EXHUMATION AND TOPOGRAPHIC UPLIFT ALONG CONTINENTAL STRIKE-SLIP AND OBLIQUE-SLIP FAULTS IN SOUTHWEST NEW ZEALAND

Within the Fiordland segment of the Southern Alps in southwest New Zealand late Tertiary strike-slip and oblique-slip faults surround areas of deeply eroded crust and anomalously high topography.  These exposures allowed us to examine how displacements on major transcurrent faults influence exhumation and uplift processes in a young, rapidly growing mountain range.  At the northern end of Fiordland a 2000 km² diamond-shaped uplift rises <1 km above the surrounding mountain topography. The uplift is bounded on all sides by seismically active fault zones. Nearly vertical oblique-slip faults and dipping reverse faults preferentially occur on the northern and eastern sides of the uplift. Normal faults occur on the southern side and dextral faults occur on the western side. Most of the largest faults reactivate old lithologic boundaries inherited from a period of Cretaceous orogenesis.  Areas of the highest elevations (up to 2699 meters) occur next to the reverse and oblique-slip faults where deeply incised, steep-walled drainages and fjords display slopes of up to 87°.  Fault-slip data, kinematic models and preliminary thermochronologic data indicate that 3.5-5.5 km of cumulative vertical displacement and 6.5-7.0 km of oblique-dextral displacement have occurred along the northeastern edge of the uplift since the Miocene (6.4 Ma). Uplift along oblique-slip faults on the northern edge is balanced by normal faulting on the southern side. Thermochronology and metamorphic data also suggest that exhumation rates are higher inside the fault-bounded uplift compared to areas outside it.  The results show that zones of anomalously high topography and exhumation along the Fiordland segment of the Australian-Pacific transform plate boundary are controlled by slip on curved, kinematically linked fault zones. Fault displacements combined with orographic effects resulted in highly localized zones of uplift and exhumation as small as 500-1000 km². The pre-existing architecture of the Fiordland crust also influenced the evolution of the uplifts by affecting fault geometry. These results show that uplift and exhumation along large continental strike-slip faults can be extremely heterogeneous and are strongly influenced by the architecture of pre-existing crust.