Isostatic Compensation on a Continental Scale: Local Versus Regional Mechanisms

Abstract

Using the techniques of linear and quadratic programming, it can be shown that the isostatic response function for the continental United States, computed by , is incompatible with any local compensation model that involves only negative density contrasts beneath topographic loads. We interpret the need for positive densities as indicating that compensation is regional rather than local. The regional compensation model that we investigate treats the outer shell of the Earth as a thin elastic plate, floating on the surface of a liquid. The response of such a model can IntroductionMeasurements of gravity over large-scale topographic features such as mountain ranges typically show negative Bouguer anomalies that have an inverse correlation with the Prescnl address: Department o f Environmental Sciences, Univcrsity of Lancaster, Lancaster, England. 432 R . J. Banks, R . L. Parker and S. P. Huestis elevation. The recognition of this relationship led to the development of the concept of isostasy; that the excess mass above sea level represented by topographic features is compensated by a deficiency of mass below sea level. The hypothesis can be given a more specific formulation in one of two ways: either in terms of the equality of mass of material in equal prisms above the 'level of compensation', or in terms of the equality of hydrostatic pressure over a 'level' surface within the earth. The second is a more flexible condition, in that it can be applied to both local and regional compensation models, and only requires that stresses within the earth are hydrostatic below some depth.The Bouguer gravity anomalies associated with topographic features contain information about the distribution of compensating mass, even though they do not determine it uniquely. If plausible limits can be placed on the maximum density differences involved, for instance, bounds can be established on the depth of the compensating mass (Parker 1975). More powerful constraints on the distribution of compensation are possible if a mechanical model of the compensation process can be specified.The earliest, and still the most commonly used, isostatic models are those of Pratt and Airy. They have tended t o dominate geophysical thinking about the processes by which isostasy is achieved, and about the definition and computation of isostatic gravity anomalies. Both are local compensation models: a topographic load is assumed to be supported only by the hydrostatic pressure at a point on the surface of compensation directly beneath it, or conversely, the compensating density at any point within the earth is determined only by the topographic elevation directly above it. The classic picture of local compensation is of a set of rigid blocks floating in a denser liquid, entirely independent of one another, and free to move vertically so as to achieve individual hydrostatic equilibrium. In the Pratt model, the blocks have different densities, the value of the density determining the elevation; in the Airy model, the blocks are of equal density, but diffe...

An accuracy assessment of the fan wavelet coherence method for elastic thickness estimation

“…Prior to the arrival of the Bouguer coherence method [Forsyth, 1985], this was usually performed by fitting a 1-D theoretically determined admittance curve to the 1-D observed Bouguer admittance, assuming only surface loading [e.g., McKenzie and Bowin, 1976;Banks et al, 1977], though McKenzie and Fairhead [1997] also used this method with multitaper estimation of the observed free-air admittance. More recently, Pérez-Gussinyé et al [2004] and Pérez- Figure 15.…”

Eustatic and tectonic change effects in the reversion of the transcontinental Amazon River drainage system

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