Gravity Flows and Rock Recycling on the Tonga Landward Trench Slope: Relation to Trench-Slope Tectonic Processes

Abstract

Dredge samples collected from the flanks of the frontal-arc platform, from the mid-slope forearc terrace, and from the lower landward slope of the Tonga Trench are dominated by arc-derived volcaniclastic sediment and calcareous ooze. Much of the sediment was delivered by sediment gravity flows. Recycling of older fossils and rocks is very widespread, with evidence for several cycles of erosion and deposition in a single sample. Steep slopes and outcropping rock have prevailed around the frontal-arc platform throughout the late Neogene, probably driven by movement on bounding normal faults. The upper trench slope is dominated by coarse-grained turbidites and debris flows. The mid-slope terrace receives mainly finer-grained turbidites, with fewer coarse turbidites and debris flows. The lower trench slope is dominated by clast-rich debris flows, containing a majority of clasts of volcaniclastic deposits derived from the upper trench slope and mid-slope terrace, mixed with arc-substrate and ocean-plate rocks ...