a) Geological map of the northeastern part of the Lanzo Massif peridotite body, including context within the Western Alps. The northern region of Lanzo features a progression of peridotite textures from proto-granular,, porphyroclastic, proto- mylonite, to mylonite that mark the evolution of a detachment fault dipping towards the northeast. The hanging-wall of this detachment fault has proto-granular textures. Having a detachment fault with sheared mylonites below it and relatively undeformed rocks above is diagnostic of metamorphic core-complex style extension (e.g., Davis, 1983). Foliation in the mylonite shear zone shows sigmoidal trends that are most prominent near the detachment fault which indicates an anastomosing detachment fault and shear zones. b) 3-D- model of the anastomosing detachment faults and shear zones based on the mapped foliation within the Lanzo Massif peridotite. c) A cross section of the Lanzo Massif with aluminum silicate, magnesium oxide, and ytterbium content as a function of textural units. Within the mylonites and proto-mylonites, aluminum oxide and Yb concentrations rise to 32.5-34 wt% and 0.35-0.38 ppm, respectively, while the magnesium oxide concentrations drop to 39.0-39.3 wt%. In the proto-granulite of the foot wall, magnesium oxide has a concentration of 39.7 wt% compared to 41 wt% in the hanging wall. The Yb concentration in the proto-granulite between the foot wall and hanging wall also changes, from 0.33 to 0.28 ppm. The variability implies the refertilization of the shear zone by melt moving through it as well as higher temperatures and melt concentration in the peridotite that composes the foot wall. Figure is modified from Kaczmarek & Müntener (2010) and from Vieira Duarte et al., (2020).