A Geophysical Investigation of Post-Alpine Granites and Tertiary Sedimentary Basins in Northern Greece

Journal of the Geological Society, London, Vol. 146, 1989, pp. 53-59, 10 figs, 1 Table. Printed in Northern Ireland A geophysical investigation of post-Alpine granites and Tertiary sedimentary basins in northern Greece F. MALTEZOU’ & M. BROOKS2 l Department of Geology, The University, Southampton S09 SNH, UK 2Department of Geology, University College, PO BOX 78, Cardiff CFl lXL, UK Ab-& The geology of the Rhodope region of northern Greece consists of crystalline basement rocks of the Rhodope Massif in which Tertiary extensional basins and granitic intrusions of Oligocene age occur widely. Geophysical modelling has highlighted the close spatial relationship of the Tertiary sedimentary basins and post-tectonic granites of closely similar age. Geochemical data for selected granites are used to support the suggestion that there is a close genetic relationship between the episodeof granite intrusion and the tectonic development of extensional basins. Regional geology Granitic intrusions of varying composition and age are also widespread in the Rhodope region. Bitzios et al. (1981) The Rhodope Massif is a crystalline basement area which classified theseintrusions into Pre-Alpine, Alpine and occupies a large part of southern Bulgaria and north-western Post-Alpinegranites. Radiometric dating of anumber of Greece and a small part of north-western Turkey (Fig. 1). Post-Alpine intrusions consistently gives an Oliogocene age Thismassif, together with theSerbomacedonian Massif, (Britzios1973; Kronberg 1974; Britzios et al. 1981; abutsthe Vardar zone and the Pelagonian and sub- Kyriakopoulos 1987). A K-Ar age of 27.9 Ma was given by Pelagonian zones of the Internal Hellenides (Marinos 1982). Kronberg (1974) forthe Xanthi granite. Kyriakopoulos The western boundary of the Rhodope Massif in Greece is (1987) gave Rb-Sr ages of 31.9 f 0.5 Ma and 31.8 f 0.6 Ma delineated by the ‘Strimon line’, a major fault zone which for the same granite. The Leptokarya granite has yielded a separates it from the Serbomacedonian Massif. The surface K-Ar age of 28Ma (Bitzios1973) and Rb-Srages of geology of the Greek Rhodope region comprises crystalline 31.9 f 0.5 Ma and 31.8 f 0.6 Ma (Kyriakopoulos 1987). The basementrocks of theRhodope Massif,which are of age of the Philippi granite (Fig. 2) is given by Bitzios et al. uncertain age,the MesozoicCircum-Rhodope Belt and a (1981) as28Ma. For the Maronia monzodiorite, in the number of Tertiary sedimentary basins. Granitic intrusions eastern part of the Rhodope region (not shown in Fig. 2), a occur widely throughout the area. Ultramafic rocks are also Rb-Sr age of 29.8 f 1.3 or 28.9 f 0.1 Ma (Kyriakopoulos present in the Eastern Rhodope region. 1987) tends to discount the Mid-Miocene age given on the Themetamorphic basement of theRhodope Massif geological map of the area. Overall, a dateof approximately consists of a variety of medium- to high-grade metamorphic 28 Ma is considered to be a realistic age for many of the rocksincluding marble,granite gneiss, amphibolite and unmetamorphosed, undeformed granites. It is also possible serpentinite.In the Eastern Rhodope the metamorphic that earlier intrusive ages may have been reset by elevated basement is structurally overlain by a sequence of low-grade temperatures during the Alpine orogeny. metasedimentsand meta-igneous rocks of Mesozoic age. In addition to acid plutonic rocks, the Rhodope region Thisunit is knownas theCircum-Rhodope Belt and contains a large number of rhyolitic and rhyodacitic dykes comprises the Phyllite (orMakri) Series and the Drimou Melia Series. The Circum-Rhodope Beltis considered to be 200 2 8O equivalent to the Peonias unit of the Vardar zone (cited in Papanikolaou 1984). Yugoslavia ’; Bulgaria The Tertiarysedimentary basins often contain interca- -42O lated volcanics of mainly acidic composition. These basins are fault-controlled and of large area1 extent. Their trends differ across the region. The Strimon and Philippi basins in the west (Fig. 2) have a NW-SE orientation, parallel to the Hellenidestructural elements. The Prinosbasin, which is the western part of the Nestos basin (Fig. 2), has a NE-SW -40° orientation cross-cutting the Hellenide elements. The above three basins areconsidered to havebeen initiated in Miocenetimes. All other major basins to the east of the Prinosbasin (Komotini-Xanthi basin, Kirki-Essimi basin, Orestiasbasin) have approximately an E-W orientation (Fig. 2) andwere initiated in Eocenetimes. A phase of Fig. 1. Main geotectonic divisionsof the Internal Hellenides and Oligocene volcanism characterizes the latter basins. adjacent areasof northern Greece. 53 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/146/1/53/4897562/gsjgs.146.1.0053.pdf by guest on 27 September 2021 54 F. MALTEZOUBROOKS & M. 24*00' 26.00' BULGARIA Fig. 2. Map of the Greek Rhodope region showing the main Tertiary basins and major intrusions.The Komotini-Xanthi-Kavala fault is also shown. B1, Strimon basin; B2, Philippi basin; B3, Prinos basin; B4, Nestos basin; B5, Komotini-Xanthi basin; B6, Kirki- Essimi basin; B7, Orestias basin; G1, Phillipi granite; G2, Xanthi granite; G3, Granite at the eastern flankof Nestos basin; G4, Leptokarya granite. The Komotini-Xanthi- Kavala and Avdhira faults are also shown. and sills. These rocks, which belong the the north Aegean havenot produced majorsedimentary basins in the volcanic province, are of Eocene-Oligocene age (Fytikas et Rhodope region. al. 1984) andappear tobedirectly related tothe Post-Alpine granitic rocks. Geophysical studies Geophysical data, in the form of gravity and aeromagnetic Tectonic evolution maps of theRhodope region, were made available by Theformation of earlyTertiary basins in theRhodope IGME.The regionalgravity maps areata scale of regionmight be relatedto extension in amarginal basin 1:25OOOO andare contoured at a 50 gu interval. The associated with a subduction zone earlier than the present average density of gravity observation points is approxim- subductionregime of theOuter Hellenic arc. Ithas ately one station every 3-4 km'. previouslybeen suggested that sucha subduction zone, Additionalgravity information was obtainedfrom a whereoceanic lithosphere was consumed, existed in the marineBouguer anomaly map of theNE Aegean area NorthAegean (Papazachos 1976; Papazachos & Papado- (Morelli et al. 1975). This map covers an area to the east poulos 1977; Fytikas et al. 1984) and this might therefore be and south of Thasos island (Fig. 2), up to a small distance theexplanation for the extensional basinsand associated (approximately 15 km) from the coastline of the Rhodope volcanicity of earlyTertiary age in theRhodope region. region. Thecontour interval is 100 gu andthe scale Robertson & Dixon (1984) also considered the possibility of 1:75OOOO. By merging the two datasets a homogenized separate oceanic tracts, in order to explain the magmatism gravitymap of the Rhodope and NE Aegean regionwas in both the Thrace and the southern Aegean areas, pointing produced(Maltezou 1987), whichcovers an area of outthat Papavassiliou & Sideris (1982) attributedthe 30 000 km2. This map has a 50 gu contour interval, but only Tertiarylavas of westernThrace to north-eastward the 100 gu interval contours are shown in Fig. 3. subduction of Vardarian oceanic crust. DetailedBouguer anomaly maps of the Phillippiand Fytikas et al. (1984) related the early Tertiary volcanicity Nestos basin areas were also made available by IGME for to subduction of theAfrican plate beneath the Eurasian studying the individualanomalies associated with these margin.In this model, asubsequent phase of collision basins. between Eurasia and a microcontinental block at the leading Theabove data sets were used in the construction of edge of the African plate resulted in the southward jump of gravity profiles across theelongate anomalies associated the volcanicity to its present location in the Cyclades arc. with the basins, which were interpreted by 2D modelling. Most information about the age of initiation of extension 3D modelling was also carried out in the case of the Philippi in eachbasin comes fromthe age of thesedimentary formations involved. The oldest sediments described in the Strimon, Prinos and Philippi basins (Fig. 2) are of Miocene age. The oldestsedimentary formations in the Komotini- Xanthibasin are of Eoceneage, whilst theMiocene is absent. In the Orestias and Kirki-Essimi basins, the Public PetroleumCorporation of Greece(DEP) and the Greek Institute of MineralExploration (IGME) boreholes show thatUpper Eocene sediments lie directly onthe metamorphic basement. Neogene sediments are absent from the Orestias basin. Normal faults of early Tertiary age exist throughout the Rhodope region andbound the extensional sedimentary basins. As an example the Komotini-Xanthi-Kavalafault (Fig. 2), whichdefines thenorthern boundaries of the Nestos basin and the Komotini-Xanthi basin, is known to haveexisted since the Eo-Oligiocene (LybCris 1984). - Eocene basins aredistributed through the eastern part of 20 km the area and are separated by a major fault zone (Avdhira Fig. 3. Bouguer anomaly gravity mapof the Rhodope fault) (Fig. 2) from the Miocenebasins in thewestern and NE Aegean area. Contour interval is 100 gu. P2,P1, Rhodoperegion. Extension has continued through the P3, P4: Profiles acrossthe Philippi, Prinos, Komotini- Neogene and Quaternary, but the later phases of extension Xanthi and Orestias basins respectively. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/146/1/53/4897562/gsjgs.146.1.0053.pdf

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