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TS7.3

Orogenic styles of the circum-Mediterranean Variscan and Alpine belts: from subduction to exhumation in time and space

The Phanerozoic orogenic belts of the circum-Mediterranean region stand out for their variety of tectonic styles, metamorphic imprints, and orogenic evolution. The similar tectono-metamorphic history of the Variscan and Alpine orogenies, as well as the distinctive traits of different sectors of these belts, make them unique to investigate orogenic processes through time and space. Circum-Mediterranean belts are, indeed, the result of the opening and closure of separate oceanic basins, of the complex interaction between microplates, and of the lateral transition to different subduction/collision zones, which marked different stages of their evolution. Over the last decades, the study of these orogenic belts showed how paleomargin geometries, type and dip of subduction zones, coupling or decoupling between the crust and the mantle, heat flux, orogenic wedge processes, interplay between erosion and sedimentation, and mantle dynamics influence mountain building processes. All these variables combined control the mechanisms of deformation of the lithosphere steering the structural and topographic evolution of collision zones. The history and complexity of the circum-Mediterranean area represent an intriguing geological puzzle whose investigation permits to address fundamental questions on tectonic and geodynamic processes and to improve our understanding of the dynamics of the lithosphere.

This section is devoted to the investigation of these processes and mechanism in the circum- Mediterranean orogenic belts of the Variscan (Maures, Corsica and Sardinia block) and Alpine (Atlas, Alps, Apennines, Pyrenees, Dinarides; Hellenides) cycles. We welcome contributions on all aspects of mountain building that cover a multiscale range of observation from meso to microscopic scale (field working to microprobe) and offer different perspectives from geology (tectonics, stratigraphy, petrology, geochronology, geochemistry, and geomorphology), geophysics (seismicity, seismic imaging, seismic anisotropy, gravity), geodesy (GPS, InSAR), modelling (numerical and analogue), natural hazards (earthquakes, volcanism). We also encourage the submission of trans-disciplinary studies that integrate multiple methods to unfold the evolution of orogenic belts.

Convener: Samuele PapeschiECSECS | Co-conveners: Giovanni Musumeci, Michele Locatelli, Paola Vannucchi
Presentations
| Thu, 26 May, 10:20–11:50 (CEST)
 
Room D1

Thu, 26 May, 10:20–11:50

Chairpersons: Paola Vannucchi, Michele Locatelli, Samuele Papeschi

10:20–10:25
Introduction

10:25–10:35
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EGU22-6002
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solicited
Guillermo Booth-Rea et al.

The Mallorca Foreland Thrust Belt (FTB) is stranded in the Western Mediterranean, isolated among deep basins from its corresponding hinterland domain. Here we integrate new structural data from Mallorca with preliminary detrital zircon data and previously published stratigraphic, paleontological, biogeographic and tectonic constraints, to provide a new tectonic evolutionary model for the Western Mediterranean. Mallorca underwent two Cenozoic rifting phases in the Oligocene and Serravallian, before and after the development of its FTB structure. The first Cenozoic extensional event produced Oligocene to Early Miocene semigrabens coeval to felsic volcanism in Mallorca and the Valencia trough (»29-19 Ma). The Oligocene extension affected a major part of the Western Mediterranean, opening the Liguro-Provençal and other back-arc basins after the collapse of the Palaeogene AlKaPeCa orogen, and Mallorca, its former hinterland. Continued plate convergence inverted the Oligocene back-arc basin, and onshore grabens during the Early-Middle Miocene (19-14 Ma), producing the Mallorca FTB and nucleating a new subduction system in the Westernmost Mediterranean. Renewed subduction probably initiated through the collapse of a NW-SE trending transform fault, inherited from the Mesozoic opening of the Tethys ocean. Development of the Mallorca WNW-directed FTB and subduction of the South-East Iberian passive margin occurred at this stage, individualizing the Betic-Rif slab that initiated its westward retreat. Moreover, detrital zircon age-population data show that the Betic and Mallorca foreland basins shared the same hinterland, equivalent to rocks of the Malaguide complex, located at the top of the AlKa orogenic domain. A later, second rifting event produced the extensional collapse of the Mallorca FTB during the Serravallian (»14-11 Ma), coeval to topographic uplift of the Island. This later rifting was polyphasic, with two orthogonal extensional systems, producing first NE-SW, and then NW-SE extension that favored the development of continental internal drainage basins. These basins shared common insular fauna with those overlying the Alboran domain in the Internal Betics, probably forming part of the same emerged archipelago, which is further supported by biogeographic data indicating a Middle Miocene common ancestry for several taxa now present in the Betics and Mallorca. Serravallian extension occurred at the northern edge of the subduction system coeval to the Algero-Balearic basin opening. Extension initiated towards the SW direction of slab tearing or detachment, and later rotated to a NW-SE direction, probably in response to flexural and isostatic rebound. This tectonic response propagated to the Betics between the Late Tortonian and Present. By these tectonic mechanisms, including slab retreat, edge delamination under continental FTB areas of the orogen and slab tearing, the Mallorca hinterland was driven towards the southwest, contributing to the present isolation of Mallorca from its Betic hinterland.

How to cite: Booth-Rea, G., Moragues, L., Ruano, P., Azañón, J. M., Hidas, K., and Garrido, C.: The Mallorca stranded and extended foreland thrust belt, its missing hinterland and the tectonic evolution of the Western Mediterranean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6002, https://doi.org/10.5194/egusphere-egu22-6002, 2022.

10:35–10:40
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EGU22-127
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ECS
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On-site presentation
Taylor Ducharme et al.

Structures that accommodate extension during orogenic relaxation significantly modify the crustal architecture of mountain belts. Discerning the tectonic significance of superimposed structures relating to extensional overprint of initially compressional features is therefore critical to the reconstruction of an orogen, and is easiest where the large-scale mechanical interactions between different crustal domains are exposed. In the Aegean region of Greece, low-angle detachment faults of early Miocene age were partially responsible for exhuming Eocene high-pressure, low-temperature (HP-LT) metamorphic rocks of the Cycladic Blueschist Unit (CBU). Extension in the Cyclades commonly occurred along multiple detachment branches at the kilometer scale, either due to arrest of older detachment planes by late Miocene plutonism, or because strain partitioning along multiple, simultaneously active structures was rheologically favourable. We document a third plausible mechanism whereby crustal attenuation is accomplished via distributed coaxial strain in the footwall of a major detachment, described previously in the Cyclades primarily for deep crustal fabrics contemporaneous with peak HP-LT conditions. This style of deformation is recorded below the basal contact of the CBU on the island of Evia, which delineates the boundary of a major tectonic window exposing an underthrust external carbonate platform known as the Basal Unit (locally Almyropotamos Unit). New structural observations, complemented by white mica 40Ar/39Ar and zircon (U-Th)/He ages, suggest that the upper structural levels of the Basal Unit accommodated flattening strain that coincided with Oligo-Miocene extension likely related to the overlying North Cycladic Detachment System. Vertical shortening, with extension in both other principal directions, is evinced by symmetric chocolate-tablet foliation boudinage and conjugate shear bands in the Basal Unit, alongside coeval type-3 refold structures in the overlying CBU. Pseudosection modelling results from Evia further corroborate a late greenschist-facies (320 ± 40 °C, 7 ± 1 kbar) paragenesis for the fabric associated with this extension that post-dates HP-LT metamorphism. Our observations indicate extrusion of the CBU and underlying Basal Unit was accomplished at least in part by coaxial vertical shortening, in contrast to the predominantly non-coaxial strain observed in the footwalls of other major Cycladic detachments.

How to cite: Ducharme, T., Schneider, D., Grasemann, B., Klonowska, I., and Soukis, K.: Exhumation of Eocene high-pressure metamorphic rock by coaxial flattening below a Miocene Cycladic-style detachment, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-127, https://doi.org/10.5194/egusphere-egu22-127, 2022.

10:40–10:45
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EGU22-158
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ECS
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On-site presentation
Christina Bakowsky et al.

Miocene extension of the Aegean region was accommodated by bivergent low-angle crustal-scale detachment systems. Folegandros island, of the southern Cyclades, lies between the SW-directed West Cycladic and S-directed Santorini detachment systems. The NW-SE oriented island is 'peanut'-shaped, consisting of a northern structural dome that exposes the structurally lower Cycladic Blueschist Unit (CBU). The CBU is characterized by coarse-grained marbles intercalated with metabasites and micaschists with a strong greenschist facies overprint. Metabasite lenses locally preserve relict HP minerals, including glaucophane and lawsonite pseudomorphs. The CBU preserves Raman spectroscopy of carbonaceous material (RSCM) peak temperatures >400°C and possesses late Eocene-early Oligocene white mica 40Ar/39Ar ages. At the corset of the island, a discrete tectonic boundary separates the CBU from a less deformed, NE-dipping homocline of Early Cretaceous to early Eocene units in the south. At the base of this package of rocks, the Eleftherios unit consists of alternating low-grade, deformed fine-grained marble and subordinate quartzitic-phyllitic sequences. The uppermost Vighlitsa unit is restricted to the SE coast and is composed of ophiolite phacoids at the base of a deformed marble and quartzitic-phyllitic sequence overlain by a metaflysch with middle Cretaceous marble olistoliths. Dispersed 40Ar/39Ar dates from these lower grade units are Early Cretaceous to early Eocene, consistent with RSCM temperatures <350°C. Based on the lower peak temperatures and Cretaceous to Eocene chronostratigraphy in conjunction with regional lithostratigraphic correlation, we propose the structurally higher Eleftherios and Vighlitsa units are hitherto unacknowledged exposures of the stratigraphically uppermost Pelagonian zone (Mesoautochthonous unit). A strong N-S lineation is dominant across the island, and a ductile top-to-S low-angle detachment system is overprinted by brittle-ductile top-to-N faults and shear bands. The differences in Raman temperatures together with ductile shear sense indicators and lepidoblastic muscovite 40Ar/39Ar ages at the detachment between the CBU and Pelagonian rocks indicate a top-to-S extensional detachment was active during the early Miocene. Middle Miocene zircon and apatite (U-Th)/He ages are comparable from both the CBU and Pelagonian zone and likely reflect cooling (<200°C) attributed to top-to-N extension exhibited by the cooler brittle-ductile structures. A similar lithostratigraphic juxtaposition between the CBU and Pelagonian zone is observed on Thera. Unlike on Folegandros, however, the middle to late Miocene ductile top-to-S low-angle detachment is overprinted by brittle-ductile top-to-S high-angle faults. These new observations reveal persistant top-to-S low-angle extension in the western and southern Cyclades throughout the Miocene. Overprinting high-angle normal faulting preserves structurally higher tectonic units, such as the Pelagonian zone, in fault relay zones.

How to cite: Bakowsky, C., Schneider, D., Soukis, K., and Grasemann, B.: Low temperature geochronology and lithostratigraphy of Folegandros, Cyclades, Greece: relationship between low- and high-angle faults results in crustal mosaic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-158, https://doi.org/10.5194/egusphere-egu22-158, 2022.

10:45–10:50
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EGU22-7989
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Virtual presentation
Christina Stouraiti et al.

The Cycladic Blueschist Unit (CBU) of the Aegean (Greece) contains several occurrences of metamorphosed Manganese mineralization within a Triassic volcaniclastic sequence. The latter includes quartz-mica schists intercalated with bimodal metavolcanics and blue-grey marble layers. SHRIMP U-Pb zircon dating has documented the Triassic age (~242 Ma) of the volcanic rocks. Herein we revisit the Mn metallogenic system of the CBU through an extensive study of Mn mineralization at Varnavas area, Northern Attica, and a similar occurrence at central Andros Island (Mparades hill). Manganese mineralogy at both localities is manifested in a typical high-P metamorphic silicate assemblage dominated by piemontite, spessartine garnet, and minor pyroxmangite (rhodonite). At Andros, Mn-rich subdomains contain brecciated braunite micronodules. The preservation of similar nodular form is documented from Varnavas, comprising dominant todorokite, lesser hollandite, pyrolusite, and minor Mn-bearing hematite. The contrasting Mn oxide mineralogy at the two sites is tentatively interpreted as the result of locally incomplete reduction of precursor Mn(IV) phases during metamorphism. Common geochemical characteristics of the Mn-rich rocks include low transition metal concentrations; positive-sloping, PAAS-normalized REE spidergrams; positive Ce anomalies of variable magnitude across individual samples; and high As, Ba, Pb. The geochemical variability recorded is ascribed to the varying mixing of a hydrothermal-sourced, hydrogenous metalliferous component that precipitated penecontemporaneously with the deposition of the host tuffs. The primary Mn precipitates are thought to have been in the form of tetravalent Mn assemblages, which may locally be partially preserved through metamorphism, as appears to be the case in the Varnavas occurrence. All these reveal the interplay between the felsic/intermediate back-arc volcanism and associated hydrothermal activity and the Mn mineralization within the rift setting of the CBU domain.

How to cite: Stouraiti, C., Lozios, S., Soukis, K., Mavrogonatos, C., Tsikos, H., Voudouris, P., Wang, H., Zamparas, C., and Kollias, K.: Hydrothermal Manganese mineralization in a Triassic back-arc rift-related volcaniclastic succession of the Cycladic Blueschist Unit, Greece, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7989, https://doi.org/10.5194/egusphere-egu22-7989, 2022.

10:50–10:55
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EGU22-11228
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ECS
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Virtual presentation
Sofia Laskari et al.

The Attic- Cycladic Crystalline Complex (Aegean Sea, Greece) is characterized by a complex history emanating from the interplay of geodynamic processes acted during successive stages of subduction zone underplating, early syn-convergence, and late extensional exhumation. Extensional tectonics strongly impacted the already formed orogenic buildup, leading to intensively denudated and locally preserved upper plate rocks that constitute the hanging walls of several detachment systems. The dominant Cycladic Blueschist Unit (CBU) and the structurally lowermost Cycladic Basement and Basal Units occupy the footwalls of these detachment systems, juxtaposed against the upper plate Pelagonian-derived fragments. Amorgos Unit is considered to occupy a structurally lower position and is correlated to the Basal Unit. This study utilizes tectonostratigraphic, detrital zircon provenance, and (U-Th)/He data to shed light on the paleogeographic and tectonic position of the Amorgos Unit in the Cycladic archipelago and potentially offer valuable insights for better understanding the architecture of the Hellenic subduction orogen.

Amorgos Unit presents a low-grade metamorphosed lithostratigraphy with a basal metaconglomerate, an intermediate carbonate sequence marked by various sedimentary facies of neritic and pelagic character, and an Eocene (meta)flysch, which preserves Nummulite fossils. The basal metaconglomerate has incorporated a metabasite block showing HP metamorphic conditions higher than the country rocks, which is either an olistolith or a tectonically incorporated slice. Detrital zircon U-Pb analysis of the basal metaconglomerate revealed a dominant Ediacaran age cluster and diverse basement rocks in the source area, including recycled Cadomian and Carboniferous affinities. MDA calculations yielded a Mid- Permian and Precambrian age for the matrix and the high-grade clasts, respectively. The metaflysch yielded Triassic- Jurassic MDAs and showed DZ distributions with a dominant Carboniferous input (Variscan affinities), suggesting Pelagonian-derived source areas. The structural evolution of Amorgos includes an early stage (Dn-Dn+1) internal tectonic imbrication in response to NW-ward advancing thrust sheets in the retro-wedge of the Late Eocene – Oligocene Hellenic subduction zone. The final structural history involves extensional low- and high-angle normal faulting (Dn+2/3) with top to SE kinematics. Zircon (U-Th)/He ages revealed exhumation below ~200 °C during the Early –Mid Miocene (18-14 Ma). Significant similarities in lithostratigraphy, structural-exhumation history, and structural position between Amorgos Unit and the Pelagonian hanging wall of the Santorini Detachment System on Santorini island suggest their close spatial relationship. From all the above, we conclude that Amorgos Unit is part of the Pelagonian upper plate, structurally above the Cycladic Blueschist Unit, and paleogeographical located at the southern Pelagonian margin.

How to cite: Laskari, S., Soukis, K., Stockli, D., Lozios, S., and Zambetakis-Lekkas, A.: Amorgos Unit: a long-lost fragment of the Pelagonian domain identified within the retro-wedge of the Oligocene Hellenic Subduction zone, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11228, https://doi.org/10.5194/egusphere-egu22-11228, 2022.

10:55–11:00
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EGU22-4126
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Virtual presentation
Frédéric Mouthereau et al.

Processes driving orogenic styles and long-term isostatic versus dynamic support of the topography have been largely debated in domains of plate convergence. Tectonics of orogens reflect the interactions between mantle flow driving plates and the inherited rheology and composition of moving plates, which are however still strikingly ill-defined. A recent review of the evolution of the weak European lithosphere, based on geological, geophysical, petrological data, has shed lights on the role played by lithospheric mantle chemo-magmatic history and structure, which inherits past subduction/collision (e.g. Variscan) and rifting events (Tethys/Atlantic), on crust-mantle coupling, plate-mantle coupling, defining Alpine-type orogens. While the details of the Cenozoic topographic history of peri-Mediterranean orogens are understood to be controlled by the rheology and architecture of rifted margins combined with changing large-scale kinematic boundary conditions (e.g. Atlas, Betics, Pyrenees, Alps), their post-10 Ma, quaternary to current surface (Insar) and tectonic (seismic) evolution appears to illustrate increasing control by magmatism and flow at the asthenosphere-lithosphere limit as well as local thermal re-equilibration. We argue that isostatic processes in western Europe linked in part to long lithosphere evolution can be first-order drivers of the post-collisional evolution of the peri-Mediterranean orogenic belts and their still active surface and tectonic evolution.

How to cite: Mouthereau, F., Angrand, P., Jourdon, A., Ternois, S., Celini, N., Lahfid, A., and Callot, J.-P.: Orogenic evolution of Western Europe controlled by lithosphere evolution, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4126, https://doi.org/10.5194/egusphere-egu22-4126, 2022.

11:00–11:05
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EGU22-3129
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Ruihong Chang et al.

The Austroalpine mega-unit contains the type locality of eclogites (Haüy, 1822) but their protolith age is largely unknown except that of the Permian Bärofen eclogite, for which three Sm-Nd ages between 275 ± 18 and 275 ± 18 Ma have been reported (Thöni & Jagoutz, 1993, Geochim. Cosmochim. Acta 56, 347–368; Miller & Thöni, 1997, Chem. Geol. 137, 283–310). Therefore, we studied the non-gabbroic eclogites from the Saualpe-Koralpe and Sieggraben Complexes, which are considered to represent a previously coherent subducted and then exhumed fragment of a continental rift, which led to the formation of the late Middle Triassic Meliata oceanic basin. A combined zircon U–Pb and Hf isotopic study, whole-rock geochemistry of two complexes revealed a protolith age of 242.3 ± 2.6 Ma (Middle Triassic) in the Sieggraben Complex, and 283 ± 5 Ma, 255 ± 3 Ma (Early and Late Permian), 251 ± 3 Ma, and 241 ± 3 Ma (Early to Middle Triassic) in the Saualpe-Koralpe Complex. Magmatic zircons from the Sieggraben eclogites have 176Hf/177Hf ratios of 0.283067–0.283174, εHf(t) values of +15.7 to +19.4, and that from Saualpe-Koralpe eclogite have 176Hf/177Hf ratios of εHf(t) 0.282935–0.283090, εHf(t) values of +10 to +17.4 showing their juvenile mantle source rather than significant crustal assimilation. In both complexes, N-MORB geochemical characteristics are established. Associated ultramafic rocks of Sieggraben eclogites as part of oceanic or Permian subcontinental mantle lithosphere suggest a depleted mantle source and a deep subduction environment. Two zircon grains of Sieggraben eclogites with low Th/U ratios yield ages of 113 ± 2 Ma and 86 ± 4 Ma and represent the approximate age of eclogite metamorphism during the Cretaceous. A trondhjemite dike cutting through the eclogite gives a crystallization age of 82.19 ± 0.4 Ma and is formed by partial melting of likely eclogite during decompression. The host metasedimentary rocks of Sieggraben and Saualpe-Koralpe Complexes are interpreted as old continental crust close to the margin of the Meliata basin and were affected by Permian migmatitic metamorphism. Metamorphic zircons of one eclogite from the Saualpe-Koralpe Complex give an age of 87–93 Ma (peak at 91 ± 1.2 Ma). The results of this study combined with previous results are used to present a new model for the tectonic evolution of the distal Austroalpine unit associated with the Meliata Ocean in a Wilson cycle: The Austroalpine Sieggraben and Saualpe-Koralpe Complexes represent a location on the distal continental margin during Permian to Middle Triassic rifting. The mafic rocks are associated with numerous Permian and potential Triassic acidic pegmatites, whereas structurally separated thick Triassic sedimentary cover successions lack any magmatism, likely excluding the present-day eclogite-bearing units as Triassic basement of the sedimentary cover successions.

The now eclogite-bearing piece of continental crust adjacent to the Meliata oceanic lithosphere subducted during Early Cretaceous times to mantle depth. The subducted continental crust was then exhumed incorporating even ultramafic mantle rocks. During exhumation and decompression of mafic rocks, partial melting took place forming the trondhjemite dike in Late Cretaceous times.

Acknowledgment: The study is financially supported by NSFC (91755212).

How to cite: Chang, R., Neubauer, F., Liu, Y., Genser, J., Yuan, S., Huang, Q., and Guan, Q.: Protolith and metamorphic ages of eclogites from the Eastern Alps: Implications for the Permian to Cretaceous Wilson cycle of the Austroalpine mega-unit, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3129, https://doi.org/10.5194/egusphere-egu22-3129, 2022.

11:05–11:10
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EGU22-4693
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ECS
Spatial correlations and slicing mechanisms during subduction of the Liguro-Piemont ocean (Western Alps)
(withdrawn)
Clément Herviou et al.

11:10–11:15
Break

11:15–11:20
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EGU22-7350
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Virtual presentation
Laura Federico et al.

The exhumation of high-pressure (HP), metaophiolitic terrains is a long-studied process, and many different models have been proposed so far, since the problem of how dense HP metamorphosed oceanic mafic and ultramafic rocks are exhumed from deep is crucial to understanding processes occurring at the plate interface and mantle wedge within subduction zones.

However, the exhumation process may obliterate peak-related structures and metamorphic associations at different degrees. In the Western Alps, one of the most studied orogen of the world, some HP terrains still preserve an almost complete stratigraphy of the oceanic lithosphere (e.g., the Monviso and Zermatt-Saas Massifs).

On the contrary, the Voltri Massif (VM), which crops out at the southern termination of the Alpine orogen (Ligurian Alps), is at places characterized by a high degree of disruption of the original stratigraphy.

The VM shows different features in the eastern and western sectors: in the eastern sector the high-pressure eclogitic-blueschist rocks are frequently embedded as bodies and lenses within serpentinite or metasediments, which act as a low strength “matrix” that accommodates most of the strain. This has led to the interpretation of the massif as a tectonic mélange, formed inside the subduction channel (Federico et al., 2007).

The western sector, on the contrary, contains relics of disrupted mélange associated to more coherent slices of metamorphic oceanic lithosphere.

Regarding the structural architecture, the VM eastern sector shows a steeply dipping foliation, steeply dipping blueschist to greenschist stretching lineation, high shear strain and prevalent structures typical of non-coaxial flow (Capponi and Crispini 2002). These structures are formed during the progressive exhumation from blueschist to greenschist facies conditions.

On the other side, the western part of the Massif is characterized by shallow-dipping fabrics and prevalent structures mostly dominated by strain flattening. Here structures related to the HP stage are better preserved and the greenschist-facies overprint is less pervasive and static at places.

Combination of new and reviewed structural data collected during several decades of fieldwork, geological mapping, PT-paths and geochronological data, points to a model of exhumation in which a non-coaxial transpressional zone played a fundamental role. Important rotation probably occurred at this stage, since the eastern high-strain zone is now perpendicular to the main orogen strike. This is likely due to the peculiar geodynamic position of the VM, at the tip of the alpine subduction zone and to the interference and lateral transition to the embryonic Apennine belt.

 

Capponi G., Crispini L. (2002) Structural and metamorphic signature of alpine tectonics in the Voltri massif (Ligurian Alps, North-Western Italy).

Federico L., Crispini L., Scambelluri M., Capponi G. (2007) Ophiolite mélange zone records exhumation in a fossil subduction channel Geology, 35/6; p. 499–502; doi: 10.1130/G23190A.1

How to cite: Federico, L., Crispini, L., Locatelli, M., and Cianfarra, P.: Oblique exhumation of HP metaophiolite at the southern termination of the Western Alps (Italy)., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7350, https://doi.org/10.5194/egusphere-egu22-7350, 2022.

11:20–11:25
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EGU22-4488
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ECS
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Virtual presentation
Qianwen Huang et al.

The Schladming-Seckau system as one part of Middle Austroalpine Unit experienced the Variscan event and overprinted by the Alpine orogeny, which is a key to reveal the tectonic evolution history of Easterrn Alps (Neubauer and Frisch, 1992). Recently, Cambrian-Ordovician magmatism and Permian-Triassic magmatism were reported in the Schladming Complex (Huang et al., 2021), they are related to the subduction of Proto-Tethys Ocean and opening of Meliata back-arc basin, respectively. Here, we present the whole-rock geochemistry, zircon U-Pb and Hf isotopic analysis of paragneiss and granitic rocks in the Schöder Valley to constrain the relationship between Schladming Complex and Variscan orogeny.

The biotite-plagioclase gneiss, hornblende-gneiss, mica-schists formed the main body of the Schladming Complex (Neubauer et al., 2018). The zircon age data of biotite-plagioclase gneiss show the peak age of 603 Ma and 487 Ma with dominated metamorphic age of 355 Ma, indicating the rocks mainly sourced from the Neoproterozoic and Ordovician rocks and suffered the strong Carboniferous metamorphism. Similar to the biotite-plagioclase gneiss, two Ordovician granitic gneisses (485Ma – 483 Ma) also comprise significance metamorphic age of 355 Ma. Their primitive mantle-normalized multiple elements patterns exhibit strong depleted in Nb, Ta, Ti, Zr, and Hf, showing a typical subduction-related features.

Not only the metamorphism shows the Carboniferous subduction event, but also two granites prove the existence of Carboniferous magmatism. These two granites have crystallization age of 353 Ma and 355 Ma, respectively. The zircon εHf(t) scatter between -1.29 and 6.04, suggesting the magma of granite derived from deplete mantle and mixed with the continent materials. Their geochemical data display subduction-related characteristics that depleted in HFSE (eg. Nb, Ta, Ti, Zr, and Hf) and enriched in LILE (eg. Ba, Th, K). The Carboniferous metamorphism and magmatism together showing the subduction of Rheic Ocean before Variscan orogeny in the Eastern Alps.

Acknowledgement: The study is financially supported by NSFC (91755212).

 

References

Huang, Q.W., Liu, Y.J., Genser, J., Neubauer, F., Chang, R.H., Yuan, S.H., Guan, Q.B., Yu, S.Y., 2021. Permian-Triassic A-type and I-type granites in the Schladming Complex, Austroalpine Unit: Constraints on subduction of Paleo-Tethys Ocean in the Eastern Alps, In: Li, S., Santosh, M. (Eds.), The 2021 Annual Convention of the International Association for Gondwana Research (IAGR) and the 18th International Symposium on Gondwana to Asia. Gondwana Research, China, Qingdao, pp. 21-22.

Neubauer, F., Frisch, W., 1992. Pre-Mesozoic geology of the Middle and Upper Austro-Alpine metamorphic basement east of the Tauern Window. 17-36.

Neubauer, F., Genser, J., Heberer, B., Etzel, A., Olive, S., 2018. Field Trip Post‐EX‐1 Transect across the Eastern Alps, XXI International Congress of the Carpathian Balkan Geological Association Salzburg, Austria, pp. 137-222.

How to cite: Huang, Q., Liu, Y., Neubauer, F., Genser, J., Yuan, S., Chang, R., Guan, Q., and Yu, S.: Early Carboniferous magmatism and metamorphism in the Schladming Complex, Eastern Alps: Constrain on the subduction of Rheic Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4488, https://doi.org/10.5194/egusphere-egu22-4488, 2022.

11:25–11:30
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EGU22-11567
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ECS
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On-site presentation
Gloria Arienti et al.

The North-Western Alps represent the better-known orogenic playground worldwide, exposing the stack of the Western Austroalpine, Penninic, and Helvetic metamorphic nappes, separated by ophiolitic sutures. However a modern and detailed 3D structural model, including the complexity of polyphase ductile and brittle structures, does not exist, and the reference 3D model is still the Argand’s (1911) block diagram.

Here we present preliminary results of a new 3D structural model of a large area (1300 km2) running along the Italian-Swiss boundary ridge, from the Helvetic Mont Blanc massif to the Penninic Monte Rosa nappe, including all main Penninic and Austroalpine units.

Input data are represented by structural surveys and detailed geological mapping, representing a truly 3D dataset thanks to a difference in elevation, from valley floors to mountain summits, of 3-4 km.

Our modelling workflow is based on a first step of conceptual modelling in vertical cross-sections, based on classical and sound structural concepts, followed by interpolation with implicit surface algorithms: advanced geomathematical tools allowing to model, under some conditions, complex structures such as those arising from multiphase ductile and brittle deformations.

This project aims at improving our understanding and our capacity to quantify some fundamental processes of Alpine tectonics. In addition to better representing and quantifying structures that were already qualitatively known, we have finally solved some problems that could not be solved in 2D.

For instance, we will present the solution to a long-lasting debate on a structure, known as the “Accident Col de Bard-Saint Nicolas”, that has been discussed for 25 years. Supported by field work, we demonstrated that the “Accident” is a brittle normal fault that represents the Miocene western continuation of the Aosta-Ranzola normal fault. This also solves problems of correlation within the Grand St-Bernard, since the “Accident” juxtaposes the highest nappe of the system (Mont Fort, to the N) to the lowermost (Ruitor) with tectonic elision of intermediate units.

A similar debate has been solved about the Aouillette ophiolitic unit, a portion of the Combin Nappe, from which it is separated by a graben limited by Oligocene and Miocene normal faults.

Another important outcome of the 3D model is the clear distinction between sections of the orogenic wedge characterized by different tectonic styles, namely (i) an inner Austroalpine-Upper Penninic domain with sub-horizontal nappes, eclogitic and greenschist peak metamorphism, and both Oligocene and Miocene brittle normal faults; (ii) an intermediate sector represented by the Grand St-Bernard nappe system, with blueschist peak metamorphism and prevailing Miocene brittle faults; and (iii) an outer system with low-T greenschist peak metamorphism, younger thrusts and no Miocene or Oligocene normal faults.

In addition, a quantitative and detailed 3D model is invaluable as a basis for applications, such as those related to the circulation and storage of deep water resources hosted in the bedrock, including geothermal fluids. We feel confident that this kind of application could result in a renewed interest for fundamental studies in tectonics and structural geology in the circum-Mediterranean Variscan and Alpine belt.

How to cite: Arienti, G., Bertolo, D., Bistacchi, A., Dal Piaz, G. V., Dal Piaz, G., and Monopoli, B.: 3D geomodelling of multiphase ductile and brittle deformations: a unique tool for quantifying structural relationships and tectonic evolution (Penninic units of the NW Alps), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11567, https://doi.org/10.5194/egusphere-egu22-11567, 2022.

11:30–11:35
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EGU22-8740
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ECS
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On-site presentation
Michele Perozzo et al.

Orogenic deformation patterns show intricate overprinting and structural relations, variations of style and orientation of folds and sense of shear, which are traditionally interpreted as due to polyphase deformation, i.e. distinct deformation phases separated by periods of tectonic quiescence. The Adula nappe in the Central Alps displays exceptional exposures of complex internal structures involving heterogeneous rocks (meta-pelitic and meta-granitic gneiss, micaschists, amphibolites, eclogites, minor quartzites and limestones). The Adula structures are distinguished through the style and the orientation of folds, schistosity and the observation of refolded folds. Structural features show a great variability within the unit, making the structures along the nappe difficult to correlate. However, the Adula deformation patterns are classically interpreted as generated by multiple, distinct deformation phases (five deformation phases; D1-5), despite only one schistosity and lineation may be clearly recognized in the field. Kinematic indicators indicate dominant top-to-N sense of shear, although local top-to-S shear is interpreted as developed during the D3 backfolding phase (e.g.  Löw 1987; Nagel 2008). In this contribution, we show a first recognition of sheath folds from the central part of the Adula nappe, the largest high-pressure nappe of the Central Alps. We performed detailed geological mapping (scale 1:10’000) and structural characterization of the spectacular outcrops of the Piz de Cressim glacial cirque. Here a large antiform is described as the main structure associated with the D3 backfolding phase. We show that the meso/leucocratic heterogeneous rocks (orthogneisses, micaschists, migmatitic gneisses, amphibolitic lenses) form highly non-cylindrical folds. Sheath folds are highlighted by several centimetre to meters scale omega and elliptical eye-structures in cross sections perpendicular to the shear direction (y-z plane). All lithological units show one penetrative foliation and a related stretching lineation with variations in orientation. We suggest that the Cressim antiform formed during progressive top-to-N deformation accomplished within rheological heterogeneous rocks, rather than as the results of multiple distinct deformation phases.

How to cite: Perozzo, M., Maino, M., Schenker, F., and Seno, S.: Discovery of sheath folds in the Adula nappe and implications for the tectonic evolution (Central Alps), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8740, https://doi.org/10.5194/egusphere-egu22-8740, 2022.

11:35–11:40
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EGU22-967
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ECS
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On-site presentation
Josselin Gremmel et al.

Oblique tectonic, including transpressional and transtensional movements, is a common feature observed at active plate boundaries. Despite being often inferred at the orogen scale, the interpretation of local structural observations in the context of oblique tectonic regimes remains challenging, especially in ductile terrains. During the last stage evolution of orogenic belts, obliquity is expected to play a key role in controlling strain partitioning and exhumation patterns of deep crustal rocks, as well as the development of brittle structures in the upper crust.

Here, we present new structural data and finite strain analyses of migmatitic rocks exposed in the Tanneron Massif in SE France. This massif, representing the most internal part of the Maures-Tanneron Variscan belt segment, was structured between 320 and 300 Ma during the late stage Variscan orogeny. This late-stage deformation, synchronous to partial melting of the middle crust, the large-scale folding of the migmatitic units and their exhumation has been interpreted to take place in a regional transpressive regime.

New detailed structural mapping carried on in two sectors of the massif highlight different strain patterns with dome-like structures in a migmatite unit to the East, and sub-vertical shear-zones with stretching lineation to the West. However, in these two sectors stretching is dominant and finite constrictional fabrics are ubiquitous. The regional lineation, corresponding to the maximum stretching direction of these L-tectonites is parallel to the large scale and local fold axes. In addition, narrow continental Carboniferous basins oriented roughly parallel to the main ductile fabric opened inside the massif contemporaneously to the exhumation of the L-tectonites. Therefore, our results suggest that local transtension might best describe the tectonic regime associated with the late-stage evolution of the massif. We will discuss these results for the Tanneron massif in the light of a series of preliminary 3D thermo-mechanical numerical models designed to investigate the horizontal and vertical partitioning of deformation in a hot orogen subject to regional oblique deformation.

How to cite: Gremmel, J., Duclaux, G., Corsini, M., Maillet, A., Jourdon, A., and Bascou, J.: Structure, strain partitioning and exhumation mechanism during the late stage oblique tectonic evolution of the Variscan Tanneron massif (SE France), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-967, https://doi.org/10.5194/egusphere-egu22-967, 2022.

11:40–11:45
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EGU22-6513
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On-site presentation
Fatih Şen

The İstanbul-Zonguldak Tectonic Unit (NW Turkey) is regarded as the unmetamorphosed continental fragment of Far East Avalonia in the Pontides. It is to the east of the Rhodope-Strandja Massif, which is a part of the metamorphosed section of Far East Avalonia, and is to the north and west of the Sakarya terrane. The Variscan orogeny in the Pontides defines as the collision of the Sakarya terrane with the İstanbul-Zonguldak Tectonic Unit during the early Carboniferous. But, the Late Carboniferous arc magmatism (c. 306-301 Ma) in the eastern sector of the İstanbul-Zonguldak Tectonic Unit rejects this view. Here, I present analytical data of basalt dykes in the western sector of the İstanbul-Zonguldak Tectonic Unit. Basalt dykes have porphyritic and holohyaline textures. Geochemically, they display calc-alkaline affinities and show depletion in Nb relative to Ce. They contain subduction components and are associated with the arc-related geodynamic setting. U–Pb dating on igneous zircons from two basalt dykes yielded Late Carboniferous ages of ca. 321.6 ± 1.6 and 311.4 ± 0.75 Ma (2σ), and their Pb-loss ages from the white spot in zircons calculated Early Permian ages of ca. 295.1 ± 1.1 to 285.0 ± 1.3 Ma and 295.5 ± 1.2 to 284.0 ± 1.4 Ma, respectively. In conjunction with the data from the literature, the Late Carboniferous arc magmatism (c. 321-311 Ma) in the western side of the İstanbul Zonguldak Tectonic Unit corresponds to the Late Carboniferous arc magmatism (c. 306-301 Ma) in the eastern side of the İstanbul Zonguldak Tectonic Unit, thus indicating that the Rheic Ocean continued to subduct under Far East Avalonia during the Carboniferous. As for the Pb-loss ages obtained from the Late Carboniferous arc basalt dykes, the earliest-latest Cisuralian ages (c. 295-285 Ma) correspond to regional deformation events at the Carboniferous-Permian boundary in the Rhodope-Strandja Massif (c. 298-296 Ma) and at the ending of the early Permian in the Sakarya terrane (c. 282-275 Ma), respectively. All in all, I suggest that the docking of Far East Avalonia, including the İstanbul-Zonguldak Tectonic Unit and Rhodope-Strandja Massif, with the Sakarya terrane formed during the early Permian instead of the early Carboniferous.

How to cite: Şen, F.: Early Permian deformational ages of Late Carboniferous basalt dykes in the İstanbul-Zonguldak Tectonic Unit: Implications for the Variscan orogeny in the Pontides, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6513, https://doi.org/10.5194/egusphere-egu22-6513, 2022.

11:45–11:50
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EGU22-9976
Giovanni Luca Cardello et al.

In north-eastern Sardinia, due to the present-day geodynamic quiescence of the island and its very low seismicity and anthropogenic seismic noise, an area has been candidate for hosting the Einstein Telescope (ET). ET is the European third-generation underground interferometric detector of gravitational waves, whose functioning requires a rocky volume virtually devoid of permeable fractures ideally not crossed by main regional faults. Hereby, we present the structural and Electrical Resistivity Tomography (ERT) features of the most relevant brittle structures in the ET candidate site.

The late- to post-Variscan tectonics in Sardinia is accompanied by extensive magmatism giving way to the Corsica-Sardinia Batholith emplacement. This is followed by a dense dyke swarm of bimodal mafic-felsic composition. These dykes are hosted in the batholith and its host metamorphic basement, being their trends reflecting the stress field of the new-formed Variscan crust during early Permian. Field evidence, shows that a ductile to brittle fault network affects both the Variscan metamorphic basement and the late-Variscan plutons. Fault zones are generally NNW-, and WSW-striking and are associated with more altered bedrock and/or occasionally pseudotachylite-bearing cataclastic bands that have been locally injected by hydrothermal fluids, as testified by thick quartz veins and chlorite-rich selvedges. Near two new drilling sites (ca. 250 m total depth), ERT shows a stratified resistivity array, that consists of up to three electrolayers with variable distribution and thickness. As supported by field observation, we have interpreted the more conductive electrolayer as regolith and alluvial units associated with minor faults, while the most resistive electro-layers correspond with the less-fractured granitoids. Overall, the large deep conductive anomalies are bounded by suddenly graded resistivity drops tracing fault systems that are NNW-, N(NE)- and WSW-striking. Upscaling the local results, which provide an accurate estimate of satured fault geometry at depth, we recognize that: i)  the post-Variscan brittle structures mirror the trend of Permian dykes, sills and veins; ii) the main fault zones that underwent strike-slip reactivation were site of later hydrothermal circulation possibly related to Oligocene-Aquitanian tectonics. Further studies are needed to constrain the actual pattern of differential uplift to exclude the presence of neotectonics in the area. Thus, direct dating of faults and dykes and new Global Navigation Satellite System data acquisition could constrain the age of faulting and the differential uplift contribution into the eventual current reactivation of the inherited Variscan structures.

How to cite: Cardello, G. L., Casini, L., D'Urso, D., Longo, V., and Oggiano, G.: Late to Post-Variscan tectonics in the Sardinia Einstein Telescope candidate site (Italy): insights from Structural Survey and Electrical Resistivity Tomography, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9976, https://doi.org/10.5194/egusphere-egu22-9976, 2022.