A Three Dimensional Radially Anisotropic Model of Shear Velocity in the Whole Mantle

Mark Panning and Barbara Romanowicz

Introduction

The 3D seismic velocity structure of the Earth's mantle represents a snapshot of its current thermal and chemical state. As tomographic models of the isotropic seismic velocity converge in their main features (Masters et al., 2000; Mégnin and Romanowicz, 2000; Ritsema and van Heijst, 2000; Gu et al., 2001), geodynamicists use them to infer the density structure, and thus the buoyancy contrasts which drive mantle convection (Hager, 1984; Ricard and Vigny, 1993; Daradich et al., 2003). This process, however, is complicated by the difficulty of separating thermal and chemical contrasts, and the lack of direct sensitivity of seismic velocities to the density contrasts which drive the convection.

In many regions of the mantle, analyzing the anisotropy of seismic velocities can give us another type of constraint on mantle dynamics. Nearly all the constituent minerals of the mantle have strongly anisotropic elastic properties on the microscopic scale. Random orientations of these crystals, though, tend to cancel out this anisotropy on the macroscopic scale observable by seismic waves, unless crystals or materials with strongly contrasting elastic properties are aligned through deformation processes. While in the relatively cold regions of the lithosphere these anisotropic signatures can remain frozen in over geologic time-scales (Silver, 1996), observed anisotropy at greater depths likely requires dynamic support (Vinnik et al., 1992). Thus, the anisotropy observed at sub-lithospheric depths is most likely a function of the current mantle strain field, and these observations can help us map out mantle flow.

Figure 28.1: $\xi$ structure at four depths in the upper mantle and transition zone.

Model Results

We have developed a degree 16 3D radially anisotropic shear velocity model of the whole mantle using a large three component surface and body waveform dataset and an iterative inversion for structure and source parameters based on Nonlinear Asymptotic Coupling Theory (NACT) (Li and Romanowicz, 1995). The model is parameterized in terms of isotropic $V_S$ and an anisotropic parameter, $\xi$, which is defined by $\xi=V_{SH}^2/V_{SV}^2$. The model shows a link between mantle flow and anisotropy in a variety of depth ranges.

The common features of S tomographic models are present in the isotropic $V_S$ model. The uppermost 200 km is dominated by tectonic features, with fast continents and slower oceans that show an age-dependent increase in velocity away from the slow velocities near ridges. Regions of active tectonic processes are, in general, slower, such as western North America, the major circum-Pacific subduction zones, and the East African rifting. In the transition zone depth range, the most prominent features are the fast velocities of subducted slabs, while the slow ridges are no longer present. Mid-mantle velocity anomalies are low in amplitude, and more white in spectrum. Finally, in the lowermost 500 km, the amplitudes of heterogeneity increase again, and become dominated by a degree 2 pattern with rings of higher velocities surrounding two lower velocity regions under the central Pacific and Africa, commonly referred to as superplumes.

In the $\xi$ model of the upper mantle (Figure 28.1), we confirm observations of regions with positive $\xi$ anomalies ($V_{SH}>V_{SV}$) starting at $\sim$80 km under oceanic regions and $\sim$250 km under old continental lithosphere, suggesting horizontal flow beneath the lithosphere (Gung et al., 2003). We also observe a $V_{SV}>V_{SH}$ signature at $\sim$200-300 km depth beneath major ridge systems with amplitude correlated with spreading rate. In the transition zone (400-700 km depth), regions of subducted slab material are associated with negative $\xi$ anomalies ($V_{SV}>V_{SH}$) (Figure 28.1), while the ridge signal decreases except under the East Pacific Rise.

We also confirm the observation of strong radially symmetric $V_{SH}>V_{SV}$ in the lowermost 300 km (Figure 28.2) (Panning and Romanowicz, 2004). The 3D deviations from this degree 0 signature are associated with the transition to the large-scale superplumes under the central Pacific and Africa, suggesting that $V_{SH}>V_{SV}$ is generated in the predominant horizontal flow of a mechanical boundary layer, with a change in signature related to transition to upwelling at the superplumes.

Figure 28.2: Isotropic $V_S$ (top) and $\xi$ (bottom) structure at 2800 km depth, centered under the Pacific (left) and Africa (right).

Source Inversions

In the process of developing the anisotropic model, we also invert for source parameters for the events in our dataset, starting from published Harvard CMT solutions, which are developed in a simpler mantle velocity model. We peformed an iterative inversion, with scalar seismic moment fixed. 964 of the 1191 events in our dataset had sufficient data coverage for a stable inversion which showed an improvement in fit. While changes in mechanism and location were quite small (horizontal location shifts averaged 0.015$^{\rm o}$), there was evidence for systematic relocation due to the improved structural model, particularly in the circum-Pacific subduction zones (Figure 28.3).

Figure 28.3: Vector-summed event relocations in $5^{\rm o} \times 5^{\rm o}$ cells.

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