TS7.3

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.

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.

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.

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 (D_n-D_n+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 (D_n+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.

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.

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.

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.

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.