HR: 0800h
AN: T11A-0359    [Abstracts]
TI: Mechanisms of Late Quaternary coastal uplift adjacent to the Hikurangi Subduction margin, North Island, New Zealand.
AU: * Wilson, K J
EM: K.Wilson@gns.cri.nz
AF: School of Earth Sciences, Victoria University of Wellington, PO Box 600, Wellington, 6005 New Zealand
AU: Berryman, K R
EM: K.Berryman@gns.cri.nz
AF: Institute of Geological and Nuclear Sciences, PO Box 30-368, Lower Hutt, 6009 New Zealand
AU: Little, T A
EM: Tim.Little@vuw.ac.nz
AF: School of Earth Sciences, Victoria University of Wellington, PO Box 600, Wellington, 6005 New Zealand
AU: Cochran, U A
EM: U.Cochran@gns.cri.nz
AF: Institute of Geological and Nuclear Sciences, PO Box 30-368, Lower Hutt, 6009 New Zealand
AU: Litchfield, N J
EM: N.Litchfield@gns.cri.nz
AF: Institute of Geological and Nuclear Sciences, PO Box 30-368, Lower Hutt, 6009 New Zealand
AB: The Hikurangi margin is one of the few Pacific Rim subduction zone segments with no historic record of great or large subduction earthquakes. Is this an artefact of the short historic record available in New Zealand (< 200 years), or is there a real absence of subduction zone seismicity? We present the results of geological investigations at two locations on the Raukumara Peninsula, North Island New Zealand, where raised marine terraces indicate that coastal uplift has taken place in the Late Quaternary. At Pakarae seven marine terraces record tectonic uplift since post-glacial stabilisation of eustatic sea level at average rates of 3.2 ± 0.8 mm yr^-1. A now-exposed fluvio-estuarine sequence beneath the highest terrace at Pakarae records infilling of an incised-valley under conditions of rapid post-glacial sea level rise in the early Holocene. The litho- and biostratigraphy of this sequence provides evidence of at least two sudden sea level regressions during the period 11--7 ka B.P., despite constant eustatic sea level rise. The sharp nature of one of the regressive event horizons suggests sudden tectonic uplift caused a change from estuarine to fluvial conditions in the paleo-valley. We attribute most of the uplift to slip on an offshore reverse fault in the accretionary complex that may merge listrically into the subduction interface. In contrast to Pakarae, drill cores of incised-valley infill sediments at Hicks Bay, 100 km north of Pakarae, show apparently continuous estuarine sedimentation at intertidal elevations throughout the early Holocene. Tectonic tilting of the region normal to the subduction zone strike is recorded by a gradient in uplift rates between Hicks Bay and Te Araroa, 10 km the SE. Radiocarbon ages from estuarine sediments at Hicks Bay yield Holocene uplift rates of 1.6 ± 0.2 mm yr^-1. Raised beach ridges at Te Araroa indicate average Holocene uplift rates of 2.5 ± 0.2 mm yr^-1. The seaward-sloping, stepped beach ridge morphology and estuarine infill stratigraphy indicate most of the uplift in this region is achieved through gradual tectonic uplift, possibly related to a continuous process of sediment underplating on the subduction interface. Intermittent sudden uplifts events, evidenced by the steps in the beach ridge sequence, causing < 1 m of coastal uplift per event have also occurred and like Pakarae, are possibly related to slip on an offshore reverse fault. These two sites illustrate contrasting uplift styles along the strike of the Hikurangi subduction zone at wavelengths of < 100 km. In contrast to the quiescent historical record, both locations have yielded rich records of Holocene paleosiesmicity. Wider spatial coverage to determine the geometry of uplift and correlation to along-margin sites of subsidence may help resolve if the uplift events were related to large or great subduction earthquakes or if they only involved the upper plate.
DE: 8036 Paleoseismology (7221)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 8175 Tectonics and landscape evolution
SC: Tectonophysics [T]
MN: Fall Meeting 2005