Abstract
Two large areas of anomalously low seismic velocities are visible in all tomographic models of the lowermost mantle. Depending on the density structure of these Large Low Shear Velocity Provinces (LLSVPs), the core-mantle boundary (CMB) will deform upwards or downwards due to isostatic and dynamic topography, the latter being sensitive to the viscosity structure of the lowermost mantle. Heterogeneities in the viscosity structure, although difficult to constrain, might be especially important if the LLSVPs are thermochemical piles with elevated intrinsic viscosity as suggested by mineral physics. Based on numerical models, we identify a short-wavelength (about 80-120 km wide, up to a few km deep) topographic depression that forms around the pile edges if the pile is more viscous than the surrounding mantle. The depression forms when a wedge of thermal boundary layer material becomes compressed against the viscous pile, and is enhanced by relative uplift of the CMB beneath the pile by plumes rising above it. The depth and asymmetry of the depression constrain the magnitude of the viscosity contrast between pile and the surrounding mantle. Furthermore, (periodic) plume initiation and pile collapse at the pile margin systematically modify the characteristic depression, with a maximum in asymmetry and depth at the time of plume initiation. Core-reflected waves or scattered energy may be used to detect this topographic signature of stiff thermochemical piles at the base of the mantle.
Highlights
Geodynamic studies have shown that low spherical harmonic degrees cannot distinguish between positive buoyancy-related topography generated by thermal plume clusters or thermochemical Large Low Shear Velocity Provinces (LLSVPs) with less than 1.5% intrinsic density excess, whereas piles with excess density higher than 1.8-2% cause a depression of the core-mantle boundary (CMB) (Lassak et al, 2007; Steinberger and Holme, 2008; Yoshida, 2008; Lassak et al, 2010; Deschamps et al, 2017; Deschamps and Li, 2019)
We present numerical models to predict shortwavelength topography, showing that topographic length scales of about 100 km are sensitive to a potential viscosity contrast between thermochemical piles and the ambient mantle
A wedge of thermal boundary layer (TBL) material becomes compressed against a stiff pile and pushes the CMB downwards at the pile edge
Summary
The presence of a dominant degree-2 structure in seismic velocities of the lowermost mantle (Fig. 1) is well established (e.g., Dziewonski et al, 2010), the origins, and the properties, of the Large Low Shear Velocity Provinces (LLSVPs) are still under debate (e.g., McNamara and Zhong, 2004; Davies et al, 2012; Li et al, 2014; Mulyukova et al, 2015; Trønnes et al, 2019). Geodynamic studies have shown that low spherical harmonic degrees cannot distinguish between positive buoyancy-related topography generated by thermal plume clusters or thermochemical LLSVPs with less than 1.5% intrinsic density excess, whereas piles with excess density higher than 1.8-2% cause a depression of the CMB (Lassak et al, 2007; Steinberger and Holme, 2008; Yoshida, 2008; Lassak et al, 2010; Deschamps et al, 2017; Deschamps and Li, 2019). It may allow for the identification of plumes at early stages
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