Abstract

A study of the Alpine fault zone and the Fiordland region of the South Island of New Zealand from February through April 1972 indicates high but diffuse microearthquake activity. Composite focal mechanism solutions show that a regional northwest-southeast compression dominates the tectonic pattern. This direction is nearly normal to the Alpine fault, indicating that the Alpine fault is now undergoing a large component of thrust faulting. This agrees with geologic data for uplift of the Southern Alps along the Alpine fault beginning in mid-Miocene time and accelerating in the Pliocene, the time of the Kaikoura orogeny. Before the Kaikoura orogeny, the Alpine fault apparently was a transcurrent fault. This major change in the New Zealand tectonic pattern could have been produced by a relatively minor migration of the nearby Indian-Pacific pole of rotation. Incipient underthrusting of the Tasman Sea appears to be occurring off the Fiordland coast, terminating at the point where the Lord Howe Rise intersects the coast. To the north is a zone of oblique continental convergence, with the Southern Alps being rapidly uplifted along the Alpine fault. North of the Alps, much of the motion is transferred to several faults that have more easterly strike; these formed in the Kaikoura orogeny and constitute a new transform fault system.

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