On the tectonic processes a long the Hel lenic Arc

A . G . G A L A N O F O U L O S

Received on Sep tember 28th, 1974

SUMMARY. — On t h e g rounds of exis t ing geophysical d a t a one migh t be allowed lo conclude t h a t t h e origin of t h e stress field in t h e Ion ian cen te r (Cepl ia lon ia-Zante-Pa t ras ) a t t h e no r thwes te rn marg in of t he Aegean mic rop la te is r a t h e r shal low and in the sou theas t e rn center (Dodecanese-Crete) is surely u n d e r t he c rus t .

In t he a rea occupied by t h e second cen te r of higher e a r t h q u a k e ac t iv i ty t h e relief of t he Moho-d i scon t inu i ty is shal lower and smoothe r in compar ison to t h a t der ived f rom g r a v i t y and seismic d a t a for t he area of t h e no r thwes t e rn center . A n o t h e r difference der ived f rom the fau l t -p lane solut ions is t h a t t he Ion ian center is seated in a region of ho r i zon ta l pressure ; t h e second cen te r in t he sou theas t e rn Aegean Sea belongs to a region of p a r a m o u n t hor izonta l tens ion .

In t h e wes te rn side of t he Hellenic arc t he high sed iment supp ly ra te , combined wi th a th icken ing of t h e E a r t h ' s c rus t along the Ion ian zone, is i n t e rp re t ed as evidence t h a t accret ion has occurred the re un t i l r ecen t ly or m a y still be occurr ing locally; p l a t e consumpt ion , if any , is r a t h e r low.

The exis tence, 011 t h e o the r h a n d , of two very deep subparal le l t r ench-es wi th l i t t le fill sou theas t of Crete (Pl iny t r ench , S t rabo t rench) , combined wi th a very high subcrus ta l ac t iv i ty in the sou theas t e rn margin of t h e Aegean Bubplate, suggest t h a t t he subduc t ion r a t e a t t h e no r the rn b o u n d a r y of the Afr ican p l a t e must be re la t ive ly h igh; 110 p l a t e accret ion is expec ted to occur the re .

RIASSUNTO. — Basandos i sui da t i geofìsici esistenti si p o t r e b b e con-c ludere che l 'origine del c a m p o di « s t ress » nel cent ro ionico (Cefalonia-Zante-P a t r a s ) al l imite nord-occidenta le della m i c r o p i a t t a f o r m a del l 'Egeo è p iu t -tos to in superficie, e nel cent ro sud-or ienta le (Dodecaneso-Creta) è sicura-m e n t e so t to la cros ta .

Ne l l ' a rea o c c u p a t a dal secondo cent ro di maggiore a t t i v i t à s ismica il rilievo della Mollo è meno p ro fondo e meno acc iden ta to in conf ron to a quello ri-cava to dai d a t i g rav imet r ic i e sismici per l ' a rea del centro nord-occidenta le .

430 a. g. g a L a x o p o u i . o s

U n ' a l t r a d i f f e r e n z a — t r a t t a da l l e so luz ioni del p i a n o di f a g l i a — è clic il c e n t r o ionico è s i t u a t o in u n a z o n a s o t t o p o s t a a press ioni o r i zzon ta l i , m e n t r e il c e n t r o ne l m a r e E g e o s u d - o r i e n t a l e f a p a r t e di u n a z o n a d i fo r -t i s s i m e t e n s i o n i o r izzon ta l i .

Ne l l a p a r t o o c c i d e n t a l e de l l ' a r co el lenico l ' a l t a ve loc i t à di a p p o r t o di s e d i m e n t i i n s i eme a d u n i s p e s s i m e n t o del la c r o s t a t e r r e s t r e l ungo la z o n a ion ica , v i ene i n t e r p r e t a t a c o m e la p r o v a clic l ' a c c r e s c i m e n t o si è ve r i f i ca to m o l t o d i r e c e n t e o è a n c o r a l o c a l m e n t e in a t t o ; l ' e ros ione de l la p i a t t a f o r m a , se es is te , è p i u t t o s t o b a s s a .

D ' a l t r a p a r t e , l ' e s i s t enza d i d u e s p a c c a t u r e s u b p a r a l l e l e m o l t o p ro -f o n d e e con piccolo a p p o r t o di m a t e r i a l e a sud -e s t di C r e t a ( s p a c c a t u r a di P l i n y , s p a c c a t u r a di S t ra l lo) a s s o c i a t a ad u n ' a t t i v i t à s u b - c r o s t a l e m o l t o i n t e n s a al m a r g i n e s u d - o r i e n t a l e de l la p i a t t a f o r m a s o t t o l ' E g e o , f a p e n s a r e che la ve loc i t à di s u b d u z i o n c al l i m i t e s e t t e n t r i o n a l e de l l a p i a t t a f o r m a af r i -c a n a d e v e essere r e l a t i v a m e n t e a l t a . Non ci si a s p e t t a q u i n d i che ivi a v v e n g a u n a c c r e s c i m e n t o de l l a p i a t t a f o r m a s tessa .

I N T R O D U C T I O N

The complex tectonics ili the eastern Mediterranean may be thought of as having been disentangled to some extent by the postulate of a number of minor plates between Europe and Africa, each in motion with respect to all adjacent plates. In the context of the plate tectonics, the tectonic zones along the southern borderlands of Europe may have been created by impingement, collision and relative motion of the postulated plates. These zones are often terminated by transform faults or rifted margins (3).

A decent number of microplates have been already identified in the Mediterranean region 011 the basis of the present-day seismicity at their margins. They are from west to east: the Messinian, the Adriatic, the Ionian, the Aegean, the Turkish and the Levantine subplates (Fig. 1). Le Pichon is of the opinion, tha t the Adriatic subplate may exist or it may be a finger of the African plate within the Eurasian plate (10).

In a section from the deep trench south of Crete to the volcanic island of Santorini, the Hellenic subduction system displays the three edgeocean typical zones: trench, arc-trench gap and arc with the Cretan ridge at the outer par t of the arc-trench gap (23).

Although the present relative motion of Africa with respect to Europe is of a northward compression, the relative motion that is occurring at the southern border of Europe is not simply compressional but varies due to the interaction of the intervening microplates. Thus, as Dewey and others pointed out, the relative plate motion in the eastern

ON T H E T E C T O N I C P R O C E S S E S ALONG T H E H E L L E N I C ARC 4 3 1

Mediterranean today is to a large extent determined not by Africa-Europe motions but by the north-south compressive motion that is occurring between the Arabian plate on the one hand and the European plate on the other. Arabia is separating from Africa along the Red Sea and the Gulf of Aden rift system. The Arabian plate, although thrusting under the Van plate along the Zagros, is pushing the Van plate north and wedging the Turkish plate westward along the north Anatolian fault (3).

Fig . I - Neot.ectonics of t he Alpine Sys t em; reproduced f rom Dewey and o the r s , 1973 (Triangles = Q u a t e r n a r y and Holocene Volcanoes; circles = ep icen te rs of e a r t h q u a k e s deeper t h a n 100 k m ; ar rows = slip direct ions of p la t e s wi th respect to (he E u r a s i a n p l a t e ; dashed lines = con tours 011 Benioff

zone in h u n d r e d s of ki lometers) .

The Aegean subplate, due to the westward squeezing of the Tur-kish subplate, moves apparently to the southwest with respect to Europe, the resulting differential motion between these plates and Africa being absorbed in the Hellenic trench south of Crete. This explains, according to Le Pichon and others, the much higher seismic activity in the eastern Mediterranean than in the western Mediter-ranean (16). As pointed out by McKenzie, the existence of the subsidiary plates may be thought of as minimizing the work to be done in the relative movement of shortening between Eurasia and Africa (--).

28

432 A. G. GALANOPOULOS

SEISMOTECTONIG FEATURES

The Aegean subplate is bounded to the West and South by the Hellenic trough and two subparallel trenches, the Pliny trench and the Strabo Trench, south of Crete. The north boundary of the plate runs along the postulated westerly prolongation of the north Anatolian shear fault zone; in the context of the prolongation, it was assumed that the north Anatolian dextral shear fault system continues through

LEVEL

IOOO

M a p s h o w i n g t h e d i s t r i b u t i o n p a t t e r n of m a j o r shocks in r e l a t i o n t o t h e t o p o g r a p h y of Greece .

the Trikkeri-Canal fault and some concealed, deep-seated transverse faults into the Gulf of Patras and the shear fault zone of Cephalonia-Zante (6). The lack of geological and morphological evidence for transverse faults between the Trikkeri-Canal and the Gulf of Patras (Fig. 2), and the migration of the earthquake activity from Magnesia

.! 1 .

ON" T H E T E C T O N I C P R O C E S S E S AI .ONO T H E H E L L E N I C A R C 4 3 3

(Pharsala, Volos, Yelestinon, 1954-1957) to the northern and northeast-ern Sporades (Alonisos, Skyros, St. Bustrate, 1965-1968), and in the same period to Eurytania and Acarnania (Kremasta, Katouna, 1966) and recently to the area of Leukas-Preveza (1973), favour the alternative tha t the Saros-Graben continues in the Trikkeri-Lamia fault zone and

Fig . 3 - Simplified f a u l t m a p of Greece.

through the northern fault escarpment of Oete mountain and the Quaternary Sperchios-Graben reaches the fault-bounded depression of the Ambrakikos Gulf (Fig. 3). The eastern boundary of the Aegean subplate is vaguely traced by a series of earthquake epicenters running from the Fethiye Gulf, between Rhodes and Kastellorizon, to the Gem-

« - i l

434 A . G . G A L A N O P O t t t O â

lik Gulf in tlie Marmara Sea. The recent seismic activity in Asia Minor, 1967-1970, was seated apparently 011 the boundary of the Aegean and Turkish microplates (Fig. 4).

Fig. 4 - E a r t h q u a k e epicenter m a p for t h e per iod 1950-1973.

The volcanic arc associated with the Hellenic subduction system starts from the volcanic islands of Nisyros-Yali-Kos, runs through the islands Santorini-Antiparos-Milos, it turns to the North and through Methana-Aegina-Isthmus reaches the group of the Quaternary vol-canoes St. Ioannis, Likhades, Achilleion, Zileria, Thebes and Porphy-r o n , in the area of Lamia-Euboea-Magnesia (24). The volcanic arc is in a roughly constant distance from the Hellenic Trough and the

ON T H E T E C T O N I C P R O C E S S E S A L O N G T H E H E L L E N I C A R C 435

Trench system tha t surrounds the Aegean subplate. Between the volcanic arc and the nearly parallel Hellenic Trough runs the sedi-mentary arc developed during the Cretaceous-Tertiary Alpine Oro-geny (x). Among the Volcanoes of the Hellenic arc, Santorini, Nisyros and Methana were active in historical times; four volcanoes at the north end of the arc, among which Likhades, were active in the Holocene and probably till the end of the Neolithic age (12).

The epicenter map for the 120-years period, 1841-1000, for which the data on damaging and destructive shocks are complete (5), does not show any marked difference in the distribution pattern from the epi-center map for the24-years period, 1950-1973, for which the parameters of the earthquakes with M ^ 5 1/2 are known with much better accuracy (Fig. 5).

In either of the epicenter maps, the boundaries of the Aegean subplate are more or less clearly delineated by earthquake foci of continuous or sporadic activity. A common feature of the epicenter maps for Greece is the continuous and relatively higher earthquake activity shown up at the northwestern and southeastern boundaries of the Aegean microplate. The two diametrically opposed centers or "poles" with maximum activity in the area of Greece are marked by a strain energy release much higher than that appeared in any other sector of the Aegean subplate or in any sector of the other subsidiary plates which presumably exist in the Mediterranean (Fig. 1).

In a general scheme, the active seismic trend of Greece is approxi-mately double A-shaped, with the A-sides perpendicular to each other, one of them being submeridian, and the perpendicular to it running subparallel, across the Hellenic Peninsula (Fig. 5).

The first a t tempt to give a good reason for the clustering of the majori ty of the epicenters around the two "poles", the northwestern and southeastern margin of the Aegean microplate, has been strongly influenced by the relatively frequent and well documented observation of higher earthquake activity at the crosspoints of major fault zones, which generally yield to forces much easier. Biased from this princi-ple, the concentration of the earthquake activity in the area of Cepha-lonia-Zante-Patras has been firstly attributed to the intersection of the Ionian fault zone with the assumed prolongation of the north Anatolian shear fault zone (").

The second center of higher strain energy release in the area of Dodecanese-Crete is of predominantly subcrustal origin; consequently, the existence of this center can not be easily explained in terms of

43(> A. G . G A L A N O P O U L O S

the above principle. The area occupied by the second center and the energy radiated from it are much higher than in the first center (9).

The Ionian center (Cephalonia-Zante-Patras) at the northwestern margin of the Aegean subplate is a site of predominantly shallow, con-tinuous earthquake activity associated with a pronounced low of

Fig. 5 - T h e ac t ive seismic t r end of Greece, 1950-1973.

negative Bouguer anomalies («). I t is believed tha t due to the high sediment supply rate of a number of rivers (for example Acheloos, Peneios, Alpheios) that flow to the Ionian Sea, a plate accretion must occur in the area. In Pacific, island arcs there is some evidence tha t accretion of crustal material occurs 011 the island side of the trench.

ON T H E T E C T O N I C P R O C E S S E S A L O N G T H E H E L L E N I C A R C 437

According to Lomnitz (17), the mechanisms of accretion and subduction are far from clear - cut; "accretion" as a mechanism of interaction be-tween two plates does not necessarily involve an increase in the surface area of either plate, "since the growth can occur as a vertical thickening rather than horizontal extension of the lithosphere".

Plate consumption at the northwestern margin of the Aegean subplate, if any, is rather low. The notion is strongly supported by the lack of agreement between gravity distribution and orography; according to H. Wunderlich (26), this lack can be explained only in terms of a shifting of orogenic activity, especially of orogenic foredeeps. Wunderlich (20) believes that shifting of negative gravity anomalies is most prominent where orogenic activity continues most strongly at the present time. On the grounds of this reasoning, the Minoides, i.e. the Peloponnese-Crete island arc is the youngest among the various Mediterranean mountain chains and the only one still undergoing oro-geny. In terms of plate tectonics, the Minoides is a double-arc structure with a range of folded mountains lying between the trench and the volcanoes (19).

Some years ago it was postulated that the large fault zone that surrounds the western and southern coasts of Greece marks the trend of a marginal geosyncline, which is now in the course of development in the eastern Mediterranean (6). I t might be added that even in the fra-mework of the now widely accepted concept of plate tectonics the Hel-lenic Trough and the Mediterranean Ridge may well be a typical pair of eugeosynclinal and miogeosynclinal structures in a "status nascendi" or embryonic stage. The trend of the Mediterranean Ridge is very poorly portrayed in the distribution pattern of earthquake epicenters; in general, the seismic activity along the Mediterranean Eidge is sparse and by far less than that occurring along the Hellenic Trough. The fact tha t 110 clear indications for horizontal compression were observed in the surveyed area of the Ionian Sea (13) is not strange for a géosyn-clinal couplet in process of formation (4). However, the question is still moot.

The second center of higher earthquake activity at the south-eastern margin of the Aegean microplate (Dodecanese-Crete) is a place of particularly high and rather continuous, intermediate focal depth seismic activity associated with a large zone of high positive gravity anomalies (8). The lack of any river of some importance indicates that the sediment supply rate in the area is relatively very low; consequent-ly, no plate accretion is expected to occur there. The existence, on

438 A . G . G A L A N O P O t t t O â

the other hand, of two very deep, subparallel trenches with little till southeast of Crete (Pliny trench, Strabo trench) combined with a very high subcrustal activity in the area considered, suggest that the sub-duction rates must be there relatively high (").

a f t e r J . Maki i s (1!)73).

Another point worth noting is that the large shock epicenters along the western segment of the Hellenic arc tend to occur near the outer or seaward side of the arc; in the southern segment there is a ten-dency of large shock epicenters to occur, as in most destructive plate

ON T H E T E C T O N I C P R O C E S S E S A L O N G T H E H E L L E N I C A R C 439

boundaries ("), near the inner or islandward side of the arc. Plots of the large shock epicenters (M > 7) along the Hellenic arc can be found in Figures 5 and 0 of a previous work of the author (7).

Fig . 7 - T h e g e n e r a l d i rec t ion of t h e c o m p r e s s i o n a l a n d t ens iona l s t ress in t h e a r e a of Greece , a f t e r A. E . R i t s e m a (1974).

DISCUSSION AND REMARKS

On t he grounds of the above considerations one might [be allowed to conclude that the origin of the stress field in the northwestern center, at the constructive margin of the Aegean subplate, is rather shallow and in the southeastern center, at the destructive margin of the African plate, is surely under the crust (10).

Tn the area occupied by the second center of higher earthquake activity the relief of the Moho-discontinuity is shallower and smoother

440 A . G . G A L A N O P O t t t O â

in comparison to that derived from gravity and seismic data for the area of the northwestern center (18). Further, the thickening of the Ear th 's crust along the Ionian zone (Fig. 6) may be interpreted as Evidence that accretion has occurred there until recently or may still be occurring locally.

Another difference derived from the fault-plane solutions (25) is tha t the Ionian center is seated in a region of predominant horizontal pressure; the second center in the southeastern Aegean Sea belongs to a region of paramount horizontal tension (Fig. 7).

Observations by geologists who have had extensive field experi-ence in the area of Greece have shown that since Pliocene the tectonics in the Cephalonia Island is compressional (20), and since the Mindel-Riss period tensional forces are predominant in the area of the Aegean Sea (21). On the grounds of these and other features of the Aegean Sea, as for instance the large positive free-air gravity anomalies, the linear magnetic anomalies and the relatively thin crust, Boccaletti and associates (2) believe the Aegean Sea, particularly the Cyclades area to be an extensional back-arc basin, i.e. a marginal sea in an early development stage formed by extension and accretion due to rising magma from the mantle.

However, by and large the compressional stress field seems to be continuous throughout the region of Greece and the general northeast-southwest direction of the compressional stresses seems to be in-dependent of the trend of the local arc structure and to be determined only by the relative motion direction of the African plate and the Anatolian or Turkish subplate. The result of these conflicting regimes is the formation of the Aegean basin by uplift and crashing of the intermediate crystalline mass and then collapsing under tension; ac-cording to A. R. Ritsema (25), the tensional stresses always seem more or less directed towards the centre of this area.

Summing up, the Dodecanese-Crete-Center at the southern side of the Hellenic arc is a subduction zone associated with tension, while the Ionian center at the western side is largely a zone of accretion related to compression; this seems to be in contradiction with the general norm of predominant stresses across convergence zones of lithospheric plates.

Assuming a N 30° E direction of the slip vector (10) and a com-bined speed of about 10 cm/year between the African plate and the Aegean microplate (15), the difference in the tectonic processes along the Hellenic arc might be attributed, after Xavier Le Pichon (10),

ON T H E T E C T O N I C P R O C E S S E S A L O N G T H E H E L L E N I C ARC 4 4 1

to the greater speed of the two plates in the X-S direction (about 8.7, i.e. more than fi cm/year) in comparison to that in the E-W direction (about 5, i.e. less than 6 cm/year).

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LIP I L , SAUVAGE J . , a n d SOREL D . , 1973 . - Déformations superficielles en compression et en extension dans la "Plaque égéenne" (Grèce) au cours du Pliocène et du Quaternaire. " R é u n i o n Annuel le des SE. de la T e r r e " , Par i s , 19-22 Mars, p . 297.

(22) MCKENZIE 1). P . , 1970. - Plate Tectonics of the Mediterranean Region. " N a t u r e " , 226, pp . 239-243.

(23) NICIIOLS I. A., 1971. - Petrology of Santorini Volcani, Cyelades, Greece. " J o u r . P e t r o l o g y " , 12, pp . 67-121.

(24) NINCOVICH I). and IIAYS 1). J . , 1972. - Mediterranean Island Arcs and Origin of High Potash Volcanoes. " E a r t h and P l a n e t a r y Science L e t t e r s " , 16, p p . 331-345.

(25) RITSEMA A. R., 1974. - The Earthquake Mechanisms of the Balkan Region. « Roya l N e t h . Met. Ins t» , D e Bil t , Scientific R e p o r t Nr . 74-4, p . 36.

(20) WUNDERLICH G. IL, 1971. - Gravity Anomalies, Shifting Foredeeps, and the Pole of Gravity in Nappe Transport as Shown by the Minoides (Eastern Mediterranean). Van Bemmelen : Volume, " G r a v i t y and Tec ton i c s " , Ed i t o r s : Kces A. De J o n g and Rober t Schöl ten , J o h n Wiley and Sons, New York , p p . 271-285.