Abstract The Upper Triassic–lower Middle Jurassic Shemshak Group is a siliciclastic unit, up to 4000 m in thickness, which is widespread across the Iran Plate of northern and central Iran. The group is sandwiched between two major unconformities: the contact with the underlying platform carbonates of the Elikah and Shotori formations is characterized by karstification and bauxite–laterite deposits; the top represents a sharp change from siliciclastic rocks to rocks of a Middle–Upper Jurassic carbonate platform–basin system. In the Alborz Mountains, the group consists of a Triassic and a Jurassic unit, separated by an unconformity, which is in part angular in the northern part of the mountain range and less conspicuous towards the south. Published lithostratigraphic schemes are based on insufficient biostratigraphic and lithological information. Here we present a new lithostratigraphic scheme for the central and eastern Alborz Mountains modified and enlarged from an unpublished report produced in 1976. Two major facies belts, a northern and a southern belt running more or less parallel to the strike of the mountain chain, can be distinguished. In the north, the Triassic part of the group is composed of the comparatively deep-marine Ekrasar Formation with the Galanderud Member (new name) at the base followed by the Laleband Formation, which represents prodelta–delta front environments. Up-section, the latter is replaced by the fluvial–lacustrine, coal-bearing Kalariz Formation. The equivalent Triassic lithostratigraphic unit in the south is the Shahmirzad Formation, redefined here, with the Parvar Member at the base. The formation represents fluvial, coastal plain and shallow- to marginal-marine environments. In the north, the Jurassic part of the group consists exclusively of the Javaherdeh Formation, coarse conglomerates of alluvial fan–braided river origin, which towards the south grades into the Alasht Formation, rocks of fluvial–lacustrine origin with coal. Further south, the Alasht Formation represents intertonguing marginal-marine–flood-plain environments and is followed by the Shirindasht Formation, sandstones and siltstones, indicative of the storm-dominated shelf, and the Fillzamin Formation (new), which is characterized by comparatively deep-marine shales. In the south, the group ends with the Dansirit Formation of deltaic–coastal-plain origin. This lithostratigraphic scheme reflects the tectono-sedimentary evolution of the Shemshak Foreland Basin of the Alborz Mountains where, during the Late Triassic, a relict marine basin in the north became gradually infilled, whereas in the south non-sedimentation and subaerial erosion prevailed and sediments record largely non-marine–marginal-marine conditions. During the early Lias, the basin was filled with erosional debris of the rising Cimmerian Mountain Chain, deposited largely in non-marine environments. During the early Middle Jurassic, in contrast, rapid subsidence in the south resulted in the deepening and subsequent infilling of a marine basin.

Abstract Bulk organic geochemical and microscopic studies (Rock-Eval pyrolysis, light transmitted–uv microscope) were carried out on the shales of the Upper Triassic–Middle Jurassic Shemshak Group in the northern, central and southern Alborz Range of northern Iran. Total organic carbon (TOC) values range from 0 to 29.4 wt% (1.2 wt% on average) indicating a generally poor–moderate organic carbon content. Upper Triassic shales in the lower part of the Shemshak Group have been mainly deposited in marine/lake settings under dysoxic–anoxic conditions, with TOC=0.7 wt% on average. Toarcian–Aalenian shales in the upper part of the Shemshak Group were deposited under comparatively deeper marine oxic–dysoxic conditions with the lowest TOC contents recorded (0.3 wt% on average). Carbonaceous shales at different stratigraphic levels of the Shemshak Group show the highest TOC contents (14.2 wt% on average). T max values range from 439 to 599 °C (average 500 °C), indicating that the organic matter has experienced high temperatures during deep burial and active post-sedimentary tectonics. The hydrogen index (HI)– T max diagram shows the presence of Type IV kerogen of altered organic matter with a very low mean HI value. The palynofacies is characterized by the dominance of amorphous organic matter probably predominately derived from degradation of marine–non-marine phytoplankton. The Upper Shemshak Group has low potential to produce petroleum, whereas the Lower Shemshak Group is an important effective petroleum source rock in the Alborz Range. The latter may have generated a considerable amount of petroleum at some localities (e.g. Tazareh and Paland) in the geological past.

Abstract The Lower–lower Middle Jurassic non-marine sedimentary succession of the Binalud Mountains of NE Iran is correlated with the Jurassic part of the Shemshak Group of the Alborz Mountains and subdivided into three formations: the Arefi, the Bazehowz and the Aghounj formations. The succession rests, with angular unconformity, on a metamorphic basement deformed during the Late Triassic Eo-Cimmerian orogeny. The lowermost unit, the Arefi Formation, is subdivided into a lower Derekhtoot Member and an upper Kurtian Member. The Derekhtoot Member (up to 750 m thick) consists of very coarse-grained, chaotic boulder beds, breccias and conglomerates representing rock-fall deposits and proximal–middle alluvial fans, deposited along steep fault scarps. The succeeding Kurtian Member (>300 m) comprises finer-grained conglomerates with well-rounded clasts, reflecting deposition in a proximal braided river system. The overlying Bazehowz Formation is more than 1000 m thick and consists of vertically stacked, decametre-scale channel-fill cycles of the middle reaches of a braided fluvial system. The uppermost unit, the Aghounj Formation, consists of at least 400 m of granule- to pebble-size, thick-bedded and large-scale trough cross-bedded quartz conglomerates and sandy interbeds of a proximal braided fluvial system. The overall succession fines upwards due to erosion, down to metamorphic basement, of a high-relief source area in the NE, and rests on Cimmerian basement, suggesting that the strata are intramontane deposits of the Cimmerian mountain chain in NE Iran. This interpretation has important implications concerning the position of the NW–SE-trending Eo-Cimmerian suture in NE Iran, which should be placed further SW than formerly assumed.

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.

Abstract The Tabas Block of east-central Iran shows very thick and well-exposed Upper Triassic–Jurassic sequences, which are crucial for the understanding of the Mesozoic evolution of the Iran Plate. The succession is subdivided into major tectonostratigraphic units based on widespread unconformities related to the Cimmerian tectonic events. As elsewhere in Iran, there is a dramatic change from Middle Triassic platform carbonates (Shotori Formation) to the siliciclastic rocks of the Shemshak Group (Norian–Bajocian), reflecting the onset of Eo-Cimmerian deformation in northern Iran. Following the marine sedimentation of the Norian–Rhaetian Nayband Formation, the change to non-marine, coal-bearing siliciclastic rocks (Ab-e-Haji Formation) around the Triassic–Jurassic boundary is related to the main uplift phase of the Cimmerian orogeny. Condensed limestones of the Toarcian–Aalenian Badamu Formation indicate widespread transgression, followed by rapid lateral facies and thickness variations in the succeeding Lower Bajocian Hojedk Formation. This tectonic instability culminated in the middle Bajocian compressional–extensional Mid-Cimmerian event. The resulting Mid-Cimmerian unconformity separates the Shemshak Group from the Upper Bajocian–Upper Jurassic Magu (or Bidou) Group. The succeeding Late Bajocian–Bathonian onlap of the Parvadeh and Baghamshah formations (Baghamshah Subgroup) was caused by increased subsidence of the Tabas Block rather than a eustatic sea-level rise, followed by the development of a large-scale platform–basin carbonate system (Callovian–Kimmeridgian Esfandiar Subgroup). Block faulting starting in the Kimmeridgian (Late Cimmerian event) resulted in the destruction of the carbonate system, which was covered by Kimmeridgian–Tithonian limestone conglomerates, red beds and evaporites (Garedu Subgroup or Ravar Formation). Virtually the same pattern of relative sea-level change, facies development and succession of geodynamic events is recorded from the Late Triassic–Jurassic of northern Iran (Alborz Mountains), suggesting that the Iran Plate behaved as a single structural unit at that time.

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.

Abstract The structurally and stratigraphically complex area of northern and central Iran holds the key to understanding the plate tectonic evolution of the South Caspian–Central Iran area. The closure of the Palaeotethys, the opening of the Neotethys, the rise and demise of the Cimmerian mountain chain, as well as the onset of Neotethys subduction and large-scale Neotethyan back-arc rifting all predated the formation of the more than 20 km-thick fill of the South Caspian Basin. This volume brings together work by specialists in different disciplines of the geosciences (tectonics, geophysics, sedimentology, stratigraphy, palaeontology, basin modelling and geodynamics) in order to elucidate the complex Late Palaeozoic–Cenozoic geodynamic history of the Iran area and the birth of the South Caspian Basin.

Abstract The Mid-Cimmerian tectonic event of Bajocian age can be documented all across the Iran Plate (Alborz Mountains of northern Iran, NE Iran, east-central Iran) and the southern Koppeh Dagh (northeastern Iran). In the Alborz area, the tectonic event consisted of two main pulses. A distinct unconformity (near the Lower–Upper Bajocian boundary) at or near the base of the Dansirit Formation is the sedimentary expression of rapid basin shallowing due to uplift and erosion. Another unconformity is developed in the early Upper Bajocian, close to or at the top of the Dansirit Formation. Locally, it is expressed as an angular unconformity due to block rotation and is overlain by a thin transgressive conglomerate followed by silty marls of the deep-marine Upper Bajocian–Callovian Dalichai Formation. This upper unconformity signals a rapid subsidence pulse. On the Tabas Block of east-central Iran, a single unconformity can be documented that is time-equivalent to those bounding the Dansirit Formation (i.e. ‘mid-Bajocian’). Local folding gives direct evidence of compressional tectonics, and conglomerates indicate subaerial denudation of older Mesozoic or Palaeozoic strata. After a stratigraphic gap, transgressive sediments of ?Late Bajocian–Bathonian age follow, suggesting a fusion of the lower and upper Mid-Cimmerian unconformities in east-central Iran. Along the southern margin of the Koppeh Dagh Mountains (NE Iran), a Late Bajocian subsidence pulse initiated the opening of the strongly subsiding Kashafrud Basin, an eastwards extension of the South Caspian Basin. In all of these areas, one phase of uplift and erosion took place followed by a pronounced pulse of subsidence running counter to trends of the eustatic sea-level curve. Thus, what is generally understood as the Mid-Cimmerian tectonic event is now thought to consist of a tectonic phase, confined to the Bajocian. This phase is explained as the expression of the onset of sea-floor spreading within the South Caspian Basin situated to the north of the present-day Alborz Mountains. This strongly subsiding basin developed close to the Palaeotethys suture during the Toarcian–Aalenian and went through a change from the rifting- to the spreading-stage during the Bajocian. The Mid-Cimmerian event therefore reflects the break-up unconformity of the South Caspian Basin.