INSIGHTS INTO THE TECTONIC EVOLUTION OF THE SOUTHERN ALPINE FAULT FROM MAPPING, PETROGRAPHY, AND ZIRCON U-PB GEOCHRONOLOGY AT KAIPO RIVER, NEW ZEALAND

The southern Alpine Fault (AF) in northern Fiordland, New Zealand is positioned between the dextral-reverse central section to the north, characterized by localized slip and high exhumation rates, and the offshore dextral section to the south, characterized by step-overs, pull-apart basins, and pressure ridges. We studied the southernmost well-exposed section of the AF at the Kaipo River, in order to provide further context to the tectonic evolution of this major oblique-transform plate boundary. We utilized lidar-based field mapping and kinematic analysis to reappraise a ~9 km² fault-parallel swath, as well as petrography and zircon U-Pb geochronology to establish correlation between displaced bedrock units and their source regions. At Kaipo, lidar and mapping show that the zone of active faulting extends ≥1.5 km NW of the AF main trace, as fault-bound plutonic slivers sandwiched between two internally deformed meta-sedimentary bands. Kinematic analysis reveals that dextral-normal displacement is strain partitioned into a negative flower structure, similar to those imaged by offshore seismic reflection lines. Zircon U-Pb laser-ablation dating of granitoids reveal mid-Paleozoic emplacement ages and Cretaceous metamorphic ages. Combined with petrographic analyses, our results indicate affinity to plutons of the Karamea Batholith and Separation Point Suite. Provenance analyses and zircon U-Pb ages of mélange-hosted meta-sandstone lozenges are consistent with a batholithic source of siliciclastic material containing both Paleozoic and Cretaceous age components. Collectively, these results indicate ~110 km of dextral displacement from a spatially restricted source region outboard of mid-latitude Fiordland. The present day position of these granitoids at Kaipo, and the presence of correlative units located in the northern West Coast suggests that their displacement post-dates early Miocene initiation of the AF. Assuming a time-invariant strike-slip rate, our preliminary results indicate that the AF has remained a segmented structure offshore since at least the mid-Pliocene. This study offers insight into the tectonic evolution of intercontinental transform boundaries generally, and allows us to understand how the offshore AF evolves with progressive slip into the onshore AF structure.