Abstract

A large, intermittently active, submarine landslide known as the Hilina slump has been interpreted along the south flank of Kilauea volcano. Seaward-dipping faults on land mark its headwall, and an offshore bench may define its uplifted toe. Geodetic data show that the entire south flank is also moving seaward, by a process referred to as volcanic spreading; this provides an alternative explanation for the bench, i.e., overthrusting along the edge of the sliding flank. The latter interpretation is consistent with new seismic reflection data across the submarine flank. A prominent reflection near the top of the oceanic plate suggests the décollement upon which the mobile flank slides. Landward-dipping reflections rise from this horizon and bound packages of bedded strata faulted and imbricated within the bench. The absence of correlative seaward-dipping faults and rotated strata on the upper flank suggests that the bench is not coupled to a slump. Moreover, kinematic reconstructions of the bench indicate that it has accommodated 15–24 km of displacement. This value is consistent with estimates for rift-zone extension but too high for shortening at the toe of a slump. We interpret the bench to result from overthrusting and accretion of volcaniclastic sediments to the edge of the mobile flank, and suggest that morphologic benches develop preferentially where landslide debris has accumulated near the base of the volcano and can be accreted to its sliding edifice.

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