Are we studying deformed rocks in the right sections? Best practices in the kinematic analysis of 3D deformation zones

A standard procedure in kinematic analyses of deformed rocks is studying samples only on the foliation perpendicular/lineation parallel section based on the assumption that the rotational component of deformation is restricted to that plane. The theoretical background for this procedure is that deformation is plane strain, which is often useful but fails to account for all relevant features observed in many natural cases. More general, 3D kinematic models reproduce several of these features and help to better understand 3D deformation, likely predominant in nature, so they are a more appropriate starting point for deformation zone analysis. Theoretical and natural 3D deformation examples suggest that the section with highest degree of asymmetry (the vorticity normal section, VNS) and the corresponding vorticity vector are independent from the finite strain fabric, and thus may present varied orientations with respect to the tectonic foliation and lineation. The VNS orientation depends on the kinematics resulting from boundary conditions whereas the strain fabric orientation also depends on accumulated strain. The orientation of the VNS is itself an essential element of shear zone kinematics, hence it cannot be pre-assumed but must be determined independently.