Is Tharsis Admitting a Plume?

The Tharsis Rise is an area of extensive volcanism containing the largest shield volcanoes in our solar system. A number of investigators have suggested that the sustained volcanism, areoid and topographic anomalies that comprise the Tharsis Rise to be the result of a mantle plume. Harder and Christensen (1996) presented a calculation for convection in a Mars-sized body that resulted in a single plume. However, their calculation evolved through stages of several plumes down to a single plume and took greater than the age of the solar system to develop into a single plume. Efforts to remove the isostatic contribution to the areoid and isolate the dynamic contribution have shown that while much of the long wavelength signal can be explained by the crust, there is a significant mantle component (Kiefer et al., 1996; Whitesell and King, 2001). Furthermore, dynamic models suggesting Tharsis is largely supported by convection (Kiefer et al., 1996; Harder and Christensen, 1996; Harder, 2000; Kiefer, 2001) can justify the young ages of the Tharsis shield volcanoes. Thus, there is reason to believe that a mantle plume may exist beneath Tharsis. Research conducted thus far has consisted of varying the Rayleigh number and rate of internal heating in an isoviscous rheology and activation energy in a temperature-dependent rheology. The areoid and topography over isoviscous plumes in a Mars-sized body are greatly reduced with increasing Rayleigh number and internal heating. Calculations over temperature-dependent plumes show that after a thick, strong lithosphere forms, the areoid and topography from a plume become even smaller. The calculated admittance over these plumes shows that the admittance over a plume forming in a temperature-dependent rheology closely resembles the shape of the observed admittance on Mars (Figure 2, Kiefer et al. 1996). Additionally, the areoid calculated using only the upper 150 km of the temperature field in our temperature-dependent rheology calculation is over 500 m. This strongly points to a large negative component of the areoid due to the presence of a plume below 150 km which would reduce the areoid anomaly. Thus, models that assume the areoid can be explained by lithospheric erosion or shallow compensation would greatly over-estimate the areoid. These results suggest that a plume may exist under the Tharsis Rise.