Strike-slip structure and sedimentary basins of the southern Alpine Fault, Fiordland, New Zealand

Abstract

The Alpine Fault is an 850-km-long, continental dextral strike-slip fault that accommodates some 60%–90% (∼20–30 mm/yr) of the obliquely convergent motion between the Pacific and Australian Plates in South Island, New Zealand. The southern 230 km of the fault traverses the continental margin off Fiordland and intersects the subduction thrust at the northern end of the Puysegur Trench. Marine seismic reflection profiles and bathymetric data are used to evaluate the late Quaternary structure and 3 m.y. evolution of the fault and five sedimentary basins. The fault offshore is more complexly segmented than its onshore counterpart, and at sedimentary basins has substantial (>1 km) bathymetric relief. Three right-stepping and overlapping active sections are identified on the basis of structural continuity and geomorphic expression. The northern, Milford-Caswell section, is 90 km long and continuous with the southern Westland section on land. The southern, Resolution section is 150 km long and developed from initially discontinuous, right-stepping segments. These sections overlap by 35 km at the large Secretary-Nancy Basin, which is currently being dissected longitudinally by the Nancy section, resulting in a straighter, almost fully linked principal displacement zone. The average strike of the fault differs from the azimuth of Pacific-Australian Plate motion by 11°–25°. Nevertheless, on the whole, the structure, tectonic geomorphology, and lateral displacements indicate predominantly dextral strike-slip displacement. The basins have the structural characteristics of pull-apart and releasing-bend basins. They commonly initiated at step-overs in the fault and evolved by development of very oblique cross- and along-basin faults that linked the principal displacement zone, resulting in longer, through-going surface traces. Localized transpressional features are developing contemporaneously with the basins and may relate to constrictional fault geometries and/or a 10° rotation of part of the fault within the last 3 m.y., rather than to changes in plate motions. The absence of significant regional contraction across the fault indicates that obliquely convergent plate motion is strongly partitioned. The convergence normal to the boundary is accommodated largely on thrust and reverse faults offshore and onshore, to the northwest and southeast of the Alpine Fault, respectively. The position of the Alpine Fault is clearly associated with the position of inherited Eocene rift structures carried in the subducting Australian Plate. Our observations and kinematic model imply a spatial-temporal evolution where the position of the southwestern end of the fault trace is transitory on a time scale of 10 5 –10 6 years, and the fault matures toward the northeast. The data are consistent with models invoking dextral reactivation of the subducting rift structures and tearing of the Australian Plate in the approximate direction of plate convergence, and/or the loading of shear stresses and strike-slip deformation on the Alpine Fault in response to spatial differences in surface roughness and interplate coupling. Several potential earthquake scenarios on the Alpine Fault are considered, ranging from rupture of individual structural sections associated with earthquakes of magnitude M 7.0–7.8, to larger, onshore-offshore composite ruptures of up to M 8.1.