LITHOSPHERIC FLEXURE AND BENDING OF THE CENTRAL ANDES

Gravity anomaly and topography data are used to define the effective elastic thickness of the lithosphere, T-e, in the bend region of the Central Andes. Values of T-e increase from nearly zero, north and south of the bend, to values greater than 50 km at the bend in Bolivia. There is a close correlation between T-e and the style and magnitude of the shortening in the Central Andes since the Late Miocene. In the bend region, where the lithosphere is flexurally strong with large T-e, foreland deformation is concentrated into a thin-skinned fold-and-thrust belt above a basal decollement which has absorbed more than 100 km of shortening. Further north and south, where the lithosphere is flexurally weak with low T-e, foreland deformation is more complex, involves basement and has absorbed less shortening. The along-strike gradients in foreland shortening have accommodated both clockwise and anticlockwise rotations about a vertical axis of almost the entire width of the Bolivian Andes. We speculate that the observed variation in T-e is related to the proximity of the Brazilian shield and has been an important factor in controlling the nature and amount of foreland deformation, and hence bending of the Central Andes. The flexural properties of the lithosphere may play an important role in determining the large-scale evolution of mountain belts.