Characterizing ductile-to-brittle exhumation of polymetamorphic units along the Sestri-Voltaggio Zone (Ligurian Alps, Italy).

The evolution of orogens is steered by complex deformation processes that act at several crustal levels, evolving over time from syn- to post-orogenic. Investigating how strain and deformation localize in the ductile domains of the deep crust and brittle domains of the shallow crust can improve our understanding of the processes ultimately controlling the exhumation of deeply seated rocks. Within this framework, the Sestri-Voltaggio Zone (SVZ) of the Italian Ligurian Alps provides a record of rocks and structures attesting to the complete subduction-exhumation cycle during the Europe-Adria convergence. The SVZ is a mature fault zone characterized by a polyphase tectonic evolution and a high lithological variability, which tectonically juxtaposes high-pressure (HP) metamorphic units to non-metamorphic rocks. It also represents an abrupt structural-metamorphic boundary between the Voltri Massif (an eclogitic domain defining a southern culmination of the Western Alps) to west, and the Northern Apennines units (anchi-metamorphic or non-metamorphic) to east. The exhumation processes that led to the current outcropping units of the SVZ occurred following a multi-stage progression from early ductile to later brittle conditions. However, open questions remain reflecting the generalized lack of systematic descriptions of structural fabrics formed during the exhumation-related events of the SVZ units. In this recently launched study we further explore the exhumation mechanisms of the SVZ by investigating how the pre-existing metamorphic fabrics helped localize the brittle deformation that occurred at later stages at shallow crustal levels. Preliminary field observations and structural analyses document N-S to NNE-SSW-striking brittle faults separating lenses of HP-mafic (metagabbros and metabasalts) and carbonate lithotypes from the enveloping phylladic schists and serpentinites. The enclosed lenses exhibit a pervasive internal schistosity that strikes either parallel or at high angle to the orientation of the main SVZ boundaries. By mapping the orientation of the rock fabric as a function of distance, perpendicular to the main tectonic boundaries, it is possible to identify systematic geometric trends between the metamorphic foliations and the bounding brittle faults. Within the matrix, the metamorphic schistosity wraps around the lenses, varying both in strike and dip. Brittle faults, with dominant oblique kinematics, are characterized by a double behavior: they truncate the metamorphic schistosity when approaching massive lenses; but they tend to rework the schistosity within the phylladic matrix. The overall structural record of the investigated units highlights the distribution of strain localization within the deeply exhumed units, suggesting a distinction between episodic vs. progressive transition from ductile to brittle during exhumation. In this sense, the SVG can be considered a useful example of the deformation history of the Western Alps-Northern Apennines tectonic junction, with noteworthy implications on the first-order mechanisms leading to the exhumation of deeply seated rocks.