Comparison of evolution and tectonic significance of the Pindos and Vourinos ophiolite suites, northern Greece

Abstract

The Pindos ophiolite, located on the western edge of the Sub-Pelagonian (Othris) zone, comprises a complex series of westward-verging thrust sheets dominantly composed of peridotite tectonite, phaneritic mafic and leucocratic rocks, and pillow lavas. Harzburgite tectonites from part of the east-central thrust sheet (Dramala Complex) show three well developed foliations. The oldest, compositional layering (S0), has been isoclinally folded, and preferred orientations of elongate grains and aggregates of olivine and orthopyroxene parallel to the axial surfaces of the isoclines define the second foliation (S1). Spinel lineations parallel fold axes of the isoclinal folds. Thermometric calculations from mineral compositions and grain sizes of olivines and pyroxenes associated with the oldest preserved foliations (compositional layering, S0 = S1) indicate that rocks of the tectonite suite equilibrated at depths of between 45 and 60 km, at temperatures between 1210°C and 925°C and at differential stresses from 5 to 10 MPa. In general, rocks indicative of higher temperatures of equilibration formed at lower values of differential stress. The third, and youngest, foliation is defined by bands of fine-grained olivine and pyroxene mylonite that cut across older structural elements. Mylonites equilibrated at temperatures between 650°C and 710°C, presumably indicative of shallower depths (30-20 km) than the older foliations, accompanied by differential stresses ranging from 200 to 300 MPa, and probably formed during obduction of the complex. The sense and orientation of shear determined from kinematic indicators within the mylonitic zones indicate an updip displacement vector inclined at an angle of 20° to 40° toward 016°, suggesting obduction to the east-northeast relative to the present position of the Pindos ophiolite. There is no direct evidence for the amount of rotation about a vertical axis that may have occurred during obduction or later tectonic transport. The composition, internal structure, environments of equilibration, stress history, and inferred direction of obduction of the Pindos ultramafites are strikingly similar to those of the Vourinos Complex located 40 km to the northeast on the eastern margin of the Sub-Pelagonian zone. It is probable that the two ophiolites shared a common origin and an identical early obduction history. During Late Jurassic times, the Vourinos and its metamorphic sole were thrust over a thick sequence of shelf carbonates and a thin, diamictitic melange, both units being metamorphosed to lower greenschist facies. Comparable units beneath the Pindos thrust sheet (the Loumnitsa Unit and the Avdella Melange) are thicker, somewhat more lithologically diverse, and comprise amphibolite, greenschist, radiolarite, pillow lava, garnetite, exotic marble, flysch, and poorly metamorphosed, fossiliferous limestone, typically occurring in tightly folded, thin thrust slices. Differences in the timing and direction of post-emplacement thrust transport of the two complexes are marked and may have begun late in the obduction phase.