QUANTITATIVE STUDY ON THE LARGE SCALE DEXTRAL STRIKE-SLIP OFFSET IN THE SOUTHERN SEGMENT OF THE RED RIVER FAULT SINCE MIOCENE

The Zhonggu Fault,located in the Yuanjiang-Yuanyang basin and active strike-slip fault since Neogene,is part of the southern segment of the Red River Fault.Its neotectonic movement resulted in the separation of the Red River basin(a Miocene basin)into two sub-basins and a dextral slip extending to the Guotoushan-Damanmi region.The concomitant mountain frontal fault,which was the dominant fault in Oligocene,is of normal faulting.Its activity resulted in the accumulation of red continental clastic sediments in the eastern and northeastern Yuanyang.The mountain frontal normal fault extended to the northwest of Honghe County and formed the deposition of conglomerate in early Miocene.Along the Zhonggu Fault,which offset Miocene sediments with high angles,the geological features,including the compressive fold with axis trending NE,the compressional deformation landforms,the distribution the Miocene to Quaternary sediments migrating from SE to NW successively and their delayed distribution at the northeastern wall of the Zhonggu Fault,all suggest that the southern segment of the Red River fault has been expanding from SE to NW and there has been the dextral strike-slip faulting since Miocene.The geological evidences,such as thick lower-mid Miocene conglomerate deposited in mountain front,the shear deformation involved in Zhonggu Fault which was more intense in Miocene than that in Pliocene,the shear zone of Zhonggu Fault mainly located in lower-mid Miocene sediments,indicate the large scale dextral slip occurring at mid Miocene,and its fission track age is 13.7Ma.The magnitude of dextral offset on the south segment of Red River Fault since Miocene was calculated by multiple means,e.g.the horizontal dimensions of the offset of Miocene sediment,the length of foreland basin and deformation width relating to the Zhonggu Fault slip,and the relationship between the offset and width of fault,with the result ranging from 62～69km(the mean is 65km).The data also suggest that dextral slip of the Red River fault experienced the process of transition(N1),initial dextral slip(N21),large scale dextral slip(N31-N12),and dextral slip propagation(N22-Q1p)phases.The related activity of the Red River fault altered between shear slip and extensional slip.