CRUSTAL-THICKNESS VARIATIONS IN THE CENTRAL ANDES

We estimated the crustal thickness along an east-west transect across the Andes at lat 20 degrees S and along a north-south transect along t he eastern edge of the Altiplano from data recorded on two arrays of p ortable broadband seismic stations (BANJO and SEDA). Waveforms of deep regional events in the downgoing Nazca slab and teleseismic earthquak es were processed to isolate the P-to-S converted phases from the Moho in order to compute the crustal thickness. We found crustal-thickness variations of nearly 40 km across the Andes, Maximum crustal thicknes ses of 70-74 km under the Western Cordillera and the Eastern Cordiller a thin to 32-38 km 200 km east of the Andes in the Chaco Plain. The ce ntral Altiplano at 20 degrees S has crustal thicknesses of 60 to 65 km . The crust also appears to thicken from north (16 degrees S, 55-60 km ) to south (20 degrees S, 70-74 km) along the Eastern Cordillera, The Subandean zone crust has intermediate thicknesses of 43 to 47 km. Crus tal-thickness predictions for the Andes based on Airy-type isostatic b ehavior show remarkable overall correlation with observed crustal thic kness in the regions of high elevation, In contrast, at the boundary b etween the Eastern Cordillera and the Subandean zone and in the Chaco Plain, the crust is thinner than predicted, suggesting that the crust in these regions is supported in part by the flexural rigidity of a st rong lithosphere, With additional constraints, we conclude that the ob servation of Airy-type isostasy is consistent with thickening associat ed with compressional shortening of a weak lithosphere squeezed betwee n the stronger lithosphere of the subducting Nazca plate and the crato nic lithosphere of the Brazilian craton.