Serpentinization has been invoked as a weakening mechanism to explain lithospheric rupture at magma-poor margins for temperature and depth conditions of less than 350ºC and less than 15 km (Lavier & Manatschal, 2006; Pérez-Gussinyé & Reston, 2001, Gillard et al. 2019). As subcontinental mantle and former asthenospheric mantle are brought near to or at the surface by detachment faults, serpentinization reactions transform the shallow rheology of the lithosphere. Serpentinite is weaker than peridotite (Escartin et al., 2001) and may allow for enhanced detachment faulting while also acting as a “seal” that prevents melt migration (and fluid migration more generally) once the fault shuts off (Skelton et al., 2005). In a well preserved ocean-continent transition in the Eastern Central Alps, the hydration of detachment faults and the conversion of peridotite to serpentinite or serpentine cataclasite in the fault gouge suggests that serpentinization is a potential weakening mechanism during extensional tectonics (Manatschal et al., 2007, Gillard et al. 2019). However, studies of serpentinized mantle exposed at the Newfoundland-Iberia conjugate margins point towards serpentinization having an insufficient effect to rupture the lithosphere to initiate seafloor spreading. Pre-seafloor-spreading magmatism (manifested as the J magnetic anomaly) was widespread even as serpentinized mantle was exhumed over >50 km, which is difficult to reconcile with a regime in which serpentinization incites final break-up (Nirrengarten et al., 2017).
In the Alpine, Pyrenean, and Uralide orogens, fossil margins preserve evidence of mantle exhumation along detachment faults and offer hints of the processes governing the deformation of the mantle lithosphere. The Err-Platta ophiolite represents a shallow domain of rifted margins where extension brought mantle rocks into contact with surficial sedimentary rocks (Manatschal & Nievergelt, 1997; Schaltegger et al., 2002). The Lanzo Massif also preserves a section of an Alpine-Tethyan magma-poor margin, specifically the intra-mantle structures associated with extension, (Fig. 2a). The Lanzo detachment fault system forms a network of anastomosing shear zones, visible from patterns of foliation in Fig. 2a and represented in 3-D in Fig. 2b. Enrichment of incompatible elements associated with the shear zone of this detachment suggest that it was a conduit for melt migration (Kaczmarek & Müntener, 2010 (Fig. 2c). This is consistent with observations in other core complexes, such as the Cemetery Ridge continental-core complex in Arizona where Miocene igneous bodies intrude into the detachment fault system (Seymour et al., 2018; Strickland et al., 2018) as well as with geothermal activity at oceanic core-complexes (Blackman et al., 2011; Harding et al., 2017; Hayman et al., 2011; Zhao et al., 2013). This melt percolation at high temperature weakens faults, shear zones, and the lithosphere as a whole and may prove to be a powerful weakening mechanism to explain lithospheric rupture and final beak-up before seafloor spreading initiates (e.g., Müntener & Piccardo, 2003; Piccardo et al., 2007).