The Rhodope Metamorphic Province in the area around the Mesta Graben (SW Bulgaria) exposes a structurally lower complex, the Pangaion‐Pirin Complex of Variscan continental crust and its cover (mostly ...orthogneiss and marble), and a higher complex, the Rhodope Terrane of mixed oceanic and continental origin with metamorphosed Jurassic arc magmatites. The boundary between the two is the top‐to‐the‐southwest Nestos Shear Zone. The regional top‐to‐the‐southwest shearing of the two basement complexes is related to the emplacement of the Rhodope Terrane over the Pangaion‐Pirin Complex along this shear zone. Syntectonic and posttectonic Alpine intrusions within the basement can provide age limits for the thrusting. Zircons from the posttectonic Teshovo (south Pirin) Granitoid Pluton in the Pangaion‐Pirin Complex yielded crystallization ages of 32 ± 0.2 Ma (LA‐SF‐ICP‐MS U‐Pb method). Zircons from two syntectonic granite plutons in the Rhodope Terrane (Dolno Dryanovo and Spanchevo) have circa 143 to 145 Ma old inherited magmatic cores and circa 55 to 56 Ma old magmatic rims. These ages, especially the presence of the Upper Jurassic/Lower Cretaceous cores in the plutons from the Rhodope Terrane, and the structural relations indicate that the southwestward thrusting of the Rhodope Terrane over the Pangaion‐Pirin Complex took place in the Paleogene. A model is proposed in which the Pangaion‐Pirin Complex represents the margin of Apulia and the structure of the Rhodopes is explained by a switch of subduction polarity from SW dipping in the Jurassic and Early Cretaceous to NE dipping in the Late Cretaceous and Paleogene.
The Nestos Shear Zone (NSZ) in the Rhodope Metamorphic Province is a major high‐strain zone between two metamorphic terranes. Microdiamond‐bearing ultrahigh‐pressure (UHP) rocks occur in the NSZ ...which was therefore interpreted as a suture zone where subduction and exhumation of these rocks and terrain accretion occurred during the Mesozoic. Our petrological study of samples from the lower part of the NSZ, together with monazite dating of a microdiamond‐bearing garnet schist, structural observations, already published results from the upper part, and other published timing constraints, results in a fundamentally different picture: the NSZ is the base of an Eocene age southwestward thrust wedge which included not only the structurally higher parts of the Rhodope Metamorphic Province but also the entire Internal Hellenides. The UHP rocks, for the peak pressure of which we derive an age of ∼200 Ma by monazite dating, are unrelated to the tectonic processes in the NSZ and probably represent slivers of a higher tectonic unit captured by thrusting along the NSZ. Pressure decrease in the footwall and regional extension and basin formation in the hanging wall during the activity of the NSZ show that the overlying thrust wedge was collapsing in late Eocene times. We hypothesize that the transition from subduction to continental collision may start with a pronounced accretion event when the first detachment horizon forms in the middle crust of the downgoing plate. Such an event could trigger slab retreat, an extensional collapse of the internal wedge, and subsequent magmatism.
Key Points
NSZ is base of an Eocene collapsing wedge on top of retreating slab
NSZ is a thrust coeval to unroofing of footwall and extension of hanging wall
UHP metamorphic assemblages in NSZ are old and unrelated exotic slivers
The metamorphosed thrust stack of the Rhodopes comprises a level with ophiolites (Middle Allochthon) underlain and overlain by continent‐derived allochthons. The Upper Allochthon represents the ...European margin, but the origin of the Lower Allochthon remains controversial, with suggestions that it may be derived from an inferred microcontinent (Drama) or from the margin of Adria. Trace element compositions and Sr and Nd isotope ratios of metagabbroic amphibolites and enclosed meta‐plagiogranites from the Satovcha Ophiolite, Middle Allochthon, show that they are cogenetic and represent suprasubduction zone ophiolites. U‐Pb dating using laser ablation sector field inductively coupled plasma mass spectrometry of zircons from two meta‐plagiogranites and a metagabbro yielded identical Jurassic ages (160 ± 1 Ma, 160.6 ± 1.8 Ma, and 160 ± 1 Ma, respectively), similar to ophiolites in the eastern Vardar Zone bordering the Rhodopes to the SW. The trace element patterns also closely resemble those of the Vardar ophiolites. The association with Late Jurassic arc‐type granitoids is another feature that applies both to eastern Vardar and Satovcha. This strongly suggests that the Middle Allochthon comprises the metamorphosed northeastward continuation of the Vardar Zone. The Jurassic age of the Satovcha Ophiolite contradicts the hypothesis of Early Jurassic suturing between Europe (Upper Allochthon) and the assumed Drama microcontinent (Lower Allochthon) but is in line with the “maximum allochthony hypothesis,” i.e., the assumption that the Lower Allochthon represents Adria and that the “root” of the Vardar‐derived thrust sheets is at the NE boundary of the Rhodopes.
Key Points
Satovcha Ophiolite (Rhodopes) is a 160 Ma old suprasubduction ophioliteRhodope nappes are metamorphosed equivalents of the Hellenide thrust sheetsHellenic subduction zone rolled back 500 km in 40 to 45 Ma
The Rhodope Metamorphic Complex is a stack of allochthons assembled during obduction, subduction, and collision processes from Jurassic to Paleogene and overprinted by extensional detachment faults ...since Middle Eocene. In the study area, the following nappes occur in superposition (from base to top): an orthogneiss-dominated unit (Unit I), garnet-bearing schist with amphibolite and serpentinite lenses (Unit II), greenschist, phyllite, and calcschist with reported Jurassic microfossils (Unit III), and muscovite-rich orthogneiss (Unit IV). U–Pb dating of zircons from a K-feldspar augengneiss (Unit I) yielded a protolith age of ca. 300 Ma. Garnet-bearing metasediment from Unit II yielded an age spectrum with distinct populations between 310 and 250 Ma (detrital), ca. 150 Ma, and ca. 69 Ma (the last two of high-grade metamorphic origin). An orthogneiss from Unit IV yielded a wide spectrum of ages. The youngest population gives a concordia age of 581 ± 5 Ma, interpreted as the age of the granitic protolith. Unit I represents the Lower Allochthon (Byala Reka-Kechros Dome), Unit II the Upper Allochthon (Krumovitsa-Kimi Unit), Unit III the Uppermost Allochthon (Circum-Rhodope Belt), and Unit IV a still higher, far-travelled unit of unknown provenance. Telescoping of the entire Rhodope nappe stack to a thickness of only a few 100 m is due to Late Eocene north directed extensional shearing along the newly defined Kulidzhik Detachment which is part of a major detachment system along the northern border of the Rhodopes. Older top-to-the south mylonites in Unit I indicate that Tertiary extension evolved from asymmetric (top-to-the-south) to symmetric (top-to-the-south and top-to-the-north), bivergent unroofing.
The Rhodope Metamorphic Province in Bulgaria and Northern Greece has been affected by significant extensional tectonics since the Middle to 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 this study, these represent an originally west-southwest-dipping, 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. 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 Breznitsa 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.
In the Central Rhodopes of southern Bulgaria, an eclogite-bearing rock sheet belonging to the Middle Allochthon (Starcevo Unit) is over- and underlain by eclogite-free, amphibolite-facies rock units ...along low-angle shear zones, the Borovica Shear Zone at the top and the Starcevo-Ardino Shear Zone at the base. The age of these shear zones is determined by U–Pb zircon dating of pre-, syn- and posttectonic magmatic rocks, mostly pegmatite veins, using LA–SF–ICP–MS. Zircons from pre- to syntectonic pegmatites within the Borovica Shear Zone yielded ages of ca. 45–43 Ma, indicating that the shear zone was active at that time, and zircons from a pretectonic pegmatite and a posttectonic granitoid body within the Starcevo-Ardino Shear Zone yielded ages of ca. 45 and ca. 36 Ma, respectively, giving a time frame for the activity of that shear zone which probably rather postdated the activity of the Borovica Shear Zone. By combining the ages with the kinematics of the shear zones and the metamorphic history of the rock units, the following scenario is sketched: Soon after the Starcevo Unit reached peak pressure (eclogite facies), it was exhumed to a mid-crustal level by top-to-the-north-west, extensional unroofing along the Borovica Shear Zone, in a kinematic framework of orogen-parallel extension. Beginning at ca. 40 Ma, the partly exhumed Starcevo Unit was underthrust from the south-west by continental crust of the foreland (Apulia), forming the Lower Allochthon of the Rhodopes, along the Starcevo-Ardino Shear Zone. These results underline the significance of orogen-parallel extension for the exhumation of high-pressure rocks. With respect to regional geology of the Hellenides and the Aegean, it is found that the tectonic architecture of the Rhodopes is essentially of Tertiary age. Cretaceous syn-metamorphic shear zones do exist but are largely restricted to higher levels of the nappe stack (Upper Allochthon). The Rhodopes do not represent an older essentially Mesozoic core of the Hellenides but are formed by the internal, higher-metamorphic portions of the same major nappe systems as occur in the Hellenides.
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