Earth Planets Space, Vol. 54 (No. 11), pp. 1011-1019, 2002
Tim Stern1, David Okaya2, and Martin Scherwath1
1Institute of Geophysics and School of Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand
2Department of Geological Sciences, University of Southern California, Los Angeles, CA 90089-0740, U.S.A.
(Received January 18, 2002; Accepted July 19, 2002)
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.