Tectonic significance of the Hellenic-Dinaric ophiolites

Abstract

Abstract Isotopic and biostratigraphic data show the Hellenic ophiolites (those SE of the Scutari-Pec transform zone in Albania) are of Middle to Late Jurassic age. They are of two contrasting types. The first type of ophiolite is complete, usually harzburgitic, forms allochthonous sheets, commonly develops metamorphic soles, generally includes significant chrome deposits and shows supra-subduction zone (SSZ) trace-element characteristics with an evolution from MORB to IAT and occasionally boninites. The second type is characterized by incomplete ophiolites which lack chrome and mantle tectonite rocks. In contrast to the first type, these are slightly younger, are exposed in situ, show contact metamorphism against the country rock, are similar to MORB and show little influence of a subduction zone. The harzburgitic ophiolites from a western and an eastern outcrop belt. The relationship between the two is unclear, principally because of large, probably strike-slip, offsets in the Vardar zone, best documented in former Yugoslavia. The western Hellenic ophiolites (Pindos, Vourinos, Othris, Evvia ( = Euboea) and Argolis) all formed above a SW-dipping subduction zone in early Middle Jurassic time. They are continuous with a similar belt in the Dinarides of Yugoslavia whose SW margin gradually becomes lherzolitic as it is traced northwards. It is totally lherzolitic at its termination in NW Yugoslavia. This gradual change in ultramafic character is speculatively linked to the changes in spreading rates in a wedge-shaped marginal basin. Emplacement took place in later Middle to Late Jurassic time probably along a low-angle detachment zone that was close to the lithosphere/asthenosphere boundary shortly after spreading ceased. The SW half of the basin was pushed over the NE half, which was subducted to the SW but has left very little trace of its former existence. In the Hellenides the ophiolites were emplaced onto the Pelagonian continent and in the Dinarides onto the Drina-Ivanjica continent. Precise analogues of the complete ophiolites appear to be absent in the present-day major ocean basins — because of their SSZ character — or any island arcs that border the present-day oceans — because their marginal stratigraphy shows no trace of arc sediments or volcanics. The incomplete ophiolites, which appear to be restricted to the Axios zone in Greece, seem to be the expression of transient motion on short extensional segments of a plate margin whose geometry resembled the early stages of the opening of the Gulf of California.