The evolution of ideas for the origin and emplacement of the western Hellenic ophiolites

Abstract

Reference to some of the western Hellenic ophiolites (Vourinos, Pindos and Othris) as serpentine and dolerite eruptions by Boue (1794-1881) dates from at least 1840. The observation of tuffs and pillow lavas (?), possibly not ophiolitic, in Othris in 1880 led Neumayr (1845-1890) to suggest their origin as large submarine eruptions. A submarine setting was reinforced by Brunn's mapping of the Pindos and Vourinos ophiolites in the 1930s and 1950s. He concluded that they were in situ, formed an entity entirely magmatic in origin, had been extruded onto the Pelagonian continent in the Jurassic along feeders concealed under the Meso-Hellenic trough and that the ophiolitic magmas had flowed outward to the NE (Vourinos) and SW (Pindos). Brunn interpreted the low-grade metamorphism at the base of the Vourinos ophiolite as contact metamorphism that provided additional evidence for their origin as huge submarine flows. In 1959, Brunn made the first analogy between these ophiolites and the rocks of the Mid-Atlantic Ridge-the oceanic rocks and ophiolitic rocks were formed by similar processes of submarine extrusion, but were not necessarily equivalent. Moores (1969) recognized the fundamental difference between magmatic and tectonized ultramafics in the Vourinos and suggested that many of the ultramafic rocks had been emplaced by solid flow to produce structures that were symmetric about a NE-trending axis. All of these interpretations were made in an essentially fixist framework. Subsequent interpretations assumed a plate tectonic setting. As with many other ophiolites, the main problems have been the root of the ophiolites; the location and dip direction of associated subduction zones; the tectonic setting(s) indicated by their marked variations in composition; the spatial and temporal relationships between extension and convergence as recorded in ophiolitic rocks; how the ophiolites were emplaced onto a continent; the significance of the metamorphic sole; and the spatial and temporal relations of the Hellenic ophiolites to contiguous ophiolites in Albania and former Yugoslavia. A modified model for the emplacement of the western Hellenic ophiolites is set out in this paper. In outline, the model starts with the creation of the Pindos Ocean, a branch of the Tethyan Ocean in Triassic time, separating western Greece from the Pelagonian continent to the east. A west-dipping subduction zone was established in mid-Jurassic time in western Greece, located in the south against the eastern margin of the Parnassos platform and in the north located within the Pindos Ocean. Initially, the hinge line of the subduction zone retreated east at a faster rate than the overall convergence. The difference in velocities caused extension between the hinge line and the continental margin that generated a back-arc basin contemporaneously with subduction. Convergence was stopped by the attempted subduction of the Pelagonian continent under the eastern part of the new ocean basin that was then emplaced as the western Hellenic ophiolites, leaving the remainder of the supra-subduction ocean basin and the Pindos Ocean to be subducted in a later phase of convergent orogeny. The model requires only one subduction zone that starts at a continental margin. It may be of general application, as sketched in a model for the emplacement of the Semail ophiolite in Oman.