Fabrizio Berra

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 Tethys Himalayan passive margin from Late Triassic to Early Cretaceous (South Tibet)

The Mesozoic succession of South Tibet, both in lithologies and stratigraphic thicknesses, compares much more closely with that of central Nepal than has been reported in the literature. Facies distribution, from relatively proximal environments in the south to more distal settings in the north, reflects the paleogeography of the Tethys Himalaya passive margin, representing the southern edge of Neotethys. As in central Nepal and NW Himalaya, accumulation rates increased greatly in the latest Triassic, when very thick shelfal siliciclastics and locally volcaniclastics (Tarap Formation) were followed by coastal sandstones (Zhamure Formation). Contrary to reports in the literature, this latter unit reaches well into the Liassic at least in proximal southern sectors. The base of the overlying Kioto Group, yielding rich faunal associations characterized by benthic foraminifers and Lithiotis, similar to those of western Tethys carbonate platforms, is dated as middle Liassic (most probably late Pliensbachian). The monotonous growth of carbonate ramps during the middle Liassic to early Dogger was interrupted by two siliciclastic episodes related to major paleogeographic changes in the Toarcian (middle part of the Kioto Group) and in the Bajocian–Bathonian (Laptal Formation). An early Callovian flooding event (Ferruginous Oolite Formation)—widespread all along the Tethys Himalaya—was locally followed by deposition of a markedly lenticular pelagic unit, previously reported only from the Thakkhola Graben (Dangar Formation). The overlying Spiti Shale is reduced to only 20 m in southern sectors, where it is not younger than the Oxfordian, whereas in northern sectors the unit is several tens of m thick and mainly ascribed to the Tithonian. Volcaniclastic deposition (Wolong Formation)—reported herein for the first time—probably began as early as the Kimmeridgian/early Tithonian and lasted until Aptian times, when it was replaced by sedimentation of relatively deep-water marlstones. This magmatic episode, related to tectonic extension associated with the detachment of India from Gondwanaland, took place earlier in South Tibet and progressively later in central Nepal and Northern India; as in Nepal volcanic products changed with time from predominantly basaltic composition to intermediate and bimodal character. The Early Cretaceous sedimentary evolution of South Tibet matches that of offshore NW Australia.

The drift history of Iran from the Ordovician to the Triassic.

Abstract 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.

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.

Neogene block rotation in central Iran: Evidence from paleomagnetic data

Paleomagnetic results from Oligocene–Miocene sedimentary units in central Iran are used to reconstruct the history of Neogene tectonic deformation of this region. Paleomagnetic data show that in central Iran, crustal blocks bounded by sets of strike-slip faults are rotated to accommodate NNE-SSW shortening related to Arabia-Eurasia convergence. Counterclockwise rotations of 20°–35° have been measured in the Tabas and Anarak areas, south of the Great Kavir fault, characterized by the presence of N-S to NNW-SSE right-lateral strike-slip faults. Conversely, in the Great Kavir and Torud areas, where ENE-WSW left-lateral strike-slip faults have been recognized, paleomagnetic results are less conclusive because the small amount of measured clockwise rotation shows a statistically uncertainty, which also includes the possibility of no rotation. Some of these faults have been active during the Quaternary up to present day, suggesting the possibility that block rotation is still occurring in central Iran.

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.

Norian serpulid and microbial bioconstructions: Implication for the platform evolution in the Lombardy Basin (Southern Alps, Italy)

SummaryThe development of peculiar margin facies and abundant talus breccias within the Dolomia Principale inner platform is commonly observed in the Lombardy Basin during the Norian. The organisms building these margins are mainly serpulids, benthic microbes, subordinate porostomata and other encrusting forms; typical margin organisms, as sponges or corals, are extremely rare or absent. The build-ups form narrow rims along the borders of tectonic-controlled intraplatform basins. Regional back-stepping and progradation of the margin facies on the talus breccias produced by the erosion of the reef is commonly observed in the uppermost Dolomia Principale depositional system. Widespread occurrence of serpulids and microbial margins in middle-late Norian times is indicative of stressed environmental conditions—fluctuation of salinity and temperature on the inner platform and in the intraplatform basins—controlled by palaeogeographic setting. Physical characteristics allowed the bloom of forms able to develop in a wide range of environmental conditions, such as serpulids.In the Late Norian, major input of fine-grained clastics is recorded; close to the Norian-Rhaetian boundary, carbonate ramps were regionally restored. Locally, small serpulid and microbial bioconstructions still persist in the lowermost part of the shaly succession, even if they are less abundant with respect to the Dolomia Principale. Patch-reefs generally do not build a platform margin, but represent isolated mounds within shaly deposits. These build-ups occur on the edge of former structural highs; the communities survived the environmental change responsible for the siliciclastic input and locally managed to produce mounds during the deposition of the lower part of the upper depositional system (Riva di Solto Shale).

Sea-level fall, carbonate production, rainy days : how do they relate? Insight from Triassic carbonate platforms (Western Tethys, Southern Alps, Italy)

The interplay of sea-level fall, climate, and sedimentological changes is recorded across two sequence boundaries at the top of two Triassic carbonate platform systems in the Western Tethys (earliest Carnian and Norian-Rhaetian boundary in age, paleolatitude 18°–25° N). The sea-level falls caused subaerial exposure of the platform top and decreased carbonate production, leading to starvation in the intraplatform basins, followed by deposition of shale. Evidence of freshwater input indicates that the change in sedimentation was driven by increased rainfall on the previously arid European hinterland and Tethys coast. A uniformitarian approach (Holocene sea-level changes and global warming are coupled with changes in the distribution of precipitation) implicates global cooling as the probable cause of the observed sea-level, climate, and sedimentological changes. Global cooling likely triggered the sea-level fall by increased storage of fresh water in continental settings and change in seawater density, probably coupled with ephemeral ice sheet development, possible even during greenhouse intervals such as the Triassic. Furthermore, global cooling caused a shift toward the equator of the poleward boundary of the arid belt. This model is supported by the traceability of the sequence boundaries and climate-sensitive facies from the Tethys shelf up to the European continent. The observed association of global climate changes, sea-level fall, sedimentological changes, and shift toward humid climate documents how climate-sensitive facies record the control exerted by global changes on local sedimentation.

U–Pb zircon geochronology of volcanic deposits from the Permian basin of the Orobic Alps (Southern Alps, Lombardy): chronostratigraphic and geological implications

U–Pb zircon ages from volcanic rocks of Early Permian age (Southern Alps, Lombardy), associated with fault-controlled transtensional continental basins, were determined with the laser ablation (LA)-ICP-MS technique. Four samples were collected at the base and at the top of the up to 1000 m thick volcaniclastic unit of the Cabianca Volcanite. This unit pre-dates the development of a sedimentary succession that still contains, at different stratigraphic levels, volcanic intercalations. Age results from a tuff in the basal part of the unit constrain the onset of the volcanic activity to 280 ± 2.5 Ma. Ignimbritic samples from the upper part of the unit show a large scatter in the age distribution. This is interpreted as the occurrence of antecrystic and autocrystic zircons. The youngest autocrystic zircons ( c . 270 Ma) are thus interpreted as better constraining the eruption age, constraining the duration of the volcanic activity in the Orobic Basin to about 10 Ma. The new geochronological results compared with those of other Early Permian basins of the Southern Alps reveal important differences that may reflect (1) a real time-transgressive beginning and end of the volcanic activity or (2) the complex mixing of antecrystic and autocrystic zircon populations in the analysed samples.

Lower Permian brachiopods from Oman: Their potential as climatic proxies

The Lower Permian of the Haushi basin, Interior Oman (Al Khlata Formation to Saiwan Formation/lower Gharif member) records climate change from glaciation, through marine sedimentation in the Haushi sea, to subtropical desert. To investigate the palaeoclimatic evolution of the Haushi Sea we used O, C, and Sr isotopes from 31 brachiopod shells of eight species collected bed by bed within the type-section of the Saiwan Formation. We assessed diagenesis by scanning electron microscopy of ultrastructure, cathodoluminescence, and geochemistry, and rejected fifteen shells not meeting specific preservation criteria. Spiriferids and spiriferinids show better preservation of the fibrous secondary layer than do orthotetids and productids and are therefore more suitable for isotopic analysis. 18 Oo f3·7 to 3·1‰ from brachiopods at the base of the Saiwan Formation are probably related to glacial meltwater. Above this, an increase in 18 O may indicate ice accumulation elsewhere in Gondwana or more probably that the Haushi sea was an evaporating embayment of the Neotethys Ocean. 13 C varies little and is within the range of published data: its trend towards heavier values is consistent with increasing aridity and oligotrophy. Saiwan Sr isotope signatures are less radiogenic than those of the Sakmarian LOWESS seawater curve, which is based on extrapolation between few data points. In the scenario of evaporation in a restricted Haushi basin, the variation in Sr isotope composition may reflect a fluvial component.