Subducted sediments, upper-plate deformation and dewatering at New Zealand's southern Hikurangi subduction margin

Abstract

The southern end of New Zealand's Hikurangi subduction margin accommodates highly oblique convergence between the Pacific and Australian plates. We carry out two-dimensional (2D) seismic reflection tomography and pre-stack depth migrations on two seismic lines to gain insight into the nature of subducted sediments and upper plate faulting and dewatering at the toe of the wedge. We also investigate the NE to SW evolution of emergent upper plate thrust faulting using 47 seismic lines spanning an along-strike distance of ∼270 km. The upper sequence of sediments that ultimately gets subducted (the MES sequence) has an anomalously-low seismic velocity character. At the southwestern end of the margin, ∼150 km east of Kaikōura, the MES sequence has experienced greater compaction (for an equivalent effective vertical stress) than it has some 200 km further to the northeast. This difference is likely attributable to greater horizontal compression in the southwest caused by impingement of the Chatham Rise on the deformation front. Relationships between velocity and effective vertical stress suggest that the MES sequence is well-drained in the vicinity of frontal thrusts, corroborated by evidence for upper plate dewatering along those thrusts. Effective drainage of the MES sequence likely promotes interplate coupling on the southern Hikurangi margin. The décollement is generally hosted near a seismic reflector known as “Reflector 7”. East of Kaikōura, however, Reflector 7 becomes accreted, indicating that subduction slip at the southwestern end of the margin is no longer hosted at (or above) this reflector. Instead, the décollement steps down to a deeper stratigraphic level further inboard. Further to the SW, approximately in line with the lower Kaikōura Canyon, the offshore manifestation of subduction-driven compression ceases.