Anatomy of an extensional shear zone leading to the exhumation of the middle crust within the Canigou dome (Eastern Pyrenees, Axial Zone)

Abstract

To investigate the mechanism of exhumation of deep crustal rocks in hot orogens, we focus on the southwestern part of the Variscan Canigou gneiss dome (Pyrenees) where the transition between the middle and upper crust can be observed. On the basis of new structural data, geological mapping, cross sections, microstructural analyses, and petrologic observations, we propose that flat-lying foliation within the gneiss dome and steep cleavage developed above it formed coevally. We note the presence of regional-scale shear zones at the boundary between the middle crust (Núria-Canigou Unit) and upper crust (Puigmal unit). At this boundary, the E-W South Canigou Shear Zone is an extensional shear zone showing top-to-the southwest kinematics. Deformation, which is localized within the andalusite-biotite transition zone, occurred under retrogressive conditions, from high-grade ductile to brittle conditions during the main deformation stage (D2). The tectonic contact was folded during a later stage (D3). We propose that the shape of the Canigou dome resulted from the activation of extensional shear zones at the top of the orthogneiss, leading to partial exhumation of high-grade rocks. The Canigou gneiss dome can be considered an extensional dome that formed during late Variscan (ca. 310-290Ma) transpression.