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Contribution to the tectonic evolution of the Rhodope massif by applying magnetic methods (AMS: anisotropy of magnetic susceptibility) in granitic rocks

By Ειρήνη Ζανανίρι

Abstract

The present study focuses on the Early Miocene Symvolon pluton, and the composite Vrondou pluton (Mid-Oligocene at its eastern and Early Miocene at its western parts). Their emplacement and tectonic evolution has been studied by a combination of field tectonic, microtextural and anisotropy of magnetic susceptibility (AMS) methods. Paleomagnetic measurements and experiments to determine the magnetic mineralogy were also performed. Samples were collected from 80 sites: 40 in Symvolon and 40 in Vrondou. In both cases the bulk susceptibility magnitude as well as the magnetic anisotropy, sometimes reaching 60%, are high, thus pointing to a dominant ferromagnetic control of the magnetic properties. Two groups of magnetic lineations were identified: a prevailing NE-SW-trending linear fabric, visible mainly in the Early Miocene plutons, and a second NW-SE-trending fabric, clearly imprinted in the Mid-Oligocene eastern part of the Vrondou pluton. Macro- and microtextural study revealed the presence of magmatic textures, mainly in eastern Vrondou, and sub-solidus plastic deformation textures, in Symvolon and western Vrondou. Paleomagnetic results displayed a complex pattern with both clockwise and counterclockwise rotations. The emplacement model we present is inspired from one already proposed by Kolokotroni & Dixon (1991) and Kolokotroni (1992). Following these authors we envisage a subhorizontal extensional ramp space at mid crustal levels formed by the regional top to the SW shear movement in the Rhodope, active during the Mid Oligocene to Early Miocene period. A box-like extensional ramp space was incrementally widening and was simultaneously being filled with batches of magma generated at deeper levels. Free space for the influx of magma within the extensional ramp was provided by the southwestward movement of its frontal edge in relation to its back edge, and this may have been accommodated by strike-slip movements on two subvertical shear zones bounding the ramp space from NW and SE and acting as paths for the ascending magma, which was therefore entering from the NW or SE the gradually developing ramp space and was filling it by steaming along a NW-SE direction. Early magmatic intrusions were attached to the back edge of the ramp space, while new space was developing in its frontal edge. A similar subvertical shear zone may have acted as a path for the ascending magma in the case of the Symvolon pluton. This same shear zone, after the exhumation of the pluton, could have developed into the normal Kavala - Xanthi - Komotini fault. The older eastern part of the Vrondou pluton is much less deformed than the Early Miocene western Vrondou and Symvolon plutons. Magmatic textures are well preserved in the older intrusion whereas they are scarcely present in the younger ones. They indicate a NW-SE direction of magmatic flow in both cases. Sub-solidus plastic deformation affected mostly the younger intrusions and only locally the older eastern one. In all cases mineral lineations trending NE-SW were developed. An increase in the shear rate by the Early Miocene or a localization of shear mainly in the west, in combination with the more silica saturated rock types present there, are probably the reasons for the different deformational behaviour of the two parts of the Vrondou pluton.

Topics: Ανισοτροπία μαγνητικής επιδεκτικότητας, Πλουτωνίτης Βροντούς, Πλουτωνίτης Συμβόλου, Μάζα Ροδόπης, Παλαιομαγνητισμός, Anisotropy of magnetic susceptibility, Vrondou pluton, Symvolon pluton, Rhodope massif, Paleomagnetism
Publisher: Aristotle University Of Thessaloniki (AUTH)
Year: 2004
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