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The Aegean micro-plate as a southwestward drifting CCW rotating lithospheric vortex. The model’s implications, to the spatial seismicity pattern of the Aegean area.

(rotational velocity calculation - last update 1st November 2005)

By

Thanassoulas¹, C., and Klentos², V.

IGME, Open File Report, Library Code No: 9907, 2005

1.   Institute of Geology and Mineral Exploration (IGME),  Department of Geophysical Research,  70, Messoghion Ave., 115-26, Athens, Greece.   e-mail : thandin@otenet.gr, URL:  www.earthquakeprediction.gr

      2.   Athens Water Supply & Sewerage Company (EYDAP), e-mail : klenvas@mycosmos.gr , URL: www.earthquakeprediction.gr

 ATHENS, JULY - 2005

ABSTRACT

The kinematics of the Aegean micro-plate is considered in relation to the forces acting upon it, through the African and Anatolian plates’ motion. The net result of these forces is a counter clockwise (CCW) rotational motion of the Aegean micro-plate combined to a SW drifting component, as far as it concerns the internal Hellenides, while a clockwise (CW) rotation is considered applicable to minor tectonic blocks for the external Hellenides. 

The physical-mechanical model postulated justifies the presence of different morphological, tectonic, volcanic, seismological and geophysical (paleomagnetic) features that have already been observed and studied by many researchers. 

Moreover, the kinematics rotational postulated model, through its tectonic implications, justifies the location and presence of the already known geothermal fields, mineralization deposition at various sites and concentration of radioactive elements at places.

Key words: Aegean plate, kinematics, paleomagnetics, gravity, tectonics, seismology, geothermal fields, mineral deposits.

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1.  INTRODUCTION

The plate tectonics theory, developed on early sixties, has been validated and adopted by the majority of the scientific community. The majority of the geological phenomena observed on the surface of the earth are due to its relative to each other movement caused by the heat convection currents located underneath. Due to this heat convection mechanism, the earth’s lithosphere has been divided into a few major plates as shown in the following figure (1).

Fig. 1. The major lithospheric plates of the earth (after USGS).

Kinematics studies of the plates, all over the world, have provided details for the direction of movement of each plate. The later is presented in the following figure (2).

Fig. 2. World plate kinematics (Heflin et al. 2001,JPL, NASA). 

The Greek territory forms part of a major seismic belt - that starts at the Indian Ocean and extends up to the Atlantic Ocean - and is bounded by the African, the Eurasian and the Anatolian plates. Due to the collision of these three plates at the Greek area, the majority of the seismic activity of the eastern Mediterranean area occurs in the Greek territory (Jackson and McKenzie, 1988).

   The earthquakes that occur in Greece are caused by stress release that has been accumulated from the local lithospheric plates motion (Cox, 1973). The focal mechanisms of the triggered earthquakes depend upon the temporary stress conditions of each focal area (compressional – extensional stress field), and therefore normal faulting, strike slip faulting, over thrust faulting or/and mixed type can be the fault plane solution of each triggered earthquake. 

    The Greek territory is compressed from the west by the Adriatic plate, from south by the African plate while from the east is compressed by the Anatolian plate. The combined effect of these compressing forces is that the Greek territory drifts towards southwest (Papazachos et al. 1989, McKenzie 1972, 1978). 

 The lithospheric plates that occupy the Greek regional area (McKenzie, 1972) are shown in the following figure (3). Arrows indicate the corresponding motion of each micro-plate. The Greek area consists of two micro-plates, the North Aegean plate and the South Aegean plate. The North Anatolian Fault separates these two plates.

 Fig. 3. Relative motion of Greek micro-plates after McKenzie (1972).

A detailed study of the movement of the same area has been presented by McClusky et al. (2000). The GPS technique has been applied and the results are presented assuming Eurasia fixed (fig. 4) and Anatolia fixed (fig. 5).

Fig. 4. GPS horizontal velocities. Eurasia fixed (McClusky et al. 2000)

Fig. 5. GPS horizontal velocities. Anatolia fixed (McClusky et al. 2000)

Detailed information and different geodynamic mechanisms have been presented by Doglioni et al. (2002) that apply on the Aegean – Anatolia area.  The key element of this model is that the Aegean – Anatolian plate are subjected to extension and therefore the horizontal velocities observed increase towards the southwest direction.  

In the same study (Doglioni et al. 2002) the main directions and tectonic meaning of Miocene-Quaternary faults in western Anatolia and Aegean, due to the southwest extension, have been calculated and are presented in the following figure (6), while a pictorial presentation of the Aegean – Anatolian extension is presented in figure (7).

Fig. 6. Main directions and tectonic meaning of Miocene-Quaternary faults in western Anatolia and Aegean (Doglioni et al. 2002).

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Fig. 7. Aegean – Anatolian extension. Greece (B) overrides the African plate (A) faster than Cyprus and Anatolia (C) (Doglioni et al. 2002).

  A different point of view, concerning the tectonics of the Aegean plate has been presented by Rotstein (1985). The key element of this model is the concept of “side arc collision”. The term is used to describe the interaction of subducted oceanic lithosphere with continental lithosphere in a subduction arc in which oblique subduction occurs. In the Hellenic arc side arc collision is proposed for the northeast corner near Rhodes. The later is presented in the following figure (8).

  Fig. 8. Simplified sketch of subduction in the eastern part of the Hellenic arc showing the zone of oceanic-continental interaction (side arc collision). Arrow indicates relative plate motion across the subduction arc (Rotstein, 1985).

The southern part of the Aegean plate, particularly the Crete region, was studied by onshore-offshore wide-aperture seismics (Bohnfoff et al. 2001). The depth of investigation reached the crust – mantle boundary. The crust of the Crete region was identified to be continental with maximum thickness of 32.5 Km below northern Central Crete, thinning towards the North and South to 15 and 17 Km.  

   A generalized map of the main tectonic elements of the south Aegean region is presented in the following figure (9).

Fig. 9. Generalized map of the main tectonic elements of the south Aegean region. Gray shades indicate 1000 m steps in water bathymetry (Bohnhoff et al. 2001).

The detailed study of the drift velocity of the Aegean micro-plates (De Bremaecker et al. 1982) indicated that the southern Aegean part drifts faster than the northern one. This is presented in the following fig. (10).

Fig. 10. Drift velocity finite element model for the Aegean area after De Bremaecker et al. (1982).

Papazachos et al. (1996) have adopted a different approach for the kinematics of the Aegean micro-plates. The deformation velocity of the regional seismic sources has been estimated and presented in a map form in the following fig. (11).

Fig. 11. Greek seismic sources deformation velocities, after Papazachos et al. (1996).

The main character of this map is the large deformation values observed at the southern Aegean plate, where large compressional forces control the stress field, while at the northern Aegean micro-plate are observed smaller deformation velocity values along with extensional forces.

   Apart from the previous studies other researchers have studied the regional Aegean area and Eastern Mediterranean too. Hatzidimitriou et al. (1985) has studied the seismic parameter (b) in relation to the geological zoning of the Greek territory, Papazachos et al. (1986) has studied the seismotectonic properties of the same area, Papadopoulos (1989), Liakopoulou et al. (1991) present the main seismotectonic features of the Hellenic Arc and the Aegean Sea, while Hanus and Vanek (1993) present a different zoning for the seismically active areas of the Greek territory. 

   In contrast to the mainly seismological methods used for the previous studies of the Greek area, Thanassoulas (1998) used gravity data in order to delineate the narrow seismically active zones where large (Ms>6R) earthquakes occur. 

   In this study, that concerns the plate kinematics of the Greek territory, an entirely different approach has been adopted and a completely different physical – mechanical model is postulated. This model complies quite well with other geotectonic observations presented to date and moreover explains some of them that were not very clear or well justified yet.     

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2. THE THEORETICAL MODEL 

   The theoretical model, which is postulated for the Aegean Sea, basically consists of two mechanical sub-models. The first one is the rotational sub-model. In this sub-model the basic driving mechanism adopted is a rotational moment. The second one is the thrust model. The thrust mechanism is adopted and considered to be applicable at the center of the stress-subjected area. 

       These two mechanisms are presented in detail as follows:

 2.1  The rotational moment model. 

    Let us consider a pair of forces F1 and F2 that act simultaneously in opposite direction upon a solid object and at different positions upon it (fig. 12). The result of this force scheme is the development of a rotational moment (P) that tends to rotate the solid body as the circular vector indicates in fig. (14).

Fig. 12. Rotational moment vector (P) developed by a pair of forces F1, F2 that act in opposite direction.

A more complicated case evolves when more than a pair of forces acts upon the same object. This is demonstrated in the following fig. (13).

Fig. 13. Synthesis of total rotational moment vector ΣΡ as a result from different forces F1, 2, 3, 4 in effect, that give rise to moment vectors P13, P23, P34.

The forces F1, 2, 3, 4, randomly oriented in space, can be analyzed into orthogonal components so that finally pairs of forces that can be utilized develop rotational moment vectors, P13, P34, P23, of the same or opposite direction. These moment vectors can be summed up in to a total rotational moment vector ΣP that characterizes the solid object. 

   Next we consider a highly viscous media in which a specified circled area is affected by the forces F1, 2, 3. The total rotational moment developed, following the previous mechanism, forces the central part of it to rotate as the thick arrow indicates.

Fig. 14. Schematic presentation of the rotational mechanical model that is generated by the application of forces F1, 2, 3 on the inner part of the host material (stable area).

As a result of the rotational action, generally, three areas can be distinguished. The first one is the stable area where the effect of the applied forces F1, 2, 3 is null. The second one is the rotating area where the rotating effect has its maximum value. In between these two a third area exists where rather rotation diminishes in value towards the stable area or increases towards the central (rotating) area. This is called the transition zone.

   Furthermore, in the brittle transition zone, the following mechanism can take place. Smaller media blocks are forced to follow a clockwise rotational movement so that there is mechanical motion compatibility between i.e. inner counterclockwise rotational motion and the outer stable area. This is schematically presented in the following fig. (15).

Fig. 15. Schematic presentation of the resulting relative rotational movement of the distinct sub-areas of the previous mechanical model,  (a) stable area, (b) transition zone, (c) clockwise (CW) rotating block, (d) counter-clockwise (CCW) rotating plate.

2.2         The thrust model.

  The next case is the thrust model. Let us consider a media in which an interface (B-B) exists and A-A is its upper surface. If forces 1 and 2 compress both sides in opposite direction, the net effect is an upward or downward movement of each side, depending on the magnitude of the two forces. This is demonstrated in the following fig. (16).

Fig. 16. Schematic presentation of forces (1, 2) applied on movable media, separated by an interface where sliding takes place. Symbols V and h denote the vertical and horizontal components of the forces 1, 2.

As long as the interface B-B is laterally extended, then fracturing lineaments, on the surface of the media, can be observed parallel to the trace of the interface B-B and the media surface. 

   Suppose now that the previous thrust mechanism is applicable at only a narrow restricted area of the media. In this case the following fracturing pattern is generated on the surface of the media due to mainly extensional mechanical reasons. This is schematically presented in the following fig. (17).

Fig. 17. Radial fracture zones developed by an upward stress field applied at the center of the medium.

These two mechanical models, the rotational model and the thrust one, are applicable, as it will be demonstrated in the following, both of them on the Aegean micro-plate. Moreover, these two models interpret many geotectonic observations made to date.

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3.  FORCES APPLIED ON THE AEGEAN MICROPLATE

     The Aegean micro-plates, as a result of the to date available geotectonic studies, are affected by three main forces. The Anatolian (Turkish) plate (TPF) through its southwest movement applies the first one. This force (TFP) moves the Aegean micro-plates towards southwest against and on top the African plate. The African plate subducts the Aegean micro-plates and therefore applies a northeast force AFPF on them. This force (AFPF), partly, opposes the southwest movement of the Aegean micro-plates, while, more or less, the Adriatic plate force (APF) exhibits the same behavior. This schematic mechanism is shown in the following figure (18).

Fig. 18. Schematic presentation of the forces applied on the Aegean micro-plates due to the Adriatic (APF), African (AFPF) and Anatolian (TPF) plates movements.

Kinematic studies of the Aegean plate have resulted to several proposed models. McKenzie (1970) introduced the “rigid blocks” model consisted of three microplates, South Aegean, NW Aegean and Anatolia. A different model consisted by two blocks, eastern and western was proposed by Taymaz et al. (1991). Further more, Le Pichon et al. (1995) proposed the one rigid block model (S. Aegean – W. Anatolia) rotating counter-clockwise.

  Papazachos et al. (1998) has presented a more detailed geotectonic configuration of the regional Aegean area. This is shown in the following figure (19). 

  The main features of this map are the collision fronts of the moving plates, while very important are the values of the velocities observed for their movement.  

   The African plate moves, as it is stated in the map, with a velocity of 10mm/y towards north. 

   The Anatolian plate moves with a velocity of 25mm/y towards west. 

   The Aegean plate moves with a velocity of 45mm/y towards southwest.

Fig. 19. Plate tectonic configuration of the area around the Aegean (Papazachos et al. 1998).

As a result of the previously presented tectonic – kinematic status of the Aegean region and the forces acting upon it, it is justified to consider the generation of a rotational moment in it and, therefore, the mechanical rotational moment - thrust model is applicable in the Aegean area. The validity of this model depends on whether it provides justification of the different geological – tectonic – geophysical and seismological observations made for the Aegean region to date. 

   To this purpose, the postulated theoretical model, in the following part of this study, will be compared to the various existing results from different studies referring to the Aegean region.

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4. MORPHOLOGICAL DATA 

   The morphology of an area depends on the tectonic processes the same area has undergone in the geological past. Orogenesis, faulting, fracturing, compression, extension of the geological units taking place in an area result in its final morphology.  In the reverse order, to some extent, the morphological features of an area are capable of revealing the tectonic processes and generating mechanisms that controlled tectonically the specific area in the geological past. 

   Consequently, the topographic and bathymetric relief of the Aegean region provides the basic elements for its initial tectonic approach. This map is presented in the following figure (20)

Fig. 20. Bathymetric and shallow seismicity map is presented of the Greek regional area (AUTH, 1985).

An overall large-scale inspection of this map reveals the presence of a largely extended circular feature. Part of it, located at the north Aegean region, conforms to the North Anatolian Fault while another part of it, at the southern Aegean region, conforms to the trench located south of the Peloponnese, Crete and Rhodes. In between these two segments there is a gap extending from north Evoia – Biotia and central Peloponnese where this circular feature is just visible due to rough change in the topographic relief.

   In the western part of Turkey this circular feature is directed towards northwest and diminishes in the Aegean Sea. This circular feature merges with the linearly extending Ionian trench close to the northwest part of Peloponnese. The circular and the linear morphological features of the Aegean area are highlighted with a thick dashed red line so that its identification is made easy. The later is presented in figure (21). Directional filtering or pattern recognition techniques can reveal the same tectonic pattern.

 Fig. 21. Main tectonic circular and linear features observed (tectonic axis, red dashed line) in the Greek regional area.

At a first approximation, the circular character of the central Aegean region is revealed. This area is separated from the Adriatic plate, the Anatolian plate and the Eurasian plate at boundaries indicated by the thick dashed red line. The fact that the Anatolian plate moves westwards while the African plate in respect to the Aegean plate generates strike slip faults (Bohnhoff et al. 2001) in the southeast part of it, is a first indication for the generation of a wider counter clockwise rotational moment applied on the Aegean plate.

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5.  VOLCANIC DATA 

   Large scale and intense tectonic events that affect the lithosphere facilitate the magma up rise to the surface and consequently of volcanic origin lava manifestations are generated. The motion of the lithospheric plates generally shifts gradually the initial location of the lava flow location. In association to this mechanism is the well known motion of the back arc volcanic arcs.

   In the case of the Aegean plate the lava manifestations have been mapped and dated. These are presented in color code in the following figure (22). Although at a first glance its spatial distribution looks rather not connected to the tectonics of the Aegean plate, if we take into account its dating and combine it to the counter clockwise rotational moment suggested earlier then the following relation outcomes.

   The lava manifestations comply in its generation time from most recent (0-11my) to the older ones (16-26 my) for the central Aegean plate with the exception of the oldest ones located northern from the north Anatolian fault. This is presented in the following figure (23).

Fig. 22. Volcanic manifestations are presented, in Greece (Tacticos, 1999, Thanassoulas et al. 1999), in relation to the deep lithospheric fractures (Thanassoulas, 1998). Color code is as follows: Purple = 0-11my (Quaternary – Pliocene), Brown = 11-16 my (Upper Miocene), Green = 16-26 my (Middle – Lower Miocene), Red = 26-36my (Oligocene).

Fig. 23. Time evolution of the volcanic activity is presented, during the past 36 my. 1 = probable center of the lithospheric vortex, 2 = estimated vortex boundaries, 3 = North Anatolian Fault, 4 = probable outer vortex boundaries (Thanassoulas et al. 1999).

    Assuming a CCW rotation of the southern Aegean plate and the corresponding original lavas manifestations, that have been originated initially westwards, have traveled along (black arrow) a path of almost 500Km. By taking into account an average velocity of 2.5cm/y for the motion of the southern Aegean plate, it results in 20my, in time needed for this travel length. This time span complies well with the dating of the lava manifestations observed in the Aegean plate.  

   Generally, the Aegean plate behaves as a lithospheric vortex (Thanassoulas et al. 1999) following a CCW rotation that modifies the location of the lava flows generated some million years ago elsewhere.

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6. GEOPHYSICAL PALEOMAGNETIC DATA 

  In studying geodynamic models and particularly in its documentation the paleomagnetism is a very effective and powerful tool (Irving, 1964; Beck, 1980; Van der Voo, 1993). Paleomagnetic studies in Greece are dated back to 1968 (Bobier) performed on Pliocene volcanics. This was followed by more intense research in the same topic by Pucher et al. (1974), Papamarinopoulos, (1978), Kondopoulou, (1982), Laj et al. (1982).

   A review paper for the paleomagnetic results obtained in Greece to date was presented by Kondopoulou (2000). Results of Cenozoic age indicate different rotating character along the Hellenic arc (Kissel and Laj, 1988). Clockwise (CW) rotation was found in the west, counterclockwise (CCW) in the east and no rotation in the south. These results comply to other data obtained by Horner (1983), Horner and Freeman (1983), Lovlie et al. (1989), Marton et al. (1990). Independent rotation of blocks, fault bounded has been reported by Mauritsch et al.(1995) for the external zones of Albanides.

   Clockwise rotation (CW) has been identified for the Ionian Islands by Duermeijer and Langereis (1999). For the Crete, Dauermeijer et al (1998) have reported counterclockwise (CCW) for the most of the studied sections of the Tortonian and Messinian (9.7 – 6.7 Ma). Similar rotations (CCW) are reported by Duermeijer and Langereis (1999) east of Crete, in Plio-Pleistocene sediments on the islands of Rhodes, Karpathos, and Kassos.

   Walcott and White (1998), suggest an East Aegean block with prevailing counterclockwise (CCW) rotations. In Eastern Aegean – Western Anatolia strong CW rotation (Karaburun) and strong CCW (Izmir) is observed (Kissel et al. (1986a). Furthermore data from Chios island indicate a CCW rotation since the Middle – Miocene (Kondopoulou et al. 1993a,b). CW rotations are observed along an E-W transverse from the Mesohellenic Trough to the Greek Rhodope (Kondopoulou and Westphal, 1986; Kissel and Laj, 1988; Westphal et al. 1991; Kondopoulou, 1994). Atzemoglou (1994) suggests CW rotation for the area extending from the Strymon valley in the west Xanthi in the East and from Kavala to the Greek-Bulgarian borders. 

   CCW rotations have been reported further to the east in the Pontides, along the North Anatolia Fault and the Biga peninsula (Saribudak, 1989; Platzman et al. 1994). For the eastern side of the Aegean Sonder and England (1989) and Taymaz et al. (1991) propose CCW rotations while Westaway (1990a,b) suggests that most of the domains in W.Turkey rotate in CCW mode. Jolivet (1993) proposes a rotation pole in the eastern part of the Aegean and extension in this area proceeds through a CCW rotation about this pole. Le Pichon et al. (1995), using geodetic data from Greece suggests a CCW rotation that agrees with paleomagnetic results obtained in Chios and Izmit (Kondopoulou et al. 1993a,b)

   In Argolis area paleomagnetic data from Jurassic carbonates indicated CW rotation (Morris, 1995). In Central Greece, data obtained on Triassic formations (sub-Pelagonian lavas) indicated a CW rotation in Tertiary while a CCW rotation before. CCW rotation was also found by Pucher et al. (1974). Sequences of Mid-late Triassic have been studied in the Pelagonian (Morris, 1995) and resulted in a CW rotation. In studying Rhyolites of Upper Carboniferous to Lower Triassic in the Serbomacedonian massif (Turnell, 1988) a CCW older rotation was followed by a CW younger one.

   Summarizing Kondopoulou (2000) all the afore mentioned rotational results concludes that the CCW rotation prevails in the internal Hellenides while CW rotations are applicable for the external Hellenides.

  This concluded paleomagnetic model in practice complies to the rotating mechanical model presented in fig. (15). The internal Hellenides correspond to the CCW (d) rotating plate, while the external Hellenides correspond to the CW rotating blocks (c) located in the transition zone (b). The pair of CCW rotation of the internal Hellenides and the CW rotation external Hellenides must exist so that mechanical rotational compatibility is maintained through out the Aegean area. 

   The fact that CW and CCW rotations are found in discomforming places is attributed to the very local rotations of independent small blocks created by the local lithospheric fracturing.

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 7. GEOPHYSICAL GRAVITY DATA

 7.1.Gravity deduced fracture zones 

     Forces applied on a solid material may produce fracturing of the subjected material and consequently motion of the blocks produced. In a larger scale, i.e. the Aegean plate, forces acting upon it produce different tectonic results as generation of faults, orogenesis, rotation of the different blocks generation of shields, relocation of geological formations just to mention some of them. As a result of this mechanism the gravity field of the earth is modified accordingly and therefore it is possible by inversely modeling of the gravity field to study the tectonics, stratigraphy and generally the mass distribution in 3D form in the study area.

   The methodology of converting the gravity field into deep lithospheric fracture zones was firstly introduced by Thanassoulas (1998). Major faulting produces large gravity field horizontal gradients into the corresponding gravity maps. The procedure is straightforward and consists of transforming the original Bouguer anomaly map, through a transformational operator, into a horizontal gradient map. Peak axes of gradient values denote the existence of a lithospheric fracture zone. The later is presented in the following fig. (24). The operator used required a 10x10 Km grid of original data and its length was 20 Km. Therefore only deep fracture zone are detected, of large wavelength, that mostly are not visible on surface by the geologists and therefore have not been mapped.

Fig. 24. Deep lithospheric fracture zones, mapped by the analysis of the gravity field, in Greece (Thanassoulas, 1998)

The inspection of the map of fig. (24) reveals two groups of deep fracturing. The first one shows a circular character while the second one shows a radial one.

  7.1.1 Circular fracture zones. 

          This group of fracturing is highlighted by the use of thick black dashed lines. It is remarkable the way concentric circles comply with almost all the fracturing of this mode. The approximate center of the circles is located in the central Aegean region while part of the circles expands over western Turkey.

Fig. 25. Circular mode of the deep lithosperic fracture zones in Greece.

    The last trace of a circular fracture zone is detected in the SW area of Ionian Sea, while at the western part of Turkey, fractures are missing due to lack of gravity data in this region.

   The question that comes up immediately is how these circular fracture zones have been generated. This is answered by the rotational mechanical model postulated for the Aegean plate (see fig. 15). As long as the central part of the Aegean plate rotates in a CCW mode then fracturing occurs tangentially along circles of rotation and therefore main fracturing of the lithosphere will occur along the circumferences of these circles. The main cause of the circular fracturing is the differential velocity of adjacent fracturing areas due to velocity change from zero (external stable area a) to some value applicable to the inner rotating plate (d) of the Aegean region.

7.1.2 Radial fracture zones

   The second fracturing group presents a radial mode and is highlighted by solid thick black lines. This is presented in the following fig. (26).

Fig. 26. Radial mode of the deep lithosperic fracture zones in Greece.

    The fracturing pattern indicated in this fig. (26) can be explained in a mechanical way (see fig. 16) by the uplift of the central Aegean region due to some uprising deeply located material. Since the lithosphere up rises there is a radial expansion developing stress that generates this radial kind of fracturing. 

   The cause of such a uprise has already been presented in the tectonodynamics literature (Doglioni et al. 2002). According to Doglioni’s proposed model, the African lithosphere subducts the Aegean plate but in the central Aegean region the African lithospheric slab is folded and upraised producing in this way radial lithospheric fracturing in the Aegean plate. The model proposed by Doglioni et al. (2002) is presented in the following fig. (27).

Fig. 27. Cross-section cartoon showing that Greece lithosphere (B) is overriding Africa fixed (A) faster than Anatolia (C), generating extension between B and C. (Doglioni et al. 2002).

    It is possible that the upraised slab of the African lithosphere undergoes simultaneously a CCW rotational movement too.

   As immediate implications of these type of deep lithospheric fracturing are the generation of a) geothermal fields along these fracture zones/faults shown in fig. (28)

Fig. 28. The known geothermal fields (red circles), in relation to the deep lithospheric fracture zones mapped in Greece (Thanassoulas et al. 1999).

b) hydrothermal manifestations shown in the following fig. (29)

Fig. 29. The known hydrothermal manifestations (red circles), in relation to the deep lithospheric fracture zones in Greece (Thanassoulas et al. 1999).

 and (c) uranium bearing deposits, shown in the following fig. (30).

  Fig. 30. The uranium bearing deposits manifestations (red circles) in relation to the deep lithospheric fracture zones in Greece (Thanassoulas et al. 1999).

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8. SEISMIC DATA 

   The rotating mechanical model postulated for the Aegean plate must be reflected too in the seismicity of the Greek territory. As a first approach the seismic hazard map of Greece (Papazachos et al. 1989) is presented. The main characteristics of this map are that large-scale discrete seismic hazard zones of the Aegean region are concentric. Zone I, of the lowest hazard is located in the center of the Aegean and is surrounded by zone II. The next zone III extends from the Ionian Islands to the Hellenic arc and the west part of Turkey while minor areas distort the more or less circular pattern of the seismic hazard zoning of the Aegean region.

Fig. 31. Seismic hazard map of Greece (Papazahos et al. 1989). Different hatching indicates seismic hazard zoning (I = lowest, IV = highest). 

   A slightly different picture is presented in the lately released (OASP, 2004) seismic hazard map of Greece. In this map only three zones are distinguished but still the first two of the lowest value of seismic hazard retain their circular character even though it is a bit distorted. This map is presented in the following fig. (32).

Fig. 32. Seismic hazard map of Greece (OASP 2004).

    Since the fracturing of the lithosphere follows a tangential pattern in accordance to the circular fracturing following the rotation of the inner micro-plate of the Aegean region, in the same way the seismicity observed in the Aegean area must exhibit a similar pattern. To this end the seismicity of the Greek area has been studied for different time intervals as follows: 

Fig. 33. Seismicity of the 13^th October 2004 registered by NOA in the Greek territory.

   A single day’s seismicity of Greece is shown in fig. (33). At a first glance these EQs seem to have occurred randomly in space. If a circle is considered in the same area (red circle) a meaningful result comes out. These EQs occurred along tangential fracturing at the circumference of the rotating micro-plate of the central Aegean.

   The same test has been applied over a longer time period of 12 days. The results are shown in the following fig. (34).

Fig. 34. Seismicity for the time period of 3^rd October – 15^th October 2004 registered by NOA in the Greek territory.

    It is clear that the seismicity of this time period can be grouped into three circumferences of circles that have approximately the same center.

Fig. 35. Seismicity (Ms >5.0 R) registered by NOA, for the time period of 2004, in the Greek territory.

    In figure (35) the seismicity for an even longer period (a year) and magnitude Ms>5.0R is considered. Similar observations can be made for this case too. 

   The same circular pattern is present in the seismic potential map (Thanassoulas and Klentos, 2003) of Greece. Actually, seismic energy stored in a strain form in the lithosphere must exhibit the same behavior as the seismicity of the same area. 

   Finally, the seismicity for the period of 1950 – 2004, 5.0>Ms>4.5R of the Greek territory has been considered and presented in the following figure (36).

Fig. 36. Seismicity (5.0>Ms>4.5 R) registered by NOA, for the time period of 1950 - 2004, in the Greek territory.

    It is evident that the circular pattern that controls the generation in space of the EQs in the Aegean plate is the rule. The hypothetical main tangential axis, along which the seismicity occurs, is presented by the blue dashed line in the following figure (37).

Fig. 37. The most of the seismicity of the Greek territory evolves along the main axis (blue dashed line) due to CCW rotation of the Aegean micro-plate.

    A final observation on fig. (37) is that intense seismicity occurs along the collision of the Adriatic plate and the Aegean one. This fact generates the linear axis of the seismicity observed at the northwest part of the Greek territory, while towards the Anatolian plate (east of the Aegean plate) northeast-southwest lineaments indicate the presence of intense tectonic elements of the same direction.

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9. THE POSTULATED KINEMATIC AEGEAN MODEL 

   By taking into account all the previous observations and theoretical physical models it is suggested that there is basically a southwestward drift of the Aegean micro-plate. Simultaneously the Aegean plate rotates CCW. This is schematically presented in the following fig. (38).

Fig. 38. Combined main southwestward motion and CCW rotation of the Aegean micro-plate.

    This model is valid for the internal Hellenides, while in the external Hellenides a simultaneous CW rotation is valid for specific peripheral tectonic blocks.

   In terms of the previously presented micro-plate models this movement is presented in the following fig. (39). Minor tectonic blocks of CW rotation are not presented.

  Fig. 39. The Aegean area postulated tectonic model. A, C = southwest movement, B = counter clockwise (CCW) Aegean micro-plate rotation.

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10. CONCLUSIONS

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