Hellenic Subduction System Research Articles

Origin of Hydrothermal Barite in Polymetallic Veins and Carbonate-Hosted Deposits of the Cyclades Continental Back Arc

Abstract Polymetallic veins and breccias and carbonate-replacement ore deposits in the Cyclades continental back arc, Greece, formed from a range of fluid and metal sources strongly influenced by the dynamics of the late Mesozoic-Cenozoic Hellenic subduction system. These complexities are recorded in the isotopic signatures of hydrothermal barite. We investigated 17 mineral occurrences on four Cycladic islands and from Lavrion on the mainland. Here, barite occurs in almost all deposit types of Miocene to Quaternary age. We used a multiple isotope and geochemical approach to characterize the barite in each deposit, including mineral separate analysis of δ34S and δ18O and laser ablation-inductively coupled plasma-mass spectrometry of 87Sr/86Sr and δ34S. Barite from carbonate-hosted vein and breccia Pb-Zn-Ag mineralization on Lavrion has a wide range of δ34S (2–20‰) and δ18O (10–15‰) values, reflecting a mix of magmatic and surface-derived fluids that have exchanged with isotopically heavy oxygen in the carbonate host rock. Sulfur (δ34S = 10–13‰) and oxygen (δ18O = 9–13‰) values of barite from the carbonate-hosted vein iron and barite mineralization on Serifos are permissive of a magmatic sulfate component. Barite from epithermal base and/or precious metal deposits on Milos has δ34S (17–28‰) and δ18O (9–11‰) values that are similar to modern seawater. In contrast, barite from vein-type deposits on Antiparos and Mykonos has a wide range of δ34S (16–37‰) and δ18O (4–12‰) values, indicating a seawater sulfate source modified by mixing or equilibration of the hydrothermal fluids with the host rocks. Strontium isotope ratios of barite vary regionally, with 87Sr/86Sr ≥ 0.711 in the central Cyclades and 87Sr/86Sr ≤ 0.711 in the west Cyclades, confirming the strong influence of upper crustal rocks on the sources of fluids, Sr, and Ba in the formation of ore.

The high-K calc-alkaline to shoshonitic volcanism of Limnos, Greece: implications for the geodynamic evolution of the northern Aegean

Genetic models for the formation of K-rich magmas in subduction-related settings range from partial melting of subduction-affected mantle sources to melting of crustal rocks depending on the local tectonic framework. The Miocene high-K calc-alkaline to shoshonitic rocks of Limnos Island reflect the magmatic activity in the northern Aegean, which migrated southwards in response to trench retreat and the collision of continental terranes in the Hellenic subduction system. New whole rock and mineral data of basaltic andesites, dacites and monzonites from Limnos indicate that the magmas underwent fractional crystallization of olivine, clinopyroxene, amphibole, apatite, and Fe-Ti oxides at 1100 to 700 °C and 0.5 to 0.1 GPa without significant assimilation of crustal rocks during the magma evolution. The strong enrichment of large ion lithophile elements and light rare-earth elements relative to depleted heavy rare earth and high-field strength elements points towards a mantle source that has been extensively hybridized by subducted sedimentary material. New Sr–Nd-Pb isotope data reveal a distinct isotopic composition of the Limnos rocks with high 207Pb/204Pb at low 206Pb/204Pb and 143Nd/144Nd ratios that is likely related to the subduction of the continental crustal succession of the Apulian block which was subducted prior to the onset of magmatism on Limnos. Partial melting models assuming a hybridized mantle source suggest that the primary melts of Limnos formed by melting of a phlogopite pyroxenite at melting degrees of 5 to 10%. Compositional differences between high-K calc-alkaline and shoshonitic magmas are explained by variable melting degrees and varying amounts of sediment supply to the mantle. The magmatic and tectonic evolution of Limnos island is typical for the Oligocene and Miocene volcanic centres of the migrating western Aegean arc front.

Normal Faulting Along the Kythira-Antikythira Strait, Southwest Hellenic Forearc, Greece

Upper-plate normal faults along forearcs often accumulate slip during >Mw 6 earthquakes. Such normal faults traverse the forearc of the Hellenic Subduction System (HSS) in Greece and are the focus of this study. Here, we use detailed field-mapping and analysis of high-resolution Digital Elevation Models (DEMs) to study 42 active normal faults on the islands of Kythira and Antikythira in the Aegean Sea. Onshore fault kinematic data are complemented by seabed bathymetry mapping of ten offshore faults that extend along the Kythira-Antikythira Strait (KAS). We find that normal faults in the KAS have lengths of ∼1–58 km and scarps ranging in height from 1.5 m to 2.8 km, accommodating, during the Quaternary, trench-orthogonal (NE-SW) extension of ∼2.46 ± 1.53 mm/a. Twenty-eight of these faults have ruptured since the Last Glacial Maximum, with their postglacial (16 ± 2 ka) displacement rates (0.19–1.25 mm/a) exceeding their Quaternary (≤0.7–3 Ma) rates (0.03–0.37 mm/a) by more than one order of magnitude. Rate variability, which is more pronounced on short (<8 km) faults, is thought to arise due to temporally clustered paleoearthquakes on individual KAS faults. When displacement accumulation is considered across the entire onshore fault network, rate variability between the two time-intervals examined decreases significantly (2.79 ± 0.41 vs 1.29 ± 0.99 mm/a), a feature that suggests that earthquake clustering in the KAS may occur over ≤16 ka timescales.

Slow Slip Triggers the 2018 Mw 6.9 Zakynthos Earthquake Within the Weakly Locked Hellenic Subduction System, Greece

AbstractSlow slip events (SSEs) at subduction zones can precede large‐magnitude earthquakes and may serve as precursor indicators, but the triggering of earthquakes by slow slip remains insufficiently understood. Here, we combine geodetic, Coulomb wedge and Coulomb failure‐stress models with seismological data to explore the potential causal relationship between two SSEs and the 2018 Mw 6.9 Zakynthos Earthquake within the Hellenic Subduction System. We show that both SSEs released up to 10 mm of aseismic slip on the plate‐interface and were accompanied by an increase in upper‐plate seismicity rate. While the first SSE in late 2014 generated only mild Coulomb failure stress changes (≤3 kPa), that were nevertheless sufficient to destabilize faults of various kinematics in the overriding plate, the second SSE in 2018 caused stress changes up to 25 kPa prior to the mainshock. Collectively, these stress changes affected a highly overpressured and mechanically weak forearc, whose state of stress fluctuated between horizontal deviatoric compression and tension during the years preceding the Zakynthos Earthquake. We conclude that this configuration facilitated episodes of aseismic and seismic deformation that ultimately triggered the Zakynthos Earthquake.

Earthquake Swarms, Slow Slip and Fault Interactions at the Western‐End of the Hellenic Subduction System Precede the Mw 6.9 Zakynthos Earthquake, Greece

AbstractThe month‐to‐year‐long deformation of the Earth's crust where active subduction zones terminate is poorly explored. Here we report on a multidisciplinary data set that captures the synergy of slow‐slip events, earthquake swarms and fault interactions during the ∼5 years leading up to the 2018 Mw 6.9 Zakynthos Earthquake at the western termination of the Hellenic Subduction System (HSS). It appears that this long‐lasting preparatory phase initiated due to a slow‐slip event that lasted ∼4 months and released strain equivalent to a ∼Mw 6.3 earthquake. We propose that the slow‐slip event, which is the first to be reported in the HSS, tectonically destabilized the upper 20–40 km of the crust, producing alternating phases of seismic and aseismic deformation, including intense microseismicity (Mw < 4) on neighboring faults, earthquake swarms in the epicentral area of the Mw 6.9 earthquake ∼1.5 years before the main event, another episode of slow slip immediately preceding the mainshock and, eventually, the large (Mw 6.9) Zakynthos Earthquake. Tectonic instability in the area is evidenced by a prolonged (∼4 years) period of overall suppressed b‐values (<1) and strong earthquake interactions on discrete strike‐slip, thrust and normal faults. We propose that composite faulting patterns accompanied by alternating (seismic/aseismic) deformation styles may characterize multifault subduction‐termination zones and may operate over a range of timescales (from individual earthquakes to millions of years).

Clusty, the waveform-based network similarity clustering toolbox: concept and application to image complex faulting offshore Zakynthos (Greece)

SUMMARYClusty is a new open source toolbox dedicated to earthquake clustering based on waveforms recorded across a network of seismic stations. Its main application is the study of active faults and the detection and characterization of faults and fault networks. By using a density-based clustering approach, earthquakes pertaining to a common fault can be recognized even over long fault segments, and the first-order geometry and extent of active faults can be inferred. Clusty implements multiple techniques to compute a waveform based network similarity from maximum cross-correlation coefficients at multiple stations. The clustering procedure is designed to be transparent and parameters can be easily tuned. It is supported by a number of analysis visualization tools which help to assess the homogeneity within each cluster and the differences among distinct clusters. The toolbox returns graphical representations of the results. A list of representative events and stacked waveforms facilitate further analyses like moment tensor inversion. Results obtained in various frequency bands can be combined to account for large magnitude ranges. Thanks to the simple configuration, the toolbox is easily adaptable to new data sets and to large magnitude ranges. To show the potential of our new toolbox, we apply Clusty to the aftershock sequence of the Mw 6.9 25 October 2018 Zakynthos (Greece) Earthquake. Thanks to the complex tectonic setting at the western termination of the Hellenic Subduction System where multiple faults and faulting styles operate simultaneously, the Zakynthos data set provides an ideal case-study for our clustering analysis toolbox. Our results support the activation of several faults and provide insight into the geometry of faults or fault segments. We identify two large thrust faulting clusters in the vicinity of the main shock and multiple strike-slip clusters to the east, west and south of these clusters. Despite its location within the largest thrust cluster, the main shock does not show a high waveform similarity to any of the clusters. This is consistent with the results of other studies suggesting a complex failure mechanism for the main shock. We propose the existence of conjugated strike-slip faults in the south of the study area. Our waveform similarity based clustering toolbox is able to reveal distinct event clusters which cannot be discriminated based on locations and/or timing only. Additionally, the clustering results allows distinction between fault and auxiliary planes of focal mechanisms and to associate them to known active faults.

Earthquake-swarms, slow-slip and fault-interactions at the western-end of the Hellenic Subduction System precede the Mw 6.9 Zakynthos Earthquake, Greece

Earthquake-swarms, slow-slip and fault-interactions at the western-end of the Hellenic Subduction System precede the Mw 6.9 Zakynthos Earthquake, Greece

Earthquake-swarms, slow-slip and fault-interactions at the western-end of the Hellenic Subduction System precede the Mw 6.9 Zakynthos Earthquake, Greece

Earthquake-swarms, slow-slip and fault-interactions at the western-end of the Hellenic Subduction System precede the Mw 6.9 Zakynthos Earthquake, Greece

Earthquake-swarms, slow-slip and fault-interactions at the western-end of the Hellenic Subduction System precede the Mw 6.9 Zakynthos Earthquake, Greece

Earthquake-swarms, slow-slip and fault-interactions at the western-end of the Hellenic Subduction System precede the Mw 6.9 Zakynthos Earthquake, Greece

Elastic Fault Interactions and Earthquake Rupture Along the Southern Hellenic Subduction Plate Interface Zone in Greece

AbstractThe importance of splay‐thrust faults in subduction seismogenesis is increasingly acknowledged; however, their elastic interaction with the plate interface remains unclear. Here, we use GPS velocities, constrained by millennial fault slip rates, to study elastic fault‐interactions between the plate interface and its upper‐plate splay‐thrust faults from the southern Hellenic Subduction System (HSS). We find that, despite its largely aseismic character, the HSS plate interface zone is kinematically segmented, with slip rate deficits locally reaching ~85% and ~45% of the plate convergence rate on the western and eastern segments, respectively, and on structures different from those that ruptured historically. Although western Crete has been more active seismically during late Holocene, we find that the eastern HSS has higher seismic potential for large‐magnitude (M > 6) earthquakes and its interface zone is closer to failure. Elastic fault interactions are responsible for both significant intersegment variability in strain accumulation and uniformity in earthquake rupture segmentation along the HSS over millennial timescales.

Mantle dynamics beneath Greece from SKS and PKS seismic anisotropy study

SKS and PKS splitting parameters were determined in the broader Greek region using data from 45 stations of the Hellenic Unified Seismological Network and the Kandilli Observatory and Earthquake Research Institute, utilizing teleseismic events that occurred between 2010 and 2017. Data were processed for shear-wave splitting with the Minimum Energy Method that was considered the optimal. The results generally confirm the existence of anisotropic zonation in the Hellenic subduction system, with alternating trench-normal and trench-parallel directions. The zonation is attributed to the upper and lower olivine fabric layers that can, potentially, be present in the subduction zone. At the edges of this zone, two possible toroidal flow cases have been identified, implying the existence of tears that allow the inflow of asthenospheric material in the mantle wedge. The high number of null measurements in the KZN and XOR stations indicates a possible anisotropic transition zone between the fore-arc and back-arc areas. SKS and PKS splitting results are jointly interpreted, given that they yield similar values in most cases.

Seismic anisotropy in the Hellenic subduction zone: Effects of slab segmentation and subslab mantle flow

The segmentation and differentiation of subducting slabs have considerable effects on mantle convection and tectonics. The Hellenic subduction zone is a complex convergent margin with strong curvature and fast slab rollback. The upper mantle seismic anisotropy in the region is studied focusing at its western and eastern edges in order to explore the effects of possible slab segmentation on mantle flow and fabrics. Complementary to new SKS shear-wave splitting measurements in regions not adequately sampled so far, the source-side splitting technique is applied to constrain the depth of anisotropy and to densify measurements. In the western Hellenic arc, a trench-normal subslab anisotropy is observed near the trench. In the forearc domain, source-side and SKS measurements reveal a trench-parallel pattern. This indicates subslab trench-parallel mantle flow, associated with return flow due to the fast slab rollback. The passage from continental to oceanic subduction in the western Hellenic zone is illustrated by a forearc transitional anisotropy pattern. This indicates subslab mantle flow parallel to a NE–SW smooth ramp that possibly connects the two subducted slabs. A young tear fault initiated at the Kefalonia Transform Fault is likely not entirely developed, as this trench-parallel anisotropy pattern is observed along the entire western Hellenic subduction system, even following this horizontal offset between the two slabs. At the eastern side of the Hellenic subduction zone, subslab source-side anisotropy measurements show a general trench-normal pattern. These are associated with mantle flow through a possible ongoing tearing of the oceanic lithosphere in the area. Although the exact geometry of this slab tear is relatively unknown, SKS trench-parallel measurements imply that the tear has not reached the surface yet. Further exploration of the Hellenic subduction system is necessary; denser seismic networks should be deployed at both its edges in order to achieve a more definite image of the structure and geodynamics of this area.

MAJOR PALEOGEOGRAPHIC, TECTONIC AND GEODYNAMIC CHANGES FROM THE LAST STAGE OF THE HELLENIDES TO THE ACTUAL HELLENIC ARC AND TRENCH SYSTEM

Present day location and geometry of the Hellenic arc and trench system is only a small portion of the previously developed Hellenic arc that created the Hellenides orogenic system. The timing of differentiation is constrained in Late Miocene, when the arc was divided in a northern and a southern segment. This is based on: a) the dating of the last compressive structures observed all along the Hellenides during Oligocene to Middle-Late Miocene, b) on the time of initiation of the Kephalonia transform fault, c) on the time of opening of the North Aegean Basin and d) on the time of opening of new arc parallel basins in the south and new transverse basins in the central shear zone, separating the rapidly moving southwestwards Hellenic subduction system from the slowly converging system of the Northern Hellenides. The driving mechanism of the arc differentiation is the heterogeneity produced by the different subducting slabs in the north (continental) and in the south (oceanic) and the resulted shear zone because of the retreating plate boundary producing a roll back mechanism in the present arc and trench system. The paleogeographic reconstructions of the Hellenic arc and surrounding areas show the shortening of the East Mediterranean oceanic area, following the slow convergence rate of the European and African plates plus the localised shortening following the rapid Hellenic subduction rate. The result is that the frontal parts of the accretionary prism developed in front of the Hellenic arc have reached the African continent in Cyrenaica whereas on the two sides the basinal parts of the Ionian and Levantine basins are still preserved before their final subduction and closure. The extension produced in the upper plate has resulted in the subsidence of the Aegean Sea and the creation of several neotectonic basins in southern continental Greece in contrast to the absence of new basins in the northern segment since Late Miocene.

Eastern Mediterranean Tectonics

extended to many other colleagues working in the Eastern Mediterranean. The papers of the present issue were written by structural geologists, sedimentologists, petrologists, geochemists, geophysics and seismologists, thus covering a wide range of disciplines. All these contributions are aiming to reveal the tectonic evolution and paleogeography of the Eastern Mediterranean. A first collection sequence of papers is dealing with the pre-Alpine tectonic evolution and paleogeography. A second collection is addressing problems related to the Eoand the Meso-Alpine orogenic phases occurring from Late Jurassic to Oligocene. Finally, there are papers dealing with the Late Miocene to Holocene evolution of the south Aegean and west Anatolian domains. Specifically, the issue starts with two papers (Chatzaras et al., and Zulauf et al.), which are mainly based on detrital zircon U–Pb ages to track the margin of northern Gondwana and the Paleotethys suture in the southern and southwestern part of the External Hellenides. Detrital zircon U– Pb ages are also used by Hinskens et al. to constrain Late Cretaceous maximum sedimentation ages and a Triassic to Neoproterozoic provenance for the metasedimentary rocks of the Cycladic blueschists of Tinos. The following two papers by Ferriere et al. and Michail et al. describe the evolution of the Vardar Ocean and suggest west-directed obduction of the ophiolitic complex to the Pelagonian zone during the Eo-Hellenic orogenic phase. Based on structural and geochronological data, Georgiev et al. suggest Jurassic to Paleogene nappe stacking in the Rhodope metamorphic complex and Middle Eocene extensional overprinting. Okay and Altiner present new constraints on the Cretaceous development of the southern part of the Pontides close to the Izmir–Ankara suture (Haymana and Ankara regions) using mainly biostratigraphical data. The Late Cretaceous to Eocene evolution of the Asterousia Most rocks exposed in the Eastern Mediterranean show evidence for a long-standing and complex tectonic history. Besides the active Hellenic subduction system, this history includes at least four major orogenic episodes, namely the Cadomian, the Variscan, the Cimmerian, and the Alpine Orogeny, accompanied by the destruction and/or formation of large oceans such as the Paleotethys and the Neotethys. Therefore, with the term “Eastern Mediterranean Tectonics” we refer to all tectonic processes that occurred from the Neoproterozoic to present and contributed to the formation of the present geological edifice of the Eastern Mediterranean. This topical issue of the International Journal of Earth Science sheds some more light on the tectonic history of the Eastern Mediterranean presenting a collection of fifteen papers, which cover processes that took place in Aegean and Anatolian domains from the Neoproterozoic to the present (Fig. 1). The idea for this issue was born during the GeoFrankfurt2014 conference, where we convened a session entitled “The Evolution of the Alpine Orogenic System in the Eastern Mediterranean.” Apart from the participants of this session, the invitation to submit papers was

Constraints on fluid flow processes in the Hellenic Accretionary Complex (eastern Mediterranean Sea) from numerical modeling

The dynamics of accretionary convergent margins are severely influenced by intense deformation and fluid expulsion. To quantify the fluid pressure and fluid flow velocities in the Hellenic subduction system, we set up 2‐D hydrogeological numerical models following two seismic reflection lines across the Mediterranean Ridge. These profiles bracket the along‐strike variation in wedge geometry: moderate compression and a >4 km thick underthrust sequence in the west versus enhanced compression and <1 km of downgoing sediment in the center. Input parameters were obtained from preexisting geophysical data, drill cores, and new geotechnical laboratory experiments. A permeability‐porosity relationship was determined by a sensitivity analysis, indicating that porosity and intrinsic permeability are small. This hampers the expulsion of fluids and leads to the build up of fluid overpressure in the deeper portion of the wedge and in the underthrust sediment. The loci of maximum fluid pressure are mainly controlled by the compactional fluid source, which generally decreases toward the backstop. However, pore pressure is still high at the decollement level at distances <100 km from the deformation front, either by the incorporation of low permeability evaporites or additional compaction of the wedge sediments in the two profiles. In the west, however, formation of a wide accretionary complex is facilitated by high pore pressure zones. When compared to other large accretionary complexes such as Nankai or Barbados, our results not only show broad similarities but also that near‐lithostatic pore pressures may be easier to maintain in the Hellenic Arc because of accentuated collision, some underthrust evaporates, and a thicker underthrust sequence.

The Hellenic Subduction System: High-Pressure Metamorphism, Exhumation, Normal Faulting, and Large-Scale Extension

The Cenozoic history of the retreating Hellenic subduction system in the eastern Mediterranean involves subduction, accretion, arc magmatism, exhumation, normal faulting, and large-scale continental extension from ∼60 Mya until the Recent. Ages for high-pressure metamorphism in the central Aegean Sea region range from ∼53 Ma in the north (the Cyclades islands) to ∼25−20 Ma in the south (Crete). Younging of high-pressure metamorphism reflects the southward retreat of the Hellenic subduction zone. The shape of pressure-temperature-time paths of high-pressure rocks is remarkably similar across all tectonic units, suggesting a steady-state thermal profile of the subduction system and persistence of deformation and exhumation styles. The high-pressure metamorphic events were caused by the underthrusting of fragments of continental crust that were superimposed on slab retreat. Most of the exhumation of high-pressure units occurred in extrusion wedges during ongoing lithospheric convergence. At 23–19 Mya large-scale lithospheric extension commenced, causing metamorphic core complexes and the opening of the Aegean Sea basin. This extensional stage caused limited exhumation at the margins of the Aegean Sea but accomplished the major part of the exhumation of high-grade rocks that formed between 21 and 16 Mya in the central Aegean. The age pattern of extensional faults and contoured maps of fission-track cooling ages do not show a simple southward progression. Our review of lithologic, structural, metamorphic, and geochronologic data is consistent with a temporal link between the draping of the subducted slab over the 660-km discontinuity and the large-scale extension causing the opening of the Aegean Sea basin.

High-resolution seismic imaging of the western Hellenic subduction zone using teleseismic scattered waves

SUMMARY The active Hellenic subduction system has long been considered an ideal setting for studying subduction dynamics because it is easily accessible and of limited spatial extent. It has been the focus of numerous seismological studies over the last few decades but, nonetheless, the detailed structure of both the slab and the surrounding mantle remain poorly constrained in an intermediate depth range from 30 to 150 km. The objective of this paper is to fill this gap. The intermediate depth regime is of particular interest because it is pivotal for improving our understanding of the dynamic interaction between subducting lithosphere and the surrounding mantle. An interdisciplinary effort aimed at addressing this challenge is currently undertaken by the ‘Multidisciplinary Experiments for Dynamic Understanding of Subduction under the Aegean Sea’ (MEDUSA) project. As part of the MEDUSA initiative, a temporary array consisting of 40 densely spaced broad-band seismometers from the IRIS-PASSCAL pool has been deployed in southern Greece. We process the teleseismic data recorded by this array with a migration algorithm based on the generalized radon transform to obtain high-resolution images of the subduction zone in 2-D. The images reveal a sharp Mohoroviy´ discontinuity (Moho) at depths ranging from 30 km beneath the western margin of the Aegean Sea to 40 km beneath the central Peloponnesus, where it outlines the crustal root of the Hellenides. To the west of the Hellenides, the continental Moho is not identified, but we interpret a pronounced discontinuity imaged at ∼20 km depth as the contact between low-velocity sediments and high-velocity crystalline basement. The images also show the subducted oceanic crust as a low-velocity layer that plunges at a constant angle of 21 ◦ from west to east. The oceanic crust exhibits low velocities to at least 90 km depth, indicating that the bulk of fluid transfer from the subducted slab into the mantle wedge occurs below this depth. A detailed comparison of images constructed for distinct backazimuthal illuminations reveals deviations in the geometry of the subducted slab. These deviations are attributed to structural and/or compositional changes taking place directly to the north of the MEDUSA array, and are consistent with the existence of a slab tear beneath the Central Hellenic Shear Zone.

From syn- to post-orogenic Tertiary extension in the north Aegean region: constraints on the kinematics in the eastern Rhodope–Thrace, Bulgaria–Greece and the Biga Peninsula, NW Turkey

Abstract The Aegean region experienced back-are extension related to the Hellenic subduction system at least from the latest Oligocene to the present. We document Tertiary extension-related kinematics in the north Aegean, in the eastern Rhodope–Thrace of Bulgaria–Greece and the Biga Peninsula of NW Turkey. A regionally consistent NNE–SSW- to NE–SW-oriented kinematic direction, delineated in both areas by stretching lineations and associated ductile–brittle shear fabrics in exhumed metamorphic domes beneath detachments, suggests that they were kinematically coupled during the Tertiary extension. This kinematic framework, combined with regional geochronological data and the stratigraphic record in hanging-wall supradetachment basins, defines an extensional history that includes syn- and post-orogenic episodes from Paleocene to Miocene times. Paleocene–early Eocene synorogenic extension in the Kemer micaschists of the northern Biga Peninsula and in the Kesebir–Kardamos dome in Rhodope–Thrace accommodated gravitationally induced hinterland-directed exhumation of the orogenic stack, coeval with the closure of the Vardar Ocean. Then, following collision within the region, it was succeeded by latest Oligocene–Early Miocene extension as recorded in the Kazdaǧ Massif in the southern Biga Peninsula, which overlaps the Aegean back-arc post-orogenic extension, widely recognized in the central Aegean and southern Greek Rhodope. The protracted record of extension is interpreted to reflect progressive exhumation of the orogenic wedge along the Eurasian plate margin. Southward migration of extension and magmatism across the study areas accounts for sequential shift and roll-back of the subduction boundary at that margin, from the latest Cretaceous in the Rhodope to its present position at the Hellenic trench. The results allow recognition of the investigated areas as an important extensional domain in the north Aegean region, which underwent Tertiary syn- and post-orogenic extension.

Origin of S-type granites coeval with I-type granites in the Hellenic subduction system, Miocene of Naxos, Greece

Origin of S-type granites coeval with I-type granites in the Hellenic subduction system, Miocene of Naxos, Greece

Origin of S-type granites coeval with I-type granites in the Hellenic subduction system, Miocene of Naxos, Greece

The island of Naxos in the Cyclades preserves a core complex developed by Miocene detachment faulting associated with extension of continental crust inboard of the Hellenic subduction zone. The extension has unroofed Miocene hornblende-biotite granite and leucogranite, previously defined as I-type and S-type granite respectively. Crystalline basement exposed by extension includes partially migmatised ortho- and paragneisses principally with Hercynian protolith ages and intercalated high-grade metasedimentary and metavolcanic rocks with Mesozoic protoliths. Most leucogranites, in addition to quartz, feldspar and biotite contain muscovite, tourmaline and garnet (almandine-spessartine). Mineral assemblages suggest crystallisation pressures < 2.5 kbar, with stability of muscovite and tourmaline resulting from high B, Mn and possibly F in the magma. Two types of leucogranite can be distinguished on the basis of rock chemistry, mineral chemistry and Nd/Sm isotopes. Earlier leucogranites (type II) have high LREE:HREE ratio, high LILE, low Y and Nb and ϵNd ∼-10 and appear to be a partial melting product of Hercynian paragneiss. Leucogranite dykes within the I-type granite appear similar. Later leucogranites (types Ia, Ib) have low LREE:HREE ratio and ϵNd ∼-7 and appear to be derived from melting of both Hercynian paragneisses and younger metasediments. The major element composition of rare leucogranite type Ic suggests derivation from partial melting of orthogneiss. These varied sources result in significant variation in mineral assemblages in the leucogranites.