Abstract

Converted phases from teleseisms recorded by a seismic array spanning the northern half of the Marlborough fault system, South Island, New Zealand, show a continuous unbroken Moho underlying a seismically anisotropic lower crust beneath the two northernmost faults of the fault system. These observations suggest that distributed deformation, not slip on a narrow vertical fault, accommodates displacement in the lower crust below the 120–480 km of right-lateral slip across the Wairau fault, one splay of the Marlborough fault system, and the northward continuation of the Alpine fault. Beneath the Wairau fault, the Moho dips 25°–30° southeast from a depth of ∼26 km northwest of the fault to a depth of ∼34 km southeast of the fault. Farther to the southeast, Ps conversions from the Moho continue under the Awatere fault (34 ± 10 km of slip) with a constant amplitude and depth of ∼34 km. Across the two faults, converted energy from 16–20 km depth varies with back-azimuth in a manner suggesting the presence of anisotropy in the lower crust. These observations imply that one of the tenets of plate tectonics, that faults defining plate boundaries pass through both crust and upper mantle, does not apply to New Zealand, or to continents in general.

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