Geoid Data and Implications for Plate Tectonic Dynamics
Abstract
It has long been recognized that the motion of the mechanically rigid lithospheric plates of the earth are the surface expression of large-scale convection in the mantle. It is also accepted that the stresses driving plate motion are an amalgam of the basal tractions associated with this convection and long-wavelength density variations within the plates themselves. Parsing the relative contribution from these two sources to the geodynamics of the lithosphere continues to be an important topic of plate dynamics research. Because geoid anomalies are directly related to the local dipole moment of the density-depth distribution, they provide an ideal method for evaluating density variations within the lithosphere and the associated tectonic stresses. The main challenge with this approach is isolating the lithospheric geoid contribution from the full geoid (which is dominated by sources from deeper in the earth, namely the lower mantle). We address this issue by using a high-pass spherical harmonic filtering of the EGM2008-WGS84 geoid (which is complete to spherical harmonic degree and order 2159), with a cosine taper between orders 9 to 13 and 78 to 82 to produce a 'lithospheric' geoid. In the present study we focus on tectonic implications of the lithospheric geoid in three different areas: 1) passive continental margins where we have evaluated over 150 margin-transects spaced roughly every three degrees. The global average geoid anomaly associated with the transition from old oceanic lithosphere to the continent was found to 6-9 meters and appears to be insensitive to a range of geoid filtering degrees and orders; 2) The geoid highs associated with the mid-ocean ridges and the cooling oceanic lithospheric, where we have examined a number of geoid profiles across ridges and find that previous estimates of a geoid anomaly of 10-15 meters associated with ridges to be valid; and 3) continental regions which are characterized by both elevated geoid anomalies (e.g., the Western U.S.) and geoid lows (e.g., the Congo Basin in Africa). All three of these geoid studies have implications for our understanding of the dynamics of plate tectonics. The 10-15 m geoid highs associated globally with ridges are consistent with a net force of ~3x1012 N/m due to 'ridge push.' Converting gradients in the oceanic 'lithospheric' geoid produce net torques on the plates consistent with this magnitude of 'ridge push.' The 6-9 meter geoid step up across passive continental margins is important for two reasons. First, it is consistent with a reduction of the ridge force acting on the continents, as evidenced by increased strike slip and normal deformation on the continents compared to oceanic lithosphere. Second, the very fact that such a small geoid step can affect tectonic style is evidence that even relatively small forces, like the ridge force compared to the negative buoyancy of subducted lithosphere, can be important in plate dynamics. Furthermore, the predicted intraplate stresses computed using a finite-element analysis of a lithospheric shell under traction from the gravitational potential energy forces associated with the lithospheric geoid provide a remarkably good fit between the predicted and observed intraplate stress field at long wavelengths (~1000km) and indicates that tectonic forces associated with the lithospheric density moment play an important role in global geodynamics.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.T51E2518R
- Keywords:
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- 8122 TECTONOPHYSICS Dynamics: gravity and tectonics;
- 8164 TECTONOPHYSICS Stresses: crust and lithosphere;
- 8120 TECTONOPHYSICS Dynamics of lithosphere and mantle: general