Papa'u seamount is an enigmatic bathymetric ridge that lies along the western boundary of the Hilina slump, off the south flank of Kilauea volcano. Several interpretations for the origin of this feature have been put forward; among these: (a) a sandy debris lobe, derived from the collapse and downslope transport of unstable shoreline; (b) a constructional volcanic feature; (c) a compressional structure resulting from oblique convergence of the Hilina slump against the lateral boundary.
In January and February of 1998, we collected several seismic reflection lines across Papa'u seamount and the adjacent seafloor on board the R/V Maurice Ewing. These data clearly reveal that Papa'u is in fact a coherent, southwest verging fold composed of 1-2 km of flank sediments, which ramped up an east dipping lateral fault. We interpret this structure to define the seaward edge of a shallow slump that developed along the upper western slopes of the submarine south flank, and overrode a structural bench downslope. A reflection line across a narrow ridge just seaward of Nali'ikakani Point shows westward dipping sediments truncated and vertically offset to the east, which we interpret to mark the upslope continuation of the east dipping lateral fault. The apparent extension along the upper slope contrasts with the compression noted in the vicinity of Papa'u, but is consistent with subsidence near the headwall region of the slump. We speculate that Papa'u seamount and adjacent features represent the submarine portion of the active Hilina slump, which is bounded on-land by the Hilina fault system. The coherent nature of the folded sediments, and the presence of several unconformities within the package suggest that the slump was emplaced slowly and incrementally. Structural comparisons across the lateral boundary along its length suggest that slumping along the upper flank may be decoupled from deformation downslope, which appears to root more deeply within the flank.