HR: 1340h
AN: T23B-0585    [Abstracts]
TI: In the Footsteps of Charles Darwin: Patterns of Coastal Subsidence and Uplift Associated with Seamount Subduction, Basal Fore-arc Erosion and Seamount Accretion in Latin America
AU: * Fisher, D M
EM: fisher@geosc.psu.edu
AF: Department of Geosciences, Penn State University, University Park, PA 16802 United States
AU: Kirby, S H
EM: skirby@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: David, S W
EM: dscholl@usgs.gov˙
AF: U. S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AB: In Geological Observations on South America (1846), Charles Darwin described beds of late Cenozoic marine seashells that were uplifted to elevations as much as several hundred meters above some localities on the western coastline of South America and implied that the whole coast was uplifting at geologic time scales. We know now that such evidence is generally restricted to coastal embayments above fore-arc basins where offshore seamounts are colliding with the South American fore arc (e.g., the Juan Fernandez seamount chain, Valpariso Basin and Valpariso Bay). We suggest that the phenomena of basal fore-arc erosion and basin formation and coastal uplift are closely related to effects of seamount subduction. Marine multibeam sonar images and multichannel seismic reflection surveys by others demonstrate that seamounts, although locally cut by normal faults in the outer-rise/near-trench region, initally subduct intact and the primary interaction with the toe of the fore arc is plowing, with material eroded from the fore arc that accumulates above and on the margins of the seamount. Submarine landslides above such regions over-steepened by plowing can lead to coastal embayments far upslope of the plowing. Such plowing interaction can therefore lead to the formation of large forearc basins and coastal embayments such as those at Valpariso, Chile, or narrow corridors of subsidence in the wake of subducting seamounts in Costa Rica. It is also known that the transition between interplate thrust seismicity, representing mechanical coupling between the plates, and aseismic slip occurs at depths of typically 30-60 km and often geographically near coastlines that mark the boundary between outer fore-arc subsidence and inner fore-arc uplift. We suggest that decoupling can occur at the base of seamounts (i.e., the originally sedimented seafloor on which the seamount lavas are laid down) and that such seamounts can be accreted to the fore arc above and lead to coastal uplift. Such basal decoupling is known to occur under active volcanic islands in the open ocean in connection with rifting and gravitational spreading, such as beneath the island of Hawaii. The spatial and temporal patterns of coastal uplift and subsidence on active margins can therefore record the local history of seamount subduction. This conceptual model explains the spatial patterns of offshore subsidence and coastal uplift in Chile and Costa Rica and also has implications for patterns of seismicity along the interplate thrust boundary.
DE: 7230 Seismicity and seismotectonics
DE: 8000 STRUCTURAL GEOLOGY (New field, replaces single entry 8165)
DE: 8105 Continental margins and sedimentary basins
DE: 8150 Plate boundary--general (3040)
DE: 3045 Seafloor morphology and bottom photography
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting