Permanent uplift in magmatic systems with application to the Tharsis Region of Mars

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Abstract
There has been considerable debate as to the importance of permanent structural uplift in attaining the immense topographic relief of the Tharsis rise on Mars. The presence of ancient terrain high on the rise suggests uplift took place, and this idea is supported by the presence of graben and fractures in the Claritas Fossae region that are orthogonal to extensional stress directions predicted by flexural uplift models of Tharsis. We derive expressions for total elevation and crustal displacement resulting from the partial melting associated with an upper mantle magmatic system. The effect of lateral mass loss is included in the derivation, as is the difference between mass balance and isostatic balance. This “isostatic effect” is due mainly to membrane stress support. The total elevation requirement for Tharsis is rather easy to meet by the choice of source region vertical dimension, but significant upward displacement of the crust requires a special condition. While most of the buoyancy for uplift is provided by a low density residuum, the crustal extrusive load will strongly counteract this effect unless it is only a small fraction of the melt products. A lower bound estimate of the fraction of intrusives necessary for any uplift at all is about 85% of the total magmatic products at Tharsis. Thus we propose that most of the magmas associated with Tharsis evolution ended up as intrusive bodies in the crust and upper mantle.

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