From Early Jurassic intracontinental subduction to Early-Middle Jurassic slab break-off magmatism during the Cimmerian orogeny in the Sanandaj-Sirjan Zone, Iran

This study aims to review the Jurassic to Cretaceous tectonic setting of the Sanandaj-Sirjan Zone (SaSZ), the inner part of the Zagros orogen. For many years, the SaSZ was interpreted as a part of the Cimmerian microcontinent carrying a Jurassic magmatic arc to the Neo-Tethyan Ocean. Recently, propagating continental rift and mantle plume models have also been considered for this magmatism without taking into consideration the presence of coeval high-pressure metamorphic rocks and S-type granites predating I- and A-type granites. In the middle part of the SaSZ, the association of high-pressure metamorphic basement rocks and S-type granites suggests a collisional tectonic setting during the Early Jurassic, followed by partial melting by decompression and exhumation. These phenomena are considered to reflect compressive forces from the collision of the Cimmerian microcontinent with the Turan plate contemporaneous with the opening of the Neo-Tethys Ocean. The heterogeneity between the rheologically stiff and heavy Upper Neoproterozoic to Cambrian basement with arc-related mafic magmatic rocks and weak Permian-Mesozoic cover rocks of the SaSZ controlled the downward motion of the former. We propose that Early Jurassic intracontinental subduction predates subduction of the Neo-Tethys oceanic lithosphere underneath the SaSZ, and the formation of Jurassic magmatism resulted from plate collision and break-off of the subducted slab of the Paleo-Tethyan Ocean and of its associated former passive margin. The NeoTethys subduction and formation of the active continental margin started only during Cretaceous times. The lower mantle Mesopotamia anomaly detected by mantle tomography could represent the subducted PaleoTethyan slab.