HR: 0830h
AN: T41C-0237    [PDF]
TI: Dynamic Topography and Edge-Driven Mantle Convection
AU: * Shahnas, H
EM: shahnas@yorku.ca
AF: Department of Geology, University of Toronto, 22 Russel St., Toronto, ON M5S 3B1 Canada
AU: Pysklywec, R N
EM: russ@geology.utoronto.ca
AF: Department of Geology, University of Toronto, 22 Russel St., Toronto, ON M5S 3B1 Canada
AB: The topological rise of Bermuda is enigmatic in that simple hot spot plume theories do not provide a clear explanation for its elevation. The broad elongated NE-SW swell exceeds its expected value by $\sim$800-1000 m, while approximately 1000 km southeast of Bermuda there exists a negative anomaly of 600 m. The rise also manifests in a geoid anomaly with a maximum value of 6-8 m; the peaks of the topography and geoid anomalies approximately coincide. It has been suggested that the anomalies may be the consequence of `edge-driven convection' (EDC), which is essentially upper mantle small-scale convection driven by the thermal gradient at a continent-ocean lithospheric discontinuity. In this study we use a numerical model to consider how EDC may influence the evolution of surface topography at a passive continental margin. We simulate an idealized plate margin by defining strong plates with a step-like thickness/temperature discontinuity at the continent-ocean boundary. Experiments are conducted with lateral velocities imposed on the lithosphere to consider how the character of EDC and associated topography may change with relative plate motions. The models demonstrate that a time-dependent EDC roll develops with downwelling flow at the lithosphere discontinuity and an upwelling beneath the oceanic lithosphere. This convection cell induces a localized subsidence at the plate boundary and uplift further outboard on the oceanic plate. The magnitude of dynamic topography is controlled by the geometry and thermal profile of the plate discontinuity, and the material rheologies. The aspect ratio of the EDC cell changes depending on the direction of the plate motion (i.e., continent- or ocean-ward) and consequently this alters the style of surface topography. The results demonstrate the behaviour of an alternative mechanism for intraplate oceanic topography anomalies that do not readily conform to simple hotspot models.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8125 Evolution of the Earth
SC: Tectonophysics [T]
MN: 2003 Fall Meeting