Mass anomaly structure of the Earth

Abstract

Gravitational interaction is the weakest among the four known forces in the universe. The particular gravity equipotential field that coincides with sea level is called the geoid, and satellite data have revealed anomalies in its pattern that are puzzling to explain. Dynamic topography solutions have offered the preferred explanation for the past 15 years, but problems remain. An alternative explanation presented here contends that very large mass anomalies lie deep in the Earth, presumably produced by topography at the coremantle boundary (CMB), and only much smaller mass anomalies occur at shallower depths. The principal distinction is whether one is willing to accept the possibility that processes within the Earth's core may generate the CMB topography. An analysis of the South American regional geoid high (spherical harmonic degrees 2-10) indicates that only mass anomalies shallower than 1200 km are its cause. This single positive source is contrary to dynamic topography solutions that would generate negative deflections of both the surface and the CMB in response to the positive subducted mass for the Earth's fourth greatest geoid high. Our alternate explanation is tested utilizing a set of 9274 subduction 5° cube slab bloblet center points, developed from reconstructed plate history, that provide estimates of the locations of material subducted into the Earth's mantle. Two global mass solutions are offered utilizing (1) only those bloblets in the outer 800 km and (2) only those bloblets in the outer 1400 km. Four point masses at 3000 km depth to simulate CMB topography, produced by processes within the core unrelated to mantle dynamic topography, complete the two mass models. Both models show reasonable agreement with patterns and magnitudes of the regional geoid and its component degree parts. The model CMB mass anomalies are only a factor of 3 greater than those initially estimated by gravity to geoid ratios from observed values at geoid anomaly center locations. Difficulties with the dynamic topography solutions, matters of the density of the subducted material, the proper hydrostatic flattening value to use for analyses, and possible causes for the CMB topography are discussed. The subduction mass models are also used to estimate the magnitude of the driving force for plate tectonics (2.78 × 10 20 to 3.23 × 10 21 N), approximately equivalent to 7.0 × 10 12 to 8.1 × 10 13 N m -1. These force values are comparable to published slab force estimates based on thermal considerations.