Transcript 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
MGS/MOLA
Broad topographic rise & center of large scale
magmatism
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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]
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Interpretation: massive volcanic pile
• Thick complex crust [Zuber, 2001]
• 3 x 108 km3 [Phillips et al., 2001]
emplaced by late-Noachian 3.5 Ga
Conventional hypothesis for Tharsis formation
Mantle convection dominated by a single thermal plume
originating at the CMB similar to a terrestrial plume but larger
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
An alternative hypothesis for Tharsis formation
Tharsis is related to early mantle dynamics associated with the
evolution of a local magma pond produced by a very large impact.
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
impact and
melt pond formation
fast crystallization
isostatic adjustment
and merger with
solid state convection
Numerical simulation of an impact induced plume
Initial conditions
Spherical shell geometry
Immobile lithosphere
• Viscous lid & rigid surface
Spatial and temporal melt
distribution is calculated
Crustal growth & spreading parameterization
• All melt is extracted to form crust
• Isostatic equilibrium maintained
• Yield strength limited topography
(e.g. 2 km relief over 1000 km)
Case 1. No bottom heating
Low interior viscosity
Animation
Case 2. Bottom heating
High interior viscosity
Animation
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