Rotational deformation in the Jurassic Mesohellenic ophiolites, Greece, and its tectonic significance

Abstract

The Jurassic Pindos and Vourinos ophiolites in the Western Hellenides of Greece are part of the Mesohellenic mafic-ultramafic slab underlying the Eocene–Miocene sedimentary basin (Mesohellenic Trough). The tectonic incorporation of this oceanic slab into the western edge of the Pelagonian subcontinent occurred via trench – passive margin collision in the late Jurassic. Much of the tectonic architecture of these ophiolites, particularly Vourinos, was acquired during progressive inhomogeneous deformation associated with the initial displacement of the Jurassic oceanic crust from its original igneous environment of formation, and its subsequent tectonic emplacement eastward onto the Pelagonian margin. The heterogeneous deformation in the mantle and crustal sequences of the Pindos–Vourinos ophiolites occurred in ductile, ductile-brittle, and brittle fields synchronously, as the Jurassic oceanic lithosphere was translated eastward; it also resulted in differential horizontal rotations within the displaced oceanic slab. Areas retaining high-temperature (diapiric) mantle fabric appear to have been “passively” translated by lower temperature ductile shearing, commonly along mylonite zones. Individual dunite bodies in the harzburgite tectonites indicate movement distances of at least kilometer scale. Pervasively mylonitic domains within the upper mantle peridotites suggest elongation on the order of five to ten times in the direction of ophiolite emplacement. Seafloor-spreading related, high-temperature mantle structures within the Pindos and Vourinos ophiolites are sub-parallel. Imprinted ductile kinematic indicators and ductile shear zones pervasive to the mantle and lower crustal sections in both ophiolites are also sub-parallel, consistent with the direction of tectonic vergence, and appear traceable across the sedimentary overburden of the Mesohellenic Trough. These geometric relations imply that the Pindos–Vourinos ophiolites retain their relative orientations from the time of cessation of ductile deformation within the ophiolitic slab. More than 90° anticlockwise horizontal rotation of the Vourinos ophiolite seems to have taken place during progressive simple shear deformation associated with the eastward emplacement of the Jurassic oceanic lithosphere. This kind of rotational deformation postdating the igneous accretion of ancient oceanic crust may be common in many ophiolites and should be carefully distinguished from tectonic extension-related vertical rotational deformation developed during their seafloor spreading evolution.