAST SICHUAN AND WEST HUBEI

The trace pattern of the Yujiang thrust bordering east Sichuan and west Hubei is measured from the geological investigation over the past five years. The trace pattern is characterized by the segmentation, zonation and the frontal imbricate thrust structure with overstep propagation. Of them, the zonation is divided into a frontal imbricate thrust sub zone with the low angle and a back normal fault sub zone with the high angle. The imbricate thrust zone, displaying overstep propagation, derived from the flat and the ramp changes of floor thrust, especially secondary fracturing branches along the ramp direction. The frontal thrust displays thrust movement from NW toward SE with the displacement of 10 m to 100 m scale, and the largest displacement is less than 1 km. The displacement of the back normal fault is in line with one of the frontal thrust. The trace pattern of the Yujiang thrust was formed in the Yanshanian movement (Late Jurassic Early Cretaceous), and the growth process of the trace pattern displays the branch faults growth from the frontal toward back along the ramp direction of the floor thrust. The back normal faults derived from the feedback relations of the trace pattern, movement accommodation and self organization in the Yujiang thrust zone, and the Himalayan orogeny enhanced the growth and development of the normal fault.