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), N E-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 an d may extend beneath the Andes as far as 150 km. Assuming that the Bra zilian Shield behaves elastically, flexural analysis shows that the is ostatic 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 x 10(24) Nm. Many small to moderate-sized earthquakes that ha ve focal depths of tens of kilometers are distributed over a broad are a from the Eastern Cordillera to the Brazilian Shield. These events ar e located well above the subducted flat Nazca plate and very likely ar e associated with the underthrusting of the Brazilian Shield. A step-l ike gravity anomaly of 150 mGal on the eastern margin of the Eastern C ordillera cannot be fully explained by the flexure model alone and req uires a sharp-edged mass deficit or discontinuous Moho beneath the Eas tern Cordillera. Our gravity modeling shows that Bouguer gravity anoma lies along the Nazca profile are best explained by a flexed Moho, whic h is deflected by the underthrusting of the Brazilian Shield beneath t he Andes along with a wedge-shaped body of low-density material direct ly beneath the Eastern Cordillera.