The “Écailles intermédiaires” of the Rio Secco-Clarée Valley and Col Longet (Western Alps)

In the western Alps, a series of discontinuous tectonic slices reported in literature as “écailles intermédiaires” (EI) locally outcrop along the contact between Piedmont and Briançonnais units. In general, these slices consist of pre-Tertiary basement rocks and/or thin Mesozoic cover sequences of Ultrabriançonnais (orAcceglio) affinity, showing then an almost complete erosion of Triassic platform carbonates by transgressive Upper Jurassic deposits (e.g. Lemoine, 1961). This paper describes the structural-geological features of the EI outcropping in the Rio Secco-Clarée Valley (north of Montgenèvre, high Susa Valley) and Col Longet (high Ubaye Valley) areas.In the Rio Secco-Clarée Valley area, the Chaberton-Grand Hoche unit of Piedmont affinity is over a tectonic unit formed by a pre-Tertiary basement and its Permo-Mesozoic cover along a major thrust plane, folded and cross-cut by N-S trending strike-slip faults. The basement consists of polimetamorphic quartz-micaschists,monometamorphic micaschists and orthogneisses showing a pervasive blueschist-greenschist facies metamorphic imprint of Alpine age. The lower portions of the metamorphic cover sequence are Permian to Lower Triassic quartzites with diffuse pink quartz pebbles and intercalated red shales (Pseudoverrucano facies), upward followed by Scythian quartzites and by rare platform carbonates considered Middle Triassic in age (Barfety et al., 1995). The upper portions of the cover sequence consist of heterogeneous detrital calcschists and marbles with intercalations of breccias, and a peculiar conglomeratic micaschist (“Permien reconstituté” of literature); clasts of these levels are made of Triassic carbonates, Permo-Triassic quartzites and basement rocks, and have sizes ranging from mm to cm. These chaotic deposits rest unconformably above both the basement rocks and the lower terms of the cover sequence and their age ranges from Late Jurassic to Cretaceous–Eocene? (Barfety et al., 1995).In the investigated sectors near Col Longet, a few hundred meters-thick succession of megabreccias crops out between Piedmont and Briançonnais units. Breccias have sub-angular clasts and blocks up to dam-sized, made up of material dismantled from the adjacent Briançonnais units (micaschists, quarztites and carbonates) and deposited on marine environments (Lemoine, 1967). The megabreccias fade upwards into gray marbles, which are tentatively considered Late Cretaceous in age on the basis of regional correlations. The most striking feature of the investigated tectonic units is therefore the occurrence of Upper Jurassic to Cretaceous levels dominated by huge breccias; the sub-angular aspect of clasts and blocks suggests rock fall processes with short distance transport. In addition, the EI exposed in the Rio Secco-Clarée Valley area show an almost complete erosion of Triassic platform deposits, conversely reaching several hundred-meters in the adjacent Briançonnais units. The tectono-depositional evolution of the investigated EI is presented and discussed within the regional tectonic scenario of the western Alps, pointing emphasis on the syn- and postrift extension occurring along the continental margin.