Geochemistry (00092819)83(3)
Studies of relict Upper Cretaceous obducted ophiolites provide information on the evolution of the Neotethys suture belts in central Iran. Studies of the sediment derived from their erosion will complement this information by addressing questions related to sandstone provenance, ophiolite emplacement history, and, perhaps most importantly, the mode and timing of their erosion. The ca. 3200 m thick Middle Eocene to lowermost Oligocene fine- to medium-grained siliciclastic beds of the Akhoreh Formation are superbly exposed along the western border of the Central-East Iranian Microcontinent. They cover the northeastern flank of the Cretaceous Nain ophiolite mélange massif and lie adjacent to the Paleogene Urmieh–Dokhtar Magmatic Arc. The common occurrence of chrome (Cr)-spinel in the sandstones indicates the presence of mafic to ultramafic rocks in the source region. However, the timing, extent and nature of contributions of the potential source rocks to the sedimentary succession are not well understood. We conducted a petrographic and chemical compositional study of Cr-spinel in samples of Akhoreh Formation sandstones to outline the type of ocean lithospheric source. Our results document variable Cr# (0.09 to 0.79) and Mg# (0.21 to 0.88) while Fe3+ and TiO2 (average 0.12 wt%) are generally low. The wide range of Cr# and Mg# indicates provenance from a variety of (ultra-)mafic rock types, including supra-subduction peridotites and partially volcanic rocks of an arc-forearc setting. Cr-spinels are chemically similar to those of the lherzolites, harzburgites and dunites of the Nain ophiolite. While 70 % of the Cr-spinel have Cr# > 0.6, suggesting a harzburgitic peridotite provenance, the minimum Cr# < 0.4, observed in the samples from the base of the succession, were likely sourced from lherzolite. The chemical composition of detrital clinopyroxene grains shows high SiO2 (52.76 wt%), CaO (20.15 wt%), and MgO (16.36 wt%), while FeO has moderate (6.03 wt%), Al2O3 has low (2.23 wt%), and TiO2 has very low (0.25 wt%) concentrations (commonly augite and diopside), indicative of crystallization from subalkaline magma in an arc setting. The provenance data trace the erosion history of the obducted ophiolite and indicate that the Nain supra-subduction ophiolite supplied detritus to the Akhoreh Basin northwards from the Middle Eocene to the earliest Oligocene. © 2023 Elsevier GmbH
Salehi, M.A.,
Wilmsen, M.,
Zamanian, E.,
Baniasad, A.,
Heubeck, C. Geological Magazine (14695081)160(2)pp. 235-259
During the early Bajocian, a conspicuous coal-bearing siliciclastic succession was deposited in the northern Tabas Bock, which is important for understanding the regional geodynamics of the Central-East Iranian Microcontinent (CEIM) as well as for the Jurassic coal genesis in this part of Laurasia. Sedimentary facies analysis in a well-exposed section of the lower Bajocian Hojedk Formation (Kalshaneh area, northern Tabas Block) led to the recognition of ten characteristic sedimentary facies and three facies associations, representing channels with point bars and floodplains of a Bajocian meandering river system. Modal analysis indicates that the mature quartz arenites and quartzo-lithic sandstones of the Hojedk Formation originated from the erosion and recycling of older, supracrustal sedimentary rocks on the Yazd Block to the west. The coal petrography and maturity show an advanced maturation stage, whereas the great thickness of these continental strata points to a pronounced extension-related subsidence in the northern Tabas Block. The rapid rate of differential subsidence can be explained by accelerated normal block-faulting in the back-arc extensional basin of the CEIM, facing the Neotethys to the south. Compared to the thick Jurassic, the post-Jurassic strata are relatively thin and played a limited role in the thermal history of the coal in the northern Tabas Block. A relatively high geothermal gradient in the tectonically highly mobile area of the northern Tabas Block and/or heating by regionally widespread Palaeogene intrusions were most probably the key drivers of the thermal maturation of the Middle Jurassic coals. © The Author(s), 2022. Published by Cambridge University Press.
Geological Quarterly (16417291)67(2)
The Middle Eocene Akhoreh Formation is superbly exposed in the western corner of the Central-East Iranian Microcontinent (CEIM). This formation covered the northeastern flank of the Cretaceous Nain Ophiolite Mélange (NOM) and is adjacent to the Paleogene Urmieh–Dokhtar Magmatic Arc (UDMA) formed in the southwest of the CEIM. This terrigenous succession is composed of a thin basal conglomerate followed by mostly pink to purple sandstones alternating with shales. The clast composition and clast imbrication of the conglomerates show local source areas towards the north-north-east. Modal components of lower Akhoreh Formation sandstones reveals immature lithic arkose (Q8F48L44) and feldspathic litharenite (Q8F44L48) sandstones that are rich in mafic and ultramafic igneous and volcanic rock fragments. Mafic to ultramafic source rocks are also indicated by geochemical data (enrichment of Mg, Cr and Ni and Cr/V) in the sandstone and shale samples analyzed. However, geochemical data suggests an intermediate igneous rock origin for the shale samples studied, most likely from the nearby continental arc. Based on petrographic data, these sandstones have characteristics of a transitional to undissected arc tectonic setting. Geochemical discrimination diagrams using major and trace elements indicate an oceanic island arc tectonic setting for the lower Akhoreh Formation sandstones and shales, probably due to a predominance of ophiolitic source rocks. Furthermore, the chemical index of alteration and modal analysis indicate a weak to moderate degree of chemical weathering with arid climatic conditions in the source area. The exhumed NOM, together with the UDMA in the southwest, were dominant sources of sediment to the lower Akhoreh Formation, that lay to the north-east in a local retroarc basin of the Central Iranian Microplate, during the Middle Eocene. © 2023, Polish Geological Institute. All rights reserved.
Journal of Structural Geology (01918141)160
The Central Iranian Micro-plate (CIM) is a dismembered piece of northern Gondwana. The aim of this study is to reconstruct the post-Early Cretaceous structural evolution of the western edge of CIM in the light of the integration of regional to the micro-scale structural data with minor Anisotropy of Magnetic Susceptibility (AMS) analyses. Our original field measurements on the structural architecture of the study area show main NW-SE and E-W structural trends that are accompanied by structural evidence for superposition. However, paleostresses obtained from fault and fold analysis (stress inversion method on faults and statistically π-plane and β-axis solution on folds), statistical Fry center-to-center analysis on the oriented thin-sections integrated with AMS results suggest that the study area has experienced a NE-SW-directed compressional regime since Paleocene time followed by a post-Early Miocene, roughly N–S-directed, regional compressional regime. Furthermore, the results of this work confirm the consistency between regional-micro structural analysis and AMS analysis. The most of samples show composite (sedimentary + tectonic) magnetic fabric and intermediate arrangement in the orientation of the magnetic fabric. Reconciling our results with published structural and AMS data suggests the changes in the regional stress regime in the western CIM has been occurred in response to the long-term stress transition from the infant Late Cretaceous–Paleogene subduction of the Neo-Tethys Ocean to the mature Cenozoic stages of the Zagros collision and the consequent Neogene tectonic reorganization in the hinterland domains of the southern Eurasian plate. © 2022 Elsevier Ltd
International Geology Review (00206814)62(11)pp. 1359-1386
The Early Cretaceous was an important epoch in the evolution of the Earth system in which major tectonic episodes occurred, especially along the Alpine–Himalayan belt. The paucity of reliable palaeogeographic data from the central segment of this geological puzzle, however, hampers the reconstruction of a panoramic view of its Early Cretaceous palaeogeography and geodynamic setting. Here we present multidisciplinary provenance data from Lower Cretaceous strata of the overriding plate of the Neo-Tethyan subduction zone (the Sanandaj–Sirjan Zone; SSZ, of central Iran), including structural, basin-fill evolution, petrographic and geochemical analyses. Sandstone provenance analysis of Lower Cretaceous red beds suggests the occurrence of sub-mature litho-quartzose sandstones attributed to an active continental arc margin in convergent setting predominantly derived from plutonic, quartzose sedimentary and metamorphic rocks exposed in the central SSZ. Weathering indices indicate moderate chemical weathering in the source area which may be related to close source-to-sink relationships or arid climate. Our palaeogeographic reconstructions and original geological mapping indicate that the erosion of uplifted basement rocks exposed in horst blocks provided the sediment sources for the syn-extensional deposition of uppermost Jurassic–lowermost Cretaceous conglomerates and Lower Cretaceous siliciclastic red beds within a continental retro-arc basin during initiation of the ‘Neo-Tethys 2ʹ. The polyphase tectonic reactivation along the principal fault of the study area controlled the syn- and post-extensional tectonostratigraphic evolution that reflect the corresponding mechanical decoupling/coupling along the northern Neo-Tethyan plate margin. © 2019, © 2019 Informa UK Limited, trading as Taylor & Francis Group.
Salehi, M.A.,
Bahrami, A.,
Moharrami, S.,
Vaziri moghaddam, H.,
Pakzad, H.R.,
Shakeri, B. Journal of African Earth Sciences (1464343X)171
The predominantly warm greenhouse climate and high global sea level during the Middle–Late Devonian resulted in fully marine conditions along the northern Gondwana margin, including the Iranian Plate. Over three-hundred meter-thick fossiliferous Middle-to Late Devonian carbonate deposits of the Bahram Formation in the Anarak region (western Central Iran) are composed of dark grey limestone, dolostone, and interbedded shale. Seventeen microfacies have been distinguished, which correspond to four depositional sub-environments, i.e., tidal flat, lagoon, shoal, and open marine. The Middle–Late Devonian carbonate deposits were deposited on a homoclinal carbonate ramp. Seven third-order depositional sequences are identified, apparently controlled by relative sea level change. Carbon and oxygen stable isotopes, and major and trace elements of micritic carbonate samples document details about the geochemical, palaeoenvironmental, and diagenetic processes; they also indicate that the carbonates underwent meteoric diagenesis within a semi-closed diagenetic system. The low concentrations of trace elements indicative of euxinic conditions, including Mo, Cr, V, and As, suggest well-oxygenated depositional environment. Carbon isotope values show a meaningful relationship to major sequence-stratigraphic key surfaces, with more negative values occurring at the sequence boundaries and positive values corresponding to maximum flooding surfaces. © 2020 Elsevier Ltd
Pourdivanbeigi moghaddam, S.,
Salehi, M.A.,
Jafarzadeh, M.,
Zohdi, A. Journal of African Earth Sciences (1464343X)161
Etesampour, A.,
Mahboubi, A.,
Moussavi-harami, R.,
Arzani, N.,
Salehi, M.A. Austrian Journal of Earth Sciences (02517493)112(1)pp. 20-41
The Upper Triassic (Norian–Rhaetian) Nayband Formation is situated at the southwestern margin of Central East Iranian Microcontinent and records Eo-Cimmerian events. The formation is composed of mixed carbonate-siliciclastic deposits. This study presents information on the tectonic reconstruction and palaeoclimate of the southwestern margin of Central East Iranian Microcontinent during the Late Triassic. Petrography and modal analyses of sandstones show a variety of quartz-rich petrofacies including subarkose, lithic arkose, sublitharenite, feldspathic litharenite and litharenite. The combined modal analysis and geochemical results of major and trace elements of the sandstone samples represents mixed sedimentary, intermediate, felsic igneous rocks and moderate-to high-grade metamorphic provenance areas. The major elements and modal analyses of the Nayband Formation sandstone samples suggest an active continental margin tectonic settings. The palaeoclimatic conditions were sub-humid to humid with relatively low to moderate weathering in the source area which is in agreement with the palaeogeography and palaeotectonic history of southwestern margin of Central East Iranian Microcontinent during the Late Triassic. © 2019 Asghar ETESAMPOUR et al.
Salehi, M.A.,
Moussavi-harami, R.,
Mahboubi, A.,
Fürsich, F.T.,
Wilmsen, M.,
Heubeck, C. Swiss Journal of Geosciences (16618734)111(1-2)pp. 51-78
The Lower Jurassic Ab-Haji Formation consists of siliciclastic strata which are widespread and superbly exposed across the Tabas and Lut blocks of east-central Iran. The formation records the geodynamic history of central Iran during the Early Jurassic in the aftermath of the main Cimmerian event (near the Triassic–Jurassic boundary) through its sedimentary facies and stratigraphic architecture and allows palaeogeographic and palaeoenvironmental reconstructions. We measured and studied three well-exposed outcrop sections and identified lithofacies and facies associations (fluvial plain, delta plain, delta front, prodelta, and shallow-marine siliciclastic shelf). The integration of all geological, stratigraphic, and sedimentological data shows a west-to-east continental-to-marine gradient within the Ab-Haji Formation. Based on thickness variations, lateral facies changes, palaeocurrent patterns, and changes in the nature of the basal contact of the Ab-Haji Formation on the Tabas and Lut blocks, we locate the fault-bounded Yazd Block in the west and the Shotori Swell at the eastern edge of the Tabas Block as provenance regions. The pattern of thickness variations, rapid east–west facies changes, and provenance is best explained by a tectonic model invoking large tilted fault blocks in an extensional basin. The basal unit shows distinct increase in grain size at the base of the Ab-Haji Formation, similar to the Shemshak Group of the Alborz Mountains (the base of the Alasht Formation) and the non-marine time-equivalent succession of the Binalud Mountains of northeastern Iran. This grain size pattern may have been caused by rapid source area uplift due to slab break-off of the subducted Iran plate in the course of the Cimmerian collision in east-central Iran. © 2017, Swiss Geological Society.
Salehi, M.A.,
Sebdani, Z.M.,
Pakzad, H.R.,
Bahrami, A.,
Fürsich, F.T.,
Heubeck, C. Neues Jahrbuch fur Geologie und Palaontologie - Abhandlungen (00777749)288(1)pp. 49-77
The “Jurassic Gap” between Upper Triassic and Lower Cretaceous siliciclastic strata at the southwestern margin of the Central East Iranian Microcontinent (CEIM) represents the Cimmerian orogenic events. However, the provenance of siliciclastic rocks above and below this prominent unconformity is poorly known, negatively affecting our ability to reconstruct paleogeography and orogenic dynamics. Here, we analyze the provenance of the uppermost Triassic Qadir Member of the Nayband Formation and the red beds of the Lower Cretaceous siliciclastic succession in central Iran, based on conglomerate clast composition, sandstone modal analysis and sandstone geochemistry. Based on the notable compositional variation, we assign the Qadir Member to a transitional continental-tectonic setting and the red beds of the Lower Cretaceous siliciclastic rocks to a recycled-orogen setting. Geochemical analyses shows that the parent rocks of the Qadir Member were intermediate igneous, felsic and metamorphic rocks; and were sedimentary quartz, felsic igneous and metamorphic rocks for the Lower Cretaceous red beds. The modal analysis also indicates semi-humid weathering conditions in the Late Triassic but more humid conditions during the Lower Cretaceous which the strata were largely influenced by the deposition of recycled Jurassic sediments. We infer the Sanandaj–Sirjan Zone (SSZ) to the south to be the main provenance region for both stratal packages. © 2018 E. Schweizerbart’sche Verlagsbuchhandlung, Stuttgart, Germany.
Salehi, M.A.,
Moussavi-harami, R.,
Mahboubi, A.,
Wilmsen, M.,
Heubeck, C. Neues Jahrbuch fur Geologie und Palaontologie - Abhandlungen (00777749)271(1)pp. 21-48
The Lower Jurassic Ab-Haji Formation which is widespread across east-central Iran but best exposed on the Tabas Block consists of sandstones, siltstones and mudstones deposited in fluviodeltaic and marginal marine settings. Provenance studies of this formation record the geodynamic history of Central Iran during and in the aftermath of the main Cimmerian event (Triassic-Jurassic boundary to Liassic) and are crucial for the reconstruction of the tectonic setting and palaeogeography of this region. Sandstone provenance analysis of six selected outcrop sections using modal analysis suggests that the predominantly quartzose and quartzolithic sandstones of the Ab-Haji Formation were largely derived from plutonic, quartzose sedimentary and low-grade metamorphic rocks, attributed to a recycled-orogen and cratonic sources. Sandstone and shale geochemistry suggests that most major and trace element patterns are near upper continental crust values, indicating felsic and (meta-) sedimentary sources. Key elemental ratios are consistent with a mixed recycled source under humid climatic conditions and moderate to intensive chemical weathering. Weathering indices indicate that recycling processes and/or humid climatic conditions contributed to the high compositional maturity of the sandstones such that they plot in craton-interior and passive-margin fields on tectonic QtFL and QmFLt ternary diagrams. Paleogeographic reconstructions and geologic mapping indicate that the erosion of uplifted pre-Jurassic rocks exposed in tilted fault blocks within an extensional continental retro-arc basin provided the principal sources. Strata of the Ab-Haji Formation in the western and southern Tabas Block were predominantly derived from low-grade metamorphic and sedimentary rocks exposed on the adjacent Yazd Block, while those in the eastern Tabas and western Lut blocks were derived from the erosion of mature sedimentary strata of the uplifted Shotori Swell, i.e., from the eastern margin of the tilted Tabas Block.©2014 E.Schweizerbartsche Verlagsbuchhandlung, Stuttgart,Germany.
Journal of Asian Earth Sciences (18785786)39(3)pp. 148-160
The Fahliyan Formation is a carbonate sequence of Lower Cretaceous (Berriasian-Hauterivian) age and was deposited in the Zagros sedimentary basin, Iran. In this investigation, the Fahliyan Formation at the type section and in the subsurface has been studied. Facies analysis and petrographic studies led to the recognition of 10 microfacies that were deposited in three facies belts: lagoon, shoal and open marine. The observed facies patterns indicate a carbonate rimmed-shelf depositional environment. Based on field observations, microfacies analysis and sequence stratigraphic concepts, two-third-order sequences in the type section and three-third-order sequences in the subsurface section were identified. The transgressive deposits display a predominance of deep subtidal facies, while highstand deposits show shallow subtidal facies. Some petrographic evidence such as an abundance of aragonite skeletal and non-skeletal components shows that this formation was deposited in a sub-tropical environment with original aragonite mineralogy. Geochemical evidence such as high Sr/Na ratios also support original aragonite mineralogy. δ18O and δ13C values suggests that alteration occurred during burial diagenesis, in a closed system, with low water/rock interaction. Palaeotemperature calculation, based on the heaviest oxygen isotope value in micritic samples of the Fahliyan limestone, shows that ambient water temperature was around 24°C during the deposition of this formation. © 2010 Elsevier Ltd.
