Reappraisal of the effective elastic thickness for the sub-Andes using 3-D finite element flexural modelling, gravity and geological constraints

Abstract

Estimates of effective elastic thickness (Te) for the western portion of the South American Plate using, independently, forward flexural modelling and coherence analysis, suggest different thermomechanical properties for the same continental lithosphere. We present a review of these Te estimates and carry out a critical reappraisal using a common methodology of 3-D finite element method to solve a differential equation for the bending of a thin elastic plate. The finite element flexural model incorporates lateral variations of Te and the Andes topography as the load. Three Te maps for the entire Andes were analysed: Stewart & Watts (1997), Tassara et al. (2007) and Pérez-Gussinyé (2007). The predicted flexural deformation obtained for each Te map was compared with the depth to the base of the foreland basin sequence. Likewise, the gravity effect of flexurally induced crust—mantle deformation was compared with the observed Bouguer gravity. Te estimates using forward flexural modelling by Stewart & Watts (1997) better predict the geological and gravity data for most of the Andean system, particularly in the Central Andes, where Te ranges from greater than 70 km in the sub-Andes to less than 15 km under the Andes Cordillera. The misfit between the calculated and observed foreland basin subsidence and the gravity anomaly for the Marañon basin in Peru and the Bermejo basin in Argentina, regardless of the assumed Te map, may be due to a dynamic topography component associated with the shallow subduction of the Nazca Plate beneath the Andes at these latitudes.