Crustal structure of the Altiplano from broadband regional waveform modeling: Implications for the composition of thick continental crust

We have modeled the full waveforms from six intermediate-depth and two shal low earthquakes recorded at regional distances by the BANJO Broadband Andea n Joint Experiment (BANJO) and Seismic Exploration of the Deep Altiplano (S EDA) portable seismic networks in the central Andes. In this study we utili ze data from those BANJO and SEDA stations located within the Altiplano and Eastern Cordillera. We used reflectivity synthetic seismograms and a grid search to constrain four parameters of the Altiplano-Eastern Cordillera lit hosphere: crustal thickness, average crustal velocity (V-p), and crustal an d upper mantle Poisson's ratios (sigma(crust) and sigma(mantle)). Using our grid search, we investigated the crustal and upper mantle structure along 36 individual event station paths and applied forward modeling to 56 event station paths. Robust models for the Altiplano that provide the best overal l fit between the data and synthetic seismograms are characterized by an av erage V-p of 5.75-6.25 km/s, crustal thicknesses of 60-65 km, sigma(crust) = 0.25, and sigma(mantle) = 0.27-0.29. We find a north-south variation in t he structure of the Altiplano, with the crust south of the BANJO transect c haracterized by either lower than average crustal P wave velocities or a sl ightly higher sigma(crust) relative to crust north of the BANJO transect. T hese results are consistent with a model of crustal thickening caused predo minantly by tectonic shortening of felsic crust, rather than by underplatin g or magmatic intrusion from the mantle.