Cenozoic Evolution Of Laolongwan Basin In The West Segment Of The Haiyuan Fault Zone, Northeastern Margin Of Tibetan Plateau, And It’s Implications

The Haiyuan fault zone is one of the greatest and the most active arcuate structural beltin the northeast of Tibetan Plateau, and is also the front deformation zone of the northeasternmargin of the Tibetan Plateau, which contains the northeastward tectonic extension of theTibetan Plateau. The sedimentary sequence of the Laolongwan cenozoic basin, which issituated in the west segment of the Haiyuan fault zone, records the formation and evolution ofthe Haiyuan fault in the northeastern margin of the Tibetan Plateau.Based on the study of cenozoic sedimentary characteristic of the Laolongwan basin andthe analysis of tectonic deformation, combined with the pre-existing paleontological andpaleomagnetic data analysis, the following implications have been obtained.1.This paper suggests the Paleogene-Neogene sedimentary sequence in the Laolongwanbasin by Paleogene-Neogene regional stratigraphic correlation with southern Ningxia,including Sikouzi Formation(Eocene), Qingshuiying Formation(Oligocene), HongliugouFormation(Miocene), and Ganhegou Formation(Late Miocene to Pliocene). The GanhegouFormation is characterized by red coarse clastic rocks, and was caused by the dextralpull-apart action along the Haiyuan fault. Pre-existing paleontological and paleomagnetic dataanalysis yields the Ganhegou Formation’s maximum deposition age of about8-10Ma, whichsuggests the Ganhegou Formation coarse clastic rocks began to fill in the Laolongwa basinsince~8-10Ma, indicating the Laolongwan pull-apart basin occurred in the Haiyuan faultzone in the Late Miocene. That is, the Haiyuan fault zone is featured by the dextralstrike-slipping at this time. There is no angular unconformilty between Eocene to Miocenestrata, so the formation of the Laolongwan pull-apart basin and the right strike-slipping of theHaiyuan fault zone in Late Miocene are the earliest sedimentary-tectonic response of thenortheastward extension of the Tibetan Plateau to the Haiyuan fault zone.2.Based on detailed field measurement of faulting deformation and analysis of thesedimentary sequence of the Laolongwan basin, the stress field sequences and thedeformation and tectonic evolution history of the Laolongwan basin in the west segment ofthe Haiyuan fault zone have been preliminarily established. The evolution history of the Laolongwan basin can be divided into four stages: Eocene to Miocene, the lacustrinesedimentary basin developed due to NW-SE-trending extension. From Late Miocene toPliocene, dextral strike-slipping occurred in the Haiyuan fault zone under the nearly N-Stectonic compression and Laolongwan pull-apart basin developed on the Eocene to Miocenelacustrine sedimentary basin. Late Pliocene to Early/Middle Pleistocene, controlled byNE-SW tectonic compression, the Haiyuan fault zone undergone strong thrust faulting and theLaolongwan basin was folded and faulted. Since Late Pleistocene, the Haiyuan fault zone isfeatured by the sinistral strike-slipping due to the ENE-WSW tectonic compression and aseries of small pull-apart basin north of Laolongwan basin along Haiyuan fault zone occurred.The sinistral strike-slipping of Haiyuan fault zone controlled by the ENE-WSW tectoniccompression transformed the local landform.3.The formation and evolution history of the Laolongwan cenozoic basin contains theevolution information of the Haiyuan fault zone and the sedimentary-tectonic response in theprocess of northeastward tectonic extension of the Tibetan Plateau. The Haiyuan fault zonehad not yet formed in Eocene to Miocene. From Late Miocene, dextral strike-slippingoccurred in the Haiyuan fault zone. The Laolongwan pull-apart basin controlled by the rightstrike-slipping of the Haiyuan fault zone in Late Miocene are the earliest sedimentary-tectonicresponse of the northeastward extension of the Tibetan Plateau to the Haiyuan fault zone,which probably indicates that the neotectonic movement in the northeast margin of theTibetan Plateau began in Late Miocene. Late Pliocene to Early/Middle Pleistocene, theHaiyuan fault zone undergone strong thrust faulting, which is the most obvious and strongeststructural activities in this area. Since Late Pleistocene, the Haiyuan fault zone is featured bythe sinistral strike-slipping, which transformed the local landform.