The isostatic residual gravity anomaly of the central Andes, 12 degrees to29 degrees S: A guide to interpreting crustal structure and deeper lithospheric processes

The isostatic residual gravity field of the Central Andean Plateau region c ontains laterally continuous, elongated anomalies that reflect the tectonic and magmatic effects of oceanic subduction in the forearc and continental subduction in the backarc. On the western side of the Andes, the residual a nomalies are similar to those found at many "Andean"-type subduction margin s. In particular, a high-low residual anomaly pair coincident with the coas t and the Peru-Chile trench marks the location when the Nazca plate underth rusts beneath western South America. This high-low anomaly couple is mirror ed in the backarc by a similar couple that tracks the location of the eastw ard-vergent Principal Frontal Thrust and results from the westward, antithe tic subduction of the Brazilian shield beneath the plateau. The residual hi gh is situated in the Eastern Cordillera of Bolivia, and is caused by a com bination of high-density basement rocks near the surface in the hanging wal l of the thrust, and by local crustal undercompensation. To the east, the p arallel low coincides with the Subandean fold-thrust belt and foreland basi n, and is due to a combination of low-density sedimentary rocks in the fore land basin and the locally overcompensated crust of the downflexed foreland lithosphere. The Eastern Cordillera and the Subandean are structural analo gs to the uplifted forearc and the Peru-Chile trench on the western side of the Andes. Along-strike variations in the amplitude and width of the Easte rn Cordillera and Subandean anomalies indicate that the foreland lithospher e is strongest at the latitude of central Bolivia, but decreases significan tly to the south where the eastern margin of the Andes appears to be nearly in a state of local isostatic compensation.