A TELESEISMIC STUDY OF THE ALTIPLANO AND THE EASTERN CORDILLERA IN NORTHERN BOLIVIA - NEW CONSTRAINTS ON A LITHOSPHERIC MODEL

The Altiplano, one of the world's great plateaus, is the most significant geomorphological unit of the Central Andes. Knowledge of its lithospheric structure is essential to the understanding of the mountain building in this region. In order to study this, we performed a teleseismic field experiment in northern Bolivia. During a 4-month period, 34 short-period seismic stations were installed along a 320-km-long profile, crossing the Altiplano and the Eastern Cordillera, from the volcanic arc zone to the sub-Andean zone, perpendicular to the main structural direction. A careful study of relative travel time residuals led us to propose a qualitative model related to the structural patterns; the maximum amplitude of these residuals along the profile reaches 3 s for P phases and have a strong azimuthal dependence. The residuals show a noticeable sudden increase, clearly associated with the fault system bordering the Eastern Cordillera to the west, the Cordillera Real fault system. The high quality of the data allows us to perform a velocity inversion and to calculate at what depth lateral variations occur. The origin of the perturbations in the upper crust is reasonably accounted for by the depth variations of the sedimentary fill, with the maximum thickness found under the Altiplano basin. The velocity perturbations in the lower crust are interpreted as variations of the Moho depth which decreases from about 60 km below the Altiplano to 50 km below the Eastern Cordillera. In the upper mantle, a high-velocity zone is observed below the Eastern Cordillera. The major feature of this model is a subvertical boundary, dipping slightly to the southwest, which separates two strongly contrasting velocity units throughout the model, from the surface down to 140 km. This boundary coincides with the Cordillera Real fault system and is interpreted as an old suture. As the Altiplano is characterized by low velocities in the crust, the high velocities beneath the Eastern Cordillera down to 120 km may correspond to the Brazilian craton; in the region studied, the western limit of the underthrusting of the craton corresponds to the western border of the Eastern Cordillera.