Successive, large mass-transport deposits in the south Kermadec fore-arc basin, New Zealand: The Matakaoa Submarine Instability Complex

Description

Four >100 km3 mass-transport deposits (MTDs) identified from their morphology and seismic facies across the Matakaoa Margin and Raukumara fore-arc basin, NE New Zealand, constitute the Matakaoa Submarine Instability Complex (MSIC). MSIC originates from a 45-km-wide, 1100-m-high reentrant in the continental slope. The deposits resulted from three mass-failure events: (1) The Raukumara Slump is identified from the collapsed NW flank of an anticline at the northern end of the reentrant and imbricate structures at its distal end, overlying a flat décollement over a >50-km distance. The slump age is roughly estimated between upper Miocene and lower Pleistocene. (2) The Matakaoa Debris Avalanche (MDA) is subdivided into a ∼260-km3 blocky unit and a ∼170-km3 weakly reflective unit, overlying a high-amplitude seismic reflector truncating the underlying sedimentary units. The MDA is dated 600 ± 150 ka. It originated as a slump, as indicated by back-tilted blocks overlying a rotational failure surface, and evolved during transport as a debris avalanche. The failure of sedimentary basement blocks released Plio-Pleistocene shelf-basin infill, thus producing the blocky and weakly reflective units. Fore-arc basin sediments deformed in front of the MDA for approximately 20 km. (3) The Matakaoa Debris Flow (MDF) occurred 38–100 ka ago and extends 200 km northward from the reentrant headwall and consists of a 150-m-thick layer with a chaotic seismic facies. Scouring beneath the MDF and a <250-m-high east bounding scarp indicate basal and lateral erosion associated with the flow displacement. Incorporation of eroded material into the debris flow accounts for >30% of the flow's 1250 km3 volume. Factors facilitating failures include the following: slope oversteepening associated with margin uplift and fore-arc subsidence, large-scale folding related to shortening between the Pacific and Australia plates, high-discharge rivers draining the region, and rapid sediment accumulation on the margin. Large to great earthquakes along the plate interface are a likely trigger mechanism controlling the recurrence of large margin failures. Unlike other MTDs along plate boundaries, which are destined to be consumed into the subduction factory, the MSIC provides an opportunity to investigate mega-instabilities at an active margin over million year timescales.

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