TIMING AND STYLE OF KAIKOURA RANGE DEVELOPMENT AND MARLBOROUGH FAULTING, SOUTH ISLAND, NEW ZEALAND, FROM LOW-TEMPERATURE THERMOCHRONOLOGY (Invited Presentation)

The ~150 km wide dextral Marlborough Fault System (MFS) and adjacent Kaikoura Mountains accommodate oblique convergence of the Australian and Pacific plates at the NE end of South Island, New Zealand. The MFS, which was the site of the 2016 Mw7.8 Kaikoura Earthquake, lies at the transition from oceanic Pacific plate subduction to the north and oblique continental collision within the Southern Alps to the south. Understanding the deformation history of the Marlborough region offers the opportunity to study topographic evolution in an oblique strike-slip fault setting and gain a fuller picture of the evolving New Zealand plate boundary. Low-temperature thermochronology from the MFS places new limits on the timing and style of mountain building and the relationship between the mountains and adjacent faults. We sampled a range of elevations spanning ~2 km within the Kaikoura Mountains, which stand high above active strike-slip faults. Young apatite (U-Th)/He ages (~2-5 Ma) on both sides of range-bounding faults are consistent with regional distributed deformation since the Pliocene initiation of strike-slip faulting. However, large differences in both zircon helium and apatite fission track ages, from Cenozoic ages within hanging walls to unreset >100 Ma ages in footwalls, indicate an early episode of fault-related vertical exhumation. These data reveal two phases of exhumation within the Kaikoura Ranges: rapid cooling at ~15-12 Ma localized to hanging wall rocks and regional rapid cooling reflected in all samples starting at ~4-5 Ma. Together, these results suggest that, despite the presence of active mountain front faults, much of the topographic relief in this region predates the onset of strike-slip faulting and portions of the Marlborough Faults were thrust faults that coincided with the early development of the transpressive plate boundary. Regional exhumation after 5 Ma likely reflects increased proximity to the migrating Pacific plate subduction zone and the buoyant Chatham Rise. The 2016 earthquake, which lifted and/or laterally shifted the surface along multiple fault strands, both onshore and off, fits well with evidence from the long-term record of a broad, complex and evolving oblique collision zone.