Tectono-Thermal Evolution of the Hope-Kelly Fault System, Southern Alps, New Zealand: Insights From Topographic Analysis and (U-Th)/He Thermochronology

Abstract

The fast-slipping Alpine (∼30 mm/yr), Hope (∼10–20 mm/yr) and Kelly (∼6 mm/yr) faults in the South Island of New Zealand form a complex intersection zone that accommodates tectonic strain along the Australian-Pacific plate boundary. Analysis of digital topography reveals evidence for stream capture, drainage divide migration, landscape responses to incipient fault development, and preserved enclaves of relic topography that collectively reflect complex interplays between active faulting and landscape evolution. (U-Th)/He thermochronology of zircon (ZHe) and apatite (AHe) is used to investigate the low-temperature thermal evolution of rocks in the intersection zone. Weighted mean sample ages for ZHe single grain ages (n = 13 samples) range from ∼9 to 2 Ma, and AHe multi-grain and single grain aliquot ages (n = 9 samples) range from ∼1.5 to 0.5 Ma. Inverse and forward thermal history modeling reveals distinct spatiotemporal variations in thermal histories. Late Miocene exhumation rates (∼0.6–3.5 km/Myr, assuming geothermal gradients of 33–40 °C/km) through crustal depths of approximately 5–6 km, are interpreted to be controlled by proximity to the Alpine fault, with rocks proximal to the fault recording faster exhumation rates relative to distal samples. Establishment of the Hope-Kelly fault system in the Quaternary structurally juxtaposed rocks with discordant cooling histories. Rocks throughout the study region record increased cooling rates from ∼2 Ma. Possible causal mechanisms include, spatial changes in rock uplift associated with transport toward the Alpine Fault, increased erosion rates associated with Quaternary climate change, or increased rock mass erodibility associated with development of the Hope-Kelly fault system.