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

The fast-slipping Alpine (similar to 30 mm/yr), Hope (similar to 10-20 mm/yr) and Kelly (similar to 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 similar to 9 to 2 Ma, and AHe multi-grain and single grain aliquot ages (n = 9 samples) range from similar to 1.5 to 0.5 Ma. Inverse and forward thermal history modeling reveals distinct spatiotemporal variations in thermal histories. Late Miocene exhumation rates (similar to 0.6-3.5 km/Myr, assuming geothermal gradients of 33-40 degrees 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 similar to 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. Plain Language Summary The Alpine, Hope, and Kelly faults are tectonic plate-boundary faults on New Zealand's South Island. We use topography analysis to show that the landscape in the area where these faults intersect was affected by the formation of the Hope and Kelly faults approximately 2 million years ago (Ma). These faults affect the paths of rivers, height of mountains, and the location of the main drainage divide. We use thermochronology of apatite and zircon crystals, which measures how quickly rocks have cooled as they progressed from depth to the Earth's surface. This analysis shows that rock cooling between 9 and 2 Ma was controlled primarily by the uplift of rocks along the Alpine Fault. At approximately 2 Ma, the rock cooling rate increased in the east, so the entire study area has the same cooling rate from 2 Ma to the present. This could have been caused by changes in rock uplift rates toward the Alpine Fault, which brought rocks more rapidly toward the surface, and/or increases in erosion associated with climate change and rock mass weakening.