GD4.4 | Convergent plate boundaries: from collisional orogens to extensional back-arc basins
EDI
Convergent plate boundaries: from collisional orogens to extensional back-arc basins
Co-organized by SM5/TS6
Convener: Zoltán Erdős | Co-conveners: Attila BalázsECSECS, Shu-Kun Hsu, Benoit Deffontaines, Valentina Magni, Julia Ribeiro, Camilla Palmiotto
Orals
| Thu, 27 Apr, 16:15–18:00 (CEST)
 
Room D1
Posters on site
| Attendance Fri, 28 Apr, 08:30–10:15 (CEST)
 
Hall X2
Posters virtual
| Attendance Fri, 28 Apr, 08:30–10:15 (CEST)
 
vHall GMPV/G/GD/SM
Orals |
Thu, 16:15
Fri, 08:30
Fri, 08:30
Convergent plate boundaries can result in a wide variety of tectonic feature from collisional orogens (e.g. the Himalaya or Pyrenees) through subduction orogens (e.g. the Andes or Taiwan) to arc-back-arc systems (e.g. Sea of Japan or the Aegean). These tectonic settings might transition from one to another like in Southeast Asia, where there is geodynamic inversion of the east dipping Manila oceanic subduction South of Taiwan, that evolves northward, first, into a Continental Subduction (collision) onshore Taiwan, then secondly, east of Taiwan, into the north dipping Ryukyu arc/continent subduction.

Recently a large volume of high quality and high resolution geophysical and geological data had been acquired that could help us better understand the processes that govern subduction, collision and back-arc extension. Our session has a special focus on overriding plate deformation as it shows a great variety between different systems from extension dominated settings to compression dominated ones.

In this session authors are encouraged to share their work on the tectonic or magmatic features convergent plate boundary settings, as well as on the study of the processes contributing to the formation, evolution, and shaping of such systems. The conveners encourage contributions using multi-disciplinary and innovative methods from disciplines such as, but not restricted to, field geology, thermochronology, geochemistry, petrology, seismology, geophysics and marine geophysics, and analogue/numerical modelling.

Orals: Thu, 27 Apr | Room D1

Chairpersons: Zoltán Erdős, Benoit Deffontaines, Camilla Palmiotto
16:15–16:20
16:20–16:30
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EGU23-5077
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solicited
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Highlight
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On-site presentation
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Irina M. Artemieva

This global study of 31 off-shore back-arc basins (BABs) identifies their principal characteristics based on a broad spectrum of geophysical and subduction-related parameters. My synthesis is used to identify trends in the evolution of BABs for improving our understanding of subduction systems in general. The analysis, based on the present plate configuration, demonstrates that geophysical characteristics and fate of the BABs are essentially controlled by the tectonic type of the overriding plate, which controls the lithosphere thermo-compositional structure and rheology. The type of the plate governs the length of the extensional zone in back-arc settings along the trench, the efficiency of lithosphere stretching, and the crustal structure, buoyancy and bathymetry of the BABs. Subduction dip angle apparently controls the location of the slab melting zone and the efficiency of slab roll-back with feedback links to other parameters. By the tectonic nature of the overriding plate (the downgoing plate is always oceanic) the back-arc basins are split into active BABs formed by ocean-ocean, arc-ocean, and continent-ocean convergence, and extinct back-arc basins. By geophysical characteristics, BABs formed on continental plates are subdivided into active BABs with and without seafloor spreading, and extinct BABs are subdivided into the Pacific BABs, possibly formed on oceanic plates, and the non-Pacific BABs with reworked continental or arc fragments. Six types of BABs are distinctly different. Extension of the overriding oceanic plate above a steeply dipping old oceanic plate, preferentially subducting nearly westwards, forms large deep back-arc basins with a thin oceanic- type crust. In contrast, BABs on the overriding continental or arc plates form at small opening rates and often by shallow subduction of younger oceanic plates with a random subduction orientation; these BABs have small sizes, shallow bathymetry, and hyperextended or transitional ~20 km thick arc- or continental-type crust typical of passive margins. The presence of a 2–5 km thick high-Vp lowermost crustal layer, characteristic of BABs in all settings, indicates the importance of magmatic underplating in the crustal growth. Conditions required for the initiation of a back-arc basin and transition from stretching to seafloor opening depend on the nature of the overriding plate. BABs formed on oceanic plates always evolve to seafloor spreading. BABs formed on continental or arc plates require long spreading duration with large (>8 cm/y) opening rates and a large crustal thinning factor of 2.8–5.0 to progress from crustal extension to seafloor spreading. On the present Earth such transition does not happen in the BABs formed behind a shallow subduction (<45o) of a young (<40 My) oceanic plate. The nature of the overriding plate also determines the fate of back-arc basins after termination of lithosphere extension: the extinct Pacific BABs with oceanic-type crust evolve towards deep old “normal” oceans, while the shallow non-Pacific BABs with low heat flow and thick crust are likely to preserve their continental or arc affinity. BABs do not follow the oceanic cooling plate model predictions. Distinctly different geophysical signatures for mid-ocean ridge spreading and for back-arc seafloor spreading are caused by principally different dynamics. https://doi.org/10.1016/j.earscirev.2022.104242

How to cite: Artemieva, I. M.: Back-arc basins: A global view from geophysical synthesis and analysis, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5077, https://doi.org/10.5194/egusphere-egu23-5077, 2023.

16:30–16:40
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EGU23-8131
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ECS
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On-site presentation
Eleonora Ficini, Marco Cuffaro, Taras Gerya, and Carlo Doglioni

Extension at back-arc basins generally occurs behind arc-trench systems and the mechanisms which act at its origin, as well as the deformation regime developed, are strongly related to the subduction of oceanic lithosphere. Here, we examine the Japan Sea back-arc basin evolution using numerical simulations along the western margin of the Pacific plate, where the subduction processes have been responsible for the deformation style during the last 57 Ma. We carried out 2D high-resolution thermo-mechanical numerical models of subduction dynamics in this area, increasing the simulation complexity integrating into the computations i) the kinematic variability of the Pacific plate over the geological past with respect to a fixed Eurasia, ii) a Low-Viscosity Zone within the asthenosphere, iii) a horizontal eastward mantle flow. Our results show a main kinematic control of the subduction trench position, which advances and retreats in time, providing stages of compression and extension in the Japan Sea back-arc basin. The obtained deformation regime is comparable with the tectonic evolution history occurred along the Eastern Eurasian margin and with analyses on paleo-volcanic front position and paleo-stress reconstructions in the Japan Sea area.

How to cite: Ficini, E., Cuffaro, M., Gerya, T., and Doglioni, C.: Role of variable plate kinematics history in the back-arc deformation regime along the western Pacific margin (Japan Sea), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8131, https://doi.org/10.5194/egusphere-egu23-8131, 2023.

16:40–16:50
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EGU23-12780
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On-site presentation
Julie Tugend, Penggao Fang, Nick Kusznir, Geoffroy Mohn, and WeiWei Ding

The formation and evolution of back-arc basins is complex controlled by subduction dynamics, lithosphere delamination, magmatism, slab roll-back and extension. In such a complex geodynamic context, it is difficult to decipher the mechanisms which controls sedimentary basin subsidence history and distinguish the contribution of lithosphere tectonics from dynamic topography.

Here we focus on one of the main basins of the Western Mediterranean, the Valencia Trough, which formed in the Cenozoic in relation with the slab roll-back of the Tethyan oceanic lithosphere. More specifically, we investigate the subsidence and geodynamic context related to the formation of a regionally observed unconformity, which separates Mesozoic from latest Palaeogene to Neogene sediments, and here referred to as the Miocene Unconformity.

Using a dense grid of seismic reflection data, well data and 3D flexural backstripping, we show that the Miocene Unconformity subsided by more than 1.5 km from ~17 Ma to the present day at an average rate of 90 m/Myr in the SW Valencia Trough. The absence of Cenozoic extensional faults affecting the basement shown by seismic data indicates that this rapid subsidence is not caused by Cenozoic rifting. This subsidence cannot be explained by flexural loading related to the adjacent thin-skin Betic fold and thrust belt either, which only affects subsidence observed near the deformation front. Subduction dynamic subsidence generated by the positive mass anomaly of the subducting slab in the mantle is another mechanism that can control the subsidence evolution of back-arc basins. However, since the formation of the Miocene unconformity, the subduction has propagated westwards and southwards and has slowed or ceased under the Valencia Trough, which would have resulted in the progressive diminution of subduction dynamic subsidence, generating a relative uplift rather than subsidence.

We propose an alternative mechanism and interpret the 1.5 km subsidence of the Miocene Unconformity as the collapse of a back-arc transient uplift event. Erosion during the uplift, resulting in the formation of the unconformity, is estimated to exceed 4 km. This transient uplift was likely caused by heating of back-arc lithosphere and asthenosphere, combined with mantle dynamic uplift, both caused by segmentation of Tethyan subduction resulting in slab tear. Rapid subsidence subsequently resulted from the removal of mantle flow dynamic support from the Tethyan subduction slab roll-back and thermal equilibration.

Our observations and interpretation of rapid back-arc kilometre-scale uplift and collapse might have global applicability to explain some of the observed vertical motions and the subsidence evolution of other back-arc regions experiencing subduction segmentation and slab tear during subduction slab roll-back.

How to cite: Tugend, J., Fang, P., Kusznir, N., Mohn, G., and Ding, W.: Rapid large-amplitude vertical motions generated by subduction slab roll-back in back-arc basins (Valencia Trough, Western Mediterranean), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12780, https://doi.org/10.5194/egusphere-egu23-12780, 2023.

16:50–17:00
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EGU23-9647
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On-site presentation
Armagan Kaykun and Russell Pysklywec

The Black Sea Basin has been a focus of interest due to its economically promising hydrocarbon reserves and complex tectonic history. Several different theories were proposed to decipher its enigmatic basin formation and tectonic evolution processes.

One important characteristic of the Black Sea Basin that makes it unique is its isolation from the world oceans, and global sea level changes for long periods during the geological time. This provides a good realm to correlate tectonic episodes with rapid sedimentation patterns in its thick sedimentary section. With the aim of modelling this sequence of events, we reviewed and reinterpreted previously proposed scenarios. We focus on the back-arc rifting and subsequent tectonic inversion that led the surrounding mountain belts to form. By reinterpreting 24 long-offset 2D seismic lines acquired by GWL in 2011, we propose a new structural framework for the Black Sea Basin.

Our structural geology analyses show that in addition to basin-bounding normal faults and inversion tectonics, numerous flower structures occur in both the western and eastern Black Sea subbasins. These flower structures are typical indicators of strike-slip fault systems and in the Black sea Basin case, the orientation of these fault systems is roughly east-west. Our interpretations align with the hinge model that Stephenson and Schellart (Geological Society London Special Publications, 2010) proposed to explain the opening of the Black Sea Basin as one basin rather than the conventional interpretation of a two separate rifted basin configuration. The proposed tectonic framework sheds light on the geometry of the Black Sea Basin’s bounding faults, complex faulting and folding recognized in the sedimentary section, and complex ridge-depression geometry.

How to cite: Kaykun, A. and Pysklywec, R.: Existence and Distribution of Basin-Wide Strike Slip Fault Systems in an Asymmetrical Back Arc Rift System: The Black Sea Basin, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9647, https://doi.org/10.5194/egusphere-egu23-9647, 2023.

17:00–17:10
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EGU23-3506
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On-site presentation
Ícaro Dias da Silva, Manuel Francisco Pereira, and Emilio González Clavijo

Devonian-Carboniferous synorogenic sedimentation is described across the Variscan orogen, as well-preserved exposures in late orogenic structures between continental blocks. Variscan marine sedimentary sequences are described in both colliding continents: Gondwana representative of the southern subducting super-plate, and Laurussia considered as the overriding block. The Variscan synorogenic basin distribution on both sides of the alleged Rheic Ocean suture zone raised questions regarding the basin geodynamic classification and possible geographycal and temporal connections. The Devonian-Carboniferous turbiditic basins of the Variscan belt have been classified as foreland, forearc, or backarc, in line with their relative geographical position in the convergent plate boundary. However, the same Variscan basin may have different classifications depending on the proposed tectonic model and its current geographic position. The standard classification of the Variscan synorogenic basins fails due to a poor understanding of their relationship with the tectono-metamorphic and magmatic evolution of their basement, which means ambiguity and controversy in defining global tectonic models.

As a world-class natural laboratory, the Iberian Massif (Portugal and Spain), at the westernmost tip of the Variscan Belt, presents itself as a place to study orogenic processes, from depth (ductile deformation, metamorphism and plutonism) to shallow (synorogenic sedimentation and volcanism) crustal levels. Recent studies in NW and SW Iberia have revealed a regional-scale relationship between Mississippian turbiditic (flysch) basins and magmatic flare-ups. Although there are many similarities between the stratigraphy of NW and SW Iberia synorogenic basins and the tectono-metamorphic and magmatic evolution of their basements, there are still many unexplored features that must be envisaged to get a better understanding of the tectonic evolution of the Variscan belt. The Mississippian basins of NW and SW Iberia show the typical rhythmic sedimentation of turbiditic sequences that are locally disturbed by large olistostrome bodies bearing different-sized olistoliths derived from the previously deformed metamorphic basement. While NW Iberia Variscan flysch-type basins have been associated with the formation of an accretionary wedge, later incorporated at the base of an unrooted slice of allochthonous units, those from SW Iberia seem to reflect their original position, only locally detached at the base due to the relative motion of their basement. SW Iberia flysch basins are also contemporaneous with voluminous bimodal volcanism, more important but not confined to the base of the synorogenic sequences. The Mississippian volcanic rocks are one of the primary sources of Variscan flysch, as evidenced by the widespread occurrence of weakly deformed olistoliths of mafic and felsic volcanic rocks and the significant input of Mississippian zircon grains found in the flysch sequences, when compared with their NW Iberia correlatives. So, considering the geological information that is known and may be used for a preliminary comparative analysis of the Mississippian NW and SW Iberia flysch basins, the following doubt stands: Did they have a common spatial and temporal geodynamic evolution? If so, what is the geological meaning of this assumption?

This work was supported by the Grant PID2020-117332GB-C21funded by MCIN/AEI/10.13039/501100011033, by the FCT-Estímulo ao Emprego Científico (Norma Transitória), by the FCT grants FCT/UIDB/50019/2020-IDL and FCT/UIDB/04683/2020- ICT.

How to cite: Dias da Silva, Í., Pereira, M. F., and González Clavijo, E.: Mississippian synorogenic sedimentation in the Variscan belt: Why are NW and SW Iberia flysch basins so different and yet so similar?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3506, https://doi.org/10.5194/egusphere-egu23-3506, 2023.

17:10–17:20
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EGU23-10333
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ECS
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Virtual presentation
Egor Koptev, Alexey Kazansky, Alexander Tevelev, Natalia Pravikova, and Borisenko Alexandra

Introduction. The Early Carboniferous Nepluyevka polyphase granitic batholith is situated in the East Ural zone. Its emplacement happened during the Sudetian orogeny, which initially shaped the structure of the southwestern segment of the Ural-Mongolian fold belt. As such, the pluton is a repository of information on tectonic evolution and geodynamics of said orogen, which can be used to enhance our understanding of interactions between Laurussia and the microcontinent of Kazakhstania during the Early Carboniferous.

Methods and materials. We have investigated the existing data on the petrology, petrochemistry, isotope systems, and U-Pb geochronology of Nepluyevka batholith, and performed our own analysis of the trace element distribution of the constituting rocks using ICP-MS method. The mechanism of emplacement and its kinematic setting were investigated through an analysis of oriented fabrics and anisotropy of magnetic susceptibility (AMS) for each phase. Paleomagnetic methods were employed for establishing the position of pluton’s host terrain during its emplacement. A total of five specimen, characterizing all of the phases of the batholith, were chosen for petrochemical analyzes, and 186 oriented specimen from 16 sites were used for rock- and paleomagnetic studies.

Results. Combinations of 87Sr/86Sr (0,70491–0,70504) and εNd (-0,29-0,5) ratios for different phases indicate that both depleted mantle and crustal sources were involved in petrogenesis. Trace element distribution is characteristic of subduction settings. AMS parameters’ spatial distribution and observed fabric features show that the batholith was emplaced in a kinematic setting of sinistral transtension. Virtual geomagnetic poles (VGPs) obtained from ChRM components of remanent magnetization do not fall anywhere on the Carboniferous-Quaternary sections of apparent polar wander paths (AWP) for Eastern Europe or Siberia.

Discussion. Combined data on geological structure of the pluton, isotope systems, petrochemistry, and rock magnetic properties of rocks lead us to the conclusion that the batholith had developed as a part of a magmatic system associated with an oblique subduction setting. Paleotectonic reconstructions of pluton’s host terrane Visean location derived from our paleomagnetic data contradict the traditional models for the region. We suggest a model featuring rotation of the host terrane in a strike-slip displacement zone to deal with the contradiction. A paleotectonic reconstruction corrected for such a rotation puts the host terrane into the Visean paleo-position of Kazakhstanian microcontinent. This reconstruction agrees well with the the model proposed by Sengor, Natalin and Burtman in [Sengor et al., 1993], featuring a single subduction system (“Kipchak arc”) stretching from Laurussia to Siberia, which existed through much of the Paleozoic and controlled the crustal growth and development of what is now known as Ural-Mongolian fold belt.

Financial support. The research has been funded by RFBR and CNF as a part of the research project № 19-55-26009 with the use of materials of the "Geoportal" Center of the Lomonosov Moscow State University.

How to cite: Koptev, E., Kazansky, A., Tevelev, A., Pravikova, N., and Alexandra, B.: The history of Nepluyevka batholith: A glimpse into Laurussia-Kazakhstania interactions during the Early Carboniferous, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10333, https://doi.org/10.5194/egusphere-egu23-10333, 2023.

17:20–17:30
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EGU23-4773
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On-site presentation
Yuan-Hsi Lee, Lucas Mesalles, and Teresito Bacolcol

The Taiwan and Mindoro islands are located on the northern and southern ends of the Malina trench, and both orogens result from the deformation of the continental margin of the Eurasia plate. Comparing the exhumation histories of both orogens allow us to discuss the mechanism of mountain building of two orogens.
In Taiwan orogen, the timing of the mountain building starts from ca. 6-8 Ma, which can be identified using ZrnFT, Ar-Ar, and the timing of the developing foreland basin. 
For Mindoro island, we combine with ZrnFT, ApaFT, and ZrnHe to constrain the timing of the exhumation. It shows oldest ZrnFT ages are ca. 6-7 Ma. We further constrain that the latest stage of granite age in the rifted continental crust is ca. 13Ma indicating the collision should be later than this age. In addition, the ApaFT and ZrnHe ages for the granite are ca. 6Ma inferring a rapid cooling age which is consistent with regional ZrnFT dates. Those data imply the timing of mountain building of Mindoro orogen is ca. 6-7Ma which is similar to the Taiwan orogen.
Considering both orogens have similar timing of mountain building, we suggest that while the Philippine Sea changes the motion to NW trending at ca. 7-8Ma and Eurasia continental margin subducts to the Philippine Sea plate and Philippine Mobile belt, respectively, that results in both orogens deforming simultaneously.

How to cite: Lee, Y.-H., Mesalles, L., and Bacolcol, T.: A tale of two orogens- Taiwan and Mindoro, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4773, https://doi.org/10.5194/egusphere-egu23-4773, 2023.

17:30–17:40
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EGU23-1587
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ECS
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On-site presentation
Falak Sheir and Wei Li

The Shangdan suture zone (SDZ) in the Qinling Orogenic Belt is a key to understanding the East Asia tectonic evolution. The SDZ gives information about convergent processes between the North China Block (NCB) and South China Block (SCB). In the Late Mesozoic, several shear zones evolved along the SDZ boundary that helps us comprehend the collisional deformation between the NCB and SCB, which was neglected in previous studies. These shear zones play an essential role in the tectonic evolution of the East Asia continents. This study focuses on the deformation and geochronology of Maanqiao shear zone (MSZ) distributed along the SDZ. The shear sense indicators and kinematic vorticity numbers (0.54–0.90) suggest MSZ have sinistral shear and simple shear deformation kinematics. The quartz’s dynamic recrystallization and c-axis fabric analysis revealed that the MSZ experienced deformation under green-schist facies conditions at ∼400–500 °C. The 40Ar/39Ar (muscovite-biotite) dating of samples provided a plateau age of 121~123 Ma. Together with previously published data, our results concluded that Qinling Orogen Belt was dominated by compressional tectonics during the late early Cretaceous. Moreover, we suggested that the Siberian Block move back to the South and Lhasa-Qiantang-Indochina Block to the North, which promoted intra-continental compressional tectonics.

How to cite: Sheir, F. and Li, W.: Structural Geology and Chronology of Maanqiao Shear Zone along the Shangdan Suture in Qinling Orogenic Belt: Implications for Late Mesozoic Intra-Continental Deformation of East Asia, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1587, https://doi.org/10.5194/egusphere-egu23-1587, 2023.

17:40–17:50
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EGU23-14180
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ECS
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Virtual presentation
Alexandra Borisenko, Ivan Gaintsev, and Alexander Tevelev

Introduction. This study examines the structural position and genesis of the Middle-Devonian Yarlykap jasper complex and associated manganese mineralization (Southern Urals).

Materials and methods. The studied sites are Gubaidullino and Mamilya potential manganese ore occurrences. They are located in the West Magnitogorsk paleovolcanic belt and are confined to the Middle Devonian sealing wax-red and grayish-yellowish jaspers and tuff sandstones of the Yarlykap formation. The Yarlykap formation is distributed as narrow extended bands and outliers stretching along the Irendyk mountain ridge, Southern Urals. The age of the Yarlykap formation is defined as the Eiffelian, which is proved by conodont finds.

Our complex study includes geochemical, geophysical (magnetic and electrical exploration) and structural (measurements of mesostructure elements).

Results. It was shown that the rock association at both sites of the Yarlykap formation underwent a single stage of deformation, while the jaspers experienced dislocations similar in type and intensity.

 Structurally, the group of Mamiliya ore occurrences is generally confined to a monocline complicated by folded-thrust mesostructures of the north-northeast strike and western vergence. It is assumed that the Yarlykap formation is limited from the east by the thrust of the western vergence.

The Gubaidullino ore occurrence is a synform complicated by a series of small folds.  Among them, there are both practically isoclinal structures and more open asymmetric folds of western vergence.

The structure of both sites can be clearly recognized according to the electrical survey data. At the Gubaidullino site, several submeridional elongated folded zones are obvious by the change of the pattern of apparent resistance. On the Mamiliya site, the isoanomals are stretched into a single submeridional zone.

Geochemical data indicates that the deficiency of light lanthanides and the Eu and Ce minima may serve as an indicator of deposits of metalliferous hydrotherms typical for volcanically active regions of the oceans.

 Discussion. Thus, a new model of formation of siliceous strata and associated manganese mineralization can be proposed. These sites represent areas of volcanic unloading of active areas of the ocean floor associated with hydrothermal vents. Most likely, the volcanoes were located to the east of the described ore occurrences, and now they are located under the allochthon composed of the Late Devonian tufopsamite strata. 

Differences in the structure of ore occurrences are probably related to differences in their position within the West-Irendyk thrust pack, which includes these fragments. Thus, the Gubaidullino site is confined to the frontal part of the thrust and the Mamiliya site is located in the rear part of this thrust, which results in its simpler structure.

Jasper formation occurred in a developed island arc environment with an intermittent chain of volcanic structures, and they were already deformed in the Late Paleozoic, during the Ural collision.

Financial support. The study was prepared with partial financial support of the RFBR, grant No. 19-55-26009.

How to cite: Borisenko, A., Gaintsev, I., and Tevelev, A.: Composition, structure and formation conditions of the Yarlykap complex of jasper of the West Magnitogorsk paleovolcanic belt (Southern Urals), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14180, https://doi.org/10.5194/egusphere-egu23-14180, 2023.

17:50–18:00

Posters on site: Fri, 28 Apr, 08:30–10:15 | Hall X2

Chairpersons: Zoltán Erdős, Camilla Palmiotto, Attila Balázs
X2.120
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EGU23-8471
Mireia Peral Millán, Manel Fernàndez, Jaume Vergés, Sergio Zlotnik, and Ivone Jiménez-Munt

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.

X2.121
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EGU23-4175
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ECS
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Ana Gomes, Attila Balázs, and Taras Gerya

While there has been a lot of work focusing on improving our understanding of divergent and convergent plate boundaries, the complex nature of the back-arc region, where convergent margins transition into large-scale extension in the upper plate, is yet to be investigated fully. Indeed, why and how extensional basins open near the boundaries between convergent plates, followed by their tectonic inversion, have long been outstanding questions in plate tectonics.

Here we investigate a wide range of factors that influence the development of back-arc extension using 2D thermo-mechanical code I2VIS employing visco-plastic rheologies, hydration and dehydration processes, melting and surface processes. We systematically vary several parameters to determine their roles and respective importance, including a)  fluid and melt induced weakening, b) upper plate geothermal gradient and c) amount of sediment in the accretionary wedge. The fluid and melt induced weakening is implemented by using the Mohr–Coulomb yield criterion that limits the creep viscosity, altogether yielding an effective visco-plastic rheology, and controlled via the melt/fluid pore fluid pressure parameters, λfluid and λmelt. The upper plate geothermal gradient is controlled by the parameter TMoho . Finally, the amount of sediment in the accretionary wedge is changed through the parameter Sedlev, which controls the minimum y-coordinate sediments can occupy, throughout the model. The higher the Sedlev, the less the height of sediment that can accumulate in the accretionary wedge.

Our extensive series of high-resolution models led to the following conclusions:

  • a) a higher upper plate geothermal gradient predictably leads to a more ductile rheology, which then results in an initial wider rift, followed by enhanced melting and earlier arc splitting; 
  • b) higher erosion and sedimentation rates lead to increasing hydration of the mantle wedge and enhancing mantle melting, and decreasing the stress transfer from the lower to the upper plate; 
  • c) λfluid controls arc rifting to a greater extent, relative to λmelt, and for λfluid smaller than 0.2, arc rifting occurs. This means that the fluid induced weakening has to be high, in order to produce arc rifting.

These initial results suggest that the upper plate geotherm has the highest magnitude effects in modulating arc rifting, but fluid and melt induced weakening are also major controls in rift development, in the sense that they regulate whether it happens at all, or not. The height of the accretionary wedge works with the fluid weakening of the upper plate, facilitating arc rifting. 

How to cite: Gomes, A., Balázs, A., and Gerya, T.: Arc splitting and back-arc spreading evolution: the control of hydration and melts, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4175, https://doi.org/10.5194/egusphere-egu23-4175, 2023.

X2.122
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EGU23-6778
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ECS
Marta Corradino, Attila Balazs, Claudio Faccenna, and Fabrizio Pepe

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.

X2.123
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EGU23-4610
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ECS
Yongjoon Park, Nyeonkeon Kang, Boyeon Yi, Gwangsoo Lee, and Donggeun Yoo

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.

X2.124
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EGU23-4690
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ECS
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Eul Roh, Yirang Jang, Areum Woo, and Sanghoon Kwon

 The South Sea of Korea has three offshore concession blocks, including a Joint Development Zone(JDZ) that is set up by the license agreement between Korea and Japan. The geological research of the offshore South Sea of Korea is insufficient to define the evolution history and its significance for petroleum accumulation. In this study, evolution of the Xihu Sag within the JDZ area at the South Sea of Korea is tackled based on re-interpretation of the seismic and well data, and are correlated tectonically with that of the ECSSB(East China Sea Shelf Basin). The ECSSB has been initially developed as a back-arc basin over the over-riding Paleo-Pacific plate, and experienced complex tectonic history by successive subduction of the tectonic plates including the Paleo-Pacific (Izanagi) Plate, the Pacific plate, and the Philippine plate since Late Cretaceous in age. The results indicate that the study area can be subdivided into three tectonic domains: Western Slope Belt, Central Uplift Belt, and East Slope Belt. The structural similarity with those of the ECSSB, although the details of structural characteristics are different in different localities, under regional influence of successive subductions of the same tectonic plates, resulting in the conclusion that the area can be assigned into the northeastern ends of the Xihu Sag of the northeastern ECSSB. This might be a common feature of oil–gas accumulation in the eastern ECSSB, and highlights the potential for petroleum exploration at the study area, although further studies on the play concept and complex petroleum system of the area are required.

How to cite: Roh, E., Jang, Y., Woo, A., and Kwon, S.: The formation and evolution of northeastern ends of the ECSSB, South Sea of Korea, and its significance for petroleum exploration, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4690, https://doi.org/10.5194/egusphere-egu23-4690, 2023.

X2.125
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EGU23-4323
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ECS
Sara Ciattoni, Matteo Basilici, Mazzoli Stefano, Megna Antonella, and Santini Stefano

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.

X2.126
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EGU23-7375
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ECS
Fabrizio Parente and Attilio Sulli

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 Mw (1950): one of the most recent hits Nicoya on September 5th, 2012 (Mw 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.

X2.127
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EGU23-15948
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ECS
Leevi Tuikka, Bérénice Cateland, David Whipp, and Miisa Häkkinen

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.

Posters virtual: Fri, 28 Apr, 08:30–10:15 | vHall GMPV/G/GD/SM

Chairpersons: Zoltán Erdős, Camilla Palmiotto, Attila Balázs
vGGGS.15
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EGU23-10992
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Francesca Burkett and James Conder

Melt production at subduction zones depends on numerous variables, including mineral composition, water content, age of the plate, dip angle of the plate subducting, rate of convergence, age of the slab, and forearc dimensions. To evaluate the importance of individual variables and their interaction with each other, we constructed 2D numerical models of subduction, tracking temperature, mantle flow, and melt production. This project examines differences in batch and fractional melting sensitivity to the changes of the different variables. Variables include modal clinopyroxene (cpx) and its exhaustion, mantle hydration, dip angle, convergence rate, and forearc depth. Models tracked total melt as parameters were altered. For this project, the dip angle of the slab varied from 45 to 60°, rate of the slab between 20 and 90 km/Myr, age of the plate between 20 and 90 Myr, forearc depth between 40-50 km, and hydration between 0.01 and 0.1 wt%. The slab age and initial modal cpx levels are held constant throughout all the trials at 60 Myr and 15%, respectively. With batch melting, melting peaks for models set with hydration content > 0.1%, a dip angle at 60°, the highest convergence rates, and the youngest ages. Melting decreases with greater ages and lower convergence rates. In both fractional and batch melting, increasing the hydration leads to an increase in melt production overall. For fractional melting with hydration less than 0.05wt%, the difference in amount of melt compared to batch melting is negligible. At greater initial hydration the difference becomes greater with less produced under fractional melting. Changes in forearc extent also affect total melt with longer forearcs resulting in less melt than shorter ones. Additionally, we explored the effects of permeability on the melt production. Most notably, a secondary region of melt begins to form for when permeability is about 0.02 or greater. The secondary region encompasses melting above the harzburgite solidus. While two melting regions were nearly always observed under batch melt conditions, typically only one region of melting was observed under fractional melt conditions. In both cases, hydration and the dip of subducting slab have the most effect on melt production, while the convergence rate and the depth of the forearc have a smaller effect on melt production.

How to cite: Burkett, F. and Conder, J.: Melt Production beneath subduction zones: Using numerical models to evaluate melt production under batch and fractional melt conditions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10992, https://doi.org/10.5194/egusphere-egu23-10992, 2023.

vGGGS.16
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EGU23-14925
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ECS
Ekaterina Volodina, Alexander Tevelev, Alexandra Borisenko, Egor Koptev, Petr Shestakov, Natalia Pravikova, and Anastasia Novikova

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.

vGGGS.17
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EGU23-6572
Alexander Tevelev, Natalia Pravikova, Alexandra Borisenko, Petr Shestakov, Egor Koptev, Ivan Sobolev, Ekaterina Volodina, Alexey Kazansky, and Anastasia Novikova

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.