Locating The Paleogene Northern Pamir Boundary:Insights From Gravel Counting

Cenozoic evolution of the Pamir salient is crucial for understanding the intracontinental deformation processes in the context of India-Eurasian collision and relevant geodynamics.However,when the Pamir salient has been established remain an issue in debate.In this study,we address this issue from the perspective of gravel grain size,carrying out gravel counting against gravels/conglomerates on Wupoer alluvial fan at the NE Pamir,Shulehe alluvial fan at the northern Tibet Plateau and Oytag Paleogene succession section at the NE Pamir foreland,the following conclusions are drawn.1.The gravels on Wupoer alluvial fan are mainly cobbles,elongated,sub-rounded and sub-angular,and well sorted.Particle size distribution conforms to a normal distribution with approximately symmetry and medium kurtosis.The gravels on Shulehe alluvial fan are mainly cobbles and pebbles,elongated,sub-rounded,and approximately well-sorted.Grain size distribution conforms to a normal distribution with approximately symmetry and medium kurtosis.The Paleogene alluvial conglomerates in the Oytag section are mainly cobbles,sub-rounded and sub-angular,subcircle shape,and well-sorted.Grain size distribution conforms to a normal distribution with roughly symmetry and medium kurtosis.The similarity between the Paleogene alluvial conglomerates in the Oytag section and the gravels at the present Wupoer and Shuluhe alluvial fans indicates that the Paleogene Oytag conglomerates had undergone medium-to short-distance transportation.These gravels are mainly caused by early Cenozoic tectonic events of the Pamir,while not necessarily to relate to climate.2.Based on the Sternberg's exponential equations(D(x)=D0*e-?x),downstream fining exponent values in Wupoer and Shulehe alluvial fan equal to 0.010 and 0.015 respectively.These results,combined with another 35 present-day alluvial fans worldwide,suggest downstream fining exponent values ranging from 0.010 to 0.630.Of these fans,the modern Wupoer fan yields the minimal downstream fining exponent of ?0.010.The results imply that the Wupoer alluvial fan can be used as a reference to constrain the maximum transport distance for the Paleogene Oytag conglomerates.3.According to the results of gravel size in present-day alluvial fans worldwide,the grain sizes of the Paleogene conglomerates in the Oytag section have transport distances less than 60 km.Limited(<60 km)sediment transport distances and limited amounts(<50 km)of Cenozoic crust shortening imply that the arcuate-shape Pamir had been largely established by the Paleogene,and the northward displacement of Main Pamir Thrust(MPT)relative to Western Kunlun in the Cenozoic is <110 km.4.If arcuate-shape Pamir had been largely established by the Paleogene,with locating at the south of the present Oytag section <110 km,a reconsideration on the affiliation of south-dipping slab underneath the northern Pamir and its geodynamic implications is required.