Distribution, style, amount of collisional shortening, and their link to Barrovian metamorphism in the European Alps

Abstract

We review estimates of collisional shortening along the Alpine Chain and reassess its amounts, showing that it increases from south to north in the Western Alps, attaining a maximum in the Central Alps, and decreasing in the Eastern Alps. We suggest that previous calculations overestimated shortening in the Western Alps, but underestimated it in the Central Alps. Based on these new determinations, we conclude that the convergence direction during Alpine collision was more likely oriented NW instead of WNW. A new map compilation of peak metamorphic temperatures related to syn-collisional, Barrovian metamorphism and of cooling ages for the Western Alps form the base for a discussion and interpretation of the spatial distribution of Barrow-type metamorphism throughout the Alpine Chain. We show that a simple correlation exists between the inferred temperature of areas where Barrow-type metamorphism is exposed at the surface and the amount of collisional shortening, which is mainly localized in the External Zone in the Western Alps, but in the Internal Zone in the Central and Eastern Alps. We conclude with a conceptual model, suggesting that major differences in the spatial distribution of shortening and exhumation of Barrovian metamorphic units across the Alpine Chain depend on the convergence direction, but also on the presence and size of the Briançonnais continental nappe stack at the onset of collision.