Abstract:
Tectonic aspects of the late Quaternary geology of the Cape Kidnappers area have been studied by mapping the distribution of late Quaternary surfaces and sediments, produced by marine, fluvial, and alluvial fan processes. Fluvial terraces preserved in the Tukituki Valley, MaraetotaraValley and in catchments of small coastal streams are generally all less than 20 000 yrsB.P. and testify to rapid downcutting as a response to tectonic uplift and river length reduction resulting from post-glacial sea level rise. Most alluvial fan surfaces have also formed since 20 000 yrs B.P., probably during the maximum of the last glaciation.
The Kidnappers terrace was formed during the maximum of the last interglacial c. 125 000 years ago and has been uplifted at a maximum average rate of 1.6 m/1000 years. It has been folded about the north-south trending, north plunging Kidnappers Anticline, which has developed during the late Quaternary on the western flank of the Lachlan Anticline. Normal faulting near the axis of the Kidnappers Anticline is possibly the result of the bending of Plio-Pleistocene sediments during folding.
A marine terrace preserved up to 6 m above sea level to the west and south of Cape Kidnappers was uplifted suddenly c. 2300 yrs B.P. and was probably accompanied by a large earthquake. Tectonic uplift was probably preceded by subsidence, as indicated by up to 2 m of landward-younging marine sediment containing an in situ fauna which overlies an uplifted shore platform.
Detailed examination of the ground deformation accompanying the 1931 Hawke's Bay earthquake provides a model for the nature and scale of co-seismic uplift in Hawke's Bay. A 90 km long, 17 km wide, northeast trending dome with a maximum uplift of 2.7 m, and 15 km of surface fault rupture at its southwestern end was formed during the earthquake. Poor preservation of both surface fault rupture and uplifted coastal deposits testifies to the minimal nature of the geologic record of large earthquakes. Studies of radiocarbon dated Holocene peat and estuarine deposits at the western margin of Ahuriri Lagoon show that tectonic subsidence occurred between c. 3500 yrs B.P. and c. 1700 yrs B.P. and again c. 500 yrs B.P. in the area that was uplifted by 1 m during the 1931 Hawke's Bay earthquake.
The growth of active folds in the Cape Kidnappers area is largely seismic, but the zone of maximum deformation for individual co-seismic events does not correspond to the axis of the major geological structures which indicate the pattern of total late Quaternary deformation.
