Introduction

Based on geological, geophysical and seismological data we propose an Excess Mass (E.M.) and Excess Mass Stress (E.M.S.) model for the Aegean Region. Excess Mass ascends obliquely at a present vertical rate of about 1.7 cm/yr. Due to gravity sliding and the gradual lowering of the angle of ascend, from ~60⁰ in the north to less than 30⁰ in the south, its horizontal velocity from ~1cm/yr in the north, reaches a value of more than 3 cm/yr in the south. The eastward lead of Africa, relative to Europe, is responsible for the anticlockwise rotation, by ~45⁰ of the Hellenides, and for the change of the direction of E.M.S. from ENE in the north to NW in the south. The Aegean Sphenohasm, the Hellenic Sphenopiezm and the Hellenic Orocline are the products of those interactions.

Ophiolites and andesites are manifestations of E.M. The high Q in mainland Greece and along the Hellenic Arc, the low Q roots in northern and southeastern Aegean Sea, the medium P/T metamorphism in the north and the high P/ low T in the south, the dextal strike -slip faulting in the North Aegean Trough, the overthrusts in the external Hellenides, in Crete and Rhodos islands, the normal faulting along and inside the Hellenic Arc, the north and south Aegean Benioff zones, the sinistral rotations in the south Aegean are manifestations of the E.M.S.

Geological and Geophysical Setting

The Aegean Region (34-42⁰ N, 19-29⁰ E) includes mainland Greece, western Turkey and the Aegean Sea. It is part of the Alpine - Himalyan - Melanesian mountain belt and of the Tethyan Torsion zone (Carey 1983). Its seismicity is the highest in Europe and of the higher in the world, lagging only to Japan and to some other areas of the circum - Pacific belt.

From east to west about ten geotectonic zones are recognized in the Aegean Region (Fig. 1). The Rhodope Massif, the Serbo-Macedonian Massif and the Axios - Vardar zone make up the Precambrian crystalline basement and Paleozoic to Mesozoic metamorphic and sedimentary rocks with magmatic intrusions and ophiolites (i.e., Chalkidiki).

The Jurassic to Eocene rocks (metamorphic , sedimentary, ophiolites) of the Pelagonian and sub-Pelagonian, the Parnasos-Giona, and the Olonos-Pindos zones, comprise the eastward low dipping Central Hellenic Nappes. They cover the central part of mainland Greece and Peloponnesos. Through Lesvos island in the north, Crete and Rhodos islands in the south, these zones extend into western Turkey.

The West Hellenic Nappes include the Gavrovo-Tripolis and the Ionianzones, and occupy the westernmost part of mainland Greece. The lower Miocene sediments and the flysch of the Ionian zone, in mainland Greece, Crete and Rhodos islands, consist the sedimentary Hellenic Arc.
 

Fig. 1 Geotectonic zones of the Aegean Region (from Jacobshagen 1977).

Up to Eocene, about three orogenic episodes are recognized. They associate with ophiolites and overthrusting and refer to the internal flanks of the Hellenides, with decreasing age toward the west. The first in upper Jurassic, ~150 m.y. ago, the second in late Cretaceous, ~80 m.y. ago, when the Pindos zone become a trough, and the third in early Paleocene, ~ 60 m.y. ago (Jacobshagen 1977).

The deformation rate successively increased in the early Eocene and in the upper Miocene episodes, that is ~50 m.y and ~10 m.y. ago. During that period the Pindos, the Gavrovo, and finally the Ionian zone were folded and uplifted. The NNW-SSE trending Hellenides and their eastward dipping nappes were formed, along with the calc-alkaline volcanics. On mainland Greece the southern limit of that episode is the ENE-WSW trending line that connects Saros Gulf, Pilio and Cephalonia island.

Andesitic volcanoes in northern Greece, and in the islands of Samothraki, Imbros, Limnos, Skyros, Lesvos, Chios, Samos, etc, are of lower Eocene to upper Miocene, (~50 m.y. to ~10 m.y.), in age. In the south the volcanoes of the inner volcanic arc, i.e., Lixades, Antiparos, Kos are of middle to upper Pliocene in age (~4 m.y. old). The activity of the andesitic volcanoes of the outer arc (i.e., Sousaki, Methana, Aegina, Poros, Milos, Santorini, Nisyros) extends to present time. Both start and finish at the same points, giving that way a measure of the extension of the south Aegean crust for the last 4 m.y., which is about 1.5-2.0 cm/yr in the NE-SW direction. Ophiolites associate with medium P/T greenschist metamorphism, while the andesites in the south Aegean, mainly with the high P and low to medium T blueschists (glaucophane schists), i.e.,

P = 13 kbar (~45 km); and,
T = 400 - 450 ⁰C  (Altherr and Seidel 1977, Dixon. 1977, Papadopoulos 1982).

In northern Crete there is a considerable subsidence during early Pliocene (~6 m.y. ago). As a result an erosional contact, with the late Miocene calcareous successions, was formed. In southern Crete the Miocene-Pliocene boundary seems to be fairly continuous (Meulenkamp et.al. 1977). K/Ar age of white micas in various types of schists in western Crete was found to be between 39 and 17.6 m.y., while in eastern Crete from 21.5 to 16 m.y. old (Seidel et.al. 1977). Similarly the K/Ar ages for the ophiolites in central Crete are found to be of middle to lower Cretaceous, that is an average age of about 93 m.y. , while in eastern Crete are of Eocene age, that is about 50 m.y. old (Delaloys et.al. 1977).

It is interesting to note that in the Aegean Region the age, the temperature, and the iron content of rocks decreases, westward in the north, southward and eastward in the south.

Crustal thickness varies from ~45 km along the western Hellenides and western Peloponnesos, to a minimum of ~20 km north of Crete, in the Cretan Sea Trough. It is ~25 km in the North Aegean Trough, and ~33 km in the eastern part of the Aegean Region. In eastern Mediterranean and west of Peloponnesos, in the Ionian Sea the crust, in certain locations, is no more than 20 km thick. The velocity of P waves in the crust is 6-6.8 km/sec, while in the upper mantle is ~7.9 km/sec (Makris 1977, Panagiotopouls et.al. 1998). Crustal thinning in an area implies present and/or past oceanization of continental crust as a result of E.M. ascend.

Similar to crustal thickness other geophysical parameters, i.e., gravity and magnetic anomalies, are parallel to the strike of the morphological units in both the northern and the southern regimes that are separated by the Saros, Pilio, Cephalonia line. For example the Bouger anomalies have their minimum values, about--140 mgals, in the Ionian and Gavrovo zones. They acquire positive values along the Pelagonian zone, and following the trend of the Hellenides and of the volcanic arc, reach a maximum of +175 mgals in the Cretan Sea, and remain positive throughout the Aegean. In the North Aegean Trough the Bouger anomaly is about +75 mgal. Northward the trend of Bouger anomalies becomes E-W. Outside the arc, in eastern Mediterranean, Bouger gravity anomalies are positive, up to +200 mgals (Makris 1977, Lagios et.al. 1994, Tsokas and Hansen 1997). This is an indication of present and/or past density differences.

On the other hand the free-air gravity anomalies are positive inside the Hellenic Arc. Outside, in the Hellenic Trench, they are, in general, negative with a minimum of -250 mgals (Fleischer 1964, Woodside and Bowin 1970). The implication is that variations from the spheroid-ellipsoid, as they are expressed by the free-air gravity anomalies and are observed in the whole Aegean Region and only locally in eastern Mediterranean, indicate recent rise of excess mass.

Along the western Hellenides and Peloponnesos the magnetic field is undisturbed. Strong magnetic anomalies, about ± 500 g , are observed in the eastern Greece, and coincide with ophiolites, andesitic volcanics and mineral occurrences. Inside the arc the greatest value, about +250 g , has been observed along the North Aegean Trough. Outside the Hellenic arc the magnetic field is undisturbed. (Morelli et. al. 1975, Makris and Tö dt 1977).This is an indication that the iron content of E.M. in the north Aegean is greater to that in the south Aegean.

Makris and Möller (1977), although in favor of the allochthonous origin of the Chalkidiki ophiolites in northern Greece, question the possibility of an old ocean to have existed, and they do not exclude the possibility of their autochthonous origin. It is also interesting to note that the ophiolites in SW Turkey are from ~50 to ~130 m.y old (Yilmaz et.al. 1982)

Heat flow values in the volcanic arc are £ 2 HFU. The highest heat flow, > 2 HFU, has been observed in the North Aegean Trough, implying a more basaltic, warmer, magma in the north, in compliance with magnetic data. Outside the Hellenic arc, in eastern Mediterranean, heat flow values are very low, i.e., 0.7 HFU (Jongsma 1974, Fytikas and Kolios 1979, Papazachos et.al. 1998). Heat flow data indicate the absence, at present, of massive ascend of mantle material outside the Hellenic Arc.

The E.M. and E.M.S. model

The isolines of macroseismic intensities, from 12 strong intermediate depth earthquakes (6 £ M_s £ 8.3) that occured in the south Aegean, indicate a high Q-low attenuation zone along the external part of the Hellenic Arc, and low Q-high attenuation in the internal part of it (Tassos 1984). Hashida et.al. (1988) calculated, for the crust and upper mantle, high Q values (~1000) in mainland Greece and along the Hellenic Arc, and very low Q values in the northern and the southeastern parts of the Aegean Sea (~40). They also found the absence of low Q roots below the south Aegean volcanic arc (Fig. 2). A LVZ between Crete and Rhodos has also been found by Tassos et.al. (1989). Panagiotopoulos et.al. (1998) observed that the attenuation, in the crust and the upper mantle, increases southward. The high Q could be a result of closer packing caused by the gravitational compression of the ascending, from the north and moving toward the south, excess mass. The southward increase of dumping indicates its primary dependence on the velocity of flow of the uppermost mantle, which increases in that direction, and not on temperature which rather decreases, as the heat flow values indicate.

In the Aegean Region, four seismogenic zones are recognized. The first is that of the External Hellenic Arc or External Hellenic Orocline. The orocline is a term of greek origin, implying ‘bending mountain’ (Carey 1976). It is the ductile deformation equivalent to strike-slip fault in brittle deformation.
 

Fig. 2 The attenuation structure of the Aegean Region, determined by inversion. Where H=High Q, and L=Low Q areas (from Hashida et.al. 1988).

Along the Hellenides and the Hellenic Orocline the stress field is tensional, in the E-W direction, and the faults strike N-S (Fig. 3). We propose that this field is the manifestation of the anticlockwise rotation of the Hellenic Arc, by ~45⁰, the axis of rotation being on Cephalonia island. This rotation is the result of the Tethyan sinistral torsion, that is of the lead of Africa relative to Europe by ~1 cm/yr. This rotation is responsible for the Hellenic sphenopiezm (term of Greek origin, meaning wedge shaped closing, Carey 1976), and for the northward movement of Crete island. Because of the Tethyan Torsion the direction of the ascending E.M. changes by about 105⁰, from ENE to NE, to N and finally to NW in the southeast Aegean Sea. This interpretation is supported by satellite data that indicate a clockwise rotation by 25⁰ the last 5 m.y. in the Pindos zone (Kalkani 1982). It is also supported by paleomagnetic data that indicate dextral rotations in the north, up to 50⁰, that reduce under the North Aegean Trough and become sinistral in the south, with a maximum in the southwest Aegean Sea (Kondopoulou 1997).

Inside the sedimentary arc, in the volcanic arc of the south Aegean, in central-west and northwest Turkey, in central and northern Greece, and in south Yugoslavia and Bulgaria, the stress field is tensional in the N-S direction (Fig. 3).

In the Aegean, an arc of about 75⁰, covering an area of about 250,000 km² and connecting Cephalonia, Crete and Rhodos islands, is formed. The angle difference of 30⁰, relative to the 45⁰ of the sinistral rotation, corresponds to an area of about 100,000 km² and to the extension caused by the excess mass, that ascends, at an angle of ~60⁰, from the ENE. The rising mantle diapir increases the ductility of the crust and causes its doming and its dilatation. It is also responsible for the oceanization of the continental crust of the area. In Carey’s terminology the Aegean can be characterized as a developing sphenochasm (Greek, wedge shaped opening), since at present only about 40% of its crust is new.

Finally a dextral strike-slip fault zone, in the north Aegean, is the continuation of the Anatolian fault. It enters mainland Greece, abruptly stops, and reappears in the Ionian islands, meets with the Cephalonia strike-slip fault and stops there (Papazachos et.al. 1998). Normal and reverse faults also exist in that zone. The plate tectonics interpretation is that the strike-slip motion is a result of the westward movement of the Anatolian plate and of the fast SW movement of the Aegean plate.

The combination of gravitational sliding and of increased ductility can increase the deformation rate of the crust. If the observed horizontal component of deformation velocity is of the order of 1 cm/yr in the North Aegean Trough (Papazachoset.al. 1998) and the dip of rising E.M. is ~60⁰, then the rate along the dip will be ~2 cm/yr and the ‘net’ uplift rate, in the vertical axis, should be ~1.7 cm/yr, plus the ~2.6 cm/yr, which is the estimated current rate of Earth’s radius increase.
 

Fig. 3 Geographic distribution of the stress field and deformation velocities in the Aegean Region (from Papazachos and Kiratzi, 1996). The gray area is to denote the E-W trending tensional field.

During the Eocene, about 50 m.y. ago, when the radius of the Earth was about 5300 km, the ascend rate of E.M. and the intensity of the Tethyan torsion increased. The manifestations of E.M. and E.M.S. of that episode are the andesitic volcanoes in north and central Greece and Aegean islands, the nappes along the Pindos zone, the dextral strike-slip fault and the intermediate depth earthquakes of the North Aegean Trough (Papazachos and Comninakis, 1978).

About 10 m.y. ago the radius of the Earth is estimated being about 6,120 km. The land-ocean distribution is well established, and the eastward lead of the ‘oceanic’ southern Hemisphere relative to the ‘continental’ northern Hemisphere is accelerated. The accretional episode that started then, and goes on today, is mostly responsible for the geological and the geophysical properties of the Aegean Region. The south Aegean Volcanic Arc was formed, the eastward bending of the Hellenic Orocline, due to the increased ductility of the area, was completed. The overriding of the south Aegean on the Mediterranean, due to gravitational sliding, produced thrust faulting and a series of low dipping (25⁰-30⁰) Benioff zones. At depths ³ 100 km their dip increases to ~45⁰ (Scordilis, et.al. 1998).

The E.M. model can very well explain the stress field in the Benioff zone of the south Aegean Sea. According to Papazachos et.al. (1998), for earthquakes with depths between 60 and 100 km the faulting is strike slip, with a strong reverse component. The compressional axis is parallel to the strike of the Hellenic Arc, while that of tension is parallel to the dip of the Benioff zone (Kiratzi and Papazachos 1995). In our model the Benioff zone is the result of the overthrustig of the Aegean on the Mediterranean and not of the underthrusting of the Mediterranean under the Aegean.

Support to the proposed model is provided by the geometry of the south Aegean Benioff zone that indicates a westward, southward and eastward advancement (in Scordilis et.al. 1997). In the island of Crete, seismotectonic investigations show that the horsts and the grabens of the island behave as isolated ‘tectonic dipoles’. They indicate a west coast uplift, by ~9 m, and a subsidence of the east side, and an increase of the thickness of the seismically active zone toward the east. The normal N-S faults of the west coast of Crete exhibit a strong anti-clockwise motion. On the eastern coast, the strike of the faults is again N-S, but the movement is dextral. Seismic energy flux data indicate that the tilting of Crete takes place about a horizontal, Heraklio-Tymbaki, axis (Delibasis et.al. 1982).

The geotectonic field of Crete could be the result of the NE-SW-SE extension of the Aegean and of the SW-NE compression, south of Crete, that causes its uplift from the SW to NE. The Heraklio-Tymbaki line could mark the 4 m.y. ago in time, and in space the change in the direction of E.M.S. towards the east, thus the low Q zone between Crete and Rhodos. The strike-slip motion is sinistral, as a result of the eastward lead of Africa relative to Europe. It coexists with reverse faulting in the external part of the Hellenic Orocline, and N-S normal faulting and anti-clockwise rotation in the western coast of Crete. During the last 4 m.y. the ascending E.M. was forced to move eastward, against the inner, concave, part of the sedimentary arc. At present it has reached Rhodos island, and acting synergistically with the SW-NE closing, causes its uplift, from south to the north, by about 1 mm/yr, (Pirazzoli et.al. 1982). As a result of that the faulting, in the eastern coast of Crete, is again normal and N-S, but the rotation is clockwise, due to the eastward turn of excess mass.
 

Fig. 4 A deformation model for the Aegean Region from Geodetic and Seismological data (from Papazachos 1998).

In our reasoning horizontal crustal movements are a result of radial extension and of drag exerted by the sliding excess mass. That happens without any appreciable decoupling between the overlying and the underlying layers. The gaps caused by the extension of the crust are filled with vertical columns of ductile mantle material, that is excess mass. Complete decoupling can only take place in the surficial process of dècollement, and it is the extreme expression of gravitational sliding. In another paper we have shown the impossibility of complete decoupling between the lithosphere and the asthenosphere, of the ridge-push/trench pull-mechanism, and/or of thermal convection.

Fig. 5 is an idealized representation of the proposed E.M. and E.M.S. model for the Aegean Region. It is a refined version of the models proposed by the late Kiskyras (1982) and by Tassos (1982, 1983_{a, b}). The Aegean Region is considered as a developing sphenohasm, and the external Hellenides an orocline, that is a product of the Hellenic sphenopiezm which suffered a 45⁰ sinistral rotation.

Excess mass ascends obliquely, through, a NE Aegean Sea-SW Turkey, channel, at an angle of ~60⁰ and at a vertical rate of ~1.7 cm/yr. Due to gravitational sliding the horizontal velocity of the crust and of the uppermost mantle increases, from ~1 cm/yr in the north to more than 3 cm/yr in the south Aegean Sea. The gaps caused by the surficial extension are filled by vertical, ‘warmer’, columns of ophiolites in the N Aegean and SW Turkey, and ‘colder’ columns of andesites in the S Aegean. Due to the Tethyan Torsion the direction of flow of E.M. in the Aegean gradually changes from NE to NW.

Thus the low Q roots in the north Aegean, and between Crete and Rhodos. Thus the absence of low Q roots under the volcanoes of the south Aegean Sea. Thus the strike-slip and reverse faulting in the south Benioff zone. Thus the southeastward increase of the thickness of the seismically active zone of intermediate depth earthquakes. Structurally the north and the south Aegean Sea troughs mark the boundaries of the doming, caused by E.M., in the Aegean Region.

Cephalonia island marks the 10 m.y. ago in time, and in space the axis of sinistral rotation by ~45⁰ of the Hellenic Orocline, which started, but at much slower than its current rate, ~150 m.y. ago. Thus the rotation by about the same angle of the extension of the Anatolian Fault in Cephalonia island.

The north Benioff zone is a direct result of E.M., while the south Benioff zone, for h £ 100 km, is a result of gravity sliding and of eastward torsion. The implication is that in the north Benioff zone, and in the south Benioff zone for h ³ 100 km, gravity is absent as a principal stress and the faulting should be reverse and oblique. If the maximum principal stress is parallel to the zone and the minimum at right angle to it, then the faulting should be near vertical or near horizontal. Strike-slip, en echelon, faulting with a strong reverse component should dominate in the south Benioff zone for h £ 100 km.

The lower frequency of both seismic and electromagnetic waves (heat-infrared radiation) in the crust and upper mantle, can be the result of rarefaction of matter. The first contact of the rising E.M. occurs in the N Aegean and in SW Turkey.
 

Fig. 5 Idealized, Excess Mass (E.M.) and Excess Mass Stress (E.M.S.), model for the Aegean Region.  a)  horizontal plane, and,  b)  cross section across the 25⁰ E meridian, extending from south of Crete to the coast of northern Greece. H=High Q, L=Low Q.

Due to gravity sliding excess mass moves southward with increasing velocity and cools through conduction. The temperature and the heat flow should be higher in the N Aegean and SW Turkey and lower in the south Aegean, and the composition of rocks should be more mafic and more felsic, respectively. In areas of increased flow Q is low. Gravitational compression results to high Q.

In summary, geotectonic and geophysical data indicate that the stress field in the Aegean Region is the vertical force of E.M., its horizontal components in the ENE to NW directions and the E-W Tethyan Torsion, and not the horizontal forces between the various microplates in the N-S and E-W directions.

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