Circum-Rhodope Allochthons

Several units of low-grade Mesozoic rock sequences were termed “Circum-Rhodope Belt” by Kauffmann et al. [1976]. This definition refers to the concept of the Rhodope being an old microcontinent island surrounded (hence the prefix “circum”) and stratigraphically covered by younger sediments [Jaranoff, 1960; Kockel et al., 1977]. However, all contacts are fault zones, and different rock types and associations were lumped under the term. Detrital heavy minerals demonstrate that indeed there are different units with different source areas [Meinhold et al., 2009; Meinhold et al., 2010b]. The fact that they occur along the western and southern margins of the high-grade Rhodope is their unifying character.

To the southwest, there are greenschist- and blueschist-facies metasediments, including deep marine Triassic sediments and metabasalts [Michard et al., 1994]. They were initially interpreted as the cover of the Vertiskos Unit [Kockel et al., 1971] but the contact is demonstrably tectonic [Meinhold et al., 2009]. The blueschist-facies [and possibly local UHP, Kostopoulos et al., 2000] metamorphism must be at least Jurassic in age because similar metamorphic rocks are found as pebbles in non-metamorphosed Tithonian to Lower Cretaceous conglomerates [Kockel et al., 1971]. Like many other blueschists, they may have been exhumed in an accretionary wedge, before any collisional event. Greenschist-facies metamorphism is Eocene [Kockel et al., 1977].

Low-grade, Late-Permian and Mesozoic sediments are associated with Jurassic arc and back-arc magmatic rocks in Eastern Rhodope [Magganas, 2002; Bonev et al., 2010a]. There, greenschist-facies metabasalts and phyllites of prehnite-pumpellyite-facies [Maratos and Andronopoulos, 1964] tectonically overlie the high-grade, eclogite-bearing Kroumovitza unit [Bonev and Stampfli, 2008]. These dominantly Jurassic rocks [see review of fossiliferous and absolute ages in Bonev and Stampfli, 2008] were emplaced by northward thrusting during the Early Cretaceous, and represent a volcanic island arc and backarc basin. Interestingly, the 155 Ma Samothraki diorite [Tsikouras et al., 1990] and the nearly 160 Ma Samothraki mafic suite [Koglin et al., 2009] could belong to this arc-backarc unit. Bonev and Stampfli [2008] argue, after Magganas [2002], that this backarc basin was linked to the Vardar system above a south-dipping subduction zone that preceded arc-continent collision. Ages of detrital zircons suggest that these sediments were deposited in front of an eroded source with Rhodopean affinity [Meinhold et al., 2010b]. Importantly, these rocks were never buried deeper than low-grade metamorphic conditions and the ca. 150 Ma fission-track ages [Bigazzi et al., 1987] show that they escaped thermal and tectonic disturbance since mid-Jurassic times. As such, they are part of the Upper Terrane(s) preserved in the Rhodope.