HR: 1330h
AN: V22C-0600    [PDF]
TI: Composition and Structure of Mauna Loa's Submarine West Flank, Hawaii
AU: * Borchers, D
EM: dborcher@rice.edu
AF: Rice University, Dept of Earth Science, 6100 Main Street, Houston, TX 77005
AU: Morgan, J K
EM: morganj@rice.edu
AF: Rice University, Dept of Earth Science, 6100 Main Street, Houston, TX 77005
AU: Clague, D A
EM: clague@mbari.org
AF: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039
AU: Moore, G F
EM: gmoore@hawaii.edu
AF: University of Hawaii, Dept of Geology and Geophysics, 1680 East West Road, Honolulu, HI 96822
AB: James Moore's pioneering work on submarine landslides in the Hawaiian Islands contributed significantly to early models for the structure and evolution of Mauna Loa's submarine western flank. The west flank experienced catastrophic failure in the past, generating massive blocks and debris fields offshore. Moore recognized that the midslope bench near the base of the submarine flank must have postdated the debris avalanche, but little data existed to determine if it formed in response to further landsliding or to deeper volcanic processes. As the processes that shaped Mauna Loa are thought to be analogous to those currently active at Kilauea, an improved understanding of Mauna Loa's history can provide valuable insight into the future of the younger Hawaiian volcanoes. Several recent marine surveys in the area, including submersible surveys conducted by MBARI and JAMSTEC, and a multi-channel seismic (MCS) survey carried out by the University of Hawaii, provide important new data about the composition and structure of Mauna Loa's submarine west flank. We carried out detailed geochemical, petrographic and structural analyses of rock samples and dive videos collected from the exposed northern wall of the midslope bench, documenting a repeated sequences of volcaniclastic sandstones and breccias. This stratigraphy contrasts with the predominantly subaerially erupted basalts composing the upper flank. Several thick ponded flows or sill-like diabase units are also interspersed in the section. The volcaniclastic units are highly cemented, and many contain hydrothermal alteration products, including chlorite, zeolites, and actinolite. The most altered rocks occur near the base of the bench and the degree of alteration decreases upward in the section. Samples collected from the outer scarp of the bench show evidence for intense shearing and cataclasis at all scales. The new MCS line crosses Mauna Loa's southern submarine flank and central bench. More than 500 m of finely layered slope strata overlie the upper flank to the south, and are truncated above the Ka Lae avalanche scar. The central bench to the north, sampled by the MBARI dives, shows only thin sediment cover above a poorly reflective interior. Strong deep reflections in both locations begin to resolve the underlying oceanic crust, as well as probable fault planes that may be responsible for flank deformation in this area. The abundance of volcaniclastic rocks with Mauna Loa affinities within the bench supports the idea that giant landslides from Mauna Loa were the source of much of the offshore debris. The stratal repetition, deformation fabrics, and cementation of the volcaniclastics also suggest that the rocks composing the bench were once deeply buried and have been subsequently exhumed by thrusting, most likely driven by deep volcanic spreading.
DE: 3022 Marine sediments--processes and transport
DE: 3025 Marine seismics (0935)
DE: 8015 Local crustal structure
DE: 8122 Dynamics, gravity and tectonics
DE: 8400 VOLCANOLOGY
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting