Impact plumes: Implications for Tharsis C.C. Reese & V.S. Solomatov Dept. of Earth & Planetary Sciences Washington University in St. Louis Saint Louis, MO 63130, USA

Tharsis province, Mars: geological & geophysical observations ● Broad topographic rise & center of large scale magmatism ● Shield volcanoes: Tharsis Montes, Olympus Mons ● Layered volcanics in Valles Marineris [McEwen et al., 1999] ● GTR consistent with surface loading & flexure of lithosphere [Zhong & Roberts, 2003] MGS/MOLA Interpretation: massive volcanic pile Thick complex crust [Zuber, 2001] 3 x 10 8 km 3 [Phillips et al., 2001] emplaced by late-Noachian 3.5 Ga

Mantle convection dominated by a single thermal plume originating at the CMB similar to a terrestrial plume but larger Conventional hypothesis for Tharsis formation The thermal plume model [e.g.Harder and Christensen, 1996] provides a heat source for early large scale magmatism could account for some present day topographic uplift [Redmond & King, 2004] might explain geologically recent volcanism [Kiefer, 2003; Hartmann and Neukum, 2001]

Conventional thermal plume model: 1 plume stabilization spinel to perovskite phase transition near the core mantle boundary [Harder and Christensen, 1996]

Reasons to consider alternative hypotheses No thermal plume model has reproduced Tharsis formation on a timescale consistent with observations which indicate emplacement by late Noachian [Banerdt & Golombek, 2000] Core radius [Yoder et al., 2003] may exclude a lower mantle perovskite layer which is key to stabilizing a single plume pattern of convection [Harder and Christensen, 1996] Geochemical heterogeneity [Kleine et al., 2004] suggests limited mantle mixing which is difficult to reconcile with vigorous mantle convection [Zuber 2001] Immobile lithosphere implies early mantle heating [Solomatov & Moresi, 2000] and core heat flow shut-off [Nimmo and Stevenson, 2001] making thermal plume formation difficult

Tharsis is related to early mantle dynamics associated with the evolution of a local magma pond produced by a very large impact. An alternative hypothesis for Tharsis formation The impact model can produce long-lived upwellings which may play a role in areoid evolution [Reese et al., 2002] can produce localized episodes of mantle magmatism [Reese et al., 2004]

Criterion for magma pond formation after [Tonks and Melosh, 1992]

Fluid dynamics of magma pond crystallization: isostatic adjustment versus crystallization Crystallization, t crys Isostatic adjustment, t iso

Fluid dynamics of magma pond crystallization: isostatic adjustment versus conductive cooling Isostatic adjustment, t sprd & conductive cooling, t cool

The idea: qualitative description of magma pond evolution fast crystallizationisostatic adjustment and merger with solid state convection impact and melt pond formation

Numerical simulation of an impact induced plume Spherical shell geometry Immobile lithosphere Viscous lid & rigid surface Spatial and temporal melt distribution is calculated Initial conditions

All melt is extracted to form crust Isostatic equilibrium maintained Yield strength limited topography (e.g. 2 km relief over 1000 km) Crustal growth & spreading parameterization

Case 1.No bottom heating Low interior viscosity Animation

Case 2.Bottom heating High interior viscosity

Conclusions 1. Evolution of impact induced local magma ponds depends on solid planet rheology, mode of crystallization, and magma pond size. Smaller melt regions  incomplete isostatic adjustment & merger with subsequent solid state evolution Large melt regions  rapid formation of a global melt layer 2. Impact induced plumes can focus magmatic activity and result in the development of a large igneous province. 3. Model predicts Tharsis development on a timescale consistent with observation