Kinematics of the Forcarei Synform (NW Iberian Variscan belt)

Abstract

The Forcarei Synform is a kilometric fold developed in the hinterland of the NW Iberian Variscan belt. A detailed analysis of the synform, based on quartz fabrics and kinematic markers, shows pervasively deformed rocks that have been continuously deformed during the last two main Variscan deformation phases (D2 and D3). Variscan D2 minor structures related to the synform were formed under general non-coaxial flow and have a sub-horizontal maximum finite stretching toward the SSE. Later a coaxial D3 produced a sub-vertical crenulation cleavage, and type 2 and 3 refolded folds figures at the meso- and microscale. Consequently, the strike of the foliations (S2 and S3) become sub-parallel and lineation (L2 and L3) are sub-horizontal. The geometry of the synform and the related structures can be interpreted in the context of D3. However the dominantly sinistral shear sense indicators, observed in both limbs of the synform (in map view), seem to be most probably developed during D2. A model is proposed involving the progressive clockwise rotation of a sub-horizontal shear, with general top-to-the-south sense, during the simultaneous development of shear zone and foliation in agreement with the kinematic indicators.