Structure and exhumation ages of the Tajik Depression (western foreland of the Pamir): toward an integrated kinematic model

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. We propose a new kinematic model that integrates an updated structural interpretation with apatite fission-track and (U-Th)/He thermochronology constraints on the timing of exhumation.

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 significant displacement. The eastern (hinterland) part accommodated less shortening and is characterized by thrust detachment folds. The thick-skinned belts surrounding the Tajik Depression originated from reactivation of pre-existing structures (Tien Shan) and recent thick-skinned thrusting (Ghissar).

Samples were collected for apatite from Early-Mid Jurassic and Early Cretaceous sandstones and from pre-Jurassic metamorphic/igneous basement. The thermochronology results are supported by vitrinite reflectance data. Ghissar AFT and (U-Th)/He ages show a similar range (12.7±1.2-4.1±1.2 Ma and

11.9±3.8-4.3±1.1 Ma, respectively), indicating rapid exhumation through the 110°C and 75°C isotherms. AFT and (U-Th)/He ages from the Tajik Depression range between 11.8±0.8-3.5±0.3 Ma and 10.6±1.3-1.0±0.8 Ma, respectively. These results suggest that exhumation was diachronous from west to east across the Tajik Depression. The central part of the Tajik Depression yielded the oldest AFT age (11.8±0.8 Ma). This is a maximum age for the activation of the detachment along the Late Jurassic evaporites. Tien Shan AFT and (U-Th)/He ages range between 145.6±12.0-1.6±0.3 Ma. The wide range of the Tien Shan ages reflects a mix of fully reset, partially reset and un-reset age components. An offset between the youngest AFT ages (12.6±0.9-3.3±0.3 Ma) and (U-Th)/He ages (8.9±1.8-1.9±0.8 Ma) indicates that exhumation was slower in the Tien Shan compared to the Ghissar.

According to AFT and (U-Th)/He constraints the frontal homocline of the Pamir started to form at~13 Ma (Figure 1), coevally with far-field uplift of the Ghissar and the Tien Shan. At ~12 Ma the slip was transferred from the Pamir to the Upper Jurassic detachment triggering thin-skinned deformation in the Tajik Depression. Until ~6 Ma, thin-skinned shortening was restricted to the external (foreland) part of the Tajik Depression where long-displacement backthrusts built a hinterland-vergent imbricate stack. Increasingly internal zones of the Tajik Depression were then successively folded. Thin-skinned deformation was largely completed by ~4 Ma. The most recent folding of the internal part of the Tajik Depression was associated with reactivation of the pre-Jurassic basement.

This research was conducted under patronage of Total.