UNRAVELING A HISTORY OF REPEATED FAULT REACTIVATIONS AND DIFFERENTIAL UPLIFT ABOVE A YOUNG SUBDUCTION ZONE IN SW NEW ZEALAND

The Fiordland region of New Zealand’s South Island is one of only a few places where it is possible to directly observe the sequence of events associated with subduction initiation. The Puysegur trench, which is part of the Pacific-Australian plate boundary near Fiordland, is thought to have formed in Miocene times. However, its evolution and relationship to other events, including episodes of crustal deformation and rapid exhumation within the upper plate, are uncertain.

We used structural observations, ²⁰⁶Pb/²³⁸U rutile and titanite dates, and high-precision ⁴⁰Ar/³⁹Ar geochronology on pseudotachylyte, mylonite, and other fault rocks to construct a multistage history of faulting and differential uplift across Fiordland since ~30 Ma. The integrated data allowed us to distinguish successive phases of faulting (i.e., reactivations) from cases where different styles of brittle and ductile deformation occurred together. The results show that central Fiordland records the beginning of pure reverse motion and uplift on faults that parallel the Puysegur trench at 8-7.5 Ma. This event followed a long period of mostly dextral strike-slip deformation that began at 25-20 Ma and intensified at 14-10 Ma as convergent rates increased. Another shortening event occurred at 6-4 Ma when trench-parallel faults were reactivated again, placing a slice of Cretaceous lower crust up and to the east over middle crust. Both events were short-lived and produced melts interlayered with mylonite, suggesting transient, high-velocity slip. After ~4 Ma, motion on trench-parallel faults in central Fiordland changed back to strike-slip, as zones of shortening and uplift migrated north and east, away from the trench.

These patterns record an advancing front of deformation, tectonic uplift, and rapid exhumation that followed the leading edge of the Australian plate as it subducted beneath Fiordland, as early as ~20 Ma. The incorporation of Eocene intra-oceanic rift segments into the trench, combined with increased convergence rates, drove fault reactivation and changed the style, location, and timing of shortening. This study highlights a multifaceted approach to unraveling the complexity of continental fault zones and linking their evolution to other processes at active plate margins, including exhumation and topographic growth.