Tectonic interpretation of an inverse gradient of zircon fission-track ages with respect to altitude: alpine thermal history of the Gran Paradiso basement

Abstract

Extensive sampling along vertical and north-south cross-sections in the Gran Paradiso basement (internal crystalline massif, Western Alps) reveals an inverse gradient of zircon fission-track ages with respect to altitude, which is explained by an “inverted metamorphism” during Eocene time. Subsequent to the obduction of the Schistes Lustrés nappe (high pressure Eoalpine phase), the Gran Paradiso massif cooled down 90-80 Myrs ago. The Austro-Alpine nappe of Monte Emilius — Dent Blanche overthrust the Gran Paradiso massif 38–40 Myrs ago, inducing in its basement an inverted metamorphism or, more precisely, a “nappe emplacement metamorphism” (greenschist facies). The last cooling below 100° C occurred as early as 35—40 Myrs ago in the southern Gran Paradiso massif; in the northern part, this last cooling occurred 20 Myrs ago, accompanied by an important uplift phase (2 mm/year).