HR: 0800h
AN: T41C-1229    [Abstracts]
TI: Observations of Seafloor Outcrops in the Oblique Subduction Setting of Adak Canyon: Implications for Understanding the Early History of the Aleutian Island Arc
AU: * Yogodzinski, G
EM: gene@sc.edu
AF: Univ. of South Carolina, Dept. of Geological Sciences, Columbia, SC 29208
AU: Scholl, D
EM: dscholl@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025
AU: Jicha, B
EM: bjicha@geology.wisc.edu
AF: Univ. of Wisconsin, Dept. of Geology & Geophysics, Madison, WI 53706
AU: Wyatt, C
EM: alinashale@hotmail.com
AF: Univ. of South Carolina, Dept. of Geological Sciences, Columbia, SC 29208
AU: Singer, B
EM: bsinger@geology.wisc.edu
AF: Univ. of Wisconsin, Dept. of Geology & Geophysics, Madison, WI 53706
AU: Kelemen, P
EM: peterk@ldeo.columbia.edu
AF: Lamont Doherty Earth Observatory, RT 9W, Palisades, NY 10964
AB: Submarine canyons in the western Aleutians (west of 177øW) are formed by oblique subduction, which has broken crustal blocks away from the arc massif and rotated them in clockwise sense, resulting in the formation of triangular-shaped summit basins and deep, structurally controlled submarine canyons (Geist et al., Tectonics v7, p327, 1988). A series of dives with the ROV Jason II on July 28-30, 2004 on Adak Canyon has provided the first-ever view of seafloor outcrops in an Aleutian canyon formed by this process. Two dives on the canyon's steep eastern wall revealed extensive exposures of blocky outcrops of volcanic rock at depths of 2900-1500 m. Samples of these units collected by the Jason II are a mixture of dark, pyroxene and plagioclase-phyric lavas and volcaniclastics. Degree of weathering/alteration is highly variable but some samples appear fresh. We anticipate that these rocks are offshore-equivalents of the Finger Bay Volcanics, which represent the earliest phase of Aleutian volcanism exposed on nearby Adak Island (e.g., Coats, 1956, USGS Bull. 1028-C). Exposures of granitic rock in Adak Canyon form low ledges of exfoliating outcrop interspersed with spheroidally weathered, bouldery sub-crop, in the depth range of 1800-1600 meters. Obtaining in-situ samples from these massive and subrounded exposures was not possible with the Jason II, but recovery of large, sub-angular slabs that litter the surface included samples of fresh diorite, fine-grained felsic intrusives and hydrothermally altered volcanic country rock. The stratigraphically highest exposures observed in Adak Canyon are gently dipping, poorly lithified `Middle Series' sedimentary rocks of probable Miocene-Oligocene age. All outcrop surfaces in Adak Canyon are covered with a uniformly dark brown, opaque coating of Mn oxide less than 1mm thick. Well-rounded cobbles and boulders interpreted to be glacial drift are largely free of Mn oxide coatings. Thick pavements of Mn-oxide were not observed. These observations indicate that combined seismic and dredging-geochemical studies will be a successful approach to unraveling the complete magmatic and tectonic history of the Aleutian arc crust, which due to the absence of significant arc-parallel rifting and backarc spreading remains largely intact and available for study. Dating and geochemical analysis of samples recovered on the July '04 Jason II expedition will provide a much-improved view of early Aleutian history.
DE: 3655 Major element composition
DE: 3045 Seafloor morphology and bottom photography
DE: 1035 Geochronology
DE: 1065 Trace elements (3670)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting