ANATOMY, STRUCTURAL EVOLUTION, AND SLIP RATE OF A PLATE-BOUNDARY THRUST - THE ALPINE FAULT AT GAUNT CREEK, WESTLAND, NEW-ZEALAND

Minimum slip rates calculated for plate-vector-parallel slickenside tr ends in cataclasite on the sole of the Alpine fault at Gaunt Creek, We stland, New Zealand, range from 18 to 24 mm/yr. Between half and two-t hirds of the total relative motion between the Pacific and Australian plates is being accommodated by movement on a single structure, the Al pine fault. During the past 14 ka, the leading edge of the Alpine faul t has changed from a moderately southeast-dipping, oblique reverse fau lt to a shallowly dipping thrust. The hanging wall (Pacific plate) is composed of a gradational sequence from basal gouge, through pseudo-ta chylite-bearing cataclasite, to progressively more coherent schist-der ived mylonite, which has been faulted against subhorizontally bedded, fluvio-glacial gravel in the footwall (Australian plate). During uplif t 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 zon e produced angular, cataclasite- and mylonite-derived, talus-fan brecc ias, building a west-dipping apron beneath the fault scarp. Wood fragm ents from near the base of the talus breccias have been C-14 dated at 12,650 +/- 90 yr B.P. Progressive tectonic shortening resulted in 180 m of over-thrusting of a schist-derived nappe across an irregular talu s fan surface composed of its own erosional debris. The structural his tory 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, an d geomorphic evolution of the fault zone.