Slip rates on the Helike Fault, Gulf of Corinth, Greece: new evidence from geoarchaeology

Palaeoseismological and archaeological analysis of a trench enabled us to estimate the Holocene slip rates on the East Helike Fault, flanking the south-western Gulf of Corinth. We recognized two major fault strands within the trench: the ‘north fault’ controls a succession of three colluvial wedges and the deposition of a 2.7 m thick sedimentary sequence. The ‘south fault’ controls the deposition of a 2.9-m thick brownish-red colluvium. Based on colluvial stratigraphy, radiocarbon dating of the sediments suggests that the slip rate was c. 0.3 mm yr⁻¹ from 10 250 to c. 1400 bp , when it increased dramatically to c. 2.0 mm yr⁻¹ after a strong earthquake event near 1400 bp . The faster slip rate evidently increased the sedimentation rate.     

Oligocene–Miocene thrusting in central Aegean: insights from the Cycladic island of Amorgos

In the central Aegean, the Cycladic island of Amorgos consists of two high‐pressure (HP) units, the marble‐rich Amorgos unit, which is correlated to the Mesozoic ‘cover’ sequence of the Menderes Massif, and the Cycladic Blueschist unit. New structural data show that the deformation history of the Amorgos HP‐rocks was principally governed by early Oligocene (or late Eocene)–early Miocene ductile to brittle thrusting (D₁–D₃) followed by middle–late Miocene oblique contractional movements (D₄–D₅). The D₁ phase caused syn‐blueschist‐facies ductile thrusting of the Cycladic Blueschist unit over the Amorgos unit, with ambiguous kinematics. Progressive deformation under continuous NW–SE compression produced a sequence of imbricate NW‐directed thrusts (D_2/3) characterized by a stratification of fault‐related rocks, with mylonitic zones (D₂) giving way downwards to cataclastic zones (D₃). Ductile D₂ thrusting synchronous to greenschist‐facies retrogression, was accompanied by mega‐sheath folding during constrictional and general shear deformation. Brittle D₃ thrusting was associated with NW‐verging F₃ folds trending at a high‐angle to the transport direction. Orthogonal contraction gave way to transpression during which the compression orientation changed from NW–SE (D₄) to NE–SW (D₅). Back‐arc related NW–SE pure extension (D₆) seems to have been established in post‐late Miocene times and related high‐angle normal faulting affected HP‐rocks only after they had already reached the uppermost crustal levels. Oligocene–early Miocene deformation history is interpreted to indicate syn‐compressional exhumation of HP‐rocks possibly in an extrusion wedge. In this case, Amorgos HP‐rocks should have occupied the base of the extrusion wedge. Copyright © 2011 John Wiley & Sons, Ltd.     

Tectonic geomorphology of the easternmost extension of the Gulf of Corinth (Beotia, Central Greece)

The easternmost sector of the Gulf of Corinth, the Beotia area in Central Greece, is an area with active normal faults located between the two major rift structures of Central Greece, the Gulf of Corinth and the North Gulf of Evia. These active normal faults include WNW to E–W and NE to ENE-trending faults affect the landscape and generate basin and range topography within the Beotia. We study four normal fault zones and drainage basin geometry in the easternmost sector of the Gulf of Corinth to document the impact of active tectonics on the landscape evolution. Fault and drainage geometry are investigated based on detailed field mapping and high-resolution digital elevation models. Tectonic geomorphic analysis using several parameters of active tectonics provides information concerning the relative tectonic activity and fault growth. In order to detect areas of lateral stream migration that could indicate recent tectonic activity, the Transverse Topographic Symmetry Factor and the Asymmetry Factor are used to analyse drainage basin geometry in six large drainage basins and a drainage domain covering the study area. Our results show that vertical motions and tilting associated with normal faulting influence the drainage geometry and its development. Values of stream-gradient indices (S_L) are relatively high close to the fault traces of the studied fault zones suggesting high activity. Mountain-front sinuosity (S_mf) mean values along the fault zones ranges from 1.08 to 1.26. Valley floor width to valley height ratios (V_f) mean values along the studied fault zones range between 0.5 and 1.6. Drainage basin shape (B_S) mean values along the fault zones range from 1.08 to 3.54. All these geomorphic parameters and geomorphological data suggest that the analyzed normal faults are highly active. Lateral fault growth was likely produced by primarily eastward propagation, with the WNW to E–W trending faults being the relatively more active structures.     

The relationship between length and width of plutons within the crustal-scale Cobequid Shear Zone, northern Appalachians, Canada

The lengths and widths have been measured for 69 component bodies of composite plutons along the Cobequid Shear Zone. Plutons on major fault strands, those with mylonite zones >0.1 km wide, exhibit evidence of multiple intrusion of magma batches. Small plutons along short faults in stepover zones appear related to rapid emplacement of magma in bodies 1.5–4 km long by 0.1–2 km wide. Such small plutons show low enrichment in incompatible elements in older component bodies, but increasing amounts in younger bodies as a result of progressive magma expulsion from crystal mush during crystallization and shear-enhanced compaction in fault zones. Wider plutons generally occur along longer fault strands accommodating more strain and penetrating deeper into the crust and show enrichment in incompatible elements. The width of the mylonitic fault zone is about 15% of the width of these plutons. The length-to-width ratio of component bodies and composite plutons varies between 2 and 11. The best-fit line describing these data has a slope of 1.056, which implies scaling behavior between plutonism and tectonic processes. Scalar properties of plutonic bodies are similar to those of faults, but scalar relationships observed in component bodies do not apply to composite plutons.     

The egion fault, earthquake-related and long-term deformation, Gulf of Corinth, Greece

On 15 July 1995, the Egion earthquake (Ms = 6.2) occurred in the vicinity of Egion, west-central Greece. Macroseismic observations along the 12 km long E-W trending Egion fault represent short-term or earthquake-related deformation characterized by fairly straight E-W trending surface ruptures with small displacements that mimic the Egion fault geologic offsets and segmentation. Hanging wall converging slip vectors along the Egion fault are clearly related to fault motions at depth. Furthermore, peak accelerations of the built-up area of Egion amount to 0.54 g, that is double the estimated peak acceleration of the Egion coastal area, showing thus close relation between fault trace and attenuation of the ground motion.The Egion fault, with a total geological throw of 200 m and dips to the north at about 55 °, accommodating active tectonic deformation of the Egion area. Its morphotectonic expression reflects long-term deformation in competition with the 1995 earthquake related deformation. The Egion fault is controlling the geomorphic evolution of the Egion area as follows: 1) The fault is defining the evolution of fan-deltas (offshore) and the Meganitas river alluvial plain (onshore). 2) The hanging-wall's greatest subsidence is observed, at the Egion bay, at the central portion of the fault. The Egion bay is located at the central part of the fault showing a strong relationship between the long term slip-rate ratio and the recent coastal morphology. The subsidence gradient or the tectonic activity along the fault is defined by the valley-floor width to valley height index (V_f) of small rivers draining the fault scarp. The Meganitas river course is tilted, when crosses the Egion fault trace, towards the area with the highest subsidence along the fault. 3) Stream incision is more important than slope recession at areas close to the fault trace.All these observations suggest that the Egion fault, which probably hosted the last earthquake, are geomorphically controlling the evolution of a 15 km-long by 5 km-wide zone, fairly similar in dimensions to the surface length of the fault.     

Intracontinental wedging and post-orogenic collapse in the mesohellenic trough

The Mesohellenic Trough is a 130 km long and about 30 km wide subsiding area which contains a thick sequence of well exposed Late Cenozoic post-orogenic sediments. This intermontane basin, located at the contact between the Apulian and Pelagonian collided margins, provides a good example of the characteristics needed to study the chronology of late orogenic intracontinental structures.The Mesohellenic Trough was developed from the Middle Eocene to Middle Miocene as a piggy-back basin along the eastern flanks of a giant pop-up structure. This structure consists of west-verging, foreland-propagating thrusts within the Apulian plate and of east-verging backthrusts within the Pelagonian plate. As a result the eastern parts of the Apulian margin were thickened and uplifted, followed by post-orogenic collapse.Internal deformation of the sedimentary infill varies widely along the trough axis. At the northern and southern terminations of the trough, two small indentors induced a tectonic escape towards the central part of the basin until the Middle Miocene. During this process of convergent wrenching, reverse strike-slip faults and pure strike-slip faults formed. Towards the central part of the trough, convergent wrenching decreased gradually until it was replaced by a post-orogenic collapse with normal and oblique normal faults trending parallel and/or perpendicular to the trough axis.     

An exhumation model of the south Peloponnesus, Greece

An exhumation model comprising forward and backward thrusting and late orogenic collapse is proposed in order to explain the kinematics of the tectonic windows in the south Peloponnesus. The model is based on mapping, mesoscopic structural data and strain analysis. Syn-compressional thickening took place throughout the Oligocene and Early Miocene which includes the subduction of the Pindos Ocean at the western margin of the Pelagonian microcontinent and the intracontinental subduction of the Phyllite–Quartzite and the Plattenkalk series. The latter subduction was associated with blueschist metamorphism, westward-directed ductile thrusting, and folding. The exhumation history of the deeper parts of the orogen began at the Oligocene–Miocene boundary with the progressive entrance of the low-density crust and the Plattenkalk carbonates in the subduction zone. Increased buoyancy caused: (a) the initiation of the Phyllite–Quartzite series extrusion; (b) vertical coaxial stretching; and (c) the evolution of two pop-up structures, i.e. the Parnon and Taygetos anticlines. This syn-compressional exhumation was taking place in the lower Miocene with decreasing rates from 7 to 1.5 mm/year. The change in the local stress field from compression to extension began in the middle Miocene with the formation of hinterland-dipping normal faults. The exhumation/denudation rate caused by the footwall uplift along these faults does not exceed 0.2 mm/year. Received: 16 April 1999 / Accepted: 19 January 2000     

Fractal analysis of normal faults in northwestern Aegean area, Greece

Normal faults within the Ptolemais coal field and large seismogenic faults in the northwestern Aegean remain fractal for displacement values larger than about 1m. The kinematic parameters on reverse drag profiles such as length of rollover, footwall uplift and wavelength of footwall uplift show that all three parameters have a power law relationship, expressed by a c exponent of about 1, with the maximum displacement which take place across the fault. Footwall uplift/hanging wall subsidence ratio is about 1/2.The displacement analysis help us to propose a growth model for larger seismogenic faults in the NW Aegean, as is the ‘Hepiros fault set’ and the ‘Aliakmon fault zone’. Faults within the ‘Aliakmon fault zone’ were independently developed, at the first stages of deformation, by tip line deformation and out-of plane bifurcation, whereas later, deformation continued by segment linkage. One of these faults the ‘Sarakina fault’ was reactivated during the 1995 earthquake to produce a 25 km long surface rupture. A long term slip rate of about 0.3 mm a⁻¹ has been estimated by taking into consideration that over the past 6 Ma a maximum displacement of 1700 m across this fault has taken place.     

Scaling properties within the Gulf of Corinth, Greece; comparison between offshore and onshore active faults

The Gulf of Corinth, Greece, is a 110-km-long by 30-km-wide active graben displaying strong seismicity hosted both on north and south dipping normal faults. This complex fault pattern consists of two fault populations, offshore and onshore. The offshore fault population is investigated by densely arranged seismic reflection profiles during the last 20 years, whereas the onshore fault population displays spectacular and well exposed faults, delineated by high accuracy mapping. We analyzed fault length and throw, in order to study the scaling properties of 136 well-determined offshore and onshore faults and the comparison between the two datasets. We examined the statistical properties on both fault populations, in order to describe the role of segmentation in the growth of faults and the different stages of the evolution of the fault networks.Our results on power law relationships associated with the scaling properties of the fault zones in the Gulf of Corinth, suggest that both fault populations are bi-fractal, providing the initiation of a sature state in deformation. In addition, the vertical throw of faults shows that both fault populations have similar properties but different distributions below and above 5 km, respectively. Displacement–length ratios, show that faults larger than 9 km appear to accumulate throw without any dramatic change to their length. These observations combined with other geophysical studies within the Gulf, suggest that the characteristic fault lengths of 5 km and 9 km can be correlated to the crustal mechanical structure and the seismicity of the Gulf.