Satellite Gravity and the Geosphere Contributions to the Study of the Solid Earth and Its Fluid Envelopes (1997) / Chapter Skim
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5 SOLID EARTH PROCESSES
5 SOLID EARTH PROCESSES
Pages 47-58
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From page 47... ...
This new measurement regime will enable a better understanding of mantle convection in the 200 to 1,000 En range and permit improved insights into Me structure of continental roots and mountain building. MANTLE CONVECTION AND PLUMES: UNDERSTANDING THE EARTH'S HEAT ENGINE Most of Me heat loss from the Earth occurs via mantle convection the process by which the Earn loses Me bulk of its internal heat and which drives the plate-tectonic cycle.
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From page 48... ...
Significant progress has been made in recent years in relating spatial variations in Me gravity field to mantle convection (e.g., Hager and Richards, 1989; King and Masters, 1992; Forte et al., 1993; Phipps Morgan and Shearer, 1993; Ricard et al., 1993~. While Mere is general agreement on the existence of an appreciable increase of viscosity win depth from the upper to the lower mantle, there is less agreement on the details of either the inferred viscosity structure or the predicted gravity field.
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From page 49... ...
At this depth, in cold regions such as continental roots, the coupling between density hetero geneities and surface deformation is essentially iso static, so that knowledge of density structure and grav ity variations, rather than non-uniqueness in mantle flow models, are the limiting factors. Neither this seismic model nor the existing gravity field are sharp enough (~200 km resolution)
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From page 50... ...
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From page 51... ...
For example, the 40-50 mGal depression in the Earth's static gravity field over northern Canada has sometimes been interpreted as a measure of incomplete post-glacial rebound, but it has also been argued to reflect mass anomalies related to large-scale convection in Me manmade (see, for example, Cathles, 1975; Peltier et al., 1992; Forte and Mitrovica' 1996~. Similarly, a secular drift in the position of the Earth's rotation axis, a non-tidal acceleration of the Earth's rotation rate, and secular changes in the lowest-order zonal spherical harmonic components of the Earth's gravity field (e.g., C20)
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From page 52... ...
A 5-year SST or SSI mission would be able easily to separate the Canadian rebound from the effects of any ongoing secular change in the volume of Greenland ice. This requires only that the effects of the rebound rise above the satellite uncertainties for values of I up to about I = 4 or 5 (the half-wavelength of an I = 4 spherical harmonic is 45°, which is about equal to the angular separation between Greenland and Hudson Bay)
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From page 53... ...
The bottom panel shows the secular signal in the geoid due to changes in continental water storage during 1987-1991, as inferred from the soil moisture data described in the text. The amplitude of this hydrological signal can be as large as 0.2 mm/yr, which is about 10% of the rebound signal.
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From page 54... ...
For example, fitting the post-glacial-rebound geoid patterns from the top two panels in Figure 5.4 to the hydrological pattern shown on the bottom panel, we find that the hydrology perturbs the regression coefficient by 1% or less, which is far smaller than the 30-40% difference between the 1 O.E2 1 and 50.E2 1 Pa-sec lower-mantle-viscosity models. The effect of a linear increase in the water mass of Hudson Bay is also likely to be small.
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From page 55... ...
is compared to that predicted for an elastic plate with the observed loads. Figure 5.6a plots the correlation coefficient of Bouguer gravity and topography, termed the coherence, as a function of wavenumber for a model of an elastic plate of various effective elastic thicknesses assuming equal amounts of surface and subsurface loading.
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From page 56... ...
On the regional scale, data gaps would best be filled in using measurements of the gravity field provided by airborne platforms (e.g., NRC, 19951. Detailed corrections to such gravity measurements to assure geodetic integrity will require accurate knowledge of spacecraft position provided by GPS, as well as a precise record of aircraft velocity.
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From page 57... ...
FIGURE 5.6 (a) Theoretical coherence, the correlation coefficient between Bouguer gravity and topography, for an elastic plate equally loaded from at the surface and bottom.
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From page 58... ...
Observations with 1 mGal accuracy at length scales of 500-1,000 km would resolve discrepancies between estimates of the depths of continental roots and would distinguish between models of mantle flow. Gravity resolution of approximately 1 mGal over length scales of order ~120 lam would help constrain the depths of origin of hotspot mantle plumes, which are a major source of intraplate volcanism and enhanced heat flow.
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