S. Kokkalas

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.

Quantitative analysis and visualization of nonplanar fault surfaces using terrestrial laser scanning (LIDAR)—The Arkitsa fault, central Greece, as a case study

Many fault surfaces are noticeably nonplanar, often containing irregular asperities and more regular corrugations and open warping. Terrestrial laser scanning (light detection and ranging, LIDAR) is a powerful and versatile tool that is highly suitable for acquisition of very detailed, precise measurements of slip-surface geometry from well-exposed faults. Quantitative analysis of the LIDAR data, combined with three-dimensional visualization software, allows the spatial variation in various geometrical properties across the fault surface to be clearly shown. Plotting the variation in distance of points from the mean fault plane is an effective way to identify culminations and depressions on the fault surface. Plots showing the spatial variation of surface orientation are useful in highlighting corrugations and warps of different wavelengths, as well as cross faults and fault bifurcations. Analysis of different curvature properties, including normal and Gaussian curvature, provides the best plots for quantitative measurements of the geometry of corrugations and folds. However, curvature analysis is highly scale dependent, so requires careful filtering and smoothing of the data to be able to analyze structures at a given wavelength. Three well-exposed fault panels from the Arkitsa fault zone in central Greece were scanned and analyzed in detail. Each panel is markedly nonplanar, and shows significant variation in surface orientation, with spreads of ~±20°–25° in strike and ±10° in dip. Most of the variation in orientation reflects decimeter- and meter-scale corrugations and longer wavelength warps of the fault panels. Average wavelengths of corrugations measured on two of the panels are 4.04 m and 4.43 m. Whereas the orientations of the three fault surfaces show significant variations, the orientations of fault striae are very similar between the panels, and are tightly clustered within each panel. Fault-slip analysis from each panel shows that the local stress field is consistent with the regional velocity field derived from global positioning system data. The oblique slip lineations observed on the fault panels represent a combination of two contemporaneous strain components: north-northeast–south-southwest extension across the Gulf of Evia and sinistral strike slip along the west-northwest–east-southeast Arkitsa fault zone.

Heterogeneous ductile deformation along a mid-crustal extruding shear zone: an example from the External Hellenides (Greece)

Abstract Petrofabric, finite strain and kinematic vorticity data were used to investigate the heterogeneous nature of ductile deformation along a 1.5–2 km thick extruding shear zone in the south Peloponnese, that formed under blueschist-facies conditions. Asymmetric quartz c-axis fabrics confirm westward thrust movements on an east-dipping shear zone and provide evidence for localized top-down-to-the-east shear sense at the front of the zone. Strain ratio (Rxz) is nearly constant (c. 3.0–4.0) along the upper structural levels of the zone but increases systematically from the middle to the bottom, approaching a value of c. 9.5 in the frontal parts, close to the basal thrust, and a value of c. 7.0 in the inner parts. The distribution of kinematic vorticity number depicts a simple-shear-dominated domain in the lower half of the shear zone and shows that the pure shear component always increases upwards in the zone, becoming dominant at the top of the inner parts of the zone. Integration of strain and vorticity data yields a shear-parallel elongation of c. 60–90% at the top and c. 40–60% at the bottom of the zone, revealing that both upper and lower surfaces of the extruding slices were ‘stretching faults’. Minimum total displacements of 25 and 41 km and slip rates of 6.5 and 10 mm/year were estimated for the basal and roof faults, respectively.

Emplacement of the Miocene west Naxos pluton (Aegean Sea, Greece): a structural study

Synmagmatic and solid-state structures within the Naxos pluton and its rim may provide insight into the interplay between plutonism and regional deformation at upper-crustal level. Within the hornblende–biotite granite of western Naxos, synmagmatic foliations display two distinct patterns, onion-skin in the north and tangential to the rim in the south. The two areas are separated by the NE-trending Glinadon fault. Deformed mafic enclaves in the pluton are prolate, with their long axes parallel to the synmagmatic lineation. In contrast, phenocryst distribution analysis, using the Fry method, defines an apparent oblate strain with a horizontal stretching lineation. Planar markers within the pluton progressively steepen through the vertical at the east pluton border. Several lines of evidence, such as dykes intruding axial areas of rim-parallel folds, foliated or folded aplite veins, folds and spaced cleavage in the mollase, and inverted stratigraphy, suggest pluton emplacement and deformation during transpressional deformation. A northward divergent flow regime with magma spreading out mainly from the Naxos fault, and the deflection of both the synmagmatic foliation pattern and the flow lines at the Glinadon fault, suggest that the NE–SW-and N–S-trending faults were active during pluton formation. In the south the pluton has grown by the expansion of dykes occupying P-shear positions with respect with the Naxos fault; in the north a piecemeal block down-drop complements this process and favours voluminous magma concentration. During the late evolutionary stages of pluton construction, the magma chamber was compartmentalized into NE-trending sectors affected by block rotation in an anticlockwise manner. Understanding the role of faults in the emplacement of the Naxos pluton is important for understanding emplacement of other plutons in the Aegean Sea region, since most of them are controlled by N–S- (Ikaria pluton) or NE- (Tinos, Serifos and Delos plutons) trending faults.

Is there a link between faulting and magmatism in the south-central Aegean Sea?

A distinct spatial relationship between surface faulting, magmatic intrusions and volcanic activity exists in the Aegean continental crust. In this paper, we provide detailed structural observations from key onshore areas, as well as compilations of lineament maps and earthquake locations with focal plane solutions from offshore areas to support such a relationship. Although pluton emplacement was associated with low-angle extensional detachments, the NNE- to NE-trending strike-slip faults also played an important role in localizing the Middle Miocene plutonism, providing ready pathways to deeper magma batches, and controlling the late-stage emplacement and deformation of granites in the upper crust. Additionally, the linear arrangements of volcanic centres, from the Quaternary volcanoes along the active South Aegean Volcanic Arc, are controlled primarily by NE-trending faults and secondarily by NW-trending faults. These volcanic features are located at several extensional settings, which are associated with the main NE-trending faults, such as (i) in the extensional steps or relay zones between strike-slip and oblique-normal fault segments, (ii) at the overlap zones between oblique-normal faults associated with an extensional strike-slip duplex and (iii) at the tip zone of a NE-trending divergent dextral strike-slip zone. The NE trend of volcano-tectonic features, such as volcanic cone alignments, concentration of eruptive centres, hydrothermal activity and fractures, indicates the significant role of tectonics in controlling fluid and magma pathways in the Aegean upper crust. Furthermore, microseismicity and focal mechanisms of earthquakes in the area confirm the activity and present kinematics of these NE- trending faults.

Vorticity of flow in ductile thrust zones: examples from the Attico-Cycladic Massif (Internal Hellenides, Greece)

Abstract Microstructural, petrofabric, strain and vorticity data from quartz-rich tectonites were used to investigate the kinematics of rock flow in the Evia and Ochi ductile thrust zones, formed during exhumation of the high-pressure nappes of the Attico-Cycladic Massif. The Evia thrust zone defines the base of the Styra nappe while the Ochi thrust zone defines the contact between the Styra and the overlying Ochi nappe. A dominant top-to-the-ENE sense of shearing along both thrust zones is indicated by several shear sense criteria. Deformation in the structurally deeper Evia thrust zone occurred under approximately plane strain conditions and was characterized by a RXZ strain ratio varying from 3 to 6. The vorticity profile above the thrust plane shows a slight down-section increase in the kinematic vorticity number (Wm) from 0.8 to 0.9, as well as the presence of local thin domains with a higher pure shear component of deformation. In the overlying Ochi thrust zone, a downward increase in Wm values from 0.6 to 0.9 is detected both above and below the thrust plane. Here, rocks have been deformed in the general constrictional field with RXZ values ranging between 5 and 8. A transport-parallel elongation of 30–90% and 50–160% has been estimated for the Evia and Ochi thrust zones, respectively, implying that ENE-directed extrusive flow controlled the formation, stacking and exhumation of the Styra and Ochi nappes.

Strain-dependent stress field and plate motions in the south-east Aegean region

Abstract In the south-eastern Aegean region, we record stress and strain established in two stages, during the convergence of the African with the Eurasian plate: the late collision of the Hellenides and the fore-arc evolution above the Hellenic subduction. Observed directions of principal stress axes associated with these stages are not in good correlation with the motion direction of the converging plates and are strongly depended to a pre-existed orthogonal fault system, which comprises WNW–ESE and NNE–SSW trending faults. At present, imposition of shear stress along the south-eastern margin of the Aegean plate causes a great variation of stress, while re-activated NNE-SSW trending faults transfer the plate motion of Anatolia south-westwards to the Hellenic Trench.

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.

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.

High-resolution seismic profiling reveals faulting associated with the 1934 Ms 6.6 Hansel Valley earthquake (Utah, USA)

The 1934 Ms 6.6 Hansel Valley, Utah, earthquake produced an 8-km-long by 3-km-wide zone of north-south−trending surface deformation in an extensional basin within the easternmost Basin and Range Province. Less than 0.5 m of purely vertical displacement was measured at the surface, although seismologic data suggest mostly strike-slip faulting at depth. Characterization of the origin and kinematics of faulting in the Hansel Valley earthquake is important to understand how complex fault ruptures accommodate regions of continental extension and transtension. Here, we address three questions: (1) How does the 1934 surface rupture compare with faults in the subsurface? (2) Are the 1934 fault scarps tectonic or secondary features? (3) Did the 1934 earthquake have components of both strike-slip and dip-slip motion? To address these questions, we acquired a 6.6-km-long, high-resolution seismic profile across Hansel Valley, including the 1934 ruptures. We observed numerous east- and west-dipping normal faults that dip 40°−70° and offset late Quaternary strata from within a few tens of meters of the surface down to a depth of ∼1 km. Spatial correspondence between the 1934 surface ruptures and subsurface faults suggests that ruptures associated with the earthquake are of tectonic origin. Our data clearly show complex basin faulting that is most consistent with transtensional tectonics. Although the kinematics of the 1934 earthquake remain underconstrained, we interpret the disagreement between surface (normal) and subsurface (strike-slip) kinematics as due to slip partitioning during fault propagation and to the effect of preexisting structural complexities. We infer that the 1934 earthquake occurred along an ∼3-km wide, off-fault damage zone characterized by distributed deformation along small-displacement faults that may be alternatively activated during different earthquake episodes.