The Eo-Cimmerian orogen results from the Late Triassic collision of Iran, a microplate of Gondwanan affinity, with the southern margin of Eurasia. The orogen is discontinuously exposed along the northern side of the Alborz Mountains of North Iran below the siliciclastic deposits of the Shemshak Group (Late Triassic–Jurassic). A preserved section of the external part of the belt crops out in the Neka Valley (eastern Alborz) south of Gorgan. Here the Mesozoic successions (Shemshak Group–Upper Cretaceous limestones) overlay a pre-Jurassic Eo-Cimmerian thrust stack with a sharp unconformity. The stack includes the Gorgan Schists, an Upper Ordovician–Lower Silurian low-grade metamorphic complex, overthrusted southward above a strongly deformed Late Palaeozoic–Middle Triassic succession belonging to north Iran. In the Talesh Mountains (western Alborz), the Shanderman Complex, previously interpreted as an ophiolitic remnant isolated along the Eo-Cimmerian suture, is considered an allochthonous nappe of deeply subducted continental crust. The new evidence for this is the occurrence of previously unknown eclogites dating to the Carboniferous, and probably related to the Variscan history of Transcaucasia. South of the Shanderman Complex, Upper Palaeozoic slates and carbonates occurring below the Lower Jurassic Shemshak Group also record the occurrence of an Eo-Cimmerian metamorphic event. Based on our new data, the Eo-Cimmerian structures exposed in the Alborz appear to be remnants of a collisional orogen consisting mainly of deformed continental crust where no ophiolites are preserved.
New structural, sedimentological, petrological and palaeomagnetic data collected in the region of Nakhlak-Anarak provide important constraints on the Cimmerian evolution of Central Iran. The Olenekian-Upper Ladinian succession of Nakhlak was deposited in a forearc setting, and records the exhumation and erosion of an orogenic wedge, possibly located in the present-day Anarak region. The Triassic succession was deformed after Ladinian times and shows south-vergent folds and thrusts unconformably covered by Upper Cretaceous limestones following the Late Jurassic Neo-Cimmerian deformation. Palaeomagnetic data obtained in the Olenekian succession suggest a palaeoposition of the region close to Eurasia at a latitude around 208N. In addition, the palaeopoles do not support large anticlockwise rotations around vertical axes for central Iran with respect to Eurasia since the Middle Triassic, as previously suggested.The Anarak Metamorphic Complex (AMC) includes blueschist-facies metabasites associated with discontinuous slivers of serpentinized ultramafic rocks and Carboniferous greenschistfacies 'Variscan' metamorphic rocks, including widespread metacarbonates. The AMC was formed, at least partially, in the Triassic. Its erosion is recorded by the Middle Triassic Bāqoroq Formation at Nakhlak, which consists of conglomerates and sandstones rich in metamorphic detritus. The AMC was repeatedly deformed during post-Triassic times, giving origin to a complex structural setting characterized by strong tectonic fragmentation of previously formed tectonic units.Based on these data, we suggest that the Nakhlak-Anarak units represent an arc-trench system developed during the Eo-Cimmerian orogenic cycle. Different tectonic scenarios that can account for the evolution of the region and for the occurrence of this orogenic wedge in its present position within Central Iran are critically discussed, as well as its relationships with a presumed 'Variscan' metamorphic event.
New Late Ordovician and Triassic palaeomagnetic data from Iran are presented. These data, in conjunction with data from the literature, provide insights on the drift history of Iran as part of Cimmeria during the Ordovician–Triassic. A robust agreement of palaeomagnetic poles of Iran and West Gondwana is observed for the Late Ordovician–earliest Carboniferous, indicating that Iran was part of Gondwana during that time. Data for the Late Permian–early Early Triassic indicate that Iran resided on subequatorial palaeolatitudes, clearly disengaged from the parental Gondwanan margin in the southern hemisphere. Since the late Early Triassic, Iran has been located in the northern hemisphere close to the Eurasian margin. This northward drift brought Iran to cover much of the Palaeotethys in approximately 35 Ma, at an average plate speed of c. 7–8 cm year−1, and was in part coeval to the transformation of Pangaea from an Irvingian B to a Wegenerian A-type configuration.
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