Curved Andes: Geoid, forebulge, and flexure

Abstract

Using geoid anomalies determined from satellite observations of the South American plate, we demonstrate the existence of a lithospheric fl exural forebulge east of the High Andes. Using the planform and location of the geoid anomalies and accounting for the curvature of the Andean orogen, we can successfully model plate fl exure using a uniform elastic thickness. Topography above 3 km elevation between -5 and -30 latitude in the Andes loads the margin of the western side of the Precambrian shield of the continental plate and drives bending of the cratonic plate. Removal of horizontal wavelengths greater than 4500 km from the geoid anomaly reveals a 5-7 m positive anomaly paralleling the trend of the orogen some 400 km east of the mountain front. We interpret the secondary geoid high as a fl exural forebulge that developed in response to topographic loading of the South American plate by the Andes. While the topographic expression of this forebulge is hidden by the alluvium shed from the Andes and the vegetative cover of the Amazon jungle, our fi ltered geoid anomalies and a three-dimensional, single-plate fl exural model in spherical geometry are both well fi t by a single model with ~50 km effective elastic thickness.