THE VARDAR ZONE AS A SUTURE FOR THE MIRDITA OPHIOLITE : CONSTRAINTS FROM THE STRUCTURAL ANALYSIS OF THE KORABI ZONE, NE ALBANIA

The Dinaro-Hellenic foldbelt is the result of Mesozoic–Tertiary convergence and continental collision between the Eurasian and Adriatic plates after the closure of the Tethys ocean. In Albania, it consists of (1) a Western zone of Early Oligocene–Middle Miocene fold-and-thrust nappes derived from proximal sedimentary facies of Adria; (2) a Central belt made up of an ophiolitic nappe, the Mirdita ophiolite, and underlying thrust slices of continental and oceanic origin; and (3) an Eastern zone, the Korabi zone, of Hercynian basement unconformably overlain by Permian–Lower Triassic rift deposits and Middle Triassic–Upper Jurassic platform carbonates. The Korabi zone and correlative rocks of former Yugoslavia are interpreted either as a micro-continent originally located between two separate oceanic basins, the Mirdita–Pindos oceanic basin to the west and the Vardar Zone oceanic basin to the east, or as a tectonic window below a single ophiolitic thrust sheet in which the most distal paleogeographic domains of Adria are exposed. The age of the Mirdita ophiolite is known from U/Pb zircon ages clustering between 160 and 165 Ma in plagiogranite and quartz-diorite intrusions (Dilek et al., 2007). The ophiolite has been thrusted over a metamorphic assemblage derived from both continental and oceanic domains, including a metamorphic sole that yield ⁴⁰Ar/³⁹Ar ages varying from 171 to ca. 162 Ma, and which record the age of obduction. The root zone of the Mirdita ophiolite and correlative terranes of the Dinarides has been, and still is, a source of debate; some argue that it is a far-travelled thrust sheet that roots eastward into the Vardar zone (Schmid et al., 2008) whereas others believe that Albanian and Greek ophiolites are doubly-verging (West- and East-directed) terranes forming the remnants of a small Jurassic oceanic basin formed between a Pelagonian block to the East (the Korabi zone) and the Apulian (Adria) passive margin to the west (Dilek et al., 2007). The metamorphic sole of the Mirdita ophiolite yields ambiguous shear-sense indicators and precludes precise and unambiguous characterization of the obduction kinematics.

A 20-km long structural transect along the Luma River of NE Albania has been performed in order to put kinematic constraints on regional deformation in the Korabi zone. The metamorphic grade in both the Hercynian basement and overlying Permian-Triassic (P-T) cover sequence is at greenschist facies. The P-T cover sequence shows two phases of regional deformation, D₁ and D₂. D₁ is associated with a NE-trending, SE-dipping regional schistosity that is axial-planar to NW-verging F₁ folds and associated with SE-dipping thrust/reverse faults, which locally over-thrust the Hercynian basement onto the P-T cover rocks. D₂ is characterized by a heterogeneously-developed, NNE-trending sub-vertical crenulation cleavage related to upright F₂ folds. There are currently no isotopic age constraints for deformation and metamorphism in the Korabi zone of Albania. Zircon fission-tracks ages yielding 150-125 Ma (Muceku et al., 2008) suggest, however, that both the P-T sequence and the Hercynian basement were at temperatures below ca. 240^oC at that time, and that the greenschist-grade metamorphism has to be Early Cretaceous or older. K-Ar muscovite and biotite ages from adjacent countries, where the eastern edge of the Korabi zone and/or the leading edge of West-Vardar terranes are exposed, vary between 148 and 130 Ma (Most, 2003). This suggests that D₁ is Late Jurassic or older, which is slightly younger than but consistent with Middle Jurassic ages preserved in the metamorphic sole of the Mirdita ophiolite. Therefore, an East-dipping subduction zone and a continent-ocean suture located East of the Mirdita ophiolite, in the Vardar Zone, can have accounted for the top-to-West structural polarity of regional deformation in the Korabi zone, in agreement with the inferred east-dipping subduction and west-directed obduction (the «Eohellenic orogeny»). Such an interpretation is moreover consistent with (1) the occurrence of a major, Mio-Pliocene west-dipping normal fault system at the western boundary of the Korabi zone (Muceku et al. 2008); the Mirdita ophiolite lies in the immediate hanging wall of these normal faults, suggesting that it was located higher in the crust and structurally above the Korabi zone in pre-Miocene times, and (2) the fact that the Mirdita ophiolite forms a tectonic sheet that bevels towards the west and thickens eastward, as expected for the emplacement of a West-verging nappe developed over east-dipping subduction.

REFERENCES. Dilek, Y. et al., 2007. Suprasubduction zone ophiolite formation along the periphery of Mesozoic Gondwana. Gondwana Res. 11, 453–475. Most, T., 2003. Geodynamic evolution of the Eastern Pelagonian Zone in NW Greece and the Republic of Macedonia. Ph.D. Thesis, Eberhardt-Karls-Universität Tübingen, 170 pages. Muceku, B. et al., 2008. Thermochronological evidence for Mio-Pliocene late orogenic extension in the north-eastern Albanides (Albania). Terra Nova 20, 180–187. Schmid, S.M. et al., 2008. The Alpine–Carpathian–Dinaridic orogenic system: correlation and evolution of tectonic units. Swiss J. Geosc. doi:10.1007/s00015-008-1247-3. 48 pp.