Abstract

Minimum slip rates calculated for plate-vector-parallel slickenside trends in cataclasite on the sole of the Alpine fault at Gaunt Creek, Westland, New Zealand, range from 18 to 24 mm/yr. Between half and two-thirds of the total relative motion between the Pacific and Australian plates is being accommodated by movement on a single structure, the Alpine fault.

During the past 14 ka, the leading edge of the Alpine fault has changed from a moderately southeast-dipping, oblique reverse fault to a shallowly dipping thrust. The hanging wall (Pacific plate) is composed of a gradational sequence from basal gouge, through pseudotachylite-bearing cataclasite, to progressively more coherent schist-derived mylonite, which has been faulted against subhorizontally bedded, fluvio-glacial gravel in the footwall (Australian plate). During uplift the hanging-wall sequence has been internally sheared and imbricated, producing duplex structures, and retrogressively veined and altered by pervasive hydrothermal fluid flow.

Erosion of the exhumed fault zone produced angular, cataclasite- and mylonite- derived, talus-fan breccias, building a west-dipping apron beneath the fault scarp. Wood fragments from near the base off the talus breccias have been 14C dated at 12,650 ± 90 yr B.P. Progressive tectonic shortening resulted in 180 m of overthrusting of a schist-derived nappe across an irregular talus fan surface composed of its own erosional debris. The structural history of the Alpine fault at Gaunt Creek illustrates the importance of the interaction between fault-induced topography and erosion, and the control these processes exert on the continued tectonic, geometric, and geomorphic evolution of the fault zone.

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