Separate Eocene-Early Oligocene and Miocene stages of extension and core complex formation in the Western Rhodopes (Bulgaria)

The basement of the Rhodope Metamorphic Province comprises four groups of tectonic units forming the Lower, Middle, Upper and the Uppermost Allochthons which were emplaced onto each other during a protracted orogenic history from Late Jurassic to Eocene. The Lower Allochthon includes the Pangaion-Pirin Complex, and the Arda, Kardamos/Kesebir, and Byala Reka/Kechros units. The units consist of Variscan basement and, partly, a metasedimentary cover dominated by marble. The overlying Middle Allochthon comprises slivers of both oceanic and continental crust and, in addition, orthogneisses derived from Late Jurassic to Early Cretaceous arc granitoids. It includes, among others, the Kerdilion unit in the Serbo-Macedonian Massif and the Sidironero-Mesta, Starcevo, and Asenica units in the Western and Central Rhodopes. The Middle Allochthon was thrusted towards southwest over the Lower Allochthon during the Palaeogene along the Nestos Shear Zone. The Upper Allochthon crops out most extensively in the Eastern Rhodopes (Kimi Complex) and in the Serbo-Macedonian Massif (Vertiskos/Ograzhden unit). These units represent Variscan continental crust which was affected by HP and partly UHP metamorphism in the Jurassic to Early Cretaceous. The Uppermost Allochthon (not exposed in the Western and Central Rhodopes) consists of low-grade metamorphic (greenschist facies, locally blueschist facies) sedimentary and volcanic rocks, partly of oceanic affinity. It includes the Circum Rhodope Belt along the SW border of the Rhodope Metamorphic Province and the Mandrica greenschists in the Eastern Rhodopes.
   The Rhodope Metamorphic Province includes, in addition to the Rhodope Mountains proper, also the Rila and Pirin Mountains and the Serbo-Macedonian Massif. These different massifs are separated by basins of Paleogene and Neogene age. The Rhodope Metamorphic Province in Bulgaria and Northern Greece has been affected by significant extensional tectonics since the Middle or Late Eocene. An important fault system active in the Eocene and Early Oligocene includes the Ribnovo Fault on the eastern side of the Mesta Basin in Bulgaria and the Vertiskos-Kerdilion Fault in Greece. Together with several minor normal fault relicts identified during our studies, these represent an originally west-southwestdipping, low-angle (at least at the end of faulting) normal fault with greenschist facies mylonites in the footwall and cataclasites along the fault plane, the Mesta-Kerdilion Detachment, exposed over ca. 150 km along strike and about 50 km parallel to the slip direction. The Mesta-Kerdilion Detachment system removed the Vertiskos-Ograzhden Unit from the top of the Sidironero-Mesta Unit. The along strike horizontal displacement amount was more or less constant. The Ribnovo, Vertiskos-Kerdilion, and Alikochov faults accommodated the collapse of a thickened orogenic wedge above the subduction zone in which the Apulian plate is retreating. In that sense, the Late Eocene Mesta-Kerdilion Detachment system corresponds to the onset of Aegean extension. During the intrusion of several plutons in the Pirin Mountains at ca. 32 Ma, the footwall of the fault was uplifted to form a large anticline parallel to fault strike, and the fault was offset by a system of antithetic, northeast-dipping normal faults along the northeastern flank of this anticline (Dobrotino and Breznica faults). The Mesta-Kerdilion Detachment was later, in the Miocene, again crosscut and offset by the southwest-dipping Strimon Valley Detachment which accommodated important, core-complex-like exhumation to the south, strongly diminishing and finally ceasing towards north. This rotational activity of the Strimon Valley Detachment represents the onset of the extension that led to opening of the Aegean Basin. The Mesta-Kerdilion Detachment can be viewed as a precursor of this, but with slightly different kinematics (i.e. not involving significant vertical-axis rotation) and separated in time from the following events by a phase of relative tectonic quiescence in the Late Oligocene.