Seismic anisotropy and P-wave delays in New Zealand imply widespread deformation in the underlying mantle, not slip on a narrow fault zone, which is characteristic of plate boundaries in oceanic regions. Large magnitudes of shear-wave splitting and orientations of fast polarization parallel to the Alpine fault show that pervasive simple shear of the mantle lithosphere has accommodated the cumulative strike-slip plate motion. Variations in P-wave residuals across the Southern Alps rule out underthrusting of one slab of mantle lithosphere beneath another but permit continuous deformation of lithosphere shortened by about 100 kilometers since 6 to 7 million years ago.

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Present‐day displacements within the Central Andes are being measured using high precision GPS geodesy. Until now, comparison of such ground motions within deforming plate boundary zones to those on a geologic time scale has not been possible due to lack of sufficient geological data. In the Central Andes, a comparable dataset for the past 25 Ma of mountain building can be reconstructed. Here, we use new interpretations of shortening rates averaged over 25–10 Ma and 10 Ma–present and find that whilst displacement directions have remained virtually constant and parallel, an acceleration has occurred synchronously with a slowing of convergence between the Nazca and South American plates. Geologic shortening rates in the Andes are initially ∼5–8 mm yr−1, and increase to ∼10–15 mm yr−1 whilst convergence slows from ∼150 mm yr−1 to ∼70 mm yr−1. Displacement and convergence rates inferred from GPS and marine magnetic data suggest that this trend may be continuing at present. Hence an increasing fraction of convergence is being absorbed by mountain building. This change may reflect increased plate coupling due to decreased sediment supply, younger subducting lithosphere, or increased normal stress at the interface from the effects of uplift.

Shear wave splitting measurements, in conjunction with studies of shear wave velocity structure, indicate that the Ontong^Java Plateau (OJP) large igneous province (LIP) has a thick, compositionally distinct root that diverts asthenospheric mantle flow beneath the Pacific plate. The OJP, the largest of Earth's LIPs, stands 2 km above adjacent Pacific abyssal plains and is composed of mantle plume derived volcanics erupted at 122 and 90 Ma. Surface wave tomography of the Plateau reveals a seismically slow upper mantle root that extends approximately to 300 km depth. The thickness and juxtaposition of the Plateau and the mantle root imply that the OJP is the preserved`head' of a rising mantle plume formed in situ when the LIP erupted. Thus, it is a far-traveled body currently moving northwestwards with the Pacific plate. Shear wave splitting at four seismic stations along the northern margin of the OJP varies systematically: the fast axis of seismic anisotropy at three stations on the NE OJP margin trend NW, parallel to hotspot-defined Pacific absolute plate motion; at a fourth station, on the NW margin of the Plateau, the fast shear wave trend is NE. Upper mantle flow directions delineated by the shear wave splitting could thus represent mantle flow diverted around the leading, northwestern face of the rheologically strong, chemically distinct OJP root. In sum, the Plateau and its deep root appear to be similar to continental tectosphere, except for contrasting seismic velocities. ß