HR: 10:55h
AN: V11G-03    [PDF]
TI: Development of the Waianae slump and Kaena Ridge, Hawaii
AU: * Coombs, M L
EM: mcoombs@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road MS 910, Menlo Park, CA 94025 United States
AU: Clague, D A
EM: clague@mbari.org
AF: MBARI, 7700 Sandholt Road, Moss Landing, CA 95039 United States
AU: Moore, G F
EM: gmoore@Hawaii.edu
AF: SOEST, University of Hawaii, 1680 East-West Road, Post 813, Honolulu, HI 96822 United States
AB: New ROV and submersible dives offshore of Waianae volcano, Oahu, provide evidence pertaining to the size and shape of the volcano's edifice as well as timing of its collapse to form the Waianae slump. Two JAMSTEC dives, K205 and S707, ascended the base of the slump and collected volcaniclcastic sandstones and pillow lavas that are relatively unfractionated (5.5 to 8.6 wt% MgO in glass), low-S ($<$300 ppm) tholeiites. These magmas likely erupted subaerially early during the volcano's shield stage, and correlate with subaerial Lualualei Member lavas. The dives are 70 km apart and their samples' compositional similarities suggest that the entire Waianae slump was derived from a single volcano. A single hyaloclastite sample from dive K205 contained abundant moderate-S (220 to 520 ppm) transitional basaltic glass grains, indicating shallow submarine eruption perhaps early during or before the volcano's shield stage. These transitional compositions do not overlap with the volcano's well-studied post-shield alkalic phase. MBARI dive T326, on a shallow portion of the slump, collected tholeiitic hyaloclastites, but glass grains are systematically higher in alkalis (2.7 to 3.8 wt% Na$_{2}$O + K$_{2}$O) and are more fractionated (5.5 to 6.7 wt% MgO) than glasses found at the slump's base. These shallow samples correlate compositionally with late shield-stage subaerial Kamailaenu lavas. None of the collected slump samples correlate with subaerial post-shield lavas. We suggest that the lower, largest portion of the slump formed during or just after the volcano's main shield building stage. A second mass-wasting event could have occurred during or after the latter portion of the shield stage, forming the upper, rotated block ascended by dive T326. MBARI dive T325 ascended a small shield atop the submarine Kaena Ridge northwest of Oahu, and encountered abundant basaltic beach cobbles and a single aa flow, all tholeiitic. Kaena Ridge may have formed as the subaerial northwest rift zone of Waianae volcano. Two additional MBARI dives along the northern flank of Kaena Ridge (T327 and T328) collected hyaloclastite fragments that contain rare alkalic glasses; these may be derived from late-shield or post-shield Waianae.
DE: 3045 Seafloor morphology and bottom photography
DE: 3655 Major element composition
DE: 3670 Minor and trace element composition
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting