Erosional processes as a control on the structural evolution of an actively deforming fold and thrust belt: An example from the Pamir‐Tien Shan region, central Asia

The foreland of the Pamir Mountains displays an unusual orogenic topography in which an active north vergent, thin-skinned fold-and-thrust belt shows no topographic foredeep in advance of the thrust sheets. Instead, the thrusts are impinging on a topographic barrier formed by the southern edge of the Tien Shan. The deformation front of this thrust belt is coincident with a deeply incised river valley occupied by a large river, the Surkhob. We suggest that this river played a key role in the evolution of this orogenic system by acting as a lateral conveyor belt for the transport of mass delivered to the deformation front by thrusting. On the basis of reconnaissance geologic mapping it is clear that the present system developed in the Late Miocene or Pliocene, when the Tien Shan were uplifted within an existing foreland basin. As the Tien Shan highland developed, a longitudinal river within the foreland basin was trapped between the north directed thrusts and the Tien Shan highland. With continued deformation the river was squeezed into a narrow canyon and since that time the system has been in an unusual equilibrium where thrusting acts to push the deformation front northward toward the Tien Shan, but fluvial erosion and transport act to keep it in a constant position. Geologic relationships indicate that the Vakhsh thrust, the frontal thrust of the north directed system, is now emerging on an erosion surface beneath a thin Quaternary cover throughout the canyon, suggesting that the river has been unable to keep pace with the tectonic mass flux, and the thrust front, as well as the river, have been pushed northward across the beveled edge of what was originally the southern edge of the Tien Shan. We suggest that the original deformation front of the Tien Shan now lies ∼10 km south of the present-day topographic front of the Tien Shan, beneath the north vergent Vakhsh thrust, as indicated by: (1) a near-vertical zone of intense basement seismicity (the “Petrovsky” seismic zone); (2) an en echelon array of backthrusts along the south flank of the Peter the First Range; and (3) footwall cutoff relationships of the Vakhsh thrust system. Order-of-magnitude, mass balance considerations based on sediment budgets support this conclusion but suggest that the mass imbalance may be small. Thus large volumes of material may have passed through the thrust fron and been carried out of the system by erosion, a relationship that may account for the narrow (30–50 km wide) band of deformed sedimentary rock between the Pamir salient and the Tien Shan. The present near equilibrium is maintained by long-term tectonic transport rates and fluvial transport rates, the latter closely linked to climate, particularly precipitation. Thus, although there is apparently a mass “pile up” within this system, other systems of this type might show grossly different structural and stratigraphic relationships depending on the amount of rainfall and corresponding variations in rates of fluvial sediment transport. Similar settings can be recognized in the active thick-skinned thrust systems of the Tien Shan and ancient systems like the Laramide belt of western North America.