The Onset of Plate Tectonics on Super-Earths Using a Damage Rheology

Details The Onset of Plate Tectonics on Super-Earths Using a Damage Rheology The Onset of Plate Tectonics on Super-Earths Using a Damage Rheology

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p21a1586f&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #P21A-1586

Physics

[5475] Planetary Sciences: Solid Surface Planets / Tectonics, [6296] Planetary Sciences: Solar System Objects / Extra-Solar Planets, [8147] Tectonophysics / Planetary Interiors, [8159] Tectonophysics / Rheology: Crust And Lithosphere

Scientific paper

Numerical simulations of mantle convection with a damage - grainsize feedback are used to develop scaling laws to predict conditions at which super-Earths would have plate tectonics. Plate tectonics on a terrestrial planet requires some form of lithospheric weakening, which is typically thought of in terms of yielding or failure. However, all mechanisms for achieving plate tectonic style convection rely on viscosity reductions in the lithosphere. Therefore, we introduce a new criterion for the onset of plate tectonics on terrestrial planets: that the viscosity of the lithosphere must be reduced to a critical value, which we assume to be the mantle viscosity. We formulate this criterion using the viscosity ratio between the pristine lithosphere and underlying mantle (μ0/μ1). These conditions are mapped out in regime diagrams of μ0/μ1 versus the damage fraction (fa). The regime diagrams show that the transition from stagnant lid to mobile surface occurs for higher μ0/μ1 as fa increases, with a power law relationship between those two variables; moreover, decreasing the healing constant (ka) at the surface shifts the transition boundary to higher μ0/μ1. A scaling law is developed assuming that the transition between regimes occurs when damage, driven by convective stresses, reduces the lithospheric viscosity to the mantle viscosity. This scaling law explains the numerical results well and can be applied to terrestrial planets. For the Earth, damage is efficient in the lithosphere, and viscosity can be reduced by 10 orders of magnitude with grains being reduced to a size on the order of a micron. When applied to super-Earths, we find that larger planets are capable of larger viscosity reductions, but the viscosity ratio increases with planetary size at roughly the same rate. Therefore, contrary to previous results, we find that the size of the planet has little effect on the convective regime that planet lies in. Factors such as surface temperature and thermal evolution may be more important in explaining the convective style of terrestrial planets.

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