Abstract

Relocation of a Fiordland-derived Pliocene conglomerate (Halfway Formation) that is currently exposed >100 km northeast of its source area indicates that the average displacement rate on the Alpine fault, New Zealand, since the Pliocene must have been >27 ±4 mm/yr and probably >35 ±5 mm/yr. This represents at least three-quarters of the currently predicted plate velocity and indicates that a high proportion of plate motion in the region has been partitioned along the Alpine fault since the Pliocene. These results, combined with the apparent absence of major displacement on the Alpine fault for at least the past 250 yr, indicate that a large earthquake may be due. The provenance of the Halfway Formation also allows crude estimates of uplift since the Pliocene of 200-800 m for the west coast (Cascade Valley) and 1-6 km for western Fiordland.

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