Timing of extensional events
The sedimentary record of basins and grabens suggests several extensional episodes, which are coeval with sets of absolute ages and volcanic events. Although grabens, which are limited in size, record local events that can be heterogeneously distributed during a wider continuum, three regional events stand out.
Sedimentary information
Late Cretaceous-Paleocene sedimentary cover
The oldest post-metamorphic cover of the Rhodope is Maastrichtian-Paleocene [Athanasov and Goranov, 1984; Goranov and Atanasov, 1992], perhaps even Upper Santonian-Campanian [Boyanov et al., 1982]. It consists of colluvial and proluvial clastic deposits with olistoliths of country gneiss and marbles [Kozhoukharov et al., 1991; Goranov and Atanasov, 1992; Boyanov and Goranov, 1994; Zagorchev, 1998]. Campanian tuffs cover both high-grade gneiss and "circum-Rhodope" low-grade sequences in the eastern Rhodope [Boyanov and Russeva, 1989]. These sediments demonstrate that much of the metamorphic Rhodope was eroded to near sea-level by ca. 60 Ma and remained a shallow sedimentation site until at least 40 Ma. This early sedimentation history spans ca. 20 Ma and seems in conflict with the many cooling ages younger than 40 reported in tables 4 and 7. Twenty million years of apparent quiescence is a long time span in this very "mobile" zone, which raises the question of polyorogeny for the upper tectonic levels of the Rhodope.
Eocene-Oligocene basins
Early Eocene, marine sediments (Nummulite-bearing limestones) are preserved in several basins in Bulgaria and Greece [Von Braun, 1993; Zagorchev, 2001]. These sediments unconformably transgressed an erosional surface in the Priabonian (Late Eocene, ca. 35 Ma) and quickly gave way to Late Oligocene and Early Miocene continental sedimentation. Extensional structures involve the European continent beyond the Rhodope Metamorphic Complex, into northern Bulgaria and the Balkan Mountains [Tzankov et al., 1996; Zagorchev et al., 1999; Burchfiel et al., 2000; Kounov et al., 2004; Tueckmantel et al., 2008; Schefer et al., 2011]. Sedimentation continued into Early Miocene times to the northeast of the Rhodope (in Thrace). The Middle and early Late Miocene witnessed a general sedimentary break possibly coeval with general erosion [Burchfiel et al., 2000].
Late Miocene to Present
A second extensional event started in the Middle Miocene. Late Miocene, alluvial and proluvial sediments rest unconformably on older rocks [Zagorčev, 1992; Dinter and Royden, 1993; Georgiev et al., 2010]. Grabens formed at that time over a broad region that extends southward to the Aegean Sea [Mascle and Martin, 1990]. This event is generally related to clockwise rotation of the Greek Peninsula with respect to Europe during southward retreat of the Hellenic trench [McKenzie, 1978; Le Pichon and Angelier, 1981; Van Hinsbergen et al., 2008].
Geochronological constraints
The decrease of ages of metamorphic rocks toward dome cores and deeper structural levels indicates distinct Cretaceous, Eocene, and Oligo-Miocene tectonic-metamorphic pulses that successively caused cooling and exhumation of gneiss complexes situated at deeper levels. The structural information from dated minerals, in particular micas, indicates that during Eocene to Miocene sedimentation on the surface, gneissic foliations and shear zones continued to form deeper in the crust.
The ca. 100 Ma Rb/Sr age [Zagorčev and Moorbath, 1986; Arnaudov et al., 1990b] from a metamorphosed granitoid with the NE-directed shear fabric would date the earliest crustal stretching. However, NE-directed extensional shearing is constrained to ca. 155 Ma by 40Ar/39Ar mica ages in allochthonous units of Eastern Rhodope [Bonev et al., 2010a]. Such ages suggest either heterogeneously distributed, protracted events, or polyorogeny already evoked, which will be discussed in the relevant paragraph later in this contribution.
For younger extensional events, important constraints come from deformed granites such as the 52.8±0.89 Ma Pripek laccolith [Gerdjikov, 2005, Table 7]. 40Ar/39Ar white mica ages of ca. 40 Ma distributed throughout the Rhodope constrain the Middle Eocene cooling history of Central [Kaiser-Rohrmeier et al., 2004] and Eastern Rhodope [Bonev et al., 2010a; Bonev and Stampfli, 2011] below ca. 350 °C. Rapid cooling of the Rhodope Metamorphic Complex is further documented by one titanite fission-track age at 55.9 ± 6.2 Ma in Central Rhodope [Wüthrich, 2009] and several zircon and apatite fission-track ages between 48 and 18 Ma in Central [Wüthrich, 2009] and Eastern [Márton et al., 2010] Rhodope. Younger (15-6 Ma) fission-track ages occur in Western Rhodope, in the direct footwall of the Strymon Detachment [Hejl et al., 1998; Wüthrich, 2009].
Magmatism
Magmatism has been very variable in composition, space and time over the Aegean and North Aegean region since the Eocene [e.g. Pe-Piper and Piper, 2006]. This magmatism has been related to either hydrous melting of the asthenospheric mantle wedge during subduction of the Pelagonian or Vardar oceanic lithosphere [Boccaletti et al., 1974b; Burchfiel et al., 2000] or to a collisional - post-collisional event [Yanev, 2003]. In details, the magmatic history is more complex. Subduction-related, calc-alkaline magmatism was dominant during the Middle to Late Eocene [35-37 Ma, Lilov et al., 1987]. That would link the coeval extension to arc--back-arc extension. During the Early Oligocene, massive calc-alkaline to shoshonitic volcanism accompanied sedimentation in fault-bounded basins [Harkovska et al., 1989; Dabovski et al., 1991; Pecskay et al., 2000]. The abundance of rhyolites and ignimbrites points to crustal melting, hence a major thermal/decompressional event at 40-30 Ma [Marchev et al., 2005]. Seismic tomography shows that there was no slab breakoff since the Mid Jurassic [Bijwaard et al., 1998]. Therefore, this magmatic event requires another interpretation. Bimodal volcanism with rhyolitic and latitic–andesitic rocks associated with few basalts [Yanev et al., 1998] was active along a 1600 km long belt, from the Eastern Alps through the Rhodope to Eastern Thrace in Turkey [Harkovska et al., 1989; Schefer et al., 2011]. The length of this magmatic belt supports a lithospheric-scale origin. For this reason, it is possible that the thermal event is lithospheric delamination and subsequent asthenopheric rise that could have melted a heterogeneously enriched subcontinental lithospheric mantle [Pe-Piper and Piper, 2006]. In any case, the cause would have been rather short-lived because magmatism ceased by the end of Oligocene times, after the effusion of alkali basalts between 28 and 26 Ma [Marchev et al., 1998].
Magmatism shifted further south during the Miocene, with very local magmatic activity [Jones et al., 1992].