The Isostatic Residual Gravity Anomaly of the Central Andes, 12° to 29° S: A Guide to Interpreting Crustal Structure and Deeper Lithospheric Processes

Abstract

The isostatic residual gravity field of the Central Andean Plateau region contains 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 anomalies are similar to those found at many “Andean”-type subduction margins. In particular, a high-low residual anomaly pair coincident with the coast and the Peru-Chile trench marks the location when the Nazca plate underthrusts beneath western South America. This high-low anomaly couple is mirrored in the backarc by a similar couple that tracks the location of the eastward-vergent Principal Frontal Thrust and results from the westward, antithetic subduction of the Brazilian shield beneath the plateau. The residual high is situated in the Eastern Cordillera of Bolivia, and is caused by a combination of high-density basement rocks near the surface in the hanging wall of the thrust, and by local crustal undercompensation. To the east, the parallel low coincides with the Subandean fold-thrust belt and foreland basin, and is due to a combination of low-density sedimentary rocks in the foreland basin and the locally overcompensated crust of the downflexed foreland lithosphere. The Eastern Cordillera and the Subandean are structural analogs 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 Eastern Cordillera and Subandean anomalies indicate that the foreland lithosphere is strongest at the latitude of central Bolivia, but decreases significantly to the south where the eastern margin of the Andes appears to be nearly in a state of local isostatic compensation.