Tectonic evolution along the Col del Lis-Trana Deformation Zone (internal Western Alps)

Gianni Balestro(*)(**), Paola Cadoppi(*)(**), Gianluigi Perrone(*) & Sergio Tallone(**)
(*) Dipartimento di Scienze della Terra - Università degli Sudi di Torino, Via Valperga Caluso, 35 - 10125 Torino. E-mail: paola.cadoppi@unito.it. (**) C.N.R. - Istituto di Geoscienze e Georisorse, Unità di Torino, Via Valperga Caluso, 35 - 10125 Torino.

his paper describes the tectonic evolution of the Col del Lis-Trana Deformation Zone (LTZ), a N-S striking structure located in the internal sector of the northern Cottian Alps, in the hanging-wall of the first-order discontinuity (the Canavese Line) and representing the eastern border of the Alpine wedge. The detailed geological mapping and structural analysis of the two tectonometamorphic Units cropping out in the area has allowed the activity of the LTZ to be defined, related to two late- to post-metamorphic deformation phases (D4 and D5). This activity strongly modified the pre-existing syn-metamorphic structural setting, resulting from three main deformation phases (D1, D2 and D3). The LTZ is defined by i) the clockwise rotation at the macroscale both of syn-metamorphic structural elements and of lithological contacts from E-W to N-S strike directions, and by ii) the structural association of faults, cleavages and folds, mainly related to the reactivation of the pre-existing syn-metamorphic anisotropies. The D4 phase is characterized by the development of N-S major faults and by NNW-SSE and NNE-SSW minor faults, linked in a through-going shear zone with dextral shearing and minor reversal component, whereas the D5 phase is associated with the activity of E-W faults and with the reactivation of the D4-related structures mainly characterized by normal and sinistral-normal movements. The kinematic analysis of the D4-related structures yields a sub-horizontal NE-SW shortening direction, comparable with the regional shortening that affected the Western Alps during the Late Oligocene-Early Miocene and which caused the dextral movements along the Canavese Line. The extensional regime related to the D5 event is interpreted as the effect of the uplift with the subsequent isostatic re-adjustment of the alpine wedge. Since the LTZ is a very persistent structure (more than 20 km in length) it may be reasonably considered as a deep-rooted shear zone, which could reach 7-10 km of depth. Based on these assumptions the LTZ may be interpreted as a sub-parallel structure of the postulated southern prolongation of the Canavese Line, both representing steep fault strands of a regional dextral transpressive flower structure that bounds the alpine wedge on the internal side.

Western Alps, Cottian Alps, faulting, kinematic analysis.