PLATE COUPLING MECHANISM AT FLAT SLAB SUBDUCTION IN THE SOUTHERN CENTRAL ANDES (28-33°S): LONG AND SHORT TERM CONSTRAINTS FOR A NUMERICAL MODEL

Plate coupling at subduction zones is a key element to understand the tectonic and magmatic evolution of the overriding plate and the competing/complementary role of the external and intrinsic control exerted by the convergence kinematics and the inherited geological features. Flat slab subduction represents a case in which plate coupling may extend further inland than a normal subduction configuration. The flat geometry of the subducting plate exerts a primary control on the tectonics and magmatism, with a secondary control from pre-existing continental geology. We present a numerical model for the shallow subduction of the Southern Central Andes (28-33°S) constrained by the regional and local geology and tectonics (long term) and the GPS (short term) data. We use a thin viscous sheet approach with basal shear traction. Shear traction at the interplate contact is controlled by the thermal state of the subducting plate. In the study area, major controlling factors of the thermal field are the Challenger FZ, and the absence/presence of the asthenospheric wedge. Plate geometry is constrained by seismicity at the interplate contact. Although the GPS data basically represents the elastic response of the interseismic cycle in the region, growing evidence points to a closer link between short and long term deformation. Velocity gradients of the GPS data show geologically instantaneous shortening and extension directions compatible with the local orientations of long term geologic structures. Additionally, there are two EW corridors where shortening occurs in all directions (at 29-30°S and 33-34°S). Geological data document the presence of WNW discontinuities at those latitudes, some of them corresponding to Neogene faults mapped in the field. Neotectonic fault-slip data support ~ NS shortening within the WNW corridor between the coast and Main Cordillera. Preferred model results require different rheological domains, controlled by thermal constraints and preexisting weak zones. Besides the general agreement with the geology and tectonics of the region, this long term approach reproduces most of the short term evidences derived from the GPS results, reinforcing the causal relationship between elastic and permanent deformation.