GSA Annual Meeting in Seattle, Washington, USA - 2017

Paper No. 114-3
Presentation Time: 8:30 AM


KLEPEIS, Keith, Dept of Geology, The University of Vermont, Burlington, VT 05405, WEBB, Laura E., Department of Geology, University of Vermont, 180 Colchester Ave., Burlington, VT 05405, BLATCHFORD, Hannah J., Department of Geology, University of Vermont, Trinity Campus, 180 Colchester Ave, Burlington, VT 05405, SCHWARTZ, Joshua J., Department of Geological Sciences, California State University Northridge, 18111 Nordhoff Street, Northridge, CA 91330, TURNBULL, Rose, GNS Science, Dunedin Research Centre, Private Bag 1930, Dunedin, 9054, New Zealand and JONGENS, Richard, Anatoki Geoscience Ltd, 64 Skibo Street, Dunedin, 9012, 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.