ALBERT

Description: Mantle convection is a fundamental planetary process. Its plate mode is established and expressed by plate tectonics. Its plume mode also is established and expressed by interregional geological patterns. We developed both an event-based stratigraphic framework to illustrate the surface effects predicted by the plume model of Griffiths et al. (1989) and Griffiths and Campbell (1990) and a methodology to analyze continent-scale geological maps based on unconformities and hiatuses. The surface expression of ascending plumes lasts for tens-of-millions-of-years and rates vary over a few million years. As the plume ascends, its surface expression narrows, but increases in amplitude, leaving distinct geological and stratigraphic patterns in the geologic record, not only above the plume-head center, but also above its margins and in distal regions a few thousands-of-kilometers from the center. To visualize these patterns, we constructed sequential geological maps, chronostratigraphic sections, and hiatus diagrams. Dome-uplift with erosion (Şengör, 2001) and the flood basalts (Duncan and Richards, 1991; Ernst and Buchan, 2001a) are diagnostic starting points for plume-stratigraphic analyses. Mechanical collapse of the dome results in narrow rifting (Burke and Dewey, 1973), drainage-network reorganization (Cox, 1989), and flood-basalt eruption. In the marginal region, patterns of vertical movement, deformation and surface response are transient and complex. At first, the plume margin is uplifted together with the central region, but then it subsides as the plume ascents farther; With plume-head flattening, the plume margin experiences renewed outward-migrating surface uplift, erosion, broad crustal faulting, and drainage reorganization. Knickpoint migration occurs first inward-directed at ½ the rate of plume ascent and later outward-directed at the rate of asthenospheric flow. Interregional-scale unconformity-bounded stratigraphic successions document the two inversions. The distal regions, which did not experience any plume-related uplift, yield complete sedimentary records of the event; Event-related time gaps (hiatuses) in the sedimentary record increase towards the center, but the event horizon is best preserved in the distal region; it may be recognized by tracing its contacts from the center outwards. We extracted system- and series-hiatuses from interregional geological maps and built hiatus maps as proxies for paleo-dynamic topography and as a basis for comparison with results from numerical models. Interregional-scale geological maps are well suited to visualize plume-related geological records of dynamic topography in continental regions. However, geological records and hiatus information at the resolution of stages will be needed at interregional scales. The plume-stratigraphic framework is event-based, interregional, but not global, with time-dependent amplitudes that are significantly larger than those of global eustatic sea-level fluctuations. Global stratigraphic syntheses require integration of plate- and plume-stratigraphic frameworks before eustatic contributions may be assessed.

Description: Highlights • Hotspot tracks occur above fast flow anomalies in the asthenosphere. • Flow channels driven by broad plumes from the African LLSVP. • Zoned intraplate sub-tracks sample shallow convection from the top of LLSVP plumes. • Multiple pulsating African LLSVP plumes drive regional plate tectonics. Abstract The location and crustal structure of hotspot tracks in the South Atlantic reflect where melts related to sluggishly flowing plume material can reach the plate surface. This raises the paradox of how long-lived, age progressive hotspot tracks can arise in the absence of closely spaced, narrow mantle plumes. Here we show that young hotspot trails in the southern South Atlantic are located above bands of seismically slow material in the asthenosphere, which we interpret as channels of fast-flowing asthenosphere fed by a large scale plume upwelling from the African LLSVP. A broad region of seismically slow asthenosphere in the vicinity of Paraná continental flood basalts may be indicative of a long-lived, large scale plume under the South American plate. We propose that hotspot tracks developed above fast flow channels in the asthenosphere that evolved between these large-scale plumes as they migrated apart with the African and South American plates, respectively. A progression from continental flood basalts to broad aseismic ridges (e.g., Walvis Ridge-Rio Grande Rise), to low volume intraplate hotspot tracks (e.g., Tristan-Gough; Discovery; Shona and Bouvet) reflects the interplay between tectonic setting and asthenosphere flow channels driven by waning pulsations from these diverging LLSVP plumes. We link the splitting of the Walvis Ridge into isotopically distinct, age-progressive intraplate sub-tracks about 72 Ma to the first sampling of material rising from the African LLSVP plume, perhaps as weak shallow long-lived plumes. Faster flowing asthenosphere enables melts related to LLSVP plumes to reach the plate surface via spreading and tectonic boundaries, and as low-volume intraplate hotspot (sub)tracks. The concept that asthenosphere flow channels and hotspot tracks evolve together between pulsating deep-seated plumes under Africa and South America suggests that LLSVPs might be a significant force in driving continental rifting and (absolute) plate motion.