Role Of Tectonic Uplift And Expansion Within And Outside The Northeastern Tibetan Plateau In The Formation And Evolution Of River And Desert Landscapes

Since Cenozoic,the India-Asian collision formed the"Third Pole in earth"-the Tibetan Plateau(TP)that has a significant impact on the tectonic pattern,geomorphic development,ecological environment and climate change there.The boundary of the northeastern Tibetan Plateau(NE TP)is controlled by Altyn Tagh fault(ATF)and Qilian Shan(QLS)thrust fault belt,where numerous strike-slip faults,thrust faults and sedimentary basins absorb most of crust deformation and control the formation and development of geomorphic features.Study of the geometry,kinematic features and tectonic transition model of these faults is the key to understand Cenozoic geologic tectonics and geodynamic process.The southern Alxa Block(AB)is located to north of the NE TP,within which the Badain Jaran Desert(BJD),the second largest flowing desert of China,and the Heihe River(HR),the second longest inland river of China are developed.However,what is the tectonic strain transition between faults within QLS and ATF?What is the structural deformation and dynamic mechanism of faults within southern AB?And how do the geomorphic formation and evolution of HR and BJD response to the deformation and expansion of active tectonics?Hence,the further study of structural deformation features,tectonic transition mechanism,tectono-geomorphic deformation,eco-environmental change and human activities not only has an important scientific value for understanding the interaction of multi-phere system,such as the lithosphere,hydrosphere,biosphere and atmosphere,but also has practical application value for excavating the international value of the United Nations Educational,Scientific and Cultural Organization(UNESCO)Alxa World Geopark and helping the BJD's landform with the coexistence of megadune-lake apply to the UNESCO world natural heritage site.Based on remote sensing,structural geology,geomorphology,geochronology,coupled with geologic interpretation of multiple satellite images and field investigation,this study focuses on the geometric segmentation,slip rates and structure evolution of key faults within NE TP and southern AB-Changma fault(CMF)and Yabrai fault(YBF),as well as the structural deformation and transition between these faults and ATF,trying to depict the tectonic deformation pattern and growth process of NE TP and southern AB since ca.33 Ma and deeply explor the role of plateau's uplift and expansion in the landform of HR,as well as the formation and evolution of megadune-lake in BJD.This study has obtained several results followed:(1)The 1932 Changma earthquake stroke and produced a 120 km surface rupture along the NW-NWW fault.Rupture can be divided into 5 first-order segments,each is14.4-39.56 km long,linked by step-overs.Rupture could jump across step-overs of 0.3-4.5-km width and 2.2-5.4-km length,but was stopped by a 6.3-km-wide compressional step-over at the easternmost end of CMF.The slip rates on mid-eastern and eastern segments are yielded to be 3.43±0.5 mm/yr and 4.49±0.5 mm/yr,suggesting CMF plays a key role in the tectonic transition of ATF by absorbing slip deformation of 3-4 mm/yr on ATF.Analyses of trenches show that CMF may experience 6 seismic events:No.1(9.4±1.0?9.6±1.0 ka),No.2(6140±30 BP?7.0±1.1 ka),No.3(3000±30?5700±30 BP),No.4(1960±30?2030±30 BP),No.5(<1960±30BP),and No.6(the 1932 Changma earthquake).(2)The NE-NEE YBF is 138 km long,where the southwestern and northeastern segment appeal left-lateral strike-slip faulting,while central segment shows normal faulting.Low temperature thermochronology results of apatite(U-Th)/He show a multi-stage evolution:the YM experienced rapid cooling and uplift in late Cretaceous(ca.135-71.5 Ma);the YM experienced a long-term denudation from late Cretaceous to Eocene(ca.70-33.9 Ma);the YBF started left-lateral faulting with a displacement of47±2 km and a long-term sinistral slip-rate of 1.40±0.06 mm/yr from Oligocene to early Pliocene(ca.33.9-5 Ma)accommodated with the slip of ATF;the YBF dominates left-lateral strike-slip with normal faulting component accommodated with the change in stain mechanism from NE-SW compression to eastward extension since Pliocene(ca.5 Ma).The normal faulting uplifts the footwall of YBF along YM,impedes and controls the migration and expansion of sand dunes,providing a unique terrain conditions for the formation of the geomorphologic landscape of worldwide highest megadune-lake in BJD.(3)The uplift of northern QLS and structural activities of the piedmont faults in the northern front of QLS have caused the channels of HR migrate from east toward west,then flow northward through the Zhengyixia and form a huge downstream alluvial fan.The monitoring of the desertification shows that the ecological environment in downstream basin of HR deteriorated resulted from the over-consumption of water resource in midstream before 2000;Since 2000,a significant restoration of?69%degraded land of downstream basin of HR has been restored because of the ecologic water dispatching,with an increase of vegetation coverage,an increase of annual average precipitation and the gradual recovery of previous dry lake,Juyan Lake.Water dispatching balance model suggests a rational threshold of?11×10~8 m~3/a water allocated from midstream to downstream with the water allocation ratio of 1.4,providing an important reference to improve the sustainable ecological restoration for keeping the economic,social and ecologic“triple bottom line”.(4)This study summarized the multi-stage tectonic-geomorphic evolution pattern of NE TP and AB since 33 Ma:First stage(33-10 Ma),slip movement of ATF partitioned into AB and influenced the left-lateral strike-slip faulting on YBF accommodated with the India-Asian collision;Second stage(10-5 Ma),the QLS began to uplift accommodated with the crust shortening of NE TP and NE-SW-trending compressional strain,within which the strike-slip and thrust faulting initiated.The strike-slip faulting became weaken in the southern AB;Third stage(5 Ma-present),the change in strain from NE-SW-trending compression to EW-trending extrusion made the slip on ATF be absorbed and transitioned by strike-slip and thrust faulting within QLS,rather than propagate to influence the YBF that is dominated by left-lateral strike-slip with normal faulting component.Meanwhile,the uplift of QLS,tectonic deformation and expansion of the region control the formation and evolution of HR's landform and unique megadune-lake landform in BJD.In conclusion,this study shows the Cenozoic tectonic strain transition,geodynamic mechanism and topo-geomorphic evolution process of“tectonic-river-desert-oasis”system in NE TP and southern AB from different spatio-temporal scales.The tectonic uplift of plateau controls the topo-geomorphic features and ecologic environment in two aspects:Internally,faults absorb the crust deformation,controlling the erosion of rivers and the formation of megadune-lake landforms in desert accommodated with the crust thickening and uplift;Externally,the plateau's uplift affects climate change and blocks the rainfall,as well as forms low reliefs where the westerlies prevails,transporting sand,promoting the formation of deserts and forming an ecological environment pattern of perennial drought and water shortage,sparse vegetation,and desertification.