Kaikoura canyon, new zealand: active conduit from near-shore sediment zones to trench-axis channel

摘要

Kaikoura Canyon is one of a few major, activeconduits between a near-shore sediment transport system and adeep-ocean channel. It is the sink for mobile zones of gravel, sandand mud that migrate northwards off northeastern South island,new Zealand. It is presently the primary source for the 1500 kmlong Hikurangi Channel, which supplies overbank turbidites to afilled trench, an ocean-plateau basin and a distal fan-drift. Swathdata, seismic profiles, side-scan sonographs, aerial photographs,cores, grab samples and current meter data are used to define theshape and texture of the canyon and adjacent shelf in order tobetter understand how coastal processes and canyon interact tosupply sediment to the deep-ocean. Kaikoura Canyon is 60 kmlong, up to 1200 m deep and generally U-shaped in profile. Itshead is within 500 m of the shore, and within 200 m of rockyprojections from the shore-platform, in a mountain-backed baywithout large rivers. The canyon head incises the 18 m depthcontour and boulders, pebble gravel and megarippled coarse sandreach the canyon rim. Fine sand migrating northward along theshelf under the influence of waves and currents is trapped in asouthward-projecting, canyon-head gully, which incises thethickest pail of die Holocene sediment prism. It is estimated thatabout 1.5 x 106 m3 of sediment falls into the canyon head eachyear. Tensional fractures around the canyon rim suggest thatsediments in the canyon-head gully are unstable. Gravel turbidites,with post last glacial age shells, are at or near the seabed in thelower canyon but are blanketed by many thin, silt and sand,possibly storm-generated, turbidites in the upper canyon. The topgravel contains a twig that is about 170 years old, suggesting thatthe last major collapse in the canyon head coincides with manyonshore rockfalls triggered by rupture of a major, strike-slip, plate-boundary fault in about 1833. An underlying gravel is about 300years old and may again coincide with fault rupture. Most of thelarge, earthquake-triggered, failures may "ignite" to form self-perpetuating, autosuspension flows, that feed a 1500 km long.deep-sea, turbidite channel. 1999 Elsevier Science B.V. All rights reserved.