New Data From the Waianae Slump Southwest of Oahu Island, Hawaii

During August 2002, we mapped and sampled the Waianae Slump offshore southwest Oahu using Seabeam, single-channel seismic (SCS) reflection and submersible diving with the JAMSTEC research ship Yokosuka and Shinkai 6500. These new data supplement Seabeam, SCS and ROV samples collected in 2001. The bulk of the slump is believed to consist of material from Waianae volcano, although the 130-km-wide slump may comprise material from Kaena ridge to the west and an older rift zone to the east. Our new Seabeam map shows that the Waianae slump consists of at least three major coherent blocks about 25-45 km long elongate parallel to the slump's arcuate toe, separated by transverse scarps. The major blocks are a series of ridges and troughs that are laterally continuous for 5-20 km and probably represent individual smaller slumps. The toe of the slump is irregular with many small embayments in the slope. Single-channel seismic data shot in 2001 show a landward-tilted block at the toe of the slump with a 1.8 km-wide basin behind filled by <100 m of sediment. Basin width increases upslope to 4-6 km and the sediment fill increases upslope to about 200m. The basin-filling sediment is tilted landward with dips progressively increasing with depth. Shinkai 6500 dive #707 traversed up the steep slope of a landslide block west of Penguin Bank, from near its base at 4300 m to near its top at 3820 m. The lowermost 200 m of upslope travel encountered mud and sand covered slopes. The upper part of the slope, characterized by landward-dipping reflections on the SCS data, is dominated by pillow lava outcrops. The pillows are bulbous and oriented down-slope, with 20-30% of the pillow faces truncated. Visual inspection of four pillow lava samples indicates that they are "typical" Hawaiian tholeiitic basalts that we interpret to be from the lower flank of an old south rift zone of Waianae volcano. The samples have ~4-6 mm thick manganese crusts, indicating that they may be about 3-4 my old.