Thermochemical structures beneath Africa and the Pacific Ocean

1. Thermochemical structures beneath Africa and the Pacific Ocean Allen K. McNamara and ShijieZhong

2. Overview • An attempt at explaining seismic anomalies beneath the Pacific and Africa • Low seismic velocities indicate a large “rounded” anomaly beneath the Pacific • Also indicate a “sharp-edged linear, ridgelike…” anomaly beneath Africa trending NW-SE • Areas of high topography and high T in the dense layer as described by Kellogg et al. 1999.

3. Review: Kellogg et al. 1999

4. Past Work • Due to technical limitations, past work primarily considered in a rectangular coordinate system introducing a geometric limiation. • Inability to understand effects of spherical geometry • “As a consequence, the actual shae of thermochemical structures predicted for the Earth could only be roughly inferred from experiments.”

5. Limitations of Rectangular Coordinate system • Cartesian coordinate system served as proxy for circum-pacific subduction • Deos not allow for the complicated geometry and “time-dependent” nature of plate boundaries • Variable linear velocities? • Complicated subduction patterns associated with the African region…..

6. Previous Work by McNamara and Colleagues • Concentrated on understanding role of spherical geometry and rheology in formation of thermochemical structures • Found that temperature-dependent rheology forms “weak”, dense piles that are “passively swept aside by cold, down-welling material”. • Rounded “superplume” structures (Pacific) and linear “piles” (Africa) are not expected to be present together within the earth • Recognized the necessity of using Earth’s plate tectonic history in order to more accurately predict the large seismic anomalies observed beneath the Pacific and Africa.

7. Figure 8. from McNamara and Zhong 2004

8. Hypothesis • “Earth’s plate motion history plays a controlling role in the development of thermochemical structures that geometrically resemble to first order the general shape and locations of the large negative seismic anomalies in the lower mantle.”

9. Parameters • Uses three models with layer thicknesses of 127 km, 255 km, and 956 km • Buoyancy ratio of 0.6 and density contrasts of 2-5% (I’m guessing in relation to overlying mantle) • 119 Ma of model time employing surface boundary conditions consistent with 11 stages of plate history • Limitations?

10. Math from McNamara and Zhong 2004

11. Resolution • Calcuations performed on a “mesh” with more than 3 million elements • Over 30 million tracers used to characterize the compositional field • “Improved algorithmic and technical computational abilities allow us to resolve viscosities consistent with Earth-like convective vigor…” with Ra = 2.7 x 108

12. Rheology • Form of Power-law creep? • Temperature and depth dependent • ηr(z)=1 for z < 663 km • ηr(z) = 0.1225z – 52.1 from 663 km to 2,867 km (core/mantle boundary) • A = 9.2103 (activation coefficient) • Assume dimensionless heat production of 10? • Weak upper mantle (30x viscosity step from upper- to lower-mantle) • 10x linear increase with depth to base of mantle

13. Boundary Conditions • Use plate velocity boundary conditions to guide subduction to occur at locations consistent with plate history • Employ convective vigor (Ra) consistent with conditions to guide rather than drive flow