This work is summarized in the following abstract, submitted to Fall AGU 1995.
Authors: Stephen C. Myers, Susan Beck George Zandt, Terry Wallace
Examination of seismic body wave spectra for earthquakes occurring in the Nazca slab and recorded at regional distance on the broadband seismic instruments of the BANJO and SEDA seismic arrays in Bolivia and Chile reveals coherent variations in seismic attenuation. When back projected, the variations in seismic attenuation can be attributed to a high attenuation zone in the mantle underneath the Altiplano and western portion of the Eastern Cordillera of Bolivia. The back projection can be formalized by performing a tomographic inversion. The figure at the top of this page is the a cross-section though the tomographic model of the attenuation structure. The zone of high attenuation extends to a depth of about 120 km. Whether the zone of highly attenuating mantle (HAM) is in contact with the base of the 60- to 70-km-thick crust of the Altiplano and Eastern Cordillera is unresolved. Because the attenuation is greatly accentuated in the S-wave spectra, we speculate that the high attenuation results from a partial melt.
Shear waves propagate efficiently to stations in the Western Cordillera, indicating that partial melt is not extensive under the volcanic arc. The conspicuous absence of HAM under the volcanic arc brings into question whether the high attenuation zone is related to normal arc magmatism. Subduction-related counter flow is not a supported by the results of attenuation tomography. The area of HAM is floored by a high Q mantle (preliminary estimates give Qs in the 800 range), which would block inflow of asthenosphere. However, the observation that HAM extends under the Eastern Cordillera, but not under the Western Cordillera, suggests that the melt is better correlated with the crustal shortening in the Eastern Cordillera than the subduction-related volcanic arc. It is possible that the lithosphere has delaminated under the Altiplano, causing inflow of asthenosphere. Alternatively, partial subduction of lower crust of the Brazilian craton may trigger the release of volatiles and subsequent melting of the crust and surrounding mantle. The relative contribution of oceanic subduction processes and continental crustal/lithospheric shortening processes to the production of HAM (melt) is currently unresolved.
