Theodor Doutsos

Geometry and kinematics of active faults and their seismotectonic significance in the western Corinth-Patras rift (Greece)

Abstract The western part of the Corinth-Patras rift is formed in an area which is being uplifted. Rifted domains have subsided relative to the adjacent areas in NW Peloponnesus and Sterea Hellas, but subsidence rates rarely exceed uplift rates. Extension is mainly accomplished by WNW-trending active faults with lengths up to 25 km. Most of these faults show an along-strike segmentation and often terminate against transfer faults. Displacement analysis, along 170 mesoscopic faults and 36 mappable faults, has been carried out to assess slip rates and to estimate the magnitudes and recurrence time intervals of earthquakes occurring along them. Earthquake magnitude ranges between 5 and 6.7. Only eight faults appear to be capable of producing earthquake magnitudes larger than 6. Recurrence times of these large earthquakes vary between 80 and 1690 years. Upper crustal deformation in the western Corinth graben can be described by six large asymmetric grabens which are inferred to bottom out to a N-dipping curved ramp. The asymmetric shape of most WNW-trending rifts in the Hellenic peninsula, including the Corinth-Patras rift promotes the simple shear model of extension against the pure shear model in the upper crustal layer.

Stress and deformation patterns in the Aegean region

The Aegean region constitutes the overriding plate of the Africa‐Eurasia convergent plate system, in the eastern Mediterranean. To explain the fault kinematics and tectonic forces that controlled rift evolution in the Aegean area, we present fault-slip data from about 900 faults, and summarise the structural analyses of five key structural “provinces”. Five regional tectonic maps are used as the basis for a new stress map for the Aegean region and for discussions on regional geodynamics. Since the Late Miocene, the central Aegean has been affected by WNW- and NE-trending faults which transfer the motion of the Anatolian plate to the southwest, synchronous with arc-normal pull acting on the boundary of the Aegean plate. At the same time, the Hellenic Peninsula has suffered moderate extension by NW-trending grabens formed due to collapse of the Hellenic mountain chain. During intense extension in the southern Aegean in the Plio-Quaternary the arcuate shape of the Hellenic Trench was established. Arcnormal pull in the Aegean plate margin, combined with transform resistive forces along the Hellenic subduction gave rise to widespread strike-slip and oblique-normal faults in the eastern segment and moderate oblique extension in the western segment of the arc. To the north, subduction involves more continental crust and consequently the push of subduction is transmitted to the overriding plate (Hellenic Peninsula), resulting in the formation of NE-trending grabens. WNW-trending grabens in this area are considered to have propagated westward from the Aegean Sea to the Ionian Sea during Plio-Quaternary times, probably acting as pull-apart structures between stable Europe and the rapidly extending southern Aegean area. q 2001 Elsevier Science Ltd. All rights reserved.

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.

Kinematics of the central Hellenides

Structural analysis along 24 cross sections crosscutting several windows in the central Hellenides provides the sense of nappe movements as well as the location of destroyed oceans lying between the Apulian and Eurasian continents from the Mesozoic. Orogeny took place in two phases: The first phase, “the Eo-Hellenic” phase, was initiated by convergence of the Apulian and Pelagonian plates with west directed subduction and closure of the Pindos Ocean. Late Jurassic obduction of oceanic lithosphere over the western margin of the Pelagonian plate was followed by footwall imbrication, mylonites and sheath folds. During the late Cretaceous, uplift was associated with ductile normal faulting at depth and tectonic unroofing at shallow crustal levels. The second phase, “the Meso-Hellenic” phase, comprised the closure of the Ambelakia Ocean at the eastern margin of the Pelagonian plate and continental subduction along the eastern margin of the Apulian plate. West directed subduction of the Ambelakia Ocean was associated with eastward directed ductile thrusting, folding and blueschist metamorphism. Blueschist formed within a simple duplex structure at depth and was subsequently overthrusted in the late Eocene onto the Olympos microcontinent, which acted as a major obstacle to the eastward directed nappe movements. Up to 150-m-thick cataclasites, kink folds and a spaced cleavage were formed during the late stage of the continental collision. “A subduction” along the eastern margin of the Apulian plate caused kink folding and reimbrication of the western parts of the Pelagonian basement. Since the Oligocene, the overthickened crust collapsed by means of low-angle normal faults.

Implications of structural segmentation during earthquakes: the 1995 Egion earthquake, Gulf of Corinth, Greece

The Egion earthquake which occurred in the Gulf of Corinth, central Greece (Ms = 6.2) on 15 June 1995 was caused by normal slip on the north-dipping and WNW-trending Egion fault. The Egion fault ruptured at depth during the Egion mainshock and probably re-ruptured at shallow level during the largest aftershock. The surface trace of the Egion fault has a segmented geometry. Linkage between three segments, which show long-term deformation differences as well as coseismic segmentation, enabled all segments to be incorporated in an earthquake segment. The surface ruptures continued to grow after the coseismic motion; the afterslip throw of the fault 10 weeks after the main event was equal to the 3 cm value for maximum coseismic slip. This afterslip was accompanied by uplift of the footwall block and a warp-like hangingwall subsidence (folding). This pattern of deformation was associated with more complex deformation at the western end of the earthquake segment. Here, afterslip was accompanied by general subsidence of the whole area (between 25th June and 30th July), followed by uplift of the whole area without afterslip (between 30th July and 2nd September). The afterslip-rate averaged over the 73 day period after the main event varied from 0.48 mm day−1 along the central part of the earthquake segment to 0.16 mm day−1 at the eastern end of the earthquake segment.

Transpression and transtension within different structural levels in the central Aegean region

In the central Aegean region, shortening structures within the Miocene molasse arc known since long ago. Nevertheless recently, most authors have recognized extensional structures within Middle to Upper Miocene granitoids, proposing a Basin and Range type model for the Late Cenozoic evolution of the area. To resolve this problem, structural mapping and mesoscopic analysis of 900 faults sampled from 12 islands have been carried out. Late orogenic uplift of the central Aegean region is the result of a continuous convergence and indentation of the Pelagonian plate by the Apulian plate, that took place throughout the Miocene. Formation of core-complexes can be associated with: (a) large oblique-upthrusts, (b) steeply dipping strike-slip faults, and (c) low-angle normal faults. The latter are produced by low, multidirectional extension, which has affected small crustal regions as adjacent areas underwent transpression. In the late stages of collision the overthickened crust began to collapse due to transtension which was replaced by extension caused by the roll-back of the Hellenic subduction zone in the Lower Pliocene time. This extensional regime has lasted until the present day.

Quaternary tectonics in the Gulf of Patras, western Greece

Abstract Air gun seismic and 3.5 kHz profiling data from the Gulf of Patras, western Greece, show that it is occupied by a small asymmetric graben with several geometric similarities to the larger-scale graben in the Gulf of Corinth to the east. Major listric faulting characterizes the southern flank of the graben whilst the northern flank represents an associated rollover structure affected by antithetic and synthetic faulting. The present phase of subsidence is of Holocene age, but buried growth faults suggest earlier subsidence in the Gulf. The average rate of subsidence through the Holocene is estimated to be 10 mm/year. The Gulf of Patras graben, together with the Gulf of Corinth graben and the Megara basin, represent a continuous system of WNW-ESE trending grabens in a broad zone of intense seismicity within the Aegean domain. Individual grabens are offset and are interconnected by NE-SW trending fault systems.

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.

Fractal analysis of normal faults in northwestern Aegean area, Greece

Abstract 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.

Thrust sequences in the central part of the External Hellenides

The model of a foreland propagating sequence already presented for the External Hellenides is significantly modified in this paper. New data are used, including structural maps, cross-sections, stratigraphic determinations and seismic profiles. In general, thrusts formed a foreland propagating sequence but they acted simultaneously for a long period of time. Thus, during the Middle Eocene the Pindos thrust resulted in the formation of the Ionian–Gavrovo foreland and acted in tandem with the newly formed Gavrovo thrust within the basin until the Late Oligocene. The Gavrovo thrust consists of segments, showing that out-of-sequence thrusting was important. Thrust nucleation and propagation history is strongly influenced by normal faults formed in the forebulge region of the Ionian–Gavrovo foreland basin. Shortening rates within the Gavrovo–Ionian foreland are low, about 1 mm/year. Although thrust load played an important role in the formation of this basin, the additional load of 3500 m thick clastics in the basin enhanced subsidence and underthrusting.