The preorogenic tectonic history of the Bracco gabbroic massif: review and news.

AbstrAct -- We present some preliminary data on the oceanic tectono-magmatic history and deformation structures of the Bracco gabbroic Massif, recently collected through detailed structural fieldwork in the Bracco Pass - Mt. San Nicolao area (Eastern Liguria). The Bracco Massif (Vara Supergroup, Internal Ligurian Units) represents a fragment of Jurassic oceanic lithosphere. It is largely made of gabbros intruded in serpentinitic lherzolites and affected by very low-grade orogenic metamorphism (prehnitepumpellyite facies); the gabbroic mass often shows a cumulitic texture and it is locally characterised by the presence of ductile shear zones, displaying a peculiar HT-LP oceanic metamorphic imprint. It is crosscut by basalt dykes and hornblende-oligoclase veins. We collected new data on the attitudes of the magmatic layering, shear zones, basalt dykes and hornblendeoligoclase veins, focusing on those areas where the Alpine to post-Alpine deformation is scarce, so that the pre-orogenic structural features were preserved and easier to interpret. The data show that the younger oceanic structures (basalt dykes and hornblende-oligoclase veins) have a homogeneous trend in the whole studied area, whereas the magmatic layering and the ductile deformations (mylonites, flaser) have very different attitudes in the eastern and western sectors of the Bracco area. These sectors are separated by a NW-SE normal fault that runs immediately westwards from Mt. San Nicolao. We also present a discussion on the tectonic significance of the ductile structures and an interpretation of the structural data in the geodynamic context of an ocean ridge near a transform fault intersection