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Marine Gravity Variations and the Strength of the Oceanic Lithosphere with Bending

  • Author(s): Garcia, Emmanuel Soliman Mortel
  • Advisor(s): Sandwell, David T
  • et al.
Abstract

The variations of the Earth's gravity field at spatial scales ranging from tens to hundreds of kilometers over the deep ocean basins can be measured with satellite remote sensing of sea surface slopes. As the composition of the crust and upper mantle comprising the oceanic lithosphere is highly uniform, these gravity variations are closely linked to geological structures formed by tectonic processes. One such process is the subduction of oceanic lithosphere into the planetary interior at deep-sea trenches. The combination of a bending moment from the slab sinking and a downward load from the overriding plate causes flexure of the lithosphere. This flexure produces the prominence of the trench outer rise and the slope of the outer trench wall. Extension of the plate at the outer rise is often accompanied by the formation of faults, which can reduce the plate strength and hence the amount of stress that the plate can support.

Data from recent radar altimetry missions of the CryoSat-2, Envisat and Jason-1 satellites was reprocessed to improve the accuracy of a global marine gravity field model. This reprocessing technique (``retracking'') refines the precision of radar range measurements by a factor of 1.5. An approximate mathematical model was formulated to enable retracking of radar waveforms collected by a novel instrument onboard CryoSat-2 in the synthetic aperture radar (SAR) mode.

A computational algorithm was developed for solving the thin plate flexure equation for spatially varying rigidity in two horizontal distributions subject to applied loads. The accuracy of the method was tested against analytic solutions. This modeling technique was applied to the oceanic lithosphere for the trench flexure case. Solving a parameter estimation problem generated flexural deformation surfaces that fit marine gravity and bathymetry observations at trench locations around the Pacific basin. Our results show that a flexure model in which the initial strength of the plate depends on age but is allowed to decreased through inelastic yielding is consistent with observations of the incoming lithosphere at Pacific subduction zones.

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