Upper Mantle and Crustal Seismic Anisotropy Across the Pacific-Australian Plate Boundary, New Zealand

Abstract

Shear wave splitting measurements are used to investigate the presence of anisotropy in the lithosphere (using local S phases) as well as in the whole mantle (using SKS phases). The study area is composed of a subduction zone in the North Island of New Zealand, a transpressive continental/continental oblique plate boundary in the South Island, and the transition region between the two in the Marlborough Fault System region, northern South Island. In the South Island, SKS phases are recorded on a portable broad-band network of 26 stations deployed at regular interval in the entire island, for a six month period (1995-1996). Fast polarisations are consistent from station to station and give an average value of 36°±8°. They parallel the geological features, in particular to the Alpine Fault. Delay times are high, and give an average value of 1.6 ± 0.2s. Except for a slight rotation of fast directions in the southern South Island, results show no variation regarding: (1) The change in tectonic processes from the termination of the Hikurangi subduction zone in the northern South Island to the Alpine fault system in the centre of the South Island; (2) The distance from the Alpine fault. This suggest that the Australian and the Pacific plates are strongly coupled at depth and that upper mantle deformation takes place on a broad area at least as wide as the island. In the Marlborough Fault System, delay times from local S phases (recorded at shortperiod permanent stations) show very little increase with depth, and it is most likely that the higher-frequency phases used in this study respond mainly to lithospheric anisotropy. Again, fast polarisations are sub-parallel to the faults. Anisotropy is attributed to the presence of a feature containing metamorphosed schist (eclogite), of 30±10 km thickness and located 50-80 km beneath the surface. This feature is composed of crustal material of the Chatham Rise subducted with the slab. On the edges of the Marlborough Fault System, fast polarizations (oriented almost east-west) are roughly parallel to the maximum compressive stress direction and are consistent with crack-induced anisotropy in the crust. The shear zone, which is as wide as the island in the mantle as inferred from SKS phases, seems to occur in a narrower zone in the lithosphere at least in this part of the South Island. In the lower half of the North Island, local phases are recorded on a network of nine broadband stations co-operated by Leeds University, Institute of Geological and Nuclear Sciences (IGNS) and Victoria University of Wellington (VUW) for a 6 month period in 1993-1994. Fast polarisation directions (ø) from deep and from shallow local events are oriented parallel to the strike of the Hikurangi subduction zone as well as to the local faults. Results are also similar in direction to those obtained using SKS phases. These results suggest that the lithosphere and the upper mantle asthenosphere deform in a coherent strike-slip shear. We calculate 1.2 ± 0.3% of velocity anisotropy in the first 200 km of the mantle from increasing delay times with depth. In order to match the SKS delay times, this result requires the presence of anisotropic material down to 580 ± 100-km depth, or a change in anisotropy with depth, or frequency dependent splitting, or all three. In the Central Volcanic Region (CVR), data from permanent stations as well as from portable networks are used: From January to June 1995, the Institute of Geological and Nuclear Sciences, the University of Memphis, and the University of Leeds jointly conducted a large-scale seismic experiment in which 64 short-period seismometers and 27 broadband sensors were deployed. In the western part of the CVR, results from local phases from 50-80 km depth give a fast polarisation in the direction of extension (120°) and deeper local events from 150-220 km depth are sub-parallel to this direction (ǿ =150°). We interpret these results as caused by the presence of a fabric of olivine with a-axes in the trench perpendicular direction, located under the back-arc extension zone and due to asthenospheric flow in the extension direction. This zone is present down to 100-150 km depth. In the eastern CVR, local phases show mixed fast polarisation directions ranging from sub-parallel to the extension direction to sub-parallel to the trench. We suggest that the presence of hydrous minerals at the slab-mantle upper interface at 90-130 km depth is responsible for the appearance of a fabric of LPO of olivine called fabric B. In this fabric, a-axes orient parallel to the trench as due to maximum shear stress oriented perpendicular to the trench along the slab. Five SKS phases recorded at broadband stations in the middle of the CVR give consistent fast polarisation directions from station to station that result in an average direction of 43°±4° and a delay time of δt = 2.5 ± 0.2s. We explain those results either by a large scale trench parallel flow in the upper mantle beneath 150 km depth, or by the presence of water at the slab interface in the 90-250 km depth range, which would trigger the appearance of fabric B of olivine at those depths. In the first scenario, the trench parallel flow may be located anywhere between 150 and 660 km depth while the second scenario implies 6% of anisotropy strength and therefore dynamic recrystallisation must be involved.