Oceanic subduction and craton underthrusting beneath the Bolivian orocline in Central Andes: Insight from anisotropic tomography

Abstract

To clarify the 3-D crustal and upper mantle structure of the Bolivian orocline in the Central Andes, we conduct azimuthal anisotropy tomography using newly measured teleseismic fundamental mode Rayleigh-wave phase and amplitude data at periods of 25-110 s. Our tomography shows that the subducting Nazca slab is imaged as a high-velocity zone beneath the study region, except for areas where the Nazca ridge is subducting. Azimuthal anisotropy in the subducting slab generally exhibits trench-parallel fast-velocity directions (FVDs), but it becomes complex in and around the subducting Nazca ridge. Low-velocity anomalies with trench-normal FVDs exist in the mantle wedge beneath active arc volcanoes and backarc regions beneath Altiplano. A significant high-velocity zone with relatively weak anisotropy exists in the crust of the overriding plate above the Peruvian flat slab in the study region. In contrast, low-velocity anomalies with trench-parallel FVDs are revealed in the crust beneath Altiplano. Furthermore, a high-velocity zone with depth-varying FVDs appears beneath Eastern Cordillera and its surrounding regions, which may indicate the westwards underthrusting cratonic lithosphere. These tomographic features well capture the primary 3-D structure of the middle-lower crust and upper mantle beneath the Bolivian orocline, which results from the subduction of an oceanic lithosphere and the delamination and underthrusting of a continental lithosphere, leading to the second-highest plateau on Earth.