We have developed a three-dimensional, lithospheric-scale model across the Bolivian Andes at ∼20°S, based on tomographic images of velocity and attenuation for both P and S waves. Observations of travel time and attenuation for this study are from regional, mantle earthquakes in the subducted Nazca plate recorded on a portable, broadband seismic array (Broadband Andean Joint Experiment and Seismic Exploration of the Deep Andes) in Bolivia and Chile. The shallow mantle under the Altiplano from ∼18°S to ∼21°S is high-velocity and moderately high Q (Vp≈8.3,Vs≈4.7, Qp≈500, and Qs≈200), suggesting lithospheric mantle. High-velocity material in the Altiplano extends to a depth of ∼125-150 km. The shallow mantle of the Western Cordillera is characterized by high Vp/Vs (∼1.83), suggesting a correlation between Vp/Vs and arc volcanism. Seismic velocity in the Western Cordillera mantle is, on average, only slightly reduced from global averages; however, velocity and attenuation anomalies are locally strong (Vp≈7.8,Vs≈4.3, Qp≈200, and Qs≈100), consistent with partial melt conditions. Under the Los Frailes volcanic field, in the Eastern Cordillera, shallow mantle velocity and Q decrease drastically from the neighboring Altiplano (Vp≈7.8, Vs≈4.3, Qp≈300, Qs≈100); however, high Vp/Vs is not as pervasive as it is in the Western Cordillera. We believe that slab-derived water, and perhaps other volatiles, strongly influence the Western Cordillera, while the Eastern Cordillera low-velocity region is more affected by partial melt and/or compositional changes. Average velocity and Q in the shallow mantle across the Bolivian Andes, where the tomographic images are best resolved, are significantly higher than in most mantle wedge environments where corresponding images are available. This is likely the result of a compressional "back arc" setting in the Andes. This implies that lithospheric shortening and thickening associated with the formation of the Andes has profoundly influenced the shallow mantle structure across the range. Shallow mantle structure is locally influenced by the subduction processes, particularly under the Western Cordillera; however, the differing volcanism and seismic character under the two Cordilleras suggest that the volcanic process in the Eastern Cordillera may be distinct from arc volcanism. Tertiary volcanism in the Eastern Cordillera is located in the region where mantle shortening is suspected to be greatest. Both the timing and location of volcanism are consistent with upward migration of mantle wedge asthenosphere following the removal of over thickened lithosphere.
ASJC Scopus subject areas
- Geochemistry and Petrology
- Earth and Planetary Sciences (miscellaneous)
- Space and Planetary Science