Tomography of the Andean crust and mantle at 20 degrees S: First results of the Lithoscope experiment

In 1994, a temporary network of 41 short-period seismic stations operated for 6 months along an east-west profile in Northern Chile and Bolivia. The profile crossed the entire Andean chain at a latitude of 20 degrees S. We present the results of the teleseismic tomography of the crust and mantle along an east-west vertical cross-section beneath the profile. The most striking feature of the tomographic image is the identification of the subducted Nazca plate. It appears as a zone of high velocities which is continuous over the aseismic part of the slab down to the 660km discontinuity. In the continental lithosphere, strong lateral P-wave velocity variations are found in the crust whereas the upper mantle shows only small velocity variations, Major lateral changes in crustal velocity coincide with the limits of the main structural units of the Andean range. The distribution of velocity anomalies in the crustal layer matches the lateral changes in crustal thickness and average velocity deduced from refraction surveys. A comparison with a previous tomographic experiment in Northern Bolivia confirms that the northern and southern segments of the Bolivian orocline have different lithospheric structures.