The structure and kinematics of the southeastern Zagros fold-thrust belt, Iran: From thin-skinned to thick-skinned tectonics

摘要

We present the first complete balanced cross section across the southeastern Zagros fold-thrust belt (ZFTB). The following main structural features emerge from this section: (1) In the south of the ZFTB, the Proterozoic-to-Recent sedimentary sequence has been decoupled from its Panafrican basement along the ductile basal evaporites and folded into a series of large detachment anticlines. Ongoing shortening of these structures has resulted in migration of the basal salt layers into the cores of the anticlines and propagation of forelimb thrusts. (2) In the north of the ZFTB, deep-seated ramps have folded the hanging wall rocks and produced imbrications and duplex structures within the higher levels of the sedimentary sequence. (3) Out-of-sequence thrusts, linked to major seismogenic basement faults, have cut through the structures in the cover of the ZFTB. A three-step incremental restoration of the section shows that two main phases of deformation can be separated in the tectonic evolution of the ZFTB: a Mio-Pliocene thin-skinned phase, in the course of which most of the structures in the cover were generated, followed by a Pliocene to Recent thick-skinned phase, expressed as out-of-sequence faulting in the cover, which is currently underlined by the seismicity within the basement. In plan view, the initial structures of the southeastern ZFTB developed with a curved shape essentially controlled by the shape and thickness of the underlying Proterozoic salt basin (i.e., the "Jura style"). In the following basement-involved phase, out-of-sequence thrusts cut at oblique angles through the preexisting structures of the cover. The total shortening absorbed in the cover amounts to at least 45 km, corresponding to a ratio of ～22%.