Deformation and alteration of a granodiorite during low-angle normal faulting (Serifos, Greece)

Abstract

On Serifos (Western Cyclades, Greece), a late Miocene I-type granodiorite pluton intruded a low-angle normal fault (LANF) during extension and exhumation of the middle crust. In the studied structural section, the LANF cuts the granodiorite and, due to the minor displacement (<3 km), the roof of the intrusion was offset onto structurally lower parts of the pluton. As a result, the (micro)structural, petrological, bulk and mineral geochemical development of the LANF, at progressively increasing strains in a relatively homogeneous granodiorite, has been studied. Field data show that a totally bleached granodiorite in the hanging wall, weakly deformed under lower greenschist-facies conditions, has been juxtaposed along the LANF against an upper greenschist-facies mylonitic granodiorite in the footwall. The latter exposes a gradual decrease in deformation intensity toward lower structural levels, into the undeformed granodiorite. Bulk major and trace element compositions of footwall samples showed almost no whole-rock compositional, mass, or volume change with increasing deformation. In ultramylonites, however, the modal mineral contents and associated bulk geochemistries have been slightly altered, as a result of straintriggered, intrinsic fluid-assisted, reaction mechanisms. Because the field observations and geochemical data do not show evidence for high-tensile fluid overpressures within the LANF, we suggest that reduction of the apparent fault strength by weak fault zone material and/or a change in the deformation mechanisms facilitated movement. © 2009 Geological Society of America.