Lithospheric-scale structure across the Bolivian Andes

Abstract

I have developed a three-dimensional, lithospheric-scale model of the Bolivian Andes at ∼20°S. The model is 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 in Bolivia. The shallow mantle under the Altiplano from ∼18°S to ∼20°S is high velocity, but seismic Q is relatively low (Vp∼8.3, Vs∼4.9, Qp∼150, Qs∼100). These seismic properties suggest lithospheric mantle, approaching solidus conditions. High velocity material in the Altiplano extends to a depth of ∼150 km at 18.5°S, shallowing to ∼100 km at 20.5°S. Anomalously low velocity and Q anomalies are imaged in the mantle beneath the Eastern (fold and thrust) and Western (volcanic arc) Cordilleras of Bolivia. In the Western Cordillera, velocity and attenuation anomalies are locally strong (Vp∼7.8, Vs∼4.25, Qp∼80, Qs∼20), consistent with partial melt conditions. However, there is a segment of higher velocity and Q between 19°S and 20°S, that is correlated with reduced Quaternary arc volcanism. In the Eastern Cordillera, shallow mantle velocity and Q generally decrease from Altiplano values, but there is a localized low velocity and Q anomaly (Vp∼7.8, Vs∼4.1, Qp∼50, Qs∼10) underneath the Los Frailes volcanic center. The strong velocity and attenuation anomalies and the spatial correlation with the volcanic complex favor an interpretation of partial melt. From the subduction trench into parts of the Western Cordillera, processes associated with Nazca Plate subduction dominate shallow mantle structure. Structural constrains from the tomographic results and the geologic history of the Bolivian Andes favor a model of lithospheric shortening for the development of shallow-mantle structure in the Eastern Cordillera and Altiplano. A delamination/partial-continental-subduction process is favored for the production of both shallow mantle structure and volcanism in the Eastern Cordillera. This process may remove mafic components in thickened lower crust, refining the crust towards a more felsic composition.