Arabia-Eurasia Collision Research Articles

Lake overspill and onset of fluvial incision in the Iranian Plateau: Insights from the Mianeh Basin

Orogenic plateaus represent a prime example of the interplay between surface processes, climate, and tectonics. This kind of an interplay is thought to be responsible for the formation, preservation, and, ultimately, the destruction of a typical elevated, low-internal relief plateau landscape. Here, we document the timing of intermontane basin filling associated with the formation of a low-relief plateau morphology, followed by basin opening and plateau-flank incision in the northwestern Iranian Plateau of the Arabia–Eurasia collision zone. Our new U–Pb zircon ages from intercalated volcanic ashes in exposed plateau basin-fill sediments from the most external plateau basin (Mianeh Basin) document that the basin was internally drained at least between ∼7 and 4 Ma, and that from ∼5 to 4 Ma it was characterized by an ∼2-km-high and ∼0.5-km-deep lake (Mianeh paleolake), most likely as a result of wetter climatic conditions. At the same time, the eastern margin of the Mianeh Basin (and, therefore, of the Iranian Plateau) experienced limited tectonic activity, as documented by onlapping sediments and smoothed topography. The combination of high lake level and subdued topography at the plateau margin led to lake overspill, which resulted in the cutting of an ∼1-km-deep bedrock gorge (Amardos) by the Qezel-Owzan River (QOR) beginning at ∼4 Ma. This was associated with the incision of the plateau landscape and the establishment of fluvial connectivity with the Caspian Sea. Overall, our study emphasizes the interplay between surface and tectonic processes in forming, maintaining, and destroying orogenic plateau morphology, the transitional nature of orogenic plateau landscapes on timescales of 106 yr, and, finally, the role played by overspilling in integrating endorheic basins.

Underplating along the northern portion of the Zagros suture zone, Iran

A 2-D absolute shear wave velocity model has been resolved beneath a seismic profile across the northeastern margin of the Arabian Plate—Central Iran by simultaneously inverting data from P receiver functions and fundamental mode Rayleigh wave phase velocity. The data were gathered by a linear seismic array crossing the Zagros fold and thrust belt, Urmia–Dokhtar magmatic arc and Central Iran block assemblage as three major structural components of the Arabia–Eurasia collision. Our model shows a low-velocity tongue protruding from upper to lower crust which, north of the Zagros suture, indicates the signature of an intracontinent low-strength shear zone between the underthrusting and overriding continents. The velocity model confirms the presence of a significant crustal root as well as a thick high-velocity lithosphere in footwall of the suture, continuing northwards beneath the overriding continent for at least 200 km. These features are interpreted as underthrusting of Arabia beneath Central Iran. Time to depth migration of P receiver functions reveals an intracrustal flat interface at ∼17 km depth south of the suture; we interpret it as a significant decoupling within the upper crust. All these crustal scale structural features coherently explain different styles and kinematics of deformation in northern Zagros (Lorestan zone) with respect to its southern part (Fars zone).

Reactivation versus reworking of the active continental margin during the Zagros collision: Mahallat–Muteh–Laybid complexes, Sanandaj–Sirjan zone, Iran

Reactivation of long-lived basement faults has significant influences on further deformation of collision zones. Three major inherited pre-collisional NW-, N- and NE-trending basement discontinuities have played important roles on the structural and tectono-sedimentary evolution of the Iranian micro-continent in the northeastern part of the Gondwana super-continent. Sanandaj–Sirjan zone (SSZ), known as the metamorphic belt of the Zagros orogeny, marks the SW margin of the Central Iran. SSZ is formed as a result of the Arabia–Eurasia collision and its general trend of deformation coincides with the NW structural trend of the collision. The NE-trending Mahallat, Muteh and Laybid complexes in the middle part of the NW-trending SSZ are the exception and have a trend almost normal to the NW-trending Zagros. A combined methodology of remote sensing, geometric and kinematics analyses complemented by field work was used to reconstruct the history of deformation in the Zagros hinterland since the earlier stages of collision to the present-day. Our results reveal the key role of the preexisting discontinuities of the Iranian basement in both the kinematics and structural pattern of the middle part of the SSZ. These basement faults have acted as main boundary conditions changing the collisional fabric perpendicular to its overall trend. Progressive deformation and the related changes during collision have caused drastic changes in the kinematics of the boundary faults. The establishment of dextral transtension in the SSZ has had secondary influences on the pattern of deformation by local clockwise rotation and localized dextral shear in the southern parts of the area of interest. This study highlights the significance of long-lived pre-existing structures in the deformation of collision zones. Such basement faults are capable to change both the pattern and kinematics of deformation of the adjacent areas involved in a continental collision.

Zircon U-Pb geochronology and emplacement history of intrusive rocks in the Ardestan section, central Iran

The Urumieh-Dokhtar Magmatic Arc (UDMA) is part of the Alpine–Himalayan orogenic belt and interpreted to be a subduction-related Andean-type magmatic arc. Along this belt, Eocene volcanics and some gabbroic to granitic bodies crop out. The main rock types of the studied intrusion are granite, granodiorite, and diorite. They have geochemical features typical of magnesian, calc-alkaline, metaluminous to slightly peraluminous granites and I-type intrusive rock that have a strong enrichment in Large-Ion Lithophile (LIL) elements (e.g. Rb, Ba, Sr), and a depletion in High Field Strength (HFS) elements (e.g. Nb, Ti, P), typical of subduction-related magmas. Zircon U-Pb dating was applied to determine the emplacement ages of the different intrusions in the Ardestan area. Among them the Kuh-e Dom diorite is 53.9±0.4Ma old; the Kuh-e Dom granodiorite is 51.10±0.4Ma old; the Mehrabad granodiorite is 36.8±0.5Ma old, the Nasrand granodiorite is 36.5±0.5Ma old, the Zafarghand granodiorite is 24.6±1.0Ma old, and the Feshark granodiorite is 20.5±0.8Ma old. These results delineate more accurately the magmatic evolution related to the Neotethyan subduction from the Lower Eocene to Lower Miocene, and the subsequent Zagros orogeny that resulted from the Arabia-Eurasia collision. The emplacement of these intrusive rocks inside the UDMA, which has a close relationship with the collisional orogeny, is transitional from a subduction-related setting to post-collisional setting in the Ardestan area.

Relict basin closure and crustal shortening budgets during continental collision: An example from Caucasus sediment provenance

Comparison of plate convergence with the timing and magnitude of upper-crustal shortening in collisional orogens indicates both shortening deficits (200-1700 km) and significant (10-40%) plate deceleration during collision, the cause(s) for which remain debated. The Greater Caucasus Mountains, which result from post-collisional Cenozoic closure of a relict Mesozoic back-arc basin on the northern margin of the Arabia-Eurasia collision zone, help reconcile these debates. Here we use U-Pb detrital zircon provenance data and the regional geology of the Caucasus to investigate the width of the now-consumed Mesozoic back-arc basin and its closure history. The provenance data record distinct southern and northern provenance domains that persisted until at least the Miocene. Maximum basin width was likely ~350-400 km. We propose that closure of the back-arc basin initiated at ~35 Ma, coincident with initial (soft) Arabia-Eurasia collision along the Bitlis-Zagros suture, eventually leading to ~5 Ma (hard) collision between the Lesser Caucasus arc and the Scythian platform to form the Greater Caucasus Mountains. Final basin closure triggered deceleration of plate convergence and tectonic reorganization throughout the collision. Post-collisional subduction of such small (102-103 km wide) relict ocean basins can account for both shortening deficits and delays in plate deceleration by accommodating convergence via subduction/underthrusting, although such shortening is easily missed if it occurs along structures hidden within flysch/slate belts. Relict-basin closure is likely typical in continental collisions in which the colliding margins are either irregularly shaped or rimmed by extensive back-arc basins and fringing arcs, such as those in the modern South Pacific.

Tertiary and quaternary marine terraces and planation surfaces of northern Oman: Interaction of flexural bulge migration associated with the Arabian-Eurasian collision and eustatic sea level changes

The northeastern Arabian passive margin is being subducted beneath the Zagros and Makran of Iran. A flexural bulge related to the weight of the Makran has migrated at 4 cm/a through the previously uplifted Hajar Mountains of Oman as this active convergence and collision between Arabia and Eurasia progresses, adding approximately another 500 meters of relief, and forming a series of uplifted marine terraces, alluvial terraces, and planation surfaces that record the passage of the bulge. We use a combination of field studies, remote sensing and GIS to map and better-understand these terraces, and elucidate how the tectonics of bulge migration, down-to-trench normal faulting, and eustatic sea level changes have interacted to produce the extant geomorphic features on the inner slope of the flexural bulge as it sinks into the foredeep of the Gulf of Oman. We speculate those terraces that were uplifted on the outer slope of the forebulge as it initially migrated through the passive margin (affected by ophiolite obduction in the Cretaceous) 3.75–7.5 Ma ago are now sinking on the inner slope of the forebulge (corresponding to the outer trench slope in the foredeep), and have been partly covered by Quaternary marine terraces related to a Weichselian sea level high stand. Both the Tertiary and Quaternary terraces are cut by faults related to the active collision, confirming that there is a significant risk of moderate earthquakes in the region.

The south Zagros suture zone in teleseismic images

The geometry of intra-continental lithosphere boundaries along the Zagros orogenic belt in the Arabia-Eurasia collision is investigated by means of teleseismic data. The data are gathered over a seismic linear profile extending across south Zagros, the Sanandaj–Sirjan metamorphic zone, the Urumieh–Dokhtar magmatic arc, Central Iran, and the Kopeh Dagh - Binalud mountains. We exploit the P and S receiver functions leading to map the geometry of the crustal and subcrustal interfaces. The migrated depth sections reveal an abrupt crustal thickening and a gentle crustal thinning ~60km north and ~30km south of the Zagros suture, respectively. Associated to the buckled antiformal Moho south of the suture, a deeper synform in the lithospheric lid of the lower Arabia plate is shown by migrated depth sections affecting the lithospheric mantle of the Arabia plate beneath the suture zone. This geometry implies an unexpected intra-lid decoupling. These features imply that the Central Iran lithosphere acts as a relatively strong backstop producing significant internal deformation expressed by shortening and thickening at the edge of the Arabian lithosphere. The 410km and 660km transition zones are imaged by P to S converted phases and showed lateral continuity implying an originally low dip angle subduction of the oceanic Arabian plate beneath Central Iran.

New age and geochemical constraints on the origin of Quaternary adakite-like lavas in the Arabia–Eurasia collision zone

Adakite-like lavas from the Iranian segment of the Arabia–Eurasia collision zone have been considered as partial melts derived from a subducted Neotethyan slab, a hypothesis that cannot be further evaluated due to the lack of systematic age and radiogenic isotopic data. Here, we revisit the Anar region, Iran where such lavas were reported by providing these data to examine their probable origin as slab melts. New SIMS zircon U–Pb ages attest to eruption at ca. 1 to 2Ma, postdating the Arabia–Eurasia collision. The lavas are characterized by dacitic to rhyolitic, metaluminous to slightly peraluminous and trondhjemitic compositions, and incompatible element patterns similar to continental crustal rocks. Extreme depletion in Nb, Ta, Y and the heavy REE relative to the light and middle REEs reflects control by garnet and rutile. Except one sample presumably affected by secondary gain of radiogenic Sr, the depleted Sr–Nd–Hf isotopic compositions relative to the Bulk Silicate Earth [(87Sr/86Sr)i=0.7040 to 0.7043; εNd(t)=+3.2 to +5.3; εHf(t)=+10.5 to +11.3] indicate magma genesis from juvenile source rocks. A slab melt origin for the Anar lavas is very unlikely and the reason is twofold. First, the Y and HREE of the source rocks estimated by inversed batch melting models assuming garnet pyroxenite residue are far too low compared with MORB-like lithologies and subducted sediments. Second, the low MgO, Ni and Cr of the lavas point to limited interaction with peridotites in a mantle wedge. Fractionation-related models are also unlikely due to the apparent lack of broadly coeval, mafic precursor magmas without adakitic signature. The low-Y and low-HREE source rocks revealed by the melting models, together with the low MgO, Ni and Cr of the Anar lavas, can be explained by lower crustal melting under eclogite-facies conditions. These source rocks were most likely high-pressure cumulates in the root of the Urumieh–Dokhtar magmatic arc, a Neotethyan arc that was active in the Paleogene. Lower crustal melting might be related to localized destruction of the continental mantle, giving rise to the temporally and spatially associated ultrapotassic magmas. The hypothesis of an ‘adakitic province’ near Anar where slab detachment and melting occurred does not hold in light of available data and observations.

Geology, zircon geochronology, and petrogenesis of Sabalan volcano (northwestern Iran)

Sabalan Volcano (NW Iran) is an isolated voluminous (4821m elevation; >800km2) composite volcano that is located within the Arabia-Eurasia collision zone. Its edifice was assembled by recurrent eruptions of trachyandesite and dacite magma falling into a relatively restricted compositional range (56–67% SiO2) with high-K calc-alkaline and adakitic trace element (Sr/Y) signatures. Previous K-Ar dating suggested protracted eruptive activity between 5.6 and 1.4Ma, and a two stage evolution which resulted in the construction of the Paleo- and Neo-Sabalan edifices, respectively. The presence of a topographic moat surrounding Neo-Sabalan and volcanic breccias with locally intense hydrothermal alteration are indicative of intermittent caldera collapse of the central part of Paleo-Sabalan. Volcanic debris-flow and debris-avalanche deposits indicate earlier episodes of volcanic edifice collapse during the Paleo-Sabalan stage. In the Neo-Sabalan stage, three dacitic domes extruded to form the summits of Sabalan (Soltan, Heram, and Kasra). Ignimbrites and minor pumice fall-out deposits are exposed in strongly dissected drainages that in part have breached the caldera depression. Lavas and pyroclastic rocks are varyingly porphyritic with Paleo-Sabalan rocks being trachyandesites carrying abundant phenocrysts (plagioclase+amphibole+pyroxene+biotite). The Neo-Sabalan rocks are slightly more evolved and include dacitic compositions with phenocrysts of plagioclase+amphibole±alkali-feldspar±quartz. All Sabalan rock types share a common accessory assemblage (oxides+apatite+zircon). High spatial resolution and sensitivity U-Pb geochronology using Secondary Ionization Mass Spectrometry yielded two clusters of zircon ages which range from 4.5 to 1.3Ma and 545 to 149ka, respectively (all ages are averages of multiple determinations per sample). U-Th zircon geochronology for selected Neo-Sabalan rocks agrees with the U-Pb ages, with the youngest zircon rims dating to ca. 110 ka. Because zircon crystallization predates eruption, this age represents the upper limit for the youngest eruptions of Sabalan. Valley-filling ignimbrites yielded variable U-Pb zircon ages which argue against these pyroclastic rocks being generated in a single caldera forming event. These results indicate that eruptions occurred more recently than previously indicated by K-Ar dating. Paleo-Sabalan and Neo-Sabalan volcanic rocks have similar geochemical characteristics, including enrichment of LILE and LREE relative to HFSE and HREE, respectively, and prominent negative Ti, Nb, and Ta anomalies. The trachyandesitic to dacitic rocks of Sabalan also share negative Eu anomalies. This, together with horizontal or slightly increasing Y vs. Rb trends, indicates fractionation of plagioclase-amphibole or plagioclase-clinopyroxene assemblages with negligible crustal assimilation (based on low and invariant Rb/Th). High degrees of mantle partial melting are inferred from high (La/Yb)N (from 28 to 48). Overall, the subduction-affinity of Sabalan volcanic rocks agrees with models of melt generation following a Quaternary slab break-off event coeval with continental collision.

3D crustal and lithospheric model of the Arabia–Eurasia collision zone

Three-dimensional joined inversion of topography, gravity and geoid data was carried out in the collision zone between the Eurasian and Arabian plates, comprising Central and Eastern Anatolia, the Black Sea Basin and the Caucasus region. The 3D algorithm, first introduced by Motavalli-Anbaran et al. (2013), is based on a Bayesian approach with Gaussian density functions in which the targeted area is divided into vertical columns, each comprised of four layers namely water (of known thickness i.e. bathymetry) if present, crust, lithospheric mantle, and asthenosphere. The inversion results are Moho depth, average crustal density and depth to the lithosphere–asthenosphere boundary, defined here as the 1350°C isotherm. Existing seismic Moho depths were used as a priori information in order to constrain crustal thicknesses. Thickest crust (up to 50km) was found underneath the Great Caucasus Mountains, thinnest crust (22–25km) underneath the Black Sea Basin. The East-Anatolian plateau is underlain by relatively thick crust (up to 45km), thinning to less than 40km towards the Central Anatolian Plateau. Eastern Anatolia and the eastern part of Central Anatolia are underlain by strongly thinned lithosphere (up to 100km). Thickest lithosphere (up to 200km) underlies the Caspian Basin and the Russian Platform.

Detrital zircon provenance analysis in the Zagros Orogen, SW Iran: implications for the amalgamation history of the Neo-Tethys

The Zagros Orogen developed as a result of Arabia–Eurasia collision. New in situ detrital zircon U–Pb and Hf isotopic analyses from a Cenozoic sedimentary sequence in SW Iran are used to unravel the amalgamation history of Neo-Tethys. Data indicate that: (1) Paleocene and Eocene strata (58 and 45 Ma, respectively) were sourced from obducted ophiolite and Triassic volcanics, (2) Lower Miocene (~18 Ma) strata indicate mixed provenance from obducted ophiolite and Iranian magmatic rocks, (3) Mid to Upper Miocene sediments (~14 to 11.2 Ma) were mainly sourced from Sanandaj–Sirjan zone granitoids to the north, and (4) Lower Pliocene (~5 Ma) sediments mainly show Arabian age characteristics, with a minor Eurasian affinity component. Two hypotheses are outlined to highlight the key events: Hypothesis A, previously published by several workers, suggests that the sequence studied lay on the Arabia passive margin and that initial collision occurred prior to 18 Ma; Hypothesis B, modified from the Makran model, which is here preferred, suggests that Paleogene to Upper Miocene sediments were sourced from the northern Neo-Tethyan accretionary complex or Eurasia, and carry no input from Arabia, whereas the Lower Pliocene sample shows a mixed provenance from both Arabia and Eurasia, suggesting that collision occurred between ~11.2 and 5 Ma.

Tectono‐sedimentary evolution of the northern Iranian Plateau: insights from middle–late Miocene foreland‐basin deposits

AbstractSedimentary basins in the interior of orogenic plateaus can provide unique insights into the early history of plateau evolution and related geodynamic processes. The northern sectors of the Iranian Plateau of the Arabia–Eurasia collision zone offer the unique possibility to study middle–late Miocene terrestrial clastic and volcaniclastic sediments that allow assessing the nascent stages of collisional plateau formation. In particular, these sedimentary archives allow investigating several debated and poorly understood issues associated with the long‐term evolution of the Iranian Plateau, including the regional spatio‐temporal characteristics of sedimentation and deformation and the mechanisms of plateau growth. We document that middle–late Miocene crustal shortening and thickening processes led to the growth of a basement‐cored range (Takab Range Complex) in the interior of the plateau. This triggered the development of a foreland‐basin (Great Pari Basin) to the east between 16.5 and 10.7 Ma. By 10.7 Ma, a fast progradation of conglomerates over the foreland strata occurred, most likely during a decrease in flexural subsidence triggered by rock uplift along an intraforeland basement‐cored range (Mahneshan Range Complex). This was in turn followed by the final incorporation of the foreland deposits into the orogenic system and ensuing compartmentalization of the formerly contiguous foreland into several intermontane basins. Overall, our data suggest that shortening and thickening processes led to the outward and vertical growth of the northern sectors of the Iranian Plateau starting from the middle Miocene. This implies that mantle‐flow processes may have had a limited contribution toward building the Iranian Plateau in NW Iran.

Geochemistry of the Eocene magmatic rocks from the Lesser Caucasus area (Armenia): evidence of a subduction geodynamic environment

Abstract This paper is focused on petrological and geochemical data obtained on a series of Middle and Upper Eocene magmatic rocks from the Lesser Caucasus of Armenia in order to elucidate magma sources and geodynamic processes. Middle–Upper Eocene magmatism is present in two main zones: the Amasia–Sevan–Hakari suture zone (ASHSZ) and the so-called South Armenian Microplate (SAM). Volcanic rocks from both places range from basalt to rhyolite and mostly display a calc-alkaline character. Trace element patterns from the SAM and ASHSZ samples show mobile-elements enrichment (Rb, Ba, Th) together with strong negative high field strength elements (Nb, Ta, Hf, Zr) anomalies. The (La/Sm) N ratio yields very close values for both areas. Conversely, the (La/Yb) N ratio is, on average, significantly higher for SAM than for ASHSZ, suggesting the presence of residual garnet at the source of the SAM volcanic rocks. Nevertheless, trace elements suggest partial melting from phlogopite- and amphibole-bearing spinel lherzolitic mantle sources. Neodymium and strontium isotopes yield εNd (40Ma) and 87 Sr/ 86 Sr (40Ma) ratios ranging, respectively, from –0.3 to +6.6 and from 0.70314 to 0.70531 for SAM samples, and from +3.4 to +6.8 and from 0.70393 to 0.70433 for ASHSZ samples. Initial Pb/Pb isotopic ratios yield close values for both areas but with slightly higher and more homogeneous 207 Pb/ 204 Pb and 208 Pb/ 204 Pb ratios for SAM samples. Such features concur with a more pronounced slab-component contribution in the frontal part of the volcanic belt, that is, in the SAM domain. No significant crustal contamination has been detected in the studied Eocene magmatic rocks from both the ASHSZ and SAM. Considering geodynamic and geochemical constraints, we propose that this magmatism is connected with a north-dipping Southern Neotethys subduction, in an extensional (back-arc) environment of orogenic belts. The Arabia–Eurasia collision and the closure of the Neotethys Ocean may have occurred after this magmatic event.

Spatial variations in late Quaternary slip rates along the Doruneh Fault System (Central Iran)

The Doruneh Fault System (DFS) is one of the major active strike-slip faults in the Arabia-Eurasia collision zone. Despite its geological activity, no large (M ≥ 6.5) historical or instrumental earthquakes have been recorded along it. To date, the rate and distribution of slip, as well as the seismic behavior of the DFS, have been unknown. We reconstructed 67 geomorphic offsets recorded by three successive alluvial abandonment surfaces (Q1, Q2, and Q3) displaced along the western (WFZ) and central (CFZ) fault zones. The determined ages of ~12, ~36, and ~120 ka, using in situ-produced 10 Be and 36 Cl cosmogenic nuclides for theses surfaces, allowed to estimate three sets of individual left-lateral slip rates and consequently to describe the spatiotemporal distribution of slip along the CFZ and WFZ. The slip rates averaged over time intervals of ~36 and ~120 ka reveal variable slip rates along length but similar slip rates at a point with a maximum rate of ~8.2 mm/yr. During the Holocene, however, the fault slip behavior appears more complex, with a maximum rate of ~5.3 mm/yr. The CFZ is divided into two ~4 km apart segments, with symmetrical slip distributions relative to a persistent boundary, which has not been ruptured over the last ~12 ka. The maximum length of seismic fault segments varies from 70 to 100 km, which could produce earthquakes with a magnitude of M w 7.2–7.4. This emphasizes the necessity of segmentation models for long strike-slip faults that may not necessarily rupture along their whole length during a single earthquake.

Alpine-Himalayan orogeny drove correlated morphological, molecular, and ecological diversification in the Persian dwarf snake (Squamata: Serpentes:Eirenis persicus)

The Persian dwarf snake Eirenis (Pseudocyclophis) persicus (Anderson, 1872) has a wide distribution range in south-western Asia. This species group was comprehensively studied here using traditional biometry, geometric morphometrics, ecological niche modelling, and genetics. Our analyses revealed that E. persicus is split into two clades. A western clade, bearing at least two different species: E. persicus, distributed in south-western Iran, and an undescribed species from south-eastern Turkey and western Iran. The eastern clade consists of at least three species: Eirenis nigrofasciatus, distributed across north-eastern Iraq, and western and southern Iran; Eirenis walteri, distributed across eastern Iran, southern Turkmenistan, and western and southern Pakistan, and Eirenis angusticeps, distributed in north-eastern Pakistan. Ecological niche modelling revealed that the distribution of the species in the western clade are mainly affected by winter precipitation, and those in the eastern clade are mainly affected by the minimum temperature of the coldest month. A molecular clock analysis revealed that the divergence and diversification of the E. persicus species group mainly correspond to Eocene to Pliocene orogeny events subsequent to the Arabia–Eurasia collision. This study confirms that specimens with the unique morphology of having 13 dorsal scale rows on the anterior dorsum, occurring in the Suleiman Mountains in central Pakistan, can be referred to Eirenis mcmahoni (Wall, 1911). However, at this moment we have insufficient data to evaluate the taxonomy of this species.

Long-lived, stationary magmatism and pulsed porphyry systems during Tethyan subduction to post-collision evolution in the southernmost Lesser Caucasus, Armenia and Nakhitchevan

The composite Meghri–Ordubad and Bargushat plutons of the Zangezur–Ordubad region in the southernmost Lesser Caucasus consist of successive Eocene to Pliocene magmatic pulses, and host two stages of porphyry Cu–Mo deposits. New high-precision TIMS U–Pb zircon ages confirm the magmatic sequence recognized by previous Rb–Sr isochron and whole-rock K–Ar dating. A 44.03±0.02Ma-old granite and a 48.99±0.07Ma-old granodiorite belong to an initial Eocene magmatic pulse, which is coeval with the first stage of porphyry Cu–Mo formation at Agarak, Hanqasar, Aygedzor and Dastakert. A subsequent Oligocene magmatic pulse was constrained by U–Pb zircon ages at 31.82±0.02Ma and 33.49±0.02Ma for a monzonite and a gabbro, and a late Miocene porphyritic granodioritic and granitic pulse yielded ages between 22.46±0.02Ma and 22.22±0.01Ma, respectively. The Oligo-Miocene magmatic evolution broadly coincides with the second porphyry-Cu–Mo ore deposit stage, including the major Kadjaran deposit at 26–27Ma.Primitive mantle-normalized spider diagrams with negative Nb, Ta and Ti anomalies support a subduction-like nature for all Cenozoic magmatic rocks. Eocene magmatic rocks have a normal arc, calc-alkaline to high-K calc-alkaline composition, early Oligocene magmatic rocks a high-K calc-alkaline to shoshonitic composition, and late Oligocene to Mio-Pliocene rocks are adakitic and have a calc-alkaline to high-K calc-alkaline composition. Radiogenic isotopes reveal a mantle-dominated magmatic source, with the mantle component becoming more predominant during the Neogene. Trace element ratio and concentration patterns (Dy/Yb, Sr/Y, La/Yb, Eu/Eu*, Y contents) correlate with the age of the magmatic rocks. They reveal combined amphibole and plagioclase fractionation during the Eocene and the early Oligocene, and amphibole fractionation in the absence of plagioclase during the late Oligocene and the Mio-Pliocene, consistent with Eocene to Pliocene progressive thickening of the crust or increasing pressure of magma differentiation. Characteristic trace element and isotope systematics (Ba vs. Nb/Y, Th/Yb vs. Ba/La, 206Pb/204Pb vs. Th/Nb, Th/Nb vs. δ18O, REE) indicate that Eocene magmatism was dominated by fluid-mobile components, whereas Oligocene and Mio-Pliocene magmatism was dominated by a depleted mantle, compositionally modified by subducted sediments.A two-stage magmatic and metallogenic evolution is proposed for the Zangezur–Ordubad region. Eocene normal arc, calc-alkaline to high-K calc-alkaline magmatism was coeval with extensive Eocene magmatism in Iran attributed to Neotethys subduction. Eocene subduction resulted in the emplacement of small tonnage porphyry Cu–Mo deposits. Subsequent Oligocene and Miocene high-K calc-alkaline and shoshonitic to adakitic magmatism, and the second porphyry Cu–Mo deposit stage coincided with Arabia–Eurasia collision to post-collision tectonics. Magmatism and ore formation are linked to asthenospheric upwelling along translithospheric, transpressional regional faults between the Gondwana-derived South Armenian block and the Eurasian margin, resulting in decompression melting of lithospheric mantle, metasomatised by sediment components added to the mantle during the previous Eocene subduction event.

Thermochronometric evidence for Miocene tectonic reactivation of the Sevan–Akera suture zone (Lesser Caucasus): a far-field tectonic effect of the Arabia–Eurasia collision?

Abstract Low-temperature thermochronological data for the Eurasian foreland north of the Bitlis–Zagros suture zone suggest that the tectonic stresses related to the Arabian collision during mid-Miocene time were transmitted efficiently over large distances, focusing preferentially at rheological discontinuities. Since the late Middle Miocene a new tectonic regime has been active as the westwards translation of Anatolia is accommodating most of the Arabia–Eurasia convergence, thus precluding the efficient transfer of stress northwards. Apatite fission-track data from the central Lesser Caucasus show that a portion of this orogen underwent a discrete phase of cooling/exhumation at 18–12 Ma (late Early–early Middle Miocene) as a result of the structural reactivation of a segment of the Late Cretaceous–Palaeogene Sevan–Akera suture zone. This inference contradicts the notion that the post-collisional history of the study area was dominated by strike-slip tectonics with relatively minor dip-slip components. Reactivation and exhumation was focused along those segments of the suture zone at high angles to the inferred collisional stress field; the remaining areas were not exhumed enough to expose a new apatite partial annealing zone and thus retained the thermochronological record of a phase of Late Cretaceous cooling/exhumation associated with ophiolite obduction and the following continental collision along the suture zone.

Progressive orocline formation in the Eastern Pontides–Lesser Caucasus

Abstract The Eastern Pontides–Lesser Caucasus fold–thrust belt displays a peculiar northwards arc-shaped geometry that was defined as an orocline in earlier studies. The Lesser Caucasus was affected by two main tectonic events that could have caused orocline formation: (1) Paleocene–Eocene collision of the South Armenian Block with Eurasia; and (2) Oligocene–Miocene Arabia–Eurasia collision. We tested the hypothesis that the Lesser Caucasus is an orocline and aimed to time the formation of this orocline. To determine the vertical axis rotations, 37 sites were sampled for palaeomagnetism in rocks of Upper Cretaceous–Miocene age in Georgia and Armenia. In addition, we compiled a review of c. 100 available datasets. A strike test was applied to the remaining datasets, which were divided into four chronological sub-sets, leading us to conclude that the Eastern Pontides–Lesser Caucasus fold–thrust belt forms a progressive orocline. We concluded that: (1) some pre-existing curvature must have been present before the Late Cretaceous; (2) the orocline acquired part of its curvature after the Paleocene and before the Middle Eocene as a result of South Armenian Block–Eurasia collision; and (3) about 50% of the curvature formed after the Eocene and probably before the Late Miocene, probably as a result of Arabia–Eurasia collision. Supplementary material: Results from rock magnetic experiments, reversal and fold tests and equal area projections of the characteristic remanent magnetizations for each site, as well as biostratigraphic ages and a table with palaeomagnetic results from the literature review (with assigned numbers referred to in the text) are available at http://www.geolsoc.org.uk/SUP18852 .

Lithospheric structure in Central Eurasia derived from elevation, geoid anomaly and thermal analysis

Abstract We present new crustal and lithospheric thickness maps for Central Eurasia from the combination of elevation and geoid anomaly data and thermal analysis. The results are strongly constrained by numerous previous data based on seismological and seismic experiments, tomographic imaging and integrated geophysical studies. Our results indicate that high topography regions are associated with crustal thickening that is at a maximum below the Zagros, Himalaya, Tien Shan and the Tibetan Plateau. The stiffer continental blocks that remain undeformed within the continental collision areas are characterized by a slightly thickened crust and flat topography. Lithospheric thickness and crustal thickness show different patterns that highlight an important strain partitioning within the lithosphere. The Arabia–Eurasia collision zone is characterized by a thick lithosphere underneath the Zagros belt, whereas a thin to non-existent lithospheric mantle is observed beneath the Iranian and Anatolian plateaus. Conversely, the India–Eurasia collision zone is characterized by a very thick lithosphere below its southern part as a consequence of the underplating of the cold and stiff Indian lithosphere. Our new model presents great improvements compared to previous global models available for the region, and allows us to discuss major aspects related to the lithospheric structure and acting geodynamic processes in Central Eurasia. Supplementary material: Residual geoid anomaly between different order and degree of filtering, our compilation of crustal thickness from publications and our resulting crustal and lithospheric thickness in .txt format are available at: http://www.geolsoc.org.uk/SUP18846

Quaternary high-Mg ultrapotassic rocks from the Qal’eh Hasan Ali maars, southeastern Iran: petrogenesis and geodynamic implications

A set of rare, high-Mg ultrapotassic rocks from the Qal’eh Hasan Ali maars, southeastern Iran, was investigated using phlogopite 40Ar/39Ar geochronology, olivine chemistry, and bulk-rock elemental and Sr–Nd–Hf isotopic geochemistry to decipher their petrogenesis and regional tectonic implications. Phlogopite separates yield inverse isochron 40Ar/39Ar ages from ca. 112 to 119 ka, indicating that magma genesis postdated the onset of the Arabia–Eurasia collision at the Late Eocene–Early Oligocene. The studied rocks are characterized by kamafugitic affinity with relatively low SiO2 and Al2O3 and high CaO and Sr. They contain relatively primitive olivine (Fo85–92) that, on the basis of olivine–liquid Fe/Mg exchange equilibrium, suggests the primary melt to be ultrapotassic with ~13 wt% MgO. Other key geochemical features include extreme enrichment in most incompatible trace elements, depletions in Nb, Ta, P and Ti and enrichment in Pb relative to elements of similar incompatibilities. The Sr–Nd–Hf isotopic ratios of the rocks do not deviate significantly from the bulk silicate Earth (87Sr/86Sr = 0.7055–0.7059, 143Nd/144Nd = 0.5125–0.5126 and 176Hf/177Hf = 0.2827–0.2829). Relatively high Zn/Fe, Gd/Yb, Rb, Rb/Sr and P2O5 and low Yb and P/P* for the rocks are consistent with derivation from a mantle source containing clinopyroxene, phlogopite, apatite and garnet that formed in response to modal metasomatism in the lithospheric mantle. Relatively low Hf/Nd and high Sr/Hf of the rocks indicate that the metasomatized lithologies from which the studied rocks formed were derived dominantly from subducted marly sediments. The Qal’eh Hasan Ali magmatism is best explained by small-scale destruction of the continental mantle in a post-collisional setting, presumably driven by localized convective instability as a result of the Arabia–Eurasia collision.