Toward A Clarification Of The Provenance Of Cenozoic Sediments In Qaidam Basin

Determining the provenance of sedimentary basin fill in the northern Qaidam Basin is a key step toward understanding the sedimentary system dynamics and mountain-building processes of the surrounding orogenic belts in Tibet.The exceptionally thick(average of 6–8 km)Cenozoic fluvio-lacustrine deposits in the northern Qaidam Basin were once thought to have been eroded from the nearby northern Qaidam Basin margin and the southern Qilian Shan and to reflect the prolonged thrust-related exhumation of these orogenic belts.However,several recent studies,based mainly on paleocurrent and detrital zircon U-Pb age data,suggested that they were derived from the distant East Kunlun Shan to the south,or the Qimen Tagh to the southwest(at least 200 and 350 km from the northern Qaidam Basin,respectively).That model assumed that the East Kunlun Shan and Qimen Tagh formed signifcant topographic barriers during the earliest sedimentation of Cenozoic strata(e.g.,the Lulehe Formation)in the northern Qaidam Basin.Therefore,the tectonic signifcance of the provenance of the Lulehe Formation remains a fundamental problem in understanding the postcollisional uplift history of the northern Tibetan Plateau.To address this issue,we conducted sedimentological and paleocurrent analyses of the Lulehe Formation and detrital zircon U-Pb dating of Mesozoic strata in the northern Qaidam Basin.The results,in combination with existing paleocurrent and seismic reflection data,collectively indicate that although the source area cannot be specifed by matching zircon U-Pb ages in sedimentary rocks with crystalline basement source rocks,other evidence points consistently to a unifed proximal northerly source area(the northern Qaidam Basin margin and the southern Qilian Shan).In addition,Our results emphasize that noncrystalline basement rocks(e.g.,Mesozoic sedimentary rocks)in fold-and-thrust belts should be taken into consideration when seeking potential source areas by correlating zircon U-Pb ages of siliciclastic detritus with related basement rocks.In addition,this study strongly supports the claim that variations in the proportions of age populations should be used with caution when determining source terrane by comparisons of age distributions.The main results achieved and understanding are as follows:(1)The detrital zircon age populations of the Cenozoic Dahonggou strata are all reported in three potential source terranes,in varying proportions,including the East Kunlun Shan and Qimen Tagh,the Qilian Shan and Nan Shan,and Mesozoic strata in the northern Qaidam Basin.Thus,the use of a single detrital zircon dating method cannot provide unique source information.(2)The sedimentological and paleocurrent analyses,in combinationwith existing paleocurrent data,seismic reflection data,and detrital apatite fission-track dating,point consistently to a unifed proximal northerly source area(the northern Qaidam Basin margin and the southern Qilian Shan).(3)The orogenic recycling effect of Mesozoic strata in the fold-and-thrust belt of the northern Qaidam Basin,which was ignored in recent studies,played a signifcant role in producing the Permian–Triassic–aged zircons observed in the Cenozoic sandstones.