Subsidence of the North Aegean Trough: an Alternative View

Journal of the Geological Society, London, Vol. 143, 1986, pp. 23-27, 4 figs. Printed in Northern Ireland

Subsidence of the North Aegean trough: an alternative view

M. BROOKS & L. KIRIAKIDIS Geology Department, University College, P.O. Box 78, Cardiff, Wales, UK.

Abstract: The North Aegean trough is a zone of deep water trending from northeast to southwest acrossthe N AegeanSea. The western part of the trough, theSporadhes basin, contains a thick sequence of late Cenozoicsediments. Le Pichon et al. (1984) suggested that theSporadhes basin coincides with a zone of extreme lithospheric stretching and that the crustal thickness is locally reducedto only 15 km. We arguethat the geophysical evidence cited by Le Pichon et al. can be equally well explained by a crustal model involving a much smaller amount of local crustal thinning.

Le Pichon et al. (1984) investigated the subsidence history of reflectionirefraction seismic experiment in the area of the the North Aegean trough (Fig. 1) using the stretching model trough. of McKenzie (1978). They concluded that it can be modelled reasonably well using astretching factor (p) of about 3.5 andstretcheda lithosphere with a very small flexural The case for extreme crustal thinning parameter so that local isostatic compensation prevails. The The arguments of Le Pichon et al. are geophysical, with an local crust is predicted to be thinned to about 15 km and to emphasis onthermal and gravity computations. In brief, be underlain by a narrowthermal perturbation in the they claim that the free-air gravity anomaly field indicates underlying upper mantle. near-isostatic equilibrium and can be modelled successfully The Aegean domain overall is widely assumed to have in terms of a very thin crust under the western part of the undergone crustal stretching of about p = 1.4 during the last NorthAegean trough; thatthe local heat flow values 10-12 million years (see Le Pichon & Angelier 1979), a (Jongsma 1974) resultfrom combinationa of lateral figure compatible with an averagecrustal thickness of conduction of heat away from the narrow zone of extreme 25-30 km under the Aegean Sea compared with 35-40 km stretching and a blanketing effect of the thick cover of young in adjacent areas of mainland Greece and western Turkey sediments within the trough; and that magnetic anomalies (Makris & Vees 1977). The associated surface subsidence is within the trough can be accounted for by young volcanic currently concentrated in several fault-controlled sedimen- rocks within the sedimentary cover and are,therefore, tary basins of which the North Aegean trough, the Skyros further evidence of extreme crustal thinning. We take a basin, the Gulf of Corinth and Gulf of Patras are notable different view of the significance of the above geophysical examples (Brooks & Ferentinos 1984; Ferentinos et al. evidence. 1985). The Sporadhes basin (Fig. 1) at the western end of the North Aegean trough is the most spectacular example, containing the deepest water zone in the Aegean Sea. An alternative view The structural zone containing the North Aegean trough A serious shortcoming in the analysis of Le Pichon et al. is extends north-eastwards into the North Anatolian fault zone their failure to acknowledge the importance of listric normal of Turkey (Dewey & Sengor 1979; Lyberis 1984) and dextral faulting in the Aegean area. The evidence for listric faulting strike-slip along thelatter must, at least in part, be hasbeen discussed by McKenzie (1978), Jackson et al. accommodated along the line of the North Aegean trough. (1982) and Brooks and Ferentinos (1980, 1984), and listric Indeed,earthquake focal mechanisms show the trough to faulting in the area of the North Aegean trough is apparent have a major component of strike-slip movement (Lyberis in seismic reflection sections(Lalechos & Savoyat 1979). & Deschamps 1982; J. A. Jackson, pers. comm., 1984). The importance of this fact is thatthe amount of basin We do not consider this aspect of the regional structure subsidence cannot be used to predict the associated crustal in the present account, not because it is unimportant to a thinning in the way that Le Pichon et al. have done. If, as is full understanding of the geology of the NorthAegean likely, the listric normal faults flatten out into a dkcollement region but because it does not bear directly on the specific horizon at some depth in the crust, the total displacement issue wewish toraise, namely,whether the existing across the faults is equivalent tothe amount of lateral geological and geophysical data from the Sporadhes basin extension. Summing the displacements on listric faults are indicative of extreme crustal thinning. According to Le across arepresentative section of crust therefore allows Pichon et al. (1984), theSporadhes basin representsa calculation of the totallateral extension, from which a p narrowzone of extreme stretchingthroughout theentire stretchingfactor forthe crust can be calculated if it is lithosphere.We arguethat onthe basis of existing assumed that the deeper crust has undergone an equivalent geological and geophysical evidence the trough can be amount of ductile stretching. satisfactorily modelledas a localized manifestation of the The western part of the Sporadhes basin contains three brittle upper crustalresponse to regional lithospheric or four major listric normal faults of WNW-ESE trend and stretching,without requiring it to beunderlain by a very the eastern part at least five major faults of SW-NE trend thin crust andan associatedzone of anomalous upper (Brooks & Ferentinos, in press). In the absence of published mantle. Resolution of this matter must, as Le Pichon ef al. deep seismic reflection data,the displacement on these also observed, await the implementation of a wide angle major faults of theSporadhes basin is notknown, but 23

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thinning under the trough. It should be noted that we have made different assumptions aboutthe Late Cenozoic sediments within thetrough, retaining approximately the overallNeogene and Quaternary thickness predicted by Lalechos & Savoyat (1977) but subdividing the sequence into a Plio-Quaternary layer with a density of 2.25 T/m3 and a Neogene layer with a density of 2.40 T/m3. Theunderlying upper crustal rocks were given a density of 2.65 T/m3. What of the magnetic anomalies attributed by Le Pichon et al. to young volcanic rocks under the floor of the trough? Figure 3 presents a magnetic map of the North Aegean area and the eastern margin of the Vardar zone compiled from aeromagnetic data kindly supplied by the Institute of Geological and Mineral Exploration, Athens, together with marinemagnetic data collected during two recent cruises supported by the U.K. NaturalEnvironment Research Council (R.R.S. Shackleton, 1978; R.R.S. Discovery, 1983). The magnetic data were continued upwards to 2 km above sea level prior to merging of the two data sets. The large anomalies at the eastern margin of the Vardar zone and within Toronaios Gulf overlie the Chalkidhiki ophiolite belt of the Internal Hellenides (Kockel & Mollat 1977). Themajor anomaly in Toronaios Gulf hasbeen interpreted in terms of a largely concealed ophiolite body about 8-10 km thick for which there is supporting evidence Fig. 1. Locality map of the NW Aegean area. Line A-B indicates from associated gravity (Kiriakidis 1984) and seismic position of free-air gravity profile illustrated in Fig. 2. Asterisks refraction (Filbrandt 1985) surveys. mark positions of heat flow measurements by Jongsma (1974). The zone of anomaly in the easternpart of the Sporadhes basin can be traced in a subdued form into the assuming an averagedisplacement of a few kilometres on western part of the basin, where linear anomalies trending each fault,the overall upper crustalextension taken up NW-SE in the offshore zone (see Fig. 3) appearto be alongthese listric faults may amountto 20-30 km. This continuous with large aeromagneticanomalies overlying crude calculation is likely to represent an underestimate of ophiolite outcrops along the western margin of the Vardar the extension since antithetic and synthetic minor faulting is zone in coastal areas of Thessaly. intense(see Ferentinos et al. 1981) and must give rise to The wide extent of magnetic anomalies in the NW significant extension within each hanging wall block to a Aegean area suggests that ophiolitic rocks occur much more major listric fault. Thus the overall extension may be 30 to extensively, under the cover of late Cenozoic sediments in 40 km within the 80 km zonecontaining the Sporadhes the Axios and Sporadhes basins, than has previously been basin, and this yields a @-value of 1.4 to 1.5. In summary, suspected. although theamount of displacement is very poorly There is, of course, widespread igneous activity of late constrained in the absence of deep reflection data, Cenozoic age in the North Aegean area (Fytikas et al. 1976; reasonable estimates are more compatible with the overall Innocenti et al. 1979; LybCris 1984), but it is not obviously Aegean crustalstretching factor of 1.4 than with the concentrated along the NorthAegean trough. Plio- extreme value of 3.5 postulated by Le Pichon et al. Quaternary volcanic activity occurs in the Atalanti Channel It may be asked whetherthe asymmetric grabens and district and around Volos (Fig. l), about 20-70 km west of graben complexes of the Aegean area, being localized zones the westward termination of the North Aegean trough. This of crustal unloading (in which ‘basement’ rocks have been activity lies along northwesta extension of the inner displaced by water and low-density sediments),are each volcanic arc of the Cyclades and is regarded by Innocenti et associated with a local upbulge of large amplitude in the al. (1979) as typical island arc calc-alkaline volcanicity underlying Moho. Weregard this as unlikely, given the related to active subduction beneath the Aegean area. They close spacing of major sedimentary basins in the Aegean suggested that the localization of this volcanic activity along area (about 100 km)and the likelihood thateach is the trend of the North Aegean trough may result from the compensated in part regionally rather than entirely locally. ‘strong distension tectonics’ along the northwest border of Figure2 shows thatthe local gravity anomaly can be the ‘Aegean microplate’. Since this volcanicity is primarily straightforwardly modelled on this assumption. Our gravity subduction related,it would not beexpected to extend interpretation is muchsimpler than that presented by Le northeastwards along the axis of the North Aegean trough Pichon et al. and in particular does not feature a very thin because the latter is not underlain by a subducted slab. crust or a narrow zone of thermally perturbed upper mantle Do young volcanic rocks cause significant magnetic underthe trough. The interpretation does involve some anomalies in the vicinity of the North Aegean trough as Le crustalthinning butthe amplitude of the local Moho Pichon et al. suggest? In considering this question, it may be upbulge is only 5 km, compared with 15 km in the crustal notedthat in the Atalanti-Volos area all zones of model of Le Pichon et al. In this model, about two-thirds of outcroppingophiolites exhibit largeaeromagnetic ano- the mass deficiency represented by thedeep waterand malies. By contrast, only some areas containing recent sediments of the trough is compensated locally by the crustal volcanic rocks(e.g. Likades, Kammena Vourla and

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mgal

801 N S

A B

Kassandra North Aegean Trough SporadhesTrough Aegean KassandraNorth Is.

'O t l CRUST 2o t

km Fig. 2. Interpretation of free-air gravity profile across the Sporadhes basinof the North Aegean trough at longitude 23"30'E. See Fig. 1 for location of profile.

Microthebe) coincide with magneticanomalies and these Le Pichon et al. used equations presented by Alvarez et areas also containophiolites. Therefore ophiolites may be al. (1984) to model two-dimensionally the thermal evolution the solecause of significant magneticanomalies in the of the Sporadhes basin. These equations are valid only for Atalanti-Volos area. This conclusion is supported by recent /%factors greaterthan about two. Since we postulate an land magnetic surveys carried out by the British Geological extensional factor of only about 1.5 across the basin, we can Survey as a part of the Anglo-Greek Volos project (A. adopt a simple one-dimensionalapproach tothe thermal Burley, pers. comm., 1984). modelling. Theother very young volcanicity recorded fromthe Conduction in the one-dimensional case can be ignored immediate vicinity of theNorth Aegean trough is a 0.5 if tP2<60Ma where t is the duration of the crustal million-year-old eruption of alkali basalt onthe island of stretching episode in millions of years (Jarvis & McKenzie Psathoura (Fig. 1, Lyberis 1984). This lies on the south flank 1980). On our view, t@' = 20 therefore conduction can safely of the major magnetic anomaly in the eastern part of the be ignored. In the case of instantaneous uniform stretching Sporadhes basin referred to above. However, the island lies of the entire lithosphere, the surface heat flow is increased 10-15 km south of the causal body of the magnetic anomaly, by p at t = 0. This represents a theoretical maximum heat as interpreted by two-dimensionalmodelling. Thus again flow in the present case. Assuming, with Le Pichon et al., a there is no direct association of young volcanic rocks with normalpre-stretching heat flow value of 55 mW/mz,the significant magnetic anomalies. We therefore conclude that theoretical maximum is thus about 80 rnW/mz. Concentra- concealed ophiolitic rocks are a sufficient explanation for the tion of ductile stretching in the lower part of the lithosphere magnetic anomalies in the NW Aegean area. with brittle deformation by block faulting in the upper part Only the heat flow in the North Aegean remains to be will reduce the surface heat flow value predited above. So considered. Thethermal modelling of Le Pichon et al. also will the blanketing effect of sediments on subsided predicted that heat flow values over the trough, even in the portions of the stretchedcrust. For example, 5 km of presence of the extreme crustal thinning, should lie within sediment deposited at a uniform rate over 10 Ma will yield a the range of 50-70 mW/m2. These near-normal heat flow surfaceheat flow that is only two-thirds of the values are accounted for by the blanketing effect of the pre-sedimentation flux, i.e. about 40 mW/m2. basin fill of young sedimentsand, more importantly, by On this simple, one-dimensional analysis most heat flow lateralconduction of heat away from the postulated local values in the North Aegean would be predicted to lie in the zone of lithospheric thinning. range from about 45-75 mW/m2, which is not significantly

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Fig. 3. Magnetic anomaly map of the NW Aegean region continued upwards to 2 km above sea-level. Contour interval: 25 nT.

different fromthe range of 50-70mW/m2derived by Le contention that the very small suite of available heat flow Pichon et al. from their two-dimensional analysis based on a values andthe single anomalous value within the trough much greater degree of crustal stretching. represent an inadequate foundation to support the edifice Of thirteen heat flow measurements in the Aegean Sea erected by Le Pichon et al. Far moreheat flow (Jongsma 1974), only three sites are locatedin the measurements are requiredthroughout the Aegeanarea immediate vicinity of the Sporadhes basin (Fig. 1). Two of before theNorth Aegeantrough can beadequately thesesites yielded values (52 and 65 mW/mz) within the modelled thermally. predicted range of both analyses, butone yielded the signicantly higher value of 105 mW/mz. The latter value was obtained from an area of irregular and shallow bathymetry Conclusions interpreted by us as a zone of shallow ophiolitic basement. We consider that the NorthAegean trough should be regard- Le Pichon et al. suggested a young volcanic source and thus ed as an integral part of an Aegean crustal regime in which conveniently accounted for the high local heat flow. It is our a listric-faulted upper crustal section overlies a lower crust

onomalous mantle zone Moho zone mantle onomalous

0 l00 km

Fig. 4. Schematic crustal section across the Aegean area. The Sporadhesbasin represents the western part of the North Aegean trough.

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that has been thinned inaductile manner,the overall western Greece. Journal of Structural Geology l, 713-717. crustal thickness being about 25 km. This is more in accord FILBRANDT, J. B.1985. Tectonic evolution of the Vardar Zone in Chalkidhiki, with the regional geological evidence than the model of Le northeastern Greece. Ph.D. thesis, University of Wales. FYTIKAS,M,, GIULIANI,O., INNOCENTI, F., MARINELLI,& G. MAZZUOLI, R. Pichon et al., and it in no way conflicts with the available 1976. Geochronological data on Recent magmatism of the Aegean Sea. geophysical evidence. Tectonophysics 31, T29-T34. A close geometrical analogy tothe upper crustal INNOCENTI,F., MANETTI,P,, PECCERILLO,A. & POLI, G. 1979. Innerarc structure of the Aegean domain is probably to be found in volcanism in the NW Aegean arc: geochemical and geochronological data. Neues Jahrbuch fur Mineralogie und monatshefte 4, 145-158. the Basin andRange province of theUnited States, as JARVIS, G. T. & MCKENZIE,D. P. 1980. Sedimentary basin formation with pointed out by Jackson et al. (1982). Figure 4 illustrates finite extension rates. Earth and Planetary Science Letter.s 48, 42-52. schematically a possible crustalsection across the Aegean JACKSON,J. A., KING, G. & VITA-FINZI,C. 1982. The neotectonics of the area. In this section the North Aegean trough is portrayed Aegean:an alternative view. Earth and Planetary Science Letters 62, 303-318. as simply the largest member of a family of major JONGSMA,D. 1974. Heat flow in the Aegean Sea. Geophysical Journal of the asymmetric graben structures generated in the upper crust Royal Astronomical Society 37, 331-346. by the prevailing north-southcrustal extension. The KOCKEL,F. & MOLLAT,H. 1977. Erlauterungen zurGeologischen Karte der equivalent extension in the lower crust is taken up by ductile Chalkidhiki und angrenzender Gebiete 1: 1OOOOO (Nord-Griechenland). stretching toproduct a rather uniform overallcrustal Bundesanstalt fur Gewissenschaften und Rohstoffe, Hannover, pp. 199. KIRIAKIDIS,L. 1984. Geophysical studies of the eastern margin of the Vardar thickness under the entire Aegean area. zone in Central Macedonia, Greece. Ph.D. thesis, University of Wales. LALECHOS,N. & SAVOYAT, E.1979. La Sedimentation Nkghe dans le Fosse Nord-EgCen. Proceedings ofthe VI Colloquium on the Geology of the References Aegean region, Athens. 2, 591-603. LE PICHON,X. & ANGELIER,J. 1979. The Hellenlc arc and trench system: a ALVAREZ, F., VIRIEUX,J. & LE PICHOW,X. 1984. Thermal consequences of key tothe neotectonic evolution of theeastern Mediterranean area. lithosphereextension over continental margins: the initial stretching Tectonophysics 60, 1-42. phase. Geophysical Journal of the RoyalAstronomical Society 78, -, LYBERIS,N. & ALVAREZ,F. 1984. Subsidence history of theNorth 389-411. Aegeantrough. In: DIXON, J. E. & ROBERTSON,A. H. F. (eds) BROOKS,M. & FERENTINOS,G. 1980. Structureand evolution of the Geologicalevolution of the Eastern Mediterranean. GeologicalSociety, Sporadhes basin of theNorth Aegean trough, northern Aegean Sea. London Special Publication, 17, 727-741. Tectonophysics 68, 15-30. LYBERIS,N. 1984. Tectonicevolution of theNorth Aegean trough. In: - & - 1984. Tectonics and sedimentation in the Gulf of Corinth and DIXON,J. E. & ROBERTSON,A. H. F. (eds) Geological evolution ofthe the Zakynthos and Kefallinia Channels, western Greece. Tectonophysics Eartern Mediterranean. Geological Society, London Special Publication, 101, 25-54. 17, 709-725. -& -Active tauts in the Sporadhes basin of the North Aegean trough, - & DESCHAMPS,A. 1982. Sismo-tectoniquedu fosse Nord-EgCen: northern Greece. Journal of Structural Geology (in press). relations avec la faille Nord-Anatolienne. Compte Rendu de I’Academie DEWEY,J. F. & SENGOR,A. M. 1979. Aegeanand surrounding regions: des Sciences t295, S&ie II, 625-628. complex multiplateand continuum tectonics in aconvergent zone. MAKRIS,J. & VEES, R. 1977. Crustal structure of the central Aegean Sea and Geological Society of America Bulletin 90, 84-92. the islands of Evia and Crete, Greece, obtained by refractional seismic FERENTINOS,G., BROOKS,M. & COLLINS,M. B. 1981. Gravity-induced experiments. Journal of Geophysics 42, 329-341. deformation on the north flank and floor of the Sporadhes basin of the MCKENZIE,D. 1978. Active tectonics of theAlpine Himalayan belt:the North Aegean Sea trough. Marine Geology 44, 289-302. AegeanSea and surrounding areas. Geophysical Journal of the Royal --, & DOIJTSOS,T. 1985. Quaternary tectonics in the Gulf of Patras, Astronomical Society 55, 211-254.

Received 20 February 1985

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