Journal ofthe Geological Society, London, Vol. 148, 1991, pp. 749-758, 7 figs, 1 table. Printed in Northern Ireland

The Oligocene pluton, northern : a granodiorite emplaced during regional extension

I. KOUKOUVELAS' & G. PE-PIPER2 Department of Geology, University of Patras, 261 10 Patras, Greece Department of Geology, St Mary 'S University, Halifax, N.S. Canada B3H 3C3

Abstract: TheXanthi pluton is one of aseries of Oligocenesubduction-related granodiorites in northern Greece. Its emplacement was controlled by major faults. It is located on the ENE-trending - fault zone, which probably originated as a strike-slip fault. Geophysical data show that the pluton is laccolith-shaped, extending many kilometres south of the fault. Andesitic dykes several million yearsolder than the pluton indicate a NE-SW-directed extensional stress field. Minerallineations plunging tothe SW reflect continuedextension during cooling of thepluton. Subsequent jointing and dykes of aplite and lamprophyre reflect continued extension. Although this could result from extensional pull-apart at a bend during dextral strike-slip motion on the Kavala- Komotini fault, there is no evidence for subsequent strike-slip deformation. Furthermore, two similar laccolithic plutonsare unrelated to the Kavala-Komotini fault and the regional extent of thick Oligocene sediments suggests an extensional environment. During the extension that created a ramp spacealong the listric faultbounding the Xanthi basin, into which thegranite was intruded, the Kavala-Komotinifault acted as a transfer fault. Reactivation of earlyjoints occurred during late Oligocene-EarlyMiocene compression. Faults within thepluton parallel to the Kavala-Komotini faultaccompanied extension during Neogene basin formation.Emplacement of subduction-related magma into ramp space developed along listric faults, to produce laccolith-like plutons, may be a common feature of back-arc extension.

The Oligocene Xanthi pluton (Christophidis 1977) intrudes consequence of local uplift and give anerroneous the southern margin of the metamorphic complex impression of the subsurface abundance of granite. of northernGreece (Fig. 1).The southern edge of the The purpose of this paper is to describe field structural exposed pluton is marked by a major fault (the evidence for thenature of emplacement of the Xanthi Kavala-Komotini fault zone);to the south lies the E-W pluton. Thisinformation is used to evaluate the role of trending Komotini-Xanthi basin, with thick Neogene strike-slip and extensional tectonics in the emplacement of sediments. The magneticsignature of the pluton suggests the Xanthi pluton, in order to develop concepts applicable that it is a laccolith with a base at about 4.5 km (Maltezou & to other back-arc extensional areas. Brooks 1989). The laccolith is bounded to the southwest by the Avdhirahorst (Fig. l), in which Eocene-Oligocene volcanic and sedimentary rocks are exposed (Fig. 2). Rocks Tertiary igneous activity of similar age are exposed north of the Kavala-Komotini Rb-Sr ages obtained forthe Xanthi intrusion are fault zone, at the eastern end of the Xanthi pluton. Such 28.8 f0.7 Ma (in the western part) and 26.3 fO.l Ma (in Upper Eocene-Oligocene marine sediments are widespread theeastern part) with initial 87Sr/86Srratio of 0.7065 f in northeastern Greece and reach a thickness of over 2 km O.OOO1 (Kyriakopoulos 1987). Severalgranodiorite plutons in the Limnos well (Schroder 1986). of similar age to the Xanthi pluton outcrop in northeastern The Kavala-Komotini fault extends westwards to at least Greece. The Philippi pluton (28 Ma, Bitzios et al. 1981) and 25 km west of Kavala and eastwards to the Greek-Bulgarian the Myrodato pluton (Kouris 1978) have subcircular outlines boundary (Dimadis & Zachos 1986). Metamorphic rocks of and similar magnetic signature to the Xanthi pluton (Fig. 1). the Rhodope massif are found north and west of the fault; The Vathiremma pluton NW of Komotini lies just north of Rhodope metamorphicrocks, ophiolites of the circum the Kavala-Komotini fault, but is considerably deformed Rhodope zone and Neogene basins are found south andeast and is interpreted by Dimadis & Zachos (1986) as an older of the fault(Dimadis & Zachos 1986). Both the age and pluton. In (Del Moro et al. 1988), there is kinematics of fault are uncertain. It is likely that such a long an ENE-trendinglinear zone of small granitoidplutons linear fault zone originated by strike-slip motion. Since the (Maronia, Kassitera, Kirki, Leptokaria, Halasmata and Tris middle to upper Miocene, the fault has moved with a major Vrisses). These consist of quartz gabbros to granodiorites, dip-slip senseduring the Neogeneextension of northern with Rb-Sr isochron ages of 28-32 Ma. Greece. Maltezou & Brooks (1989) have shown from The Kavala pluton (Fischer 1964; Kokkinakis 1980; geophysical datathat severalplutons form laccolith-like Kyriakopoulos 1987) lies onthe trend of the Kavala- bodies beneath Tertiary basins, and suggest that there may Komotini fault. The pluton is mostly on the south side of the be a geneticbea link betweenextensional tectonics and fault, but the northeastern end of the pluton near Kavala emplacement of the plutons. The surface outcrops of appears to lie north of the fault, as if there is an offset in the plutons in such an extensional regime may be the fault in this region. The pluton consists of relatively 149

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Fig. 1. Geological map of northeastern Greece, showing Rhodope Massif, prin- cipal faults, granite plutons, and Tertiary basins. Based on IGME 1 :500 OOO Geological map of Greece (Bornovas & Rondogianni-Tsiambaou 1983); Ne- ogene basins from Lalechos & Savoyat (1977); faults at sea from Martin(1987). Magnetic anomalies based on IGME t :+ -9 ** 1 : 50 OOO aeromagnetic maps of Rho- dope massif (ABEM 1967): contoured at Is. 100 nT, only +300 nT and greater contours shown.

Fig. 2. Geology of Xanthi pluton and adjacent areas based in part on Christophidis (1977) and Eleftheriadis et al. (1984). Faults based on air photo- D' graph interpretation. Cross sections show nature of the margin of the pluton at selected locations. Magnetic anomalies based on ABEM (1967): contoured at 100 nT.

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Table 1. Summary of structural events in the Xanthi pluton

Andesitedykes, strike NWRegional NE-SW-directed extension Radiometric date 33.5 Ma (early Oligocene) Mineral lineationContinued extension duringintrusion Radiometric dates west,29 in Ma 26 Ma in east (late Oligocene) Early joints Continued extension during cooling Aplo-granites Intruded along early joints Aplite veins, lamprophyre Intruded along early joints, radial dykesand sills to margin of pluton,and along SE-striking subvertical planes associated with continued regional NE-SW-directed extension Faults mostlywith strike-Associated with regionalcompression: Regionally late Oligocene- slip motion many are reactivated early joint surfaces early Miocene Hydrothermal alteration Developed along early joint surfaces and ?Early Miocene faults associated with compression Conjugate faults, strike NW* Regional extension Steep-dipping faults, Parallelto Kavala-Komotini fault, associated Mid-Miocene and later strike Estrike or ENE with extension

* These faults possibly predate the faults with strike-slip motion.

leucocratic granodiorite; and it hasa different magnetic (Koukouvelas 1989). The age of this event is constrained by signature from the Xanthi pluton. The pluton cuts the early the youngest radiometric age for volcanism in the area of NNW-trending major folds in the Rhodope basement rocks, 23.6Ma (Eleftheriadis et al. 1984; Fytikas et al. 1984) and but ishighly deformed by structuresthat parallel the the lack of compressive deformation in mid-Miocene rocks in Kavala-Komotini fault. K-Ar datinghas yielded dates the Komotini-Xanthi basin. Neogene rocks are largely between 26.3 and 15.5 Ma, which are partly or entirely undeformed, but are affected by a series of syn-sedimentary cooling ages. Kokkinakis (1980) obtained highly discordant normal faults.Near Xanthi,these faults follow two main zircon ages (three of which were Cretaceous,one structuraldirections. One is thetrend of the Kavala- Hercynian). The pluton post-dates the main deformation of Komotini fault; the other is the NNW trend which bounds the Rhodope massif the age of this deformation is disputed, the Avdhira horst. There are two main hypotheses for the but is most probably Cretaceous (on the basis of ages of origin of thesestructural trends.They may represent syn-tectonic granites: Soldatos 1985) or Eocene (on the basis extensional faultsdeveloped in post-Eocenetime by of radiometricdating of metamorphic amphiboles:Liati extension of an overthickened nappe pile (Doutsos & 1986). In this case, the Kavala pluton may bea late Ferentinos 1984). Alternatively,they may represent Cretaceous or early Tertiary shear zone pluton. respectively dextralstrike slip and syntheticextensional Early Tertiary volcanic rocks are widespread in Thrace faults which experiencedpredominantly dip-slip motion and eastern (Innocenti et al. 1984). Most during Neogene extension (Dewey & Sengor 1979; Martin radiometric age determinations fall in the Oligocene to 1987). Early Miocene(Soldatos 1961; Eleftheriadis & Lippolt The Kavala-Komotini fault in places separatesthe 1984), but volcanic rocks are also interbedded with Upper Rhodope massif from the circum-Rhodope zone: Kockel et Eocene sediments(Fytikas et al. 1984). These rocks are al. (1971) suggest thatthe latter was deposited on a principally andesites,dacites and rhyolites with relatively continental slope seaward of a Rhodope continent. Eocene high K content: they form a typical subduction-related rocks north of the Kavala-Komotini fault tothe east of volcanic suite(Boccaletti et al. 1974; Fytikas et al. 1984; Xanthi are of flysch facies; those to the south are mainly Innocenti et al. 1984). The geochemical similarity of the molassse facies and shelf limestones (Kronberg & Eltgen rhyolites tothe Xanthipluton indicates that theyhave a 1971), suggesting that either the fault marked the margin of common origin (Kotopouli & Pe-Piper 1989). Volcanic theEocene flysch basin, or experiencedsubstantial activity ceased at about the base of the Miocene (Fytikas et post-Eocene strike slip motion. The Kavala-Komotini fault al. 1984, table 1). may have originatedas astrike slip fault in theLate Cretaceous or Early Tertiary, separating these two geologic terranes and bounding the thick flysch of the Eocene basin. Regional structural development The Kavala pluton appearsto have been intrudedand TheRhodope massif consists of multiply deformed and deformed in this fault zone. The linear NNE trend of the metamorphosed rocks in a series of thrust sheets (Kockel & Kavala-Komotini fault is offset in two places: at the Kavala Walther 1965; Ivanov 1981; Liati 1986). Regionalmapping Pluton andat the Xanthi Pluton,where a 20 km-long shows prominent folds with NE-trending axes (B1 of Meyer segment of the fault trends east (Fig. 2). 1968) and in some areas a later set (B2) with WNW-trending Major extension has taken place since the mid-Miocene. axes. Eoceneto Oligocenesedimentary and pyroclastic The Komotini-Xanthi basin has at least 1.2 km and rocks south of theRhodope massif were deformed in a probably more than 2 km thickness of Neogenesediment compressive phase nearthe Oligocene-Miocene boundary (Lalechos & Savoyat 1979). TheDEP borehole near

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General geology of the Xanthi pluton pluton (confirmed by an observed intrusive contact of granodiorite). Andesitic dykes with a NW or N trend also The mountainous terrain makes it difficult mapto cut lower Tertiarysediments in the Avdhirahorst. The systematically the pluton and in many areasthe plutonic orientation of the andesite dykes indicatea regional rocks are deeply weathered. Therefore systematic structural extensional stress field with NE-SW extension. observations are largely restricted to forestroads. The Xanthi pluton is intruded into marbles and gneisses of the Rhodope massif on its northernand western sides. The Marginal structures of the pluton eastern end of the plutoncutsEocene-Oligocene In general,structural deformation of the margin of the sedimentary rocks (dominantly flysch) and andesite dykes pluton has not been seen. Sharp igneous contacts without (Fig. 2). There is a contact metamorphic aureole typically a ductility contrasts and locally with veining are found near few hundred metres wide (Liati 1986). The southern margin Xanthi. The northwesterncontact of the pluton with of the pluton appears to be in fault contact with Neogene mylonitic rocks is poorly exposed, but the rest of the and Quaternary rocks,except for a small area of gneiss northerncontact of the pluton has irregular igneous described in detail below. contacts, with local evidence for veining and stoping (Ayne Granodiorite, containing hornblendeand biotite, and et al. 1979). Thesouthern margin of the pluton is fault locally passing into granite makes up three quarters of the controlled; brittle structures are seen in the pluton, but the outcroparea. The eastern end of the pluton consists fault zone is masked by Plio-Quaternary sediments. Veining principally of monzonite (Fig. 2), with lesser quartz is also seen on part of the southeastern margin, where the monzodiorite, quartz monzonite and monzogabbro,con- granodiorite is in intrusive contact with gneiss (of uncertain taining biotite and pyroxene. The granodiorite has a chilled affinity but with subvertical foliation) cut by andesite dykes, contact with monzonite. At the eastern end of the pluton is whichin turnare slightly offset by faults along which a pyroxene-olivine gabbro: it is unclear whether this is part granodiorite veins have been intruded. Thus with the of thepluton, since Christophidis (pers. comm. 1988) has exception of the major fault along the southern margin of obtained a radiometric age of 157 Ma from the gabbro: this the pluton, there is no evidence for significant deformation date is very similar to that obtained from circum-Rhodope of the margins of the pluton. ophiolites (Spray et al. 1984). The granodioritecontains mafic inclusions of rounded Mineral foliations and lineations shape and these are several centimetres in diameter. They occur mainly near the margin of the granodiorite. They have Locally, indistinct primary foliation surfaces are defined by sharp boundaries,a fine-grained texture,quartz diorite biotite. In most places, however, the only fabric visible in compositions andcontain biotite, hornblende andrare the field is a lineation picked out by mafic minerals such as augite.They areinterpreted either ascognate inclusions hornblende.Lineations generally plunge tothe SW, (Christophidis 1977) or the result of mingling of mafic and typically between 30" and 65" (Fig. 3). Plunges nearthe felsic magma.Aplo-granites are found in subhorizontal pluton margin are more variable, particularly at the extreme sheets in the western part of the pluton.Aplite veins are western end of the pluton. In thin section, lineation appears found throughout the pluton. A few lamprophyric dykes and picked out by hornblende and in some cases by plagioclase; sills also cut the pluton. quartz and K-feldspar show apparently random orientation. The intermediate and acid rocks from the Xanthi pluton The lineation thus resembles the 'pre-full crystallization yield relatively flat REEpatterns, consistent with early fabric' of Hutton (1988,).Mafic cognate xenoliths also fractionation of olivine andclinopyroxene from a mafic occur, generally elongated in the samedirection as the parent magma (Kotopouli & Pe-Piper 1989). These rocks mineral orientation (cf. Hutton 1982). Only rarely are show incompatibleelement patterns indicative of active 'crystal plastic strain'fabrics seen, in which crystals show continental margin-type tectonicenvironment and plot in evidence of creep, dislocation and bending: these occur in the upper part of the volcanic arc field on the Rb-(Y + Nb) samples that also show fracture deformation. Brittle fracture discriminant diagram by Pearce et al. (1984). The deformation is widespread in the pluton. subduction-related origin of the Xanthi pluton is indicated by its geochemical distribution patterns. Early joints A distinctive set of closely-spaced joints,referred to as Detailed observations on the Xanthi pluton 'early joints' appearto be the oldest post-crystallization Structural features have beenmapped throughout the features in the pluton, being cut by all other features. well-exposed areas of the main granodioritephase of the Locally, joints are developed along the poorly defined Xanthi pluton, principally on road cuts. These features are biotite foliation planes. Representative orientations of early described below in approximate order of their age, and a joints in the pluton are shown on the map and all data are brief interpretation of each is provided (Table 1). plotted in stereograms in Fig. 4. In the western part of the pluton (stereogram A, Fig. 4), the joints dip consistently to the east, commonly at angles of20" to 70". In the eastern Andesite dykes in country rock part of the pluton,joint orientations are morevariable Maficsills anddykes (striking approximately NNW,steep (stereogram B, Fig. 4). Some joints have a similar strike but dips toENE or WSW) intrudethe Eocene-Oligocene lower dipthan those in the west. In addition, there is a

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Fig. 3. Map of the Xanthi pluton showing representative dipsof mineral lineations, with stereogram showing pattern generalized for entire pluton. The stereogram contours are at 2.8, 5.7,8.5%; n = 70.

1 Fig. 4. Map of the Xanthi pluton showning representative strikes and dipsof early joint surfaces,with stereograms showing distribution of early joint surfaces in the (A) western and (B)eastern parts of the pluton. Contours for (A) at 4.7, 9.5, 14.2,19.24%, n = 42 and for (B) at 2.9, 5.8, 8.8%, n = 34.

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conjugate set of joints dipping SW and NE at angles of 30 to margins of the pluton (map, Fig. 5), or in the case of the 50": this conjugate set is discussed in more detail below. In western end of the pluton, are parallel to the early joints bothparts of thepluton, there are somesubhorizontal (compare stereogram A, Fig. 4 with stereogram A, Fig. 5). joints. Aplites atthe faulted margin of the pluton immediately north of Xanthi either show steep dips parallel to the fault or lesser dips orthogonal to the fault and parallel to local Aplite veins cooling surfaces. Inthe east part of thepluton, several Aplo-granites occur in the western part of the pluton. Most kilometres from the margin (stereogram B in Fig. 5), a few form subhorizontal sheets dipping gently eastwards that aplites show low dips subparallel to earlyjoints. Most, parallel earlyjoint surfaces inthis area.In addition, however, dip at high angles to the southwest or northeast. high-angle aplite veins are common in some parts of the Similar orientations are found to isolatedaplite dykes granodiorite pluton (Fig. 5). They can be distinguished from cutting thegabbro and theEocene sedimentseast of the the aplo-granites by the lack of biotite. These aplite veins pluton. are sharp sided and generally occur in parallel sets. They cut The orientation of many of the aplite dykes, particularly the earlyjoint surfaces at a low angle andare therefore in the west of the pluton, appears to be sub-parallel to early younger than the cooling phase that produced these joints. joints (and therefore in places concentric with the margin of The orientation of these veins varies in different parts of the the pluton). Subvertical, E-W-trending aplite veins are also pluton: representative veins are shown on the map in Fig. 5, common: these are radial to the extreme western margin of whereas the stereograms are acomposite of all measure- the pluton and parallel with the Kavala-Komotini fault. In ments. Aplites have been mapped near the present margin the eastern part of the pluton, subvertical veins striking SE of the pluton at the western end of the pluton and in the dominate. south, east of Kimmeria. In both areas, the present southern margin of the pluton is marked by a fault, although the observations east of Kimmeria are close tothe intrusive Lamprophyre dykes contact with Eocene sediments. In these areas,the A few lamprophyre dykes are found within the pluton. Most orientations of the aplites tend to be radial to the intrusive dykes cut all fabrics in the granodiorite including the early

Fig. 5. Map of Xanthi pluton showing orientation of representative aplite veins. Stereograms show orientations of aplite veins in the (A) western and (B) eastern parts of the pluton, and (C) the orientation of lamprophyric dykes throughout the entire pluton. Contours for (A) at 2.8, 5.7%, n = 71, for (B) at 3.7, 7.4, 11.2%, n = 53 and for (C) at 4.3, 8.6, 13, 17.3%, n = 23.

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jointsand the aplite veins, althougha few lamprophyre motion. The lesser development of hydrothermal alteration bodies show a sill-like relationship tothe earlyjoint compared with the re-activatedearly joint surfaces may surfaces. Somedykes contain granodiorite inclusions, with indicate that the conjugate fault set is younger, but might sharp boundaries showing no evidence of assimilation. result from these faults forming less efficient conduits than Dykes are typically 1 m wide and can be traced for about the reactivated joints. Cross-cutting relationshipsbetween 30 m along strike. The 23 lamprophyre dykes mapped in the the two sets of faults donot provide unequivocal evidence of pluton show similar orientation to the aplite dykes (Fig. 5). relative age.

Faults associated with hydrothermal activity Other faults Small faults have been mapped that are associated with wall Other minorfaults visible in the field havebeen mapped rock alteration (with the formation of chlorite) and local Fe (Fig. 6a). Since these lack hydrothermal alteration, they are and Mn oxidation (Fig. 6c). Alteration is also developed in presumably younger than the hydrothermally affected faults. places on early joint surfaces that show no evidence of fault Most areeither subvertical N-S or NNW-SSE, or offset. Hydrothermal activity is more intense in the western subvertical ENE-WSW (Fig. 6a). There is a minor set that part of the pluton. Sulphide-bearing quartz veins occur NW is parallel to the hydrothermally altered faults. These minor of Kimmeria near the southern margin of the pluton. faults are mostly parallel tothe E-W- or ENE-WSW- Almost all the faults with hydrothermal alteration appear trending majorfaults, mapped from air-photolineations to be re-activated early joint surfaces: evidence of faulting is (Fig. 2), which are parallel tothe dominant regional provided by the presence of slickensides and in places the directions of the Kavala-Komotini fault. offset of aplite veins. Inthe eastern part of thepluton, Theserelationships suggest thatthe hydrothermally where most faults strike N-S, slickensides indicate active fault set pre-dates significant post-intrusion movement subhorizontal sinistral movement. Inthe area north of onthe Kavala-Komotini fault.Faults parallel tothe Kimmeria, similar N-S-striking faults show subhorizontal Kavala-Komotini fault are most common near the southern dextralmovement, whereas E-W-striking faults show dip margin of the pluton, near Xanthi. Such faulting is much slip to the north (Fig. 2). The few data in the east of the less pronounced in the interior of the pluton. pluton are similar to those from the central part,except that dip slip on the E-W faults tends to be to the south. Controls on the siting of emplacement Conjugate NW-striking normalfaults General setting A conjugate set of minor faults dips SW and NE at angles of Interpretation of the aeromagnetic data showed thatthe 30 to 50" (CS in Fig. 6a). Some faults on this trend show Xanthi plutonextends southwards beneaththe Komotini poorly developedhydrothermal alteration.Others show basin (Figs 1 & 2), terminating in the southwest at the edge somewhat curved and irregularsurfaces. Although these of the Avdhira horst, and forming a laccolith with a base at faults are parallel to some of the joints illustrated in Fig. 4, about 4.5 km (Maltezou & Brooks 1989). It is not known the faults offset aplitedykes and high angle dippingearly whether the granitesubcrops beneath the Miocene in the joints: joints on this trend in Fig. 4 may therefore be minor basin, or if it occurs beneatha roof of basementrock. faults. Slickensides on the conjugate faults indicate dip slip Granodioritehasbeen found beneath the40m of

Fig. 6. (a) Stereogram showing orientation of small faults in Xanthi pluton that are recognizable in the field, and are not generally associated with hydrothermal activity. CS, conjugate systemof faults striking NW (shown as 2a, 2b in Fig. 7) (show minor hydrothermal alteration); RE, reactivated early joint surfaces;DF, predominantly strike slip faults correlatedwith the late Oligocene-early Miocene phase of compression; XF, faults parallel to the Kavala-Komotini fault. Contours at 1.4, 4.2, 7.0 and 9.8%, n = 70. (b) Sphere diagram showing slickensides (arrows) on faultswithin the Xanthi pluton interpreted as resulting from the late Oligocene-early Miocene phase of compression. n = 34. (c) Stereogram showing small scale faults within the western part of the Xanthi pluton associated with hydrothermal activity. Contours at 3.2, 6.4, 9.6, 12.8, 16.0, 19.270, n =62.

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Neogene-Quaternary sediment in a borehole in the town of cooling surfaces at the top and sides of the pluton. These Xanthi. Maltezou & Brooks (1989) suggested on the basis of joints therefore are unlikely to represent cooling surfaces or magnetic modelling thatthe deeper part of the pluton be dueto stress relief during unroofing, as suggested by extends many kilometres north of the Kavala-Komotini Meyer (1968). These surfaces may therefore be a response fault; but provide no sensitivity analysis for their modelling. to the same extensionalstress field that is inferred from Comparison of thedata for the Xanthi and Leptokarya mineral lineations and later aplite veins. plutons (which they analysed in more detail and interpreted Aplite veins close to the intrusive margins of the pluton to show no subsurface extension beyond a bounding fault) are either radial to the margin, or intruded along early joint suggests thatthe postulatednorthward extension of the surfaces. Those close to the faulted southern margin of the Xanthi pluton is not well constrained by the available data. pluton are parallel tothe E-W trend of the Kavala- This would be particularly true if the Kavala-Komotini fault Komotini fault. Many aplite dykes in thecentre of the were the site of enhanced intrusion of mafic magma. western part of the exposed pluton are subvertical, striking Much of the northern and the southeastern part of the NW-SE. Such atrend is parallel tothat produced by Xanthi pluton shows intrusive contacts that trend approxim- extension duringdextral shear along E-Wmaster faults ately parallel to the fault, and dykes are abundant in the such as the Xanthi-Komotini fault. However, no evidence flysch east of thepluton, indicating thatthe fault was of strike slip motion is seen in the orientation of mineral probablyazone of weaknessprior to intrusion of the lineations or early joint planes in most of the pluton. It is pluton. The aeromagnetic data suggest that the southwes- also parallel tothe trend of the andesitedykes, and tern margin of the pluton is marked by the NNW-trending probably indicatesa continuation of the same extensional margin of the Avdhirahorst andthat the northeastern stress field. Although many lamprophyre dykes are margin of the pluton follows a similar trend, Thus the main subparallel to aplite veins, there is a mode with a steep dip geometry of the pluton appearsto befault controlled. to the WSW. This may reflect the same extensional stress Similar evidence for fault control of the location of plutons field to that producing the aplite veins in the centre of the is seen in the linear belt of plutons in western Thrace (Del eastern part of the pluton. Moro et al. 1988). Thus the syn-intrusion structures can all be accounted for The pronounced bend in the Kavala-Komotini fault near by a persistent extensionalstress field oriented NE-SW. Xanthimarks thenorthern limit of the Xanthi pluton. There may be other explanationsfor individual features, Dextral slip onthe Kavala-Komotini fault zone would particularly those developed near the margin of the pluton, create extension in thearea of theXanthi pluton, in a but the consistency of the orientations of all the structures in manner similar to that postulated by Hutton (1988b) for the the central part of the pluton is striking. There is no formation of the biotite granite at Strontian near the Great evidence for syn-intrusion strike-slip motion within the Glen Fault in Scotland. Theinterpretation of theXanthi pluton or along its margins (although the critical graniteasa pull-apart structure in thebend of the northwestern margin is not well exposed). Such extensional Kavala-Komotini fault cannot be excluded. Three lines of features could result eitherfrom a regional extensional evidence,however, suggest that a regional extensional stress field, or from local extensionrelated to pull-apart setting is more likely. The similar Philippi and Myrodato during dextral slip on the Kavala-Komotini fault. plutons are unrelated to the Kavala-Komotini fault (Fig. 1). The thick regionally extensive Upper Eocene to Oligocene sediments of the northeastern Aegean region imply basinal Post-intrusion structures subsidence. There is no evidence for subsequent strike-slip Several phases of post-intrusion structure can be recognized. motion on the Kavala-Komotini fault since the intrusion of The main hydrothermally-alteredfaults cut syn-intrusion the Xanthi pluton. structures andappear to be reactivated early joints. The lack of hydrothermalalteration along other fault planes indicates that unalteredfault planes are probably later. Syn-intrusion structures Slickensides on the hydrothermally altered fault planes show The most distinctive feature of the pluton is the reasonably geographic variation in sense of motion: for example, N-S consistent WSW dip of mineral lineations with orthogonal faults in places show sinistral and in places dextral earlyjoint surfaces. This regularity implies some form of sub-horizontal motion. structuralcontrol onthe intrusion. The elongation of the Later structural trendsare represented by the faults pluton along the Kavala-Komotini faultand the cross- lacking hydrothermal alteration. The oldest is probably the cutting of basement trends (Fig. 2) indicates that this conjugate set of dip slip normal faults striking NW, which structuralcontrol is unlikely to be simply the foliation probably represent a phase of extension. Since some show direction within theRhodope basement. The absence of poorly developed hydrothermal alteration, it is possible that radial mineral lineations and associated early joint surfaces this fault set is older. The ENE-WSW and NNW-SSE fault indicates that oblique doming and ballooning (Castro 1984) trendsseen in the analysis of minorfaults in the Xanthi is also unlikely to be an important mechanism. pluton are parallel to the most prominent fault directions Mineral and xenolith lineations such as occur in the defining the Neogene basins tothe south. The Kavala- Xanthi pluton generally indicate the direction of maximum Komotini fault has cut the southern margin of the exposed extension during magma cooling (Hutton 1988~).The lack pluton with predominant dip-slip motion. of shear fabrics and the general SW plunge suggests that the lineations may have resultedfrom the sameextensional stress field as recorded by the pre-intrusion dykes. Interpretation of tectonic history The consistent steep dips of the early joints in the The andesitic dykes dated at 33.5 Ma indicatea regional easternpart of the pluton are almostorthogonal to likely stress field with NE-SW directedextension immediately

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prior to intrusion of the pluton (at about 29 Ma). Regional movement on the fault. Most dip steeply to the SW or NE, considerations(widespread and thick UpperEocene to probably forming aconjugate set of fractures associated Oligocene sediments; the location of the Phillipi and with continued extension. Myrodato plutons) suggest that this extension was regional, The reactivation of early joint planes within the pluton, rather than being due to local effects of stike-slip motion on producing predominantlyhorizontal dextral slip on N-S the Kavala-Komotini fault. Aplite and lamprophyre dykes trending faultplanes, must reflect a significant change in with a similar trend cut the pluton,suggesting a continuation regional stress pattern. It probably corresponds to the late of thesame stress field. Theorientation of the mineral Oligocene-early Miocenephase of compression (D4 of lineations and early joint planes can also be most simply Koukouvelas & Doutsos 1990). The geographic variability in explained as the result of NE-SW directedextension slip vectors in the reactivatedearly joint planes is creating a ramp space (Fig. 7A). This ramp space is similar characteristic of the response of a variably faultedbrittle to thatpostulated by Hutton (1988b) forthe Strontian body to compression. granite, except that the causal tectonic process at Strontian The absence of hydrothermalalteration distinguishes was strike-slip motion. In the Xanthi pluton, the regional later faults within the pluton. The distinctive NW striking data does not support strike-slip motion along a master fault conjugateset with dip-slip motionprobably reflects initial zone. Theramp space was boundedto the NW by the extension following thelate Oligocene-early Miocene ENE-trending segment of the Kavala-Komotini fault (Fig. compression, although it might bedue to vertical 7C) that we interpret as acting as a transfer fault. Intrusion compressive stress during the late magmatic stages. Most of was largely passive in the western part (Fig. 7B) of the the later faults trend either E-W or ENE-WSW, parallel to pluton, but minor doming occurred at the open end of the the dominant regional directions of the Kavala-Komotini ramp tothe east (Fig. 7C).The Kavala-Komotini fault fault. These presumably developed at the same time as the facilitated the high-level rise of magmas atthe northern faulting thataccompanied the formation of the Neogene edge of the pluton. This fault and the faults bounding the basins. ramp space were sealed by the intrusion of the pluton across Plutons similar to the Xanthi pluton are to be expected the faults. Thereappears to havebeen nosubsequent in extensional settings where there is abundant supply of strike-slip motion on the Kavala-Komotini fault. granitoid magma. Our study suggests that extension,not The emplacement of aplitic veins and lamprophyric strike-slip motion, has been the dominantcontrol onthe dykes probably corresponds to continuing volcanic activity Xanthi pluton. The Kavala-Komotini fault may have andtherefore probably predatesthe termination of originated as a strike-slip fault, but the emplacement of the volcanism at about 24Ma. Most are parallel to early joint pluton was accompanied only by passive transfer motion on surfaces or cross-cut themat a low angle.A few aplites the fault in order to create space for the pluton; the pluton parallel the E-W section of the Kavala-Komotini fault, itself was not deformed by continuing strike-slip motion. suggesting thatthere may have beenminor (? transfer) Conclusions The Xanthipluton is one of seriesa of subcircular granodioritic plutons of Oligocene age in northern Greece, related to Early Tertiary subduction. The earliest phase of igneous activity was the intrusion of andesitic dykes under a NE-SW-directed extensional stress field at about 33.5 Ma. Subsequent intrusion of the pluton (26-29 Ma) was localized by the formation of a ramp spaceduring continued extension, with the ENE-trending Kavala-Komotini fault acting as a transfer fault bounding the ramp space to the north. Intrusion was largely passive in the western part of the pluton,but minor doming may haveoccurred in the east. The Kavala-Komotini fault facilitated the high-level P rise of magmas atthe northern edge of the pluton. Extension during crystallization of the magma is reflected in the orientation of mafic minerals and mafic xenoliths. Continuedextension during the cooling of the pluton Q resulted in emplacement of aplitic veins and lamprophyric dykes. Thelate Oligocene-early Miocenephase of compression (D4 of Koukouvelas &L Doutsos 1990) is represented by the reactivation of early joint planes within the pluton.Neogene basin extension was accompanied by 7. Interpretative cross sectionsof the southern, central and ig . new faulting within thepluton, largely parallel tothe northern partsof the exposed Xanthi pluton: sectionsare located on the aeromagnetic map (cf. Figs 1 & 2). (A) General geometry of the Kavala-Komotini fault. ramp space createdby regional extension and its relationship to This case study extends our knowledge of plutons whose deeper structures (modified from Hutton, 1988a). (B) Central part emplacement is primarily controlled by major faults. The of pluton, showingmagma filling rampspace to form a laccolith. Xanthi pluton is a laccolith-like plutonemplaced along a (C) Northern part of pluton, showinghigher level intrusion (with major listric faultduring regional extension. Pre-existing minor doming andballooning) facilitatedby the presence of the structures, including an old strike-slip lineament,were Kavala-Komotini fault acting as a transfer fault. reactivated and control the location of the pluton. Granitoid

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Received U) March 1990; revised typescript accepted 10 October 1990

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