PARALLELS IN LOSS OF CRUSTAL WIDTH BY FOREARC SUBDUCTION EROSION AND CRUSTAL SHORTENING ALONG THE SOUTH CENTRAL ANDEAN MARGIN

An analysis of the Early Miocene to Recent Andean arc and backarc in the Southern Volcanic Zone (SVZ) shows parallel north to south changes in amounts of backarc crustal shortening, frontal arc migration, and forearc subduction erosion with all three varying from ~ 85 km at 34°S to ~ 18 km at 38°S. Crustal balancing by Ramos and others show that the along strike differences in backarc shortening can largely account for the southward decrease in crustal thickness beneath the Main Cordillera without significant magmatic addition. Assuming a nearly constant arc trench gap like that along the SVZ arc today, the migration of the frontal arc requires a parallel loss of forearc width. As contemporaneous crustal shortening is absent west of the magmatic arc, a minimum of 85 km of forearc subduction erosion is required near 34°S since 20 Ma. The fact that the forearc Chilean Central Valley is 85 km narrower at 34°S than at 38°S is consistent with uplift of the northern part by underthrusting of forearc crust removed by subduction erosion. The combination of lost forearc width and backarc shortening since 20 Ma implies that the Chilean trench is now ~ 170 km closer to the undeformed foreland at 34°S and ~ 30 km closer at 38°S. The symmetry of forearc loss and backarc shortening indicates that forearc loss, arc migration, and crustal shortening are all linked by mantle processes related to plate convergence parameters. The regional picture shows that subduction of the Juan Fernandez ridge on the Nazca plate cannot be a major driving force and that most of the crust lost due to forearc subduction erosion is recycled back into the mantle.