Geotectonic Significance of the Deep Seismic Zones in the Aegean Area

The most important of these zones is a well defined very active Benioff zone which is associated with the Hellenic arc. This zone has an amphitheatrical shape and dips from the convex (Mediterranean) to the concave (Aegean) part of the Hellenic arc at a mean angle of about 30°. The shallow and intermediate shocks of this zone are produced by thrust faulting, parallel to the dip of this Benioff zone, while the shallow earthquakes in the inner part of the Hellenic arc (Aegean) are produced by normal faulting. The second deep zone is associated with the broader northern Aegean - Marmara region. The shallow part of this zone is active but the deeper part is of very low seismicity. This second zone is interpreted as a Benioff zone in a dying phase of development. These two zones suggest a temporal migration of the deep tectonic activity in the Aegean from about north to south. The generation of shallow shocks as well as of shocks with focal depths larger than normal in the convex side of the Hellenic arc show that this activity has reached the Hellenic trough complex and the Mediterranean ridge. A geodynamic model is proposed to interpret the active tectonic and geophysical properties of the Aegean area. This model consists of a descending slab, of convective currents in the mantle above the slab and of a back-arc expanding lithosphere.

INTRODUCTION

The Aegean area and the Hellenic arc associated with it are of special geotectonic importance. Their basic properties resemble similar properties of large marginal seas and typical island arcs but they also have some other special properties. Due to the smaller dimensions of this area in respect to those of other areas of similar tectonic structure, geotectonic theories can more easily be tested here.

Morphological features of tectonic significance, associated with the Aegean area, from south to north, are the eastern Mediterranean ridge, the Hellenic arc and the northern Aegean structure.

The Mediterranean ridge, called by Finetti (1976) the Mediterranean Chain and by Stride and his colleagues (1977) the Hellenic Outer Ridge, is a swelling of the eastern Mediterranean crust parallel to the Hellenic arc.

Between the Mediterranean ridge and the Hellenic arc a system of linear trenches (Hellenic trench, Stravo trench, Pliny trench) and small troughs occurs.

It is called the Hellenic trough complex or Hellenic trench because it resembles the trenches associated with island arcs. Its depth reaches 5 km.

The Hellenic arc consists of the outer sedimentary arc, which is a link between the Dinaric Alps and the Turkish Taurides, and the inner active volcanic arc. Between the sedimentary and the volcanic arc the Cretan trough exists with a maximum water depth of about 2 km.

An interesting morphological feature in the northern part of the area is the northern Aegean trough with maximum water depth of about 1.5 km.

Most of the geophysical properties in the inner (Aegean) part are different from those in the outer (Mediterranean) part of the Hellenic arc. High heat flow (Jongsma 1974), magnetic anomalies (Vogt and Higgs 1969), predominantly positive free air anomalies (Allan and Morelli 1971) and inefficient transmission of seismic body waves (Papazachos and Comninakis 1971) have been observed in the Aegean. On the contrary, low heat flow, undisturbed magnetic field, negative free air gravity anomalies and efficient transmission of body waves have been observed in the convex side of the Hellenic arc.

The crustal structure is continental in the broader Aegean area (Papazachos et al. 1966, 1967, 1969; Makris 1973) except for the Cretan trough; where the crust has an atypical thickness of about 20 km (Payo 1969, Makris 1973). The mean crustal thickness in the eastern Mediterranean is also about 20 km (Payo 1969; Papazachos 1969) but the opinions about the existence of a granitic layer are controversial (Comninakis and Papazachos 1977).

Several attempts have been made to explain the deep tectonics of this area (Ritsema 1969; McKenzie 1972; Boccaletti et al. 1974; Papazachos & Comninakis 1976; Stride et al. 1977). Although these attempts have thrown much light on the matter, several of the opinions expressed in the above mentioned papers are controversial and many basic problems are still unsolved.

In the present paper large samples of old and new data which concern the distribution of the earthquake foci and recent data of focal mechanism have been used to further investigate the deep tectonics of the Aegean area.

These data are of known accuracy, complete and homogeneous to a satisfactory degree. A geodynamic model is proposed to interpret these seismic data and other geophysical properties of the Aegean area.

DISTRIBUTION OF SEISMIC FOCI

The data concerning the distribution of the earthquake foci can contribute much to the understanding of the deep tectonic process. The same data, however, can be misleading, if certain properties of these data (accuracy, homogeneity, completeness) have not been carefully examined.

In the present study, only data concerning the spatial distribution of the earthquakes having occurred since 1911, when the first reliable seismometer was put into regular operation in Athens, have been used. Almost all these data have been taken from a recently revised catalogue of earthquakes (Comninakis and Papazachos 1977b). The accuracy, homogeneity and completeness of this catalogue has been carefully examined. Some additional data concerning small shocks of intermediate focal depth have been taken from the Bulletin of the ISC.

The errors of the epicenters and focal depths of the earthquakes, for which data are used in the present study, are less than 35 km in more than seventy per cent of the cases.

Figure (1) shows an epicenter map of the earthquakes with intermediate focal depth (h ≥ 60 km) in the Aegean and the surrounding area. Twelve symbols have been used to denote three ranges of focal depths, four ranges of magnitudes and four sample periods. The completeness of the data for each sample period and for the corresponding magnitude ranges has been proved by plotting the frequency distribution of the magnitudes. Data concerning earthquakes or small earthquakes (M < 4.5) of the period 1964 - 1974 have been used. We have, however, excluded the data refering to small shocks with magnitudes about 4 if the number of stations which have recorded each of these shocks is smaller than 15.

Figure (2) shows a plot of the focal depths, h, versus the approximate distance, x, of the epicenters from the isodepth of 150 km which almost coincides with the southern Aegean volcanic arc (Nisyros - Thera - Milos - Methana).

Five symbols have been used to denote three ranges of magnitudes, three sample periods and two kinds of focal mechanisms.

Complete data of shallow shocks (h < 60 km) with M ≥ 5.0, and for the period 1964 - 1974, have been used for this plot. This sample is sufficient to show the vertical distribution of the shallow shocks. In the case, however, of the intermediate shocks (h ≥ 60 km) complete data of three sample periods have been used. Intermediate shocks with M ≥ 5.5 for the period 1911 - 1974, with M ≥ 5.0 for the period 1950 - 1974 and with M ≥ 4.0 for the period 1964 - 1974 have been plotted.

We have used such large and complete samples of data for the intermediate shocks to determine the deep seismic zones as accurately as possible. Black circles, in this figure, show shocks occurring in areas of horizontal or almost horizontal compression (thrust faulting), and white circles show shocks in areas of horizontal or almost horizontal tension (normal faulting).

From figures (1) and (2) we can conclude the following:

First, a well defined very active Benioff zone is observed in the southern Aegean. This zone has an amphitheatrical shape and dips from the convex (Mediterranean) to the concave (Aegean) part of the Hellenic arc at a mean angle of about 30°.

Second, a less active and much scattered deep seismic zone is observed in the northern Aegean - Marmara region. The shallow activity of this zone is considerable but it decreases rapidly with depth. At depths larger than 100 km only very small shocks occur. It is possible that this is a Benioff zone in a dying phase of development, and in such a phase is at present the deep tectonic process associated with this zone. There is magmatic and other geological evidence supporting the opinion that the tectonic activity in the northern Aegean and the surrounding area has been very intense in the past.

Third, some intermediate shocks have been observed in the outer part of the Hellenic arc up to the Mediterranean ridge. This, probably, indicates a deep tectonic activity in its initial phase of development.

According to the above-quoted interpretation of the deep seismic zones, a temporal migration of the tectonic activity from about north to south is observed in the Aegean area. This is in accordance with similar migrations in other marginal seas (Coleman 1975).

DIRECTION OF STRESS COMPONENTS AND FAULTING

Reliable fault plane solutions of earthquakes can give useful information about the direction of tectonic stress components and the kind of faulting at the foci of earthquakes. Theoretically, the P and T axes of the fault plane solutions give the directions of the maximum compression and maximum tension, respectively, if the medium at the focal region is homogeneous. It has been shown that, in practice, the stress directions, determined by this method, are in good agreement with the stress directions determined by other methods or expected from other geological and geophysical evidence. We will present here further evidence which supports this conclusion.

The axis A or C of the fault plane solutions show the directions of slip on the seismic faults. When P is horizontal and T is vertical the faulting is reverse (thrust), and when T is horizontal and P is vertical the faulting is normal (gravity).

When the two axes (P and T) make equal angles with the horizontal plane, the faulting is strike-slip. For other orientations of P and T axes, the faulting is a combination of the above mentioned kinds of faulting.

The best available fault plane solutions have been used to determine the stress pattern in the Aegean and the surrounding area. Figure (3) shows the directions of the P axis (compression) and of the T axis (tension), which are based on the fault plane solutions of the shallow (h < 50km) shocks (Papazachos & Comninakis 1976). The number close to each symbol is the last two figures of the year of occurrence of the corresponding shock. Thick symbols mean that the corresponding axis is horizontal or almost horizontal and that the difference between the plunges of the two axes is larger than 20^o. Thin symbols mean that the corresponding axis makes a smaller angle with the horizontal plane than the other axis makes, but the difference between the two axes is smaller than 20°.

It is observed that, along the external (convex) side of the Hellenic arc, the stress field is compressional (thrust faulting), while, in the inner part of the Aegean area, the field is tensional (normal faulting). However, in the northernmost part of the Aegean - Marmara area the compressional field reappears.

Horizontal compression is also exerted at the foci of the earthquakes with intermediate focal depth in the inner part of the Hellenic arc. The thrust faulting remains always parallel to the dip of the Benioff zone which dips from the Mediterranean to the Aegean area.

INTERPRETATION

On the basis of the above mentioned results, a geodynamic model is proposed to interpret the basic geophysical observations in the Aegean area (fig. 4).

This model consists of a lithospheric slab, called the Mediterranean slab and being part of the African lithospheric plate, a back-arc broken lithosphere, which is the lithosphere under the broader Aegean area, and a low Q region between these lithospheres where convection takes place in a manner suggested by Sleep and Toksöz (1971).

A direct result of the interaction of the two lithospheres (Mediterranean - Aegean) is the occurrence of earthquakes along the convex side of the Hellenic arc by compressional mechanism (thrust faulting). This convergence is responsible for the occurrence of the Mediterranean ridge (Finetti 1976). It is not easy to answer the question whether the Mediterranean or the Aegean part is active.

It is probable that both are active. Due to northward motion of the African plate its front oceanic part descends under the Eurasian at a rate of about 2.5 cm/year in this region. This rate which has been found by dividing the length of the Benioff zone (~ 250 km) by 10 myr (Papazachos and Comninakis 1971) is in accordance with the rate expected there from magnetic observations (Le Pichon 1968).

However, the compressional field due to this convergence of the two big plates is strongly modified along the convex side of the Hellenic arc (fig. 3), becoming everywhere perpendicular to the arc. This modification can be caused by the active outward motion of the southern Aegean in accordance with the opinion of Stride and his colleagues (1977).

The existence of a slab, dipping from about south to north under the Aegean, is not only indicated by a well defined Benioff zone but also its dimensions have been determined by measurements of P-wave travel time residuals (Gregersen 1977). As this slab goes down, friction along its top surface produces heating and the slab becomes warmer near its top surface. Due to gravitational pull shear zones are generated within the slab. The warmer parts of the slab, near its top surface, tend to rise while the colder parts of the slab, farther away from the top surface, tend to sink. This explains the generation of intermediate earthquakes by thrust faulting along the dipping Benioff zone.

The frictional heating at the top surface of the Mediterranean slab or the hydrodynamic forces caused by the sinking of the slab induce convection currents in the soft asthenosphere between the top surface of the Mediterranean slab and the back-arc Aegean lithosphere. These hot materials (low Q, and low v) are responsible for the inefficient transmission of the short period body waves (Molnar and Oliver 1969; Papazachos and Comninakis 1971) as well as for the reduction of the wave velocity in this asthenosphere relatively to the velocity in the slab (Agarwal et al 1976; Gregersen 1977).

The convective cells exert horizontal forces at the bottom of the back-arc southern Aegean lithosphere and force it to expand. Under the action of these forces the Aegean lithosphere breaks and hot material intrudes into it. This results in volcanic activity, high heat flow, magnetic anomalies, modification of crustal structure, subsidence of crustal blocks and generation of shallow earthquakes in the Aegean lithosphere by tensional mechanism. These convective cells also drag the southern Aegean lithosphere to the Hellenic trench.

The extensional properties at relatively large distances from the descending slab to the north can be attibuted to secondary convection centers such as those expected by the theory suggested by Toksöz (1975). These tensional stresses gradually give way to compressional in the northernmost part of the Aegean - Marmara region (fig. 3).

The occurrence of small shocks at depths larger than normal in the northern Aegean area can be attributed to remnants of an old lithospheric slab which are still assimilated in the mantle, and the northern Aegean trough can be considered as the remnant of an old back-arc marginal sea.

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