ABSTRACT

Bimodal volcanogenic sediments dredged from depths greater than 7 km on the Kermadec Trench inner slope at latitude 31°S comprise diamictons of mudstone, basalt, and dolerite clasts enclosed in noncalcareous mud. The mud and mudstones are medium-to-high-K rhyolitic vitric mud of continental-arc derivation that has been transported 900 km from New Zealand. All contain minor shards of Kermadec arc low-K rhyolitic glass. All mudstones appear to be Quaternary, but one basalt clast gave a latest Miocene K-Ar age (7.84 ± 0.64 Ma). There have been at least three periods of deposition of mud, and two or three episodes of emplacement of diamict. The present trench slope is 10-24°, and diamicts are interpreted to be deposits of debris flows of varying fluidity. Graded mafic volcanic sand beds are interpreted to be deposits of turbidity currents or sandy debris flows. The degree of consolidation of mudstone clasts, along with the presence of compaction faults, suggests burial followed by re-exhumation, implying that redeposition processes may be deep-reaching. Shallow-water fossils and calcareous mudstone clasts originating above the carbonate compensation depth (c. 4.5 km), plus clasts of basalt and dolerite sourced locally on the crest of the Kermadec arc, reached the lower trench slope in gravity flows that bypassed the mid-slope terrace.

There was extensive mixing of products from various eruptions and sources during multiple episodes of transportation, and during deposition and redeposition on the lower trench slope. A range of agencies and routes is available to carry New Zealand (Taupo Volcanic Zone)-derived vitric mud to the Kermadec Trench--both low-level and high-level winds plus the north-flowing Deep Western Boundary Current (DWBC) of Antarctic Bottom Water. The latter washes the Raukumara Plain forearc basin and the western trench slope to depths as shallow as 2-3 km. We infer that ashfall tephra carried over the Raukumara Plain by the prevailing low-level southwesterly wind is the principal source of the muds found in the trench. The relative purity of the vitric mud in the trench might be explained by the tendency of rhyolitic material to dominate sediment dispersal systems in the aftermath of very large caldera eruptions. Confinement of the mud to the lower trench slope is probably caused by the tendency of DWBC surges of >20 cm/s to veer 10°-30° clockwise of the trench axis. The continent-derived volcanic ash on the subducting slab may influence oceanic arc magma composition.