Alpine fault and related geology of the Kokatahi Valley, Westland, New Zealand

Wright, Craig A. (Craig Andrew)

Recent displacements on the Alpine Fault have uplifted a terrace which is laterally equivalent to the gravels of the Hokitika - Kokatahi alluvial plains. Vertical displacement near Granite Creek was calculated as 63± 7 m. The dip on the fan gravels exposed in Muriel Creek demonstrates that the maximum uplift does not occur at the main trace of the Alpine Fault, but rather to the south-east of it. A compressional jog to the north of Doughboy Creek has caused enhanced uplift in the Region of Round Top. The continuation of the fault north of the jog is exposed as a mylonite-derived ultracataclasite thrust over granite in Canada Creek on the eastern edge of Mt. Harry. The compressional jog is currently being sheared off by the formation of new fault planes in the north-west face of Round Top, resulting in the re-alignment of the Alpine Fault to 051° - the average strike for the studied area. Alpine Fault mylonites are exposed in a narrow zone north of the jog. The re-alignment of the Alpine Fault has left them stranded on the footwall of the fault. The high uplift rate in the Round Top area, resulting from the compressional jog, has caused oversteepening of the mountain face. Four large avalanches have occurred from the north-west face of Round Top. The major cause of the avalanches is the high uplift rates; it is likely that they have been triggered by strong vertical accelerations during seismic events on the Alpine Fault. The largest avalanche deposit prior to its partial erosion, consisted of a volume of(4.5±2.8) x 107m3 spread over an area of 5.6±0.7 km2. Coring of trees on the deposit demonstrated that the Round Top Debris Avalanche occurred no less than ~530 years ago.

xii, 142 p., [25] leaves of plates : ill. (some col.), maps ; 30 cm.

1994Wright

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