Southern Margin Of Eurasia Research Articles

Multiple Exhumation Phases in the Central Pontides (N Turkey): New Temporal Constraints on Major Geodynamic Changes Associated With the Closure of the Neo‐Tethys Ocean

AbstractThe Central Pontides of N Turkey represents a mobile orogenic belt of the southern Eurasian margin that experienced several phases of exhumation associated with the consumption of different branches of the Neo‐Tethys Ocean and the amalgamation of continental domains. Our new low‐temperature thermochronology data help to constrain the timing of these episodes, providing new insights into associated geodynamic processes. In particular, our data suggest that exhumation occurred at (1) ~110 to 90 Ma, most likely during tectonic accretion and exhumation of metamorphic rocks from the subduction zone; (2) from ~60 to 40 Ma, during the collision of the Kirşehir and Anatolide‐Tauride microcontinental domains with the Eurasian margin; (3) from ~40 to 25 Ma, either during the early stages of the Arabia‐Eurasia collision (soft collision) when the Arabian passive margin reached the trench, implying 70 to 530 km of subduction of the Arabian passive margin, or during a phase of trench advance predating hard collision at ~20 Ma; and (4) ~11 Ma to the present, during transpression associated with the westward motion of Anatolia. Our findings document the punctuated nature of fault‐related exhumation, with episodes of fast cooling followed by periods of slow cooling or subsidence, the role of inverted normal faults in controlling the Paleogene exhumation pattern, and of the North Anatolian Fault in dictating the most recent pattern of exhumation.

Evidence from detrital chrome spinel chemistry for a Paleo-Tethyan intra-oceanic island-arc provenance recorded in Triassic sandstones of the Nakhlak Group, Central Iran

Detrital chrome spinel (Cr-spinel) is for the first time described from the Nakhlak Group, which is a distinctive Triassic (Late Olenekian‒?Early Carnian) sedimentary succession in Central Iran. Previously published data from the Nakhlak Group suggest deposition along an active margin during the Triassic and Eurasian affinity. Presently, the Nakhlak Group is located in the northwestern region of the Central-East Iranian Microcontinent, a major segment of the Cimmerian block. The Nakhlak region is believed to have been dislocated from its prior position at the Turan Plate from the southern Eurasian active margin after deposition in Triassic time. Almost all of the analyzed detrital Cr-spinels have high Cr-numbers (0.6–0.85), variable Mg-numbers (0.33–0.78), and low Fe3+ (<0.25), Al2O3 (<20 wt%) and TiO2 (<0.4 wt%) contents, suggesting a magmatic source for the Cr-spinels formed both within oceanic (mainly harzburgitic) mantle and in a supra-subduction zone (SSZ) tectonic setting. The data suggest that mafic‒ultramafic rock assemblages with SSZ signatures were generated in the Paleo-Tethyan realm before their obduction as an ophiolite. By comparing data from the present study with results from previous work on the Triassic in Central Iran it is evident that Paleo-Tethyan ophiolites of intra-oceanic island-arc (IOIA) origin supplied detrital material to the Nakhlak Group at the southern margin of Eurasia during pre-to syn-Eocimmerian tectonics in the Triassic.

Cooling, exhumation, and deformation in the Hindu Kush, NW Pakistan: New constraints from preliminary 40Ar/39Ar and fission track analyses

Asian crust in the Hindu Kush region in northern Pakistan records a protracted history of rifting, subduction and collision not commonly preserved within the Himalaya. Because of this, it is key to understanding the development of the southern Eurasian margin both prior to and after collision with India. New mica 40Ar/39Ar and apatite fission track geochronologic data from this region provide constraints on the kinematics of the Hindu Kush. 40Ar/39Ar muscovite and biotite ages from the late Cambrian Kafiristan pluton are 379.7 ± 1.7 Ma and 47.2 ± 0.3 Ma, respectively. The muscovite age may record cooling or partial resetting, while the biotite age is interpreted to record a thermal disruption associated with the early stages of continental collision in the Himalayan system. A 111.0 ± 0.6 Ma muscovite age from the northern part of the Tirich Mir pluton (∼123 Ma old; U-Pb) is interpreted to indicate a recrystallization event ∼12 Myrs after its intrusion. In addition, a younger muscovite age of 47.5 ± 0.2 Ma was derived from the opposite side of the same pluton in the immediate hanging wall of the Tirich Mir fault. This Eocene age is interpreted to represent the time of recrystallization during fault (re)activation in the early stages of India-Asia continent-continent collision. 40Ar/39Ar biotite analysis from the Buni-Zom pluton yields an age of 61.6 ± 1.1 Ma and is interpreted to reflect cooling at mid-upper crustal levels subsequent to the pluton’s emplacement in the middle Cretaceous. Finally, 17.1–21.3 Ma 40Ar/39Ar ages from the Garam Chasma pluton and surrounding metapelites indicate cooling immediately following crystallization of the leucogranite body in the earliest Miocene/latest Oligocene.The younger cooling history is resolved by fission track dating of apatite (AFT). In the vicinity of the bounding Tirich Mir fault, the Tirich Mir pluton yields an AFT age of 1.4 ± 0.3 Ma, which is consistent with active exhumation associated with the surface uplift of the 7700+ m Tirich Mir peak. The Garam Chasma pluton has a young age of 3.5 ± 0.2 Ma, which also records rapid rock uplift and exhumation in the area. Finally, an AFT age of 9.1 ± 2.1 Ma was extracted from a metapelite in the footwall of an east verging thrust fault separating it from the Garam Chasma pluton to the west. The difference in ages, Pliocene vs. late Miocene, reflect differential cooling/exhumation paths across that structure.

New zircon U-Pb LA-ICP-MS ages and Hf isotope data from the Central Pontides (Turkey): Geological and geodynamic constraints

The Central Pontides in northern Anatolia is located on the accretionary complex formed by the closure of Neotethyan Intra-Pontide Ocean between the southern Eurasian margin (Istanbul-Zonguldak Terrane) and the Cimmerian Sakarya Composite Terrane. Among other components of the oceanic lithosphere, it comprises not yet well-dated felsic igneous rocks formed in arc-basin as well as continent margin settings. In-situ U-Pb age results for zircons from the arc-basin system (öangaldağ Metamorphic Complex) and the continental arc (Devrekani Metadiorite and Granitoid) yield ages of 176 ± 6 Ma, 163 ± 9 Ma and 165 ± 3 Ma, respectively. Corresponding in-situ average (initial) 176Hf/177Hf initial ratios are 0.28261 ± 0.00003, 0.28267 ± 0.00002 and 0.28290 ± 0.00004 for these units and indicative of a subduction-modified mantle source. The new U-Pb ages and Hf isotope data from these oceanic and continental arc units together with regional geological constraints support the presence of a multiple subduction system within the Intra-Pontide Ocean during the Middle Jurassic.

Facies variability of a lower Aptian carbonate platform succession, Tirgan Formation, eastern Kopet Dagh Basin, northeast Iran

The Lower Cretaceous (Hauterivian-lower Aptian) Tirgan Formation is a carbonate platform succession in the Kopet Dagh Basin and is up to nearly 800 m thick. This study focuses on facies variability and depositional settings of the Tirgan in the eastern part of eastern Kopet Dagh where the formation constitutes the basal part of the lower Aptian deposits. The succession developed during the final stage of evolution of Cretaceous carbonate platform in the southern Eurasian margin and could demonstrate a synchronicity with comparable successions in other areas. In the study area, the Tirgan (48 m thick) overlies Berriasian–Barremian continental strata and underlies the lower Aptian deep marine deposits. Facies analysis of Tirgan determined several facies related to open marine, ooid grainstone shoal, protected lagoon, and lower intertidal facies belts of a carbonate system. Abundant ooids, calcareous green algae, mud size carbonate, lack of evaporite evidence, and the absence of basinal deposits record deposition in warm water shallow marine settings of a carbonate ramp under a relatively humid climate. The Tirgan encompasses a single third-order depositional sequence composed of transgressive and highstand systems tracts. The transgressive package consists of intertidal, lagoon, and open marine facies intercalated with storm and shale layers showing an overall retrogradational trend. However, the highstand package consists mainly of ooid grainstone facies and displays prograding clinoforms. The lowermost lower Aptian Tirgan sequence in the study area corresponds to the basal Aptian global third-order sea level cycle. Contact of Tirgan sequence with the overlying middle to late Aptian deep marine strata is abrupt, suggesting a drowning event, which was likely the result of early Aptian extraordinary global warming.

Thermochronological insights into the structural contact between the Tian Shan and Pamirs, Tajikistan

AbstractMulti‐method thermochronology along the Vakhsh‐Surkhob fault zone reveals the thermotectonic history of the South Tian Shan–Pamirs boundary. Apatite U/Pb analyses yield a consistent age of 251 ± 2 Ma, corresponding to cooling below ~550–350°C, related to the final closure of the Palaeo‐Asian Ocean and contemporaneous magmatism in the South Tian Shan. Zircon (U–Th–Sm)/He ages constrain cooling below ~180°C to the end of the Triassic (~200 Ma), likely related either to deformation induced by the Qiangtang collision or to the closure of the Rushan Ocean. Apatite fission track thermochronology reveals two low‐temperature (&lt;120°C) thermal events at ~25 Ma and ~10 Ma, which may be correlated with tectonic activity at the distant southern Eurasian margin. The late Miocene cooling is confirmed by apatite (U–Th–Sm)/He data and marks the onset of mountain building within the South Tian Shan that is ongoing today.

On position of the Black Sea region in general scheme of Cenozoic tectonic movement in Eastern Europe and the Middle East

Purpose. To clarify a structural position of Cenozoic Black Sea basin in the overall scheme of geodynamics of Ukraine, Eastern Europe and the Middle East.Methodology. Within these regions structural relations between geological complexes (stratified and unstratified) and the analysis of change were identified. Modern geodynamic models and data of present-day seismology tectonic movements in the region were additionally used. The results are presented in the form of paleotectonic schemes.Findings. Cenozoic movement in Eastern Europe and the Middle East reflects the global tectonic processes. The closure of the paleo-ocean Tethys occurred in these regions by collision between the southern passive margin of the East European and microcontinents Tisia and Anatolia, approaching their northern active margins in a north-western direction. This is evidenced by the spatial distribution of paleoarc volcanics, melange zones and the degree of pre-Eocene sedimentary formation folding in Europe. As a result of collisional processes in the Paleocene there formed a vast area of Laramy folding in Europe. The northern border of this folding area on the territory of Ukraine and Poland has been established. Its shape shows the progress of fold deformations from south to north from the two microcontinents-indentors.Due to separation of Tisia from Anatolia a large marine gulf (future Carpathians) was formed between them on the southern edge of Eastern Europe, devoid of Laramy deformations and filled by Paleogene flysch from the north-east and south-west sides. In the Miocene general direction of tectonic movement in Europe has changed to the north-east, associated with opening of the Atlantic. It formed the attic orogenic structure of the Carpathians and complicated structure in other regions of Ukraine. At the same time closure of the Eastern Mediterranean basin continued with the subduction of the heavy oceanic plate under the Cretan-Hellenic continental edge. This led to Anatolia rifting from the southern margin of Eurasia in opposite southwest direction. As result, the Black Sea basin opened as a back-arc spreading sea between the oppositely moving plates.Implications. Good agreement between the structural and geological observations with seismic data, regional geological material and global models are shown.

The thermal evolution of Chinese central Tianshan and its implications: Insights from multi-method chronometry

The Chinese Tianshan is located in the south of the Central Asian Orogenic Belt and formed during final consumption of the Paleo-Asian Ocean in the late Palaeozoic. In order to further elucidate the tectonic evolution of the Chinese Tianshan, we have established the temperature-time history of granitic rocks from the Chinese Tianshan through a multi-chronological approach that includes U/Pb (zircon), 40Ar/39Ar (biotite and K-feldspar), and (U-Th)/He (zircon and apatite) dating. Our data show that the central Tianshan experienced accelerated cooling during the late Carboniferous- to early Permian. Multiple sequences of complex multiple accretionary, subduction and collisional events could have induced the cooling in the Tianshan Orogenic Belt. The new 40Ar/39Ar and (U-Th)/He data, in combination with thermal history modeling results, reveal that several tectonic reactivation and exhumation episodes affected the Chinese central Tianshan during middle Triassic (245–210Ma), early Cretaceous (140–100Ma), late Oligocene-early Miocene (35–20Ma) and late Miocene (12–9Ma). The middle Triassic cooling dates was only found in the central Tianshan. Strong uplift and deformation in the Chinese Tianshan has been limited and localized. It have been concentrated in around major fault zone and the foreland thrust belt since the early Cretaceous. The middle Triassic and early Cretaceous exhumation is interpreted as distal effects of the Cimmerian collisions (i.e. the Qiangtang and Kunlun-Qaidam collision and Lhasa-Qiangtang collision) at the southern Eurasian margin. The Cenozoic reactivation and exhumation is interpreted as a far field response to the India-Eurasia collision and represents the beginning of modern mountain building and denudation in the Chinese Tianshan.

Permo–Triassic and Liassic Tethyan Oceanic Tracts within the Pontide Belt Along the Southern Margin of Eurasia, Northern Anatolia

The Pontide belt in northern Turkey includes three major tectonic terranes, the Strandja Massif (SjM), and the Istanbul (ISZ) and Sakarya Zones (SZ) (Fig. 1). We present new age and geochemical data from ophiolites and ophiolitic mélanges within the Sakarya Zone and show that these mafic–ultramafic rocks are the remnants of Tethyan oceanic lithosphere formed in different tectonic settings. The main ophiolite occurrences investigated in this study along the Karakaya Suture (KS) are associated with the latest Triassic Cimmeride orogeny, and in the Küre–Yusufeli ophiolite belt are part of the Alpide orogeny.The Karakaya Suture Zone ophiolites in northern west Turkey are comprised mainly of the Denizgoren (Çanakkale) ophiolite, Boğreve;azköy (Bursa), Geyve (Sakarya), Almacık (Düzce) and Çele (Bolu) metaophiolites. The Denizgören ophiolite largely contains upper mantle peridotites, which are equivalents of the Permo–Triassic Lesvos peridotites and mélange units farther SW in the northern Aegean Sea. The Boğreve;azköy ophiolite includes serpentinite and metagabbro, and the Almacık and Geyve ophiolites display an almost complete Penrose–type sequence consisting of serpentinized upper mantle peridotites, cumulate ultramafic–mafic rocks, isotropic gabbros, dolerite and plagiogranite dikes, and extrusive rocks. U–Pb zircon dating of plagiogranite dikes from Çele has revealed an igneous age of 260 Ma, and 255, 235, 227 Ma from Almacık (Bozkurt et al., 2012a, b). Consistent with the previously published Permo–Triassic age, we obtained a 268.4±6.3 Ma U–Pb zircon age from a plagiogranite dike within the Almacik ophiolite to the west. This KS ophiolite belt containing the Çele, Almacık, Geyve ophiolites within the SZ extends westward into the Armutlu Peninsula and then into the Biga Peninsula (i.e. Denizgören ophiolite) and most likely connects with the remnants of the Triassic Meliata–Meliac ocean basin (Stampfli and Borel, 2002) in the Balkan Peninsula. The KS ophiolites also continue eastward within the Pontide Belt into the Elekdağreve; ophiolite (eastern Kastamonu) and then to the Refahiye ophiolite in NE Anatolia.Triassic granites in the SZ represent a magmatic arc that formed as a result of the northward subduction of the Izmir–Ankara–Erzincan oceanic lithosphere existing during the late Paleozoic through Cretaceous (Sarifakioglu et al., 2014) beneath the Pontides. We obtained a U–Pb zircon age of 231±2 Ma from a metagranitic intrusion into the Variscan basement of the SZ in the Kastamonu region of the central Pontides. This metagranite is enriched in LILE (Rb: 63 ppm; Ba:65 ppm; Sr: 200 ppm) and depleted in HFSE (Y: 12.58 ppm; Yb: 1.26 ppm; TiO2: 0.2 wt.%; Nb: 7.6 ppm; Hf: 3.9 ppm), characterizing it as subduction–related calc‐alkaline pluton. Lead (3.9 ppm), U (1.6 ppm) and Ce (59 ppm) contents are interpreted as evidence for contamination by continental crust. The Küre basin to the north opened during the late Triassic to Liassic, following a backarc rifting episode in the central Pontides. Metabasic dike intrusions in the Devrekani metamorphic massif represent the first magmatic stage of this backarc rifting event. Whole‐rock 40Ar‐39Ar dating of the metabasic dikes has yielded cooling ages of 160.5±1.2 Ma. We infer that this age was reset due to thermal heating during the emplacement of the Middle Jurassic granitoids as the Küre oceanic basin was closing. The Küre ophiolite contains upper mantle peridotites and oceanic crustal rocks composed mainly of pillow–massive–breccia basalts, dacitic and rhyolitic lavas–tuffs, diabase dyke swarms, massive gabbros and a limited extent mafic cumulates. We obtained 182.6±1.9 Ma as a whole‐rock 40Ar‐39Ar age from a pillow basalt and a U–Pb zircon age of 171±1 Ma from the granitic intrusion cross‐cutting the peridotites. The easternmost representatives of the Küre ophiolite occur in the Yusufeli (Artvin) area in the eastern part of the Pontide belt. Here, oceanic crustal rocks are tectonically related to metamorphic rocks of the Variscan basement of the SZ. The ophiolitic crustal rocks contain isotropic gabbro and mafic and felsic dikes. Serpentinized upper mantle peridotites are scarce. Pillow lava basalts are overlain by a thick metasandstone–metashale association with locally foliated meta–lava and some manganiferous chert and mudstone interlayers. We obtained a U–Pb zircon age of 172.5±1.4 Ma from the granitic intrusion cross‐cutting the Yusufeli ophiolite and of 181.9±0.9 Ma from a felsic dike (plagiogranite) in the Yusufeli ophiolite. The Middle Jurassic granites are related to the closure of the Küre‐Yusufeli marginal ocean basin. The Küre and Yusufeli ophiolites have been previously interpreted as the remnants of the Paleotethys or the Intra‐Pontide Ocean. However, we posit that these ophiolites represent a marginal, short‐lived (∼10 Ma) ocean basin, which opened during the late Triassic through Liassic, and then closed in Dogger. This oceanic lithosphere is similar to the Evros ophiolite in the northeastern Greece in terms of its ages and geochemical characteristics.

Palaeotethys-related sediments of the Karaburun Peninsula, western Turkey: constraints on provenance and stratigraphy from detrital zircon geochronology

Detrital zircon U–Pb geochronology of 15 Late Palaeozoic to Early Mesozoic siliciclastic sandstones from the Karaburun Peninsula in western Turkey determines maximum sedimentation ages, identifies possible source areas, and anchors the study area within the Palaeotethyan realm. Siliciclastic sandstones yielded ages from Triassic to Archean with major input from Palaeozoic to Neoproterozoic sources and very few Mesoproterozoic zircons. The youngest age groups set the new limit of the maximum depositional ages to Late Carboniferous–Early Permian for the Kucukbahce and Dikendagi formations. Detrital zircons from Triassic sandstones are mainly Neoproterozoic and Palaeozoic in age. Zircons from the Scythian–Anisian Gerence Formation are predominantly Devonian and Carboniferous in age, while also Permian and Triassic zircon grains occur in the Carnian–Rhaetian Guvercinlik Formation. According to the zircon age populations and the data available from possible source regions, the Karaburun siliciclastic sediments, with the exception of two samples from the Dikendagi Formation, record sediment supply from units located at the southern margin of Eurasia during Late Palaeozoic and Early Mesozoic times. This interpretation is in agreement with palaeotectonic reconstructions for the closely related Greek islands of Chios and Inousses. The presence of Devonian accompanied by Carboniferous zircons in some of the Karaburun samples reveals similarities with Karakaya Complex sandstones of the Sakarya Zone in NW Turkey.

Inclined transpression in the Neka Valley, eastern Alborz, Iran

Three major nappes in the Neka Valley in the eastern Alborz Mountains of Iran allow the Cimmerian to present convergence following the oblique collision between Iran and the southern margin of Eurasia. This work reports the identification of an inclined transpression zone recognized by field investigations and strain analyses of the geometries of formations and detailed mesoscopic structural analyses of multiple faults, folds and a cleavage. The main structures encountered include refolded recumbent asymmetric fold nappes, highly curved fold hinges, in a transpression zone that dips 37° to the NW between boundaries thrusts striking from N050° to N060°. The β angle (the angle between the zone boundary and direction of horizontal far-field shortening) is about 80°. The north-west and south-east boundaries of this zone coincide with the Haji-abad thrust and the Shah-Kuh thrust, respectively. Fold axes generally trend NE–SW and step to both right and left as a result of strike–slip components of fault displacements. Strain analyses using Fry’s method on macroscopic ooids and fusulina deformed into oblate ellipsoids indicate that the natural strain varies between 2.1 and 3.14. The estimated angle between the maximum instantaneous strain axis (ISAmax) and the transpression zone boundary (θ′) is between 6° and 20°. The estimated oblique convergence angle (α), therefore, ranges between 31° and 43°. The average kinematic vorticity number (W k ) is 0.6, in a zone of sinistral pure shear-dominated inclined triclinic transpression. These results support the applicability of kinematic models of triclinic transpression to natural brittle–ductile shear zones.

Obduction of old oceanic lithosphere due to reheating and plate reorganization: Insights from numerical modelling and the NE Anatolia – Lesser Caucasus case example

The ophiolites of NE Anatolia and of the Lesser Caucasus (NALC) evidence an obduction over ∼200km of oceanic lithosphere of Middle Jurassic age (c. 175–165Ma) along an entire tectonic boundary (>1000km) at around 90Ma. The obduction process is characterized by four first order geological constraints:(1)Ophiolites represent remnants of a single ophiolite nappe currently of only a few kilometres thick and 200km long. The oceanic crust was old (∼80Ma) at the time of its obduction.(2)The presence of OIB-type magmatism emplaced up to 10Ma prior to obduction preserved on top of the ophiolites is indicative of mantle upwelling processes (hotspot).(3)The leading edge of the Taurides-Anatolides, represented by the South Armenian Block, did not experience pressures exceeding 0.8GPa nor temperatures greater than ∼300°C during underthrusting below the obducting oceanic lithosphere.(4)An oceanic domain of a maximum 1000km (from north to south) remained between Taurides-Anatolides and Pontides-Southern Eurasian Margin after the obduction.We employ two-dimensional thermo-mechanical numerical modelling in order to investigate obduction dynamics of a re-heated oceanic lithosphere. Our results suggest that thermal rejuvenation (i.e. reheating) of the oceanic domain, tectonic compression, and the structure of the passive margin are essential ingredients for enabling obduction. Afterwards, extension induced by far-field plate kinematics (subduction below Southern Eurasian Margin), facilitates the thinning of the ophiolite, the transport of the ophiolite on the continental domain, and the exhumation of continental basement through the ophiolite. The combined action of thermal rejuvenation and compression are ascribed to a major change in tectonic motions occurring at 110–90Ma, which led to simultaneous obductions in the Oman (Arabia) and NALC regions.

Late Triassic paleogeographic reconstruction along the Neo–Tethyan Ocean margins, southern Tibet

Sandstone petrographic and U–Pb detrital zircon analyses of Upper Triassic sedimentary rocks from the northern margin of India (Tethyan Himalaya Sequence) and southern margin of Eurasia (Lhasa terrane) provide new constraints on the Mesozoic paleogeography of Neo–Tethyan Ocean basins. The Upper Triassic Nieru Formation of the Tethyan Himalaya Sequence (THS) near Lazi city (∼29°N, 87.5°E) is dominated by Indian-affinity, Precambrian detrital zircons, which are typical of the majority of the THS. However, the Upper Triassic Langjiexue Formation of the THS exposed to the east (at 90–93°E longitude) includes significant populations of Permian to Early Jurassic (291–184 Ma) detrital zircons for which there is no known Indian source. In addition, the Upper Triassic Nieru Formation near Kangma town (∼28.5°N, 90°E), located ∼200 km to the southeast of Lazi city, yielded detrital zircon age spectra that are similar to those of Langjiexue Formation. Based on detrital zircon age spectra comparisons, we propose that both the Langjiexue and Nieru formations were derived from continental crustal fragments that were adjacent to the northwestern margin of Australia. Furthermore, we suggest that these THS units, and age-equivalent strata in Northwest Australia, West Sulawesi, Timor and West Papua, comprised a Late Triassic submarine fan along the northern Australian shelf. The Upper Triassic Mailonggang Formation in the southern Lhasa terrane (35 km northeast of Lhasa city, ∼30°N, 91.5°E) is dominated by Permian detrital zircons, which were likely derived from proximal Lhasa terrane sources. The Mailonggang Formation differs from all age-equivalent strata in the Tethyan Himalaya; therefore we interpret that it was separated from Greater India by the Neo–Tethyan Ocean.

Oblique convergence during the Cimmerian collision: Evidence from the Triassic Aghdarband Basin, NE Iran

The Lower-Middle Triassic Aghdarband Basin, NE Iran, consists of a strongly deformed arc-related marine succession deposited along the southern margin of Eurasia in a highly mobile tectonic context. This basin is a key-area for the study of the Cimmerian events, as the Triassic units show severe deformations, which occurred short time after the collision of Iran with Eurasia, and were sealed by the Middle Jurassic succession. In this work, we document the structural setting and evolution of this area, based on detailed mesoscopic structural analyses of faults and folds, paleostress reconstruction and revision of the Triassic stratigraphy. The Triassic sequences are deeply involved in a N-verging thrust stack interacting with an important left-lateral transpressional fault zone characterized by strike-slip faults, vertical folds and high angle reverse faults generating intricate positive flowers. Systematic folds asymmetry indicates that they developed in a left-lateral transpressional zone coeval to thrust imbrication to the south, due to a marked strain partitioning.The extent of the transpressional zone shows that important left-lateral movements developed parallel to the belt during the Cimmerian collision, in response to oblique convergence between Iran and Eurasia. Inversion of Triassic syn-sedimentary faults, possibly inherited from Palaeozoic structures of the Kopeh Dagh basement and favouring strain partitioning, is suggested by unconformities, significant differences in the sedimentary successions, repeated olistoliths, scarp-related coarse breccias and rapid tectonic drowning, occurring especially along the northern tectonic boundary of the basin. Paleostress analyses point to a complex stress pattern showing a 45° rotation of the stress field along the left-lateral fault system, related to a complete deformation partitioning in two domains respectively characterized by pure reverse dip-slip and strike-slip motions. The main direction of compression, possibly oriented NE–SW in present days coordinates, favoured the development of large shear zones disrupting the eastern portion of the Cimmerian orogen.

Mesozoic reactivation of the Beishan, southern Central Asian Orogenic Belt: Insights from low-temperature thermochronology

The Beishan Orogenic Collage (BOC) is located in the southeast of the Central Asian Orogenic Belt (CAOB) and formed during final consumption of the Palaeoasian Ocean in the late Palaeozoic. This study applies low temperature thermochronology to constrain the Meso-Cenozoic history of the BOC.Apatite fission track and U–Th–Sm/He data obtained for granitoid samples along a north–south transect through the BOC indicate three distinct phases of exhumation during (1) the late Triassic–early Jurassic (~225–180Ma), (2) early Cretaceous (~130–95Ma) and (3) late Cretaceous–early Palaeogene (~75–60Ma). Samples from the northern BOC reveal a more profound early Cretaceous signal and a weaker late Triassic–early Jurassic signal than those in the southern BOC. A possible explanation for this discrepancy is the presence of crustal-scale fault zones in the northern BOC that are interpreted to have undergone repeated reactivation throughout the Mesozoic, exposing deeper exhumed sections of the BOC. These faults may thus have acted as a major control on exhumation in the region. This pattern is consistent with results from elsewhere in the CAOB, such as in the Tianshan and the Altai, where regional widespread exhumation occurred since the early Cretaceous while major fault zones record localised exhumation during the late Cretaceous and Cenozoic. Late Cretaceous–early Palaeogene cooling ages were found only in the south of the BOC, suggesting that exhumation at that time was more localised and did not reach the northern margins of the study area.The Meso-Cenozoic cooling events described here are thought to be related to the progressive closure of the (Palaeo-)Tethys ocean to the south. Associated collision and accretion of micro-continental blocks and island–arcs at the southern Eurasian margin are interpreted to have induced more widespread reactivation and exhumation in Central Asia than previously anticipated, extending to the northern margin of the Tarim Craton. This observation hence refines the existing tectonic history models for Central Asia.

Implications of U–Pb and Lu–Hf isotopic analysis of detrital zircons for the depositional age, provenance and tectonic setting of the Permian–Triassic Palaeotethyan Karakaya Complex, NW Turkey

New zircon U–Pb age data, combined with Lu–Hf isotopic data, are presented here for sandstones of mainly arkosic composition from the Permian–Triassic Karakaya Complex. Predominantly, Carboniferous, Triassic and Devonian zircon age groups are recognised, most of which have a Late Triassic (Carnian–Norian) maximum depositional age. Carboniferous- and Devonian-aged zircon populations exhibit intermediate e Hf(t) values (−11 to +2), consistent with formation in a continental margin arc setting where juvenile mantle-derived magma mixed with (recycled) old crust of Palaeoproterozoic Hf model age. In contrast, the Triassic-aged zircon population exhibits higher e Hf(t) values (−5 to +4), consistent with mixing of juvenile mantle-derived melts with (recycled) old crust of Neoproterozoic Hf model age. Potential igneous source rocks for the sandstones of the Karakaya Complex exist in the Devonian and Carboniferous granitic rocks of the Sakarya continental basement to the north. Their e Hf(t) and corresponding model ages are nearly identical to the age-equivalent zircon populations within the Karakaya Complex sandstones. However, the Triassic granitic rocks of the Sakarya continental crust differ significantly in e Hf(t) and corresponding model age from the sandstones of the Karakaya Complex. Late Triassic sandstones from the Tauride continental unit to the south lack the dominant Late Palaeozoic and Triassic zircon populations of the Karakaya Complex sandstones. Triassic granitic bodies and intermediate-composition extrusive rocks in the Tauride continental unit also differ in e Hf(t) and corresponding Hf model ages from the Karakaya Complex sandstones. In addition, Late Triassic sandstones of the Kocaeli Triassic unit (Istanbul Terrane) in the north differ strongly from the Karakaya Complex sandstones in zircon population ages and e Hf(t). In the regional context, the new zircon age and lutetium–hafnium isotopic data are consistent with derivation of the Late Triassic Karakaya Complex sandstones from a Late Palaeozoic–Triassic continental margin arc located somewhere along the southern margin of Eurasia, although its exact position cannot be pinpointed at present owing to lack of suitable outcrop and comparable isotopic data.

Rifting, subduction and collisional records from pluton petrogenesis and geochronology in the Hindu Kush, NW Pakistan

New U-(Th)/Pb geochronology and geochemical analyses of plutonic bodies in the Hindu Kush range, NW Pakistan, provide insight on the crustal growth and tectonic evolution of the southern Eurasian margin. These new data outline a protracted magmatic history that spans the Cambrian to the Neogene (ca. 538 to 23Ma) and record a variety of petrogenetic associations variably influenced by within plate, volcanic arc, and collision tectonic environments. The Kafiristan pluton (538±4 to 487±3Ma) yields geochemical signatures consistent with extensional plutonism and rifting of the Hindu Kush terrane from Gondwana. The Tirich Mir (127±1 to 123±1Ma) and Buni-Zom (110±1 to 104±1Ma) plutons have geochemical signatures that can be attributed to a subduction related continental volcanic arc system that developed along the southern margin of Eurasia in the Mesozoic. The Garam Chasma pluton, the youngest body in the study area (27.3±0.5 to 22.8±0.4Ma), yields a geochemical signature consistent with widespread anatexis during crustal thickening related to the development of the Himalaya. The present geochemical and geochronological analysis from the Hindu Kush have produced important new constraints on the timing of tectonic events and variable tectonic settings along the south Eurasian margin before and after the continued India–Asia collision.

Late-Paleozoic emplacement and Meso-Cenozoic reactivation of the southern Kazakhstan granitoid basement

The Ili–Balkhash Basin in southeastern Kazakhstan is located at the junction of the actively deforming mountain ranges of western Junggar and the Tien Shan, and is therefore part of the southwestern Central Asian Orogenic Belt. The basement of the Ili–Balkhash area consists of an assemblage of mainly Precambrian microcontinental fragments, magmatic arcs and accretionary complexes. Eight magmatic basement samples (granitoids and tuffs) from the Ili–Balkhash area were dated with zircon U–Pb LA-ICP-MS and yield Carboniferous to late Permian (~350–260Ma) crystallization ages. These ages are interpreted as reflecting the transition from subduction to (post-) collisional magmatism, related to the closure of the Junggar–Balkhash Ocean during the Carboniferous–early Permian and hence, to the final late Paleozoic accretion history of the ancestral Central Asian Orogenic Belt. Apatite fission track (AFT) dating of 14 basement samples (gneiss, granitoids and volcanic tuffs) mainly provides Cretaceous cooling ages. Thermal history modeling based on the AFT data reveals that several intracontinental tectonic reactivation episodes affected the studied basement during the late Mesozoic and Cenozoic. Late Mesozoic reactivation and associated basement exhumation is interpreted as distant effects of the Cimmerian collisions at the southern Eurasian margin and possibly of the Mongol–Okhotsk Orogeny in SE Siberia during the Jurassic–Cretaceous. Following tectonic stability during the Paleogene, inherited basement structures were reactivated during the Neogene (constrained by Miocene AFT ages of ~17–10Ma). This late Cenozoic reactivation is interpreted as the far-field response of the India–Eurasia collision and reflects the onset of modern mountain building and denudation in southeast Kazakhstan, which seems to be at least partially controlled by the inherited basement architecture.

The Bogeda Shan uplifting: Evidence from multiple phases of deformation

The current topography of Central Asia is considered to have resulted from crustal shortening related to the ongoing India–Asia collision that started in the Early Tertiary period. The Bogeda Shan, a northeastern branch of the Tian Shan located in northwest China, has several summits higher than 5000m. The northern piedmont of the Bogeda Shan exhibits remarkable deformation features that remain poorly studied. This study documents three phases of tectonic deformation in the Bogeda Shan that are related to the far-field effects of collisions along the southern Eurasian margin. Detailed field observations show that the intensively folded Upper Permian strata are covered by a subhorizontal Lower Jurassic layer, thus indicating a Triassic tectonic event that is probably related to the onset of the uplift in the Bogeda Shan during a Cimmerian orogenic episode (D1). A seismic profile suggests that this deformation episode was followed by the Late Jurassic–Early Cretaceous deformation (D2), which is characterized by wide wavelength folds overlaid by undeformed Cretaceous layers. Cenozoic thrusts were also observed in the northern piedmont of the Bogeda Shan (D3). This study demonstrates that the onset of uplift in the Bogeda Shan began in the Early Mesozoic and separated the Turfan–Hami Basin from the Junggar Basin. A new tectonic evolution model of the Bogeda Shan was reconstructed by these three tectonic phases. This work indicates that the current topography of the Bogeda Shan and its surrounding area was the result of multiple phases of deformation related to successive collisions at the southern Eurasian margin.

Geochronology and isotope geochemistry of Eocene dykes intruding the Ladakh Batholith

In order to determine the extent and timing of dyke formation related to possible E–W extension along the southern margin of Eurasia during Early Cenozoic time, we examined ca. 30 mostly andesitic dykes intruding the Ladakh batholith from 10 to 50 km west of Leh (NW India). The dykes in the east of the area trend E–NE and those in the west trend N–NW. The difference in orientation is also evident in the petrography and isotopic signatures. The eastern dykes contain corroded quartz xenocrysts and show negative ε0(Nd) and positive ε0(Sr) values, whereas the western dykes do not contain quartz xenocrysts and exhibit positive ε0(Nd) and near-zero ε0(Sr) values. The variability in Sr–Nd isotopes (ε0(Nd) = 3.6 to − 9.6, ε0(Sr) = 0.4 to 143) and the quartz xenocrysts can best be explained by (differing degrees of) crustal assimilation of the parent magma of the dykes. Separated minerals from five dykes were dated by 40Ar–39Ar incremental heating: amphibole ages range between 50 and 54 Ma, and one biotite separate dated both by Rb–Sr and 40Ar–39Ar gave an age of 45 Ma. One dated pseudotachylyte sample attests to brittle faulting at ca. 54 Ma. The combination of structural field evidence with petrographic, isotopic and geochronological analyses demonstrates that the dykes did not form from a single, progressively differentiating magma chamber, despite having formed in the same tectonic setting around the same time, and that processes such as crustal assimilation and magma mixing/mingling, also played a significant role in magma petrogenesis.