High-resolution imaging of lowermost mantle structure under the Cocos plate

Broadband seismic shear waves are analyzed to investigate the fine-velocity structure in the lowermost mantle beneath the Cocos plate, a region where previous studies have indicated the presence of a shear velocity increase about 200-300 km above the core-mantle boundary. Data from 14 South American earthquakes recorded at California broadband networks provide dense ray path sampling of the lowermost mantle in an approximately 700 km long north-south corridor, roughly 150 km wide. Application of a simplified seismic migration method that uses a homogeneous background velocity model suggests topography of the previously imaged positive impedance jump, varying in depth from north to south by as much as 150 km, with a weakly reflecting transition zone in between. The migration approach enables examination of small-scale spatial variations and out-of-plane scattering effects. Topography of the discontinuity may account for observed variations in the amplitude of reflected arrivals or there may be lateral variations in the velocity contrast across the boundary. Lateral variations of the shear velocity structure within the D″ layer may produce some apparent topography in the discontinuity image, but any such volumetric structure is not yet well enough determined to incorporate in the migration. A localized negative impedance contrast reflector or scatterer is imaged at depths about 100 km below the positive reflector in the northern portion of the study area. Several scenarios can explain these results, including (1) a slab that reaches the lowermost mantle, (2) the birth of an upwelling beneath a recumbent slab, or (3) chemical layering in this region.