Postglacial ( after 18 Ka ) Deep - Sea Sedimentation along the Hikurangi Subduction 1 Margin ( New Zealand ) : Characterisation , Timing and Origin of Turbidites 2

10 Recent sedimentation along the Hikurangi subduction margin off northeastern New Zealand is 11 investigated using a series of piston cores collected between 2003 and 2008. The active Hikurangi 12 Margin lies along the Pacific-Australia subduction plate boundary and contains a diverse range of 13 geomorphologic settings. Slope basin stratigraphy is thick and complex, resulting from sustained high 14 rates of sedimentation from adjacent muddy rivers throughout the Quaternary. Turbidites deposited 15 since c. 18 ka in the Poverty, Ruatoria and Matakaoa re-entrants are central to this study in that they 16 provide a detailed record of the past climatic conditions and tectonic activity. Here, alternating 17 hemipelagite, turbidite, debrite and tephra layers reflect distinctive depositional modes of marine 18 sedimentation, turbidity current, debris flow and volcanic eruption, respectively. Turbidites dominate 19 the record, ranging in lithofacies from muddy to sandy turbidites, and include some basal-reverse 20 graded turbidites inferred to be derived from hyperpycnal flows. Stacked turbidites are common and 21 indicate multiple gravity-flows over short time periods. The chronology of turbidites is determined by 22 collating an extremely dense set of radiocarbon ages and dated tephra, which facilitate 23 sedimentation rate calculation and identification of the origin of turbidites. Sedimentation rates 24 range from 285 cm/ka during late glacial time (18.5-17 ka) to 15 to 109 cm/ka during postglacial time 25 (17-0 ka). Turbidite deposition is controlled by: (1) the emplacement of slope avalanches reorganizing 26 sediment pathways; (2) the postglacial marine transgression leading to a five-fold reduction in 27 sediment supply to the slope due to disconnection of river mouths from the shelf edge, and (3) the 28 Holocene/Pleistocene boundary climate warming resulting in a drastic decrease in the average 29 turbidite grain-size. Flood-induced turbidites are scarce: nine hyperpycnites are recognized since 18 30 in su -0 07 00 82 0, v er si on 1 24 M ay 2 01 2 Author manuscript, published in "Marine Geology 295-298 (2012) 51-76" DOI : 10.1016/j.margeo.2011.11.002