Making Continental Transforms – Lessons from New Zealand and California

The development of the San Andreas (SA) transform system through California and the Alpine Fault (AF) transform through New Zealand provides insight into the processes and rates by which continental transform plate boundaries form. Both systems are young, minimizing the obscuring effects of subsequent tectonic overprints. These continental transforms differ from oceanic transforms, perhaps most importantly in that their geometry and geography changes substantially through time – in contrast to the quasi-steady state configurations of Mid-Ocean transforms. In both the SA and AF systems, the development of the plate boundary structure has involved not only the formation of a crustal fault system, but also requires the formation of a lithospheric-scale shear zone, whose orientation and behavior reflects the plate motions of the bounding plates, and can differ from the upper crustal structures that also reflect inherited crustal properties. There are several fundamental differences between these two systems. The kinematics of the AF system is significantly transpressive while the SA (with the exception of the Transverse Ranges) is nearly purely translational. Perhaps more important is that the SA system has developed in a systematic way with the steady migration of triple junctions, while the New Zealand plate boundary has developed and evolved in response to substantial changes in Pacific-Australia plate motions manifest in a rapidly migrating rotation pole, resulting in quite different time scales and processes for plate boundary modification in the two cases. The SA system forms and lengthens systematically at plate motion rates (in a rapidly evolving thermal regime), while the AF system formed rapidly (at essentially its present length) approximately 23 Ma in response to the plate motion changes and its role as the linking structure between the two subduction zones, which formed at that time – but its kinematics require it to reform essentially continuously.