Joint seismic, geodynamic and mineral physical constraints on three-dimensional mantle heterogeneity: Implications for the relative importance of thermal versus compositional heterogeneity

Abstract

S U M M A R YThe joint interpretation of seismic and geodynamic data requires mineral physical parameters linking seismic velocity to density perturbations in the Earth's mantle. The most common approach is to link velocity and density through relative scaling or conversion factors: R ρ/s = dlnρ/dlnV S . However, the range of possible R ρ/s values remains large even when only considering thermal effects. We directly test the validity of several proposed depthdependent conversion profiles developed from mineral physics studies for thermally-varying properties of mantle materials. The tests are conducted by simultaneously inverting shear wave traveltime data and a diverse suite of convection-related constraints interpreted with viscous-flow response functions of the mantle. These geodynamic constraints are represented by surface spherical harmonic basis functions (up to harmonic degree 16) and they consist of the global free-air gravity field, tectonic plate divergences, dynamic surface topography and the excess ellipticity of the core-mantle boundary (CMB). The tests yield an optimum 1-D thermal R ρ/s profile that is generally compatible with all considered data, with the exception of the dynamic surface topography that is most sensitive to the shallow upper mantle. This result is not surprising given that cratonic roots are known to be compositionally-distinct from the surrounding ambient mantle. Moreover, it is expected that the temperature-dependence of the thermal R ρ/s in the upper mantle is significant due to the temperature-dependence of seismic attenuation or Q. Therefore, a simple 1-D density-velocity relationship is insufficient. To address this problem, we obtained independent estimates of the first-order correction factors to the selected R ρ/s profile within the cratonic roots and in the ambient (thermal) upper mantle. These correction factors, defined as ∂ R ρ/s /∂lnV S , are highly negative within the cratons signifying considerable compositional buoyancy. This result confirms that the negative buoyancy associated with the low temperatures in the cratons is significantly counteracted by the composition-induced positive buoyancy resulting in near-neutral buoyancy of the cratonic roots. Within the ambient upper mantle, the correction factors are found to be positive which is consistent with the expected behaviour of the R ρ/s relationship in thermally-varying upper-mantle material. We obtain a significantly greater reconciliation of the global gravity anomalies and dynamic surface topography when applying these laterally-varying corrections compared to a simple 1-D R ρ/s relationship. Inversion for a fully 3-D R ρ/s relationship reveals secondary effects including additional compositional variation within the cratonic roots and the deep-mantle superplume structures. We estimate the relative magnitude of the thermal and compositional (non-thermal) contributions to mantle density anomalies and conclude that thermal effects dominate shear wave and density heterogeneity throughout the non-cratonic mantle....

Anisotropy from SKS splitting across the Pacific-North America plate boundary offshore southern California

“…Below we discuss the effects of APM alone, buoyancy driven mantle flow alone, and full mantle convection from both APM and buoyancy driven flow. (Simmons et al 2009;Schmandt & Humphreys 2010, 2011Becker 2012), (c) HMSL-p06 (Houser et al 2008) and (d) SEMum (Lekić & Romanowicz 2011).…”

“…Four models are shown, calculated by subtracting the effects of density-buoyancy driven mantle flow (with no-slip boundary conditions) from a total model that has velocity boundary conditions of APM together with density-buoyancy driven mantle flow effects. APM effects computed from models that had density-buoyancy flow obtained from (a) model SAW642AN (Panning & Romanowicz 2006), (b) model SH11 TX2008 (Simmons et al 2009;Schmandt & Humphreys 2010, 2011Becker 2012), (c) model HMSL-p06 (Houser et al 2008) and (d) model SEMum (Lekić & Romanowicz 2011). Profile A-A shows location of cross-sections in Fig.…”

“…Becker et al 2003Becker et al , 2006. (Simmons et al 2009;Schmandt & Humphreys 2010, 2011Becker 2012), (c) model HMSL-p06 (Houser et al 2008) and (d) model SEMum (Lekić & Romanowicz 2011). Predicted anisotropy fast polarization directions together with SKS splitting directions (red vectors: this study; purple vectors: compiled by Becker et al 2012).…”

“…11. (Simmons et al 2009;Schmandt & Humphreys 2010, 2011Becker 2012), (c) model HMSL-p06 (Houser et al 2008) and (d) model SEMum (Lekić & Romanowicz 2011). computed by first calculating the full mantle flow field (including effects of buoyancy driven mantle flow) with imposed surface boundary conditions of APM [HS3-NUVEL-1A (Gripp & Gordon 2002)], and then second by subtracting out the effects of buoyancyrelated flow alone, computed using no-slip boundary conditions. This procedure yields identical results for plate motion effects, regardless of which tomography model is used in the original full calculation of mantle flow (Figs 6 and 7).…”

The limits of ray theory when measuring shear wave splitting in the lowermost mantle withScSwaves

“…In brief, the elastic constants were created by Walker et al (2011) using the flow field of the TX2008 model of Simmons et al (2009) to produce strain histories at each point on a 5 • -by-5 • longitudelatitude grid, 100 km above the CMB. They used estimates of postperovskite (ppv) deformation mechanisms, to perform viscoplastic self-consistent modelling along these strain histories, yielding the texture of a wholly-ppv lowermost mantle.…”