METAMORPHIC OPHIOLITES OF THE LIGURIAN ALPS: STATE OF THE ART ON AN EXHUMED SUBDUCTION INTERFACE

Abstract

The metamorphic ophiolites of the central Ligurian Alps, primarily exposed in the Voltri Massif and the Sestri-Voltaggio Zone, represent an extensive and well-studied record of oceanic lithosphere subducted to high-pressure (HP) and locally ultrahigh-pressure (UHP) conditions during Alpine orogenesis. In this review, we critically synthesize published structural, petrological, geochronological and geophysical constraints, and organize the discussion by the main tectono-metamorphic units of the Voltri Massif and by the composite slices of the Sestri–Voltaggio Zone. These meta-ophiolites originated from the Jurassic Ligurian-Piedmont Ocean, a branch of the western Tethys characterized by mantle-mediated extension and MORB-type magmatism within a magma-poor margin and ocean-continent transition (OCT) setting.

Overall, the metamorphic evolution documents a cold subduction regime, with peak conditions spanning from pumpellyite-actinolite to eclogite facies. For the Voltri Massif (Voltri Unit), the best constrained in terms of P–T–t,  the eclogite peak is estimated between ca. 1.8-2.5 GPa and 450-550 °C, and the recent discovery of coesite suggests localized UHP metamorphism reaching at least ca. 2.8 GPa. Geochronological data constrain the main eclogite-facies peak between ca. 50 and 40 Ma, followed by exhumation-related cooling and a pervasive greenschist-facies overprint near the Eocene-Oligocene boundary (ca. 34-29 Ma).

The regional structural architecture reflects a polyphase deformation history. Earlier deformations are associated with subduction, whereas composite transposition fabrics developed during initial exhumation; later stages record transpressional reactivation within the "Ligurian collisional knot". This late activity is commonly linked to the counterclockwise rotation of the Corsica-Sardinia block and the interaction between the Alpine and Apennine orogenic system.

Current tectonic interpretations debate the relative importance of serpentinite-hosted subduction channel processes versus the exhumation of large coherent slices. Evidence of strong strain partitioning between weak serpentinite matrices and competent mafic/ultramafic blocks suggests that both mechanisms likely operated at different scales and times, with late-stage coupling promoting map-scale structural coherency. Geophysical data are consistent with this view, imaging a fossil plate interface characterized by a shallow Adriatic mantle wedge and a hydrated, low-velocity zone.

In conclusion, the Ligurian meta-ophiolites provide a key natural laboratory to investigate the mechanical behavior of subduction interfaces and the tectonic reorganization at the junction of two major orogenic belts.