Structural setting of the easternmost Dauphinois: stratigraphic and tectonic characters from the Ventimiglia area.

Since 2010, a detailed structural and stratigraphic mapping has been performed north of Ventimiglia (IM), between the Roya and Nervia valleys, by the Pavia University surveyors for the Risknat-Interreg project (Regione Liguria). The investigated area is constituted by a thick Dauphinois succession of sedimentary rocks. The post- Cretaceous Dauphinois units near the boundary between Italy and France are formed by a well-known succession: the “Priabonian trilogy”. It is formed by rocks that deposited in the Alpine underfilled peripheral foreland basin. A thick succession of Cretaceous marly limestones (Calcari e calcari marnosi di Trucco) forms the pre-Cenozoic basement, top-bounded by a regional unconformity (mostly an angular unconformity with rarer paraconcordance evidences) sealing up late Cretaceous and Paleocene deposits. The following transgression caused the deposition of marine-continental transitional deposits (conglomerates, sandstones and marls of the Microcodium Formation) during the Early Eocene. The vertical evolution of these deposits in the developing Alpine foreland basin is represented by the nummulitic limestones and calcarenites of the Calcareniti di Capo member, belonging to a shallow platform environment (about few to a hundred meters depth). The deepening trend bring to the sedimentation of marls and pelites of Marne di Olivetta San Michele Formation and finally to the turbidite sandstones of the Ventimiglia Flysch (corresponding to the Italian sub-basin deposit of the Gres d’Annot turbidite system). The field mapping of the study area evidenced the presence of several kilometric thrusts (Abellio thrust system) that juxtaposes two tectonic elements made up by the same, above described Cretaceous-Eocene succession. The units has been informally named “M. Terca unit” (the upper) and “Roja unit” (the lower). The thrusts are roughly NNE-SSW oriented; the amount of westtransport of the M. Terca unit is probably limited but significantly diminishes toward the South. This assumption is supported by observing the amplitude and geometry of the associated drag folds. The thrust is interrupted by a lateral ramp located northward near the M. Gouta area and reaches the coast near Ventimiglia toward the South. The geometry of the thrust has been also reconstructed through digital modelling in order to understand the real implications of its anticlockwise rotation. The Abellio thrust has been tentatively dated as Late Oligocene-Early Miocene, when the north-west Alpine compressive thrust fronts propagated in the Helvetic-Dauphinois zone. Interesting implications may come from its position, as the Abellio thrust could be considered the southermost onland structure affecting the Dauphinois succession. Its transport was probably also influenced by the interactions between the complex geometry of dispersal blocks that followed the rifting of Greater Iberia (causing the anticlockwise rotation of the Corsica- Sardinia) and the Alpine-Apennine orogens.