Forming Mechanism Of The Pamir Salient:insights From Physical Analogue Modeling

The Indian-Eurasian collision has established the Tibetan Plateau,at northwest of which lies the Pamir salient.The Pamir salient,with vast crustal shortening,doubled crust thickness,huge topographic elevation,synorogenic extension,and profound impact on the structural deformation,sedimentation and paleoclimate in southwestern Tarim basin,supplies an ideal region to reveal intracontinental orogenic processes and its impact on paleoclimate in the context of continent-continent collision.Despit the significance,the forming mechanism of the Pamir is still controversial.It is unclear whether the North Pamir was previously situated at the linear along-strike continuation of the Western Kunlun and subsequently has been indented northward since the Oligo-Miocene,or was situated at the location very close to its present locality with its arcuate shape been established by the Paleogene.In this study,we used physical analogue modeling to investigate the forming mechanism of the Pamir salient and explore the main factors which controlled the deformation of the Pamir arcuate structural belt.The modeling results will be then applied to evaluate the applicability of two competing models,gradually-deformed model v.s.pre-existing model,on the formation of the Pamir salient.By setting different conditions of physical analogue modelings,we draw the following conclusions.1 Physical analogue modeling results indicated that the pre-existing arcuate-shape weak zones played a key role in forming the arcuate shape of the Pamir.The modeling outcomes produced by the condition of four pre-existing arcuate-shape weak zones(correspongding to the Main Pamir Thrust,and Tanymass,Rushan-Pshart and Shyok suture zones)were consistent with the present Pamir salient.2 Detachment accelarated deformation propagation onto the northern Pamir boundary.Without a detachment,deformation would show a break-forward mode,in which the deformation reached the northern Pamir margin during the later deformation stage.In contrast,if a detachment existed,the northern Pamir margin,i.e.the Main Pamir Thrust,was activated at the very beginning of the deformation.3 The difference of basement strength might be one of the factors controlling the formation of the Pamir salient.The arcuate boundary between the hard Tajik-Tarim basement and the weak Pamir basement could result in formation of the arcuate-shape Pamir.However,the modeling results suggested that,in this case,the arcuate-shape structural belt was only distributed in the north edge of the Pamir,with other thrust belts within the interior of the Pamir showing linear extension,which was inconsistent with the Pamir salient we see today.4 Northward indentation of the western Himalayan syntaxis could form many arcuate-shape sturcutral belt in the Pamir salient.However,the modeling results suggested,in this condition,radians of the structural belts would decrease outwards,which was inconsistent with the present Pamir.5 Combining our own modeling results with previous studies,we conclude that,given four pre-existing arcuate-shape weak zones and a detachment,it is completely acceptable that the Pamir salient had been largely established by early Cenozoic.Our study lends support to the claim of reactivation of pre-existing arcuate-shape weak zones during the Cenozoic Indian-Eurasian collision.