Single or double subduction? New constraints from structural analysis and magnetic fabric from the Vardar suture zone, Serbia

The NNW–SSE-trending Vardar suture zone was formed as a result of the Late Triassic to Early Jurassic opening of the Vardar Ocean as a branch of Neotethys, followed by subduction and collision of the Adria- and Europe-derived continental microplates during Early Cretaceous. The paleogeography and kinematics of the closure of the Vardar Ocean are still a matter of debate, and two main models exist based on the differences between Eastern and Western Vardar ophiolites. A simpler, one-ocean model invokes one oceanic domain subducted beneath the European plate, with a slab-rollback generating Eastern Vardar ophiolite in a back-arc basin. In contrast, a two-ocean model assumes the existence of two separate oceanic basins (relict Paleotethys and Vardar) consumed by doubly-vergent subduction. To test these two contrasting models, we present new tectonic, kinematic, and anisotropy of magnetic susceptibility (AMS) data from the Vardar zone in southern Serbia, where a complete section from one continental margin to another is superbly exposed. From east to west, the main lithotectonic elements across the suture are: (1) the Serbo-Macedonian Massif, composed of medium- to high-grade orthogneisses and paragneisses with lenses of marble, quartzite, and amphibolite, representing the European margin; (2) the Cretaceous flysch, younging westward; (3) the Goč–Kopaonik and Radočelo metamorphic complexes, composed of Early Paleozoic volcano-sedimentary successions (phyllites, marbles, and metabasites), that crop out within the suture; (4) obducted ophiolites dominated by serpentinized peridotite (Ibar being the largest) and their metamorphic soles; (5) deep-marine, pelagic successions (shales, cherts, limestones); and (6) Drina–Ivanjica belt, dominated by low-grade albite–sericite slates, representing the Adria-derived microplate. The flysch successions have been strongly shortened and folded into upright, tight to isoclinal folds and thrust over the ophiolites in some places with top-to-the-WSW kinematics. The ophiolites are frequently cross-cut by a dense network of brittle-ductile shear zones anastomosing around rigid (less serpentinized) peridotite blocks. The sense of shear locally varies, but generally is top-to-the-WSW(SW). The AMS data in the Ibar ophiolite indicate simple-shear-dominated strain along the eastern ophiolite–flysch contact with magnetic foliations moderately dipping to the NE to SW and bearing down-dip magnetic lineations, whereas magnetic fabric of the inner part of the ophiolite indicates pure-shear-dominated strain with more variable orientation of the principal susceptibilities. The easterly sub-ophiolitic Goč–Kopaonik and Radočelo complexes preserve an older fabric predating ophiolite emplacement, except near-contact zones that are strongly sheared. In contrast, the westerly metamorphic sole and pelagic successions dip uniformly beneath the ophiolite. Finally, the Drina–Ivanjica belt is dominated by a pre-collisional flat fabric. Taken together, the obtained data suggest regionally consistent top-to-the-WSW(SW) kinematics of thrusting and ophiolite emplacement across the Vardar suture zone and along with westward-younging of deformation and flysch deposition support the one-ocean model.