Illite (KI) and chlorite (AI) crystallinity indices as a constraint for the evolution of the External Brianconnais Front in Western Ligurian Alps (NW Italy)

Analyses of illite (KI) and chlorite (AI) crystallinity indices, mainly from Cretaceous to upper Eocene-lower Oligocene clay-rich formations, have been performed to constrain the metamorphic history of Brianconnais, Dauphinois/Provencal, and Western Ligurian flysch tectonic units at the Ligurian-Maritime Alps junction. The illite and chlorite mineralogical parameters point to very low-grade conditions for both the Brianconnais and Dauphinois/Proven al units, characteristic of the middle anchizonal grade. Also some tectonic slices of Western Ligurian flysch involved in the major transpressive shear zones developed along the External Briangonnais front share the same metamorphic grade as the Briangonnais and Dauphinois/Provencal 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, underwent only diagenetic conditions. This shows that: a) in Western Ligurian Alps no significative change of metamorphic grade occurs across the External Briangonnais Front and thus the Briangonnais and Dauphinois/Provencal 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 Briangonnais front; other units, such as the S. Remo-M. Saccarello unit, show a very different thermal history consistent with a shallower crustal path and an emplacement at the top of the tectonic stack in a final stage of the contractional regime.