Study On The Geologic And Geomorphic Evolution Of The Late-Cenozoic Basins In The Southwest Margin Of The Ordos Block

The late Cenozoic uplift and expansion of the Tibet Plateau has resulted in intense tectonic deformation within the Asian continent and posed a significant impact on the geomorphologic pattern and environmental evolution in the surrounding areas.Located at the intersection of the northeast margin of the Tibet Plateau,the North China Craton and the Qinling Orogenic Belt,the southwest margin of the Ordos Block belongs to the frontier of the northeast expansion of the Tibet Plateau and is also the transition area of the Haiyuan-Liupanshan-Baoji fault zone(HLBF)and the north margin fault zone of the Qingling mountains(NQF).Moreover,in terms of geographic location,the southwest margin of the Ordos Block on where the Liupanshan thrust belt with strong compression shortening and the extensional faulted Wei River Basin distributed from northwest to southeast was the transition site from extrusion thrust stress system to strike-slip stretching stress system.Therefore,the southwest margin of the Ordos Block is a critical area to understand the time and mechanism of the horizontal growth of the Tibetan Plateau and the regional tectonic deformation transition.In this region,the late Cenozoic faulted basins——the Qian River Basin and the western Wei River Basin(west of the Longxian-Qishan-Mazhao fault(LQMF))controlled by HLBF and NQF fault systems play a role of bridge and link in the northeastward expanding of the Tibet Plateau and the regional tectonic deformation transformation.However,the understanding of the tectonic-sedimentary evolution process of the basins,especially in the aspects of time node,structural transformation,and dynamic mechanism,is still vague and controversial.Also,even though some achievements have obtained in geometry,kinematics and dynamics research of the Longxian-Baoji fault zone,but because of the widely distributed thick Quaternary deposits,the study objects are mainly faulting records since middle-late Pleistocene,and lack of systematic understanding about fault evolution process and dynamic mechanism since the later Cenozoic.The sedimentary fillings and tectonic landforms of the Qian River Basin and Wei River Basin preserve much information about basin evolution and tectonic activity,which make them an irreplaceable carrier for the study of regional neotectonic movement.Therefore,the geologic and geomorphic evolution study based on the sedimentary filling and tectonic geomorphologic process is a good starting point to solve the problems mentioned above.Based on the comprehensive investigation,we have analyzed the regional neotectonic background,sedimentary filling characteristics,and Quaternary geomorphologic surfaces process,and discussed the sedimentary-tectonic evolution and dynamic mechanism of the late-Cenozoic basins in the southwest margin of the Ordos Block.The main findings obtained are as follows:(1)Since the late Cenozoic,the Gansu group(N_1-2G),the Sanmen formation(N₂-Q₁s),and the Quaternary loess-paleosol sequence have been developed in the Qian River Basin.In contrast,the Bahe formation(N₁b),the Lantian Formation(N₂l),the Sanmen formation(N₂-Q₁s),and the Quaternary loess-paleosoil sequence have been deposited in the western Wei River Basin.Based on the results of the magnetostratigraphy study of the Fengxiang borehole in the western Wei River Basin together with the previous magnetostratigraphy and the Electron Spin Resonance dating result,the late-Cenozoic stratigraphic ages of each formation are redefined.In Qian River Basin,the age of the Gansu group is 8.26?3.6Ma,the lacustrine sediments of the lower Sanmen formation extend from 4.5 to 3.6Ma which is a heterotopic product synchronous with the upper Gansu group,while the conglomerate deposit of the upper Sanmen formation roughly formed between 3.6 and 2.0Ma,and the Quaternary loess began to accumulate at about 2.0Ma at the earliest.In the western Wei River Basin,the Bahe Formation was deposited in 11?7Ma,the Lantian formation spans the interval of 8.26?2.5Ma,the Sanmen formation extends from 4.5 to 2.5Ma,and the loess-paleosoil sequence deposited during the Quaternary(2.5?0Ma).Among them,the Sanmen formation is contemporaneous difference facies with the Lantian formation and the Quaternary loess,and can be subdivided into two lithostratigraphic units:the lacustrine sediments of the lower Sanmen formation extends from 4.5 to 3.6Ma,while the fluvial-eolian strata of upper subunits deposited in 3.6?2.5Ma.(2)Between 8.26 and 4.5 Ma,under the foreign influence of the northeast expansion of the Tibet Plateau,the tectonic activity in the southwest margin of the Ordos Block is dominated by overall uplift.Based on stable tectonic activity and relatively flat landform,the Gansu group and the Lantian formation,which was mainly composed of eolian red clay,were deposited at the Qian River Basin and the western Wei River basin respectively,and developed part of the"red clay plateau."Around 4.5 Ma,affected by local NE-SW tension stress caused by counterclockwise rotation of the Ordos Block,fault depression occurred along the LQMF fault in the southwest margin of the Ordos Block and developed the ancient Sanmen lake.(3)From the late Pliocene to the early Pleistocene,two important sediment-tectonic interfaces are representing two significant tectonic events developed in the Qianhe basin.The first is the planation surface at the top of the Gansu group,which formed at about 3.6Ma and represented that the differential lifting movement and highly differentiated landforms occurred as the response to the“phase A of the Qingzang movement."Under its influence,the further fault depression occurred in the Qian River Basin,and the Qianyang uplift rose rapidly and separated the late Cenozoic basin into the Qian River Basin and the western Wei River Basin.The second is the piedmont denudation surface,developed at about 2.0Ma,which represents the response to the“phase C of the Qingzang movement."This further tectonic uplift made the study area to accept eolian loess deposition,and then entered the stage of modern water system formation and evolution.(4)During the Quaternary period,influenced by the pulsating uplift of the Tibet Plateau and the climatic cycle,asymmetric river terraces developed on both sides of the Qian river.The five terraces on the north bank of the river were developed at 1.176 Ma,0.778 Ma,0.504 Ma,0.131 Ma,and 0.039 Ma,respectively,while the four terraces on the south bank were formed at 0.778 Ma,0.375 Ma,0.131 Ma,and 0.039 Ma,respectively.(5)In response to the northeast growing of the Tibet Plateau,the geologic and geomorphic evolution process of the late Cenozoic basin in the southwest margin of the Ordos Block can be divided into five stages:the late Miocene-early Pliocene“red clay plateau”evolutionary stage,the early Pliocene basin initial rifting stage,the late Pliocene-early Pleistocene differential lifting stage,the early Pleistocene loess plain and modern river system rudiment developing stage,and the gradual uplift and river terrace development stage since the middle early Pleistocene.Combination with the background of regional neotectonic movement,this paper proposed that the east extrusion of Qinling Orogenic belt and the Longxi block are the main ways of Tibet Plateau expansion,and shows the characteristics of gradual spatio-temporal north-eastward expansion.These growth features do not support the“Rigid block model”with discontinuous deformations,but more inclined to the“continuum model”.