The Evolution of the Andes of Central Chile and Argentina: Constraints using Structural Cross Sections, Geophysics, and Thermochronology
Marcelo Farias, Departamento de Geologia, Universidad de Chile
Mark Brandon, Department of Geology and Geophysics
The Andes at the latitude of Santiago of Chile (33º30'S) have been recently on the focus structural discussion with two different models explaining differently block accommodation during mountain building: on one hand, one model suggests that shortening has been accommodated mostly by west-vergent thrusts synthetic to the Nazca slab subduction; on the other hand, in our preferred model, shortening has been accommodated by east-vergent thrusts rooted into the mantle and connecting with the slab at depths. Here, we do a review on this problematic including new constraints using thermochronological modeling. Modern evolution of this region began with the inversion of a Paleogene extensional basin at ~22Ma, focusing shortening mostly in the western side of the belt. At 16-15 Ma, deformation migrated and propagated eastward into the east-vergent belt Aconcagua fold-and-thrust. Deformation continued in this area until ~10 Ma, when immediately to the east started the uplift of the basement that forms the Argentinean Frontal Cordillera through deep rooted east-vergent ramps with the underthrusting of the Cuyania-Precordillera terrain. Simultaneously, western sectors of the inverted extensional basin and fold-thrust belt suffered out-of-sequence thrusting and backthrusting shortening. During this time, surface uplift was higher and produced the current topography of the cordillera. Considering the available data, shortening migrated from west to east and its final results shows that the eastern side of the belt did accommodate about 70% of total shortening mainly by east-vergent thrusts. Estimates of shortening balance well with crustal thickening along this time, implying a deep distribution of thickening from east to west.
Therefore this evolution is evidencing that the western flank of the Andes is the retro-wedge and the eastern side is the pro-wedge. Advances in structural-thermochronology modeling shows that despite folding and thrusting of the western side of the belt, the western Andes behave relatively passive to deformation, being mainly tilted to the west. Regarding the structure at depths visualized by seismic tomography, mountain building will be the result of the South American lithosphere subduction beneath the Andes, where the Nazca subduction will act as a boundary condition rather than deforming directly the continental lithosphere. Similar results can be extrapolated elsewhere along the Chilean Andes, where shortening has been largely smaller than that accommodated in the eastern flank of the belt, where interaction of old basement with stratified series produced the underthrusting that distributed thickening in a simple shear mode.