Strain softening in a continental shear zone: A field guide to the excursion in the Ferriere-Mollières shear zone (Argentera massif, western Alps, Italy)

Fieldwork, integrated with several techniques for microstructural and geochronological analyses, plays a fundamental part in the study of shear zones. In this guide, we describe a two-day field trip in the Argentera External Crystalline Massif (Western Alps). The Massif is cross-cut by the NW-SE oriented Ferriere-Mollières Shear Zone, a spectacular example of a nearly 25-km-long regional transpressive zone with a maximum thickness of 2 km and a complex and long-lasting evolution during the Variscan time. Recent detailed geological mapping coupled with structural, microstructural, and petrochronological studies reveal that the Ferriere-Mollières Shear Zone was characterized by strain softening that localized strain in its central part during the syn-shearing exhumation and retro-metamorphism. The aim of the field trip is to visit some key outcrops that allow to recognize such evolution and to observe the main features of the Ferriere-Mollières Shear Zone that can be regarded as a good example of a strain softening shear zone in the continental crust.