Modeling the Thermal and Melting Evolution of the Martian Mantle: Implications of the Hemispheric Dichotomy for Tharsis and Other Volcanic Regions

We use geodynamic models to study the thermal and melting history of Mars, and explore a possible link between the hemispheric dichotomy and the development of volcanic regions, in particular Tharsis. First, we consider a reference case without a dichotomy. We tested all combinations of a range of initial CMB temperature, crustal HPE enrichment, and mantle activation energies. Based on Christensen (1983), activation energy is divided by the stress exponent n to approximate a power law rheology for olivine. Typically, n is taken to be 3, but we vary the effective activation energy by also testing values of n = 1.5 and 1, keeping the nominal activation energy at 350 kJ/mol. To test whether an early lithospheric/crustal dichotomy structure is more likely to produce Tharsis-like melting anomalies, we repeated these model runs with a hemispheric (degree-1) dichotomy structure in the form of a thicker, thermally older southern hemisphere lithosphere.

We find the geoid, topography, and thermal structure results from models with n = 1 (i.e., diffusion creep) to be more consistent with present day Mars. Of the three parameters varied, the results are most sensitive to the activation energy, and least sensitive to the initial CMB temperature. Higher activation energies, and thus higher temperatures in the mid-lower mantle, result in a cooler and thicker lid, but also an overall hotter mantle. Corresponding with the higher average temperatures, lower enrichment and higher activation energy lead to more melt being produced for longer. Models with higher (10-15x) crustal enrichment tend to develop geoid and topography highs at the poles that persist for billions of years. This is often more pronounced in the dichotomy models, with antipodal polar plumes evident in temperature cross-sections. Many of our models, both with and without the dichotomy, also develop several smaller geoid and topography highs, especially near the equator. Melting occurs primarily in the middle of plume heads, especially after the first ~2 Gyr. In the dichotomy models, melting is more abundant in the northern hemisphere, where the lithosphere is thinner.