CRUSTAL BALANCE AND CRUSTAL FLUX FROM SHORTENING ESTIMATES IN THE CENTRAL ANDES

The Central Andes of South America form the second largest high elevation plateau on earth. Extreme elevations have formed on a non-collisional margin with abundant associated arc magmatism. It has long been thought that the crustal thickness necessary to support Andean topography was not accounted for by known crustal shortening alone. We show that this may in part be due to a 2 dimensional treatment of the problem. A 3 dimensional analysis of crustal shortening and crustal thickness shows that displacement of material towards the axis of the bend in the Central Andes has added a significant volume of crust not accounted for in previous comparisons. We find that present day crustal thickness between 12oS and 25oS is accounted for (~ -10% to ~ +3% ) with the same shortening estimates and the same assumed initial crustal thickness as had previously led to the conclusion of a ~25–35% deficit in shortening relative to volume of crustal material. Additional shortening may still be necessary to account for crustal volumes when the effects of erosion and possible delamination of the lower crust are included in mass balance calculations and depending on the initial conditions assumed. We suggest that the distribution of crustal material due to shortening may not match that of the present day and substantial redistribution of crust may have occurred by both erosion and deposition at the surface and lower crustal flow in regions of the thermally weakened middle and lower crust. This is the equivalent of relaying upper crustal shortening from the Cordilleras to beneath the Altiplano-Puna Plateau at lower and middle crustal depths.