HR: 1340h
AN: V43E-1463    [Abstracts]
TI: Evidence That Three Seamounts Off Southern California Were Ancient Islands
AU: * Paduan, J B
EM: paje@mbari.org
AF: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039
AU: Clague, D A
EM: clague@mbari.org
AF: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039
AU: Davis, A S
EM: davisa@mbari.org
AF: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039
AB: Eleven ROV Tiburon dives in 2003 and 2004 explored Rodriguez and San Juan Seamounts, and Northeast Bank off southern California. Now submerged, these seamounts appear to have been subaerially exposed while the volcanoes were active. The summit of Rodriguez seamount, now at 630 m depth, is a smooth, gently domed platform. The shallowest points are low, rough hills standing above the platform and consist of thick, dense, degassed `a`a flows erupted and oxidized subaerially. Coarse bedded and cross-bedded sandstones and rounded cobbles interpreted to be beach deposits occur near the top of a major break-in-slope at about 700 m. Rodriguez stood at least 70 m above sea level and formed a small island 6.8 km$^{2}$ in area prior to subsiding at least 700 m. The summit of Northeast Bank, now at about 360 m depth, is another smooth, gently domed platform. Wave-sculpted lava flows, sandstone, and pebbly conglomerate beach deposits were observed between 510 and 554 m depth, below the sharp break-in slope at about 500 m depth. Although the bathymetry of the entire summit is poorly known, Northeast Bank was a large island perhaps 200 m above sea level and 90 km$^{2}$ in area. The summit of San Juan Seamount, now at 560 m depth, is a northeast-southwest trending series of rough ridges rather than a dome. However, subaerially oxidized `a`a-like lava flows were found above 700 m. San Juan's summit ridge emerged as a line of eight small islands with a total area of about 2.8 km$^{2}$, and the tallest island rose 140 m above sea level. The 550 to 700 m of subsidence recorded by Northeast Bank within the California borderland, Rodriguez Seamount on the continental slope, and San Juan Seamount on the adjacent ocean crust occurred since these seamounts formed, most likely since the late Miocene.
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting