Landscape Response to a Developing Plate Boundary Fault: Hope-Kelly-Alpine Fault Interaction Zone, South Island, New Zealand

Mature plate boundary strike-slip faults often have a distinct influence on landscape evolution, which varies with fault slip rate and erosion rate. This effect is generally expressed as fault parallel valleys and saddles, mountains formed through differential uplift and fault complexities, or juxtaposition of pre-existing features. The topography of the central Southern Alps of New Zealand is dominated by the reverse dextral Alpine Fault, which produces a narrow, asymmetric mountain range. The Marlborough Fault System (MFS) is a series of dextral strike-slip faults in the northern South Island which transfer slip off the Alpine Fault. As the MFS has formed new faults over the last ~7 myr, the topography is less influenced by the Alpine Fault and develops into the lower, broader MFS topography with re-routed drainage system resulting in fault parallel river valleys. The Hope fault is the southernmost and fastest slipping of the MFS faults and the Kelly fault is thought to be its youngest, south-westernmost extension. The Hope-Kelly fault system connects with the Alpine fault via a geometrically and kinematically complex horsetail splay which also marks the transition zone between the two orogenic domains. While the Hope fault and older MFS faults have developed strike parallel river valleys, the Kelly fault has not yet influenced the river system but crosses multiple valleys and ridges associated with the preexisting central Southern Alps style topography. Using new apatite and zircon (U-Th)/He thermochronology, lidar and airphoto fault mapping, and large scale topographic analysis, we examine the landscape response to this young, plate boundary fault. Preliminary ZHe ages suggest that the region within the Hope-Kelly-Alpine fault triangle is exhuming more slowly than the central Southern Alps, which likely reflects the reduced slip rate on the northern Alpine fault since inception of the Hope and Kelly faults. Preliminary AHe ages show no trend with respect to the Alpine fault, and fault mapping reveals extensive fracturing and mountain collapse associated with the Kelly fault. These results suggest that the incipient fault breaks down incompatible Alpine fault hanging-wall topography, resulting in topographic collapse and the emergence of lower, fault parallel MFS topography.