Illite (KI) and chlorite (AI) "crystallinity" indices as a constraint for the evolution of the External Briançonnais Front in Western Ligurian Alps..

Analyses of illite (KI) and chlorite (AI) 'crystallinity' indices,mainly from Cretaceous to upper Eocene-lower Oligocene clay-richformations, have been performed to constrain the metamorphic history of Briançonnais, Dauphinois/Provençal, and Western Ligurian flysch tectonic units at the Ligurian-Maritime Alps junction.The illite and chlorite mineralogical parameters point to verylow-grade conditions for both the Briançonnais and Dauphinois/Provençal units, characteristic of the middle anchizonal grade. Alsosome tectonic slices of Western Ligurian flysch involved in the major transpressive shear zones developed along the External Briançonnais front share the same metamorphic grade as the Briançonnais and Dauphinois/Provençal units. Conversely the S. Remo-M. Saccarello unit, another Western Ligurian flysch unit overthrust at the top of the tectonic stack of the Western Ligurian Alps, under went only diagenetic conditions. Thisshows that: a) in Western Ligurian Alps no significative change ofmetamorphic grade occurs across the External Briançonnais Front and thus the Briançonnais and Dauphinois/Provençal units were parts of a single foreland fold-and-thust-belt; b) Western Ligurian flysch units, although referable to a common palaeogeographic Ligurian domain, occur in two different tectonic settings: some units were placed along the European continental margin since before the beginning of Alpine tectonics and later on, from the Oligocene onwards, were involved and intensively sheared within transpressive shear zones along the External Briançonnais front;other units, such as the S. Remo-M. Saccarello unit, show a very different thermal history consistent with a shallower crustal pathand an emplacement at the top of the tectonic stack in a final stage of the contractional regime.