Rotation of ductile fabrics across the Alpine Fault and Cenozoic bending of the New Zealand orocline

This study examines the role and kinematic mechanism or vertical-axis rotat ions and folding during early development phases of a transform plate bound ary in continental crust. Near the Alpine Fault in New Zealand. this involv ed an initial period of widely distributed shear (including wrench folding) , together with vertical-axis rotations and orocline development, followed by a focusing of slip onto a single fault. In the South Island. the Haast S chist has been displaced horizontally in the Neogene by c. 480 km of dextra l-slip on the Alpine Fault. LS tectonites on either side of the fault lie o n different limbs of a profound Z-shaped orocline. Rotation of these limbs took place about a vertical axis. resulting in 25-30 degrees of clockwise r otation of fabrics to the NW of the Alpine Fault relative to those to the s outh. Foliation anisotropy caused bending to take place locally by folding about an inclined hinge in the limbs of a pre-existing synform, which tight ened during the deformation. On both sides of the fault. involvement of pos t-Oligocene age structures in wrench-folding and bending Suggests that the bending took place entirely, in the Cenozoic, before or during Miocene deve lopment of the Alpine Fault as a through-going structure, During this early phase of plate boundary deformation, shear was distributed across a zone > 100 km wide. Vertical-axis rotation affected. fault-bounded basement slive rs that shortened perpendicular to their strike, a style of deformation lik e that in the Transverse Ranges in California today.