The geodynamic evolution of the Western Mediterranean related to the closure of the Ligurian-Tethys ocean is not yet fully resolved. We present a new 3D numerical model of double subduction with opposite polarities fostered by the inherited segmentation of the Ligurian-Tethys margins and rifting system between Iberia and NW Africa. The model is constrained by plate kinematic reconstructions and assumes that both Alboran-Tethys and Algerian-Tethys plate segments are separated by a NW-SE transform zone enabling that subduction polarity changes from SE-dipping in the Alboran-Tethys segment to NW-dipping in the Algerian-Tethys segment. The model starts about late Eocene times at 36.5 Ma and the temporal evolution of the simulation is tied to the geological evolution by comparing the rates of convergence and trench retreat, and the onset and end of opening in the Alboran Basin. Curvature of the Alboran-Tethys slab is imposed by the pinning of its western edge when reaching the end of the transform zone in the adjacent west-Africa continental block. The progressive curvature of the trench explains the observed regional stress reorientation changing from N-S to NW-SE and to E-W in the central and western regions of the Alboran Basin. The increase of the retreat rates from the Alboran-Tethys to the Algerian-Tethys slabs is compatible with the west-to-east transition from continental-to-magmatic-to-oceanic crustal nature and with the massive and partially synchronous calc-alkaline and alkaline magmatism.

How to cite: Peral Millán, M., Fernàndez, M., Vergés, J., Zlotnik, S., and Jiménez-Munt, I.: Numerical modelling of opposing subduction in the Western Mediterranean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8471, https://doi.org/10.5194/egusphere-egu23-8471, 2023.

The evolution of backarc and forearc basins is usually treated separately, as the volcanic arc represents a clear barrier between them. We analyse their spatial and temporal relationships in the Tyrrhenian subduction system, using seismic profiles and numerical modelling. Our results highlight that the Marsili volcano, commonly interpreted as the spreading centre of the Marsili backarc basin, was instead a part of an old (Pliocene) volcanic arc associated with the development of the Vavilov backarc basin (4.3-4.1 to 2.6 Ma). The old volcanic arc was successively affected by arc rifting. This process caused the shift of the Marsili volcano eastwards and the formation of an oceanic backarc basin (~ 1.8 Ma) located between the Marsili volcano and the old remnant arc, which remained fixed. The eastern side of the Marsili basin, previously considered as the other half of the oceanic backarc basin, is instead a part of the forearc domain floored by serpentinised mantle. As slab rollback continued, volcanism migrated towards the trench and a new volcanic arc (Aeolian Island) formed at ~1 Ma in the forearc domain. The formation of the new volcanic arc represents the onset of the forearc-rifting that could lead to the opening of a new backarc basin between the old and young volcanic arc, resulting in the decrease of the initial forearc region extension.

The example of the Tyrrhenian Sea illustrates how the evolution of forearc and backarc domains is intimately interconnected. Fluids, released from the downgoing plates, control lithospheric hydration and mantle serpentinisation as well as asthenospheric mantle melting. Fluids and melts induce weakening of the volcanic arc region and drive the arc-rifting that led to the backarc basin formation. Later, the slab rollback causes the trench-ward migration of volcanism that led to the forearc- rifting under the control of fluids released from the downgoing plate.

How to cite: Corradino, M., Balazs, A., Faccenna, C., and Pepe, F.: Arc and forearc rifting in the Tyrrhenian subduction system, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6778, https://doi.org/10.5194/egusphere-egu23-6778, 2023.

The tectonostratigraphic evolution in the western margin of the Ulleung back-arc basin was reconstructed based on the seismic reflection data. According to our stratigraphic and structural analysis, the study area developed via four tectonostratigraphic stages, one extensional and two subsequent tectonic inversions. Together with the back-arc opening of the East Sea, most fault-controlled depocenters (e.g., half-grabens) were formed mainly in the western margin of the Ulleung Basin during the Early–early Late Miocene. This syn-extensional sedimentation occurred in non-marine to deep-marine environments analogous to typical rift-related linked depositional systems. During the early Late Miocene, the Ulleung back-arc basin had changed entirely into a compressive regime (NW–SE compression). Under the inversion tectonics, NNE–SSW and N–S trending extensional faults were mainly reactivated as reverse faults. The Hupo Basin was likely created by the regional flexural response to the crustal or thrust loading. As the formation of the Hupo Basin began, hemipelagic sedimentation accompanied by episodic gravity-controlled slope failures prevailed in the deep-water environment. Since the late Early Pliocene, the subsidence of the Hupo Basin was enhanced by the crustal shortening. The sedimentary condition became shallower gradually upward and coarse-grained terrigenous input into the Hupo Basin began, leading to deposition in shallow- to deep-marine environments. During the Quaternary, although the tectonic activity was subdued, the Hupo Fault was reactivated as a reverse fault, maintaining the uplift of the Hupo Bank and coeval flexural subsidence of the Hupo Basin. During this depositional period, shallow- to deep-marine deposition continued but a greater quantity of coarse-grained terrestrial sediments was transported into the Hupo Basin. The Quaternary depositional systems are likely the result of the interplay between tectonics and eustasy.

How to cite: Park, Y., Kang, N., Yi, B., Lee, G., and Yoo, D.: Tectonostratigraphic evolution of the Hupo Basin in the western margin of the Ulleung back-arc basin, the East Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4610, https://doi.org/10.5194/egusphere-egu23-4610, 2023.

The Nazca Ridge is a wide aseismic ridge subducting beneath the South American margin at latitude about 15°. The buoyancy of the thickened oceanic crust of the Nazca Ridge produces localized flat subduction influencing the geometry and the geological history of the whole area.  With the aim of analysing the spatio-temporal evolution of the deformation and uplift/subsidence history of the lithosphere above the Nazca Ridge flat slab, we have started from the study of the geothermal structure of the upper plate. We have built a crustal section with a length of 1000 km that reaches a depth of about 130 km. The section runs from the top of the Nazca Ridge in the west to the Amazonian Basin in the east, progressively crossing the Peru-Chile trench, the East Pisco Basin and the Andean Cordillera. Thereafter we have elaborated a 2D geothermal model based on the crustal section. We have considered the whole lithosphere composed of two main geological units: (i) crystalline basement, (ii) sedimentary cover (including the whole lithostratigraphic succession). For each unit we have assigned the following parameters: thickness, density, heat production and thermal conductivity. Moreover, we have also taken into account the friction coefficient, the convergence rate of the plates, the heat flux of the Moho, and the slip rate of the megathrust. Model parameters have been set up in order to obtain the best simulation of the heat flow contribution due to the large reverse fault responsible for the coastal seismic event of November 12, 1996, with epicentre on the section trace. Using these parameters and applying an analytical methodology we have calculated isotherms and geotherms. The resulting model may provide an important contribution on the investigation of the effects of the Nazca Ridge subduction and the associated flat slab geometry on the tectonic evolution of the area.

How to cite: Ciattoni, S., Basilici, M., Stefano, M., Antonella, M., and Stefano, S.: 2D Geothermal model across the Peru-Chile trench and the Andean Cordillera above the Nazca Ridge subduction, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4323, https://doi.org/10.5194/egusphere-egu23-4323, 2023.

The Middle American Trench (MAT) is one of the most complex subduction margins all over the earth surface. Its geodynamical complexity is due to the interaction between five major lithospheric plates: North America, Caribbean, Cocos, Nazca and South America; between them is the Panama microplate.
We focused on the Costa Rica subduction margin, which is a portion of the MAT and it is characterized by some peculiarities with respect to the other portions of the MAT. Along the Costa Rica offshore the subduction of the Cocos Plate is currently developing towards NE, beneath both the Caribbean Plate and the Panama Microplate, with a rate that increases from NW (87 mm/yr), in correspondence of the Nicoya Peninsula, to SE (92-95 mm/yr), in correspondence of the Osa Peninsula.   
The Cocos Plate formed, together with the Nazca Plate, about 28 Ma from the Farallon Plate fragmentation in turn due to the formation of the East Pacific Rise (EPR). The subduction process is extremely seismogenetic and caused some earthquakes up to 7.8 M_w (1950): one of the most recent hits Nicoya on September 5th, 2012 (M_w 7.6). The migration of the Cocos Plate towards the Galapagos plume generated, about 14 Ma, the Cocos Ridge, a strip of oceanic ridge that is currently subducting beneath the southeastern margin of Costa Rica, in correspondence of the Osa Peninsula. The beginning of subduction, dated between 8 and 1 Ma, generated an isostatic rebound that gave rise to a general uplift generating the Cordillera de Talamanca, which emerged between 4.5 and 3 Ma and representing the extinct portion of the volcanic arc.    
The main aim of this study is to provide a reliable model about the evolution of the Costa Rica subduction margin, paying attention on the Cocos Ridge subduction and to understand how this affects the evolution of the margin. Through the seismostratigraphic interpretation of several multichannel seismic reflection profiles, together with morphobathymetric data, well data from ODP Leg 170, focal mechanisms and oceanic crust age variation chart along the MAT, as well as the Costa Rica geological map, produced by USGS, we recognized some evidence and mechanisms responsible for the uplift that affected the area (e.g. underthrusting process and strike-slip faults) and how this could be related to the subduction of the Cocos Ridge and of several seamounts recognized along the Costa Rica subduction margin. The Cocos Ridge subduction is also responsible for the magmatism recognized along the Nicoya Peninsula offshore, as well as of the variation of the slab geometry recognized through the realization of a 3D model of the Wadati-Benioff Plane.

How to cite: Parente, F. and Sulli, A.: Geological evolutionary model of the Costa Rica subduction margin, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7375, https://doi.org/10.5194/egusphere-egu23-7375, 2023.

In the Paleoproterozoic era (2.5-1.6 Ga ago), the mode of the plate tectonics was shifting from Archean plume-lid tectonics to modern tectonics, with colder and deeper subduction due to a decreasing mantle potential temperature. Hence, the geodynamic regime was different as well; subduction was more episodic and characterised by frequent slab breakoffs, while weaker lithosphere resulted in wider and lower-relief orogens. Metamorphic rocks also recorded a fingerprint of these conditions, generally lacking evidence of UHP metamorphism and indicating higher temperatures in the lithosphere.

However, studying Paleoproterozoic orogens is challenging, as metamorphic rocks at the present-day erosional level often represent the middle-to-lower crustal orogenic interior. We aim to overcome this issue using pressure-temperature-time (P-T-t) paths extracted from generic, geodynamic continent-continent collision models and comparing them to P-T-t paths reconstructed from metamorphic minerals. The models are loosely based on Paleoproterozoic Svecofennian orogen, which formed the majority of the bedrock in southern Finland. It is well studied by number of geological and geophysical means, but physics-based geodynamical models are still lacking.

The models were run using the 3D thermo-mechanical, finite-element geodynamic modeling code DOUAR (Braun et al., 2008), which uses the PETSc version of the direct matrix equation solver MUMPS and the landscape evolution model FastScape. The work explored the effects of various continental collision obliquity angles, temperature conditions, and crustal thicknesses in a set of 13 different models. The spatial dimensions of the models are 1000×1000×70 km and crustal thickness values of 35 km and 45 km were used. In the Svecofennian orogeny, continent-continent collision was an event between colder and hotter continental blocks, which is implemented in the models by including a temperature difference of 100ºC along the model base at 70 km depth. Along this boundary, heat production is varied laterally to explore three different temperature scenarios. The convergence obliquity angle is also varied between 0º, 30º and 60º, while the subduction dip angle is constant at 45º.

With the thinner 35 km crust, the models do not show much difference in the dynamics between the temperature scenarios, as the crust is too thin to develop wide orogens, and eventual partitioning of strain due to oblique collision. Similarly, the P-T-t paths represent only straightforward retrograde metamorphism, due to simple model dynamics and the lack of large-scale internal orogenic heating. Increasing the crustal thickness to 45 km significantly affects the orogenic development. The Paleoproterozoic temperature scenario with a 45 km crust creates both wide and lower-relief orogens, also producing clear strain partitioning for the 60º obliquity angle. This difference in dynamics further results in more variation in the recorded P-T-t paths, suggesting potential for their use to explore Paleoproterozoic orogen dynamics. Ongoing work is exploring which stable mineral assemblages these P-T-t paths would correspond in metamorphic rocks.

How to cite: Tuikka, L., Cateland, B., Whipp, D., and Häkkinen, M.: Records of continent-continent collisions in the Paleoproterozoic: exploring the effects of convergence obliquity and temperature on P-T-t paths, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15948, https://doi.org/10.5194/egusphere-egu23-15948, 2023.

Introduction. This work is devoted to the study of the sources of   drift material during the formation of Late Paleozoic deposits of the southern part of the Pre-Ural trough. Sample for the study was taken in a quarry near the Urgala region, Bashkortostan area. The section is represented by conglomerates with a sand matrix. These deposits belong to Ural forland basin. The age of this conglomerate formation – Moscovian (Middle Carboniferous).

Materials and methods. The most reliable determination of sources is possible due to U-Pb zircon dating. We also analyzed some thin sections for detailed studying of sandstone composition.

Results and discussion. Zircon grains vary greatly in shape and size. In some grains, the core and edges are clearly visible; others are full of inclusions, cracks, and zones of metamict decay. The size of the crystals varies from 60 to 400 microns. Most of the ages obtained fall in the interval from the Ordovician to the Devonian, less on the Lower and Middle Riphean. Single grains are of Cambrian, Vendian and Late Riphean age. Early Proterozoic and Archean grains are absent in the sample.

The most difficult interval is from the Cambrian to the Devonian, it accounts for the majority of the ages (410-430 Ma). Within the studied territory, the volcanic rocks closest to the sampling site are located in Nyazepetrovsk and Bardym allochthons, as well as in the Tagil arc. In addition, Devonian granitoids are found within the Ufalei anticlinorium. The largest number of Precambrian dates falls on the Middle Riphean. The source of zircons during the middle Riphean could be the Mashak formation, whose age is 1350-1346 Ma, however, there are no grains with the age of the Mashak formation in the sample.

A relatively large number of grains have the early Riphean age of 1650-1500 Ma, which correlates perfectly with the age of the Ai formation. However, almost all Riphean formations, including the Ai formation, contain zircons with the peak at 2050 Ma (the age of migmatization in the Taratash block),but  the studied sample contains no zircons of 2050 million years age or older. This means that the Taratash block and the surrounding Riphean formations were not exposed at that time.

Also, the largest number of lithoclasts in the studied sandstones are represented by siliceous rocks. The similar rocks compose the Ordovician-Devonian section of the Mayaktau Allochthon, which is located closely to the sampling site. Also, the thickness Aziam  formation   increases towards Mayaktau Allochthon. In addition to the sources described above, there is jne more source – Asha series (Vendian), because there are quite a large number of Middle-Riphean dates in the sample, which are typical for the rocks of the Asha series.

Financial support. The research has been funded by RFBR and CNF as a part of the research project № 19-55-26009 Czechia_a

How to cite: Volodina, E., Tevelev, A., Borisenko, A., Koptev, E., Shestakov, P., Pravikova, N., and Novikova, A.: Sources of material drift into the Ural foredeep at the beginning of collision (Southern Urals), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14925, https://doi.org/10.5194/egusphere-egu23-14925, 2023.

Introduction. Determination of the age of igneous roc Comparative analysis of U-Pb dating of zircons from Early Carboniferous volcanites and Middle Triassic alkaline granitoids of the Magnitogorsk zone (Southern Urals)

ks by the U-Pb isotope method using zircons is currently one of the main dating methods. Here we present new isotopic data of zircons from alkaline granitoids of the Cheka massif and zircons from acidic volcanites of the lower Carboniferous of the Magnitogorsk zone (Southern Urals).

Materials and methods. The Middle Triassic isotopic age of the Cheka massif was determined by the Rb-Sr isochron method. Currently, we obtained new seven U-Pb dates based on zircons isolated from various phases of the massif. Early Carboniferous volcanites are represented by a contrast moderately alkaline series. Volcanites have been sampled at two points. The U-Pb dating was performed at the All-Russian Geological Research Institute using SHRIMP-II.

Results. At least two zircon populations of early Carboniferous isotopic age have been identified in acid volcanites. The first population is represented by full   crystals and their fragments 100-200 microns in size. They have a short-prismatic habit and a clear oscillatory zonation. This population is predominant in all samples. Zircons have a moderate content of U and Th. The population is homogeneous with average concordant age is 348.5 ± 3.1 Ma.

Zircons of the second population were found in all samples. They are small (about 50 microns), perfectly faceted crystals with an increased content of U and Th. Their isotopic ages (344 and 351 Ma) are entirely fit the age range of the first population. Thus, completely different in morphology and composition, zircons have the same isotopic age.

Two most representative samples of alkaline granitoids, provide zircons 150-250 microns in size. They are light in the cathodoluminescent image, with a clear fine oscillatory zonation and weakly expressed sectorial. The range of isotopic ages of these zircons in is 342.6–376.6 Ma, and their average concordant age is almost the same: 353.9±4.0 and 352.7±3.9 Ma.

Discussion. U-Pb dating of zircons from acidic volcanites confirmed their Tournaisian age. The morphology and composition of zircons turned out to be an important key to understanding the age of volcanites intruded by the alkaline granitoids.

Inherent zircons in alkaline granitoids may not be crystallized at all, since all zirconium should be concentrated in alkaline dark-colored minerals. In this case, only the inherited zircon will remain in the rock. In addition, the dissolution of inherited zircons can also occur in alkaline melts.

Early Carboniferous zircon grains in all samples of alkaline granitoids are similar to those from volcanites. They have a typically magmatic appearance and zonation and the concentration and ratio of uranium and thorium are also typical. At the same time, alkali-rich fluid-saturated magmatites are usually characterized by a Th/U ratio close to or significantly higher than 1. Uranium and thorium concentrations are usually very high. The described features most likely indicate the xenogenic nature of Early Carboniferous zircons in relation to granitoids.

Financial support. The research has been funded by RFBR (research project № 19-55-26009).

How to cite: Tevelev, A., Pravikova, N., Borisenko, A., Shestakov, P., Koptev, E., Sobolev, I., Volodina, E., Kazansky, A., and Novikova, A.: Comparative analysis of U-Pb dating of zircons from Early Carboniferous volcanites and Middle Triassic alkaline granitoids of the Magnitogorsk zone (Southern Urals), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6572, https://doi.org/10.5194/egusphere-egu23-6572, 2023.