S-wave splitting in the transpressional zone of New Zealand plate-boundary: implications for deformation and dynamics
Seismic anisotropy in the transpressional plate-boundary zone in New Zealand is investigated with shear-wave splitting to gain insights into lithospheric deformation and mantle flow. Constraints on plate-boundary deformation in the lithosphere of the oblique-collision and subduction regimes in South Island have been estimated from the local and regional shear-wave splitting parameters that are made at both inland and offshore seismographs. Mantle and lithospheric anisotropy of the southernmost Hikurangi subduction zone in the southern North Island is examined from SKS, ScS and teleseismic S-phases. The splitting of these phases measured on a recent transect crossing the Wellington region is analyzed to understand the lateral anisotropic structure of the fore-arc Hikurangi subduction zone. Local and regional splitting reveal both laterally and depth varying anisotropy in South Island. The scatter in splitting parameters at individual stations suggests the splitting of high-frequency S-phases is mainly controlled by heterogeneous anisotropic structure and S-wave propagation direction within those heterogeneities. When the average results are examined as a whole through 2-D delay time tomographic inversion and spatial averaging, consistent patterns in delay times and fast azimuths exist. Spatially averaged fast azimuths indicate a localized high strain zone in the southern central region of the South Island. Based on fast azimuths observed above 100 km depth, we suggest that the plate-boundary sub-parallel anisotropy that is produced by pervasive shear is mainly distributed within a zone extending ~130 km SE of the Alpine fault in the southern South Island and is widely distributed (at least 200 km wide) in the northern South Island. Average station delay times (δt) of ~0.1 - 0.4 s compared to 1.7 s SKS δt from previous studies in South Island further suggest a deep seated anisotropic zone or sensitivity of S-wave splitting to the layered and/or heterogeneous anisotropic structure of the plate-boundary zone in the inland South Island. The heterogeneous anisotropic structure further suggests that the lithosphere is not only characterized by the plate-boundary parallel shear related to Cenozoic deformation, but is also affected by anisotropic imprints from the other tectonic episodes and anisotropy that is governed by the contemporary stress. A shear-wave splitting anisotropy investigation in the offshore South Island regions is an extended study of the inland experiment and aims to provide a broad-scale understanding of the plate-boundary deformation. Individual splitting of local and regional S-phases yield a range of δt that varies between very small δt (~0.02 s), which may represent a nearly isotropic medium, and large δt (~0.6 s), which corresponds to lithospheric anisotropy. The average station δt of ~0.25 s and variable delays of the individual splitting measurements imply that the observed splitting is most likely controlled by the geometry of the ray paths. Long ray paths that are detected at the stations further away from the plate-boundary appear to penetrate to deeper lithosphere and capture a significant portion of the upper-mantle anisotropy to produce fast azimuths parallel to the plate-boundary shear (NE-SW). Thus, the long and deep ray paths respond to the deeper structure, but may not be re-split by the upper-most crustal structures. However, the observed variable delays suggest that changes in ray propagation direction with respect to the orientation of symmetry axes of the anisotropic media may have an effect on the measured anisotropy. Offshore measurements that are close to the land are consistent with the inland measurements and appear to be controlled by the regional stress field. This implies that short and shallow ray paths are mostly sensitive to the crustal anisotropy. The uneven distribution of ray paths from the shallow and deep events, therefore, plays a dominant role in controlling the observed splitting depending on their depth sensitivity and/or extent of anisotropy. Consequently, when fast directions are spatially averaged along with the inland measurements consistent patterns appear to correlate with the possible depth contribution of anisotropy in the region. We are unable to provide accurate constraints on the offshore extent of plate-boundary parallel shear because of the shallow stress-controlled anisotropy that likely overlies the mantle-shear zone. However, the splitting parameters from long and deep ray paths suggest a deep-seated, plate-boundary sub-parallel shear in a broad zone at least in the northern and upper-central South Island. Mantle anisotropy detected from teleseismic earthquakes recorded across the southern North Island displays NE-SW fast axis alignment, consistent with the strike of the Hikurangi trench and the predominant upper-plate faulting trends, with a range of δt (~0.5 - 3.0 s) and small-scale variation in NE-SW fast azimuths. When combined with the previous measurements in the western side of the array, δt from long period (>7 s) S-phases indicate an abrupt lateral variation across the fore-arc Hikurangi subduction zone. This lateral variation together with frequency dependence suggest that the shear wave splitting in the fore-arc of the Hikurangi subduction zone in the southern North Island is governed in part by the laterally varying crustal contribution of anisotropy or isotropic velocity variations within the shallow crust. Frequency dependent splitting also suggests that the anisotropic structure is governed by either multilayer or more complex anisotropy perhaps due to the combined effects of laterally varying multilayer structure. If the variations are due to lateral changes in crustal anisotropy, then mantle and crustal deformation are most likely coupled in the east of the Wairarapa fault where there is a possibility of strong crustal contribution.