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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.
In most oceanic regions, plates of lithosphere move past one another along narrow boundaries (width <20km), and a single major fault can define transform and convergent boundaries; active deformation within continental regions, however, commonly spans dimensions of hundreds to thousands of kilometers. The much greater creep strength of olivine than of crustal minerals makes oceanic lithosphere strong in the depth range where continental lithosphere appears to be weakest (1). What then is the role of the mantle lithosphere beneath continents? Is it cut by faults, or narrow shear zones, that separate effectively rigid bodies, as if plate tectonics occurred but was blurred by the easily deformed overlying crust (2, 3); or does the mantle lithosphere deform continuously over a wide area, behaving as a continuous medium (4, 5) (Fig. 1)? New Zealand offers tests of these possibilities. Since --45 million years ago (Ma), the Pacific and Australian plates have slid 850 (+/-100) km past one another, with --460 km of right-lateral slip on a single major fault, the Alpine fault, dominating Cenozoic deformation of New Zealand's crust (6, 7). Does the Alpine fault cut through the entire lithosphere, or does widespread simple shear parallel to the fault occur throughout the underlying mantle lithosphere? At -6-6 to 7 Ma, the direction of relative movement between the Pacific and Australian plates changed somewhat, resulting in - 100 km of convergence across the South Island of New Zealand (8). Has "intracontinental subduction" of mantle lithosphere occurred (2, 9), or has mantle lithosphere deformed continuously (4, 5)? We compare magnitudes of relative plate motion across the Pacific-Australian plate boundary in New Zealand with estimates of strain in the underlying...