Journal ofthe Geological Society, London, Vol. 151, 1994, pp. 413-415, 0 figs. Printed in Northern Ireland

Discussion on a major Oligocene-Miocene detachment in southern Rhodope controlling extension Journal, Vol. 150, 1992, pp. 243-246

Ivan S. ZagorEev writes: The problem of the Late Cenozoic basin in the hanging wall. They trace the detachment extension in theNorthern Aegeanhas been discussed for north along the eastern boundary of the basin and on more than 30 years. The approach by Sokoutis et al. (1993) Bulgarian territory,along theeastern boundary of the and by Dinter & Royden (1993) seeks the control of this Sandanski graben. In both cases, the ‘Strymon valley extension in the displacements along very low-angle normal detachment system’ is uniting wrongly into a single fault faults of great extentand offset (detachments) which surface different normalfaults from the NW-SE and resulted also in a fast unroofing of deep crustal levels in the NE-SW sets which have been active in different times. footwall, and intensesediment deposition onthe down- Some of the faults, e.g. the West-Pirin fault zone (ZagorEev thrown side (i.e., formation of depocentres). This problem 1992a) together with its splay fault (Melnik fault), displace, is traditionally discussed withouttaking into account the the whole section of the Neogene whichin the Sandanski abundant evidence from theadjacent to the north, and graben embraces the time interval from the Sarmatian (or well-exposed andmapped Central-Balkan neotectonic even theLate Badenian) tothe Dacian (or even the region (ZagorEev 19926). Romanian) with a major event at the base of the Pontian The following important facets of this problem should be corresponding to a new phase of intense uplift of the Pirin taken into consideration. horst in the footwall. Maeotian and Pontian formations are (1) Age of the faulting event, and identification of fault also proven at the eastern side of the Pirin horst (along the movements of a given age. The region is characterized by Mesta valley in ) i.e. far in the ‘footwall block of several Alpinephases of thrustingproven with fossil- the detachment’. Thus, both the detachmenttrace (front) documentedsediments within the following limits: (i) and the time of displacement are inaccurately determined; Berriasian (Aptian?)-Turonian (Cenomanian?); (ii) Cam- the displacement is neither Oligo-Miocene (Sokoutis et al. panian (Maastrichtian?)-Late Eocene(Palaeocene?); (iii) 1993) nor mid-Miocene-early Pliocene (Dinter & Royden Mid-Eocene-Late Eocene; (iv) Mid-Oligocene (?); (v) 1993), but extensional faults were active at the periphery of earliest Miocene-Badenian. Normalfaulting is proven the horsts since the end of the Badenian (or the Sarmatian) (again with fossiliferous sediments) in the following times: to thelate Pliocene, and even during the Pleistocene (i)Late Triassic (Carnian-Rhaetian); (ii) EarlyJurassic (ZagorEev 1992~). (Hettangian-Pliensbachian); (iii) Bathonian; (iv) Callo- (3) Computation of displacementand extension. The vian; (v) Tithonian-Berriasian; (vi),., Cenomanian- computation of the extension is very importantfor Turonian; (vii) Coniacian-Campanian; (VIII)Maastrichtian; palinspastic neotectonic estimations, and different methods (ix) Palaeocene-Early Eocene;(X) Late Eocene; (xi) have been applied by the previous researchers. Many Mid-Oligocene-Early Miocene; (xii) Late Badenian- authors compute enormous extensionfactors (upto 3.5) Maeotian; (xiii) Pontian-Romanian; (xiv) Pleistocene- which is inconsistent with the geological evidence of others Holocene(e.g. Boyanov et al. 1989; ZagorEev 1992a, b). (Stiros 1991; ZagorEev 1992a, b). The visible extensions on Experience in southwest Bulgaria shows that somefault the deep seismic sections (e.g. Sokoutis et al. 1993, fig. Ib) surfaces have been activated two or three times after the are of theorder of 5 km along the master fault or of first movement, with different kinematics (e.g. first 10-15% of the graben section length, and ZagorEev (1992a, movement thrusting, second movement normal faulting with b) computedup to 30% extension in someparts of the a slight wrench component). Kockel & Walther (1965) have Sandanski graben but an overall extension varying from 3 to shown thatthe Strimonoverthrust (dated by ZagorEev & 10-15% over sections embracing several adjacent horsts and Moorbath 1983 as pre-Late Cretaceous) is intersectedand grabens. The computationmethods differ significantly displaced by Late Cenozoicnormal faults, and ZagorEev according to the accepted model. The ‘domino-style model’ (1992~)described low-angle normal faults that displaced the (Jackson & McKenzie 1987; Jackson & White 1989) admits Pontian-Dacian sediments of the Sandanskigraben block and fault rotations within the small blocks in the (Strumagraben system) and theolder folded mylonites hanging wall of the master (detachment) fault, and Dinter& bound to thrust surfaces of the Strimon overthrust. Royden (1993) make the assumptionthat the minimum (2) Tracing of the faultsurfaces. Both Sokoutis et al. extension is equal to the exposed width of the ‘detachment (1993) and Dinter & Royden (1993) accept the trace of the fault’, i.e. about 25km forthe ‘Strymon detachment Strimon overthrust of Kockel & Walther (1965) as the system’. The geological approachevaluates the vertical surface of a low-angle normal fault which represents a part difference betweeen the base of the sedirnents with a given of a detachment dipping to southwest. The detachment front age in the graben, and the corresponding in age planation is traced by Sokoutis et al. (1993) tothe south of the surface (peneplain, pediment) in theadjacent horsts (e.g. outcrops of the Strimon overthrust first to the east from the ZagorEev 1992a, tables 2, 3, 4). The extension is then Orfanos Gulf towards , and then south (west of the calculated in function of the dip angle of the border faults. island of Thasos), at the eastern edge of the Orfanos basin. In this case, with minimum dip angles of 20-25”, the Dinter & Royden (1993) trace thedetachment south extension reaches maximum values of 5-8 km. approximately in the same manner but include also the West (4) Conditions and age of the metamorphism; exhuma- 413

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tion of lower crust. Most of the generalizations about the complex system of faults belonging to several major fault fast unroofing of ‘ductile lower crust exhumed and deformed sets. Palaeogene granitoid plutons have been exhumed since along a major detachment’ or ‘the Rhodope metamorphic thePontian time. The maximum extension in Miocene- core complex’ are based on wrong assumptions about a very Pleistocene times was about 10-15% of the length of most young age (Mid-Cretaceous to Palaeogene or even Early profile sections but reached locally up to 30-50% along Miocene) of the metamorphism of theRhodope complex low-angle normal faults. No single detachment surfaces of (Rhodopian Supergroup) exposed in the footwall of the greatextent may be proven. TheRhodope metamorphic detachment. Even the eclogite relicts within the complex (Rhodopian Supergroup in Bulgaria) together with amphibolite-facies metamorphic rocks are regardedas the cross-cutting Hercynian granitoids was exhumed no later Jurassic toLate Cretaceous (Liati & Mposkos 1990). The than Late Cretaceous times. amphibolite-facies metamorphism is considered to be of 21 May 1993 Palaeogeneage (e.g. Arnaudov et al. 1990). These assumptions are based exclusively on K-Arand Pb-Pb D. Sokoutis, J. P. Brun, J. Vanden Driessche & S. ‘ages’ which are strongly influenced by late tectonothermal Pavlides reply: We thank ZagorEev for his discussion which events (ZagorEev 1991). In fact, weathered fragments from challenges our age for extension of the southern Rhodope all Rhodope metamorphic rock varieties including eclogites massif in rather than the extension itself. and serpentinites are present in non-metamorphic sediments ZagorEev’s first paragraph implies that the 19 phases of of Palaeogene age (fossil-proven Palaeocene-Mid-Eocene faulting so complicate his deformation history thatthe and Late Eocene-EarlyOligocene) which implies an possibility of identifying an extensionaldetachment of exhumation not only of the ‘ductile lower crust’ but also of regional extent is excluded. However, his second paragraph the amphibolite-facies crust not later than the Palaeocene. seems to contradict this implication by implicitly admitting Pebbles fromthe Rhodopian amphibolite-faciesmetamor- such astructure by discussing contours of adetachment phic rocks are present also in theUpper Cretaceous which, by his third paragraph is merely ‘pretended’. (Turonian-Maastrichtian) deposits north of theRhodope ZagorEev’s main objection ourto work is the massif (in the Srednogorie volcanic island arc), and even in Oligo-Miocene age we assign to the metamorphism and the the Upper Palaeozoic deposits in the same area (Kozhouk- related ductile-brittle extension of the Rhodope massif. He harov et al. 1980). The most important metamorphic events appears to consider that we and other workers would not be in the Rhodopian complex (Kozhoukharova 1988) are aware of problemsrelated to the resetting of radiometric proven as pre-Hercynian, and Hercynian granites (Kavala, data duringTertiary thermal events unaccompanied by SpanEevo and Krupnik plutons) are cross-cutting the folded penetrative strain. However, he himself claims that ‘a very Rhodopianmetamorphic rocks in the Pirin -Pangaion importanttectonothermal event’ was unrelated to any structural zone, in the footwall of thepretended contemporaneous ductile deformation. detachment. To insist thatthe main exhumation of theRhodope (5) Radiometricresults. Many authors working in massifwas ‘no later than late Cretaceous’, ZagorEev must northern Greece make far-gone conclusions from the scarce have overlooked a large body of recent work documenting data obtained in theRhodope massif by isotopic studies. geological as well isotopic,as evidence for ductile Most of thesestudies have been performed by theK-Ar deformationduring metamorphism related to Alpine method, and the data obtained are being taken at their face thrusting (e.g. Burg et al. 1990; Kilias & Mountrakis 1990) values asages of metamorphism,intrusion or effusion, or the Tertiary extension (e.g. Kolokotroni & Dixon 1991; respectively. Jones et al. 1992). We consider that Kokkinakis (1980), TheRhodope region underwenta very important Eleftheriadis & Lippolt (1984), Kyriakopoulos et al. (1989) tectonothermalevent in Late Eocene-Early Oligocene have convincingly argued thatthe Kavala pluton was still times (Harkovska et al. 1989; ZagorEev 1991). About 50% being mylonitized as late as Early Miocene, between 18 Ma of the territory was covered by lava sheets and flows with a and 14Ma. thickness oftenexceeding 500m, and large Palaeogene We (Sokoutis et al. 1993) were very careful to use plutons are exposed in the youngest uplifted blocks as the several lines of geological evidence todemonstrate that Pirin horst (ZagorEev 1992a, fig. 6). The unroofing of the ductile extension in the Kavala pluton and its country rocks Palaeogene plutons occurred in the Pontian, and the vertical was, clearly andundoubtedly, coeval with,and kinemati- uplift reachedprobably 5-7 km since the Mid-Oligocene, cally related to the development of the surrounding basins and 2.5 km since the Sarmatian. The heat flow is still very containing Serravallian sediments in the basin high in the region (e.g.Stiros 1991). Temperatures of the (Kousparis 1979) and Tortonian sediments in the Serres and order of 300-500°C have been maintained for a long time in Prinos basins (Buttner & Kowalczyk 1978; Kousparis 1979; the basement, and thus the K-Ar and Pb-Pb data obtained Lalechos 1986). The same relationships hold for the ductile can be due to ‘mixed ages’, intenseradioactive argon loss extension on the island of Thassos. during Palaeogene heating, and prolonged cooling (cf. York Although ZagorEev addresses most elements of his & Farquhar 1972; Jager & Hunziker 1977). K-Ar dates discussion equally to bothDinter & Royden (1993) and cannot be taken as reliable even in the case of the youngest Sokoutis et al. (1993), these two essentially contem- (Palaeogene) volcanic rocks unless they are confirmed by poraneousinterpretations of the same extension were independent geological evidence (fossil-dated volcano- completely independent. As long as ZagorEev ignores these sedimentary formations). lines of evidence, he will beunable to recognize that the Thenorthern Aegeanextensional province developed metamorphic rocks of the southern Rhodope massif were since the end of the Mid-Miocene time simultaneously with exhumed by Mid-Tertiary extension. vertical displacementsreaching up to 3-5 km along a 23 June 1993

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I. S. ZAGOR~EV,Bulgarian Academy of Sciences, Geological Institute 'Strashimir Dimitrov', Sofia 1113, Str. aead. G. Bonehev block 24, Bulgaria D. SOKOUTIS& J. P. BRUN,Laboratoire de Tectonique, UPR 4661, CNRS,Universitt Rennes I/BeauCeu, 35042 Rennes CCdex, France J. VAN DEN DRIESSCHE, Laboratoirede Tectonique et MCcanique de la Lithosphtre, UA-CNRS 1093, IPGP et Universitt Paris 7, 2 Place Jussieu, 75251 Paris CCdex 05, France S. PAVLIDES, Aristotelian Universityof Thessaloniki, School of Geology, 54006 Thessaloniki, Greece

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