Reconstructing extension, subduction, and lateral extrusion in the Central Tethysides (Iran, Afghanistan)

Nalan Lom, Douwe J.J. van Hinsbergen 

The supercontinent Pangea enclosed an internal Tethys ocean whose destruction by subduction formed the Tethysides orogenic belt, exposed in the Alpine-Himalayan mountain range. The Tethyan realm consisted of multiple oceanic basins separated by microcontinents, and their tectonic reconstruction has proved instrumental in understanding subduction geodynamics, back-arc basin evolution, and continental deformation. The Central Tethysides, mostly in Iran and Afghanistan, hosts remnants of distinct, non-parallel ocean basins, including the Jurassic South Caspian-Kopet Dagh basins in the north, the Jurassic Sabzevar-Nain-Baft basins in Central Iran, and the Early Cretaceous Sistan Basin in eastern Iran, whose opening and closure have not been kinematically reconstructed in detail. Here, we review geological constraints on Central Tethysides structure and provide a GPlates-based reconstruction of its tectonic history. Our reconstruction indicates that the South Caspian oceanic basin (pinching eastwards in the continental Kopet Dagh Basin) and the oceanic Sabzevar-Nain-Baft basins (pinching westwards in the continental Central Basin) together partitioned Late Jurassic-Early Cretaceous N-S back-arc extension behind the main Neo-Tethyan subduction zone. The Sistan Ocean formed at a long-lived plate boundary that accommodated final Eurasian amalgamation. In the Early Cretaceous, these continents shared a rotation pole at the junction of the closing Waser Suture leading to extensional reactivation of the southwestern part of the collision zone, where the Sistan Ocean opened. The Sabzevar-Nain-Baft basin subsequently closed in Late Cretaceous-Eocene time by westward extrusion of the Central-East Iranian Microcontinent and Farah-Rud Zone from western Tibet and the Pamirs. The Sistan Ocean closed simultaneously by westward extrusion of the Helmand Block, originally part of the Tibetan Qiangtang Block. Extrusion exceeded 1000 km, likely initiated by Cretaceous Tibetan Plateau shortening.