Geometry and impact of transpressional faulting in polyphasic metamorphic orogenic belts: the Viù Deformation Zone (inner Western Alps)

Abstract

This article describes the geometry and structural architecture of the Viù Deformation Zone (VDZ), a brittle-ductile to brittle structure affecting the metamorphic units of the inner Western Alps, and its role in modifying the pre-existing syn-metamorphic structural setting. The VDZ reactivates and displaces the contact between two different oceanic units, the Lanzo Ultramafic Complex and the Lower Susa–Lanzo Valleys Unit, characterized by polyphase syn-metamorphic deformation. It shows a strike-slip duplex geometry, constituted by N–S reverse-dextral faults linked by NW–SE antithetical sinistral-reverse faults, and represents a contractional step-over zone along a N–S regional dextral-reverse structure, the Col del Lis-Trana Deformation Zone. Formation of these transpressional structures steepened the Lanzo Ultramafic Complex during the last stages of its exhumation. The 3D geometry of the VDZ was strongly controlled by the reactivation of pre-existing structures, such as the buried western edge of the Ivrea body and metamorphic foliations. Brittle reactivation also induced block rotation along the VDZ, causing anomalous kinematic relations between the VDZ-associated faults. This study, hence, shows that in metamorphic orogens the mechanisms generating strike-slip duplexes may be different from those classically provided by the literature, with brittle reactivation and block rotation strongly prevailing on newly formed faults. In such orogens, moreover, rotations induced by transpressional faulting may sometimes be mistaken for steep syn-metamorphic shear zones. Underestimating the effects of later brittle deformation and associated rotations may cause erroneous interpretations of the tectonic evolution of orogens.