ABSTRACT: Maturation of Large Scale Mass-Wasting along the Hawaiian Ridge
Michael E. Torresan, David A. Clague, Colin L. Jacobs, James G. Moore
Extensive GLORIA side-scan sonar mapping of the Hawaiian Ridge from Hawaii to St. Rogatien Bank shows that massive slumps and blocky debris avalanches are the major degradational processes that affect the island and ridge areas. About 30 failures have been imaged in the region surveyed; they range in area from 250 to >6000 km2
and in volume from 500 to >5000 km3. Four are rotational slumps, and the rest are blocky debris avalanches. Such deposits cover 125,000 km2 of the Hawaiian Ridge and adjacent seafloor.
The slumps are wide (up to 110 km), short (30-35 km), thick (about 10 km), and slow moving. They are broken into comparatively few major rotational blocks that have not moved far and are characterized by steep toes and transverse ridges. Back rotation of the blocks has elevated their seaward edges, producing transverse ridges and perched basins filled with 5 to >35 m of sediment. Compared to the slumps, the debris avalanches are lobate, long (up to 230 km), thin (0.5-2 km), and fast-moving. These deposits cross the Hawaiian Trough and run upslope onto the Hawaiian Arch (up to 550 m in elevation over a distance of 140 km). These failures commonly have amphitheaters and subaerial canyons at their heads. Their distal ends are hummocky, and blocky debris litters the seafloor adjacent t the ridge.
As one proceeds west from Hawaii to St. Rogatien Bank, the GLORIA sonographs and seismic reflection profiles show a progression from youthful to mature failures and from active to about 12 Ma volcanoes. The Alika and Hilina slide complexes are examples of youthful failures on active volcanoes. Slumping in the Hilina slide is ongoing (7.2 magnitude earthquake in 1975). Little to no sediment covers the blocks and hummocky terrane of the Alika (about 100 ka), whereas the older deposits along the western part of the ridge are covered by up to 30 m of transparent sediment. Pixel-size blocks, which dominate the young slides, are obscured by post-slide sediment on the older slides. The slope failures that produce all these deposits culminate during the shield-building phase and apparently en soon after the volcanoes become dormant.