Changing kinematics of the Ouse Fault during the Cretaceous–Neogene tectonic evolution of eastern Marlborough

The Ouse Fault and adjacent strata record the tectonic evolution of part of the East Coast Basin since the latest Mesozoic. The Ouse Fault is a significant inactive structure that separates contrasting domains of Cretaceous strata within the northern Clarence valley, northeastern South Island. Detailed structural mapping along the trace of the Ouse Fault, in combination with examination of oriented fault rock samples, has helped to resolve the kinematic history of the fault. Results suggest that extensional activity on the Ouse Fault commenced sometime between ~103--95 Ma during deposition of the Split Rock Formation. Activity on the Ouse Fault continued until about 86 Ma, with the fault possibly experiencing a brief reversal of dip separation around 90 Ma. The Ouse Fault was reactivated as a thrust fault during development of the Early Miocene oblique-convergent Pacific-Australia plate boundary along the east coast of New Zealand, when the fault likely formed part of the southeast-verging (in present-day coordinates) Flags Creek Fault System. The Ouse Fault accommodated a late phase of minor dextral-slip or oblique-reverse motion, most likely as a result of the transpressional deformation that has affected Marlborough since the Late Miocene.

The Champagne Formation constitutes a significant portion of the hanging wall of the Ouse Fault. The unit hosts spectacular deformation structures that formed penecontemporaneously with deposition. Previous studies postulate that the unit forms part of a mass-transport complex that was syn-tectonically emplaced with activity on the Ouse Fault. Results from this study suggest that the Champagne Formation was emplaced on a northeast-dipping paleoslope within the interval ~113–103 Ma, and that soft-sediment deformation within the unit predates the onset of motion on the Ouse Fault. The relationship between the Champagne Formation and the underlying Pahau Terrane basement remains ambiguous, with the unit possibly representing the youngest strata of the Pahau Terrane that were affected by widespread soft-sediment deformation. Results in this study accord well with a model of subduction shutdown beneath the Zealandia segment of eastern Gondwana sometime between ~105–100 Ma.