Negative Radial Anisotropy Absent Below Cratons: Insights from Bayesian Inversion


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  • Global radial anisotropic models are inconsistent meaning geological interpretation of active/ancient mantle flow/deformation is challenging.
  • One recent interpretation (Priestley et al., 2020) suggests that negative radial anisotropy in the CAM2016 model at ~150km depth within cratons reflects their formation by horizontal shortening/vertical thickening. This mechanism is not easily reconciled with other available models however.
  • Using variable parameterizations within both LSQR and Bayesian inversions of Rayleigh and Love (R&L) surface wave dispersion curves, we test whether negative radial anisotropy is reliably recovered at upper mantle depths using synthetic models (Section 3) and whether anisotropic anomalies are required below cratons using real data inversions (Section 4).
  • Both algorithmic and parameterization choices can affect ‘recovered’ radial anisotropy (Xi) meaning existing geological interpretations may be biased.

Take Home Messages

  1. Negative Xi below cratons (~150km depth), reproduced using LSQR inversion, is likely an artefact…
  2. Bayesian inversion with free VPH parameterization yields no negative radial anisotropy below cratons, only +5-10% Xi above 150km depth, similar to PREM.
  3. Episodic craton formation (e.g., Yuan & Romanowicz 2010; Darbyshire et al., 2013) preferred over horizontal shortening (Priestley et al., 2020).
  4. Set VPH free in surface wave inversions, preferably with Bayesian algorithms!