Postglacial (after 18 ka) deep-sea sedimentation along the Hikurangi subduction margin (New Zealand): Characterisation, timing and origin of turbidites

摘要

Recent sedimentation along the Hikurangi subduction margin off northeastern New Zealand is investigated using a series of piston cores collected between 2003 and 2008. The active Hikurangi Margin lies along the Pacific-Australia subduction plate boundary and contains a diverse range of geomorphologic settings. Slope basin stratigraphy is thick and complex, resulting from sustained high rates of sedimentation from adjacent muddy rivers throughout the Quaternary. Turbidites deposited since c. 18 ka in the Poverty, Ruatoria and Matakaoa re-entrants are central to this study in that they provide a detailed record of the past climatic conditions and tectonic activity. Here, alternating hemipelagite, turbidite, debrite and tephra layers reflect distinctive depositional modes of marine sedimentation, turbidity current, debris flow and volcanic eruption, respectively. Turbidites dominate the record, ranging in lithofacies from muddy to sandy turbidites, and include some basal-reverse graded turbidites inferred to be derived from hyperpycnal flows. Stacked turbidites are common and indicate multiple gravity-flows over short time periods. The chronology of turbidites is determined by collating an extremely dense set of radiocarbon ages and dated tephra, which facilitate sedimentation rate calculation and identification of the origin of turbidites. Sedimentation rates range from 285cm/ka during late glacial time (18.5-17 ka) to 15 to 109cm/ka during postglacial time (17-0 ka). Turbidite deposition is controlled by: (1) the emplacement of slope avalanches reorganising sediment pathways; (2) the postglacial marine transgression leading to a five-fold reduction in sediment supply to the slope due to disconnection of river mouths from the shelf edge, and (3) the Holocene/ Pleistocene boundary climate warming resulting in a drastic decrease in the average turbidite grain-size. Flood-induced turbidites are scarce: nine hyperpycnites are recognised since 18 ka and the youngest is correlated to the largest ENSO-related storm event recorded onland (Lake Tutira). Other turbidites contain a benthic foraminiferal assemblage which is strictly reworked from the upper slope and which relates to large earthquakes over the last c. 7 ka. They yield a shorter return time (270-430 years) than the published coastal records for large earthquakes (c. 670 years), but the offshore record is likely to be more complete. The deep-sea sedimentation along the New Zealand active margin illustrates the complex interaction of tectonic and climate in turbidite generation. Climate warming and glacio-eustatic fluctuations are well recorded at a millennial timescale (18 ka), while tectonic deformation and earthquakes appear predominant in fostering turbidite production at a centennial timescale (270-430 years).