Structure and strength of a continental transform from onshore-offshore seismic profiling of South Island, New Zealand

Abstract

Seismic images of deformation beneath South Island, New Zealand, are provided by a form of seismic exploration uniquely suited to the study of “continental islands”—double-sided, onshore-offshore seismic methods in conjunction with onshore refraction and teleseismic P-wave delay data. Four sets of independent observations and analysis are use to infer rock properties within this plate boundary zone: seismic and electrical indications of high-fluid pressures within the crust; P-wave delays from teleseismic anisotropy to show a high-speed zone in the mantle directly below the crustal root; Pn anisotropy of 11±3% distributed over a region > 100 km-wide; and an effective elastic thickness (Te) that is vanishingly small beneath the Southern Alps and surface outcrop of the Alpine Fault, but increases to values of Te > 20 km beyond the coastlines of the South Island. Together, these observations show that deformation in the crust and mantle becomes progressively wider with depth. A region of distributed deformation > 200 km wide is inferred for the upper mantle. We propose that the weakness and the wide zone of deformation are phenomena of plate boundaries where both strike-slip and convergence have persisted for several millions of years.