A multidisciplinary approach to the study of the fault networkin the internal Cottian Alps (Western Alps)

Abstract

The architecture of the fault network and the late- to post-metamorphic structural evolution in the internal Cottian Alps have been studied by an integrated approach combining morphostructural analysis, field mapping and detailed mesostructural analysis between the Susa and Pellice Valleys. Available apatite fission track data indicate that this sector of the Western Alps reached shallow crustal levels, where brittle deformation mechanisms prevail, by Late Oligocene times. The analysed area comprises two main N-S striking regional, polyphasic semi-brittle to brittle deformation zones: the Colle delle Finestre Fault Zone (CFFZ) and the Col del Lis-Trana Deformation Zone (LTZ). The activity of those long-lived structures evolved, respectively, from dextral and dextral-reverse to extensional. In the block bounded by these two regional structures, the late- to post-metamorphic tectonic evolution is characterized by two faulting stages: (a) the development of an E-W left-lateral fault zone with a minor normal component of movement; at the regional scale, this fault zone may be considered as an antithetical shear of the major LTZ and the CFFZ; (b) the extensional reactivation and development of N-S normal faults, coeval with those recorded both by the LTZ and the CFFZ. The geometry of the fault and fracture systems has been compared with the remote sensed lineaments pattern, characterized by four main systems: Ln1 (N0°-30°E), Ln2 (N45°-70°E), Ln3 (N80°-100°E) and Ln4 (N100°-120°E). The lineament systems display similarities in terms of orientation, geometry and length with the fault systems observed at different scales. These similarities allowed us to extrapolate the structural model outlined in the mapped area to the adjoining sectors. Comparison between the different data suggests that the pattern of the lineaments detected between the LTZ and the CFFZ agrees with the architecture of a strike-slip fault network, which accommodated the internal deformation of the block bounded by these two major N-S transcurrent deformation zones. The transcurrent activity of these faults is here interpreted as coeval with the late dextral movements along the Canavese Line and with the propagation, in the Late Oligocene-Early Miocene, of the roughly E-vergent South-Alpine Thrusts beneath the Tertiary Piemonte Basin. The extensional event that followed may be related to the uplift with the subsequent isostatic readjustment of the chain which may have also induced the normal reactivation of the Penninic Frontal Thrust. This extensional regime is also in agreement with focal mechanisms of recent earthquakes.