Geometry,Kinematics And Deformation Mechanism Of The Keping Fold-thrust Belt In The South Tian Shan,NW China

The Keping fold-thrust belt occurs between the Southern Tian Shan Mountains in the north and the Tarim Basin in the south,and consists of a series of ENE-WSW-oriented and S-directed thrust imbricates.The Keping fold-thrust belt,which is the only margin of the Tarim Basin to deform in thin-skinned style with Cambrian gypsum mudstone detachment,plays an important role in basinward deformation propagation of the Southern Tian Shan Mountains in the Cenozoic.In this study,we use the field investigation data and seismic data in the study area,combined with growth strata analysis,low temperature thermochronology and sandbox modeling,to study the geometry,kinematics and deformation mechanism in the Keping fold-thrust belt.Based on these researchs,we propose a reasonable tectonic evolution model about the Keping fold-thrust in the Cenozoic.The following results have been achieved:The Keping fold-thrust belt is divided into three segments.The western segment is characterized by deep and flat detachment layer and thick overburden.The minimum shortening in the western segment is estimated at 47.3km(38.0%).The middle segment is chatacterized by shallow and flat detachment layer and thin overburden.The minimum shortening in the middle segment is estimated at 32.5km(27.7%).The eastern segment is characterized by steep detachment layer,which is deeper in the north and shallower in the south.The overburden is thicker in the north and thinner in the south.The minimum shortening in the eastern segment is estimated at 42.7km(34.0%).Based on the analysis of growth strata and the inverse modeling of apatite fission track data,the South Tian Shan Fault start thrusting at about 23-16Ma.Then the deformation propagated forward to the Tuokesanatanengbaile thrust(T4)in the middle-late Neogene.After about 3Ma the activity of the South Tian Shan Fault accelerated,resulting in the thrust of the Kepingtage thrust(T1)and the Aozigeertawu thrust(T2)in the middle-late Pleistocene(1.4-0.04Ma).Based on the fact that the thickness of overburden in the middle segment of the Keping fold-thrust belt is less than that in the west segment,we conducted three sets of sandbox modelings with the thickness of sand layers varies between two sides.The results of sandbox modeling confirm that if the shortening is constant,fewer thrust faults with wider spacing develop in thicker cover units.Because the propagation of deformation front is faster and farther in the thicker cover units,a tear fault will form to adjust the deformation variations.We propose a tectonic evolution model about the Keping fold-thrust in the Cenozoic.The South Tian Shan Fault start thrusting at about 23-16Ma.Then the deformation propagated forward to the Aoyibulake thrust(T8),the Kalabukesaitage thrust(T9),the Kekebukesanshan thrust(T7)and the Piqiangshan thrust(T6).In the middle-late Neogene the defoemation propagated to the Tuokesanatanengbaile thrust(T4).At this time,the Kekebukesanshan thrust(T7)in the west and east of the Piqiang fault,the Tuokesanatanengbaile thrust(T4)and the Piqiangshan thrust(T6)are connected.Then,the Bachu uplift formed in the late Neogene.The Piqiang-Selibuya fault acting as the western boundary of the Bachu uplift cut through the Kekebukesanshan thrust(T7),the Tuokesanatanengbaile thrust(T4)and the Piqiangshan thrust(T6).The formation of Bachu Uplift caused the varation of depth and dip of detachment layer and overburden thickness along E-W striking in Keping fold-thrust belt.After about 3Ma the activity of the South Tian Shan Fault accelerated.The deformation propagated forward rapidly and cause the thrusting of the Kepingtage thrust(T1),the Aozigeertawu thrust(T2),the Tataaiertage thrust(T3)and the Yimugantawu thrust(T5)in the Quaternary.Because the thrusting started above the uneven basement caused by the Bachu Uplift,the Keping fold-thrust belt is divided into western,middle and eastern segments according to the characteristics of the basement.