Approximately 105,000 yr ago (based on uranium-series dating), waves in a giant wave train swept up to an elevation of about 375 m on the island of Lanai. The waves deposited the Hulopoe Gravel, which near the present shoreline consists of basalt boulders, coral fragments, and calcareous beachrock slabs, and near the upper limit of the deposit consists of sand and shell fragments. The maximum heights of similar but lower deposits on nearby islands, when adjusted for their estimated subsidence due to volcanic loading during the past 105,000 yr, indicate that the source of the wave was about 50 km southwest of Lanai. We hypothesize that failure and downward movement of the huge Lanai submarine landslide created an ocean disturbance, which produced waves that rushed across the Lanai reef and beach, picked up limestone and lava fragments, and deposited them high on the island as the Hulopoe Gravel. Backwash from the waves stripped soil and rock from the islands and carried much of it to the sea. The Hulopoe Gravel is 8 m thick in a gulch 200 m inland from the Lanai shoreline, where it consists of three beds, successively, 2, 4, and 2 m thick. These beds are considered to have been laid down by successive waves in the wave train. Each bed consists of two units: a lower unit of basalt and limestone boulders, cobbles, and sand, and an upper bimodal unit of large basalt boulders with a pebbly sand matrix. These subunits are inferred to be deposited from the runup and backwash of each wave. At the upper surface of the Hulopoe Gravel, basalt boulders averaging 0.5 m in diameter are arranged in dunelike ridges about 1 m high and 10 m apart. Nearby, where young streams have cut into and exposed the lower beds of the Hulopoe, clasts at the boundaries between the beds are locally imbricated and dip landward. We interpret these features as aspects of torrential flow and crossbedding created during the high-speed backwash of the great waves.

ABSTRACT About 40 subduction-zone segments have been identified worldwide on the basis of intermediate-focus earthquakes, calc-alkalic volcanic arcs, and lines of rapid tectonic uplift. The total length of these actively convergent plate boundaries is 57,000 km. Of this length, 42% is of the Japan type, in which the upper plate is relatively stable with respect to the subduction zone; 37% is of the Andes type, in which the upper plate actively overrides the trench; and 21% is of the Himalaya type, in which continental plates or microplates collide with other continental bodies. Subduction zones of both the Japan and Andes types are marked by basement highs at the trench-slope break. Uplift of the crust and upper mantle at the edge of the upper plate causes these basement highs where a relatively low-density prism of accreted oceanic material is emplaced below. The accretionary prism for each cycle of subduction forms within 5 m.y. after a new subduction zone is established, while the megathrust is evolving from an initial dip of about 30° near the zone's seafloor outcrop to a steady-state dip of about 10°. Except during this relatively brief period of accretion, most oceanic sediment at subduction zones is believed to be carried deeply into the lithosphere. Elongate sedimentary basins form on both sides of the uplift at the trench-slope break: forearc basins toward the arc, and trench-slope basins toward the trench. Depending on the balance between sedimentation and tectonic displacement, the topographic shelf-slope boundary may be located anywhere in the forearc basins, approximately to the upper edge of the trench-slope basins. At the lower trench slope, compression usually removes the seawater involved in primary oil migration before thermal maturation of oil precursors can occur. Elsewhere at the active margin, although a low geothermal gradient caused by the subduction of cool oceanic crust delays hydrocarbon maturation, such thermal maturation can nevertheless resume when a normal geothermal gradient is reestablished after continental collision or after the subduction-zone alignment moves to a new position.