USING UPPER PLATE NORMAL FAULTS TO UNDERSTAND SUBDUCTION ZONE DYNAMICS IN THE NORTHERN CHILEAN FOREARC

The northern Chilean Coastal Cordillera is dominated by normal faults demonstrating finite extension in the direction of plate convergence. Models of Coulomb stress change on these fault planes resulting from interseismic strain accumulation and its release during strong underthrusting earthquakes on the plate boundary provide insight into the role of the subduction zone earthquake cycle in shaping the forearc. Interseismic Coulomb stress changes encourage failure on these faults, suggesting that the flexure of the forearc influences extensional faulting near the surface. Models of stress changes due to the 1995 M_W = 8.1 earthquake near the city of Antofagasta show complicated distributions of stress, but in general indicate a lesser degree of encouraged failure on extensional faults, arguing against coseismic elastic rebound as the primary trigger for normal slip. Complex deformation patterns are also observed in the field: Though the movement sense on faults is dominated by normal motion, which has generated substantial fault scarps throughout the forearc, some evidence for superposed reverse and strike-slip faulting also exists. The multiple senses of fault movement are consistent when viewed in terms of the complicated stress patterns predicted for the entire seismic cycle. Further complications to the stress field result from subduction erosion processes which play a substantial role in the forearc structural evolution but are not included in this study.

The pattern of near-surface deformation predicted by Coulomb stress change models depends on the distribution of interplate slip. Thus, if the positive Coulomb stress predicted near the surface during the interseismic period is the dominant control on the evolution of normal faulting in the Coastal Cordillera, as is suggested by the modeling, then the traces of the faults provide information about the extent of coupling between the Nazca and South American plates. The distribution of faults suggests a variable extent of interplate coupling along strike: models that predict interseismic loading of normal faults towards failure require a decrease in the maximum depth of coupling from ~50 km north of the Mejillones Peninsula (23ºS) to ~40 km south of the peninsula. This variation is consistent with results of seismic and geodetic studies in this region.