Anisotropic structure under a back arc spreading region, the Taupo Volcanic Zone, New Zealand

Abstract

We investigate anisotropy surrounding a continental back arc spreading region, the Central Volcanic Region (CVR), in the North Island of New Zealand and present a simple method for spatial averaging to examine heterogeneous anisotropy. S phases from local earthquakes yield consistent trench‐parallel fast directions (ϕ) in the southern, compressional region but varied directions in the northern, extensional region. In the forearc east of the CVR, ϕ is nearly trench‐parallel, suggesting shear in the mantle or trench‐parallel flow. In the western CVR, local phases from 50 to 80 km depth give ϕ parallel to extension (120°), and phases from 150 to 220 km depth yield similar ϕ (150°). These results suggest asthenospheric flow with olivine a axes oriented in the extension direction down to 100–150 km depth. In the eastern CVR, local phases show mixed ϕ, suggesting that it is a transition between the western CVR and the forearc. Trench‐parallel ϕ for shallow local events within the CVR may be caused by fluid‐filled cracks. Trench‐parallel ϕ for deeper local events and for SKS phases can be explained by standard olivine fabrics developed by lithospheric shearing, by trench‐parallel flow, and by fossil anisotropy in the subducting oceanic lithosphere. Alternatively, in the CVR they could be caused in part by hydrous minerals at the slab‐mantle upper interface, which could cause olivine a axes to orient parallel to the trench due to a maximum shear stress oriented perpendicular to the trench along the slab. We suggest that trench‐perpendicular flow in the extensional region helps to drive fluids away from the slab, while in the compressional region, trench‐parallel flow fails to distribute the fluids, explaining the location of changes in geophysical properties.

Seismic anisotropy tomography: New insight into subduction dynamics

“…They applied this approach to the Marlborough region in central New Zealand, and their results show variations in P-wave anisotropy in the brittle upper crust, ductile lower crust, mantle wedge and the subducting Pacific slab. The patterns are consistent with the SWS observations (Klosko et al, 1999;Audoine et al, 2000Audoine et al, , 2004, but show some additional features. The maximum anisotropy is approximately 12% oriented northeast, between the Awatere and Wairau faults.…”

Characteristics of mantle fabrics beneath the south-central United States: Constraints from shear-wave splitting measurements

“…Averaged shear-wave splitting parameters for a station (Table 1) are computed using standard Gaussian statistics as weighted mean values of the splitting parameters from the individual events, and the standard deviations (STDs) of the mean values are the weighted STDs when the number of events is two or greater, and are taken as the STDs from the F-test for stations with only one event. We also computed weighted means of the fast directions using circular (von Mises) statistics (Mardia and Jupp, 2000; Audoine et al, 2004) and found that the results are virtually identical to those computed using Gaussian statistics.…”

“…Relative to typical Global Seismographic Network (GSN) stations, the stations used in this study have high noise levels, most likely due to local site effects of the thick loose sediment layer beneath most of the stations. In addition, less rigorous installation standards relative to GSN stations (except for HKT, which is a GSN station), high cultural activities, and waveform distortion associated with lithospheric structural complexity in the study area (Audoine et al, 2004) could all have contributed to the small number of high-quality measurements. Although stations AMTX and KVTX have only one measurement each, the results are reliable, as evidenced by the clear SKS signals, the similarity between the computed fast and slow components, and the linearity of the corrected particle motions (Fig.…”

Shear-wave splitting analysis of the upper mantle at the Niigata-Kobe Tectonic Zone with the data of the Joint Seismic Observations at NKTZ

“…The flow lines below an island arc are parallel to the earth's surface. The anisotropy of peridotite samples originating in the upper mantle has also been investigated (Peselnick et al, 1974;Peselnick and Nicolas, 1978;Nicolas and Christensen, 1987;Audoine et al, 2004). In the Kuril-Kamchatka subduction zone, seismic anisotropy was studied by Fischer and Yang (1994).…”