On the structure of the Tajik Depression (western foreland of the Pamir) and the problem of the amount of convergence between the Pamir and the Asia interior

The Tajik Depression is the westernmost and widest part of the Neogene-Recent foreland basin of the Pamir. It is confined between the Pamir, Tien Shan and Ghissar mountain belts. In the Neogene, the Cretaceous-Recent fill of the Tajik Depression was detached along Late Jurassic evaporites, folded and thrust towards the W-NW in front of the advancing Pamir.

The structural interpretation is based on a series of balanced cross-sections tied to field observations and new seismic data. The cross-sections show how thin-skinned folds and thrust sheets interact with thick-skinned structures in the basement. The western (foreland) part of the thin-skinned system features thrust sheets stacked along backthrusts that accommodated large displacements up to ~20 km each. The eastern (hinterland) part accommodated less shortening and is characterized by thrust detachment folds. The thick-skinned belts that surround the Tajik Depression originated from reactivation of pre-existing structures (Tien Shan) and recent thick-skinned thrusting (Ghissar).

According to our new AFT and (U-Th)/He constraints folding in the Tajik Depression started at ~13 Ma and was largely completed by ~4 Ma, although the area is still tectonically active. The succession of deformation reveals a non-Coulomb behavior of the thin-skinned belt and control imposed by the surrounding thick-skinned structures.

The amount of thin-skinned shortening first increases from ~89.4 km in the south to ~117.8 km in the centre, but drastically drops to ~16.4-7.8 km in the northeastern and northern sectors of the Depression. These low shortening estimates represent a challenge for the palinspastic restoration. To sustain the commonly accepted ~300 km of convergence between the Pamir and the Asian Plate one has to accept stationary, erosional front of the thin-skinned system in the northeastern and northern parts of the Tajik Depression.

This research was conducted under the patronage of Total.