Hydration and petrologic changes of inner portions of the subducting oceanic lithosphere facilitated by intermediate-depth faulting

The subduction-zone rheology and seismic behaviour of hydrated oceanic slabs have been widely studied, while the unaltered, dry portions of the subducting lithosphere remain less understood despite their role in earthquake generation and slab pull through eclogitization. We present field-based evidence from the ophiolitic Lanzo Massif (Western Alps), a slice of oceanic lithosphere composed of mantle peridotite and small volumes of gabbro which largely escaped hydration and Alpine metamorphism, representing a mechanically rigid block in the subduction complex. At intermediate depths, this dry lithosphere locally developed pseudotachylyte-bearing faults (e.g. the Moncuni locality) and widespread meso- to micro-faults (e.g., the Mt. Arpone locality). At Mt. Arpone, faults offset the gabbroic dykes by several centimetres: they contain sub-micrometric “annealed” ultramafic ultracataclasite of olivine, pyroxene and spinel, locally overgrown by chlorite and orthopyroxene. These features indicate faulting under dry conditions, which produced a tectonic porosity and enabled localized fluid infiltration. This led to discontinuous hydration of peridotite and to the widespread transformation of cataclastic gabbroic plagioclase into high-pressure zoisite-paragonite symplectites. Thermodynamic modelling suggests that plagioclase breakdown during faulting occurred under blueschist-facies conditions. Recent petrological studies show that this reaction is associated to volume reduction and the formation of reactive porosity, making plagioclase the most intensely eclogitized mineral in the studied samples. Trace element analyses reveal that fluid infiltration drove the internal (closed-system) redistribution of fluid-mobile elements. Eclogitization of the Lanzo peridotite and gabbro only occurred where fluids were present, either from limited oceanic hydration, or from subduction-related fluid infiltration through faults and the porosity created by the metamorphic breakdown of gabbroic plagioclase. The Arpone blueschist-facies ultracataclasites may represent seismic structures that did not evolve into frictional melting; they may be the precursors to the earthquake-generating faults that developed at Moncuni.