Surface breakthrough of a basement fault by repeated seismic slip episodes: The Ostler Fault, South Island, New Zealand

Abstract

The Ostler Fault is one of the major active reverse faults in the piedmont of the Southern Alps, SE of the Alpine Fault. We present a new geological and morphotectonic map of the southern Ostler Fault, integrated with two seismic reflection profiles across the active central segments of the fault. Segmented, subparallel scarps define a N‐S belt (∼40 km long and 2−3 km wide) of pure reverse faults, which upthrow and back‐tilt a panel of Plio‐Pleistocene terrestrial units (2.4−1.0 Ma) plus the overlying glacial outwash (<200 ka). Uplift gradients, the chronology of newly faulted markers, and tectonically controlled diversion of paleodrainages, all indicate progressive S to N breakthrough of the surface trace of the Ostler Fault in the last 2.4 Ma. The new seismic data define a main fault segment dipping 50°−60°W to depths of ∼1.5 km, with a vertical throw of 800 m, and a shortening of ∼30%. The fault geometry and kinematics and the subsurface data favor the interpretation that the Ostler Fault propagated updip across the Plio‐Quaternary terrestrial sequence as the emerging, high‐angle splay of an inherited Late Cretaceous−Paleocene normal fault, that underwent repeated cycles of compressional reactivation in the last 2.4 Ma.