Mathematics – Logic
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.p31b0144k&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #P31B-0144
Mathematics
Logic
5430 Interiors (8147), 5480 Volcanism (6063, 8148, 8450), 8121 Dynamics: Convection Currents, And Mantle Plumes, 8147 Planetary Interiors (5430, 5724, 6024), 8148 Planetary Volcanism (5480, 8450)
Scientific paper
Several attempts have been made to produce a crustal dichotomy followed by a single, stationary mantle plume, giving rise to the Tharsis volcanic province using 3D mantle convection models. If Mars evolved in such a scenario, two degree-1 (i.e., hemispherical) patterns would be required to develop at 90° to each other in the span of a few hundred million years. This has not yet been accomplished although recent numerical models (e.g. Roberts and Zhong, 2006) have made significant strides towards a solution including the use of both a lower mantle phase change and a layered viscosity mantle. Our goal is not to determine how the crustal dichotomy formed, but to assume it was already in place within the first 0.5 Ga of Mars' evolution. The presence of an early dichotomy boundary likely affected the planform of mantle convection and may have played a role in the formation of the Tharsis Rise based on the proximity of Tharsis to the dichotomy boundary. We model Martian mantle convection using the 3D finite element code CitComS (Zhong et al., 2000). Previous laboratory experiments have shown that an insulating lid overlying part of a temperature-dependent mantle will generate a large, stationary upwelling beneath the center of the lid. Thus, we incorporate a dry, Newtonian rheology with E*=300 kJ/mol and simulate a dichotomy boundary by integrating a high viscosity lid over the "southern hemisphere" of our model. Redmond and King (2004) illustrate that the effect of a strongly temperature-dependent rheology is the development of a stiff, rheological lithosphere. Thus, for small Rayleigh numbers, the initially imposed dichotomy boundary does not have an effect on the location of mantle upwellings because of the global lid that forms as a result of the temperature-dependent rheology. Our results suggest that an initially larger Rayleigh number, on the order of 107, will produce stationary upwellings beneath the imposed southern hemisphere lid before a global lid develops while upwellings in the northern hemisphere are short-lived and/or migratory until the rheological lithosphere strengthens. Thus, if the dichotomy was emplaced early on, it may be possible for mantle flow that migrates away from the initial upwelling(s) to interact with the edge of the imposed dichotomy boundary generating small-scale edge-driven convection. An edge-driven convection instability may be responsible for long-lived mantle convection at and around the Tharsis Rise volcanic province.
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