How have northern Hikurangi Margin gravity flow processes evolved in the past 42 ka? Insights from IODP Expedition 372B/375, Site U1520.

Abstract

Gravity flow processes are one of the most significant drivers of sediment transport and deposition in submarine environments. The characteristics of their deposits can provide valuable records of how sedimentary, tectonic, oceanographic, and atmospheric processes function and how they vary over time. A >500 m-thick sediment record was recently acquired from Site U1520 in the northern Hikurangi Subduction Margin (HSM) as part of the International Ocean Discovery Program (IODP) Expedition 375. The upper ~106 m of this record (Unit I) provides a continuous, high-resolution record of a turbidite succession that contains sediments associated with glacial-interglacial climate cyclicity. An analysis of this core material, integrated with bathymetric and seismic reflection data, was undertaken to provide insight into what sedimentary processes have been active in this region and how they have varied over time. Stream network and morphometric analyses were performed on the bathymetric dataset to provide information on the morphological setting of Site U1520. 2D seismic reflection data was used to assess the seismic stratigraphic character of the region, with a focus on Seismic Unit 1 as defined by Expedition 375 scientists. Core analyses included the utilisation of numerous datasets acquired during Expedition 375, accompanied by computed-tomography (CT) scanning, X-ray fluorescence (XRF) scanning, principal component analyses, tephrochronology, AMS radiocarbon dating, Bayesian age-depth modelling, grain-size analyses and carbonate content analyses. These datasets were used to produce a lithofacies scheme of the core to assess and determine the sedimentological processes responsible for the formation of the preserved sequence. The results of this study show that the northern Hikurangi Trough has undergone significant changes through the past three marine isotope stages (MIS). Gravity flows, interpreted as being predominantly sourced from the Māhia Canyon and from overspill of the Hikurangi Channel, have infilled a depression that was blocked by a mass transport deposit (MTD) in MIS3. In MIS2, cyclic steps began to evolve downslope of the mouth of the Māhia Canyon and a weakly-confined channel-belt system developed above Site U1520. This channel system was subsequently infilled in MIS1, forming the gently-sloping, featureless plain seen today. The sedimentary succession preserved in the core is primarily composed of siliclastic turbidites, from which six lithofacies were identified, representing a continuum of clay-rich, low-density turbidites (1 – 4) to transitional- or high-density turbidites/low-strength debrites (5 – 6); most deposits show a bi-partite character, implying a transition from a higher-to-lower density regime with time. Sedimentary characteristics suggest stricto-sensu hemipelagite was unlikely, while evidence is insufficient to rule out the occurrence of mixed turbidite-contourite deposition but such processes, if present, are likely to be restricted to the tops of beds. Sedimentation rates during the last glacial maximum (LGM) are comparable to the highest on record (up to 9.73 m/ka), while interglacial sedimentation rates are significantly lower (0.5–1.5 m/ka) but still high compared to most recorded rates on the HSM. This study will provide a sedimentological dataset and stratigraphic event record that will support future studies in the evaluation of the occurrence of geological hazards and how these events, and their requisite processes, may respond to changing environmental conditions.