Gravity anomaly and flexural model: Constraints on the structure beneath the Peruvian Andes

Bouguer gravity data along the Nazca profile of Fukao et al. (1989), NE-SW striking across the Peruvian Andes, are used to investigate the flexure of the Brazilian Shield. Modeling of the observed gravity data indicates that the Brazilian Shield lithosphere is flexed downward and may extend beneath the Andes as far as 150 km. Assuming that the Brazilian Shield behaves elastically, flexural analysis shows that the isostatic gravity inequilibrium in the eastern Andes can be explained by deflection of the Moho due to the bending of an elastic plate beneath the Sub-Andes. The elastic thickness of the plate is estimated to be between 25 and 55 km, with corresponding flexural rigidity between 0.1 and 1.7 × 10²⁴ Nm. Many small to moderate-sized earthquakes that have focal depths of tens of kilometers are distributed over a broad area from the Eastern Cordillera to the Brazilian Shield. These events are located well above the subducted flat Nazca plate and very likely are associated with the underthrusting of the Brazilian Shield. A step-like gravity anomaly of 150 mGal on the eastern margin of the Eastern Cordillera cannot be fully explained by the flexure model alone and requires a sharp-edged mass deficit or discontinuous Moho beneath the Eastern Cordillera. Our gravity modeling shows that Bouguer gravity anomalies along the Nazca profile are best explained by a flexed Moho, which is deflected by the underthrusting of the Brazilian Shield beneath the Andes along with a wedge-shaped body of low-density material directly beneath the Eastern Cordillera.