P. Xypolias

Kinematics of rock flow in a crustal-scale shear zone: implication for the orogenic evolution of the southwestern Hellenides

Combined shear-sense criteria, finite-strain data and vorticity analyses were used to study the deformation path in a curved crustal-scale shear zone (Phyllite–Quartzite Series) of the southwestern Hellenides. The results are combined with data on the structural evolution of a cover nappe (Pindos thrust belt) to provide new insights into the orogenic evolution of this region. Ductile deformation within the Phyllite–Quartzite Series was associated with a top-to-the-west-southwest shearing and was partitioned into two structural domains: a root zone and a frontal domain. The root zone is characterized by vertical coaxial stretching, high strain and upward movement of the material, while the frontal domain comprises simple-shear deformation at the base and pure shear at the top. This pattern suggests superposition of pure shear on simple-shear deformation, and implies tectonic extrusion of the material from the root zone. The initiation of brittle deformation in the Pindos thrust belt was associated with westward translation above the sub-horizontal Pindos Thrust. Later, as the mountain range elevated, normal faulting at high altitudes and migration of thrusting to the west occurred, while east-directed folding and thrusting in the belt started to the east. According to the proposed model, crustal thickening was taking place throughout the Oligocene and early Miocene, including the subduction of the Apulian beneath the Pelagonian microcontinent and the intracontinental subduction of the Phyllite–Quartzite Series. During the lower Miocene, vertical buoyancy forces led to the successive steepening of the shear zone and the simultaneous duplexing of its basement, facilitating tectonic extrusion of the material from its root zone. Finally, an indentation process caused vertical expulsion of the orogenic wedge and gravity collapse in the brittle crust.

Kinematic vorticity and strain rate patterns associated with ductile extrusion in the Chelmos Shear Zone (External Hellenides, Greece)

Abstract Kinematic vorticity, strain rate, finite strain and structural data have been used to describe the heterogeneous nature of ductile deformation in an extruding crustal-scale shear zone, the Chelmos Shear Zone, (CSZ) of the External Hellenides. The CSZ has a curved shape, which is marked by a frontal-flat and a steep zone. The spatial variation in the kinematic vorticity number is strongly correlated with the geometry and the structural depth of the shear zone. The pure shear component of deformation decreases from back to front and from top to bottom. The ductile thinning normal to the shear zones boundaries is higher in the steep zone and decreases westwards. Quartz grain size and strain rate in the CSZ is distributed into two approximately homogeneous domains. The higher strain rate is recorded in the steep zone and the base of the frontal-flat zone with lower rates in the upper parts of the frontal-flat zone. Based on mesoscopic structural data and the spatial distribution of the shear strain rate, the existence of a west-dipping back-flow zone in the boarder zone between these two domains is inferred. The overall pattern of deformation in the CSZ implies that tectonic extrusion from the steep zone occurred at the final stages of ductile deformation and was associated with the formation of the backflow zone in the front of the shear zone. Ductile deformation of rocks probably started close to simple shear on a planar low-angle shear zone, progressively became penetrative, and more general shear occurred as the CSZ steepened up. This progressive increase in pure shearing may have occurred in a heterogeneous manner and was higher close to both the eastern margin of the steep zone and the top of the frontal-flat zone. Solid-state extrusion in the CSZ was also associated with a heterogeneous and progressive increase in strain rate.

Upward extrusion and subsequent transpression as a possible mechanism for the exhumation of HP/LT rocks in Evia Island (Aegean Sea, Greece)

Abstract Structural, kinematic and strain-path analyses were used to elucidate how strain was accommodated at deep tectonic levels during the exhumation of high-pressure/low-temperature (HP/LT) units of the Attico-Cycladic-Massif (ACM), which are exposed on Evia Island (West Aegean Sea, Greece). These analyses are combined with data from the structural evolution of the overlying non-metamorphic belt (Pelagonian) to provide new insights into the orogenic evolution of this region. According to the proposed model, the exhumation of the HP/LT-rocks in the Evia area occurred under a mechanism which includes upward extrusion and subsequent transpression. In the study area, a continent–continent collision began during the Eocene involving the subduction of the protolith of the Evia Blueschist Unit (EBU) beneath the Pelagonian microcontinent. Continued compression and progressive underthrusting of the Almyropotamos passive continental margin through Oligocene resulted in the successive east-directed ductile extrusion of the allochthonous EBU, derived from the deepest underthrust crustal parts. Therefore, at the Oligocene/Miocene boundary the EBU emplaced tectonically over the Almyropotamos Unit (AU) and the latter underwent a mild HP-metamorphism. During this extrusion process, the EBU underwent intensive deformation under plane strain conditions, which partitioned into two homogeneous domains: (a) a root zone characterized by pure shear dominated deformation, and (b) a frontal flat-lying domain where deformation includes a high simple shear component. Starting at the early Miocene and extending into the Middle Miocene the nappe pile was caught in transpression, which led to the development of the Pelagonian Fault. Dextral transpressional shearing along this major fault caused further eduction and doming of the HP-units, juxtaposing these against weakly metamorphosed rocks of the uppermost tectonic unit (Pelagonian). Transpression occurred under continuous cooling and transferred into localized, distinct deformation zones close to the Pelagonian Fault. Strain-path analyses show an increasing component of flattening strain within these shear zones.

Exhumation of high-pressure rocks under continuous compression: a working hypothesis for the southern Hellenides (central Crete, Greece)

Combined kinematic, structural and palaeostress (calcite twinning, fault-slip data) analyses are used to study the exhumation mechanism of the high-pressure rocks exposed on the island of Crete (southern Aegean, Greece). Our study shows that the evolution of windows in central Crete was controlled by two main contractional phases of deformation. The first phase (D 1 ) was related to the ductile-stage of exhumation. NNW–SSE compression during D 1 caused layer- and transport-parallel shortening in the upper thrust sheets, resulting in nappe stacking via low-angle thrusting. Synchronously, intracontinental subduction led to high-pressure metamorphism which, however, did not affect the most external parts of the southern Hellenides. Subsequent upward ductile extrusion of high-pressure rocks was characterized by both down-section increase of strain and up-section increase of the pure shear component. The second phase (D 2 ) was associated with the brittle-stage of exhumation. D 2 was governed by NNE–SSW compression and involved conspicuous thrust-related folding, considerable tectonic imbrication and formation of a Middle Miocene basin. The major D 2 -related Psiloritis Thrust cross-cuts the entire nappe pile, and its trajectory partially follows and reworks the D 1 -related contact between upper and lower (high-pressure) tectonic units. Eduction and doming of the Talea Window was accompanied by gravity sliding of the upper thrust sheets and by out-of-the-syncline thrusting. Late-orogenic collapse also contributed to the exhumation process. Therefore, it seems that the high-pressure rocks of central Crete were exhumed under continuous compression and that the role of extension was previously overestimated.

Surface deformation during the Mw 6.4 (8 June 2008) Movri Mountain earthquake in the Peloponnese, and its implications for the seismotectonics of western Greece

The Movri Mountain earthquake (Mw 6.4), western Greece, was likely caused by dextral‐slip along a blind high‐angle fault, and generated a complex pattern of co‐seismic surface ruptures southwest of the Gulf of Corinth. The mapped Nisi, Michoi, and Vithoulkas rupture segments have similar lengths (5–6 km) and vertical offset on the order of 25, 10, and 5 cm, respectively. They are commonly expressed as straight or jagged linear traces with secondary cracks radiating from the main segments. Horizontal slip vector analysis indicates extensional faulting processes for all rupture segments. Although these faults exert some control on the fluvial drainage pattern and at least one of them was ruptured during past events, their escarpments are poorly preserved. The indistinct topographic expression of the studied faults and their complex rupture patterns can be attributed to the distribution of the deformation over a blind fault.

Ductile nappe stacking and refolding in the Cycladic Blueschist Unit: insights from Sifnos Island (south Aegean Sea)

New geological and structural mapping combined with kinematic and amphibole chemistry analyses is used to investigate the deformation history of the Cycladic Blueschist Unit (CBU) on Sifnos Island (Cyclades, Aegean Sea). We concentrate on north Sifnos, an area characterized by exceptionally well-preserved eclogites and blueschists. Our data show that the early, main phase (D2) of ductile deformation in the CBU occurred synchronous with the transition from prograde to close-to-peak retrograde conditions. This deformation phase took place at middle Eocene and is related to ESE-directed thrusting that emplaced the metavolcano-sedimentary subunit over the Marble subunit. The subsequent exhumation-related (D3) deformation is characterized by gently NE-plunging folds and NE-directed contractional shear zones that formed parallel to the axial planes of folds. NE-directed shearing occurred under blueschist and transitional blueschist-/greenschist-facies conditions during late Eocene–Oligocene and caused the restacking of the early nappe pile. We suggest that a mechanism of ductile extrusion of the CBU in a tectonic setting of net compression could explain better the recorded exhumation-related deformation than a mechanism of syn- and post-orogenic extension. Our new kinematic results in combination with previous works in the Cyclades area reveal a regional scale change in tectonic transport direction from (W)NW–(E)SE at Late Cretaceous–middle Eocene to (E)NE–(W)SW at late Eocene–Oligocene times. The observed change in transport direction may be governed by the relative motion of Africa with respect to Europe during Alpine orogeny.

Paleoseismic investigations along a key active fault within the Gulf of Corinth, Greece

The study of paleoseismological and archaeological excavations provide clues for the evolution of Helike Fault, located along the westernmost end of the Gulf of Corinth, that displays high activity and exerts control on the landscape. In this study we present evidence from paleoseismic trenches which revealed well defined fault strands and clear colluvial stratigraphy. We focus on the two main segments of the Helike Fault and their implications on strong earthquake activity. The Helike Fault is a major tectonic structure that influenced the evolution of ancient settlements on the Helike Delta, from the Early Bronze Age through the Byzantine period, till present times. The eastern fault segment appears to control the southern Gulf morphology, while the western segment is controlling the large Aigion basin. Interbedded organic-rich soils and gravels dominate in all trenches. Fault strands that control successive scarp-derived colluvial deposits were identified within the trenches and indicate the continuous seismic activity along the fault trace. Co-seismic offsets, open cracks filled with debris and liquefaction related deformation was also recognized. At least seven seismic events were identified inside the excavated trenches, during the last 10 ka. The estimated vertical throw along the fault segments, observed within the trenches, is on the order of 1 meter per event. Based on dating of colluvial wedges we estimated the Holocene slip rate on the Helike Fault, which shows an increase from ~0.3 mm/yr to 2 mm/yr in the last 2 ka. We consider the derived slip rates to be minimum values due to the implication of erosional effects and sediment accumulation from the upthrown block. The Helike fault appears to play a crucial role both in subsidence of the Helike delta plain and in shifting Kerynites river course that runs between the two Helike fault segments. The Helike Fault activity and the clustering of surface rupturing events on the Helike fault seems to fit well with the subsidence of the Helike Delta plain and its change from marsh to lake or pod over the last 5 Ka.

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