Open AccessAnisotropic structure under a back arc spreading region, the Taupo Volcanic Zone, New Zealand
Author(s) -
Audoine Etienne,
Savage Martha K.,
Gledhill Ken
Publication year - 2004
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2003jb002932
Subject(s) - geology , trench , lithosphere , forearc , seismology , slab , subduction , anisotropy , petrology , volcano , geophysics , tectonics , materials science , physics , layer (electronics) , quantum mechanics , composite material
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.