Along-strike variations of foreland thrusting in the Pamir

The arcuate, northward convex Pamir mountains are fringed in the west and north by foreland fold-and-thrust belts forming the External Pamir (EP). In the west, the wide thrust belt occupies the entire Tajik Basin. Towards the northeast, it grades into a narrow, W-E-trending belt along the southern margin of the intermontane Alai Valley which separates the Pamir front from the Tian Shan in the north. In the westernmost Tarim Basin, the thrust belt widens again to form a pronounced northward salient. We present three cross sections to illustrate along-strike variations of the EP in topography, erosion level, stratigraphy, and structure. Possible variables that controlled structural styles and evolution include: (1) Basement rheology and structure: mechanical strength and buoyancy probably increase eastward from Pamir lithosphere forming a steeply dipping continental slab to the rigid Tarim basin. The basement beneath Alai makes a large antiform plunging west- and eastward towards the Tajik and Tarim basins. (2) Pre-Cenozoic structure: in the Tian Shan the Talas-Fergana fault is a major NW-SE-striking discontinuity that was already active in Paleozoic and Mesozoic time and probably links up with large faults in western Tarim. (3) Facies and thickness variations of Mesozoic and Cenozoic deposits: in Alai, multiple décollement levels in Cretaceous and Paleocene rocks favor short-wavelength folding. In W Tarim, few large thrusts splay from a single basal décollement in thick Paleocene gypsum. Cenozoic strata thicken markedly eastward. Mutual influence and feedback between these tectonic-stratigraphic parameters and variations in climate driving erosion intensity is probable.