The Structural Framework Of Duohemao, Qinghai Province

Duohemao of Zeku County in Qinghai is located in Northeastern Tibetan Plateau and the compound site where the Jungong Fault extending northeastern meets the Guide-Duohemao Fault extending north and south.The Anemaqen continential rift lies to the south of Duohemao, Qilian - North Qinling fold system to the norh, the West Qinling orogenic belt to the west and the Qaidam terrane to the west ,separated by the Gonghe Basin.The tectonic position of Duohemao is very special.Since the formation of the Altun-Qilian-Karakorum-Qinlin complex in the early Paleozoic ,Duohemao has experienced the closure of Paleo Tethys ocean and Neo-Tethyan ocean and the subduction and collision between the India plate and the the Eurasian plate,so the formation of the structural framework of Duohemao was related to the closure of Paleo Tethys ocean and Neo-Tethyan ocean and the subduction and collision between the India plate and the the Eurasian plate.The superposition of different tectonic stress field in different directions related to multiple tectonic events will inevitably lead to the diversity of the structural stytles and the structural feature in the study area.East-west trending fault, north-south trending faults and northeast trending faults and the first-phase folds, box-shaped folds with almost vertical hub, nearly east-west trending folds and north-south trending folds compose the structural framework of Duohemao.. During Early Triassic and Middle Triassic , the oblique collision of the Prototethys cause the formation of the first-phase folds and the east-west trending strike-slip fault,and the embryo of structural formation of Duohemao shapes up.The Northen Prototethys closed in the early Late Triassic which leaded to the collision of the Altun-Qilian-Karakorum-Qinlin terrane and the Bayan Har terrane,which caused strong deformation and the formation of box-shaped folds in the Duohemao.During the Late Triassic and the Early Cretaceous,the remote effects of the subduction of oceanic crust and closure of the north Neo-Tethys Ocean leaded to the generation of the east-west trending isoclinal fold in the late Triassic,tectonic overprinting on the first and the sceond stage folds.In the Jurassic,the stretching from east to west brought about the generation of four north-south trending normal faults in the Duohemao.During the late Jurassic and the Cretaceous,northward subduction of south Neo-Tethyan oceanic basin led to violent east-west extension in Duohemao,and then Duohemao Passive Rift Basin generated in the Early Cretaceous.Becasuse of the impact of the south-north compressive stress,the east-west stike-slip fault activeted again and changed into thrust fault thrusting from south to north.Since Eocene, the eastward extrusion of Qaidam terrane since the Oligocene caused the east-west bearing stress in Duohemao,and leaded to the generation of north-south open folds and the structural framework of Duohemao.The generation of the north-south trending faults is earlier than the Duohemao Basin,and the faults lying on the east side and the west of the basin control the size of the basin. The bearing stress exerting on Duohemao made the occurance of tectonic inversion of the north-south trending normal faults and thethe north-south trending normal faults changed into thrust faults,leading to the atrophy of the Duohemao Basin.