Fluid Dynamics of Local Martian Magma Oceans: Implications for Hemispherical Dichotomy and Tharsis Formation

Both the hemispherical crustal dichotomy and Tharsis province were emplaced early in Martian history. Several hypotheses for formation of early Mars' large scale crustal structure have been proposed. One possibility is that planetary formation processes and initial thermal and compositional conditions play an important role. Large impacts occurring during late stage accretion can produce localized, intact melt regions, i.e. local magma oceans (LMOs). A LMO evolves to a uniform global layer, i.e. a global magma ocean, only if buoyant melt is rapidly extruded prior to any significant solidification. A simple fluid dynamical model suggests that if cooling and crystallization are fast compared to melt extrusion, the driving force for isostatic adjustment diminishes, melt region viscosity increases significantly, and formation of a globally symmetric, fully molten layer can be difficult. Instead, a localized, mantle upwelling develops in response to the buoyant, partially molten LMO. Formation of the hemispheric dichotomy is associated with isostatic adjustment of a relatively large, partially molten LMO. A subsequent, smaller LMO results in rapid development of much of the Tharsis rise. Continued melting at Tharsis throughout planetary evolution can be due to coupling of LMO evolution with internally and bottom heated mantle convection.