LATE OROGENIC STRUCTURAL EVOLUTION IN THE LIGURIAN ALPS: CASE HISTORY FROM THE VOLTRI GROUP

Abstract

This work deals with ductile, brittle and brittle-ductile shear zones developed in ultramafic rocks of the Voltri Group (VG), Eastern Ligurian Alps (Italy), during the late Alpine orogeny. The late orogenic events (upper Eocenelower Miocene) are characterized by a complex superposition of syn-metamorphic (greenschist facies) and non-metamorphic structures linked to the exhumation of the subducted slabs during Alpine orogeny. In the VG these deformations are represented by low angle reverse shear zones and associated vein networks in mafic and ultramafic rocks. Many of these shear zones show multiple reactivation and thus several types of fault rocks, linked to different structural levels and stress regime, overprint in a single shear zone. In the ultramafites shear zones are associated with significant vein networks and wallrock metasomatic alteration and are characterized by a full range of fault rocks from serpentinite mylonite to fault gouge. Fault zones thickness and displacement, recrystallization of the host rocks and the brittle vs ductile style of the structures are strictly controlled by the fluid-induced reactions and are only partially linked to the structural level of development. Highest thickness of the fault zone occurs where fault rocks have been affected by huge quartz-carbonate vein networks and carbonate dominated metasomatic alteration; in this case cataclasite and fault breccia develop and brittle deformation overprint ductile ones. Superposition of ductile over brittle structures occurs along discrete low thickness shear zones, where sepiolite and phyllosilicate minerals replace serpentine minerals and pyroxene and/or in presence of very fine grained fault gouge. We investigated the complex interaction between brittle and ductilestructuresby constructing detailed structural maps, and logs of fault rock distribution and textures and studying the associated vein networks in order to understand the role of fluids in deformation. We discuss the implication for regional structural analysis and the late orogenic evolution.