Development and passive exhumation of high-pressure shear zones (Blueschist Unit, Syros): Insights from quartz and columnar calcite microstructures

Aravadinou, E.; Gerogiannis, Nikolaos; Xypolias, P.

doi:10.1016/j.jsg.2022.104607

Cited by 9 publications

(13 citation statements)

References 88 publications

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“…Our observations are not consistent with previous studies that propose subduction occurred along a west-dipping slab with a prograde top-to-the-ESE thrust sense of shear in a tapered wedge (Aravadinou et al, 2016(Aravadinou et al, , 2022Xypolias et al, 2012). This geometry would imply that the thrusting direction was nearly parallel to the current structural grain of the Cyclades on a regional scale, as opposed to perpendicular (Ridley, 1982;Xypolias et al, 2012).…”

Section: Subduction Kinematics and Exhumation Processescontrasting

confidence: 99%

“…Aravadinou et al. (2022) interpreted the opposite rotation from E‐W to NE‐directed transport through time based on the evolution of quartz and calcite petrofabric analysis from two shear zones in Northern Syros (Kastri; this is part of the same structure we interpret as the contact between the northern and central slices) and Northeastern Syros (Agios Dimitrios). However, they noted that amphiboles in both shear zones were all sodic (no calcic amphibole present) and unzoned, making it difficult to discern the relative conditions or timing of operative deformation between exposures.…”

Section: Discussionmentioning

confidence: 94%

“…Furthermore, our kinematic model describes rotation in transport directions during progressive exhumation from NE-(blueschist facies) to E-W (dominantly greenschist facies), supported by amphibole zonations, metamorphic replacement reactions, and isochron geochronology (e.g., Azolimnos vs. Delfini). Aravadinou et al (2022) interpreted the opposite rotation from E-W to NE-directed transport through time based on the evolution of quartz and calcite petrofabric analysis from two shear zones in Northern Syros (Kastri; this is part of the same structure we interpret as the contact between the northern and central slices) and Northeastern Syros (Agios Dimitrios). However, they noted that amphiboles in both shear zones were all sodic (no calcic amphibole present) and unzoned, making it difficult to discern the relative conditions or timing of operative deformation between exposures.…”

Section: Subduction Kinematics and Exhumation Processesmentioning

confidence: 89%

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Subduction, Underplating, and Return Flow Recorded in the Cycladic Blueschist Unit Exposed on Syros, Greece

et al. 2022

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Exhumed high‐pressure/low‐temperature (HP/LT) metamorphic rocks provide insights into deep (∼20–70 km) subduction interface dynamics. On Syros Island (Cyclades, Greece), the Cycladic Blueschist Unit preserves blueschist‐to‐eclogite facies oceanic‐ and continental‐affinity rocks that record the structural and thermal evolution linked to Eocene subduction. Despite decades of research, the metamorphic and deformation history (P‐T‐D) and timing of subduction and exhumation are matters of ongoing discussion. We suggest that Syros comprises three coherent tectonic slices and that each slice underwent subduction, underplating, and syn‐subduction return flow along similar P‐T trajectories, but at progressively younger times. Subduction and exhumation are distinguished by lineations and ductile fold axis orientations, and are kinematically consistent with previous studies that document top‐to‐the‐S‐SW shear (prograde‐to‐peak subduction), top‐to‐the‐NE shear (blueschist facies exhumation), and then E‐W coaxial stretching (greenschist facies exhumation). Amphibole zonations record cooling during decompression, indicating return flow above a cold slab. Multi‐mineral Rb‐Sr isochrons and compiled metamorphic geochronology show that the three slices record distinct stages of peak subduction (53–52, ∼50, and 45 Ma) that young with structural depth. Retrograde blueschist and greenschist facies fabrics span ∼50–40 and ∼43–20 Ma, respectively, and also young with structural depth. Synthesized data sets support a revised tectonic framework for Syros, involving subduction of structurally distinct coherent slices and simultaneous return flow of previously accreted tectonic slices in the subduction channel shear zone. Distributed, ductile, dominantly coaxial return flow in an Eocene‐Oligocene subduction channel proceeded at rates of ∼1.5–5 mm/yr and accommodated ∼80% of the total exhumation of this HP/LT complex.

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Section: Subduction Kinematics and Exhumation Processescontrasting

confidence: 99%

Section: Discussionmentioning

confidence: 94%

Section: Subduction Kinematics and Exhumation Processesmentioning

confidence: 89%

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Subduction, Underplating, and Return Flow Recorded in the Cycladic Blueschist Unit Exposed on Syros, Greece

et al. 2022

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“…Syros Island is composed of rocks belonging to the Blueschist Unit, the only exception being its SE area, where rocks of the Uppermost Unit are also exposed (Figure 1a). The Blueschist Unit generally consists of calcite marble, mica/calcite schist, eclogite and blueschist, and meta-tuffitic schist, as well as greenschist and paragneiss (Figure 1a) (e.g., [2,12]). The Uppermost Unit (i.e., Vari Unit) is represented by a sequence of quartz, mica, and metabasic schists overlaying by felsic orthogneiss [11] (Figure 1a).…”

Section: Geological and Structural Settingmentioning

confidence: 99%

“…HP rocks originated during Hellenides formation in the Eocene-Oligocene in the course of the Alpine orogenesis. Although these rocks have been the subject of extensive study for about four decades, with petrological, geochronological, and structural studies, diverse interpretations, mainly regarding the structural evolution and the exhumation structures of the HP rocks, hinder our understanding of the processes that controlled the exhumation and preservation of these rocks [2][3][4][5][6][7][8][9][10][11][12][13]. Currently, two main contrasting models are proposed for the exhumation of the HP rocks of the Blueschist Unit in the Cyclades and by extension on the island of Syros.…”

Section: Introductionmentioning

confidence: 99%

Deformation Pattern of Well-Preserved High-Pressure Rocks (SE Syros, Cyclades)

Gerogiannis,

Aravadinou,

Xypolias

2023

Geosciences

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New, detailed geological/structural mapping and field-based structural analysis were carried out to investigate the deformation pattern of well-preserved high-pressure rocks of the Blueschist Unit exposed in SE Syros (Cyclades, Greece). Geological mapping revealed the occurrence of extensive alternations between different rock groups, as well as interfingering patterns in map-scale that are possibly the result of folding. The earlier ductile deformation phase recognized in the mapped area is associated with the development of a penetrative foliation, which was formed at eclogite/blueschist-facies conditions under peak metamorphism. The subsequent main deformation phase occurred under blueschist facies conditions synchronous with the early stages of exhumation of the high-pressure rocks. This phase is mainly associated with the formation of WNW-trending folds and a pervasive axial planar foliation linked with ESE-directed shearing. The main deformation ceased under blueschist-facies conditions, and exhumation of the rocks to greenschist-facies conditions took place under very weak and localized deformation. Greenschist retrogression observed in the southwestern part of the mapped area seems to be controlled by fluids, rather than by intense deformation and formation of major syn-greenschist shear zones.

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Subduction–exhumation cycle recorded by calcite deformation microstructures: blueschist-facies metacarbonates and kinematic implications for deformation of the Meliata Unit (Western Carpathians)

Potočný,

Jeřábek,

Plašienka

2023

Int J Earth Sci (Geol Rundsch)

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The Meliata Unit (Meliaticum) is a tectonic superunit of the Western Carpathians that incorporates the blueschists-facies Bôrka Nappe and the low-grade metasediments and polygenetic mélange, Meliata Unit s.s., both occurring as scattered tectonic slices overlying the Gemeric Superunit. Calcitic marbles were sampled in a wide area within the Bôrka Nappe and blocks embedded in Jurassic oceanic sediments (Meliata Unit s.s.). Based on the microstructural evaluation and electron backscatter diffraction analysis (EBSD), the carbonates of the Bôrka Nappe experienced differential post-subduction P–T–D paths related to a collisional/exhumation setting following closure of the Neotethys-related “Meliata Ocean”. Variations in the calcite deformation microstructures were used to distinguish three principal microstructural groups. The first group (G1) contains large columnar and lobate calcite grains (≥ 1 mm) reflecting peak P–T conditions during subduction of the Meliata oceanic lithosphere. The second group (G2) exhibits dynamic recrystallization of the original G1 grains resulting in grain size reduction (< 0.5 mm) and shape-preferred orientation related to exhumation and formation of the accretionary complex. The third group (G3) shows a ‘foam’ microstructure with a uniform grain size (0.4–1 mm), sharp grain boundaries and triple junctions. The G3 microstructure may have been caused by a static recrystallization at elevated temperatures postdating the main deformation, and it is restricted to peripheries of the underlying Veporic metamorphic dome and probably is not associated with the Meliata sequences nor its tectono-metamorphic evolution. The corrected kinematic sections indicate dominantly ESE–WNW-trending lineations suggesting top-to-WNW kinematics of the Meliata subduction–exhumation process for G1 and G2 microstructures, and Gemeric–Veporic E–W orogen-parallel stretching for G3.

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Development and passive exhumation of high-pressure shear zones (Blueschist Unit, Syros): Insights from quartz and columnar calcite microstructures

Cited by 9 publications

References 88 publications

Subduction, Underplating, and Return Flow Recorded in the Cycladic Blueschist Unit Exposed on Syros, Greece

Subduction, Underplating, and Return Flow Recorded in the Cycladic Blueschist Unit Exposed on Syros, Greece

Deformation Pattern of Well-Preserved High-Pressure Rocks (SE Syros, Cyclades)

Subduction–exhumation cycle recorded by calcite deformation microstructures: blueschist-facies metacarbonates and kinematic implications for deformation of the Meliata Unit (Western Carpathians)

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