Impact of the Large Low Shear Velocity Provinces in the deep mantle on long-term mantle dynamics

Category Geophysic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Beuchert, M.J.۱; Podladchikov, Y.Y.۱; Simon, N.S.C.۱; Torsvik, T.H.۲
Holding Date 21 September 2008

Recent studies have shown that the seismically observed Large Low Shear Velocity Provinces (LLSVPs) in the lower mantle under the African continent and the Pacific basin have remained stationary for at least 300 million years. This is based on reconstructing the location of Large Igneous Provinces (LIPs) at the time of eruption using both hotspot and palaeomagnetic reference frames (removing the effect of plate tectonics) and the observation that most of the LIP-forming plumes appear to originate from the edges of the seismically observed LLSVPs in the deep mantle.
The low seismic velocities imply anomalously high temperatures in the LLSVPs, a fact that can only be reconciled with their long-term gravitational stability if these regions are compositionally denser than the surrounding mantle or, alternatively, contain fractions of partial melt. Melt is known to be denser than solid at high pressures due to its higher compressibility. Such a dense partial melt fraction is commonly assumed to be present in the Ultra Low Velocity Zone (ULVZ) near the core-mantle-boundary (CMB) and this might as well be the case for the LLSVPs.
The long wavelength heterogeneity introduced by the LLSVPs has a strong impact on the patterns of mantle convection. We investigate this effect in a new numerical model. It is most intriguing, from a dynamical point of view, that the LLSVPs have remained in an equatorial and antipodal position throughout such long geological times. The question is whether centrifugal forces play a role in stabilizing the LLSVPs in their position, given the existence of low viscous, dense material in the ULVZ and possibly in the LLSVPs.
The influence of the LLSVPs on mantle convection might provide an explanation for the yet unexplained long amplitude pattern of plate tectonics.