HR: 16:50h
AN: T34A-04    [Abstracts]
TI: Geoid Constraints on Density Varations Between Continental and Oceanic Lithosphere and on Associated Geodynamic Forces
AU: * Richardson, R M
EM: rmr@u.arizona.edu
AF: Department of Geosciences, University of Arizona, Tucson, AZ 85721-0077 United States
AB: The Earth's Geoid directly reflects density variations within the Earth, and therefore may be used to constrain density variations within the Earth. One important horizontal density variation with important geodynamic implications is the average density change between continental and oceanic lithosphere. Long wavelength density variations are associated with variations in gravitational potential energy, and hence tectonic forces. In the present study we focused on passive continental margins where density and geoid variations can best be used to constrain differences in lithospheric density. A total of 167 geoid profiles were generated perpendicular to passive continental margins on six continents globally. The geoid profiles were created using the EGM96 data set, but were also tested against the newer but not fully released GRACE geoid coefficients. The profiles, each just over 1000 km in length, were centered on the continental margin, and taken approximately every three degrees along the passive continental margins of North America, South America, Africa, Australia, India, and Antarctica. The geoid was tapered from degrees 11 to 85 in an attempt to eliminate long wavelength geoid features commonly assumed to originate in the lower mantle, and to use only those shorter wavelengths for which local isostasy is a good assumption. The average geoid step up from old oceanic lithosphere to continental lithosphere is 6 meters, based on all 167 profiles. The step up for each continental margin varied from 5 to 7 m. While the averages are very robust, the individual profiles are fairly noisy, in part because the edge of the continent is often rather arbitrarily defined by a particular bathymetric level. To overcome this, we allowed the center point of each profile to move up to 100-200 km if it increased the coherence of the profiles. Using these adjusted profiles, the average geoid step was 9m. This is likely an upper bound for the geoid step from oceanic to continental lithosphere. The geoid step found in this study has geodynamic implications. In the absence on any other forces acting on the system, the continents would have a tendency to fail in extension due to the excess gravitational energy associated with the geoid step. The so-called ridge force also arises from horizontal density variations within oceanic lithosphere, and has previously been associated with a 15-20m geoid step, leading to compression in the lithosphere. The 6-9m geoid step found in this study from oceanic to continental lithosphere indicates that the ridge force is reduced by one third to one half on the continents. This is consistent with the increase of strike slip and extensional deformation on continents compared to oceanic lithosphere.
DE: 8110 Continental tectonics--general (0905)
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8122 Dynamics, gravity and tectonics
DE: 8164 Stresses--crust and lithosphere
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting