Porphyroblast non-rotation during crustal extension in the Variscan Lys-Caillaouas Massif, Pyrenees

The dominant foliation (S2) in the Variscan Lys-Caillaouas massif (Central Pyrenees, France/Spain) formed in a subvertical orientation during crustal thickening (D2)-Subsequent non-coaxial crustal extension produced a subhorizontal crenulation cleavage (S3), whereby S2 was folded and rotated. Andalusite, staurolite, cordierite and biotite porphyroblasts grew very early during this extension (D3) and included S2 as straight, curved and weakly crenulated inclusion trails. These inclusion trails exhibit a strong subvertical preferred orientation, which is independent of the local magnitude of D3 strain, the dip angle of the external main foliation (S2), or the aspect ratio and shape-orientation of porphyroblasts. This indicates that porphyroblasts did not rotate in the D3 deformation field and hence, preserve the orientation of S2 at the time it was trapped in the porphyroblast, relative to the bulk D3 flow plane. Porphyroblasts could maintain a stable orientation in the D3 flow due to complete accommodation of the bulk flow vorticity by heterogeneous shear strain. The lack of vorticity of local volumes of zero (shear) strain, such as porphyroblasts, was balanced by concentration of shear strain in planar zones anastomosing around them. It is likely that S2 maintained a subhorizontal orientation throughout the D3 crustal extension, which would imply that porphyroblasts also remained stationary relative to the earthʹs surface during D3, whereas S2 in the deforming matrix experienced shear-induced rotation. A late brittle-ductile folding event (D4), involving differential rigid block rotations, dispersed the orientation of inclusion trails to some degree. These late rotations could be corrected for by artificial ‘unfolding’ of the F4 folds.