A comparison between the transpressional plate boundaries of South Island, New Zealand, and Southern California, USA: the Alpine and San Andreas fault systems

摘要

There are clear similarities in structure and tectonics between the Alpine Fault system (AF) of New Zealand’s South Island and the San Andreas Fault system (SAF) of southern California, USA. Both systems are transpressional, with similar right slip and convergence rates, similar onset ages (for the current traces), and similar total offsets. There are also notable differences, including the dips of the faults and their plate-tectonic histories. The crustal structure surrounding the AF and SAF was investigated with active and passive seismic sources along transects known as South Island Geophysical Transect (SIGHT) and Los Angeles Region Seismic udExperiment (LARSE), respectively. Along the SIGHT transects, the AF appears to dip moderately southeastward (~50 deg.), toward the Pacific plate (PAC), but along udthe LARSE transects, the SAF dips vertically to steeply northeastward toward the North American plate (NAM). Away from the LARSE transects, the dip of the udSAF changes significantly. In both locations, a midcrustal decollement is observed that connects the plate-boundary fault to thrust faults farther south in the PAC. udThis decollement allows upper crust to escape collision laterally and vertically, but forces the lower crust to form crustal roots, reaching maximum depths of 44 km (South Island) and 36 km (southern California). In both locations, upper-mantle udbodies of high P velocity are observed extending from near the Moho to more than 200-km depth. These bodies appear to be confined to the PAC and to represent oblique downwelling of PAC mantle lithosphere along the plate boundaries.