Fossil thermal structure of the southern Sanandaj-Sirjan zone (SW Iran): Implications for regional-scale tectonics

The Sanandaj-Sirjan Zone (SSZ) is one of the Cimmerian blocks accreted to Eurasia during the Late Triassic and the only tectonic unit of the Zagros orogen affected by Barrovian metamorphism. Both the age and absolute temperature of this metamorphic event are still poorly defined. In the present study, we use the Raman Spectrometry of Carbonaceous Material (RSCM) to estimate maximum temperatures attained in the southern SSZ, along two SW-NE cross-sections. While maximum obtained temperature for Precambrian to Jurassic rocks ranges from similar to 200 to similar to 600 degrees C, all Precambrian and Early Paleozoic samples record temperatures 500 degrees C. Results thus reveal the existence of a major temperature contrast between Precambrian to Devonian samples (showing T-max > 500 degrees C) and Late Devonian to Cretaceous ones (T-max < 400 degrees C). This temperature contrast can be assigned to two distinct end-member scenarios, either to a so far unreported Variscan thermal event or to differential exhumation along later thrusts. The stepwise decrease of temperatures with age, and the tectonic style common to rocks older than the Early Paleozoic, strongly support the existence of a thermal (and likely) deformation event at similar to 380-360 Ma. The SSZ, which was part of the northern edge of Gondwana at that time, was thus affected by Variscan tectonics, similar to what is known in western Europe or NW Africa.