Analysis of a complex high-strain zone at Cap de Creus, Spain

The structural analysis of a high-strain zone developed in medium- to high-grade metamorphic micaschists from the Cap de Creus area, Spain provides an example of the complex relationships between geometry, strain and kinematics to be found in deep crustal shear zones. This high-strain zone is composed of E-W trending structural domains characterized by different strain intensities and associated with syntectonic emplacement of pegmatite dykes. The main phase of deformation discussed here, D-2, affects steeply dipping bedding, boudinaged quartz veins and S-1 developed parallel to bedding. D-2 deformation of these features and of syn-D-2 pegmatite dykes gives rise to fold/boudin structures. In map view, a D-2 high-strain zone coincides with a km-scale dextral flexure of S-0, S-1 and S-2, although all small-scale structures on outcrop surfaces close to horizontal indicate a prevalent sinistral shear sense. In addition, a subvertical stretching lineation is present approximately parallel to the axis of the foliation-deflection and thus normal to the apparent 'displacement' direction in the high-strain zone. It is proposed that this high-strain zone did not form by horizontal dextral simple shear with a flow plane parallel to the boundaries of high- and low-strain zones but acted as a zone of highly vortical sinistral non-coaxial flow with a strong vertical extension component. The dextral flexure of S-0, S-1 and S-2 can be explained by considering that sinistral non-coaxial flow in the low-strain zones was either slower or earlier that in the high-strain core. This example stresses the necessity of being careful when interpreting displacement directions from the deflection of older fabric elements in deep-seated high-strain zones.