Deflating the LLSVPs: bundles of mantle thermochemical hot instabilities rather than thick stagnant "piles"

The large-low shear velocity provinces (LLSVPs) present at the base of the Earth's mantle beneath the Pacific and Africa cover about 25% of the core-mantle boundary. At long-wavelength, they appear as compact, uniform structures, and have often been interpreted as hot but denser thermochemical piles. In contrast, based on the higher resolution SEMUCB-WM1 tomographic model and fluid mechanics constraints, we show that LLSVPs are in fact bundles of hot thermochemical instabilities. Each LLSVPs contains roots of a number of well-separated, low velocity conduits that extend vertically throughout most of the lower mantle. However, their thicker size, uneven amplitude and sometimes contorted shapes do not belong to the classical purely thermal plumes. Instead, they can be explained by the presence of compositional heterogeneities in the deep lower mantle, which strongly influence the development, shape, and time-dependence of hot instabilities there. Comparing tomographic images, completed by gravity informations and surface volcanism evolution wherever possible, with an experimental data base of thermochemical instabilities shapes and evolution, we can begin to infer the magnitude of the compositional density anomaly relative to the density anomaly of thermal origin in the deep mantle. The diversity of shapes and evolutions suggests a large distribution of rather small compositional density contrasts. This could result from convective stirring of several mantle components. Moreover it shows that far from being stagnant zones, LLSVPs are the siege of active upwellings and downwellings.