Journal of the Geological Society, London, Vol. 150, 1993, pp. 243-246, 3 figs. Printed in Northern Ireland A major Oligo-Miocene detachment in southern Rhodope controlling north Aegean extension D. SOKOUTIS 1, J. P. BRUN l, J. VAN DEN DRIESSCHE z & S. PAVLIDES 3 1Laboratoire de Tectonique, UPR 4661 CNRS, Universit~ Rennes I/Beaulieu, 35042 Rennes C~dex, France 2Laboratoire de Tectonique et M~canique de la Lithosphkre, UA-CNRS 1093, IPGP et Universit~ Paris 7, 2 Place Jussieu, 75251 Paris Cddex 05, France 3Aristotelian University of Thessaloniki, School of Geology-Department of Geology & Physical Geography, 54006 Thessaloniki, Greece Plutonic and metamorphic rocks of the southwest part of the Rhodope massif in Greece correspond to ductile lower crust ex- humed and deformed along a major detachment during post- thickening extensional tectonics. Extension started during the Olig- ocene and is responsible for the development of Miocene- Quaternary sedimentary basins. Both brittle and ductile defor- mations result from gravity collapse of previously thickened fithosphere, as proposed for others large extended terranes. This interpretation disagrees with the previous models which attributed Tertiary ductile deformation to Alpine thrusting and brittle exten- sional deformation to back arc tectonics above a subdnction zone. The Rhodope massif, in northern Greece and Bulgaria, is bounded by the Balkan belt to the north and the Dinarides-Hellenides to the south, two branches of the Alpine-Himalayan collision system (Dewey et al. 1973; Hsii et al. 1977). Recently, it was demonstrated in Bulgaria (Burg et al. 1990) that the ductile deformation of the massif is not Precambrian or Hercynian as previously though but corresponds mainly to Alpine tectonics leading to crustal thickening. During the last two decades the Aegean has been a critical area for the understanding of crustal extension, demonstrated on the basis of fault analysis in basins (Angelier et al. 1982; Mercier et al. 1989), focal mechanisms of earthquakes (McKenzie 1978; Taymaz et al. 1991) and basin stratigraphy (Biittner & Kowalczyk 1978). It has been related to back arc tectonics above a subduction zone (Angelier et al. 1982; Mercier et al. 1989), whereas ductile deformation was considered to occur earlier during Alpine compression. More recently, exhumation of metamorphic terranes during extension has been identified in the Cyclades in South Aegean (Lister et al. 1984; Gautier et al. 1990; Buick 1991). We emphasize here the existence of a similar domain in the Greek Rhodope (North Aegean). In both regions ductile extension is extremely strong implying that their significance at the scale of the whole Aegean is especially relevant. 243 Kinematic data. The Kavala and Thassos areas in the southwestern Rhodope massif display a strongly deformed basement overlain by Miocene basins (Fig. la). Basement rocks consists of ortho- and paragneisses, amphibolites, miscaschists and an uppermost thick sequence of marbles. These units are classically related to the Lower Tectonic Unit of the massif (Papanikolaou & Panagopoulos 1981). Metamorphic grade ranges from middle greenschist to amphibolite facies (Kroneberg et al. 1970; Dimitriadis 1989; Kilias & Mountrakis 1980). The widespread occurrence of Jurassic to Late Cretaceous eclogites (Liati & Mposkos 1990) argues for high pressure (15kbar) metamorphism during Alpine thrusting (Burg et al. 1990). The Kavala granite, which intrudes the Lower Tectonic Unit (Kok- kinakis 1980), is elongated in a NE-SW direction and strongly deformed. The mylonitization increases towards the contact with the country rocks. Thassos island is a broad domal structure, similar to Kavala, with intrusive granitoids in the central part. Both granites and the upper most unit layered marbles, display strong mylonitization. Ductile deformation in Kavala-Thassos results from a major shearing deformation giving: (a) a fiat-lying foliation nearly parallel to the lithological layering, (b) a strong NE-SW trending stretching lineation and (c) shear criteria indicative of top-to-the southwest direction of movement. Small folds are generally isoclinal with axes parallel to the stretching lineation. Common sheath folds (Cobbold & Quinquis 1980) indicate large finite strains. Widespread C-S shear fabrics in the granites indicates syntectonic emplacement (Gapais 1989). The upward increase in finite strain intensity, observed in the granitoid core of Thassos, corresponds to the shear gradient associated with a major shallow southwest-dipping shear zone. Radiometric data for the mylonitization (Kokkinakis 1980; Kyriakopoulos et al. 1989; Eleftheriades & Lippolt 1984) provide ages between 18 and 14 Ma for the mylonititzation. Upper Miocene (Martin 1987) to Pleistocene (Mercier et al. 1989) sediments which overlie marbles in the southwestern part of Thassos show evidences of syn- sedimentary tectonics as block tilting associated with listric normal growth faults, roll-over folds and slumps during NE-SW extension. Constant tilting towards the northeast of the sediments argues for a major SW-directed shear within the underlying basement as observed in the rest of Thassos. This overall consistency of kinematic inidicators argues for a cogenetic relationship between brittle and ductile deformations in the study area. Syntectonic sedimentation is also evident in seismic profiles within the Nestos, Prinos and Orfanos basins (Martin 1987) (Fig. lb). Drill-hole and off-shore seismic data show that the age of the sediments, which directly overlie the metamorphic basement, are of Servallian age (Kousparis 1979) in the Nestos basin, and Tortonian (Btittner & Kowalczyk 1978; Kousparis 1979; Lalechos 1986) for the rest, i.e. close to the age of the shearing deformation recorded in the basement. The Orfanos and Prinos basins are separated by a SW-dipping major normal fault which strongly affects the pre-Messinian and at least part of the Pliocene formations. Tilted blocks, northeast of this fault are considered equivalent to those of the southwest part of Thassos. In the Prinos basin. SW-dipping normal faults delimit large tilted blocks, while small listric normal faults associated with roll-over anticlines affect the basin fill. Transverse seismic lines (Martin 1987)