Variations in lithospheric structure and slab geometry under northwestern Bolivia and southern Peru from teleseismic tomography

The central Andes in South America are characterized by along-strike variations in magmatism, upper crustal shortening, crustal thickness, and slab geometry that make it an ideal region to study the relationship between the subducting slab, the mantle wedge, and the overriding plate. The north central Andes between 10° and 19°S is characterized by a change in the geometry of the subducting slab, shifting from normal (~30° dip) subduction in the south to near horizontal subduction in the north. We deployed ~85 broadband seismometers in this region as part of the CAUGHT and PULSE temporary seismic deployments to image the Earth beneath the array. We use finite frequency teleseismic tomography to image P-wave velocity variations in the mantle down to ~600 km. P- and PKIKP- (diffracted PKP) arrivals were picked in multiple frequency bands for earthquakes at distances between 30° and 90° and between 155° to 180° from the array respectively. The tomographic algorithm used calculates approximate finite frequency kernels for each ray, providing additional sampling for each model layer to potentially increase the resolution of our images. Preliminary results from the finite frequency tomography inversion show a clear trench-parallel fast velocity anomaly with approximately +3% P-wave velocity perturbation migrating to the northeast with increasing depth in the southern part of the study area. This dipping fast anomaly, which penetrates the 410 km discontinuity, corresponds to the dipping Nazca slab under Bolivia. In the northern part of the study area, around 14°S, this fast anomaly bends away from the trench below 200 km depth as the presence of the Peruvian flat slab causes the slab to descend into the mantle farther inland. Below about 400 km depth the slab anomaly is almost uniformly trench-parallel in map view and the influence of the flat slab region is no longer observed in the shape of the subducting slab. The northern Altiplano is consistently underlain by slower P-wave velocities in the upper mantle while the upper mantle beneath the Subandean Zone is characterized by fast (+3-4% velocity perturbation) anomalies at shallow depths (<150 km) consistent with the presence of fast cratonic lithosphere. While this fast anomaly is consistently observed under the Subandean Zone, it also extends under the Eastern Cordillera at 14° and 16°S. The western edge of this fast anomaly is characterized by a sub-vertical boundary which is observed down to ~150 km depth, separating the strongly contrasting fast anomaly under the Subandean Zone and Eastern Cordillera and the slower anomaly under the Altiplano. Dorbath et. al. (1993, JGR v. 98, n. B6) observed this sub-vertical boundary in their 2-D tomographic study at ~17°S and interpreted the fast velocities under the Subandean Zone and Eastern Cordillera as the Brazilian craton underthrusting the Eastern Cordillera. Our results indicate that this high velocity material is laterally extensive and extends to the north to at least 12°S. However, the western limit of this anomaly is variable, with fast velocity perturbations only observed under part of the Eastern Cordillera and, based on our tomographic results, there is no indication that the Brazilian craton extends further west under the Altiplano.