Long-wavelength stagnant lid convection with hemispheric variation in lithospheric thickness: Link between Martian crustal dichotomy and Tharsis?

A dynamic link between the early evolution of Tharsis and the crustal dichotomy on Mars was recently proposed by Zhong (2009). We address in detail the fundamental aspects of the proposed model using 3-D spherical shell modeling of convection. We investigate the conditions under which a spherical harmonic degree 1 flow is produced in the mantle of Mars in layered viscosity models with different depths of viscosity layering and different viscosity ratios between the bottom and top layers. We find that a thinner weak layer requires a larger viscosity decrease in order to produce degree-1 planform, in qualitative accordance with existing analytical studies. A lithospheric keel of hemispheric extent, which may represent melt residue after dichotomy formation process, is then introduced in the models. As a consequence, thermal upwellings are first formed and oriented below the center of the melt residue cap. For keels of maximum thickness greater than or similar to 100 km, a rotation of the entire single-plate lithosphere relative to the underlying mantle follows until the plume is stabilized near the edge of the keel, which can be identified as the dichotomy boundary. This model may explain the early migration of the Tharsis volcanic center from southern latitudes to the vicinity of the dichotomy boundary. For keels greater than or similar to 200 km thick, the modeled time scales agree with observationally inferred migration rate. If the Tharsis migration is caused by the rotation of the lithosphere, the requirement on the amount and location of upper mantle melting would argue for an endogenic origin of the dichotomy.