Paleoseismicity and structure of the alpine fault between Haast River and Arawhata River

Wright, Trevor (Trevor Dean)

The Alpine Fault, a continental transform boundary linking two opposite polarity subduction systems, extends c. 480 km along the length of the ·south Island, New Zealand. Plate boundary parallel motion accommodated by the Alpine Fault is 26±3 mm/yr. This thesis presents a structural descr1ption of the southern section of the fault, and an analysis of recent fault activity, between the Haast and Waiatoto Rivers. A variety of methods were deployed for this purpose: Field mapping, aerial photograph in erpretation, fault trenching, tree ring analysis, landslide dating and analysis of a tectonically controlled swamp. Additionally, a petrological study of the only outcrop in the field area suggests the Caples/Torlesse Terrane boundary is South of Turnbull River. From Haast River to 2 km southwest of Turnbull River, the fault forms a near-linear feature trending 052° - 05T with a morphologically controlled scarp height between 0- 16 m. On a local scale the fault trace is discontinuous consisting of at least 23 east and west stepping en echelon segments (separation< 25m) that reflect subtle changes in fault orientation. The fault bifurcates into two active traces from Compass Creek, c. 16 km southwest of Turnbull River, to at least Arawhata River. Numerous channels are predominantly dextrally offset across the fault and their morphology may suggest three recent seismic events. A shutter ridge identified 4 km southwest of the Turnbull River has been dextrally offset 415 m post-20-to-12.5 ka. South Turnbull Swamp fills a pull-apart on the Alpine Fault and provides a sediment record indicative of tectonic events. Three main stages of sediment accumulation are recognised, the second of which coincides with the previously recognised c. 1.2 ka Muriel Creek event in the northern section of the Alpine Fault. Six sediment pulses are recognised in the South Turnbull Swamp record post-1.2 ka. Accumulation rates range from 5.9 - 7.3 mm/yr - nearly an order of magnitude higher than in nearby nontectonically controlled swamps. Total average subsidence rates are 3.1 - 8.45 mm/yr, calculated from a model based on an individual Alpine Fault displacement over the past c. 1.2 ka. A paleoseismic record of recent fault activity indicative of three ground ruptures over the last c. 1000 years was established by trenching a terrace riser at Turnbull River and dating a slip over the Alpine Fault at Drizzle Creek. The earliest event was at 1155±35 A.D., the pre-penultimate event was not constrained, and the penultimate event occurred post-1655±20 A.D. Slope failure at Drizzle Creek post-1725-to-1815 A.D. provides evidence for a further event to the south of the field area and correlates with the 1826 Fiordland earthquake. A model is developed to determine the magnitude and rupture length of the 1826 Fiordland earthquake. The probable magnitude was between M 7.6-7.7, located just offshore of the Fiordland coast. The epicentre was not further south than Coal River. The absolute minimum northern rupture limit is George Sound and it is likely to have ground ruptured at least slightly north of Milford. A southern Alpine Fault chronology is presented estimating southern rupture limits (northern for the 1826 Fiordland event) and magnitudes for the last 4 events. Comparison with the northern chronology suggests the earliest identified event occurred at approximately the same time as the c.1200 A.D Muriel Creek event. The Pre-penultimate event is likely to have been either the 1620±20 A.D Crane Creek event or a southern event not recognised in the north, and the last ground rupturing event, the penultimate event, is most likely to be the 1718±5 A.D Toaroha River event.

xiv, 137 leaves : ill. (some col., some folded), maps (some col., some folded) ; 30 cm.

2001Wright

Abstract Only

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