• Main Authors: Ocean Here, Lianxing Wen B, Tianyu Zheng A
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 2006
• Subjects: shear wave velocity structure; core–mantle boundary; Pacific anomaly; compositional anomaly; African anomaly; very-low velocity province; Indian; Pacific; South Atlantic Ocean
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.507.3676; http://geophysics.geo.sunysb.edu/wen/Reprints/HeEA06EPSL.pdf

ar velocity jump of about 2 % at about 100–145km above the core–mantle boundary and a thin (30-km thick) basal layer with a shear Seismic results have consi stently shown two prom- anomaly ” and the “Pacific anomaly”. The geographic boundary, structural features and velocity structure of the “African anomaly ” have been extensively studied and Earth and Planetary Science Lettersinent low-velocity anomalies in the lower mantle, withwave velocity reduction of −13%. Stacked seismic data sampling the middle of the anomaly, however, show no evidence for any internal discontinuity with a velocity decrease greater than −2 % in the middle of the anomaly. Overall, the seismic data sampling the base of the “Pacific anomaly ” can be explained by a negative shear-velocity gradient from 0 % to −1 % (top) to −13 % (bottom) in the lowermost 220 km of the mantle, similar to those of a very-low velocity province beneath the South Atlantic Ocean and the Indian Ocean. Such a strong negative shear-velocity gradient can be explained by partial melting of a compositional anomaly produced early in the Earth's history located within a bottom thermal boundary layer. Our travel time data also exhibit small-scale