The heat flow constraint on mantle tomography‐based convection models: Towards a geodynamically self‐consistent inference of mantle viscosity

Abstract

We present a detailed study of the advective contribution to the radial flow of heat in the mantle as deduced using a compressible internal loading theory in which the flow is assumed to be driven by the density heterogeneities implied by recent global seismic tomographic models. We calculate the radial flow velocity response of a viscous mantle and find that for reasonable values of the parameters which enter the theory, the heat flow correlation integral delivers the correct areaintegrated value of the heat flow observed at Earth's surface. This result is unlikely to be significantly affected by the low-degree truncation of the tomographic models we employ as in both the upper mantle and lower part of the lower mantle, heat is primarily transported by the degree 2 components of the flow. We propose that the radial profile of heat advection is a particularly useful diagnostic with which to "prospect" for the existence of thermal boundary layers in the mantle. For split mantle tomographic models, we find a sharp drop in advected heat at a depth of 670 kin. We investigate in detail the theological consequences of a circulation that is layered at this depth by the action of the endothermic phase transformation of spinel to a mixture of perovskite and magnesiowiistite. On physical grounds this is expected to lead to the development of a dipolar viscosity structure centered on the internal thermal boundary layer. We investigate the impact that such a structure has on the predicted aspherical geoid. A sequence of forward modeling calculations of this geophysical observable demonstrates that a viscosity profile which includes a dipolaf structure centered at 670 km depth and a significant increase of viscosity below mantle depths of 1200-1500 km optima!ly reconciles the long-wavelength GEM-T2 observations. The increase in viscosity in the lower part of the lower mantle is also required by the heat flow data whereas the introduction of the dip olaf structure in the viscosity profile allows the upper mantle value of the viscosity which is close to that inferred by Haskell of 10 •'• Pa s to be continued to a depth of 1200-1500 km in accord with the requirements of recent postglacial rebound inferences. In the context of a whole mantle model of the circulation, we axe also able to accommodate the constraints imposed by the data by introducing only a low-viscosity notch at 670 km depth rather than the dipolaf structure. The viscosity models derived herein therefore provide a fully self-consistent reconciliation of these distinctly different geodynamic data and would appear to resolve a previously unresolved conflict between them. Recent numerical simulations of mantleconvection which incorporate the effects of the endothermic phase transformation of ff-spinel to a mixture of perovskite and magnesiowfistite at the depth of the 670-kin seismic discontinuity have revealed a style of circulation that is episodically layered [Macbetel and Weber, 1991; Peltier Paper number 95JB01078. 0148-0227/95 / 95J B-010785 05.00 and Solhelm, ...

Regional Correlation Analysis between Seismic Heterogeneity in the Lower Mantle and Subduction in the Last 180 Myr: Implications for Mantle Dynamics and Rheology

“…Recent investigation of the grain-size evolution in the subducted oceanic lithosphere (RIEDEL and KARATO, 1997) indicates that the strength of the slabs depends on the temperature in a very unusual way, and that the resultant rheological weakening may be especially significant for cold and fast slabs. If the major viscosity jump in the mantle is not associated with the 660-km discontinuity but is located somewhat deeper in the lower mantle (FORTE and PELTIER, 1991;PARI and PELTIER, 1995;FORTE and MITROVICA, 1996;Č ÍŽ KOVÁ et al, 1996), then it is quite natural that the slabs in the West Pacific, which are fast and cold, are not able to penetrate through the viscosity interface and accumulate above it. In contrast, the fast slabs in the East Pacific penetrate to the mid-mantle because they are warmer and, thus, less affected by the rheological weakening.…”

Global surface heat flux anomalies from seismic tomography‐based models of mantle flow: Implications for mantle convection

“…Consider, for the sake of discussion, the case in which the delta function load is an approximate order of magnitude through the lower mantle, is given by Pari and Peltier [1995]. A feature of this profile that has not been considered in the previous work of Pari and Peltier, however, is the 75 km thick low-viscosity asthenosphere which appears immediately beneath the high-viscosity lithosphere.…”

Radial profiles of temperature and viscosity in the Earth's mantle inferred from the geoid and lateral seismic structure

“…The gravity signal generated by topographies of these interfaces contributes to the observed geoid and its sign and magnitude strongly depend on the viscosity structure [9]. The attempts to infer the radial profile of viscosity from the geoid date back to the mid-eighties [3,5,[11][12][13][14][15][16][17][18]. In this paper we follow the traditional inversion scheme described, e.g., by King [12] to determine the viscosity profile parameterized as described above.…”