We present a table giving the “present-day” (average over most recent ∼5 m.y.) azimuths of tracks for fifty-seven hotspots, distributed on all major plates. Estimates of the azimuth errors and the present-day rates for those tracks with age control are also given. An electronic supplement contains a discussion of each track and references to the data sources. Using this table, the best global solution for plates moving in a fixed hotspot reference frame has the Pacific plate rotating about a pole at 59.33°N, 85.10°W with a rate that gives a velocity at this pole's equator of 89.20 mm/yr (–0.8029 °/m.y.). Errors in this pole location and rate are on the order of ±2°N, ±4°W, and ±3 mm/yr, respectively. The motions of other plates are related to this through the NUVEL-1A model. The large number of close, very short tracks in the Pacific superswell region precludes all hotspots being rooted near the core-mantle boundary. In general, we think the asthenosphere is hotter than the mantle just below it (in the sense of potential temperature). Asthenosphere is very hot—it is brought up from the core-mantle boundary by plumes. The mantle is cooled by downgoing slabs, and a convective stability is established whereby mantle rises only at plumes and sinks only at trenches. We propose that this normal mantle geotherm is overwhelmed by much-larger-than-average mantle upwelling in superswell areas, making many short-lived instabilities in the upper mantle. Because soft asthenosphere so decouples plates from the mantle below, instabilities in the upper mantle (even above the 660-km discontinuity) are relatively fixed in comparison to plate motions. With the mantle velocity contribution being minor, tracks are parallel to and have rates set by plate velocities.

This study explores a conceptual model for mantle convection in which buoyant and low-viscosity asthenosphere is present beneath the relatively thin lithosphere of ocean basins and regions of active continental deformation, but is less well developed beneath thicker-keeled continental cratons. We start by summarizing the concept of a buoyant plume-fed asthenosphere and the alternative implications this framework has for the roles of compositional and thermal lithosphere. We then describe the sinks of asthenosphere made by forming compositional lithosphere at ridges, by plate cooling wherever the thermal boundary layer extends beneath the compositional lithosphere, and by drag-down of buoyant asthenosphere along the sides of subducting slabs. We also review the implied origin of hotspot swell roots by melt-extraction from the hottest portions of upwelling plumes, analogous to the generation of compositional lithosphere by melt-extraction beneath a spreading center. The plume-fed asthenosphere hypothesis requires an alternative to “distinct source reservoirs” to explain the differing trace element and isotopic characteristics of ocean island basalt (OIB) and mid-ocean ridge basalt (MORB) sources; it does so by having the MORB source be the plum-depleted and buoyant asthenospheric leftovers from progressive melt-extraction within upwelling plumes, while the preferential melting and melt-extraction of more-enriched plum components is what makes OIB of a given hotspot typically fall within a tubelike geometric isotope topology characteristic of that hotspot. (The distinct plum components result from the subduction of chemically differing sediments, basalts, and residues to hotspot and mid-ocean ridge melt extraction.) Using this conceptual framework, we construct a thin-spherical-shell finite element model with a ∼100-km-scale mesh to explore the possible structure of global asthenosphere flow. Lubrication theory approximations are used to solve for the flow profile in the vertical direction. We assess the correlations between predicted flow and geophysical observations, and conclude by noting current limitations in the model and the reason why we currently neglect the influence of subcontinental plume upwelling for global asthenosphere flow.

Asthenosphere plume-to-ridge flow has often been proposed to explain both the existence of geochemical anomalies at the mid-ocean ridge segments nearest an off-axis hotspot and the existence of apparent geochemical provinces within the global mid-ocean spreading system. We have constructed a thin-spherical-shell finite element model to explore the possible structure of global asthenosphere flow and to determine whether plume-fed asthenosphere flow is compatible with present-day geochemical and geophysical observations. The assumptions behind the physical flow model are described in the companion paper to this study. Despite its oversimplifications (especially the steady-state assumption), Atlantic, Indian, and Pacific mid-ocean ridge isotope geochemistry can be fit well at medium and long wavelengths by the predicted global asthenosphere flow pattern from distinct plume sources. The model suggests that the rapidly northward-moving southern margin of Australia, not the Australia-Antarctic discordance, is the convergence zone for much plume material in the southern hemisphere. It also suggests a possible link between the strike of asthenosphere flow with respect to a ridge axis and along-axis isotopic peaks.

During the roughly year-long Seismic Wave Exploration in the Lower Lithosphere (SWELL) pilot experiment in 1997/1998, eight ocean bottom instruments deployed to the southwest of the Hawaiian Islands recorded teleseismic Rayleigh waves with periods between 15 and 70 s. Such data are capable of resolving structural variations in the oceanic lithosphere and upper asthenosphere and therefore help understand the mechanism that supports the Hawaiian Swell relief. The pilot experiment was a technical as well as a scientific feasibility study and consisted of a hexagonal array of Scripps Low-Cost Hardware for Earth Applications and Physical Oceanography (L-CHEAPO) instruments using differential pressure sensors. The analysis of eighty-four earthquakes provided numerous high-precision phase velocity curves over an un-precedentedly wide period range. We find a rather uniform (unaltered) lid at the top of the lithosphere that is underlain by a strongly heterogeneous lower lithosphere and upper asthenosphere. Strong slow anomalies appear within ∼300 km of the island chain and indicate that the lithosphere has most likely been altered by the same process that causes the Hawaiian volcanism. The anomalies increase with depth and reach well into the asthenosphere, suggesting a sublithospheric dynamic source for the swell relief. The imaged velocity variations are consistent with thermal rejuvenation, but our array does not appear to have covered the melt-generating region of the Hawaiian hotspot.