GLOBAL SURFACE HEAT-FLUX ANOMALIES FROM SEISMIC TOMOGRAPHY-BASED MODELS OF MANTLE FLOW - IMPLICATIONS FOR MANTLE CONVECTION

We investigate the very long-wavelength, global pattern of surface hea t flux anomalies within the context of whole-mantle and layered-mantle anelastically compressible internal loading theories. Since the inter nal loading framework does not yield a direct estimate of the geotherm , we argue that accurate predictions for the surface heat flux may nev ertheless be obtained by assuming that it is linearly related to the r adial component of flow velocity at shallow depth in the mantle. The m antle convective circulation is assumed to be driven by density hetero geneity inferred from global seismic tomography models. Best results f or the pattern of surface heat flux anomalies are obtained for models that significantly impede the circulation at a depth of 670 km. Total variance reductions of 60-65% (degree 1-5) are obtained when the visco sity profile includes a low-viscosity asthenosphere. Within the contex t of our modeling assumptions, however, whole-mantle circulation model s provide best descriptions of the long-wavelength nonhydrostatic grav ity data. In order to resolve the gravity-heat flux impasse that is re vealed herein, we consider the possibility of modifying the a priori g lobal seismic models employed in the calculations. We show that the ri gidly layered-mantle internal loading theory is equivalent to a theory in which no explicit flow-blocking boundary condition is imposed at 6 70 km but in which the buoyancy field inferred from the a priori tomog raphic model is supplemented by flow-blocking heterogeneity in the for m of an appropriately constrained sheet mass load. We develop a genera l mathematical formalism describing how the introduction of appropriat ely constrained sheet mass loads allows the exact reconciliation of a number of a priori constraints or hypotheses concerning the structure of the circulation. Using this formalism, we explore the extreme nonun iqueness that not only characterizes internal loading theory inference s of the depth profile of mantle viscosity but also inferences of the radial style of the circulation. On this basis, we suggest that great caution is warranted with respect to tomography-based inferences of ma ntle properties. Based on a viscosity profile whose depth dependence i s close to that independently inferred within the context of postglaci al rebound studies, we present plausible resolutions of the gravity-he at flux impasse effected either within the framework of whole-mantle o r layered-mantle circulation models.