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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS006 Oral Presentation 1925
Beyond marine terraces: investigating Holocene coastal uplift using
transgressive marine sediments and fluvio-tectonic terraces at the
Pakarae River mouth, North Island, New Zealand.
Dr. Kate Wilson
Geohazards Solutions GNS Science IASPEI
Kelvin Berryman, Nicola Litchfield, Ursula Cochran, Tim Little
This study describes two new techniques used to extend the paleoseismic record and constrain the
geometry of an active offshore fault at the Pakarae River mouth locality, East Coast, North Island, New
Zealand. Seven marine terraces dating back to 7 ka have previously been used to estimate the
magnitude and frequency of coastal uplift events at Pakarae, the site of highest uplift (3.2 mm yr-1)
along the Hikurangi margin. To attain a longer record and to verify the sudden, coseismic nature of
uplift events, a method of using the fluvio-estuarine sedimentary sequence underlying the highest
marine terrace was developed. We compared the paleoenvironmental facies architecture of the uplifted
fluvio-estuarine sequence with typical incised valley sequences deposited on stable coastlines. Two
sharp estuarine-to-fluvial transitions dated at 9 ka and 8.5 ka indicate rapid sea level regressions. Their
occurrence during a period of eustatic sea level rise suggests coseismic uplift events caused temporary
estuary abandonment. A further uplift event between 8.5 and 7.35 ka is inferred from the significant
difference between the amount of sediment preserved from this period and the predicted sediment
thickness according to the eustatic sea level curve. This technique enables us to extend the record of
coastal uplift events back to 10 ka and overcome the limitation of marine terraces, which only record
uplift since eustatic SL stabilisation. The second technique uses fluvial terraces that appear to grade to
the marine terraces and thus are also interpreted to be coseismic. A longitudinal profile shows these
terraces fan out downstream and thus their distribution defines the upstream limit of uplift. These data,
coupled with along-coast correlations with marine terraces at Puatai Beach and Waihau Bay (9 and 15
km northward), provide constraints on the geometry of uplift. Forward-elastic dislocation modelling
indicates the fault is likely to be a steeply northwest-dipping reverse fault lying close to the shoreline;
there is possibly involvement of plate interface slip. Techniques such as these are vital for
understanding seismic hazard of subduction zones such as the Hikurangi margin. With no historical
large subduction interface earthquakes, investigations along the coastline need to be able to distinguish
upper plate faulting effects, if we are to isolate plate interface events. Furthermore, the methods
described here contribute to defining an offshore fault that is too close to the coast for marine
geophysical investigation.
Keywords: terraces, subduction, geomorphology