Subduction zones generate volcanic arcs, but there are many examples where magmatism in convergent plate boundaries occurs in unexpected locations relative to the subducting slab. These magmas are commonly also geochemically anomalous relative to the composition of neighbouring typical subduction-related rocks. The origin of such Spatially and Geochemically Anomalous arc Magmatism (SGAM) may correspond to local variations in subduction parameters, the presence of crustal and lithospheric heterogeneities, or the potential contribution of melts generated by slab tearing and slab edge effects. Using the Holocene volcanoes in South America as a case study, we investigated spatial and geochemical patterns of volcanism along the Andean volcanic belt. Based on a series of geochemical indices, we developed a scoring system for the composition of volcanic rocks, with the lowest and highest scores indicating ‘typical’ and ‘anomalous’ arc melting processes, respectively. The results show that a number of Holocene volcanoes in South America can be unambiguously defined as SGAM. Volcanism in these localities may correspond to disruptions in the geometry of the subducting slab, or to areas affected by mantle flow in the proximity of the slab edge. To test the potential applicability of this method for plate tectonic reconstructions, we calculated geochemical anomaly scores for whole-rock analyses of volcanic rocks from other convergent boundary settings. The results show that high geochemical anomaly scores are obtained in areas where slab tearing has been documented or postulated, such as in Mount Etna (Sicily). The occurrence of anomalous magmatic rocks in older convergent plate boundary settings (e.g., Neogene rocks from the Gibraltar area) corroborates plate tectonic reconstructions that incorporated processes such as subduction segmentation, slab tearing, and the development of asthenospheric windows. Accordingly, we suggest that the recognition of SGAM from other modern and ancient arc settings may inform on similar types of processes, even in cases where the three-dimensional slab structure is no longer detectable.

How to cite: Rosenbaum, G., Caulfield, J., Ubide, T., Ward, J., Sandiford, M., and Sandiford, D.: Recognising spatially and geochemically anomalous arc magmatism (SGAM) and implications for plate tectonic reconstructions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6932, https://doi.org/10.5194/egusphere-egu21-6932, 2021.

The West Mediterranean Alpine belts of the Rif and its northern counterpart, the Betics, are famous for the subcontinental peridotites exposed in their Internal zones (Alboran Domain), the Beni Bousera (BB) and Ronda massifs, respectively. The Beni Bousera Marbles (BBMs) here described are known for long in the northern Rif, but remained overlooked so far. Since Kornprobst (1974), these marbles have been considered as simple intercalations within the kinzigites (migmatitic granulites) envelope of the BB peridotite. Based on the integration of field mapping, structural and petrology investigations and supported by SHRIMP U-Th-Pb geochronology, we present a new interpretation of these marbles and infer geodynamic implications at the local and regional scale. The field data show that the BBMs form minor, dismembered units within a ~30 to 300 m-thick mylonitic contact zone between the kinzigites and the overlying gneisses of the Filali Unit (Filali-Beni Bousera Shear Zone, FBBSZ). They display bedding structures marked by more or less siliceous marbles and some mica-rich or conglomeratic beds. The FBBSZ includes secondary ductile thrusts that determine kinzigite horses carried NW-ward over the marbles. Within the latter, NNE-trending folds are conspicuous. Brittle, northward-dipping normal faults crosscut the FBBSZ ductile structures. An unconformable contact, either of stratigraphic or tectonic origin, onto the kinzigites can be locally observed. The petrological investigation allows us to define pebbles and/or detrital grains, including K-feldspar, quartz, garnet, and zircon in these high-grade marbles. Peak mineral assemblage consists of forsterite, Mg-Al-spinel, phlogopite, and geikielite (MgTiO3) in dolomite marbles, phlogopite, scapolite, diopside, and titanite in calcite marbles. This characterizes a peak HT-LP metamorphism at ~700-750°C, 4-8 kbar. The BBMs compare with the Triassic carbonates deposited over the crustal units of the Alpujarrides-Sebtides. The detrital cores of the zircon grains from the BBMs yield two U-Th-Pb age clusters of ~270 Ma and ~340 Ma, distinct from the 290-300 Ma age of the zircon grains from the kinzigites (Rossetti et al., 2020), and supporting a Triassic age of the protoliths; the zircon rims yield ~21 Ma ages. The BBMs protoliths may have been deposited onto the kinzigites or carried later as extensional allochthons over a detachment in the frame of the incipient formation of the Alboran Domain continental margin, which is dated from the late Liassic-Dogger in the “Dorsale calcaire” detached units (Chalouan et al., 2008). Thus, the Beni Bousera mantle rocks would have been exhumed at shallow depth during the early rifting events responsible for the birth of the Maghrebian Tethys, i.e., as early as the Triassic-late Liassic.

Keywords: BBMs/ FFBSZ/ HT-LP metamorphism/ SHRIMP U-Th-Pb geochronology / hyperextended margin/ mantle rocks exhumation / Gibraltar Arc

References :

Please use this link for access to the cited references:  https://www.docdroid.net/hPSheTG/references-farah-et-al-2021-vegu-pdf 

How to cite: Farah, A., Michard, A., Saddiqi, O., Chalouan, A., Chopin, C., Montero, P., Corsini, M., and Bea, F.: Early exhumation of the Beni Bousera granulites and peridotites at the northern margin of the westernmost Tethys (Rif belt, Morocco); new constraints from overlying marbles, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-581, https://doi.org/10.5194/egusphere-egu21-581, 2021.

Orogens closely linked to 3-D subduction dynamics are frequently non-cylindrical and the Mediterranean region is a perfect natural laboratory to observe several of them, as well as their interactions. Through the succession of extension, subduction and sometimes collision events, the kinematic reconstructions of such orogens can be difficult and the subject of active debates. The internal zones are often non-consensual, especially when their long-term Pressure-Temperature-time-deformation (P-T-t-d) evolutions are studied. This complexity is mostly due to pre-orogenic inheritance or complex interactions between the subducting lithosphere, the overriding plate and the asthenosphere. All these elements are described and documented in Mediterranean orogens, i.e., a complex shape of the Eurasian and African margins in pre-orogenic times and a complex slab retreat and tearing dynamics. Their 3-D geometry results in strongly arcuate belts, such as the Betic-Rif Cordillera, located in the westernmost part of the Mediterranean region.

Focused on the Internal Zones of the Betic-Rif Cordillera and based on recent findings (Orogen Project framework), a synthesis of the tectono-metamorphic evolution shows the relations in space and time between tectonic and P-T evolutions. The reinterpretation of the contact between peridotite massifs and Mesozoic sediments as an extensional detachment leads to a discussion of the geodynamic setting and timing of mantle exhumation. Based on new ⁴⁰Ar/³⁹Ar ages in the Alpujárride Complex (metamorphic formations of the Betic Internal Zones) and a discussion of published ages in the Nevado-Filabride Complex (metamorphic formations of the Betic Internal Zones), we conclude that the age of the HP-LT metamorphism is Eocene in all the Internal Zones. A first-order observation is the contrast between the well-preserved Eocene HP-LT blueschists-facies rocks of the Eastern Alpujárride-Sebtide Complex and the younger HT-LP conditions reaching partial melting recorded in the Western Alpujárride. We propose a model where the large longitudinal variations in the P-T evolution are mainly due to (i) differences in the timing of subduction and exhumation, (ii) the nature of the subducting lithosphere and (iii) a major change in subduction dynamics at ~20 Ma associated with a slab-tearing event.

How to cite: Bessière, E., Jolivet, L., Augier, R., Scaillet, S., Précigout, J., Azañon, J. M., Crespo-Blanc, A., Masini, E., and Do Couto, D.: Lateral variations of pressure-temperature evolution in non-cylindrical orogens and 3-D subduction dynamics: the Betic-Rif Cordillera example, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7837, https://doi.org/10.5194/egusphere-egu21-7837, 2021.

Jurassic shallow-intrusive basic bodies within the Permian-Triassic Tethyan passive margin sedimentary sequences of the Lower Alpujarride units (Internal Betic Zone, Spain) locally show Alpine low-grade metamorphism in the greenschist and blueschist facies. A small sill-like mafic body near Redován town (Callosa Range) partially preserves igneous ophitic/subophitic texture and relics of augite, ferrohornblende-ferroedenite, kaersutite and K-feldspar (orthoclase). The metamorphic overprint corresponds to high-pressure and low-temperature mineral assemblages that comprise magnesioriebeckite, actinolite, albite, stilpnomelane, phengite and chlorite, with rutile, apatite and titanite as accessory minerals. Major and trace element geochemical data reveal igneous protoliths derived from magmas of alkaline basalt composition enriched in incompatible elements and E-MORB geochemical affinity. The intrusion emplacement occurred at shallow crustal levels in an extensional geodynamic setting (within-plate basalts) related to the breakoff of Pangea. Pressure-Temperature (P-T) conditions estimated by means of pseudosection calculations and the intersection of phengite (Si) and chlorite (Mg#) isopleths indicate a cold thermal gradient with calculated peak metamorphic conditions of ca. 8 kbar at 310 ºC. These conditions are consistent with metamorphism during burial down to ca. 24 km depth and a thermal gradient of ca. 13 ºC/km. Although the easternmost Lower Alpujarride units have been traditionally described as reaching only lower-greenschist to greenschist metamorphic peak conditions, the textures, mineral compositions and P-T conditions of the studied metagabbroic body reveal blueschist facies conditions that attest for a regional early stage (Eocene) of subduction of the lower Alpujarride units. This event predates the late Oligocene - early Miocene subduction-related metamorphism of the Intermediate and Upper Alpujarride units.

How to cite: Santamaría-Pérez, E., Blanco-Quintero, I. F., Martín-Algarra, A., Benavente, D., Cañaveras, J. C., González-Jiménez, J. M., and Garcia-Casco, A.: Blueschist facies conditions in Tethyan passive margin metabasaltic rocks of the easternmost Lower Alpujarride Units (Internal Betic Zone, Spain), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10913, https://doi.org/10.5194/egusphere-egu21-10913, 2021.

The Alps preserve abundant oceanic blueschists and eclogites that exemplify the selective preservation of fragments of relatively short-lived, small, slow-spreading North Atlantic-type ocean basins (here the ~400-700 km wide Alpine Tethys), whose subducting slabs reach down to the Mantle Transition Zone. Whereas none of the subducted fragments were returned during the first half of the subduction history, those exhumed afterwards experienced conditions typical of mature subduction zones worldwide. Sedimentary-dominated units were underplated intermittently, mostly at ~30-40 km depth, while mafic/ultramafic-dominated units subducted to ~80 km (In the W. Alps), whose protoliths had formed close to the continent, were offscraped from the slab only a few Ma before continental subduction. Spatiotemporal contrasts in burial and preservation of the fragments reveal how along-strike segmentation of the continental margin affects ocean subduction dynamics.

How to cite: Agard, P. and Handy, M.: Ocean subduction dynamics in the Alps, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5235, https://doi.org/10.5194/egusphere-egu21-5235, 2021.

The Alps as part of the Alpine mountain belts are one of the classical examples of orogenesis where the Mesozoic-Cenozoic tectonic evolution is well known, but not of the basement because poor age data. New data from the  pre–Alpine basement of the Austroalpine megaunit indicate that this basement is composed of a series of continental rocks, island arcs, ophiolites and subduction mélanges，accretionary wedges, and seamounts with different metamorphic, but often amphibolite facies grade. This study presents new results of LA–ICP–MS U–Pb and MC–ICP–MS Lu–Hf dating of zircons from three key areas of Austroalpine basement units: i) the Wechsel–Waldbach–Sieggraben, (ii) the Saualpe–Koralpe –Pohorje, and (iii) the Schladming Mts. areas. As a result, the Wechsel unit is a continental magmatic arc during 500-560 Ma, and 2.1 to 2.2 Ga-and 2.5 to 2.8 Ga age show the close relationship to northern Gondwana, with depleted mantle model ages as old as 3.5 Ga. Even the Wechsel Phyllite Unit overlying the Wechselgneiss, but they have partly different sources, include juvenile crust formed at ca. 530 Ma. The Waldbach Complex is constantly added new crustal material during 490-470Ma, and considerably more positive εHf(t) values from 700 to 500 Ma interpreted being part of a magmatic arc during closure of the Prototethys and got metamorphosed during Variscan orogenic events. We consider that Schladming to Wechsel Complexes represent a Cambrian-Ordovician volcanic-magmatic arc system followed proto-Tethys subduction, and the ophiolitic Speik complex represent a back-arc basin. Many granites were formed during Permian (Grobgneiss and various granites in Pohorje Mts.) likely in an extensional environment, remelting a crust with mainly Middle Proterozoic model Hf isotopic model ages. The Plankogel Complex represents accreted oceanic, ocean island and continental-derived materials, it should belong to the accretion complex formed during Permotriassic closure of Paleotethys. We argue that the various basement complexes of the Austroalpine are different sources of ages of different tectonic evolutionary histories and likely represent, different locations before drifting. Consequently, the Austroalpine meagunit represents a composite pre-Alpine mega-unit likely assembled not earlier as Permian or Triassic times.

How to cite: Chang, R., Neubauer, F., Genser, J., Liu, Y., Yuan, S., Huang, Q., Guan, Q., and Yu, S.: Hf isotopic constraints for Austroalpine basement evolution of Eastern Alps: review and new data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10909, https://doi.org/10.5194/egusphere-egu21-10909, 2021.

The pre-Mesozoic basements in the Eastern Alps overprinted by the Variscan and alpine metamorphism (Neubauer and Frisch, 1993), which still remained the pre-Variscan tectonic evolution evidences. Many of these basements left away from their lithospheric roots due to large-scale tectonic activities (von Raumer et al., 2001), whereas their origin and tectonic history can be recorded by detailed geochemistry and geochronology. Here we present a study on the Schladming Complex, one part of Silvretta-Seckau nappe system in Austroalpine Unit, that located in the northern part of Alps to discuss their ages, origin, and tectonic relationship with the Proto-Tethys Ocean.

The Schladming Complex basement mainly comprises biotite-plagioclase gneiss, hornblende-gneiss, mica-schists, together with some amphibolites, orthogneisses, paragneisses, metagabbro and migmatites, which covered by sequence of metasedimentary (Slapansky and Frank, 1987). It underwent the medium- to high-grade metamorphism during the Variscan event and is overprinted by the greenschist facies metamorphism during the Alpine orogeny (Slapansky and Frank, 1987).

Granodioritic gneisses (539~538 Ma) and fine-grained amphibolite (531±2 Ma) in the basement represent a bimodal magmatism. Geochemical data show that the granodioritic gneisses belong to A₂-type granite and originated from the lower crust, while the fine-grained amphibolites have an E-MORB affinity and the magma origined from the lithospheric mantle and contaminated by the arc-related materials. The data implies that the Schladming Complex formed in a back-arc rift tectonic setting in the Early Cambrian.

A medium-grained amphibolite gives an age of 495±5 Ma, exhibits ocean island basalt-like geochemical features and zircons positive εHf(t) values (+5.3~+10.9) indicating that the medium-grained amphibolite derived from a depleted mantle source. The monzonite granitic gneiss and plagioclase gneiss yields ages of 464±4 Ma for and 487±3 Ma, respectively. The monzonite granitic gneiss derived from the mixing of melts derived from pelitic and metaluminous rocks. The protolith of plagioclase gneiss is aplite, which has positive εHf(t) values of +5.9~+7.9, indicating it derived from the lower crust sources. The monzonite granitic gneiss and plagioclase gneiss exhibit S-type and I-type geochemical features, respectively. They are geochemically similar to the volcanic arc granite.

In summary, our data presents record of the Cambrian to Ordovician magmatism in the Schladming Complex, which provided new evidence of tectonic evolution history between Proto-Tethys and Gondwana. According to the data, we proposed that a series of rift developed in the northern margin of Gondwana during 540-530 Ma, the rifts continually expanded into a back-arc ocean in ~490 Ma and was closed around 460 Ma with S-type granitic magma intruded.

References

Neubauer, F., Frisch, W. 1993. The Austroalpine metamorphic basement east of the Tauern window.  In: Raumer, J. von & Neubauer, F. (eds.): Pre-Mesozoic Geology in the Alps. Berlin (Springer), pp. 515–536.

von Raumer, J., Stampfli, G., Borel, G., Bussy, F., 2001. Organization of pre-Variscan basement areas at the north-Gondwanan margin. International Journal of Earth Sciences 91, 35-52.

Slapansky, P., Frank, W. 1987. Structural evolution and geochronology of the northern margin of the Austroalpine in the northwestern Schladming crystalline (NE Tadstädter Tauern). In: Flügel, H. W. & Faupl, P. (eds.), Geodynamics of the Eastern Alps, pp. 244-262.

How to cite: Huang, Q., Liu, Y., Genser, J., Neubauer, F., Yuan, S., Yu, S., Bernroider, M., Guan, Q., Wei, J., and Chang, R.: Cambrian-Ordovician evolution of Eastern Alps: New evidences constrain from magmatic rocks in the Schladming Complex (Austroalpine unit), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8373, https://doi.org/10.5194/egusphere-egu21-8373, 2021.

Timing of the opening of the West Paleo-Tethys Ocean in Eastern Alps remains controversial. The debate over the timing has revolved mainly around three possible periods, namely Silurian, Early Devonian and Middle–Late Devonian. To constrain this event, we present new zircon U-Pb ages, Hf isotopic compositions, and whole-rock major- and trace-element data for the meta-mafic rocks in the southern Saualpe crystalline basement, Eastern Alps. Zircon U-Pb dating results from three samples yielded crystallization ages of 418 ± 6 Ma, 417 ± 3 Ma and 415 ± 3 Ma, indicating that they formed during the Early Devonian. Geochemically, these meta-mafic rocks have relatively low SiO₂ and MgO contents and high TiO₂ contents. They are enriched in light rare earth elements (LREE), particularly in Nb and Ta, and show relatively flat heavy rare-earth elements (HREE) patterns, indicating that they have affinities with the alkaline oceanic island basalts (OIB). The geochemical characteristics, together with the positive ε_Hf(t) values of 0.7–11.1, imply that the OIB-like meta-mafic rocks originated from partial melting of a lherzolite source including spinel and garnet. And the primary magma showed complex sources involving the asthenospheric, lithospheric mantle and subducted slab components and subsequently modified by crustal contamination, revealing that the magma formed in a slab window environment associated with mid-ocean ridge subduction. The contemporaneous OIB-like alkaline amphibolites were also found in the central Austroalpine basement and Northwestern Turkey. We suggest that the Late Silurian–Early Devonian OIB-like magmatism is related to a back-arc extension setting in the northern margin of Gondwana leading to the detachment of the European Hunic terranes and hence placing an age on the opening of the West Paleo-Tethys Ocean.

How to cite: Liu, Y., Guan, Q., Neubauer, F., Genser, J., Yuan, S., Chang, R., and Huang, Q.: Opening of the West Paleo-Tethys Ocean: New insights from Early Devonian meta-mafic rocks in the southern Saualpe crystalline basement, Eastern Alps, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8301, https://doi.org/10.5194/egusphere-egu21-8301, 2021.

In the geodynamic context of the slow Africa-Europe plates convergence, the Central-Western Mediterranean region has been involved in a complex subduction process, which in the last 30 Myr was characterized by the rapid retreat of the Ionian slab, the opening of back-arc extensional basins (i.e., Liguro-Provençal, Algerian, Alboran, and Tyrrhenian basins) and episodes of slab lateral tearing, segmentation and break-off.  A proper modelling of 3-D mantle flow evolution beneath the Mediterranean could provide important clarifications about the complex mantle dynamics of this region and help us understanding the interaction between surface tectono-magmatic processes and mantle convection patterns. 

The mantle flow and its relations with plate horizontal and vertical motions can be determined by measuring seismic anisotropy generated by strain-induced lattice/crystal preferred orientation (LPO/CPO) of intrinsically anisotropic minerals. Seismic anisotropy is widespread in the Mediterranean and it shows an intricate pattern that likely has some relations with the recent (20-30 Myr) behavior of subducting slabs. The extrapolation of the mantle flow from seismic anisotropy is neither simple nor always warranted, especially at subduction zones where complex and non-steady-state 3D flow patterns may establish.  A promising approach, which helps reducing the number of plausible models that can explain a given anisotropy dataset, is to compare seismic measurements with predictions of numerical and experimental flow models (Long et al.,2007). Recently, Faccenda and Capitanio (2013) and Faccenda (2014) have extended this methodology to account for the non-steady state evolution typical of many subduction zones, yielding mantle fabrics that are physically consistent with the deformation history.

In this study, we apply a similar modelling approach to the complex Central-Western Mediterranean convergent margin. We use the wealth of observations from the Mediterranean region available in the literature to design and calibrate 3D thermo-mechanical subduction modelling. We test different initial configurations defined at 30 Ma according to the paleogeographic and tectonic reconstructions derived from (Lucente and Speranza, 2001; Carminati et al., 2012; van Hinsbergen et al., 2014) in order to optimize the fit between predicted and observed slabs position and obtain a final model configuration resembling the present-day surface and deeper structures.

In particular, here we want to evaluate the influence on rollback rates, trench shape and the occurrence and timing of slab tears (Mason et al., 2010) of structural heterogeneities within the Adria plate as proposed by (Lucente and Speranza, 2001). In all models, subduction migrates south-eastward driven by the subducting oceanic lithosphere, and slab lateral tearing or break-off occurs when a continental margin enters the trench. More importantly, we show that the presence of a stiffer continental promontory in central Adria together with a thinned continental margin in the Umbria-Marche region plays a fundamental role on (i) the development of a slab window below the Central Apennines, (ii) the retreat of the Northern Apenninic trench till the Adriatic Sea, and (iii) the retreat of the Ionian slab till the present-day position.

How to cite: Lo Bue, R., Faccenda, M., and Yang, J.: Role of the Adria plate structural heterogeneities on the dynamics of the Central-Western Mediterranean region, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9978, https://doi.org/10.5194/egusphere-egu21-9978, 2021.

Here we present the first 4D tectonic reconstruction that models the Vrancea slablet and incorporates the floated slab as a constraint on the magnitude of slab rollback during collapse of the Palaeo-Pannonian Basin. Seismic tomographic images provide insight into the geometry and tectonic history of subducted slabs. High velocity anomalies can be interpreted as ‘cold’ lithosphere penetrating ‘warmer’ lower velocity asthenosphere, and 3D models created using the SKUA-GOCAD modelling software. Combined with information from the 3D distribution of earthquake hypocentres, we thereby obtain a simple approximation to slab geometry beneath the Vrancea region. The resultant DXF was imported into the Pplates tectonic reconstruction software, and floated back to the Earth’s surface. The method utilised assumes no significant deformation (stretching, buckling, folding, shortening) during or after subduction, so that the obtained geometry estimates the pre-subduction configuration. The resultant floated slab is then incorporated as a constraint on 2D + time tectonic reconstructions. We apply a double-saloon-door rollback model, which involves propagation of a slab tear along the mid-Hungarian lineament. Each saloon-door rolls back independently of the other and this leads to two epochs of extension. AlPaCa is ‘pulled’ eastwards and rotated counter-clockwise as the western saloon-door rolls back. The Tisza-Dacia unit is then ‘pulled’ eastward, and rotated, but in a clockwise sense as the eastern saloon-door rolls back. Once the subduction hinge reached the East European Platform, the slab was left hanging. Gravitational forces then drove slab-boudinage and detachment in a similar fashion as occurs today beneath the Hindu Kush. This model explains the large opposing-sense vertical-axis rotations that occurred during convergence of the AlPaCa and Tisza-Dacia terranes. The zipper fault model rotates the microplates without requiring large-scale thrusting. Interpretation of the Mid-Hungarian lineament as a zipper-fault system is also consistent with the geodynamic effects expected because of tearing in a subducting plate leading to a double-saloon-door rollback. The vertical extent of the slab is roughly 300 km, which only fills half of the basin, consistent with the double-saloon-door roll-back model interpretation.

How to cite: Muston, J., Spakman, W., and Lister, G.: Floating the Vrancea slab and tectonic reconstruction of the collapse of the PalaeoPannonian Basin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13235, https://doi.org/10.5194/egusphere-egu21-13235, 2021.

The Cretaceous sedimentation along the NE Dinarides margin was associated with subduction and collision of the Neotethys Ocean located between continental units of Adria and Europe (i.e., the Sava subduction system). In this region, we have performed a coupled kinematic and sedimentological study in order to understand the main controlling mechanism of deposition in basins situated above the Sava subduction zone.

The Cretaceous sedimentation on the upper plate of the Sava subduction system took place in a fore-arc basin developed in frontal parts of the active European continental margin. The sedimentary facies indicate three cycles of deposition during Early Cretaceous–Cenomanian, Turonian–Santonian, and Campanian-Maastrichtian. Lower Cretaceous–Cenomanian deposition was associated with regional contraction and characterized by the clastic-carbonatic cyclic shelf and slope deposits (i.e., the “para-flysch”). The European fore-arc “para-flysch” sequences, deposited during Berriasian–Aptian times, presently outcrop in the Gledićke Mts and Rudnik area in central Serbia. Following the Albian–Cenomanian regression that created regional unconformity across the entire fore-arc domain, Turonian–Santonian extension resulted in subsidence and syn-depositional bimodal magmatism. Fore-arc syn-subductional extension was triggered by retreating and steepening of the subducting Neotethys lithosphere. The final Campanian–Maastrichtian regression was initiated by large-scale shortening during the onset of Adria-Europe collision.

Unlike the European fore-arc domain, the Cretaceous sedimentation over the passive continental margin of the Dinarides was exclusively controlled by continuous shortening and overall transgression over the subducting Adria plate. Deposition starts with transgressive Albian–Cenomanian coarse-clastics and gradually deepens into the clastic-carbonatic shelf deposits. Rapid subsidence since the late Turonian resulted in deposition of slope carbonates followed by the deep pelagic sedimentation of Coniacian to Campanian–Maastrichtian limestones with cherts (i.e., the Struganik facies). The onset of deposition in the Sava subduction trench, as well as the accelerated subsidence in the entire lower Adria plate domain was coeval with Turonian–Coniacian switch to syn-subductional extension in the European fore-arc basin. The trench sedimentation starts with Turonian distal mudstones overlain by Coniacian–Maastrichtian clastic-carbonatic turbidites, as observed in the Rudnik Formation in Central Serbia. The westward expansion and migration of trench deposition towards the lower Adria plate culminated with Middle Campanian–Late Maastrichtian deposition of siliciclastic trench turbidites observed in the Ljig Formation.

The onset of the latest Cretaceous–Paleogene Adria-Europe continental collision resulted in large-scale W-wards thrusting that inverted the Cretaceous basins along NE Dinarides margin and emplaced sedimentary infill and basement of the European fore-arc over the Sava trench turbidites. The continued continental collision led to the propagation of thrusting during Eocene, which was characterized by formation of the large offset out-of-sequence thrusts. The eduction that followed break-off of the Neotethys slab beneath the Dinarides triggered Oligocene–Miocene extension which reactivated the inherited thrust contacts as extensional detachments along the entire Dinarides margin. The extension exhumed the lower Adria plate and additionally fragmented and deformed the former Cretaceous basins. The rates of extensional exhumation are decreasing to the NE, from the Dinarides margin towards the Carpathians.

How to cite: Stojadinovic, U., Krstekanić, N., Kostić, B., and Bogdanović, T.: Balance between tectonics and sedimentation during geodynamic evolution of the Adria-Europe convergence zone in central Serbia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-177, https://doi.org/10.5194/egusphere-egu21-177, 2020.

The Carpatho-Balkanides of south-eastern Europe is a double 180° curved orogenic system. It is comprised of a foreland-convex orocline, situated in the north and east and a backarc-convex orocline situated in the south and west. The southern orocline of the Carpatho-Balkanides orogen formed during the Cretaceous closure of the Alpine Tethys Ocean and collision of the Dacia mega-unit with the Moesian Platform. Following the main orogen-building processes, the Carpathians subduction and Miocene slab retreat in the West and East Carpathians have driven the formation of the backarc-convex oroclinal bending in the south and west. The orocline formed during clockwise rotation of the Dacia mega-unit and coeval docking against the Moesian indenter. This oroclinal bending was associated with a Paleocene-Eocene orogen-parallel extension that exhumed the Danubian nappes of the South Carpathians and with a large late Oligocene – middle Miocene Circum-Moesian fault system that affected the orogenic system surrounding the Moesian Platform along its southern, western and northern margins. This fault system is composed of various segments that have different and contrasting types of kinematics, which often formed coevally, indicating a large degree of strain partitioning during oroclinal bending. It includes the curved Cerna and Timok faults that cumulate up to 100 km of dextral offset, the lower offset Sokobanja-Zvonce and Rtanj-Pirot dextral strike-slip faults, associated with orogen parallel extension that controls numerous intra-montane basins and thrusting of the western Balkans units over the Moesian Platform. We have performed a field structural study in order to understand the mechanisms of deformation transfer and strain partitioning around the Moesian indenter during oroclinal bending by focusing on kinematics and geometry of large-scale faults within the Circum-Moesian fault system.

Our structural analysis shows that the major strike-slip faults are composed of multi-strand geometries associated with significant strain partitioning within tens to hundreds of metres wide deformation zones. Kinematics of the Circum-Moesian fault system changes from transtensional in the north, where the formation of numerous basins is controlled by the Cerna or Timok faults, to strike-slip and transpression in the south, where transcurrent offsets are gradually transferred to thrusting in the Balkanides. The characteristic feature of the whole system is splaying of major faults to facilitate movements around the Moesian indenter. Splaying towards the east connects the Circum-Moesian fault system with deformation observed in the Getic Depression in front of the South Carpathians, while in the south-west the Sokobanja-Zvonce and Rtanj-Pirot faults splay off the Timok Fault. These two faults are connected by coeval E-W oriented normal faults that control several intra-montane basins and accommodate orogen-parallel extension. We infer that all these deformations are driven by the roll-back of the Carpathians slab that exerts a northward pull on the upper Dacia plate in the Serbian Carpathians. However, the variability in deformation styles is controlled by geometry of the Moesian indenter and the distance to Moesia, as the rotation and northward displacements increase gradually to the north and west.

How to cite: Krstekanic, N., Matenco, L., Stojadinovic, U., Willingshofer, E., Toljić, M., and Tamminga, D.: Strain partitioning around an indenter during oroclinal bending: kinematics of the Circum-Moesian fault system of the Carpatho-Balkanides, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1570, https://doi.org/10.5194/egusphere-egu21-1570, 2021.

We report updated results of our ongoing research on constraining geodynamic conditions associated with the final closure of the Vardar branch of the Tethys Ocean by means of application of numerical simulations (previous interim results reported in EGU2020-5919).

The aim of our numerical study is to test the hypothesis that a single eastward subduction in the Jurassic is a valid explanation for the occurrence of three major, presently observed geological entities that are left behind after the closure of the Vardar Tethys. These include: ophiolite-like igneous rocks of the Sava-Vardar zone and presumably subduction related Timok Magmatic Complex, both Late Cretaceous in age as well as Jurassic ophiolites obducted onto the Adriatic margin. In our simulations we initiate an intraoceanic subduction in the Early/Middle Jurassic, which eventually transitions into an oceanic closure and subsequent continental collision processes.

In the scope of our study numerical simulations are performed by solving a set of partial differential equations: the continuity equation, the Navier-Stokes equations and the temperature equation. To this end we used I2VIS thermo-mechanical code which utilizes marker in cell approach with finite difference discretization of equations on a staggered grid [Gerya et al., 2000; Gerya&Yuen, 2003].

The 2D model consists of two continental plates separated by two oceanic slabs connected at a mid-oceanic ridge. Intraoceanic subduction is initiated along the ridge by assigning a weak zone beneath the ridge. Time-dependent boundary conditions for velocity are imposed on the simulation in order to model a transient spreading period. The change of sign in plate velocities is found to be useful for both obtaining obduction / ophiolite emplacement [Duretz et al., 2016] and causing back-arc extension. Changes in velocities are linear in time. Simulations follow a three-phase evolution of velocity boundary conditions consisting of two convergent phases separated by a single divergent phase where spreading regime is dominant. Effect of duration and magnitude of the second phase on model evolution is also explored.

Our so far obtained simulations were able to reproduce the westward obduction and certain extension processes along the active (European) margin, which match the existing geological relationships. However, the simulations involve an unreasonably short geodynamic event (cca 15-20 My) and we are working on solving this problem with new simulations. 

How to cite: Stanković, N., Cvetkov, V., and Cvetković, V.: Numerical simulation of the Late Jurassic closure of the Vardar Tethys, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9893, https://doi.org/10.5194/egusphere-egu21-9893, 2021.

Southern Evia in Greece exposes an inverted high pressure-low temperature (HP-LT) metamorphic sequence that has been loosely correlated with the Cycladic Blueschist Unit (CBU). On the island, the CBU is divided into the metavolcanic and ophiolitic Ochi Nappe and predominantly metacarbonate Styra Nappe. A lower-grade unit, the Almyropotamos Nappe, is exposed in the core of a N-S trending antiform and comprises Eocene platform carbonates overlain by metaflysch. The Almyropotamos Nappe occupies a tectonic window defined by the Evia Thrust, a brittle-ductile fault zone that emplaced the Ochi and Styra nappes atop the Almyropotamos Nappe. New multiple single-grain white mica total fusion ⁴⁰Ar/³⁹Ar ages indicate that deformation occurred along the Evia Thrust at 25-23 Ma. White mica ⁴⁰Ar/³⁹Ar data on either side of the tectonic window record Eocene dates between 40 and 32 Ma, consistent with previously published ⁴⁰Ar/³⁹Ar dates and a single Rb-Sr age of c. 30 Ma. These ages broadly coincide with estimates for the timing of NE-directed thrusting of the Ochi Nappe over the Styra Nappe. Strain associated with thrusting localized as cylindrical folds in Styra marbles, with fold axes parallel to the stretching lineation and a clear strain gradient increasing toward the upper contact with the Ochi Nappe. The most prominent structures in the Ochi Nappe are a strong L-S fabric defined by acicular blue amphibole and type-3 refold structures with fold axes trending parallel to the NE-SW oriented stretching lineation. Whereas the Ochi Nappe and Styra Nappe locally preserve peak blueschist facies mineral assemblages, all three units commonly display evidence only for retrogressed initial HP-LT assemblages in the form of ferroglaucophane inclusions in albite porphyroblasts. Isochemical phase diagrams calculated in the Na₂O-CaO-K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O-TiO₂±O₂ system support minimum peak metamorphic conditions of 12.5 ± 1.5 kbar and 465 ± 75 °C for an Ochi Nappe blueschist, and 6.0 ± 0.5 kbar and 315 ± 15 °C for an albite mica schist from the Evia Thrust. Peak P-T conditions for the Ochi Nappe support a metamorphic history more closely resembling that of the Lower Cycladic Blueschist Nappe, indicating that the entire section of the CBU exposed on Evia lies below the Trans-Cycladic Thrust. The Early Miocene ages from the Evia Thrust overlap with the proposed timing for the initiation of bivergent greenschist facies extension in the Cyclades. The remainder of the region, including high-strain corridors within individual nappes such as the Almyropotamos Thrust, uniformly records Eocene deformation ages. The similarity in ⁴⁰Ar/³⁹Ar ages across the tectonic window contrasts with age relationships observed in similar tectonic packages on Lavrion, and suggests that regional scale deformation persisted until the Late Eocene before strain became localized in brittle-ductile corridors by the Early Miocene. 

How to cite: Ducharme, T., Klonowska, I., Schneider, D., Grasemann, B., and Soukis, K.: Synchronous Eocene deformation recorded on either side of a major Miocene thrust bounding the Almyropotamos tectonic window in Evia, Greece, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3275, https://doi.org/10.5194/egusphere-egu21-3275, 2021.

How to cite: Yeung, S., Forster, M., Skourtsos, E., and Lister, G.: The Late Cretaceous Asteroussia event as recorded in the Cyclades: a potential key to Western Tethys tectonic evolution, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3890, https://doi.org/10.5194/egusphere-egu21-3890, 2021.

In the Aegean region (Cyclades - Greece), the island of Anafi island comprises Late Cretaceous intermediate and felsic granitoids that intruded within exhumed HT/LP metamorphic sequences that include amphibolites, serpentinites and metasediments. The granitoids correspond to I-type arc-related rocks with calc-alkaline geochemical affinities. Variations in their petrography mineral chemistry and geochemical features are attributed to magma differentiation with removal of plagioclase and/or K-feldspar, but also amphibole and biotite. Differentiation processes of the upwelling granitoid magma included fractional crystallization accompanied with crustal assimilation, pointing to interaction with the overriding continental crust. Mineral chemistry and geochemical results display that the Anafi granitoids are highly comparable with the Late Cretaceous granitoids of East Crete and Donousa island suggesting that this magmatic activity was not a local event. Geothermometric results show relatively moderate temperature crystallization conditions (~790 °C) for the compositionally intermediate granitoids, which are and lower for the felsic granitoids (~630 °C). Geobarometric calculations suggest shallow intrusion conditions (~2.0-6.5 kbar), which corresponds to a depth of crystallization of ~12 ± 4 km.

The thrust sheets that overly the flysch constitute a subducted and metamorphosed oceanic sequence, that after the intrusion of the granitoids was exhumed from the Late Cretaceous to the Late Oligocene. These metamorphic units likely represent a part of the Pindos - CBU domain that was subducted at an earlier pre-Campanian stage. In the hydrated mantle wedge, incorporation of shallow level granitoids within metamorphic units was likely facilitated via corner flow intrusion mechanisms. Ongoing underplating of subducted material gradually brought the granitoids along with the host units to shallow structural levels and on top of the parautochtonous flysch.

How to cite: Koutsovitis, P., Soukis, K., Voudouris, P., Lozios, S., Ntaflos, T., Stouraiti, C., and Koukouzas, N.: Geodynamic constraints deciphered from the petrology and geochemistry of the Late Cretaceous granitoids from Anafi island (Cyclades - Greece), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3522, https://doi.org/10.5194/egusphere-egu21-3522, 2021.

The birth and death of oceanic areas have often proved to involve contemporaneous destruction of previously created and evolved oceanic domains and the initiation of new ones in back-arc areas. As a result, several and often competing geodynamic processes, have been taking place at the same time, thus creating a complex tectonostratigraphy.

The Attic-Cycladic Crystalline Complex (ACCC), in the Aegean Sea (Greece), the outcome of the formation and destruction of Paleotethyan and Tethyan oceanic domains, is one such case. Four major units have been identified in the ACCC. These are from top to bottom, the complex Upper Cycladic Nappe, the Cycladic Blueschist Unit, the pre-alpine Cycladic Basement, and the Basal Unit. The present-day configuration has resulted from an Eocene stage of subduction and metamorphism under blueschist to eclogite facies and an Oligocene-Miocene exhumation and metamorphic core complex formation, through a combination of contractional and extensional mechanisms. Original relations between these four units have been obscured from the Cenozoic tectonometamorphic processes and several conflicting views have been expressed in the literature, regarding the nature of the Cycladic Blueschist domain, the relation between the Cycladic Blueschist Unit and the Cycladic Basement.

In this paper, we make a reconstruction of the domain, from which the Cycladic Blueschist Unit originated, based on a synthesis of structural, tectonostratigraphic, geochemical, and geochronological data. Through this reconstruction, we attempt to reconcile existing controversies and differences of views in the literature and to highlight the major structures that controlled the main features and geological evolution of this remarkable area.

How to cite: Soukis, K. and Stockli, D.: A reconstruction of the Cycladic Blueschist Domain (Cyclades, Greece), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15918, https://doi.org/10.5194/egusphere-egu21-15918, 2021.

Across the Tethyan realm, subduction zones are characterized by phases of forearc and backarc extension, and subsequent collisions are protracted and polygenetic, often resulting in significant discrepancies among proxies of collision age. The closure of the northern branch of the Neotethys Ocean along the İzmir-Ankara-Erzincan suture in Anatolia has been variously estimated from the Late Cretaceous to Eocene. It remains unclear whether this age range results from a protracted, multi-phase collision or disparities between proxies and geographic location. Near-continuous Jurassic through Eocene deposition in the forearc-to-foreland Central Sakarya Basin system in western Anatolia makes it an ideal location to integrate pre-collisional extension and multi-stage collision into a holistic reconstruction of subduction through collision. The Central Sakarya Basin system is located north of the Izmir-Ankara-Erzincan suture, where the Gondwanan-derived Anatolide and Tauride terranes to the south collided with the Laurasian-derived Pontide terrane in the north. By integrating new sandstone petrography and detrital zircon U-Pb and Hf isotopes with other geologic proxies, we demonstrate four phases of evolution of subduction and collision. (1) Magmatism began on the Pontides at 110 Ma, potentially the signal of subduction (re-)initiation, and is coincident with extension in the forearc. (2) Forearc obduction began around 94 Ma with initial subduction of lower plate continental lithosphere. Extension migrated to the backarc and opened the Black Sea. (3) The onset of intercontinental collision at 76 Ma is marked by gradual arc shutdown, basement exhumation, and uplift of the suture zone. (4) This first contractional phase is followed by thick-skinned deformation and basin partitioning starting around 54 Ma, coeval with regional syn-collisional magmatism. The 20-Myr protracted collision in western Anatolia could be explained by three non-exclusive mechanisms that produced a change in plate coupling: relict basin closure, progressive underthrusting of thicker lithosphere, and slab breakoff.

How to cite: Mueller, M., Licht, A., Campbell, C., Ocakoğlu, F., Akşit, G., Métais, G., Coster, P., Beard, K. C., and Taylor, M.: Closing the Neotethys Ocean in western Anatolia: Insights from forearc and foreland sedimentary basin records, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13757, https://doi.org/10.5194/egusphere-egu21-13757, 2021.

Upper Cretaceous arc-related volcanic and volcanoclastic units overlying the Paleozoic sedimentary rocks of the Istanbul Zone are a key unit related to the opening of the Black Sea as a back-arc basin. They formed as a result of north dipping subduction of the Neo-Tethys Ocean beneath Laurasia. We studied the Upper Cretaceous volcanic units north of Istanbul along several stratigraphic sections, and present new geochemical data from the volcanic rocks in order to understand Cretaceous geodynamic evolution of the İstanbul Zone.

The Upper Cretaceous  volcanic units north of Istanbul are divided into two formations. At the base there is a fore-arc turbidite succession,the İshaklı Formation, which is made up of volcaniclastic sandstone, shale, marl, tuff, debris flow horizons and epiclastic rocks of Turonian age. The İshaklı Formation is conformably overlain by the volcanoclastics,  tuffs, andesite and basalt lavas and agglomerates- the Riva Formation, which represents the arc/ intra-arc series.

Geochemically, basalts and basaltic andesites of the Riva Formation are low K calc-alkaline to medium-high K calc-alkaline and with magnesium numbers ranging from 32.6% to 51.5% Primitive mantle normalized spider diagram of trace elements show  enrichment in LILE elements (K, Rb, Sr, Cs, Ba, Th and U) and depletion in HFS elements ( Nb,Ta and Ti) . The high ratio of LILE/ HFS and negative Nb-Ta anomalies indicate that the volcanism evolved in subduction setting. Chondirite-normalized REE pattern display slight negative Eu anomalies and the La/Yb ratios of the samples range between 2,76 and 4,89. Our new geochemical, stratigraphical and the regional geological data suggest that north of Istanbul there was a transition from fore-arc deposition to arc volcanism during the Late Cretaceous opening of the Western Black Sea.  Considering the whole Pontide – Sredna-Gora Upper Cretaceous magmatic arc, it can be stated that calc-alkaline volcanism developed in relation to northward subduction of the Neo-Tethys oceanic lithosphere during the Turonian, and may have passed into high-K calc alkaline and shoshonitic magmatism as a result of the progressive extentional tectonism during the Campanian.

How to cite: Ay, C., Sunal, G., and Okay, A. I.: Stratigraphy, geochemistry, and petrology of the Upper Cretaceous volcanic arc sequence north of Istanbul, Pontides, NW Turkey, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6240, https://doi.org/10.5194/egusphere-egu21-6240, 2021.

The Upper Cretaceous volcanic and volcaniclastic rocks crop out along the Black Sea coastline in Turkey. They are part of a magmatic arc that formed as a result of northward subduction of the Tethys ocean beneath the southern margin of Laurasia. The lower part of the Upper Cretaceous volcanism in the Kefken region, 100 km northeast of Istanbul, is represented by basaltic andesites, andesites, agglomerates and tuffs, which have yielded Late Cretaceous (Campanian, ca. 83 Ma) U-Pb zircon ages. The volcanic and volcanoclastic rocks are stratigraphically overlain by shallow to deep marine limestones, which range in age from Late Campanian to Early Eocene.  Geochemically, basaltic andesites and andesites display negative anomalies in Nb, Ta and Ti, enrichment in large ion lithophile elements (LILE) relative to high field strength elements (HFSE). Light rare earth elements (LREE) show slightly enrichment relative to heavy rare earth elements (La_cn/Yb_cn =2.51-3.63) and there are slight negative Eu anomalies (Eu/Eu* = 0.71-0.95) in basaltic andesite and andesite samples. The geochemical data indicate that Campanian volcanic rocks were derived from the partial melting of the mantle wedge induced by hydrous fluids released by dehydration of the subducted oceanic slab.

There is also a horizon of volcanic rocks, about 230 m thick, within the Late Campanian-Early Eocene limestone sequence.  This volcanic horizon, which consists of pillow basalts, porphyritic basalts,  andesites and dacites, is of Maastrichtian age based on paleontological data from the intra-pillow sediments and U-Pb zircon ages from the andesites and dacites (72-68 Ma).  The Maastrichtian andesites and dacites are geochemically distinct from the Campanian volcanic rocks. They show distinct adakite-like geochemical signatures with high ratios of Sr/Y (>85.5), high La_cn/Yb_cn (16.4-23.7) ratios, low content of Y (7.4-8.6 ppm) and low content of heavy rare-earth elements (HREE). The adakitic rocks most probably formed as a result of partial melting of the subducting oceanic slab under garnet and amphibole stable conditions.

The Upper Cretaceous arc sequence in the Kefken region shows a change from typical subduction-related magmas to adakitic ones, accompanied by decrease in the volcanism.

How to cite: Duzman, T., Sağlam, E., and Okay, A. I.: Petrogenesis of the Upper Cretaceous volcanism in the Kefken region, Western Pontides, NW Turkey, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5108, https://doi.org/10.5194/egusphere-egu21-5108, 2021.

Continental deformation can be described in two end-member approaches: block (or microplate) and continuum models. The first considers a strong lithosphere with deformation localized in fault zones. For the latter, however, the lithosphere is weak and deforms as a thin viscous sheet. The Anatolia – Aegean domain represents both continuum and plate-like deformation. Furthermore, recent modeling studies suggest a dynamic support mechanism of the Anatolian plateaus, with dynamic topography estimates ranging from 1 to 3 km for various crustal models and geodynamic scenarios, although the gravity and crustal thickness data support predominant Airy isostasy. The solution to both intricacies relies on the thermal structure of the crust and the lithosphere. Available thermal considerations stem from either the uppermost mantle velocity structure or thermal modeling with assumptions on radiogenic heat production and boundary conditions. Yet, homogeneous and independent constraints on the lithospheric structure are scarce. We aim to contribute to this knowledge gap by providing Curie Point Depths (CPDs), which corresponds to the depth at which rock-forming minerals lose their magnetization at the Curie temperature, ~580 ^oC.

Resolution of deep magnetic sources requires spectral methods with large windows, which reduce the CPD resolution. Moving & overlapping smaller windows have been used in order to increase the resolution, but these introduce spectral leakage and bias. In previous studies, subjective wavenumber ranges of the magnetic anomaly spectra were used, often combined with wrong scaling factors between map units and the equations. This resulted in generally erroneous CPD estimates. Furthermore, CPD uncertainties have often been unquantified for the study area. We use a wavelet transform method, which overcomes the artifacts due to segmentation of magnetic signal to finite windows, results in higher spatial resolution as well as enabling uncertainty estimation. We used as large an area as possible for constraining the edge effects away from the study area. The resultant CPD map spatially correlates well with low Pn velocity areas, locations of volcanoes, and thermal springs.

How to cite: Ozbakir, A. D. and Karabulut, H.: New thermal constraints for the Anatolian lithosphere from Curie depth point and Pn tomography, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9926, https://doi.org/10.5194/egusphere-egu21-9926, 2021.

Tectonically the considered area of junction of four lithospheric plates (Nubian, Arabian, Aegean-Anatolian and Sinai) belongs to the Eastern Mediterranean, with its Cyprus-Levantine marine and Anatolian-Nubian-Arabian continental framing. The anomalousness of the region is manifested in the tectono-structural features of the mantle, lithosphere, hydrosphere and specifics of atmospheric, biospheric processes, and Hominid evolution.

The study region is distinguished by a complex junction of elements of the continental and oceanic crust. This intricate structure is caused by the simultaneous development of collision processes associated with the latitudinal zone of the Neotethys Ocean closure and manifestation of the initial stages of spreading of the Red Sea – Indian Ocean submeridional rift system. This area is characterized by presence of several geological-geophysical phenomena: (1) anomalous thickening of the mantle lithosphere in the Cyprus-Levantine zone, (2) development of the most ancient oceanic crust block with the Kiama paleomagnetic hyperzone, (3) presence of significant in size and amplitude gravitational and magnetic anomalies and lowest values of thermal flow, (4) presence of mantle diapirs, (5) high seismic activity, (5) development of a counterclockwise circular rotation of the GPS vectors, and (6) the location of the apical part of the oval structure occurring in the Earth's lower mantle. The study area is also distinguished by unique geomorphological and paleogeographic features. At present, the lowest elevations of the earth's surface relief developed here reach –430 m on the Dead Sea coast, and the deepest zones of the Mediterranean Sea almost reach the ultra-abyssal depth of –5267 m in the Calypso depression in the Ionian arc of Greece. In the epoch of the Mediterranean Sea drying out in the end of the Miocene (the Messinian crisis), the earth's surface marks (taking into account the hydro-isostatic effect) could reach 3000-4000 m below the hydrosphere level; this was probably the lowest land hypsometric minimum in the Earth geological history.

The aforementioned phenomena make it possible to conclude that this region is a giant geodynamic node formed in the northern hemisphere at the intersection of the latitudinal critical parallel (35^о) in the Eurasia and Gondwana junction zone and the meridional step of the Ural-African geoid anomaly. The combined use of systematic data analysis, geodynamic constructions, structural-tectonic zonation, and cyclic analysis enabled to clarify  the history of geodynamic development and genesis of the tectono-physical formation of individual geological structures, and the region as a whole. 

A special importance was paid to the satellite gravity data analysis with the subsequent modeling and transformation and identification of the heterogeneous structures in the Earth's crust, mantle lithosphere and lower mantle. Paleomagnetic mapping of the region indicates an increase of the frequency and diversity of magmatic complexes from the west to east. Obviously, this manifestation is due to the counterclockwise rotation of the Earth's crust relative to elongated axis of the discovered deep mantle structure (Eppelbaum et al., 2021).

Eppelbaum, L.V., Ben-Avraham, Z., Katz, Y., Cloetingh, S. and Kaban, M., 2021. Giant quasi-ring mantle structure in the African-Arabian junction: Results derived from the geological-geophysical data integration. Geotectonics, 55, No. 1, 1-28

How to cite: Eppelbaum, L. and Katz, Y.: Integrated geological-geophysical study of the junction zone of Eurasia and Gondwana, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3201, https://doi.org/10.5194/egusphere-egu21-3201, 2021.

Young back-arc rift basins, because of the not yet dissipated extensional thermal signature, can be easily inverted following changes in the geodynamic regime and/or far-field stress transmission. Structural inversion of such basins mainly develops through reactivation of normal faults, particularly if the latter are favourably oriented with respect to the direction of stress transfer. The Adjara-Trialeti fold-and-thrust belt of SW Georgia is an example of this mechanism, resulting from the structural inversion of a continental back-arc rift basin developed on the upper plate of the northern Neotethys slab in Paleogene times, behind the Pontides-Lesser Caucasus magmatic arc. New low-temperature thermochronological data [apatite fission-track (AFT) and (U-Th)/He (AHe) analyses] were obtained from a number of samples, collected across the Adjara-Trialeti belt from the former sedimentary fill of the basin and from syn-rift plutons. AFT central ages range between 46 and 15 Ma, while AHe ages cluster mainly between 10 and 3 Ma. Thermal modelling, integrating AFT and AHe data with independent geological constraints (e.g. depositional/intrusion age, other geochronological data, thermal maturity indicators and stratigraphic relationships), clearly indicates that the Adjara-Trialeti back-arc basin was inverted starting from the late Middle Miocene, at 14-10 Ma. This result is corroborated by many independent geological evidences, found for example in the adjacent Rioni, Kartli and Kura foreland basins and in the eastern Black Sea offshore, which all suggest a Middle-Late Miocene phase of deformation linked with the Adjara-Trialeti FTB building. Adjara-Trialeti structural inversion can be associated with the widespread Middle-to-Late Miocene phase of shortening and exhumation that is recognised from the eastern Pontides to the Lesser Caucasus, the Talysh and the Alborz ranges. This tectonic phase can in turn be interpreted as a far-field effect of the Arabia-Eurasia collision, developed along the Bitlis suture hundreds of kilometres to the south.

How to cite: Gusmeo, T., Cavazza, W., Alania, V., Enukidze, O., Zattin, M., and Corrado, S.: Miocene structural inversion of the Adjara-Trialeti back-arc basin as a far-field effect of the Arabia-Eurasia collision, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-179, https://doi.org/10.5194/egusphere-egu21-179, 2020.

The Iranian plateau is a natural laboratory for studying the early stage of continental collision and plateau development. The collisional front and northern plateau are the major areas accommodating the Arabia-Eurasia convergence. GPS observations suggest that the blocks of central Iran with minor shortening may be relatively rigid. However, recent seismic imaging results suggest that the lithosphere in this region might not be rigid for it is thin and not seismically fast. Widespread mantle-derived magmatism since Middle Miocene also lends support to a relatively hot and weak lithosphere. It may raise a question of why these blocks could behave rigidly when transmitting stresses to the north.

Deformation patterns of the lithosphere and asthenosphere in the northeastern and eastern Iranian plateau, which can be constrained by seismic anisotropy, may help to understand the nature of the lithosphere within the continental interior and its responses to the Arabia-Eurasia collision. We studied the seismic anisotropy of the region via teleseismic shear-wave splitting analysis on dense array data and compared the new results with multidisciplinary observations, particularly the surface strain rates and the structure of the lithosphere-asthenosphere system. In northeastern Iran around the Paleo-Tehtys suture, the dominant fast polarization direction (FPD) is NW-SE, subparallel to the strikes of thrust faults and orogenic belts. This combined with the relatively higher strain rates and thicker crust and lithosphere suggests that northeastern Iran with pre-existing weakness may have experienced considerable lithospheric shortening. The Lut block, which is a major block of eastern Iran bounded by large-scale strike-slip faults and previously assumed rigid, shows a complex anisotropic structure. In its northern part where the strain rates are low, the average NE-SW FPD has no obvious link to active faults but is roughly parallel to the collision-induced asthenospheric flow. The area to the south around the Dasht-e-Bayaz fault shows high strain rates and a complex structure of Moho. The generally NW-SE FPDs are subparallel to the direction of the surface right-lateral shear, possibly reflecting a fault-controlled lithospheric deformation pattern. Further south is the central Lut area with moderate strain rates. It is characterized by a two-layer structure of anisotropy, with the FPDs in the upper and lower layers being similar to those of the area around the Dasht-e-Bayaz fault and the northern Lut block, respectively. This feature indicates that the anisotropy and deformation of the central Lut area could be affected by both large-scale strike-slip faults and collision-induced mantle flow.

Collectively, our observations suggest that both the collisional processes at the plate boundary and the nature and structural heterogeneities of the continental lithosphere may control the intracontinental deformation of the Iranian plateau. The observed minor deformation of the Lut block and also other blocks within this young plateau does not necessarily mean that these blocks are rigid, but is probably because of significant deformation preferentially taking place at not only the collision front but also mechanically weak zones in the hinterland, which may have accommodated most of the Arabia-Eurasia convergence.

How to cite: Gao, Y., Chen, L., Talebian, M., Wu, Z., Wang, X., Lan, H., Ai, Y., Jiang, M., Khatib, M. M., Xiao, W., and Zhu, R.: Spatial variation in seismic anisotropy beneath the eastern and northeastern Iranian plateau and its geodynamic implications, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8590, https://doi.org/10.5194/egusphere-egu21-8590, 2021.

Iran is a mosaic of continental blocks that are surrounded by Tethyan oceanic relics. Remnants of these oceanic rock assemblages are exposed around the Central Iranian Microcontinent (CIM), discretely along the Sanandaj-Sirjan Zone and in Jaz-Murian. The ophiolite belts surrounding the CIM are mainly assumed to represent narrow back-arc basins that opened in Cretaceous and closed before the Eocene. Although these ophiolites are exposed as small pieces on continental crust today, they represent oceans wide enough to form supra-subduction ophiolites and arc-related magmatic rocks which suggest that their palaeogeographic width was at least some hundreds of kilometers. Current models for the palaeogeographic dimension, opening and closure of these basins are highly schematic. They usually seem plausible in two-dimensional reconstructions, however a single three-dimensional model explaining whole Iran and its surrounding regions has not been fully accomplished.  This is mostly because while the geological record provides constraints on the origin and ages of the subducted ocean floor, it provides limited information about onset and cessation of the subduction and almost no constraints on the dimension of these oceans and the subduction zones that consumed them.

In this study, we follow a novel approach in estimating the dimension and evolution of these back-arc basin by using seismic tomography. Seismic tomography has revealed that we can image and trace subducted lithosphere relics. Imaged mantle structure is now being used to link sinking slabs with sutures and to define shape of a slab. Systematic comparison of regions where the timing of subduction is reasonably well constrained by geological data showed that slabs sink gradually through the mantle at rates more or less the same. This perspective enabled us to study slab shape as a function of absolute trench motion. While mantle stationary trenches tend to create steep slabs or slab walls, the flat-lying segments are formed where the overlying trenches are mobile relative to the mantle, normal facing during roll-back, overturned during slab advance.  Under the assumption of vertical sinking after break-off, it is also possible to locate the palaeo-trenches.  When combined with absolute plate motion reconstructions, tomographically determined volume and size of the subducted lithosphere can also be used to estimate the size/width of the prehistoric oceans. To this end, we build on and further develop concepts that relate absolute trench motion during subduction to modern slab geometry to evaluate the possible range of dimensions associated with opening and closure of the Iranian back-arc basins.

How to cite: Lom, N., Qayyum, A., Gürer, D., van der Meer, D. G., Spakman, W., and van Hinsbergen, D. J. J.: Opening and closure of Iranian back-arc basins: A seismic tomography view, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5209, https://doi.org/10.5194/egusphere-egu21-5209, 2021.

The Kashan-Ardestan sedimentary basin in Central Iran was initially formed by back-arc extension due to the subduction of Neo-Tethys oceanic lithosphere beneath the Iranian Plate during Eocene time. Following rifting and the onset of the Arabian-Central Iranian continental collision in the Oligocene, the basin was infilled by a sequence of continental clastic and evaporitic sediments referred to as the Lower Red Formation. Post-rift cooling and thermal subsidence led to the development of a shallow marine environment for the accumulation of Qom Formation carbonates and shales in the late Oligocene–early Miocene. The Qom Formation is the most significant hydrocarbon target in Central Iran, containing both source and reservoir rocks. The continental collision triggered the reactivation of pre-existing normal and strike-slip fault systems. The basin was subjected to compressional tectonism during the deposition of the Miocene Upper Red Formation and overlying Plio-Quaternary sediments. This long-lasting and multi-episodic tectono-sedimentary evolution of the Kashan-Ardestan Basin has led to the formation of a complex structural style, which must be resolved before petroleum system modeling and drilling of prospects can take place.

In this study, several transverse and longitudinal 2D seismic lines were converted to depth and interpreted to define the deep-seated geometry of structures in the basin. The seismic lines were tied to the data from three exploration wells, reaching depths of ~ 4 km. In addition, ~ 15000 gravity and magnetic stations, covering the entire Kashan-Ardestan Basin, were integrated into our model.

The results of our study indicate that two major strike-slip fault systems, including the Qom-Zefreh and Ardestan faults in the south and the Gazu fault zone in the north, control the geometry and evolution of the Kashan-Ardestan Basin. In this basin, the rheological profiles of the sedimentary sequences control the folding style and deformation mechanisms. Both basement-involved and thin-skinned faults developed in the basin and formed different types of fault-related anticlines. The reactivation of pre-existing strike-slip faults has produced positive flower structures during compression. There is some evidence that the Navab Anticline in the SW developed as a forced fold, with basement involvement. In addition, several thin-skinned detachment folds are observed above the evaporites of the Lower Red Formation at the base of the sedimentary cover. The Lower Red Formation thins and pinches out toward the eastern limit of the basin, where the Qom carbonates directly overly the Eocene volcanic basement. Meanwhile, the Upper Red Formation thins toward the north and northeastern limits of the basin, and towards the crests of anticlines. These syntectonic thickness variations allow us to define the geometric evolution of the Kashan-Ardestan Basin through geologic times, allowing for the burial history of the source rock and timing of trap formation at the reservoir level to be described.

How to cite: Gholamian, F., Najafi, M., Welford, J. K., Ghods, A., and Bakhtiari, M. R.: Structural style of the Kashan-Ardestan syn-tectonic sedimentary basin in Central Iran, Arabian-Eurasian collision zone, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13537, https://doi.org/10.5194/egusphere-egu21-13537, 2021.

A Cretaceous paleo-accretionary wedge (the Ashin Complex) now exposed along the Zagros suture zone in southern Iran exhibits mafic and metapelitic lithologies. Field, geochemical and petrological observations point to a high-temperature event that gave rise to the formation of peritectic (trondhjemitic) melts associated with restitic garnet-bearing amphibolites in the structurally highest sliver of the Ashin Complex. SHRIMP U-Pb zircon dating of grains crystallized in trondhjemitic leucosomes yields a ²⁰⁶Pb/²³⁸U weighted mean age of 104 ±1 Ma, interpreted as the peak temperature event, which occurred in the amphibolite facies (c. 640-650°C at 1.1-1.3 GPa), based on thermodynamic modeling. Rutile crystals from several leucosomes yield Zr-in-rutile temperatures between 580-640°C and LA-ICP-MS U/Pb ages of 87-94 Ma. This rutile generation may be related to the observed static formation of Na-clinopyroxene and Si-rich phengite rims, as well as the growth of lawsonite in late fractures. The latter paragenetic sequence has been previously interpreted as reflecting a long-term isobaric cooling that occurred at least until the end of the Cretaceous (ages in Angiboust et al., 2016).

While the latter observations point to a long-term cooling of the Zagros subduction thermal gradient down to 7°C/km during late Cretaceous times, this first report of an earlier melting event in the Zagros paleo-accretionary wedge indicates an abnormally high thermal gradient of 17-20°C/km. GPLATES paleogeographic reconstructions of the Tethyan realm evolution during Cretaceous times reveal the presence of a spreading ridge jump followed by the subduction of the formerly active ridge-segment between 105-115 Ma, which possibly left an imprint marked by the unusually hot gradient seen in Ashin amphibolites. The model further predicts the subduction of progressively aging oceanic lithosphere, possibly explaining the observed cooling of the subduction thermal regime.

How to cite: Holtmann, R., Muñoz-Montecinos, J., Angiboust, S., Cambeses, A., Bonnet, G., Glodny, J., Gharamohammadi, Z., Kananian, A., and Agard, P.: The Zagros Suture Amphibolites Record the Cretaceous Thermal Evolution of the Closing Tethyan Realm, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3259, https://doi.org/10.5194/egusphere-egu21-3259, 2021.

The Alborz Mountains in N Iran underwent several tectono-metamorphic events that reflect the opening and closure of the Paleo- and Neotethys Oceans. Metamorphic rocks that recorded these are rare and discontinuously exposed. They range from the HP-LT Asalem-Shanderman Complex in the west, to the Gasht Metamorphic Complex (GMC, this study), to the Gorgan Schists, and Fariman Schists near Mashhad in the east. They are considered to have formed during the closure of the Paleotethys Ocean. The GMC comprises poorly exposed metasediments and amphibolite metamorphosed under greenschist- to amphibolite-facies conditions. In addition, smaller volumes of granite occur. As the evolution of the basement rocks of the Alborz Mountains is still poorly known and their radiometric ages are very limited, we applied different dating methods to selected samples of the GMC basement to better understand the geological evolution of this part of the Alborz Mountains.

The granite yielded an Ediacaran 551 ± 2.5 Ma LA-ICP-MS U-Pb pooled zircon age. Monazites in two amphibolite-facies metapelites (Bt-Ms-St ± And schists) yielded Triassic 226 ± 24 and 229 ± 25 Ma CHIME U-Pb ages. Foliation-defining biotite and retrograde white mica replacing andalusite porphyroblasts in metapelites and peak-metamorphic amphibole from an amphibolite yielded much younger 175.1 ± 0.5 Ma to 177.0 ± 0.4 Ma ⁴⁰Ar/³⁹Ar plateau ages.

The Ediacaran crystallization age of the granite agrees with the late Neoproterozoic to Cambrian zircon age of the Lahijan granite in the eastern GMC reported by Guest et al. (2006) and indicates that the Alborz basement was a part of the northern margin of Gondwana at that time. It rifted and drifted away from Gondwana due to the opening of the Neotethys, probably in the Permian, along with other Iranian blocks (the so-called Cimmerian terranes). The mid to late Triassic monazite ages date the Barrovian peak metamorphism of the GMC and mark collision and accretion of a Cimmerian terrane following closure of the Paleotethys. The monazite ages overlap with the early Late Triassic age of deposition of the lowest parts of the unconformably overlying Shemshak Group in the central and eastern Alborz Mountains (ca. 213 Ma, Horton et al. 2008). Younger and very similar Toarcian ⁴⁰Ar/³⁹Ar ages for both pro- and retrograde minerals with different nominal closure temperatures, reflect very rapid cooling of GMC basement below the Shemshak Group due to extension-triggered uplift. This late Toarcian to Aalenian extension event can be correlated with the regional Mid-Cimmerian unconformity of mid-Bajocian age (c. 170 Ma) that resulted from the tectonic movements causing rapid uplift and erosion (Fürsich et al. 2009). Extension probably started in the western Alborz Mountains in the Toarcian and culminated in the Aalenian in the eastern Alborz with the formation of a deep-marine basin and was triggered by the onset of the subduction of Neotethys oceanic crust beneath the Central Iranian Microcontinent (Wilmsen et al. 2009).

Fürsich et al. 2009, Geol. Soc., London, Spec. Publ. 312, 189-203. Guest et al., 2006, GSA Bulletin 118, 1507-1521. Horton et al., 2008, Tectonophysics 451, 97–122. Wilmsen et al. 2009, Terra Nova 21, 211–218.

How to cite: Rezaei, L., Timmerman, M. J., Altenberger, U., Moazzen, M., Wilke, F. D. H., Günter, C., Sudo, M., and Sláma, J.: Ediacaran to Toarcian evolution of the Gasht Metamorphic Complex, Alborz Mountains, N Iran, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9755, https://doi.org/10.5194/egusphere-egu21-9755, 2021.

The timings of the onset of oceanic spreading, subduction and collision are crucial in plate tectonic reconstructions, but not always straightforward to resolve. The evolution of the Paleo-Tethys Ocean dominated the Paleozoic-Early Mesozoic tectonics of West Asia, but the timeline of events is still poorly-constrained. In this study we present detrital zircon ages from NE Iran, in order to determine the timing of tectonic events in the region, and the wider implications for regional tectonics, paleogeography and climate change. Paleozoic clastic rocks record two major age peaks at ~800 Ma and ~600 Ma. The consistency in age patterns shows a dominant provenance from the Neoproterozoic basement of northern Gondwana. We interpret deposition on a long-lasting passive continental margin after the initial spreading of the Paleo-Tethys Ocean. Initial collision between the South Turan (Eurasia) and Central Iran (Gondwana) blocks caused coarse clastic deposition, the protolith of the Mashhad Phyllite, in a peripheral foreland basin on the Paleozoic passive margin. The Mashhad Phyllite yields major zircon age clusters at 450-250 Ma and 1900-1800 Ma, with a clear provenance from the active, Eurasian, margin. The Paleozoic ages reveal a long-lived subduction zone under the South Turan Block began in the latest Ordovician. Analysis of the age spectra allows us to constrain the timing of initial collision as no later than 228 Ma, which is also a constraint on the maximum depositional age of the Mashhad Phyllite. Based on our new results and previous data, we discuss the interaction between the Rheic and Paleo-Tethys oceans, and explain how a new subduction zone may have initiated after continental collision. The timing of collision is similar to the Carnian Pluvial Event (CPE). Paleo-Tethys collision has previously been suggested as the trigger for this climatic change, and our study provides timing evidence that reinforces Paleo-Tethys closure as a causal mechanism for the CPE.

How to cite: Chu, Y., Wan, B., Allen, M. B., Chen, L., Lin, W., and Talebian, M.: Tectonic evolution of Paleo-Tethys in NE Iran, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3557, https://doi.org/10.5194/egusphere-egu21-3557, 2021.

The Eastern Iranian Orocline provides us several opportunities to study magmatism in relation to tectonic events. The buckling of this orocline is accompanied by an extreme extension in its Khorasan outer arc during which a calc-alkaline dike swarm, generally andesite to dacite, intruded in a radial pattern into the Paleocene-Eocene volcano-sedimentary units, belonging to the platform of the Lut block. The azimuth of these dikes shows a declination of 30 degrees, from N300^o to N330^o. The U²³⁵/Pb²⁰⁷ age of ~41±74 Ma from zircon crystals taken from the dikes represents a considerable buckling with an extension occurred during the middle-upper Eocene. In fact, this time refers to the buckling in the boundary of the inner- and outer-arc of the orocline. This could be a noticeable document of syn-orocline magmatism in the Tethyan realm in the east of the Iranian plateau. The dikes and their host rocks are also sampled for AMS analysis and paleomagnetic measurements to test the amount of the oroclinal buckling in the Qayen area.

How to cite: Rojhani, E., Bagheri, S., Hinsbergen, D., Azizi, H., Ghaemi, F., Lom, N., and Qayyum, A.: Age of the Eastern Iranian oroclinal Buckling inferred from a U235/Pb207 dating on radial dikes in the Qayen Area, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14258, https://doi.org/10.5194/egusphere-egu21-14258, 2021.

The geology of the Oman Mountains was shaped by the SW-directed obduction of allochthonous deep-sea rocks (Hawasina), trench-facies rocks (Haybi) and oceanic lithosphere (Semail Ophiolite) onto Arabian autochthonous shelf carbonates during the Late Cretaceous. Locally, the resulting obduction orogen was overprinted by significant post-obductional extension. NNE-directed extension occurred during at least two episodes which took place from the latest Cretaceous to early Eocene and late Eocene to Oligocene/Miocene, respectively. Moreover, the Oman Mountains, between the eastern Batinah Coastal Plain and the Sur area (Qalhat Fault) display numerous ~N/S-oriented folds and reverse faults. These structures overprinted mid-Eocene to at least Oligocene/Miocene formations (i.e., the Seeb to Barzaman formations).

Detailed structural/field work and satellite image analyses provide ample evidence that these ~N/S-compressional features are cogenetic with ~WNW to NW-striking sinistral faults. All these post-mid-Eocene structures are part of one major left-lateral WNW- to NW-striking shear zone from the Batinah Coastal Plain in the NW to the Batain area in the SE. Sinistral shearing is localized along the southwestern margin of the Saih Hatat Dome, crosses the Fanja area and continues to the northern part of the Jabal Akhdar Dome (Jabal Nakhl Subdome). The straight southwestern margin of the Saih Hatat Dome may correlate with a Permo-Triassic major extensional fault, active during the Pangea rifting. Shearing also affected rocks northeast of this zone, i.e., within the Salma Plateau and the Rusayl Embayment. Thus, shearing affected an area of 250 km by 40 km in width. We term this shear zone hereafter the “Hajar Shear Zone” (HSZ). The amount of sinistral shearing is unknown due to the absence of markers and wide strain distribution, but is likely to be at the order of a few tens of kilometers.

The cause for the WNW-directed sinistral shearing is the overall E/W-directed shortening between the Arabian and Indian plates. During shortening, a pre-existing WNW-striking basement fault zone was reactivated, creating the HSZ. A G-Plates reconstruction between the two plates reveals an ~8° counter-clockwise rotation of India (with respect to fixed Arabia) between 32.5 and 20 Ma, resulting in ~150 km E/W-shortening between both plates at the easternmost tip of Arabia. The area northeast of the HSZ underwent most E-W-shortening. The 150 km interplate E/W-shortening is the maximum value for sinistral shearing along the HSZ and other faults. Some of the shortening may have been absorbed offshore Oman across the Owen Basin and/or along the continental/oceanic transitions of both plates.

How to cite: Mattern, F., Bolhar, R., Scharf, A., Scharf, K., Mattern, P., and Callegari, I.: Novelly discovered post-mid-Eocene sinistral slip in the eastern Oman Mountains: widely distributed shear with wrench-fault assemblage related to Arabia-India convergence, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8321, https://doi.org/10.5194/egusphere-egu21-8321, 2021.

The Semail Ophiolite is the world‘s largest and best exposed oceanic lithosphere on land and a primary reference site for studies of creation of oceanic lithosphere, initiation of subduction, geodynamic models of obduction, subduction and exhumation of continental rocks during obduction. Five decades of geological mapping, structural, petrological and geochronological research provide a robust understanding of the geodynamic evolution of the shallow continental crust in northern Oman and how the late Cretaceous obduction process largely shaped the present-day landscape. Yet, prior to obduction, other first-order tectonic processes have left their imprint in the lithosphere, in particular the Neoproterozoic accretion of Arabia and Permian breakup of Pangea. Due to the scarcity of deep structure imaging below the ophiolite, the presence and significance of inherited structures for the obduction process remain unclear.

We discuss a new 3-D anisotropic shear wave velocity model of the crust below northern Oman derived from ambient noise tomography and Receiver Function analysis which allows to resolve some key unknowns in geodynamics of eastern Arabia: (1) Several NE-trending structural boundaries in the middle and lower crust are attributed to the Pan-African orogeny and align with first-order lateral changes in surface geology and topography. (2) The well-known Semail Gap Fault Zone is an upper crustal feature whereas two other deep crustal faults are newly identified. (3) Permian rifting occurred on both eastern and northern margins but large-scale mafic intrusions and/or underplating occurred only in the east. (4) While obduction is inherently lithospheric by nature, its effects are mostly observed at shallow crustal depths, and lateral variations in its geometry and dynamics can be explained by effects on pre-existing Pan-African and Permian structures. (5) Continental subduction and exhumation during late Cretaceous obduction may be the cause for crustal thickening below today‘s topography. (6) Thinning of the continental lithosphere below northern Oman in late Eocene times – possibly related to thermal effects of the incipient Afar mantle plume - provides a plausible mechanism for the broad emergence of the Oman Mountains and in particular the Jabal Akhdar Dome. Uplift might thus be unrelated to compressional tectonics during Arabia-Eurasia convergence as previously believed.

How to cite: Weidle, C., Wiesenberg, L., Scharf, A., Agard, P., El-Sharkawy, A., Krüger, F., and Meier, T.: Architecture of the crust and lithosphere beneath the Semail Ophiolite from ambient noise tomography and receiver functions: insights on the tectonic evolution of eastern Arabia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9890, https://doi.org/10.5194/egusphere-egu21-9890, 2021.

Knowledge of the timing of India-Asia collision and associated Tethyan closure in the region is critical to advancement of models of crustal deformation.   One of a number of methods traditionally used to constrain the time of India-Asia collision is the detrital approach. This involves determination of when Asian material first arrived on the Indian plate, with most recent estimates documenting collision at ca 60 Ma (e.g. Hu et al, Earth Science Reviews 2016). However, more recently, such data and a number of other approaches providing data previously used to determine the timing of India-Asia collision, have been controversially re-interpreted to represent collision of India with an Island arc, with terminal India-Asia collision occurring significantly later, ca 34 Ma (e.g. Aitchison et al, J. Geophysical Research 2007). Clearly, for the detrital approach to advance the debate, discrimination between Asian detritus and arc detritus is required. Such a discrimination was proposed in Najman et al (EPSL 2017), dating the timing of terminal India-Asia collision at 54 Ma. However, this evidence is far from universally accepted.  For example, such data are at variance with various palaeomagnetic studies which suggest that an oceanic Transtethyan subduction zone existed 600-2300 kms south of the Eurasian margin in the Paleocene  (e.g. Martin et al, PNAS 2020) and therefore these authors propose different explanations to explain the detrital data.  This presentation will discuss the uncertainties associated with our current understanding of the timing of India-Asia collision.

How to cite: Najman, Y. and Li, S.: Contributions from the detrital approach to unravelling the timing of India-Asia collision and Himalayan evolution, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1916, https://doi.org/10.5194/egusphere-egu21-1916, 2021.

The classical model for the collision between India and Eurasia, which resulted in the formation of the Himalayan orogeny, is a single-stage continent-continent collision event at around 55 – 50 Ma. However, it has also been proposed that the India-Eurasia collision was a multi-stage process involving an intra-oceanic Trans-Tethyan subduction zone south of the Eurasian margin. We present paleomagnetic data constraining the location the Kohistan-Ladakh arc, a remnant of this intra-oceanic subduction zone, to a paleolatitude of 8.1 ± 5.6 °N between 66 – 62 Ma. Comparing this result with new paleomagnetic data from the Eurasian Karakoram terrane, and previous paleomagnetic reconstructions of the Lhasa terrane reveals that the Trans-Tethyan Subduction zone was situated 600 – 2,300 km south of the contemporaneous Eurasian margin at the same time as the first ophiolite obduction event onto the northern Indian margin. Our results confirm that the collision was a multistage process involving at least two subduction systems. Collision began with docking between India and the Trans-Tethyan subduction zone in the Late Cretaceous and Early Paleocene, followed by the India-Eurasia collision in the mid-Eocene. The final stage of India-Eurasia collision occurred along the Shyok-Tsangpo suture zone, rather than the Indus-Tsangpo. The addition of the Kshiroda oceanic plate, north of India after the Paleocene reconciles the amount of convergence between India and Eurasia with the observed shortening across the India–Eurasia collision system. Our results constrain the total post-collisional convergence accommodated by crustal deformation in the Himalaya to 1,350 – 2,150 km, and the north-south extent of the northwestern part of Greater India to < 900 km.

How to cite: Martin, C. R., Jagoutz, O., Upadhyay, R., Royden, L. H., Eddy, M. P., Bailey, E., Nichols, C. I. O., and Weiss, B. P.: Paleomagnetic results from the western Himalaya indicate multi-stage India-Eurasia collision, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13684, https://doi.org/10.5194/egusphere-egu21-13684, 2021.

Pre-early Eocene volcaniclastic rocks exposed in the Indus Suture Zone (Ladakh, India) are key to deciphering the complex magmatic and tectonic evolution of the convergent margins that existed between India and Eurasia. Several hypotheses exist regarding the provenance of the middle Cretaceous to early Cenozoic Jurutze and Nindam formations yet there is presently no consensus. Leading models propose that: (a) they were either formed in neighbouring sub-basins at one convergent margin consisting of the Kohistan-Ladakh-Dras arc; or (b) they became stratigraphically superposed after the collision between the Kohistan-Ladakh and Dras arcs. Here we present new U-Pb detrital zircon, major and trace element geochemical, and petrographic datasets from the Nindam and Jurutze formations that support a disparate provenance and thus necessitate an alternative model. The Jurutze Fm. has a geochemical composition typical of arcs built on continental crust, whereas the Nindam Fm. presents a geochemical signature compatible with that of an intraoceanic arc. The significant age gap between these formations (>20 m.y.) in the Zanskar Gorge further precludes the possibility that the Jurutze Fm. was deposited on top of the Nindam Fm. We propose that the Nindam and Jurutze formations were deposited in distinct forearc basins and explore scenarios for their formation at separate convergent margins, i.e. the separate Kohistan-Ladakh and Dras arcs, respectively.

How to cite: Andjic, G., Zhou, R., Jonell, T. N., and Aitchison, J. C.: Dissimilar age and provenance of the Nindam and Jurutze volcaniclastic formations, Zanskar Gorge, Ladakh (India), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7181, https://doi.org/10.5194/egusphere-egu21-7181, 2021.

Knowing the original size of Greater India is a fundamental parameter to quantify the amount of continental lithosphere that was subducted to help form the Tibetan Plateau and to constrain the tectonic evolution of the India-Asia collision. Here, we report Early Cretaceous paleomagnetic data from the central and eastern Tethyan Himalaya that yield paleolatitudes consistent with previous Early Cretaceous paleogeographic reconstructions. These data suggest Greater India extended at least 2,675 ± 720 and 1,950 ± 970 km farther north from the present northern margin of India at 83.6°E and 92.4°E, respectively. The paleomagnetic data from Upper Cretaceous rocks of the western Tethyan Himalaya that are consistent with a model that Greater India extended ~2700 km farther north from its present northern margin at the longitude of 79.6°E before collision with Asia. Our result further suggests that the Indian plate, together with Greater India, acted as a single entity since at least the Early Cretaceous. An area of lithosphere ≥4.7 × 10⁶ km² was consumed through subduction, thereby placing a strict limit on the minimum amount of Indian lithosphere consumed since the breakup of Gondwanaland. The pre-collision geometry of Greater India’s leading margin helped shape the India-Asia plate boundary. The proposed configuration produced right lateral shear east of the indenter, thereby accounting for the clockwise vertical axis block rotations observed there.

How to cite: Meng, J., Gilder, S., Li, Y., and Wang, C.: Expanse of greater india in the cretaceous, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-636, https://doi.org/10.5194/egusphere-egu21-636, 2021.

The India-Asia collision is one of the most globally significant tectonic events of the Cenozoic era. It is widely cited as providing a unique natural laboratory for studying collisional tectonics, offering invaluable insights of processes associated with continental collision across a multitude of scales. Yet despite its importance, significant debate continues to surround the validity of three mutually exclusive models to explain the India-Asia collision. These include: (1) the single subduction model; (2) the double subduction model; and (3) the Greater India Basin hypothesis. In our recent review (Parsons et al. 2020, Earth-Science Reviews) we demonstrated that available constraints from the Himalayan orogen and Tibetan plateau, including tomographic analysis of subducted slabs beneath these regions, are unable to robustly define the relative likeliness of each model. In this contribution, we expand upon the work of Parsons et al. (2020), with geological, geophysical, and plate kinematic constraints from the southern Eurasian margin between Myanmar and Sulawesi.

Our analysis focuses on the interpretation of subducted oceanic lithosphere beneath Myanmar to Sulawesi and includes a cross sectional area-based restoration of actively subducting India-Australia plate oceanic lithosphere. Our results provide a new restoration for the southern Eurasian margin and the India-Australia plate boundary (the Wharton ridge) during the India-Asia collision. Our integration of plate kinematic constraints with tomographic interpretation of subducted slabs suggests that the plate boundary between the Indian continent and southern Tibet migrated ~1000-2000 km northwards during collision. This includes ~1000 km lateral migration of subducted Indian plate oceanic lithosphere, now imaged beneath northern India and southern Tibet.

Our reconstruction proposes that northward migration of the India-Tibet suture and subducted Indian plate oceanic lithosphere initiated at ~43 Ma and reflects a major plate network reorganisation event. At this time, “hard collision” of the Indian continent with southern Eurasia occurred synchronously with (1) reduction in Indian plate velocity; (2) cessation of the Wharton ridge and coupling of the Indian and Australian plates; (3) subduction initiation of Australian plate oceanic lithosphere beneath southeast Eurasia, and onset of northeast migration of the Australian continent; (4) accelerated ocean spreading between Australia and Antarctica; and (5) southwest ridge jump of the Central India spreading ridge. Buoyancy of the Indian continent kept it afloat, whilst oceanic lithosphere to the east continued to drive wholesale motion of the coupled India-Australia plate. This forced the Indian continent to migrate northwards, dragging the subducted Indian plate oceanic slab with it, which effectively unzipped the coupled India-Australia plate along the extinct Wharton ridge, during subduction.

Our findings are most consistent with models (2) and (3), which are characterised by two collisions, the latter of which occurred between India and the Eurasian margin at ~45-40 Ma. More generally, our study demonstrates how changes in the balance of forces within a plate network, caused by events such as continental collision, can lead to significant plate network reorganisations. Such events can have dramatic effects on the position and geometry of subducted slabs and should be considered when interpreting plate restorations from deep-mantle structure.

How to cite: Parsons, A., Sigloch, K., and Hosseini, K.: Plate kinematic and mantle seismic tomography constraints for the India-Asia collision, Part II: Insights from Myanmar to Sulawesi, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10737, https://doi.org/10.5194/egusphere-egu21-10737, 2021.

Recent paleomagnetic data from early Late Cretaceous and late Eocene rocks from Myanmar (1,2) demonstrate that the Burma Terrane (BT) underwent an important northward translation alongside India in the Cenozoic. We present new paleomagnetic results from Paleocene to Eocene sediments that confirm the slightly southern to equatorial paleolatitudes during the Paleocene to mid Eocene. However, these paleomagnetic results imply a new paleogeography not compatible with the typical view of the geology of Myanmar as an andean-type margin above an active subduction of the Tethys/India oceanic crust below Sundaland.  Most previous models proposed an active subduction below Myanmar during the Paleogene but a slab anchored in the mantle would impede the large northward motion of the BT implied by our paleomagnetic data. We thus review the geology of the BT in light of the new latitudinal constraints provided by the paleomagnetic data. The BT contains >10km thick Cenozoic basins (Central Myanmar Basins (CMBs)) recording the Cenozoic geological evolution of the BT. The CMBs were previously interpreted with sediment sources located within the Myanmar magmatic arc and to the east in Sibumasu. The numerous studies on detrital zircons from the Late Cretaceous - Paleogene sediments  of  the CMBs highlight a clear correlation in the distribution of the ages of the pre-Cretaceous zircons (~40% of the zircons in the sediments) with the one from the Triassic turbidites (Pane Chaung Formation) of the Indo-Burman Ranges and the Triassic sediments from the Tethyan Himalaya (Langjiexue Fm.). Thus, the source of sediments is unlikely to be in Sibumasu but proposed to be in an actively eroding north-western extension of the Indo-Burman ranges (Greater Burma block, (2)) possibly linked to the Tethyan Himalaya and consistent with a BT position within the India plate during the Cenozoic. In any case, we find little evidence for a nearby active magmatic arc in the detrital zircon record supporting the hypothesis of an active subduction below the BT. Thus this review of the geology of the BT supports a rapid northward moving BT alongside India during the Cenozoic. We will discuss the implication of this new paleogeography on the India-Asia collision models.

(1) Westerweel et al. « Burma Terrane Part of the Trans-Tethyan Arc during Collision with India According to Palaeomagnetic Data ». Nature Geoscience 12, no 10 (octobre 2019): 863‑68. https://doi.org/10.1038/s41561-019-0443-2.

(2) Westerweel et al. « Burma Terrane Collision and Northward Indentation in the Eastern Himalayas Recorded in the Eocene‐Miocene Chindwin Basin (Myanmar) ». Tectonics 39, no 10 (octobre 2020). https://doi.org/10.1029/2020TC006413.

How to cite: Westerweel, J., Roperch, P., Dupont-Nivet, G., Licht, A., Cogne, N., and Poblete, F.: Northward motion of the Burma Terrane alongside India during the Cenozoic., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15621, https://doi.org/10.5194/egusphere-egu21-15621, 2021.

Continental collision, which leads to mountain building (e.g. Himalayas, Alps), has been under the geodynamic modelling lenses for the last few decades. Such processes subjected to physical and numerical investigations, in conjunction with observational studies, enrich knowledge on mountain belts and have worked out the general architectural large-scale structure and crustal shortening in such regions. The intent to understand the driving forces of long term (~50 Ma) and consistent convergence at the India-Eurasia collisional zone is the goal of the dynamic self-consistent buoyancy-driven whole-mantle scale 2D and 3D models presented in this contribution. The maximum post-collisional convergence rate (~0.362 cm/year) in 2D models, is less than 2 cm/year convergence of India considering it advanced ~1000 km in about 50 Ma.  Additionally, the 2D models are inadequate in exploring the spatio-temporal evolution and dynamics of natural systems, thus necessitating modelling large scale subduction and subsequent continental collision resolving the 3D components of mantle flow.  With a whole mantle reservoir and buoyancy-driven 2D models, the observed trench advance rate, with a large and fixed overriding plate, is relatively novel and higher than previous studies and the high resolution in 2D models also shows crustal-scale localisation in conjunction with large scale mantle flow. The computationally intensive simulations have significantly large (11520 km) trench-perpendicular (in 2D and 3D) and parallel (in 3D) lengths, include two sets of modelled depths: whole mantle (2880 km) and, upper mantle + partial lower mantle (960 km) and use the Underworld2 framework. In 3D, the interaction of an adjacent subducting oceanic plate(s) significantly aids the indentation and trench advance in the collisional margin. These would help understand the dynamics of analogues system(s) in nature such as the Sunda subduction zone and the India-Eurasia collision zone.

How to cite: Laik, A., Schellart, W. P., and Strak, V.: Trench Advance in Collisional settings: insights from large scale 2D and 3D models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7210, https://doi.org/10.5194/egusphere-egu21-7210, 2021.

Deciphering the exhumation mechanism of high-pressure, low-temperature (HP-LT) metamorphic rocks can provide important insights into the tectonic evolution of oceanic subduction zones at active continental margins. Here we present a multidisciplinary study examining the exhumation tectonics of the Permo–Triassic eclogite-bearing Lanling HP-LT terrane within the Central Qiangtang metamorphic belt (CQMB). Field relations and microscopic observations show that the HP-LT rocks are separated from the Permian ophiolite mélange of the hanging wall by low-angle detachments and exhibit five stages of deformation. The pervasive top-to-the-SW and -S shearing structures imply that the Lanling HP-LT terrane was exhumed as a transtensional metamorphic core complex (mcc). The results of the petrological and mineralogical analysis and pseudosection modeling of eclogites indicate that the eclogites and blueschists are characterized by synexhumation mineral growth pulses with decompressional P-T trajectories. A compilation of previous geochronological data and our ⁴⁰Ar/³⁹Ar dating results of shearing structures in HP-LT rocks indicate a continuous exhumation at ca. 244–210 Ma. Moreover, the CQMB experienced lithospheric transtension, as shown by the Middle–Late Triassic geological events, which include mantle upwelling at ca. 237–230 Ma and abyssal basin development in the Anisian–middle Norian. These observations indicate that the CQMB is likely a autochthonous accretionary wedge resulting from northward subduction of the Paleo-Tethys Ocean beneath the North Qiangtang Block (NQB). Moreover, the transtension of the CQMB occurred in the late stage of the oceanic subduction, which was probably triggered by oceanic slab rollback.

How to cite: Liang, X., Wang, G., Cao, W., Forster, M., and Lister, G.: Lithospheric Extension of the Accretionary Wedge: An Example From the Lanling High-Pressure Metamorphic Terrane in Central Qiangtang, Tibet, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2055, https://doi.org/10.5194/egusphere-egu21-2055, 2021.

Early Mesozoic development of Southeast Asia involved oceanic subduction, closure, accretion and collision of discrete terranes rifted from Gondwana. South China, as an important continental terrane, is bound to the north by the Qinling-Dabie collisional orogenic belt, to the south by the Indochina Block, and to the east by the Pacific Plate. The role of continental collision and subduction during the Early Mesozoic development of South China has sparked the interest of geologists worldwide and stimulated considerable research. The Triassic tectonic history of the southwestern South China Block is marked by the Indosinian orogeny that records amalgamation of the Indochina and South China blocks during the late Permian to Triassic as a result of closure of the eastern branch of the Paleo-Tethys Ocean. In South China, there is widespread granitic magmatism, metamorphism and deformation. The closure of eastern Paleo-Tethys Ocean and subsequent collision between the South China block and Indochina Block has caused the collision zone metamorphism and formation of granites during the Permo-Triassic, with the Song Ma fault zone as the collision boundary. The Indosinian magmatism in the Pingxiang region was the magmatic products in this period. We report the new results of bulk-rock major and trace element, Nd, Hf isotopic compositions and zircon U–Pb dating of granites and rhyolites in the Pingxiang region in Guangxi Province, Southwest China, to decipher their petrogenesis and tectonic settings. The granites and rhyolitics in the Pingxiang area have low Mg^# values (11.1–36.7), low Nb/Ta ratios (9.26–13.74) exhibiting a both affinity from S-type to I-type granaite. The isotopic features of these rocks show negative ε_Hf(t) with the values ranging from -9.89 to -6.09, negative ε_Nd(t) values ranging from -12.89 to -12.02 and T_2DM values of 1.8–3.3 Ga, suggesting that the Pingxiang granites and rhyolites was derived from partial melting of paleoproterozoic crust rocks. The granites yielded ²⁰⁶Pb/²³⁸U ages ranging from 243 to 241 Ma, and the rhyolites yielded ²⁰⁶Pb/²³⁸U ages ranging from 247 to 245 Ma, which are both within the age range of the subduction to collision. Combine the regional geology, we suggest these granitoids and rhyolites were formed by the partial melting of crustal rocks during a transition from subduction to post-collisional environment with closure of Paleo-Tethys Ocean between the South China block and Indochina Block.

This study was financially supported by Guangxi Natural Science Foundation for Distinguished Young Scholars (2018GXNSFFA281009) and the Fifth Bagui Scholar Innovation Project of Guangxi Province (to XU Ji-feng).

How to cite: Huang, W., Liu, X., Li, Z., Zhao, B., and Han, Y.: Tectonic evolution of the Paleo-Tethys Ocean in Southwest Guangxi: evidence from acidic magmatic rocks in the Pingxiang area, SW China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11976, https://doi.org/10.5194/egusphere-egu21-11976, 2021.