Multiscale Geomorphic Surface Formation And Its Relationship With Tectonic Forcing

Deformed geomorphic markers are used ubiquitously in tectonic geomorphology to constrain rates and spatial distribution of deformation.The river terrace and planation surface are the most widely used markers to study active tectonics and neotectonics respectively.While,the formation mechanism of the river terrace and the planation surface remains uncertain,which impedes using them as markers to understand deformation.In the northern foreland basin of the Tian Shan,three sub-parallel roughly east-west-striking fold and thrust belts have deformed Mesozoic to Cenozoic strata,forming a basinward migrating deformation front.Northward flowing perennial rivers,originating from glaciers,transversely incise these fold and thrust belts and in many cases cut deep valleys through them towards the Junggar Basin.The Dushanzi anticline lies in the northernmost deformation belt where the Kuitun River transversely incises the uplifting Dushanzi anticline and builds a series of outstanding late Quaternary terraces.Here,the Kuitun river terraces honestly record important information of tectonic uplift,and/or climatic variations during their evolution,which provides an excellent natural laboratory to probe driving forces of river terrace formation.In order to probe the formation mechanism of the Kuitun river terraces,the longitudinal profile was rigorously reconstructed and the uplift history of the Dushanzi frontal fault was constrained.Via detailed interpretation of high-resolution lidar-derived topographic data and photographic data,and field observations across the Dushanzi anticline,a Pleistocene alluvial fan?fp?and sixteen river terraces?T1-T16?decreasing systematically in elevation along the Kuitun River were identified.There are also a series of sub-terraces within the regional terraces in the upper reach of the Kuitun River.If taking account of these sub-terraces as well,there would be circa³5river terraces in total along the Kuitun River.This work then integrates a high-quality seismic reflection profile and field observations across the Dushanzi anticline to resolve the Holocene deformation history of the anticline.The reconstructed longitudinal profile along the Kuitun River flowing across the Dushanzi anticline demonstrates a Holocene deformation of both folding and faulting.The total shortening accommodated by faulting is 4.4±0.1 m,and by folding is 12.6±1.1 m since 7.5-10 ka B.P.A total of17.0±1.1 m of shortening has occurred at a rate of 2.0±0.2 mm/a across the Dushanzi anticline in the Holocene.This shortening represents up to 18%of the>11 mm/a total shortening rate across the entire eastern Tian Shan at this longitude.The Holocene shortening rate of the Dushanzi anticline is⁶ times greater than its long-term average shortening rate?0.3±0.1 mm/a?since 4.8 Ma.Furthermore,this work analyzed a displaced terrace sequence at a site along the Dushanzi frontal fault and found that the average coseismic dip-slip displacement is 2.5±0.1 m,with an average recurrence interval of 2.8±0.3 ka.This work reveals that tectonic uplift events generated by the Dushanzi frontal fault cannot explain terrace formation of the Kuitun River.Rather,the results hold that these terraces are triggered by climate variations.In the upper reach,almost each regional terrace consists of a suit of ladder-shaped subterraces,indicating that these regional terraces were abandoned from the upstream to downstream.Such terrace formation pattern is related to climate variation.Moreover,within the Dushanzi anticline,there are roughly¹0 terraces preserved,while only³ surface rupture events occurred during their formation.Only¹ surface rupture events occurred during the terrace formation from T2 to T14.Apparently,there are much more river terraces formed than paleoearthquakes occurred.These findings suggest that river terraces along the Kuitun River are all controlled by climate variation.This work tend to infer that these regional terraces are controlled by millennial scale climate change,while these sub-terraces are possibly related to decadal to multi-century scale climate variation.Noteworthy that a flight of small terraces within the Dushanzi anticline are specially preserved in the convex-bank,this work argues that the existence of the meander,combined with rhythmic climate changes,has controlled the formation of these small terraces.The Shanxi Graben System in North China,separating the North China Plain to the east from the Ordos block to the west,is the most outstanding continental graben systems in China.On the upland of the Shanxi Graben System,low-relief,high elevation patches so-called planation surfaces are widely preserved and several prominent tectonic events have been reported in the nearby area,which offers a natural case study on the interplay of the planation evolution and its relationship with tectonic uplift.This work firstly reconstracted the planation surface sequence that preserved on the Shanxi upland and found two levels of planation surfaces in total,the Beitai planation surface and the Tangxian planation surface.Using diagnostic criteria that developed by Whipple et al.?2016?,this work reconfirms that the Beitai planation surfaces are remnants of a formerly continuous low-relief continental-scale peneplain.Secondly,this work presented low-temperature thermochronological observations using joint apatite fission track and apatite?U-Th-Sm?/He data from exhumed graben shoulders in the N-S trending Huo Shan and the E-W trending Zhongtiao Shan.Joint inverse modeling of these data reveals two phases of rapid exhumation during the Cenozoic circa 50 Ma and 8 Ma respectively.Thirdly,the sys-rifting basal deposits in the Linfen basin have been dated based on cosmogenic nuclide ¹⁰Be-²⁶Al,which indicates that the Linfen basin initiated circa 5.8±0.6 Ma.The consistence between the onset of the Linfen basin?5.8±0.6 Ma?and the last cooling recorded in the Zhongtiao Shan?8 Ma?,suggesting that the Shanxi Graben System was initiated circa 8 Ma.This work then compares the evolution history of planation surface preserved on the Shanxi upland with the tectonic history recorded by thermochronology and cosmogenic ¹⁰Be-²⁶Al dating,suggesting that the planation surface formation correlates well with the tectonic events occurred during the Cenozoic.The Beitai and Tangxian planation surfaces were abandoned circa 50 Ma and 8 Ma respectively.From the late Mecozoic to mid-Eocene,the North China underwent longterm smooth denudation,forming the Beitai peneplain.In response to the Pacific subduction,the North China started to rift circa 50 Ma and the Beitai planation surfacec formed.After this early Cenozoic tectonic event,the North China underwent the other relatively peaceful stage and the Tangxian planation surface started to form.A renewed tectonic event abandoned the Tangxian surface and forced the initiation of the Shanxi Graben System.Furthermore,the onset of the Shanxi Graben System was broadly synchronous with the rise of mountain ranges along the periphery of the Tibet Plateau,suggesting a coordinated pulse of driving force.