Global tomographic images of mantle plumes and subducting slabs: insight into deep Earth dynamics

A new model of whole mantle P-wave tomography is determined with a novel approach. A grid parameterization instead of blocks and spherical harmonics is adopted to express the Earth structure. Depth variations of the Moho, 410 and 660 km discontinuities are taken into account in the inversion. Ray paths and travel times are computed with an efficient 3-D ray tracing scheme. This new approach was applied to a large data set of ISC (International Seismological Center) travel times (P, PP, PcP, pP, Pdiff) to determine a whole mantle P-wave tomography. For the shallow mantle, the new model contains the general features observed in the previous models: a low-velocity ring around the Pacific Ocean basins and high-velocity anomalies under the old and stable continents in the depth range of 0-300 km. One significant difference from the previous models is that stronger and wider high-velocity anomalies are visible in the transition zone depths under the subduction regions, which suggests that most of the slab materials are stagnant in the transition zone before finally collapsing down to the lower mantle as a result of very large gravitational instability from phase transitions. Very slow anomalies exist in the upper mantle right beneath the Wudalianchi and Changbai active volcanoes in Eastern China, right above the stagnant Pacific slab in the transition zone, suggesting that the origin of the intraplate volcanism in East Asia is closely related to the Pacific plate subduction process, such as deep slab dehydration and convective circulation in the mantle wedge. Plume-like slow anomalies are clearly visible under the major hotspot regions in most parts of the mantle, in particular, under Hawaii, Iceland, South Pacific and Africa. The slow anomalies under South Pacific and Africa have lateral extensions of over one thousand kilometers and exist in the entire mantle, representing two superplumes. The Pacific superplume has a larger spatial extent and stronger slow anomalies than that of the Africa superplume. The Hawaiian plume is not part of the Pacific superplume. The slow anomalies under hotspots usually do not show a straight pillar shape, but exhibit winding images, suggesting that plumes are not fixed in the mantle but can be deflected by the mantle flow. As a consequence, hotspots are not really fixed but can wander on the Earth's surface, as evidenced by the recent paleomagnetic and numeric modeling studies. Wider and more prominent slow anomalies are visible at the core-mantle boundary (CMB) than most of the lower mantle, and there is a good correlation between the distribution of slow anomalies at the CMB and that of hotspots on the surface, suggesting that most of the strong mantle plumes under the hotspots originate from the CMB. However, there are some small-scaled, weak plumes originating from the transition zone or mid mantle depths. (C) 2004 Elsevier B.V. All rights reserved.