CRUSTAL THICKNESS VARIATION IN THE ANDEAN FORELAND, ARGENTINA, FROM CONVERTED WAVES

Local network three-component digital data from the San Juan area, Arg entina, provide the first seismological images of the deep crustal str ucture in the Andean foreland above a horizontal segment of the subduc ted Nazca plate. We have identified S-to-P seismic phases converted on the Moho by analysis of seismograms formed by taking the product of t he radial and vertical components ( R Z) from intermediate-depth ear thquakes in the Benioff zone directly beneath the network. Under the S ierras Pampeanas, the Moho is estimated to be at a depth of about 52 k m. Beneath the eastern Precordillera, the Moho deepens to 55/57 km and further west, beneath the central Precordillera, to 60 km. We therefo re estimate a 5-degrees to 10-degrees westward dip to the Moho under t he Andean foreland. In both the Sierras Pampeanas and Precordillera pr ovinces, the thickness of the ductile lower crust is comparable to tha t for a stable continental crust, while the brittle upper crust, impli ed 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 e ast-west deepening of the seismicity, the Moho, and the depth to basem ent under the Precordillera all suggest that the Andean foreland litho sphere is underthrusting beneath the Andes. The associated shortening is estimated to be about 15% across the Sierras Pampeanas. It is estim ated to be at least 38% across the Precordillera for the part of the c rust beneath the decollement, while it is up to 70% for the part above the decollement.