Crustal extension
The collisional nappe pile is overprinted by extensional detachment faulting, and intruded by pre-, syn-, and post-tectonic granitoid plutons. Sense-of-shear criteria indicate bulk top-to-south-southwest shear, consistent with magmatic fabrics of late, often calc-alkaline granitoids [Kolocotroni and Dixon, 1991; Zananiri et al., 2004] that deform the nappe system into large dome and basin structures (Fig. 1). SW-NE trending folds and lineations (Fig. 4) conventionally related to the thrusting event have been attributed to crustal extension [Dinter and Royden, 1993; Sokoutis et al., 1993; Dinter, 1994]. The distinction between thrusting- and extension-related shearing is, in many places, structurally difficult. Extension-related ductile deformation seems coeval with thrusting structures because both evolved under similar metamorphic conditions. Such interacting features are easier to interpret as gravitational adjustment of an unstable orogenic wedge during its tectonic accretion. Late Eocene marine sediments which unconformably overlie the metamorphic Rhodope Complex markedly separate previous structures from normal faults active since the Oligocene throughout the Aegean realm [e.g. Angelier et al., 1982; Lister et al., 1984; Gautier and Brun, 1994].
Backward crustal stretching
The first evidence for crustal stretching and extension was inferred from both structural considerations and kinematic indicators opposed to (i.e. northeastward to subparallel to the strike of the orogen) and overprinting those denoting southwestward thrust tectonics. Evidence for backward shear with respect to the bulk regional shear was particularly discussed for some of the imbricate units. It is attributed to syn-orogenic extension.
Gneiss-marble sequence and eclogite-metabasic-gneiss sequence
In the structurally high gneiss-marble imbricate (so-called Asenitsa, Fig. 5) the foliation contains a stretching lineation related to top-to-east-northeast shear-sense criteria [Burg et al., 1990; Burg et al., 1996a]. This backward (top to NE) shear is linked with the exhumation from depths greater than 30 km, at a rate fast enough to prevent significant retrogression of the white-schist parageneses at the basal contact of the gneiss-marble imbricate [Guiraud et al., 1992], and the eclogites in the underlying imbricates (Fig. 6).
Foliation-parallel scars
The shear-inversion zones between SW-directed and NE- or NW-directed sheared units are a few metres thick and show no attitude change of the low-dip foliation across the zone in question. Such zones were reported at several levels [Ricou et al., 1998] where they may represent the foliation-parallel scar left by a missing unit displaced south-westward with respect to both the footwall and hanging wall units. Such contacts have locally been reduced to thin brittle zones where thick crustal segments are missing, for example between the high-grade allochthonous Kroumovitza hanging wall and medium-grade Biela-Reka-Kardamos footwall gneisses in Eastern Rhodope [Ricou et al., 1998; Krohe and Mposkos, 2002]. The metamorphic gap is less pronounced but also exists between the Asenitsa and the underlying Arda2 (Madan) and Borovitsa units (Fig. 5). Forward extrusion of high grade units [Chemenda et al., 1995] involves thinning in the scar area coeval with thickening further southwest, towards the foreland, where the extruded unit was transferred.
Low angle normal faults and basins
Semi-ductile shear zones active under greenschist-facies and overprinting, very low-grade brittle fault zones mark the late Cretaceous Gabrov Dol Detachment, which is conveniently taken here as the roof boundary of the Serbo-Macedonian-Rhodope Metamorphic Complex [Bonev et al., 1995]. Other low-dip faults cut the foliation of the high-grade metamorphic rocks and show low-grade and brittle conditions [e.g. Ivanov, 1988]. They are particularly associated with Eocene-Oligocene half grabens and basins into which large olistoliths of metamorphic rocks have glided [Ivanov et al., 1979; Burchfiel et al., 2000; Burchfiel et al., 2003].
The Strymon low-angle detachment is the most cited one. Dinter et al. [1995] noted that a major ductile shear zone overprints ductile structures associated with thrust tectonics along the eastern side of the Strymon Valley. Observing that the extensional fabric is pervasive in the Kavala Granodiorite and in the southwestern part of the Vrondou Granodiorite (Fig. 5), they distinguished a 21-22 Ma ductile mid-crustal extension stage from a ca. 16 to 3.5 Ma ductile to brittle extensional movement [see also Dinter, 1998]. The Tertiary activity of the Strymon Detachment is documented by the contrasting 50 versus <26 Ma cooling ages between the hanging wall and the footwall in Thasos [Wawrzenitz and Krohe, 1998]. Contrasting cooling ages (K-Ar and 40Ar/39Ar on micas) between younger (Eocene-early Oligocene) footwall and older (mostly Cretaceous) hanging wall were also obtained for the west-dipping shear zone between the Vertiskos and Kerdylion units [Harre et al., 1968; De Wet et al., 1989; Frei, 1996]. Zircon and apatite fission-track ages on both sides of the Strymon Detachment on the eastern side of the Strymon Valley are also younger than those on both sides of the Vertiskos/Kerdylion contact [Wüthrich, 2009]. Ages do not allow correlating these two west-dipping fault zones.
Gneiss domes
Most of the large antiforms described in the early literature and ascribed to late compressional folding have now turned out to be (for the most part) extensional core complexes [Dinter and Royden, 1993; Sokoutis et al., 1993; Brun and Sokoutis, 2004; Kounov et al., 2004; Bonev et al., 2006a]. Core granitoids and low-pressure anatexites hint at crustal melting during extension-related decompression of the gneiss exhumed below ductile, normal shear zones.
The Kesebir-Kardamos is one of the best-documented extensional gneiss domes [Fig. 8, Bonev et al., 2006a; Krenn et al., 2010]. A low-angle (Tokachka) detachment separates the core of intermediate-pressure, amphibolite-facies orthogneisses and migmatites of the Lower Terrane [0.3-0.9 GPa, 550-620°C, Mposkos et al., 1989; Krohe and Mposkos, 2002] from the eclogite-metabasic-gneiss sequence of the Kimi-Kroumovitsa imbricate. Maastrichtian–Paleocene sediments [Goranov and Atanasov, 1992] rest directly on the fault contact. 40-35 Ma 40Ar/39Ar mica ages date cooling below 350-300°C of the core migmatites [Bonev et al., 2006b; Márton et al., 2010]. Zircon fission-track ages (all from 39.2 ± 4.2 to 35.6 ± 5.2 Ma) and apatite fission-track ages (35.8 ± 8.6 to 28.1 ± 6.2 Ma) show that both the hanging and the footwall of the Tokachka detachment cooled rapidly together from ~300°C down to 60°C without significant displacement after ca. 33 Ma [Wüthrich, 2009]. Reset zircon and apatite FT ages from both overlying sediments and subjacent basement rocks consistently indicate that the onset of high-angle normal faulting is placed between 33 and 24 Ma [Wüthrich, 2009; Márton et al., 2010].
Further east, the very flat Biela Reka-Kechros Dome (Fig. 9) is also interpreted as an extensional feature [Krohe and Mposkos, 2002; Bonev, 2006]. Likewise, most of the antiforms exposing the lower terrane are tectonic windows surrounded by ductile detachments, often reworking previous thrusts. The general structure of very flat domes such as the Biela-Reka-Kechros raises the question as to whether there should not be more caution in identifying extensional core complexes based on few normal senses of shear, since those are possible in any dome [Burg et al., 2004] and a bent thrust may appear like a normal fault. In most cases only the contrasting thermal history of the upper plate in comparison to the lower plate will reveal whether any particular structure is related to extensional detachments or to thrusting.
Granitoids
Voluminous plutonism is one of the Rhodope characteristics noted in the earliest work [Viquesnel, 1853] and was used to infer an old basement at times when granitoids were professed to be rare in Alpine orogens. Geochronology (Table 7) has been the key to establishing the Rhodope peculiarity.
The Kavala [Dinter and Royden, 1993] and Vrondou [Kolocotroni and Dixon, 1991] plutons are dated at 21 and ca. 30 Ma, respectively (Fig. 10; Table 7). They display pervasive C/S type fabrics that generally indicate top to the SW or WSW, normal shearing [Dinter and Royden, 1993; Sokoutis et al., 1993]. Owing to their Oligocene to Early Miocene intrusion age, they are likely the most convincing argument to demonstrate ductile, extensional deformation in the intruded middle crust at times when sediments were being deposited on the Rhodope surface.
The early to mid-Tertiary granitic intrusions in the Rhodope mostly represent calc-alkaline, deep crustal melts attributed to elevated temperatures in the mountain root and emplaced during extensional collapse of the thickened crust [Jones et al., 1992]. The upwelling, decompressing asthenosphere would have been the heat source and produced melts with strong mantle signature [Koukouvelas and Pe-Piper, 1991]. Hydrothermal base- and precious-metal deposits are mostly related to the Oligocene magmatism [Singer and Marchev, 2000; Kaiser-Rohrmeier et al., 2004; Marchev et al., 2005].