Conditions Necessary for a Mantle Plume to Initiate Large Rapid True Polar Wander
Abstract
True polar wander (TPW) is the motion of the crust and mantle relative to the spin axis due to changes in the Earth's moment of inertia. Paleomagnetic studies assume that plate tectonic speeds have been relatively constant through time, and thus observations of large rapid continental movements have been attributed to TPW. Plumes arriving at the Earth's surface have been linked to large igneous provinces and supercontinent breakup. Mantle density anomalies such as plumes can affect Earth's moment of inertia and hence TPW. In attempting to link surface observations and geodynamic processes, it has been suggested (Li et al., EPSL, 2004) that a superplume rising under Rodinia triggered an episode of large (close to 90°) TPW in the Neoproterozoic as well as helping to initiate supercontinent breakup. Previous studies of TPW initiated by mantle density anomalies (plumes or slabs) considered many anomalies in the mantle, anomalies reflecting tomographic studies, or a single slab or plume at a fixed location. This study explores the conditions necessary for a single plume to initiate large rapid TPW. Our model builds upon previous work involving dynamic geoid calculations and rotational dynamics. Response functions for fixed and free surface boundary conditions, and various viscosity structures were calculated and convolved with the density anomaly of a plume to calculate the inertia perturbations. These perturbations were then incorporated into the nonlinear Liouville equations that govern rotation. Using this model, the speed and magnitude of TPW was explored for a large parameter space. The plume radius was varied from 400 km to 1000 km, the upper-lower mantle viscosity contrast from 1 to 100, and plume location from the equator to the pole. The effects of including a phase transition in the mantle, free and fixed surface boundary conditions, and the presence or absence of a viscoelastic lithosphere were examined. We find, as in previous studies, that the speed of TPW is dependent on the viscosity structure of the mantle. However, this study demonstrates that the speed and magnitude also are dependent on the latitude of the density anomaly and the surface boundary conditions. For a plume with an approximate radius of 1000 km rising at the pole, large rapid TPW was found for a low viscosity contrast between the upper and lower mantle. In contrast, for a 1000 km plume rising at the equator, large magnitude TPW with speeds faster than common plate velocities was only found for a large viscosity contrast between the upper and lower mantle. Fixed surface boundary conditions enhanced initiation of large rapid TPW for a smaller polar plume but severely retarded TPW for an equatorial plume. When the upper-lower mantle transition was treated as a phase boundary, more of the parameter space resulted in larger magnitude TPW for both the polar and equatorial plumes, but the TPW speed for the equatorial plumes was reduced. When a viscoelastic lithosphere was included, the magnitude and speed of TPW initiated by both the polar and equatorial plumes was drastically reduced. In summary, our results show that large rapid true polar wander can be initiated by a plume of sufficient size, provided that it is near the pole or equator and viscosity conditions are satisfied.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMDI21A2252E
- Keywords:
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- 5450 PLANETARY SCIENCES: SOLID SURFACE PLANETS Orbital and rotational dynamics;
- 8121 TECTONOPHYSICS Dynamics: convection currents;
- and mantle plumes;
- 1500 GEOMAGNETISM AND PALEOMAGNETISM