MAKING NEW PLATE BOUNDARIES: LESSONS FROM NORTHERN CALIFORNIA AND NEW ZEALAND

Plate boundaries are relatively stable features on Earth, and once established may persist for 10’s-100’s of million years. Whether these new structures exploit existing plate boundaries or develop independent of the regional deformational history, their development requires the formation of a localized, lithospheric scale structure to host the new plate boundary. In both northern California (associated with the formation of the San Andreas plate boundary system), and New Zealand (with the development of the Alpine Fault - Hikurangi trench composite plate boundary), the recent and ongoing formation of the plate boundary allows us to explore the processes that reform the plate boundary structure to accommodate the changing nature of plate interactions. In the San Andreas plate boundary case, the North American lithosphere that will host the new plate boundary undergoes some pre-treatment (while still serving as the upper plate of the subduction system) ahead of the cessation of subduction, which combined with post-triple junction tectonics produces an initially broad plate boundary deformation zone that coalesces over ~ 5 Ma into the stable San Andreas plate boundary. Much of this evolution is controlled by the interplay of thermally controlled ductile shear and structurally controlled brittle deformation. Although in some ways the resulting plate boundary structure has similarities with California, the evolution of the New Zealand plate boundary follows a significantly different route. In that case, a period of regional extension preceded the changes in plate motion that has produced the current transpressive plate boundary system. The systematic southward propagation of the transition from extension to transpression over the past ~ 25 Ma has led to the development of an evolving plate boundary structure that currently consists of oblique subduction (to the north) and dominantly translational transpression further south. Over time the subduction segment of the NZ plate boundary will lengthen at the expense of the Alpine Fault structures. These two locations with currently forming new plate boundaries provide direct evidence of the processes at work as the lithosphere is transformed from one mode of deformation into another.