GRAVITY FLOWS AND ROCK RECYCLING ON THE TONGA LANDWARD TRENCH SLOPE - RELATION TO TRENCH-SLOPE TECTONIC PROCESSES

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 finger-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 exposed on the lower trench slope. There are probably several stages of transport involved in delivering volcaniclastic rocks to the deep trench slope. The mid-slope forearc basin has subsided 5 km since the late Eocene (ODP Site 841), in response to subarc tectonic erosion of about 6 km3/km length of arc/m.yr. The general steepness of the lower trench slope appears to be maintained by minor tectonic erosion at the toe. However, volcaniclastic and pelagic rocks delivered to the lower trench slope by debris flows fill downgoing ocean-plate grabens and largely cancel out tectonic erosion of the base of the slope. The graben capacity of about 30 km3/km of trench/m.yr. is thought to be filled by about 15 km3 of volcanic products and about 15 km3 of pelagic carbonate.