Local network three-component digital data from the San Juan area, Argentina, provide the first seismological images of the deep crustal structure in the Andean foreland above a horizontal segment of the subducted Nazca plate. We have identified S-to-P seismic phases converted on the Moho by analysis of seismograms formed by taking the product of the radial and vertical components (R * Z) from intermediate-depth earthquakes in the Benioff zone directly beneath the network. Under the Sierras Pampeanas, the Moho is estimated to be at a depth of about 52 km. Beneath the eastern Precordillera, the Moho deepens to 55/57 km and further west, beneath the central Precordillera, to 60 km. We therefore estimate a 5° to 10° westward dip to the Moho under the Andean foreland. In both the Sierras Pampeanas and Precordillera provinces, the thickness of the ductile lower crust is comparable to that for a stable continental crust, while the brittle upper crust, implied from the spatial distribution of the seismicity, shows an important thickness increase. This rheological behavior is interpreted to be a consequence of the flattening of the dip of the subducted plate. The east-west deepening of the seismicity, the Moho, and the depth to basement under the Precordillera all suggest that the Andean foreland lithosphere is underthrusting beneath the Andes. The associated shortening is estimated to be about 15% across the Sierras Pampeanas. It is estimated to be at least 38% across the Precordillera for the part of the crust beneath the décollement, while it is up to 70% for the part above the décollement.

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