Geometries, structural features and velocity structures of the Pacific Anomaly and comparison with the African Anomaly (Invited)

Seismic tomography has revealed two large low shear velocity anomalies in the Earth's lower mantle beneath the Pacific and Africa (we term them the Pacific Anomaly and the African Anomaly). For further understanding the origin and evolution of these anomalies, we map their geometries, structural features and velocity structures in detail. Previous waveform modeling and travel time analysis revealed that the African Anomaly exhibits a bell-like feature in the lower mantle extending to 1300 km above the core mantle boundary (CMB), and steeply dipping edges, rapidly varying thicknesses and a strong negative shear velocity gradient from -2% (top) to -9% to -12% (bottom) in the lowermost 300 km of the mantle. All those features clearly indicate that the African Anomaly is chemically distinct and geologically stable. The Pacific Anomaly, on the other hand, shows quite different features from those of the African Anomaly. Differential travel times indicate that the average shear velocity reduction inside the anomaly reaches -6% in the lowermost 300 km of the mantle and small-scale velocity heterogeneities exist inside the anomaly. On the basis of forward travel time and waveform modeling, the seismic data sampling a great arc across the anomaly from eastern Asia to southern South America suggest that the Pacific Anomaly along the great arc consists of at least two separated portions with a 740-km wide gap between them. The western portion of the anomaly extends about 700 km above the CMB and exhibits a trapezoidal shape with a negative shear velocity gradient from -3% (top) to -5% (bottom) in the lowermost 100 km of the mantle. The eastern portion of the anomaly has an 1800-km wide base and reaches at least 340 km above the CMB beneath the mid-Pacific with a uniform velocity reduction of -3%. Waveform modeling further suggests a high-velocity layer adjacent to the northwestern edge of the anomaly. The detailed structures of the transition from the base of the Pacific Anomaly to the northern high velocity zone are constrained along three perpendicular cross sections, with one across the Anomaly from New Zealand to Alaska and the others from Solomon Islands to North America. Waveform analysis suggests that the northern boundary is best characterized by a shear velocity model with low-velocity structure reaching 300 km above the CMB with its shallowly dipping edge extending beneath the surrounding high-velocity region. While the structural and velocity features of the African Anomaly can be best explained by a chemical anomaly produced early in the Earth's history and largely stabilized over the history, the complex morphology of the Pacific Anomaly, though clearly indicating a compositional origin, suggests several possible origins for the anomaly. It may similarly be a chemical anomaly produced in the Earth's history as the African Anomaly but with less intrinsic density buoyancy; it may also be accumulation of subducted oceanic crust or a mix of a primordial chemical anomaly with the subsequent accumulation of subducted oceanic crust, with the geometric features modified by mantle flow.