Effects of loads in the upper crust on estimates of the elastic thickness of the lithosphere

The admittance function and coherence relating gravity and topography are important constraints on the elastic thickness (T-e) of the lithosphere. Forsyth (1985) modelled them with a lithosphere loaded both externally by topography and internally by density contrasts such as initial undulations on the Moho. His approach has yielded thicknesses as high as 150 km for the continental lithosphere. McKenzie & Fairhead (1997) argued that this method can only supply an upper bound on T-e because the response is biased by 'noise' due to density variations in the upper crust. We have used Forsyth's approach to analyse the effect of shallow heterogeneities such as sedimentary basins and igneous intrusions, simulating them by a thin layer with a laterally variable density. The information content of the admittance and coherence is concentrated in the wavenumbers at which the two responses roll-off to half their zero-wavenumber values. These two parameters are used to explore the interdependence of the retrieved elastic thickness and internal/ external loading ratio f. Provided the internal load is less than 10 per cent of the external, it does indeed bias the response so that T-e is overestimated. However, for predominantly internal loads, the reverse is the ease, and the value of T-e obtained by assuming equal internal and external loads is actually a lower bound. It is particularly important to allow for the flexure due to upper-crustal loads when the lithosphere is old and topography subdued due to prolonged erosion. Application of our approach to McKenzie gr Fairhead's (1997) Western Australia data shows that the loading ratio depends strongly on wavenumber, but that it is still possible to use the response data to constrain T-e, which must be at least 90 km if the results obtained by conventional spectral estimation are accepted, at least 40 km using the multitaper method.