Forearc structures and tectonic regimes at the oblique subduction zone between the Hikurangi Plateau and the southern Kermadec margin

Abstract

The convergent southern Kermadec margin is examined to study the structural development and mechanical parameters of the inner trench slope in response to the oblique subduction of the oceanic Hikurangi Plateau. Swath bathymetric data have been combined with other geophysical data to reveal that the South Kermadec inner trench wall is segmented into distinct upper, middle, and lower margins. The lower margin is a nongrowing, compressionally unstable accretionary wedge that is dissected by transcurrent faults. The wedge has been thinned and narrowed as a result of tectonic erosion. The middle margin is stable by comparison and separated from the lower margin by the 220‐km‐long Awanui Fault. This fault is a landward dipping, strike‐slip, reverse fault. The middle upper margin, including East Cape Ridge (ECR), forms a deforming backstop with a sharp along‐strike change in tectonic regime. South of 36°40′S, transpression characterizes the ECR shear zone. North of 36°40′S, the ECR is marked by uplift, by extension, and farther north by subsidence. It is suggested that subsidence and subsequent tectonic erosion of the margin north of 35°50′S, in the wake of Hikurangi Plateau subduction, have broken the link between once continuous segments of the ECR‐Tonga Platform. The unstable nature of the southern Kermadec wedge under compression, together with a comparison of its taper characteristics with those of modern convergent margins, implies that the wedge deforms under a relatively high internal pore pressure but high effective basal friction regime. Strike‐slip deformation in the wedge is attributable to a relatively strong interplate coupling related to the buoyancy and ruggedness of the Hikurangi Plateau.