Arabia-Eurasia Continental Collision Research Articles

The P–T–X conditions of staurolite-garnet metabasites, NW Iran: Implications for metamorphism during Arabia-Eurasia collision

The staurolite-bearing Avajiq metabasites (AMB) crop out near the NW border of Iran with Turkey and contain the peak assemblage garnet + Ca-amphibole + staurolite + biotite + chlorite1 + ilmenite. The AMB record peak pressure–temperature (P-T) conditions of >1 GPa at ∼650 °C; the moderately high pressure is consistent with the presence of staurolite in metabasalt. Garnets exhibit a weak prograde zoning, with relatively homogenous cores and a slight increase in Mg and decrease in Ca, Fe, and Mn toward the rim, consistent with garnet growth during a clockwise P-T path. Texturally late margarite + biotite + muscovite correspond to retrograde P-T conditions of ∼0.5 GPa at ∼500 °C. This stage was followed by further retrogression under greenschist facies conditions recorded by late pumpellyite + paragonite ± calcite. Based on the MORB-like composition of the AMB, it has been unclear whether the metabasites are associated with Neo-Tethyan ophiolites in the area or formed within a continental setting. However, the petrologic association of metabasites with micaschist, gneiss and marble and upper amphibolite facies conditions of metamorphism indicate that the basaltic rocks likely metamorphosed during Arabia-Eurasia continental collision, following consumption of the intervening Neo-Tethys Ocean and emplacement of the ophiolitic rocks in the region.

Vanished evaporites, halokinetic structure, and Zn-Pb mineralization in the world-class Angouran deposit, northwestern Iran

The Angouran deposit (19.3 Mt at 23.4% Zn and 4% Pb) is the second-largest Zn-Pb deposit in Iran. The deposit is hosted in a Neoproterozoic−Cambrian marble-schist sequence within a breccia pipe in a domal structure, with sulfide mineralization under low-temperature hydrothermal conditions (<200 ºC). The features of the ore-hosting breccias are similar to known halokinetic diapir breccias in the world but evaporite minerals are subtle. The common types of breccia clasts in the Angouran breccia pipe include a matrix-supported angular clast (float breccia) with highly variable sizes and orientations and exotic volcanic clasts. The volcanic clasts were derived from the underlying Miocene volcanic rocks, evidenced by the consistent petrography and zircon U-Pb ages dated at 20−19 Ma. Abundant smithsonite pseudomorphs after anhydrite and anhydrite inclusions within sphalerite and pre-ore marcasite in the breccia matrix indicate that the breccia pipe contains abundant anhydrite prior to the Zn-Pb mineralization. The enrichment of evaporite is also supported by the occurrence of considerable double-terminated quartz crystals that contain spherical and tabular carbonate inclusions and anomalously high Li, Na, and K concentrations, relatively high B concentration, and high δ18O values (up to 28.3‰). These observations suggest the Angouran deposit formed in a former halokinetic diapir breccia pipe. The halokinetic diapirism was possibly triggered by thrust loading of the marble-schist sequence over the Miocene evaporite beds during the Arabia-Eurasia continental collision. Halokinetic structures elsewhere in the Angouran region warrant this consideration. Most of the evaporite minerals in the breccia pipe were dissolved and replaced before and/or during subsequent Zn-Pb sulfide and smithsonite mineralization events. This study provides a good example for the identification of vanished evaporites, halokinetic structure, and associated Mississippi Valley-type mineralization.

The upper crustal shear wave structure of the Zagros collision zone from local earthquake Rayleigh wave tomography

The Zagros fold-and-thrust belt is the principal response to the Arabia–Eurasia continental collision, which generates significant seismic activity in Iran. Surface wave tomography can be a proper tool for crustal structure imaging in this region. We used observed dispersion curves of Rayleigh wave derived from high-quality local-earthquake waveforms to obtain 2D tomographic maps in the period range 1–28 s beneath the Zagros collision zone. Subsequently, local pseudo-dispersion curves for individual grid points are inverted for shear-wave velocity structure, in order to obtain a 3D velocity model of the crust. Our results indicate shallow low-velocity structures near the suture zone. Moreover, a low-velocity flexure shape in the south of the Zagros Mountain Front Fault could be due to the thick sedimentary basin beneath the Dezful embayment. Relatively an uplifting of the deeper structures beneath Sanandaj-Sirjan and the high Zagros might could be led to high-velocity structures at mid-crustal depths. The variation of shear-wave velocity along the Zagros indicates the inhomogeneous structures along this folding and thrust belt.

From Tethyan subduction to Arabia-Eurasia continental collision: Multiple geo-thermochronological signals from granitoids in NW Iran

NW Iran, situated between the Arabian and Eurasian plates, carries a record of both Paleo-Tethyan and Neo-Tethyan tectonic evolution. During the Wilson Cycle of Tethys Ocean opening and closing, several episodes of magmatism from Late Paleozoic to latest Cenozoic generated a massive volume of intrusive rocks. These intrusives, which record cooling histories from high-temperatures to final exhumation, are ideal for multiple geo-thermochronological studies. Our new zircon and apatite U-Pb results suggest three regional magmatic events in the Late Carboniferous, mid-Cretaceous and middle Eocene, which could be related to Paleo-Tethys subduction, Neo-Tethys subduction and Neo-Tethyan ridge subduction, respectively. New apatite fission track and (U-Th)/He data reveal post-magmatic cooling and differential exhumation related to subduction and collision. By integrating published regional thermochronological data from northeastern part of the Middle East, a broader tectono-thermal framework is outlined as follows: 1) Cretaceous cooling signals are most pronounced in the Alborz and Caucasus, and reflect back-arc extension during Neo-Tethyan subduction; 2) diachronous Neo-Tethyan evolution led to the Paleocene assemblage of Anatolia in the west, and subduction associated with a late Paleocene-Eocene magmatic flare-up in Iran. Subsequent westward escape of Anatolia and extensive surface uplift were driven by propagation of Arabia-Eurasia collision in the Miocene; 3) Continuous northward indentation of Arabia into Eurasia triggered late Miocene-Pliocene fast exhumation of the Zagros, Alborz and the Caucasus.

Internal deformation of continental blocks within converging plates: insights from the Ovacık Fault (Anatolia, Türkiye)

The active tectonics of Anatolia is mostly characterized by its westward motion with respect to Eurasia between the Hellenic subduction in the west and Arabia-Eurasia continental collision in the east. Although most of the deformation is suggested to be confined along Anatolia?s boundary elements, viz. the North and East Anatolian shear zones, recent studies indicate a higher magnitude of internal strain accumulation, especially along the parallel/subparallel strike-slip faults of its central province. We present the first morphochronology-based slip rate estimate for one of these strike-slip structures, the Ovacık Fault, by using cosmogenic 36Cl dating of offset fluvial deposits. At the Köseler Site (39.3643°N, 39.1688°E), two faulted risers, bounding the alluvial fan with its subplanar surface (NF1/NF1?) and the inset terrace tread (NF1/T2), are offset 19?24 and 15?22 m, respectively. The scattered surface ages and variability of 36Cl concentrations in depth profiles suggest strong evidence for inheritance in alluvial fan and terrace deposits; thus, we used modelled depth-profile ages for both surfaces. The modelled ages 8?10 ka for NF1 and 6?8 ka for T2 yield slip-rate estimates 2.4 +0.5/?0.4 mm/a and 2.8 +0.7/?0.7 mm/a, respectively, for the upper-tread reconstruction of the NF1/NF1?and the lower-tread reconstruction of the NF1/T2. Our results together with previous slip-rate estimates for other structures show a significant internal deformation for Anatolia, especially along its subparallel strike-slip faults. These secondary faults slice Anatolia into several pieces giving rise to the formation of the Malatya-Erzincan, Cappadocian, and Central Anatolian slices, where the geometry is strongly controlled by the distribution of the Tethyan accretionary complexes.

Tectono-thermal evolution of central Transcaucasia: Thermal modelling, seismic interpretation, and low-temperature thermochronology of the eastern Adjara-Trialeti and western Kura sedimentary basins (Georgia)

Structural interference along the boundaries of adjacent tectonic domains generates complex subsidence and exhumation patterns. In this paper we study a key segment of the convergence zone between the interfering south-verging Greater Caucasus and north-verging Lesser Caucasus retro-wedge. The study area comprises the along-strike transition between the Paleogene Adjara-Trialeti back-arc basin and the Oligocene-Neogene Kura foreland basin of eastern Georgia. During the Neogene, despite their difference in age and subsidence mechanisms, both basins have been progressively inverted to various extent within the overall context of the Arabia-Eurasia continental collision. Thermal modelling of borehole data from the Adjara-Trialeti basin allowed to reconstruct two phases of rapid subsidence, late Paleocene-Early Eocene and Middle-Late Eocene, respectively correlated to flexural loading by the Lesser Caucasus retro-wedge and continental rifting. Integration of thermal modelling, apatite fission-track statistical inverse modelling, and seismic interpretation detected a third subsidence phase in the Early Miocene, possibly related to strike-slip tectonics. Thermal maturity data dictate that 1.0–1.3 km of the Adjara-Trialeti sedimentary succession has been eroded since the onset of structural inversion in the mid-Miocene. The burial history of the western Kura Basin outlines intermittent and asymmetrical episodes of flexural subsidence from the Oligocene to the Late Miocene, due to competing loading by the Lesser Caucasus, the Adjara-Trialeti fold-and-thrust belt and the Greater Caucasus. Finally, during latest Miocene times southward propagation of deformation from the Greater Caucasus induced an additional tectonic loading (1.3–1.8 km) due to the emplacement of thin-skinned thrust sheets, mostly eroded during the Plio-Quaternary.

Detrital zircon provenance record of the Zagros mountain building from the Neotethys obduction to the Arabia–Eurasia collision, NW Zagros fold–thrust belt, Kurdistan region of Iraq

Abstract. Recognition of a new angular unconformity and the synthesis of new detrital zircon U–Pb and (U–Th) / He provenance records, including zircon (U–Th) / He double dating, from the NW Zagros region elucidate the basin dynamics of the foreland wedge-top and intermontane units, as well as the tectonic processes in the source terranes in response to the different geodynamic phases. In this contribution, we present field observations and detrital zircon provenance data from hinterland basins to reconstruct the basin dynamics and the underlying tectonic controls in the NW Zagros area in the Kurdistan region of Iraq. Results reveal that the deposition of the suture zone units of the Red Beds Series (RBS; Suwais Group, Govanda Formation, Merga Group) occurred in an intermontane basin on top of folded Upper Cretaceous units and that the RBS deposits rest with an angular unconformity on the underlying older strata. The RBS provenance data point to the Paleogene Walash–Naopurdan–Kamyaran (WNK) complex as a source area and imply a substantial decrease in magmatism by ∼ 36 Ma, as reflected by the youngest age peaks. New detrital zircon provenance data from the hinterland wedge-top units of the proto-Zagros foreland basin (the Tanjero, Kolosh, and Gercus formations) exhibit exclusive derivation from the Upper Cretaceous Neotethys ophiolitic terranes, which differs from the provenance of the older Lower Cretaceous and Paleozoic units that are dominated by the Paleozoic and Neoproterozoic age spectra. These shifts in provenance between different tectonostratigraphic units argue for a sediment source reversal from the west to the east in response to ophiolite obduction, arrival of the WNK complex, and commencement of the Arabia–Eurasia continental collision during the latest Eocene (< 36 Ma). According to the provenance data, the incipient collision was followed by the deposition of the RBS in the hinterland of the proto-Zagros fold–thrust belt as well as the connection of drainages with the collision-related Neogene foreland basin.

High resolution reconstructions of the Southwest Indian Ridge, 52 Ma to present: implications for the breakup and absolute motion of the Africa plate

SUMMARY We reconstruct the post-52 Ma seafloor spreading history of the Southwest Indian Ridge at 44 distinct times from inversions of ≈20 000 magnetic reversal, fracture zone and transform fault crossings, spanning major regional tectonic events such as the Arabia–Eurasia continental collision, the Arabia Peninsula’s detachment from Africa, the arrival of the Afar mantle plume below eastern Africa and the initiation of rifting in eastern Africa. Best-fitting and noise-reduced rotation sequences for the Nubia–Antarctic, Lwandle–Antarctic and Somalia–Antarctic Plate pairs indicate that spreading rates everywhere along the ridge declined gradually by ≈50 per cent from ≈31 to 19–18 Ma. A concurrent similar-magnitude slowdown in the component of the Africa Plate’s absolute motion parallel to Southwest Indian Ridge spreading suggests that both were caused by a 31–18 Ma change in the forces that drove and resisted Africa’s absolute motion. Possible causes for this change include the effects of the Afar mantle plume on eastern Africa or the Arabia Peninsula’s detachment from the Somalia Plate, which culminated at 20–18 Ma with the onset of seafloor spreading in the Gulf of Aden. At earlier times, an apparently robust but previously unknown ≈6-Myr-long period of rapid kinematic change occurred from 43 to 37 Ma, consisting of a ≈50 per cent spreading rate slowdown from 43 to 40 Ma followed by a full spreading rate recovery and 30–40° clockwise rotation of the plate slip direction from 40 to 37 Ma. Although these kinematic changes coincided with a reconfiguration of the palaeoridge geometry, their underlying cause is unknown. Southwest Indian Ridge abyssal hill azimuths are consistent with the slip directions estimated with our newly derived Somalia–Antarctic and Lwandle–Antarctic angular velocities, adding confidence in their reliability. Lwandle–Antarctica Plate motion has closely tracked Somalia–Antarctic Plate motion since 50 Ma, consistent with slow-to-no motion between the Lwandle and Somalia plates for much of that time. In contrast, Nubia–Somalia rotations estimated from our new Southwest Indian Ridge rotations indicate that 189 ± 34 km of WNW–ESE divergence between Nubia and Somalia has occurred in northern Africa since 40 Ma, including 70–80 km of WNW–ESE divergence since 17–16 Ma, slow to no motion from 26 to 17 Ma, and 109 ± 38 km of WNW–ESE divergence from 40 to ≈26 Ma absent any deformation within eastern Antarctica before 26 Ma.

Mantle-flow diversion beneath the Iranian plateau induced by Zagros\u2019 lithospheric keel

Previous investigation of seismic anisotropy indicates the presence of a simple mantle flow regime beneath the Turkish-Anatolian Plateau and Arabian Plate. Numerical modeling suggests that this simple flow is a component of a large-scale global mantle flow associated with the African superplume, which plays a key role in the geodynamic framework of the Arabia-Eurasia continental collision zone. However, the extent and impact of the flow pattern farther east beneath the Iranian Plateau and Zagros remains unclear. While the relatively smoothly varying lithospheric thickness beneath the Anatolian Plateau and Arabian Plate allows progress of the simple mantle flow, the variable lithospheric thickness across the Iranian Plateau is expected to impose additional boundary conditions on the mantle flow field. In this study, for the first time, we use an unprecedented data set of seismic waveforms from a network of 245 seismic stations to examine the mantle flow pattern and lithospheric deformation over the entire region of the Iranian Plateau and Zagros by investigation of seismic anisotropy. We also examine the correlation between the pattern of seismic anisotropy, plate motion using GPS velocities and surface strain fields. Our study reveals a complex pattern of seismic anisotropy that implies a similarly complex mantle flow field. The pattern of seismic anisotropy suggests that the regional simple mantle flow beneath the Arabian Platform and eastern Turkey deflects as a circular flow around the thick Zagros lithosphere. This circular flow merges into a toroidal component beneath the NW Zagros that is likely an indicator of a lateral discontinuity in the lithosphere. Our examination also suggests that the main lithospheric deformation in the Zagros occurs as an axial shortening across the belt, whereas in the eastern Alborz and Kopeh-Dagh a belt-parallel horizontal lithospheric deformation plays a major role.

Petrogenesis of the Late Oligocene Takht batholith, Southeastern Iran: Implications for the Diachronous Nature of the Arabia–Eurasia Collision

Zircon U-Pb age, mineral compositional, elemental and Sr-Nd isotopic data are documented for intermediate to felsic rocks in the Takht batholith, a Late Oligocene igneous complex in the southeastern segment of the Urumieh-Dokhtar belt, Iran, to investigate magma genesis in the context of regional tectonics. A large part of the magmatic belt formed by northward subduction of the Neotethys before the Arabia-Eurasia continental collision. Zircon U-Pb age data indicate the batholith crystallized at ~25 Ma, an age consistent with previous results. Geochemical data indicate that the rocks share features typical of calc-alkaline magmas and I-type granitoids. The least evolved magma inferred from the data has basaltic andesite composition, consistent with either of the following origins: (i) a partial melt of the mantle followed by differentiation, or (ii) a partial melt of the lower crust. With increasing SiO2, (87Sr/86Sr)25 Ma increases from 0.7053 to 0.7073, and εNd25 Ma decreases from –0.3 to –2.9, consistent with increasing effects of assimilation–fractional crystallization, although processes such as magma mixing and melting of heterogeneous source rocks cannot be entirely ruled out. Aluminum-in-hornblende thermometry indicates that the batholith might have emplaced at ~6.5 km or shallower paleo-depths. Moho depth proxies Sr/Y and (La/Yb)N yield crustal thickness of ~20–30 km, showing no evidence for magma processing within thick crust. Overall, our results indicate that the Takht batholith is a Cordilleran-type batholith formed beneath a continental or transitional arc of normal crustal thickness. This is consistent with the notion that Arabia and Eurasia have collided in a diachronous manner, propagating from the northwest to the southeast since the Late Eocene–Earlty Oligocene.

Pleistocene ignimbrites of western Armenia - Paleomagnetic and magnetic anisotropy constraints on flow direction and stratigraphy

Voluminous early to middle Pleistocene pyroclastic flow deposits were generated from Mount Aragats (Aragats Volcanic Province, AVP), one of the largest volcanoes in the Arabia-Eurasia continental collision zone. Here we report paleomagnetic results, in combination with anisotropy of magnetic susceptibility (AMS) analysis, from 31 outcrops belonging to six ignimbrite units of the western part of the AVP (covering an area of more than 5000 km2). Paleomagnetic directional analysis of samples from 26 outcrops of the three younger ignimbrite units yields a well-defined normal polarity direction of D = 339.2°, I = 55.7°, k = 278.4 and α95 = 1.7°. The virtual coincidence of paleomagnetic directions points to an emplacement of the units within a restricted time of similar geomagnetic secular variation (i.e., few hundred years). However, new isotope 40Ar/39Ar ages of the three units suggest two separated events at ~0.65 and ~ 0.75 Ma. The possibility of similar paleomagnetic directions at 0.65 and 0.75 Ma or the occurrence of a widespread remagnetization event at 0.65 Ma cannot be fully excluded, but both scenarios are highly unlikely. In contrast to that, five sites from the three older units show reversed magnetic polarity, which implies an age of emplacement older than the last magnetic polarity reversal at ~780 ka. This agrees with recent 40Ar/39Ar ages of the three older units at ~0.93 Ma. The paleomagnetic scatter of the older units is high compared to the younger ones. Even if paleomagnetic directions and radiometric age are similar, available data cannot unequivocally indicate if the three older units were generated from the same or three different eruptions. The high quality AMS data result in three different types of fabric (lineated, foliated and undetermined) that are all commonly observed in the studied pyroclastic deposits. Accordingly, we use lineation, tilt of foliation, or both for the determination of paleoflow direction. Results reveal a radial flow pattern surrounding Mt. Aragats, which is, however, highly influenced by topographic features, such as valleys, even in proximity of the eruption center.

Timing of the Arabia-Eurasia continental collision—Evidence from detrital zircon U-Pb geochronology of the Red Bed Series strata of the northwest Zagros hinterland, Kurdistan region of Iraq: COMMENT

Timing of the Arabia-Eurasia continental collision—Evidence from detrital zircon U-Pb geochronology of the Red Bed Series strata of the northwest Zagros hinterland, Kurdistan region of Iraq: COMMENT

Timing of the Arabia-Eurasia continental collision—Evidence from detrital zircon U-Pb geochronology of the Red Bed Series strata of the northwest Zagros hinterland, Kurdistan region of Iraq: REPLY

Timing of the Arabia-Eurasia continental collision—Evidence from detrital zircon U-Pb geochronology of the Red Bed Series strata of the northwest Zagros hinterland, Kurdistan region of Iraq: REPLY

Timing of the Arabia-Eurasia continental collision—Evidence from detrital zircon U-Pb geochronology of the Red Bed Series strata of the northwest Zagros hinterland, Kurdistan region of Iraq

One of the major debated aspects of the Zagros orogenic system in the Middle East is the timing of onset of continental collision between Arabia and Eurasia. The Zagros hinterland in the Kurdistan region of Iraq contains an ~2-km-thick clastic depositional sequence of the Red Bed Series (RBS) that rests unconformably on the Arabian foreland and structurally below the Main Zagros fault, which carries the allochthonous volcaniclastic rocks of the Walash and Naopurdan groups in its hanging wall. Detrital zircon (DZ) U-Pb geochronol- ogy constrains both the depositional age and the provenance of the RBS and pinpoints the timing of initial arrival of Eurasian sediment on the Arabian plate. The youngest DZ U-Pb ages for the laterally extensive (~150 km) basal RBS (Suwais unit) imply a middle Oligocene (ca. 26 Ma) maximum depositional age. The provenance data reveal dominant DZ U-Pb age modes of late Paleocene (ca. 55–60 Ma) and middle Eocene (ca. 37–44 Ma) and, importantly, the presence of ~10%–15% DZ grains that are unequivocally derived from Eurasia, includ- ing of Jurassic (150–200 Ma) and late Paleozoic (270–380 Ma) DZ age modes. These data suggest that the RBS deposits were mainly sourced from forearc- and/or arc-related terranes along the southwest margin and hinterland of Eurasia. We advocate that by ca. 26 Ma, Neo- Tethys oceanic crust had been consumed and that Arabia-Eurasia continental collision well was underway as indicated by deposition of strata with Eurasian provenance on the Arabian margin. These DZ U-Pb data from the RBS highlight the significance of provenance data from synorogenic deposits in revealing the timing of initial continent collision by document- ing the earliest arrival of upper-plate sediment on the lower plate.

Incremental Growth of Mid- to Upper-Crustal Magma Bodies During Arabia–Eurasia Convergence and Collision: A Petrological Study of the Calc-Alkaline to Shoshonitic Meghri–Ordubad Pluton (Southern Armenia and Nakhitchevan, Lesser Caucasus)

The composite Meghri-Ordubad pluton (MOP) is located in the southernmost Lesser Caucasus and outcrops over ~1000 km2 in southern Armenia and Nakhitchevan. It is characterized by nested intrusions incrementally emplaced over ~30 Myr from the Middle Eocene to Early Miocene, coeval with the closure of the Neotethyan Ocean and the regional Arabia-Eurasia continental collision. We recognize three compositionally distinct intrusive series in the MOP: a medium- to high-K calc-alkaline series 1 dominated by gabbro, quartz diorite and tonalite, a shoshonitic to high-K calc-alkaline series 2 including gabbro, monzogabbro, monzodiorite, monzonite and syenite, and a high-K calc-alkaline adakitic series 3 characterized by the emplacement of lamprophyre dikes followed by porphyritic granodioritic intrusions and dikes. Thermobarometry calculations together with detailed petrography, mineral chemistry and experimental mineral stability P-T diagrams indicate a polybaric crystallization history and magma emplacement at mid- to upper crustal levels (0.1-0.3 GPa). Mineral textures and compositions reveal open-system magmatic processes such as mafic magma replenishment and reactivation of crystal mushes prior to transport towards shallower crustal levels in the MOP. The evolution of magma chemistry (calc-alkaline vs shoshonitic) and mineral assemblages (amphibole-bearing vs amphibole-free) indicate fluctuation in primary melt alkali and water contents over time. Such chemical evolution is ascribed to a decrease in the degree of partial melting of a chemically heterogeneous, but isotopically homogeneous, mantle source. Overall, the MOP is characterized by the successive emplacement of hydrous magma batches in the mid- to upper crust and petrological processes that were dominated by fractional crystallization (± crustal assimilation) upon cooling and, or, decompression (± degassing). We propose a new temporal and spatial petrogenetic model for the MOP, encompassing three long-lived, chemically distinct magmatic series incrementally assembled over 30 Myr within a pre-, syn- and post-collisional geodynamic setting.

Interaction between active tectonics, erosion and diapirism, a case study from Habble-Rud in Southern Central Alborz (Northern Iran)

The Alborz mountain chain is a region of active deformation within the Arabia–Eurasia continental collision zone. The southern part of central Alborz Mountains, in the north of Iran, represents complex tectonics because it is located at the border of two developing continental sedimentary basins between southern central Alborz and Central Iran. An arid and semi-arid climate, a large extent of Quaternary sediments, rugged topography, salt domes and faults with historical seismicity influence the Habble-Rud River catchment. In the present research, a number of tectonic geomorphologic indices were extracted from satellite imagery and 10m DEM (digital elevation model) data in order to identify relative tectonic activity within the basin. The indices include: stream length-gradient index (Sl), drainage basin asymmetry (Af), index of mountain front sinuosity (Smf), hypsometric integral (Hi), index of drainage basin shape (Bs), ratio of valley-floor width to valley height (Vf), and fault density (Fd). Due to the presence of heterogeneous indices for all sections of the catchment causing large extension of Habble-Rud (3260km2), all of the variables such as extremely erodible formations, faults and folds and salt tectonics on the Southern part; were put into a matrix table. As a new approach, the variables were put into the SAW (simple additive model) model as one of MADM (multi-attribute decision-making models) techniques. The study area was divided into four regions according to the values of SAW. These classes include very high (%11), high (48.3%), moderate (34.7%), and low activity (3.4%). The result of the model suggests that the study area is located on a changing tectonic trend in central Alborz from NW-SE to NE-SW. The regions with high relative tectonic activity in HR catchment correspond to the active Garmsar and Sorkhe-Kalout faults and diapirs.

30 Myr of Cenozoic magmatism along the Tethyan margin during Arabia–Eurasia accretionary orogenesis (Meghri–Ordubad pluton, southernmost Lesser Caucasus)

Three magmatic series of substantially different ages and compositions were successively emplaced to form the composite Meghri–Ordubab pluton (MOP) in the southernmost Lesser Caucasus. The protracted incremental assembly during 30Myr, from Middle Eocene to Early Miocene, renders this location particularly suitable to characterize the petrogenetic evolution of Cenozoic magmatism during the final stage of the Neotethyan subduction. Based on whole-rock geochemistry, two main transitions tightly constrained in time are recognized. The first transition from Middle Eocene medium-K calc-alkaline to Late Eocene–Middle Oligocene shoshonitic magmatism corresponds to a marked increase in LREE and MREE and more juvenile 87Sr/86Sr and 143Nd/144Nd ratios. The second transition to Late Oligocene–Early Miocene high-K calc-alkaline “adakite-like” magmatism is coeval with a marked increase in Mg#, and Ni and Cr contents together with a depletion in MREE and HREE. Although the three differentiation series are derived from lower to mid-crustal hydrous magma fractionation, temporal variations of the magmatic source conditions are required to explain the contrasting chemistry of the parental magmas over time. Medium-K calc-alkaline parental magmas were generated by high degree of partial melting (~15%) of a garnet lherzolite. The shoshonitic and adakitic magmatic series represent magmas produced by low degree (1–5%) of partial melting of a garnet lherzolite, but a higher amount of residual garnet is required to reproduce the "adakite-like" signature. The timing of the two main geochemical transitions in the MOP is correlated with a progressive evolution from a compressional to an extensional stress regime linked to (1) the final stage of the Neotethyan subduction followed by the Arabia–Eurasia continental collision during the Eocene–Oligocene, and (2) the transition toward post-collisional magmatism, combined with a switch toward transcurrent tectonics during the Late Oligocene–Early Miocene.

Constraining quantitatively the timing and process of continent-continent collision using magmatic record: Method and examples

Based on the main driving force of plate motion (the slab pull force generated by the descent of the oceanic plate in subduction zones) and the three primary mechanisms for magma generation (adding fluid, increasing temperature, and decreasing pressure), the continent-continent collisional process has been divided into three stages, including initial collision, ongoing collision, and tectonic transition. These stages are characterized by normal calc-alkaline andesitic magma (dehydration of the oceanic crust to release fluids), the migration of calc-alkaline magma toward the trench (dehydration of the oceanic crust or an increase in temperature) or small-scale crust-derived peraluminous magma (heat from intra-crustal shearing), and extensive magmatism with compositional diversity induced by slab break-off (increasing temperature and decreasing pressure), respectively. On the basis of the obtained age of slab break-off, the timing of the initial continent-continent collision can be quantitatively back-dated using the convergence rate, depth of slab break-off, and subduction angle. The spatio-temporal migration of the magmatic activity of the Gangdese Batholith, the onset of magmatic flare-up, and the increase of magma temperature at 52–51 Ma documented by the volcanic rocks of the Linzizong Pana Formation were most likely the result of the break-off of the Yarlung-Zangbo Neo-Tethyan oceanic lithosphere at approximately 53 Ma. This proposed age of slab break-off suggests that the initial India-Asia collision likely occurred at approximately 55–54 Ma, which is consistent with the collision ages constrained by other abundant geological data (60–55 Ma). This magmatic method has been applied to the Bitlis orogenic belt in southern Turkey in the Arabia-Eurasia continental collision zone, yielding an age range of approximately 29–22 Ma for the initial Arabia-Asia continental collision that is close to the collision ages recently obtained by apatite fission-track dating (approximately 20 Ma) and regional tectonic shortening (approximately 27 Ma). The intense folding of the Upper Cretaceous and the angular unconformity between the overlying Linzizong volcanic rocks in the southern Lhasa Terrane (90−69 Ma) are not related to the initial continental collision between India and Asia, but can be interpreted as the consequences of the strong coupling between the hot and young subducting oceanic crust immediately south of the spreading ridge and the overriding lithosphere or the subduction of the Neo-Tethys oceanic plateaux or seamounts. The tectonic event documented by the angular unconformity between the Linzizong Dianzhong Formation and the Nianbo Formation lasted approximately 3 Ma and likely marks the initial India-Asia collision. The significant deceleration of the Indian continent at approximately 51 Ma can be attributed to the disappearance of the slab pull force in the subduction zone due to the break-off of the Yarlung-Zangbo Neo-Tethyan oceanic lithosphere. The descent of the eclogitized lower crust of the northern Indian continent provides the main driving force for the current northward motion of Indian plate. The weak deformation of the lithospheric plate in the overriding plate of the India-Asia collisional zone between 60 and 40 Ma can be attributed to the high-angle subduction related to the rollback of the Yarlung-Zangbo Neo-Tethyan oceanic lithosphere after the initial India-Asia continental collision, the presence of the thick crust and high elevation on the southern margin of the Lhasa Terrane, and the decoupling between the mid-upper and lower crust and between the lower crust and lithospheric mantle of the Indian continent.

U–Pb zircon geochronology of the Daraban leucogranite, Mawat ophiolite, Northeastern Iraq: A record of the subduction to collision history for the Arabia–Eurasia plates

AbstractThe Mawat ophiolite is part of the Mesozoic Neo‐Tethyan ophiolite belt of the Middle East and is located in the Zagros Imbricate Zone of Iraq. It represents fossil fragments of the Neo‐Tethyan oceanic lithosphere within the Alpine collisional system between the Arabian and Eurasia Plates. The first U–Pb zircon dating of the Daraban leucogranite from the Mawat ophiolite provides a 207Pb–206Pb age of 96.8 ± 6.0 Ma. The age is 59.0 ± 6.0 m.y. older than the previously published age of the Daraban leucogranite obtained by 40Ar–39Ar muscovite dating method. The U–Pb dating of magmatic zircons collected from the Daraban leucogranite, which intrudes into the Mawat ophiolite, reveals that melting of the pelagic sediment beneath the hot Zagros proto‐ophiolite in an intra‐oceanic arc environment led to anatexis at the subduction front and the generation of granitic melts at 96.8 ± 6.0 Ma, which were emplaced in the overlaying mantle wedge. This process was a response to the initial formation of the Neo‐Tethys ophiolite above a northeast‐dipping intra‐oceanic subduction zone at 96.8 ± 6.0 Ma. Published 40Ar–39Ar muscovite dating from the same leucogranite dike yields plateau ages of 37.7 ± 0.3 Ma, reflecting that the age was reset during the Arabia–Eurasia continental collision. Therefore, the bimodal age populations from the granitic intrusion in the Mawat ophiolite preserve a record of the subduction to the collision cycle of the Zagros Orogenic Belt. The 59.0 ± 6.0 m.y. age difference from the Daraban leucogranite represents the duration of the subduction‐collision cycle of the Zagros Orogenic Belt in the Kurdistan region of Iraq and the time span for the closure of the Neo‐Tethys Ocean along the northern margin of the Arabian plate.

Geochemistry and Ar–Ar muscovite ages of the Daraban Leucogranite, Mawat Ophiolite, northeastern Iraq: Implications for Arabia–Eurasia continental collision

Daraban Leucogranite dykes intruded discordantly into the basal serpentinized harzburgite of the Mawat Ophiolite, Kurdistan region, NE Iraq. These coarse grained muscovite-tourmaline leucogranites are the first leucogranite dykes identified within the Mawat Ophiolite. They are mainly composed of quartz, K-feldspar, plagioclase, tourmaline, muscovite, and secondary phologopite, while zircon, xenotime, corundum, mangano-ilemnite and cassiterite occur as accessories.The A/CNK value of the granite dyke samples varies from 1.10 to 1.22 indicating a strongly peraluminous composition. CaO/Na2O ranges from 0.11 to 0.15 and Al2O3/TiO2 from 264 to 463, similar to the strongly peraluminous (SP) granites exposed in ‘high-pressure’ collision zones such as the Himalayas.Ar–Ar muscovite step-heating dating yields 37.57±0.25 and 38.02±0.53Ma plateau ages for two samples which are thought to reflect either their magmatic emplacement or resetting during collision-related metamorphism. Mineral chemistry shows evidence of both primary and secondary types of muscovite, with cores favouring the magmatic interpretation and slight effects of a late syn-serpentinization fluid seen at the rims.Geochemical features of Daraban Leucogranite dykes favour a syn-collisional tectonic setting. They probably formed in response to the continental collision between Eurasia and Arabia during the initial stage of the opening of the Gulf of Aden at 37Ma. The muscovite ages and geochemical features of Daraban Leucogranite are strong evidence for the timing of the continental collision between northeastern Arabia and Eurasia in Kurdistan region of Iraq.