Regionalized Properties of the Lowermost Mantle From Spherical Slepian Analysis

Date: 4/25/2019

Time: 06:00 PM

Room: Grand Ballroom

Lower mantle structure is dominated by a pair of large, antipodal, low shear velocity provinces (LLSVPs) located beneath Africa and the Pacific Ocean. Though LLSVPs are dominantly long-wavelength (degree 2) features imaged in earliest tomographic models, their nature and origin remain enigmatic. A number of hypotheses have been proposed to address their origin, summarized by two end-member scenarios: (1) they represent thermochemical piles that are either primordial or have grown over time, such as by the accumulation of subducted oceanic lithosphere, (2) they are purely thermal features, seen through the lens of tomographic imaging.

Characterizing smaller-scale structures within and outside the LLSVPs may yield insights allowing us to distinguish between these two interpretations. Therefore, we compare the amplitude and length-scales of velocity heterogeneities within and outside the LLSVPs and analyze their variation with height above the core-mantle boundary. This requires estimating the wavenumber spectrum of heterogeneity by localizing it from a global tomographic model. Using spatially abrupt windowing functions leads to unreliable spectral estimates due to their non-compact spatiospectral concentration. We adopt a spherical Slepian approach that balances the trade-off between spatial localization and spectral leakage. For an optimal Slepian taper, the useful bandwidth is controlled by a trade-off between tomographic-model resolution, taper-bandwidth, and LLSVP surface-area. We divide the lower mantle into 6 regions enabling us to extend the useful spectral bandwidth and compare heterogeneity across LLSVP and non-LLSVP regions. We identify robust features by quantitatively assessing the similarities and differences in the spectrum of heterogeneity across a collection of global tomographic models. We show that structure within the African LLSVP is more vertically continuous that in the Pacific.

Presenting Author: Tolulope M. Olugboji

Authors