EXHUMATION, MOUNTAIN BUILDING, AND LANDSCAPE EVOLUTION ACROSS THE MARLBOROUGH FAULT SYSTEM, SOUTH ISLAND, NEW ZEALAND

The Marlborough Fault System (MFS), a system of parallel strike-slip faults at the boundary between the Australian and Pacific plates, connects the Hikurangi Subduction Zone to the north with the dextral-reverse Alpine Fault to the south. Presently, the strike-slip Marlborough faults bound or cut across mountain ranges, which increase in relief toward the Pacific plate. We use low-temperature thermochronology to constrain the tectonic history and landscape response across the MFS as it has transitioned from convergence to strike-slip over the last 25 Ma. Recent thermochronology data from the Inland and Seaward Kaikōura ranges, in the eastern MFS, revealed two major phases of exhumation. Rapid cooling in hanging wall rocks during the Miocene reflect a phase of thrust faulting at that time, when the Marlborough faults were oriented more parallel to the subduction zone. A second phase of regional exhumation began at 2-5 Ma and is thought to reflect distributed deformation associated with the southward migration of the buoyant Chatham Rise, which marks the southern end of the Hikurangi Subduction Zone. This study presents cooling ages for an expanded area covering nearly the entire MFS, including the western MFS, which is outside of the zone of Pacific underplating. Thirty apatite and zircon (U-Th)/He samples and twelve apatite fission-track samples provide new constraints on the spatial extent of exhumation caused by Miocene thrusting, as well as the extent and timing of more recent regional exhumation. Comparison of thermochronology ages with landscape features across the width of the MFS, such as zones of high topography, high-relief slopes, and steep rivers, sheds light on the landscape response to deformation during MFS evolution.