Joint Japan-USA cruises in 1998-99 explored and sampled the previously unstudied deep offshore region south of Kïlauea. Bathymetric features, dive observations, and recovered samples indicate that the 3-km-deep mid-slope bench, bounded seaward by a 2-km-high lower scarp, is underlain by massive turbidite sandstone and interbedded debris-flow breccia. Debris-flow clasts are submarineerupted (high-S) alkalic basalt, distinctive fine- to coarse-grained alkalic gabbro and nephelinite (some containing phlogopite), and subordinate transitional basalt. No recovered clasts are similar to recent Kïlauea tholeiite. Primary volcanic deposits (pillow basalt, hyaloclastite breccia) are absent. The sandstone and breccia matrix, a large fraction of the volcaniclastic apron, are mainly subaerially erupted (low-S) tholeiitic glass sand generated by shoreline processes on pre-Kïlauea volcanoes. Fractures, shears, slickensided clasts, and open folds indicate widespread deformation low in the lower scarp
upward-decreasing proportions of alkalic materials define a gross stratigraphy. Alkalic high-S compositions of many basalt clasts and some sandstone glass indicate derivation from the submarine "Lö‘ihi" stage of ancestral Kïlauea, prior to growth of its tholeiitic shield. Slopes (3300-2800 m depth) above the mid-slope bench contain submarine-erupted (S &gt
750 ppm) pillow lava of transitional basalt, defining the initial flank of subalkaline Kïlauea. The geometry and diverse constituents of the bench and lower scarp require initial landsliding during alkalic volcanism at 200-300 ka, prior to shield growth at Kïlauea and inception of Hilina faults. The active Hilina slump structures on Kïlauea’s south flank are in an early growth stage, thus possibly posing greater potential for future large-scale landsliding and tsunamis.