Continental transforms: A view from the Alpine Fault

New shear wave splitting measurements made from stations onshore and offshore the South Island of New Zealand show a zone of anisotropy 100–200 km wide. Measurements in central South Island and up to approximately 100 km offshore from the west coast yield orientations of the fast quasi-shear wave nearly parallel to relative plate motion, with increased obliquity to this orientation observed farther from shore. On the eastern side of the island, fast orientations rotate counterclockwise to become nearly perpendicular to the orientation of relative plate motion approximately 200 km off the east coast. Uniform delay times between the fast and slow quasi-shear waves of nearly 2.0 s onshore continue to stations approximately 100 km off the west coast, after which they decrease to ~1 s at 200 km. Stations more than ~300 km from the west coast show little to no splitting. East coast stations have delay times around 1 s. Simple strain fields calculated from a thin viscous sheet model (representing distributed lithospheric deformation) with strain rates decreasing exponentially to both the northwest and southeast with e-folding dimensions of 25–35 km (approximately 75% of the deformation within a zone 100–140 km wide) match orientations and amounts of observed splitting. A model of deformation localized in the lithosphere and then spreading out in the asthenosphere also yields predictions consistent with observed splitting if, at depths of 100–130 km below the lithosphere, typical grain sizes are ~ 6–7 mm.

/pdf/upper-mantle-seismic-anisotropy-at-a-strike-slip-boundary-2imqqsa03t.pdf

Upper mantle seismic anisotropy at a strike-slip boundary: South Island, New Zealand

We performed forward modelling of seismic anisotropy beneath a migrating strike-slip plate boundary to: (1) test if such a geodynamic context might explain teleseismic shear wave splitting data in the vicinity of the central part of the San Andreas Fault and (2) constrain the power of such data to unravel vertical and lateral variations in deformation patterns in the upper mantle. The modelling involves five steps: (1) thermomechanical modelling, using a finite-element code, of the deformation field, (2) viscoplastic self-consistent modelling of the resulting olivine and pyroxene crystal preferred orientations, (3) calculation of the elasticity tensors for different domains of the finite elements (FEs) model, (4) forward modelling of seismic wave propagation through the model using ray theory, finite-frequency theory and a full-wave approach and (5) performing splitting measurements on the synthetic seismograms. SKS splitting data in central California are best fitted by a model with a hotter geotherm within 60 km of the plate boundary accounting for the opening of an asthenospheric window due to the northward migration of the Mendocino Triple Junction. The westward motion of the plate boundary cannot however explain the rotation of fast polarization directions east of the San Andreas Fault in central California. Comparison between modelled and measured individual shear wave splitting also implies that the homogeneity of the two-layer models accounting for the observations in the vicinity of the San Andreas Fault indicates a sharp transition between lithospheric and asthenospheric deformations beneath this plate boundary. The ability of different synthetic approaches to localize horizontally and vertically the plate boundary-related deformation differs significantly. Splitting in ray theory synthetics closely follow variations in olivine crystallographic preferred orientations in the model. In contrast, splitting analysis on full-wave synthetics, which should be more representative of actual long-period SKS waves, results in smooth apparent lateral variations of anisotropy; the exact location and width of the plate boundary may only be retrieved by comparing fast-polarization profiles obtained using a multichannel analysis on waves with different periods.

Numerical modelling of the upper mantle anisotropy beneath a migrating strike-slip plate boundary: the San Andreas Fault system

Seismic anisotropy is one of the few tools for assessing the orientation of mantle deformation. We use it to study the plate boundary through New Zealand, which includes two oblique subduction zones of opposing polarity linked by the transpressional Alpine Fault. The mantle under South Island has strong, coherent deformation as evidenced by shear-wave splitting of SKS phases with fast polarizations oriented subparallel to the plate boundary, and with delay times averaging 1.7 s. SKS delay times are consistent through most of South Island and southern North Island, and fast polarizations are independent of distance from the plate boundary across the landmass. A fundamental change in anisotropy is nevertheless observed, which does not correspond in a simple way to changes between subduction and transform faulting. Instead, the change corresponds roughly with the northward narrowing of South Island (the "Waistline"). North of the Waistline, SKS splitting is parallel to the plate boundary, and the region of anisotropy is narrow (60-100 km wide) in the uppermost mantle as evidenced by Pn wave speeds and by shear wave splitting of local events. Lithospheric strain with dynamic recrystallization and sub-slab asthenospheric flow may both be contributing to the anisotropy. South of the Waistline, SKS splitting averages 20°, about 25° to the shear plane, and along the Waistline, Pn anisotropy extends about 200 km across the plate boundary. Lithospheric shearing without dynamic recrystallization, appropriate for smaller strain in this region, or asthenospheric flow directed at 20° or 200° could form the anisotropy.

Seismic Anisotropy in South Island, New Zealand

/pdf/numerical-modelling-of-the-upper-mantle-anisotropy-beneath-a-5blatqzcjh.pdf

Numerical modelling of the upper-mantle anisotropy beneath a migrating strike-slip plate boundary: the San Andreas Fault system

Near-surface deformation between the Australian and Pacific plates in South Island, New Zealand is concentrated in a narrow zone marked by the Alpine Fault, but strong and widespread anisotropy inferred across New Zealand from shear wave splitting suggests diffuse deformation at mantle depths. To constrain this interpretation, we calculate temperature- and stress-dependent strain fields, crystal preferred orientation, anisotropy and resultant shear-wave splitting beneath a lithospheric fault deforming by either pure strike-slip or transpression. In pure strike-slip experiments, strain localizes in the high temperature regions under the thick continental landmass. Under the oceanic regions, which have thin crust, anisotropy is weak (delay times (dt) 1400 km. In transpression experiments that have initial structure based on estimates of the present crustal thickness and temperature, the cold root inhibits strain beneath the fault and leads to anisotropy patterns that do not explain the observations. This suggests that (a) a weak zone in the mantle lithosphere is needed to explain present-day deformation; and (b) most of the anisotropy measured may be "frozen in" from strike-slip deformation that occurred before the present mantle root developed.

Modeling Strain and Anisotropy Along the Alpine Fault, South Island, New Zealand

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Modeling Strain and Anisotropy Along the Alpine Fault, South Island, New Zealand