Early Cretaceous Sedimentary Characteiristics And Tectonic Evolution Of Pingshanhu Basin,Hexi Corridor

Pingshanhu basin,located on the north of Hexi Corridor,is bounded by the Longshou mountain to the south,the Beida mountain to the north,and Heli mountain to the west.The deposition and evolution of Pingshanhu basin was controlled by Longshoushan fault and Beidashan fault in the Early Cretaceous.In this thesis,we deepened the study on the Early Cretaceous Miaogou Group,mainly about the geometry of the basin,the tectonic stress field and detrital zircon.We make a detail discussion on the tectonic evolution of Pingshanhu Basin.In Pingshanhu Basin,the age of Miaogou Group is the Early Cretaceous which appear an upward-fining sedimentary sequence.The lower strata consist of conglomerate,coarse sandstone,etc.The upper strata consist of siltstone,argillaceous siltstone,mudstone,shale,etc.The sedimentary facies of Pingshanhu Basin consist of alluvial fan facies,fluvial facies and lacustrine facies.The sedimentary sequence represents the rapid accumulation in front of mountain according to the transformation of sedimentary facies.In early-to-mid Early Cretaceous,the basin was formed under a NE-SW trending compressive stress field resulted from the extrusion of Longshoushan fault and Beidashan fault which formed a series of compressional structures.The Longshoushan fault was also a NW-SE trending strike-slip fault.The zircon age of five clastic rocks range from 129.3Ma to 3214 Ma.The peak ages are 200Ma-300 Ma,400Ma-600 Ma,800Ma-1000 Ma,1800Ma-2000 Ma and 2400Ma-2600 Ma.It indicates that the provenance of the lower rock formation of Miaogou Group may origin from Alashan Block(The north mountain);the provenance of the upper rock formation of Miaogou Group may origin from Qilianshan Orogenic Belt(Longshou mountain)and Alashan Block(the Beida mountain).The latest zircon age of sample CQL2016-L1-88 is 129.3±1.8Ma.It may represent the sedimentation age of the upper rock formation and the formation age of graben.On the basis of the sedimentary facies of Early Cretaceous Miaogou Group,the tectonic style,tectonic stress field and zircon ages of clastic rocks,Pingshanhu Basin was a compressional basin in the early Early Cretaceous.The growth strata restricted the compressional structure ages.The tectonic evolution of Pingshanhu Basin in Early Cretaceous could be divided into four steps:(1)Beidashan fault trending thrusted from NE to SW(2)Longshoushan fault trending thrusted from NE to SW with NW-SE trending strike-slip(3)Beidashan fault trending thrusted from NE to SW(4)the basin EW expansion.