Late Mesozoic Tectonic-Sedimentary Evolution Of The Northern Qaidam Basin And Its Dynamic Meaning

The Early Cretaceous collision between the Lhasa and Qiangtang terranes is a critical tectonic event that shaped the southern margin of the Eurasian continent,and that impacted the formation of the Cenozoic Tibetan Plateau.This collision also resulted in crustal shortening and surface uplift in the Tibetan Plateau before Cenozoic.However,the northward influence range of the collision and its contribution to the Cenozoic uplift of the Tibetan Plateau remains to be further studied.To address this issue,we present the results of sedimentary characteristics,structural deformations,and basin properties from an integrated study of Mesozoic strata in the Northern Qaidam Basin(NQB),northern Tibetan Plateau,based on field investigations(including description of lithology and sedimentary facies,paleocurrent measurement),detrital zircon U-Pb analyses,seismic profile interpretations,and fault distribution plane analysis.We futher reveal the sedimentary and structural response records of the NQB to the Early Cretaceous Lhasa-Qiangtang collision.On the basis of the above studies,combined with previous studies,we analyzed the similarities and differences of Jurassic and Cretaceous basin evolution in NW China,and the dynamic mechanism of this phenomenon.In addition,based on the Mesozoic and Cenozoic deformation characteristics of the NQB,we preliminarily evaluated the impact of the collision on the Cenozoic uplift and deformation of the Tibetan Plateau.The conclusions are summarized as follows:1.The Upper Jurassic in the NQB is composed of lacustrine red mud and sandstone,whose lithology is relatively unified in the whole basin,lacking of marginal sedimentary facies.The upper Jurassic has a wide range of original deposits with obvious characteristics of extensional depression.The provenance is mainly from the East Kunlun Shan,the North Qaidam UHP belt,and the South Qilian Shan.The Lower Cretaceous mainly consists of deltaic,fluvial,and alluvial sandstone and conglomerate.It has typical orogeny-related molasse deposits and generally presents upward coarsening sedimentary sequence.The sedimentary characteristics(including sedimentary facies,lithology and thickness)change rapidly laterally,and piggy-back basins related to thrust are developed.The analysis of paleocurrent and sandstone zircon age indicates that the provenance of Lower Cretaceous is South Qilian Shan.These phenomena indicate that there was an important change of sedimentary environment in the NQB from the end of Late Jurassic to the beginning of Early Cretaceous,from extensional setting to active compressional orogenic setting.2.A series of marks for identifying Mesozoic faults in the NQB are put forward in this paper,and the Jurassic normal fault system and Cretaceous reverse fault system are depicted.The main strike of the two fault systems is NW to NNW,and most of them are developed at high angle.The spatial correlation shows that there is a certain inheritance between the Jurassic normal fault system and the Cretaceous reverse fault system.Most of the Jurassic normal faults turned into high angle reverse faults in the Cretaceous,and the two fault systems jointly controlled the residual distribution of Mesozoic.As the major thrusts in the South Qilian Shan thrust belt,these faults hace affected the Cenozoic sedimentation and deformation characteristics of the main structural belts.The reconstructions of balanced cross section show that the horizontal shortening caused by reverse fault system in Cretaceous is only 20% of the total deformation shortening of Mesozoic and Cenozoic.3.Combined with previous studies,we consider that the NQB was local rift basin– extensional depression in the Early-Middle Jurassic.In the late Jurassic,it was an extensive extensional depression basin,and turned into a foreland-like basin in Cretaceous due to the compression and uplift of the South Qilian Shan.The basin attribute characteristics in the NQB are basically consistent with that of Hexi Corridor and Yingen-Ejinaqi Basin in the Early-Middle Jurassic,but there are obvious differences in the Early Cretaceous.Regional correlation shows that in the Early Cretaceous,the basins in NW China was obviously differentiated with the Middle Qilian Shan as the boundary.The basins in the south were compressional basins,and the northern basins were extensional fault basins,which reflected that thay had completely different geodynamic mechanisms in the Early Cretaceous.The compressional basins in the south were mainly controlled by the collision and continuous convergence between Lhasa terrane and Qiangtang terrane,and the NQB is the northern boundary of its influence area.The extensional Hexi Corridor basin group and Yingen-Ejinaqi basin in the north have similar characteristics with North China,suggesting that the two regions have the same dynamic mechanism,which is related to the subduction of the paleo Pacific and/or Mongolia Okhotsk plate towards Eurasia.4.The South Qilian Shan and the NQB are the northern edge of the Early Cretaceous paleoplateau caused by the Lhasa-Qiangtang collision,which indicates that the deformation caused by this collision has been transmitted northward for thousands of kilometers.The compressive deformation intensity of the NQB in Cretaceous is weaker than that of Cenozoic,which is only 1/4 of that of Cenozoic.If the deformation intensity is extended to the whole Qinghai-Tibet region,it can be inferred that the impact of the Early Cretaceous collision on the crustal thickening in the Qinghai-Tibet region is not significant,which may be related to the bivergent double subduction the soft collision between Lhasa and Qiangtang terranes in the Early Cretaceous.However,such a weak convergence caused the compression deformation of the South Qilian Shan and NQB thousands of kilometers away from the north,and its extent was almost had same as that caused by the strong collision between India and Eurasia in Cenozoic.Considering that these deformations are mostly concentrated in(or near)the Early Paleozoic collisional orogenic belts,we speculate that the pre-existing weak lithospheric zones played an important role in the propagation and concentration of strain.