Fossil thermal structure of the southern Sanandaj-Sirjan zone (SW Iran): Implications for regional-scale tectonics
Résumé
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 ~200 to ~600 °C, all Precambrian and Early Paleozoic samples record temperatures >500 °C. Results thus reveal the existence of a major temperature contrast between Precambrian to Devonian samples (showing Tmax > 500 °C) and Late Devonian to Cretaceous ones (Tmax < 400 °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 ~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.