Tectonic Characteristics And Formation Mechanism Of Cangmucuo-Duosang North-South Graben In Tibet

A series of nearly north-south graben and fault basin structure are the one of the most typical structural features of Qinghai-Tibet Plateau since the Cenozoic. These nearly north-south graben structures are prevailing in the Qinghai-Tibet Plateau, especially in the tectonic belt of Gangdise. Mass information about evolution of the Tibetan Plateau is recorded by the graben structure. Therefore, the study on structures and dynamics mechanism of more typical nearly north-south graben structure in Gangdise belt will have important theoretical significance on research of the causes of north-south graben structure and the Tibetan Plateau tectonic environment since the late Cenozoic.This paper selects Cangmucuo - Duosang north-south graben structure of western Gangdise as an object of study. Research thinking is in line with the macro to micro, from the distribution of the plane to the characteristic of horizontal section along the graben, from the deformation of shallow to the dynamic effect of deep. By the route geological survey, remote interpretation, structural geomorphology analysis, fission track dating and so on, while collecting seismic geological, geophysical data, we compile and analyze the characteristics of north-south fault, and more discussing its formation process and kinetic mechanisms. So we can further provide information to elaborate the tectonic characteristics and evolution of Gangdise-Lhasa block .Study area is located at the Qinghai-Tibet Plateau, belonging to the Gangdise - Nyainqentanglha plate of tectonic units on classⅠ, and the GeJi - Cuoqin complex back-arc basin of sub-tectonic units. The active tectonics belt is formed by Cangmucuo-Duosang grabens and Longgeer graben mainly distributing in the longitude belt: 83°25'-83°45 ', latitude: 31°20'-32°14'. The overall extension is in NE15°~ 20°, the length of about 127km, width 10 ~ 15km. According to its plane structure alignment, it can be divided into northern, central, and southern section, was "Z" shaped line, and remote sensing image clarity.Canmucuo- Duosang graben is mainly maked up by east and west boundary faults and the internal active faults. Steep normal faults, the section with the right combination of tilting ladder are the chiefly characters of most faults. Fault rocks are mostly tectonic breccia, near the fracture zone, rocks can be seen near vertical tensile cracks, reflecting the characteristics of brittle deformation.Canmucuo- Duosang graben with the oldest strata is Miocene Mangxiang group. Fault with internal graben are active which are being significantly cut the Holocene Quaternary terrace deposits.The lateral variation of sediments show that the east side of the graben boundary faults universally early and then the west.According Longgeer fault uplift mountain apatite, zircon fission track and normal geothermal gradient of Duosang graben, the uplift ablation rate and normal geothermal gradient is 35℃/ km , comparing with the average uplift rate during the same period, the rapid uplift of both sides off the table fault uplift mountain region and rapid subsidence of the graben basin should be after 16.8Ma.By the Quaternary geology of normal faults with east and west sides, geomorphology profile measurement and sampling a large number of Moraine dating (ESR method) studies of Duosang graben obtained the most intense period of its activities during the late Pleistocene, its uplift rates is up to 4.44mm / a, reflecting the region is subjected to the role of stretching east to west since the late Pleistocene. Collected gravity detection, explosive detection and land seismic data integrated electrical test probe analysis shows the study area is likely to exist within a low-speed high-conductivity layer in the lower crust, and the mantle melt mixed. According to Bouguer gravity anomaly inversion changes from the depth of Moho surface, suggesting that this region deep within the graben is likely to face a deep trough in the Moho, and the northern part of the Bangong - Nu River suture zone Moho surface was uplifted of about 10km. Combined with the exposed of the Himalayan rocks and dikes in the late development sides of the graben of the facts, infer the formation of graben normal faults is likely to have the role of the deep motivation.Cangmucuo– Duosang graben formation mechanism may be due to pre-formed X-conjugate shear fracture by the north-south regional compression; late to squeeze into the crust from to south north because the north resisting cause the east and west both sides of migration, then form a deep east-west extensional stress field, so that tensile stress transfer to the main process of the tensile .pre-formed conjugate shear fracture X-track and part of the link, eventually forming Construction of the current form.