Cristina Malatesta

Fluid-controlled deformation in blueschist-facies conditions: plastic vs brittle behaviour in a brecciated mylonite (Voltri Massif, Western Alps, Italy)C. MALATESTA AND OTHERSDeformation and fluid flow in subduction zones

A blueschist-facies mylonite crops out between two high-pressure tectono-metamorphic oceanic units of the Ligurian Western Alps (NW Italy). This mylonitic metabasite is made up of alternating layers with different grain size and proportions of blueschist-facies minerals. The mylonitic foliation formed at metamorphic conditions of T = 220–310 °C and P = 6.5–10 kbar. The mylonite shows various superposed structures: (i) intrafoliar and similar folds; (ii) chocolatetablet foliation boudinage; (iii) veins; (iv) breccia. The occurrence of comparable mineral assemblages along the foliation, in boudin necks, in veins and in breccia cement suggests that the transition from ductile deformation (folds) to brittle deformation (veining and breccia), passing through a brittle–ductile regime (foliation boudinage), occurred gradually, without a substantial change in mineral assemblage and therefore in the overall P–T metamorphic conditions (blueschist-facies). A strong fluid–rock interaction was associated with all the deformative events affecting the rock: the mylonite shows an enrichment in incompatible elements (i.e. As and Sb), suggesting an input of fluids, released by adjacent high-pressure metasedimentary rocks, during ductile deformation. The following fracturing was probably enhanced by brittle instabilities arising from strain and pore-fluid pressure partitioning between adjacent domains, without further external fluid input. Fluids were therefore fixed inside the rock during mylonitization and later released into a dense fracture mesh that allowed them to migrate through the mylonitic horizon close to the plate interface. We finally propose that the fracture mesh might represent the field evidence of past episodic tremors or ‘slow earthquakes’ triggered by high pore-fluid pressure.

Geology of the Pontinvrea area (Ligurian Alps, Italy): structural setting of the contact between Montenotte and Voltri units

This geological map at the 1:10,000 scale shows the structural setting of two poly-deformed metaophiolite units, with different metamorphic peak conditions, i.e. the blueschist facies Montenotte Unit and the eclogite facies Voltri Unit, in a selected area of 8.2 km2 within the Ligurian Alps (northern Italy). This study focuses on the tectonic contact between the two tectono-metamorphic units and on their relationships with the Oligocene sediments of the Tertiary Piedmont Basin. The map is a composite report of our field and laboratory study of structures and metamorphism, that explains our interpretation of the tectonic history of the study area. It shows that the two units were coupled during their exhumation path, along a blueschist facies mylonitic contact. This contact has been later involved in thrust faults that caused the superposition of the metamorphic basement on top of the Oligocene sediments.

Geology of the Eastern Ligurian Alps: a review of the tectonic units

The Alpine and Apennine belts come into contact in Central Ligu ria, in an extremely complex structural arrangement; this junction is usually referred to as the Ligurian Knot. The geological mapping for the 213-230 Genova and 212 Spigno Monferrato 1:50,000 quadrangles necessitated the updating of the stratigraphic, structural and metamorphic descriptions of this area, and a redefinition of some of the units and geological ensembles already established in Central Liguria, i.e. the Voltri Group, the Sestri-Voltaggio Zone and the Val Polcevera Unit. We propose a new definition of tectonic and tectono-metamorphic units, grouped in three main associations, as follows: a) units derived from oceanic crust and mantle; b) units derived from a continental margin; c) Flysch units derived from the sedimentary cover of an oceanic basement. In group a), we propose to keep the already established Palmaro-Caffarella, Cravasco-Voltaggio, Varazze and Figogna units and to establish a new Voltri tectonometamorphic Unit, including all the rocks showing a metamorphic climax in eclogite facies conditions, with a variably developed greenschist facies overprint. In group b) we propose to keep the already established Gazzo-Isoverde Unit and to define a new Angassino- Terma tectono-metamorphic Unit, encompassing limited outcrops of quartzite and dolomitic limestone associated with the Voltri rocks. In group c) we propose to replace the former Val Polcevera Unit with new Ronco, Montanesi and Mignanego tectonic units. We propose to abandon the term Voltri Group, for the term Voltri Massif that can be used to define a set of units, including the Voltri, Varazze, Palmaro-Caffarella and Angassino-Terma units, irrespective of their different lithology and/or paleogeographic derivation and/or metamorphism. For the Sestri-Voltaggio Zone, we propose to use this term only for the Cravasco-Voltaggio and Gazzo-Isoverde units (and not the Figogna Unit), to emphasize their common tectonometamorphic evolution.