Gravity anomalies and flexure of the Brazilian Shield beneath the Bolivian Andes

Abstract

Seismological and geological data suggest that the Brazilian Shield is now underthrusting beneath the sub-Andes. In order to constrain the minimum amount of shortening that results from this underthrusting, we analyzed Bouguer gravity anomalies over the shield and the Bolivian Andes, assuming that they are due primarily to variations in crustal thickness that in turn are due to the flexure of an effectively elastic plate (Brazilian Shield) loaded by the weight of the Andean mountain range. Gravity anomalies show that Airy-type isostatic equilibrium does not exist; the sub-Andes are overcompensated and the Chaco Plain to the east is undercompensated. The flexure of the plate can help explain both the overcompensation and the undercompensation if the Brazilian Shield behaves as an essentially elastic plate that extends at least 150 km, and probably 200 km, beneath the sub-Andes and the Cordillera Oriental and if the flexural rigidity of the plate is between 0.1 and 2 × 10 24 N m. The fit of observed and calculated anomalies, however, is far from perfect, and additional sources of short-wavelength anomalies appear to be present. Assuming that the sediments and sedimentary rocks in the sub-Andes and Cordillera Oriental were deposited on the Brazilian Shield and that later they were overthrust eastward onto one another causing the present topography and the deflection of the plate, we computed a family of estimates for the minimum amount of crustal shortening. These estimates depend strongly on the amount of sedimentary rock deposited before deformation, which does not seem to be well constrained by published geologic mapping. Nevertheless, the calculations suggest that at least 100 km of crustal shortening have occurred, and probably more, implying that the Andes may have been built mostly by crustal shortening.