A geodynamic model of mantle density heterogeneity

Abstract

Using Cenozoic and Mesozoic plate motion reconstructions, we derive a model of present-day mantle density heterogeneity under the assumption that subducted slabs sink vertically into the mantle. The thermal buoyancy of these slabs is estimated from the observed thermal subsidence (cooling) of oceanic lithosphere. Slat) velocities in the upper mantle are computed from the local convergence rate. We assume that slabs cross the upper/lower mantle interface and continue sinking into the lower mantle witIx a reduced velocity. For a velocity reduction factor between :2 and 5, our slab heterogeneity model is as correlated with current tomographic models as these models are correlated with each other. We have also computed a synthetic geoid from our density model. For a viscosity increase of about a factor of 40 from the upper to lower mantle, our model predicts the first 8 spherical harmonic degrees of the geoid witIx statistical confidence larger than 95% and explains 84% of the observed geoid assuming that the model C21 and S21 terms are absent due to a long relaxation time for Earth's rotational bulge. Otherwise, 73% of the geoid variance is explained. The viscosity increase is consistent witIx our velocity reduction factor for slabs entering the lower mantle, since downwelling velocities are expected to scale roughly as the logarithm of viscosity (loge 40 -3.7). These results show that the history of plate tectonics can explain the main features of the present-day structure of the mantle. The dynamic topography induced by this heterogeneity structure consists mainly of about 1-kin amplitude lows concentrated along the active continental margins of the Pacific basin. Our model can also be used to predict the time variation of mantle heterogeneity and the gravity field. We find that the "age" of the geoid, defined as the time in the past herore which the geoid becomes uncorrelated witIx the present geoid, is about 50 m.y. Our model for the history of the degree 2 geoid, which is equivalent to the history of the inertia tensor, should give us a tool to study the variations in Earth's rotation pole indicated in paleomagnetic studies.

Towards a quantitative interpretation of global seismic tomography

“…Heterogeneity in the upper-mantle transition zone (between the seismic discontinuities near 410-and 660-km depth) and near the base of the mantle is manifest at very long wavelengths (Plate 2), with a predominance of fast wave propagation in the mantle beneath the circum-Pacific "Ring of Fire" and slow speeds beneath Africa and the central Pacific. This structure explains the long-wavelength variations in the gravity field [Richards and Hager, 1984;Hager et al, 1985;Cazenave et al, 1989] and correlates well with sites of post-Mesozoic subduction (recycling) of oceanic lithosphere [Richards and Engebretson, 1992;Ricard et al, 1993]. The shallow and lowermost mantles are marked by relatively strong heterogeneity, but the inferred magnitude of elastic heterogeneity decreases away from these boundary regions.…”

Plate tectonics conserves angular momentum

“…Ricard et al, 1993;Bercovici et al, 2000). Although the concept of plate tectonics was new, there was recognition that there exists a vast store of geologic information about past periods of mountain building and the changing patterns of Earth deformations.…”

Great earthquakes and slab pull: interaction between seismic coupling and plate–slab coupling

“…Because Conrad and Lithgow-Bertelloni [13] found that only slabs in the upper mantle contribute to the slab pull force, we sum the excess weight of all upper mantle slab material that is part of a continuous history of subduction at each subduction zone. We use a model of slab locations [11,36] that is derived from estimates of Cenozoic and Mesozoic plate motions [11,37]. We then calculate the slab suction forces on each plate from a model of instantaneous mantle £ow driven by lower mantle slabs [10,11].…”