Flexural response of the llthosphere ano regional subsioence in the Amazon Cone area

Abstract

The effective elastic thickness of the lithosphere in the Amazon Cone area was obtained using the direct relation between estimates of basement flexure and sediment load. In order to predict thermal contribution to present depths to basement, this approach required an a priori choice of a thermo-mechanical model of evolution of the Amazon continental margin. Based on structural data obtained in earlier studies, McKenzie's pure-shear uniform-extension mode! was used to estimate tectonic subsidence in the Amazon Cone area. Subject to the assumptions implicit in that model, estimates 01 sediment load and basement flexure may be made. The crustal stretching factor(the only Iree parameter in the uniform extension model} was estimated by assuming a 32-km-thick unstretched crust as reference. The isostatic filter response was then estimated Irem the Fourier coefficients of the sediment load and subsidence estimates. Flexural rigidity (O) and corresponding elastic thickness (Te) were then obtained by least squares fitting of filter estimates to the thin elastic plate mode/. The analysis yielded an elastic thickness estimate of 12 km (0=1.lxl022 Newton-meters). This time-average estimate was used together with Parsons an Sclater's cooling plate model and with available chronostratigraphic constraints to estimate the time evolution of the elastic litosphere beneath the Amazon Cone area. The value of Te, determined Irem the direct relation between load and flexure in the Amazon Cone, is not affected by uncertainties as to the depth of compensation, as commonly occurs in flexural isostasy analyses based on gravity and bathymetry data.