HR: 17:40h
AN: T34A-07    [Abstracts]
TI: Anomalous Topography and Heat Flow in the Western Atlantic Caused by Small-Scale Convection at the Passive Plate Margin
AU: * Pysklywec, R
EM: russ@geology.utoronto.ca
AF: Department of Geology, University of Toronto, 22 Russell Street, Toronto, ON M5S 3B1 Canada
AU: Shahnas, M H
EM: shahnas@yorku.ca
AF: Department of Geology, University of Toronto, 22 Russell Street, Toronto, ON M5S 3B1 Canada
AB: Although the topography of the ocean floor is dominated by cooling and subsidence of the plate as it moves away from the mid-ocean ridge, anomalous topographic features can be superimposed on this long-wavelength plate subsidence. As an example, we focus on the western North Atlantic region and derive `residual' topography which corrects observed bathymetry data for sediment loading, ocean plate cooling and isostatic compensation of the continental crust (using Crust2.0). The residual topography is characterised by long-wavelength intraplate anomalies that are defined by the NE-SW trending Bermuda Rise and adjacent lows. These features can not be reconciled with simple hot spot theory. We test the hypothesis that the observed topography signal reflects the existence of small-scale convective flow induced by the sharp lateral thermal gradient at the passive continent-ocean margin of North America, or `edge-driven convection'. A series of coupled lithosphere-mantle numerical experiments are conducted where an idealized plate margin is modeled by prescribing strong plates with a step-like thickness/temperature discontinuity at the continent-ocean boundary. A primary edge-driven convection cell and secondary flow circulation develops at the margin. This induces a lithospheric deflection that matches with the observed residual topography. Namely, both show subsidence at the continent-ocean margin, an off-shore peak/plateau of high topography on the ocean plate, and distal ocean plate subsidence. The small-scale mantle convection also manifests in anomalous surface heat flow, which is consistent with observations. Unlike hot spots, the edge-driven convection cell and associated topography and heat flow anomalies migrate with moving lithospheric plates. The flow cell and wavelength of the topography anomalies are broadened with continent-ward motion of the lithosphere relative to the mantle, whereas a migration in the ocean-ward direction suppresses the formation of the edge-driven convection cell and surface anomalies. We show that the coupled crust-mantle dynamics are consistent with the measured motion of the North American plate relative to a fixed hot spot reference frame.
DE: 9325 Atlantic Ocean
DE: 8105 Continental margins and sedimentary basins
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8155 Plate motions--general
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting