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

This paper describes the geometry, 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 a polyphasic syn-metamorphic deformation. It shows a strike-slip duplexes geometry, constituted by N-S reverse-dextral faults linked by NW-SE antithetical sinistral-reverse faults, and represents a contractional step-over zone along an N-S regional dextral-reverse structure, the Col del Lis-Trana Deformation Zone. The activity of these transpressional structures caused the steepening of the Lanzo Ultramafic Complex and drove the last stages of its exhumation. The 3D geometry of the VDZ seems to have been strongly controlled by the reactivation of different pre-existing anisotropies, like the buried western edge of the Ivrea body and the metamorphic foliations. Brittle reactivation also induced blocks rotation mechanism along this structure, 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 literature, with brittle reactivation and blocks rotation strongly prevailing on newly formed faults. In such orogens, moreover, rotations induced by transpressional faulting may be sometimes mistaken with steep syn-metamorphic shear zones. Therefore, the overlook and the underestimation of the effects of the brittle deformations and of its associated rotations may cause erroneous interpretation in the tectonic reconstructions