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.

A scheme of deep mantle convection is proposed in which narrow plumes of deep material rise and then spread out radially in the asthenosphere. These vertical plumes spreading outward in the asthenosphere produce stresses on the bottoms of the lithospheric plates, causing them to move and thus providing the driving mechanism for continental drift. One such plume is beneath Iceland, and the outpouring of unusual lava at this spot produced the submarine ridge between Greenland and Great Britain as the Atlantic opened up. It is concluded that all the aseismic ridges, for example, the Walvis Ridge, the Ninetyeast Ridge, the Tuamotu Archipelago, and so on, were produced in this manner, and thus their strikes show the direction the plates were moving as they were formed. Another plume is beneath Hawaii (perhaps of lesser strength, as it has not torn the Pacific plate apart), and the Hawaiian Islands and Emperor Seamount Chain were formed as the Pacific plate passed over this “hot spot.” Three studies are presented to support the above conclusion. (1) The Hawaiian-Emperor, Tuamotu-Line, and Austral-Gilbert-Marshall island chains show a remarkable parallelism and all three can be generated by the same motion of the Pacific plate over three fixed hot spots. The accuracy of the fit shows that the hot spots have remained practically fixed relative to one another in this 100 m.y. period, thus implying a deep source below the asthenosphere. (2) The above motion of the Pacific plate agrees with the paleo-reconstruction based on magnetic studies of Pacific seamounts. The paleomotion of the African plate was deduced from the Walvis Ridge and trends from Bouvet, Reunion, and Ascension Islands. This motion did not agree well with the paleomagnetic studies of the orientation of Africa since the Cretaceous; however, better agreement with the paleomagnetic studies of Africa and of seamounts in the Pacific can be made if some polar wandering is permitted in addition to the motion of the plates. (3) A system of absolute plate motions was found which agrees with the present day relative plate motions (deduced from fault strikes and spreading rates) and with the present trends of island chains-aseismic ridges away from hot-spots. This shows that the hot spots form a fixed reference frame and that, within allowable errors, the hot spots do not move about in this frame.