We have used broadband seismograms recorded at portable and permanent seismic stations in South America to image the three-dimensional S velocity structure beneath central and western South America. We have interactively fitted the waveforms of S and surface wave trains in >500 seismograms from 90 earthquakes and 60 different stations. The constraints on upper mantle S velocity structure provided by the waveform fits have been combined with independent estimates of depth to the Mohorovicic discontinuity, yielding model SA99. The S velocity structure of SA99 reveals a highly heterogeneous upper mantle with a maximum peak to peak difference of 1 km/s above 100 km. The heterogeneity seems to be correlated with tectonic and geologic domains. SA99 shows high lithospheric S velocities for the Precambrian cratons. The Amazon basin is also underlain by high S velocities, while three other intraplatform basins are underlain by low S velocities at various depths. SA99 shows extremely low velocities in the mantle wedge beneath the volcanically active Andes. We explain these low S velocities as originating from realistic amounts of volatiles, released by the subducting Nazca crust, and resulting amounts of partial melt within the peridotitic mantle wedge, without requiring elevated temperatures. While SA99 could be consistent with diffuse influx for corner flow in the mantle wedge, it does not support models that require influx of subcontinental upper mantle into the mantle wedge through a laterally uniform low-viscosity channel. ¿ 2001 American Geophysical Union