46th Lunar and Planetary Science Conference (2015) 2439.pdf
EFFICIENCY OF DUCTILE SHEAR ZONE LOCALIZATION BY GRAIN SIZE REDUCTION ON EARTH, VENUS, AND MARS L.G.J. Montesi1, F. Gueydan2, and J. Précigout3, 1University of Maryland, Depart- ment of Geology, College Park MD 20742, [email protected]. 2Géosciences Montpellier, Université de Montpellier 2, CNRS UMR 5243, Montpellier, France 3Institut des Sciences de la Terre d'Orléans, Université d'Orléans, CNRS UMR 7327, Orléans, France
Introduction: The Earth is unique in the solar sys- also possible that micas are rare in the Martian interior tem in that it displays clear evidence of plate tectonics. because even the crust is dominantly mafic. How to explain this characteristic remains a major Ductile shear zones in exhumed mantle rocks often challenge in planetary sciences. Previous studies show feature a reduced grain size compared to undeformed that considering convective vigor in the planet’s interi- rocks, which leads to the many shear zones being clas- or is not sufficient to explain why the lithosphere of sified as mylonites [15–18]. Reducing grain size is the Earth is broken [1, 2]. The strength of Earth’s lith- possible only when dislocations are active inside the osphere must be reduced compared to that of other rocks [e.g., 19]. However, it leads to weakening only if terrestrial planets. This links the possibility of plate grain boundary sliding or diffusion creep are dominant, tectonics with the formation of localized shear zones, which leads to a connendrum: grain size reduction is which are expected to develop when a material looses not possible under condition where it would be an effi- strength as it deforms [3]. In this study, we focus on cient weakening process (Figure 1). However, at low the formation of ductile shear zones through grain size enough temperature, olivine aggregates may deform in reduction and discuss the conditions under which sig- the dislocation-accommodated Grain Boundary Sliding nificant weakening (defined as an increase in strain regime (dis-GBS, [20, 21]), which is grain size sensi- rate for a given stress) is possible. tive and during which dislocations are active. Simula- tions of simple shear that include grain size evolution Localization processes: Rocks deform following result in ductile shear zones only at relatively low tem- brittle processes at relatively shallow pressure and peratures, when dis-GBS is important (Figure 2, [22]). temperature and plastic mechanisms are greater depth. ) Geological observations on Earth show that defor- Dislocation creep + diffusion creep Dislocation creep + diffusion creep + dis-GBS mation can localize under both conditions [4], even Dislocation creep + diffusion creep + dis-GBS from Hansen et al. (2011)