Tectonic Uplift And Deformation Mechanism Of The Altun Structural System Since The Middle-Late Period Of Late Pleistocene Time

Strath terrace and thrust scarp are two typical tectonic geomorphy and the geomorphicresponses to tectonic uplift. They imply abundant information of tectonic uplift. Based on theinterpretation to satellite images and field investigations, I found that there are obvious thrustcomponents in the Altun Fault zone, and the latest thrust movements of thrust faults in thenorthern margin of the Altun Mountains. In the meantime, there are strath terraces with cleargeomorphic characters in a lot of streams of the Altun structural system. Therefore, I regard thestrath terrace and thrust scarp in the Altun structural system as subjects, research the latest upliftrate of the structural system, and combine these data with other kinematic characters to discuss thepresent deformation mechanism of the Altun structural system.Based on the investigating of field geological of fifteen sites, testing of age samples andcalculation of uplift rates, interoperating of satellite images, analyzing of other data, I can obtainseveral major conclusions as below.1) In the Altun structure system, the thrust fault can be divided into three segments, thewestern segment, middle segment and eastern segment. The western segment is toward west fromthe debouchment of the Cheerchenhe River. The characters of the segment are that there are thrustcomponents on the major fault of Altun and there are a few of thrust faults in the system.The middle segment is located from the debouchment of Cheerchenhe River to Qingyazi. Themajor fault forms the boundary between the Qaidam Basin and the Tarim Basin, the thrust faultsspread on the northern margin of the Altun Mountain, and there are thrust components on themajor fault of Altun.The eastern segment is towards east from Qingyazi. There are thrust components on thenorthern marginal fault of Altun. There are thrust faults with different scales between theSanweishan fault and the major fault of Altun. There are latest thrust faults with small scales onthe alluvial fan of pediment, which have intermittent, non-linear and convex shapes and extendtoward north.On the whole Altun structure system, there are thrust components in strike-slip faults andmost thrust faults have strike-slip components.2) Based on analyzing and contrasting the ninety-eight ages of stream terraces, combinedwith regional paleoclimate data, nine forming periods of stream terraces are determined: Ⅰ.2-4kaBP;Ⅱ.4-8ka BP,concentrated about 6ka BP;Ⅲ.9-14ka BP,concentrated between 11-13ka BP;Ⅳ.16-23ka BP,concentrated about 18ka BP;Ⅴ.25-32ka BP,concentrated between 27-29ka BP;Ⅵ.35-42ka BP;Ⅶ.45-53ka BP;Ⅷ.65-78ka BP;Ⅸ:103ka BP.The ages of terrace surface indicate synchronism of the terraces formation, especially on theyoung terraces. In the meantime, the terraces formation is related with the changes of regionalpalaeoclimate. The major terraces formed in the periods when the paleoclimate changed from cooland dry to warm and humid, controlled by the changes of regional palaeoclimate. The formation ofstrath terrace also indicates the same characters.3) Based on the surveying of geomorphy, testing of ages and calculating of uplift rates onstrath terraces of eight investigated sites, the uplift rates of every site are obtained.On the western segment of the Altun structural system, the uplift rate is 6.67±0.50mm/yr inNaoqi since18.98±1.42ka BP, 1.75±0.09 mm/yr in Tuohepayi since 18.31±0.99ka BP.On the middle segment of the Altun structural system, the uplift rate is 0.64±0.05mm/yr atthe debouchment of the Milan River since 10.70±0.81ka BP.On the eastern segment of the Altun structural system, the uplift rates are between 4.21±0.64mm/yr to 2.15±0.16mm/yr since 40.5±3.16ka BP in Tuanjiexiang, the averaging rate is3.38±0.24mm/yr;The uplift rates are between 1.26±0.23mm/yr to1.2±0.03mm/yr since11.44±0.86ka BP in Changcaogou, the averaging rate is 1.23±0.11mm/yr;The uplift rates arebetween 4.89±0.59 mm/yr and 4.46±0.19 mm/yr 4.21±0 since 16.07±0.27ka BP in Yandantu,the averaging rate near the front of the mountain is 4.68±0.31mm/yr since 12.83±0.97ka BP, theaveraging rate near the margin of the platform is 4.54±0.08mm/yr, the averaging rate is4.61±0.16mm/yr since 16.07±0.27ka BP in the Yandantu area;The uplift rate is 1.05±0.08mm/yrsince 53.26±4.05ka BP in Erjiatai;The uplift rates range from 1.09±0.48 mm/yr to 0.40±0.03mm/yr since 103.47±9.42ka BP in the area around the debouchment of Shulehe River, theaveraging rate is 0.75±13 mm/yr. In the middle part of Yingzuizhan, the uplift rate is 0.79±0.06mm/yr since 23.71±1.73 ka BP.The uplift movements indicated by strath basically took place during the time when thepalaeoclimate changed from warm and humid to cool and dry. Among the eldest ages of the treadsabove for all sites, the youngest is 10.70±0.81ka BP of debouchment of the Milan River. Therefore,the uplift rates or averaging rates at these sites can be regarded as the distribution of uplift rates inthe Altun structural system since 10.70±0.81ka BP, representing the distribution of uplift ratessince Holocene. And there is a tendency that the rates dwindle from west to east in the wholesystem, and the same in the every segment.4) Based on the surveying of geomorphy, testing of ages and calculating of thrust rates onthrust scarps of nine investigated sites on the middle and eastern segments of Altun StructuralSystem, the thrust rates of all sites are obtained.On the middle segment, the thrust rates are 0.33±0.04 mm/yr and 0.09±0.01 mm/yrrespectively since 16.09±1.24ka BP in Jianggalasayi;the rate is 1.42±0.11 mm/yr since31.96±2.40ka BP in Jianggalasayi;0.57±0.05mm/yr since 72.36±5.28ka BP in Liuchengzi;and0.15±0.04mm/yr since 74.51±5.41ka BP in Akesaigou.On the eastern segment, the thrust rate is 0.91±0.18mm/yr since 18.52±1.01ka BP inTuanjiexiang;0.57±0.04mm/yr since 47.43±3.51ka BP in Dalebayi;0.37±0.07mm/yr since35.38±2.58ka BP in Bangeba;0.13±0.01mm/yr since 65.41±4.71ka BP in west of Shulehe;and0.05±0.01mm/yr since 8.99±0.68ka BP in Hongliugou.In all ages of treads related with thrust scarps above, the youngest is 8.99±0.68ka BP at theHongliugou site. Therefore, these rates represent the distribution of thrust rates in the Altunstructural system since 8.99±0.68ka BP, namely since early Holocene.5) In general, in the area where strath terrace and thrust scarp coexist, the height of strathabove channel and the uplift rate are bigger than or equal to those of thrust scarp and thrust rate,respectively.6) It is a boundary structure and a sinistral transpression zone for the Altun structural systembetween the Qing-zang Massif and Xinjiang Massif. All of data, including GPS, inversion of stressfield, surface and deep structures, indicate oblique slip convergence between the bilateral massifsof the Altun structural system. The boundary structures of this system are sinistral strike-slip faults.The northwestern boundary is the Cheerchenhe River Concealed faults and the Sailikesayi fault,and the southeastern boundary is the major fault of Altun. The southwestern segment of thissystem generates structural conversion with Aqike Structure. The northeastern segment generatesstructure conversion with Danhenanshan and Qilianshan Fold Systems. There are thrustcomponents in the strike-slip faults and there are strike-slip components in most thrust faults.7) In the Altun structural system, the surface structure appears as a combination of strike-slipfaults and thrust faults. The lithosphere has lateral discontinuity and vertical lamination.Deformation partitioning exists in the Altun structural system. In the meantime, the major fault ofAltun extends downwards to the upper mantle and forms a shear zone with 40km width, otherthrust faults end on the major fault at depth. Therefore the deformation partitioning in the systemaccords with the oblique slip partitioning model. However the system's deformation is notpartitioned into single deformation domains of simple shear (strike-slip) or pure shear (contract).There are thrust components in the strike-slip faults and there are strike-slip components in mostthrust faults. The deformation partitioning of the Altun structural system is an incompletedeformation partitioning.8) In the background of the India plate underthrusting and pushing towards north, the obliqueconvergence between the Qingzang massif and Xinjiang massif is the major driving force of thepresent deformation in the Altun structure system. When the southeastern lateral Qingzang massifof the structural system moves, the northwestern lateral Xinjiang massif also does.