impact plumes: implications for tharsis

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 108 km3 [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 perovskitephase 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, tcrys

Isostatic adjustment, tiso

Fluid dynamics of magma pond crystallization:isostatic adjustment versus conductive cooling

Isostatic adjustment, tsprd &

conductive cooling, tcool

The idea: qualitative description of magma pond evolution

fast crystallization isostatic adjustmentand 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 heatingLow interior viscosity

Animation

Animation

Case 2. Bottom heatingHigh 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