Stefano Zanchetta

The Eo-Cimmerian (Late? Triassic) orogeny in North Iran

Abstract 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.

The Cimmerian evolution of the Nakhlak–Anarak area, Central Iran, and its bearing for the reconstruction of the history of the Eurasian margin

Abstract 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 20°N. 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 greenschist-facies ‘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.

The Shanderman eclogites: a Late Carboniferous high-pressure event in the NW Talesh Mountains (NW Iran)

Abstract The Shanderman Metamorphic Complex, exposed along the Caspian foothills of the Talesh Mountain, western Alborz, Iran, has always been interpreted as an ophiolitic fragment of the Palaeotethys Ocean. According to our new data, this unit consists of metamorphic rocks mainly represented by garnet–staurolite micaschists with large bodies of metabasites containing well-preserved eclogitic-phase assemblages. The Shanderman Complex (SC) was later intruded at middle crustal levels by intermediate–basic intrusive bodies. New Ar/Ar ages of paragonitic white micas in equilibrium with the high-pressure assemblages have given a Late Carboniferous age (315±9 Ma). Our new data suggest that the SC was equilibrated in high-pressure conditions during an orogenic event that predates the Eo-Cimmerian orogeny by more than 100 Ma and that may be tentatively ascribed to the Variscan orogeny sensu latu. We suggest that the Shanderman Complex represents a fragment of the Upper Palaeozoic European continental crust. The occurrence of eclogites in these regions can be explained by two different hypotheses: (1) the SC high-pressure rocks can be related to the accretion of Gondwana-related Transcauscasian–Moesian microplate to the southern margin of Eurasia; or (2) the SC eclogites can represent a fragment of the Late Palaeozoic ‘Variscan belt’ sensu latu of central Europe, which has been translated eastwards during Permian along a dextral megashear zone taking from a Pangea-B to a Pangea-A plate configuration. This metamorphic unit was stacked southwards on the northern edge of the Iran Plate during the Eo-Cimmerian events occurring at the end of the Triassic. The eclogite-bearing basement of the SC was finally exhumed at the end of the Eo-Cimmerian orogeny, as suggested by the composition of the basal layers of the Shemshak Group dated here Middle Jurassic, that cover the crystalline rocks of the SC along a regional non-conformity. The SC was probably displaced further southwards during the Mesozoic opening of the South Caspian Basin and the Tertiary thrust stacking and dextral shearing accompanying the formation of the Alborz intracontinental belt.

The Triassic stratigraphic succession of Nakhlak (Central Iran), a record from an active margin

Abstract An important, 2.4 km-thick Triassic succession is exposed at Nakhlak (central Iran). This succession was deformed during the Cimmerian orogeny and truncated by an angular unconformity with undeformed Upper Cretaceous sediments. This integrated stratigraphic study of the Triassic included bed-by-bed sampling for ammonoids, conodonts and bivalves, as well as limestone and sandstone petrographic analyses. The Nakhlak Group succession consists of three formations: Alam (Olenekian–Anisian), Bāqoroq (?Upper Anisian–Ladinian) and Ashin (Upper Ladinian). The Alam Formation records several shifts from carbonate to siliciclastic deposition, the Bāqoroq Formation consists of continental conglomerates and the Ashin Formation documents the transition to deep-sea turbiditic sedimentation. Petrographic composition has been studied for sandstones and conglomerates. Provenance analysis for Alam and most of the Ashin samples suggests a volcanic arc setting, whereas the samples from the Bāqoroq Formation are related to exhumation of a metamorphic basement. The provenance data, together with the great thickness, the sudden change of facies, the abundance of volcaniclastic supply, the relatively common occurrence of tuffitic layers and the orogenic calc-alkaline affinity of the volcanism, point to sedimentation along an active margin in a forearc setting. A comparison between the Triassic of Nakhlak and the Triassic succession exposed in the erosional window of Aghdarband (Koppeh Dag, NE Iran) indicates that both were deposited along active margins. However, they do not show the same type of evolution. Nakhlak and Aghdarband have quite different ammonoid faunal affinities during the Early Triassic, but similar faunal composition from the Bithynian to Late Ladinian. These results argue against the location of Nakhlak close to Aghdarband.

Geological-structural map of the Orobic and Porcile thrust junction, central Southern Alps (N Italy)

The central Southern Alps retrobelt was built trough a polyphase tectonic evolution that developed in the hangingwall of the SE-directed Alpine Tethys subduction. In the northern sector of the central Southern Alps the earliest stages of shortening resulted in the inversion of structures inherited from the Permian and Triassic rifting phases, together with southward thrusting and large-scale folding. In the San Marco Pass area (N Italy) the Variscan basement overrides the Permian-Triassic sedimentary cover along regional thrusts. Here the E-W trending Orobic Thrust joins the NE-SE Porcile Thrust. Initial thrusting phases formed greenschist facies mylonites in the basement and S-verging folds in the sedimentary covers. The Porcile line, a feature likely inherited from the Permian extension, was re-activated as a steep mylonite shear zone. Fault activity continued at brittle conditions: cataclastic shear zones overprinted the mylonites. During this phase, shortening was preferentially accommodated along the Orobic Thrust. A final transpressive re-activation observed along both thrusts is linked to the Oligocene activity of the Insubric Fault.

Contrasting subduction–exhumation paths in the blueschists of the Anarak Metamorphic Complex (Central Iran)

The Anarak Metamorphic Complex, localized in Central Iran, is a fossil accretionary wedge composed of several tectonometamorphic units. Some of these, the Chah Gorbeh, the Morghab and the Ophiolitic complexes, contain mafic rocks that have been metamorphosed at high-pressure–low-temperature conditions. Such units have been stacked together and later refolded during the final stages of exhumation. Structural analysis at the mesoscale recognized at least three deformation events. Microstructural analyses, mineral chemistry and thermodynamic modelling reveal that the mafic schists followed contrasting P–T paths during their tectonometamorphic evolutions. In the schists of the Chah Gorbeh and Ophiolitic complexes an early greenschist-facies stage was later overprinted by blueschist-facies phase assemblages with suggested peak conditions of 390–440°C at 0.6–0.9 GPa for the meta-basalt within the Ophiolitic Complex and 320–380°C at 0.6–0.9 GPa for the blueschists of the Chah Gorbeh Complex. P–T conditions at metamorphic peak were 410–450°C at 0.78–0.9 GPa for the Morghab blueschists, but they are reached before a greenschist-facies re-equilibration. Compositional zoning of amphiboles and epidotes of this greenschist-facies stage suggests a renewed pressure increase at the end of this metamorphic stage. Based on these data we reconstructed a clockwise P–T path for the Morghab mafic schists and a counter-clockwise path for the Chah Gorbeh blueschists and ophiolitic meta-basalts. Such contrasting metamorphic evolutions of tectonic units that were later accreted to the same wedge are indicative of the complex tectonic dynamics that occur within accretionary–subduction complexes.