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

Abstract

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 metamorphichistory of Brianconnais, Dauphinois/Provencal, and Western Ligurianflysch tectonic units at the Ligurian-Maritime Alps junction.The illite and chlorite mineralogical parameters point to verylow-grade conditions for both the Brianconnais and Dauphinois/Provencal units, characteristic of the middle anchizonal grade. Alsosome tectonic slices of Western Ligurian flysch involved in the majortranspressive shear zones developed along the External Brianconnaisfront share the same metamorphic grade as the Brianconnaisand Dauphinois/Provencal units.Conversely the S. Remo-M. Saccarello unit, another WesternLigurian flysch unit overthrust at the top of the tectonic stack of theWestern Ligurian Alps, underwent only diagenetic conditions. Thisshows that: a) in Western Ligurian Alps no significative change ofmetamorphic grade occurs across the External Brianconnais Frontand thus the Brianconnais and Dauphinois/Provencal units wereparts of a single foreland fold-and-thust-belt; b) Western Ligurianflysch units, although referable to a common palaeogeographicLigu rian domain, occur in two different tectonic settings: someunits were placed along the European continental margin sincebefore the beginning of Alpine tectonics and later on, from theOligocene onwards, were involved and intensively sheared withintranspressive shear zones along the External Brianconnais front;other units, such as the S. Remo-M. Saccarello unit, show a verydifferent thermal history consistent with a shallower crustal pathand an emplacement at the top of the tectonic stack in a final stageof the contractional regime.