Aegean and Surrounding Regions: Complex Multiplate and Continuum Tectonics in a Convergent Zone

Aegean and surrounding regions: Complex multiplate and continuum tectonics in a convergent zone J. F. DEWEY I Department of Geological Sciences, State University of New York at Albany, Albany, New York 12203 A. M. CELAL §ENGOR j ABSTRACT in intracontinental convergence be resolved strike-slip faults and fold-thrust systems into the relative motion of a number of whose close spacing prohibits their being The tectonics of the Aegean region in- small plates, or, conversely, does a con- considered as plate boundaries. The evolu- volves complex slip patterns across the tinuum or semicontinuum behavior domi- tion of arrays of close-spaced grabens, boundaries of several microplates that seg- nate, with short-term strains distributed strike-slip faults and fold-thrust systems is ment the end of the Anatolian plate, which more uniformly within the convergent zone, characteristic of orogenic belts and wide is moving in a westward direction from the or is there a combination of the two modes zones of intracontinental foreland defor- Bitlis zone, an intracontinental suture zone, of behavior? Another way of asking this mation associated with collisional suturing to consume oceanic lithosphere in the east- question is (1) are the large finite strains ob- (Dewey, 1977). These relations suggest that ern Mediterranean. The segmentation of served in intracontinental orogenic belts the although stresses may be transmitted for the western end of the Anatolian plate into result of relatively continuous small-scale large distances into orogenic forelands and scholles with adjacent zones of grabens, deformation over the whole convergent re- major high-strain zones and in such areas strike-slip, and thrust semicontinuum gion, (2) are they the sum of incremental may be considered as the boundaries of tectonics results from "locking" across the strains developed along plate boundaries scholles in relative motion, many of these two North Anatolian transform strands that constantly change their position, (3) do smaller scholles undergo continuum or where they change orientation at the west- they develop by exceedingly heterogeneous semicontinuum strain on a scale that pre- ern end of the Sea of Marmara. This fast strain distribution with high strain zones vents the useful employment of plate lateral motion of buoyant continental sliv- bounding scholles1 within which strains are tectonics sensu stricto within intracontinen- ers is a transient phase of the early stages of very small, or (4) do they result from a tal convergent zones. continental collision resulting from the ir- combination of the above? The Aegean region is an area of intense regularity of colliding margins. It is, how- Within the Alpine-Himalayan system, the seismic activity compared with surrounding ever, a tectonic phase that leaves a funda- distribution of seismicity is not homoge- regions and one in which seismic first mo- mental signature on the convergent zone by neous, indicating that, at least instantane- tions (Fig. 1, C) and Neogene-Quarternary imprinting a complex widespread series of ously, convergent strain is not homogene- geology indicate, spatially, a very complex structures that mask, and make difficult the ous but concentrated in high strain zones and rapidly varying tectonic picture. interpretation of, earlier structures. (for example, Das and Filson, 1975), many The broad tectonic framework of the of which are major strike-slip faults, such as Aegean, Turkey, and the eastern Mediter- INTRODUCTION the North Anatolian fault (Ketin, 1969; ranean region (Fig. 2, C) is dominated, as McKenzie, 1972) and Altyn Tagh fault shown by McKenzie (1972), by the rapid It is well known that intracontinental (Molnar and Tapponnier, 1975). It is westward motion of the Anatolian plate zones along which plate convergent dis- equally clear that although many of these relative to the Black Sea plate and west- placement is converted into convergent high strain zones are very long features with southwestward motion relative to the Afri- strain are usually wide diffuse regions large displacement rates that mandate their can plate. The Anatolian plate may be re- within which complex relationships exist penetration of the lithosphere as plate garded as a buoyant continental sliver plate between extensional, compressional, and boundaries, there are many more smaller being driven westward from the intraconti- strike-slip deformations (McKenzie, 1972; nental Bitlis suture zone, the southern edge Dewey and Burke, 1973; Molnar and Tap- 1 The term Scholle (German: lump, clod of of the Arabian-European convergent strain ponnier, 1975; $engor, 1976). This con- earth, or flake) has been used by German- zone. This kind of lateral escape of buoyant trasts with the narrower strain zones of speaking geologists since the early days of this continental fragments from intracontinen- oceanic and continental margin convergent century to describe the deformational pattern of tal convergent strain zones is common, and the Alpine foreland, caused by the interaction of boundaries and probably is related to the rigid and semirigid crustal blocks bounded by re- it involves motion on major transforms buoyancy and lower shear strength of con- verse, normal, and/or strike-slip belts (see Kober, whose strike is at a high angle to the continental rocks compared with the higher 1933). This term is appropriate to describe also vergent slip vector (Molnar and Tappon- density, stronger oceanic rocks (McKenzie, the crustal fragments and splinters that form, as a nier, 1975). The Anatolian plate is clearly result of complex strain patterns, during conti- 1969, 1972). A critical question, addressed nental collision, and by the virtue of their "escaping" westward into the western by McKenzie (1972) is, To what extent can tectonic nature and behavior cannot be termed Mediterranean oceanic tract, where its mo- instantaneous or short-term deformations "plates." tion, relative to Africa, is taken up by sub- Geological Society of America Bulletin, Parti, v. 90, p. 84-92, 3 figs., January 1979, Doc. no. 90113. 84 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/1/84/3434326/i0016-7606-90-1-84.pdf by guest on 24 September 2021 AEGEAN AND SURROUNDING REGIONS 85 duction at the Hellenic Trench. However, sion, the Izmit-Sapanca graben, and the Cephalonia and Zante is suggested by the the widespread seismicity roughly encir- Gemlik-Iznik graben north of Bursa (Ketin, northward termination of the Hellenic cling the Aegean region and the numerous 1968). These grabens are the northernmost Trench and an offset in seismicity. The active faults of western Anatolia and the of the east-west—striking graben swarm of upper part of the Gulf of Corinth is a west- northern Aegean indicate that the Aegean is western Anatolia (Fig. 1, C). There has been northwestward—striking graben (M'uller not a rigid integral western part of the considerable debate over the westward and Logters, 1974), compatible with its in- Anatolian plate, which itself is currently in- continuations of the two strands. Alptekin terpretation as a second-order extensional ternally deforming along east-west— (1973) considered the northern strand in- feature in a zone of northeastward-striking oriented normal faults and thus forms a active and continued the southern strand right-lateral shear. North of the Gulf of more feeble easterly continuation of the westward as a single transform across the Corinth, normal fault first-motion solutions Aegean strain zone (Salomon-Calvi, 1936; northern Aegean to cut Greece between (Fig. 1, C) are consistent with second-order Alptekin, 1973), and that a multi—small- Euboea and the entrance of the Gulf of extensional faulting in a zone of right- plate and/or continuum tectonic behavior Corinth. Bingol (1976) completely disre- lateral shear. These features therefore dominates. garded the southern strand and connected suggest a mechanism whereby right-lateral McKenzie (1972, Fig. 2) suggested that the northern one with a hypothetical east- motion is taken up from a triple junction at two plate boundaries dominate Aegean southeastward-trending extensional zone. the northern termination of the Hellenic tectonics: an Anatolian-Aegean boundary Brinkmann (1976) continued both strands Trench to the southwestern end of the diffusely defined by the east-west graben into, and terminated them in, the northern Sporades Trough: a wide zone of second- complexes of western Anatolia, and an Aegean. We suggest that seismicity and order extensional grabens striking roughly Aegean-Greek boundary defined more pre- morphologic expression (Ketin, 1966a; west-northwestward (Fig. 1, A). cisely by a ridge-transform system running Allen, 1975, Fig. 12) indicate that both Between the Gulf of Saros and Rhodes from the Dardanelles to the Gulf of strands are active and continue into the (Fig. 1, C), western Anatolia and the north- Corinth. He proposed that these two boun- Aegean — the northern strand into the eastern Aegean are dominated by a swarm daries meet a quadruple junction at the Saros and Sporades Troughs and the south- of roughly east-west grabens (Philippson, western end of the Sea of Marmara. In this ern strand into the Skiros Trough. Both 1918; Ketin, 1968,1970; Arpat and Bingol, paper we show how relative motion be- strands, however, show a deflection to a 1969; Brinkmann, 1976) that close east- tween the western edge of the Anatolian more southwesterly strike at the western ward and terminate westward along a line plate and adjacent

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