The Alps-Apennines Interference Zone: A Perspective from the Maritime and Western Ligurian Alps

In SW Piemonte the Western Alps arc ends off in a narrow, E-W trending zone, where somegeological domains of the Alps converged. Based on a critical review of available data, integrated withnew field data, it is concluded that the southern termination of Western Alps recorded the Oligocene-Miocene activity of a regional transfer zone (southwestern Alps Transfer, SWAT) already postulatedin the literature, which should have allowed, since early Oligocene, the westward indentation ofAdria, while the regional shortening of SW Alps and tectonic transport toward the SSW (Dauphinoisforeland) was continuing. This transfer zone corresponds to a system of deformation units andkm-scale shear zones (Gardetta-Viozene Zone, GVZ). The GVZ/SWAT developed externally to thePenninic Front (PF), here corresponding to the Internal Briançonnais Front (IBF), which separatesthe Internal Briançonnais domain, affected by major tectono-metamorphic transformations, from theExternal Briançonnais, subjected only to anchizonal metamorphic conditions. The postcollisionalevolution of the SW Alps axial belt units was recorded by the Oligocene to Miocene inner syn-orogenicbasin (Tertiary Piemonte Basin, TPB), which rests also on the Ligurian units stacked within theadjoining Apennines belt in southern Piemonte. The TPB successions were controlled by transpressivefaults propagating (to E and NE) from the previously formed Alpine belt, as well as by the Apenninethrusts that were progressively stacking the Ligurian units, resting on the subducting Adriaticcontinental margin, with the TPB units themselves. This allows correlation between Alps andApennines kinematics, in terms of age of the main geologic events, interference between the mainstructural systems and tectonic control exerted by both tectonic belts on the same syn-orogenic basin.