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Efficiency of Ductile Shear Zone Localization by Grain Size Reduction on Earth, Venus, and Mars L.G.J 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) though plastic rheologies, being strain-rate hardening, 10 are fundamentally stable in the laboratory [5]. Locali- Initial grain size 10,000 micron Final grain size 1.0 micron zation can be understood in either case if a state varia- 10 ble enables rock strength to weaken as strain accumu- 10 lates [2, 5]. ) 10 The best-studied way to generate ductile shear zones is by shear heating. In that model shearing gen- 10 erates heat, which increases temperature and weakens 10 the rock, leading to more shear [6-8]. This mechanisms is particularly efficient at low temperature and initial Strain rate (s 10 strain rate. However, field evidence for shear heating 10 in ductile shear zones remains scant [9] possibly indi- 10 cating that heat is evacuated faster than considered in 10 these models. 400 500 600 700 800 900 1000 1100 1200 Field evidence supports the notion that localization Temperature (C) is facilitated by structure evolution in two ways. First, Figure 1: Shear zone strain rate after grain size reduction to 10 mm to 1 micron (red) or to the value predicted by the van sheared rocks tend to develop layers that allow inter- de Wal piezometer [23] assuming an initial strain rate of 10- connection of a relatively weak phase. The intercon- 15 s-1 and considering dislocation creep, diffusion creep and nection of phyllosilicates, in particular, seems to play a (dashed line and dotted line) dis-GBS creep according to [20] role in the weakening of brittle faults [10-12] and duc- or [21]. tile shear zones in the middle crust [13, 14]. This pro- cess is particularly efficient when one of the rock Consequences for the strength of the litho- phases obeys a highly nonlinear rheology, like mica sphere. Gueydan et al. [22] recently discussed how the [5]. Because phyllosilicates are often hydrated, we do strength profile of the continental lithosphere changes not expect this process to be efficient on Venus. It is with strain. Fabric evolution reduces strength in the upper crust (brittle fabric), middle crust (fabric in pres- 46th Lunar and Planetary Science Conference (2015) 2439.pdf ence of mica) and uppermost mantle (grain size reduc- References: [1] O’Neill C. et al. (2007) EPSL, tion). The reduced strength of the lithosphere as a 261, 20–32. [2] Bercovici, D. et al. (2000) Geophysical whole is accompanied by increased strain rate if the Monograph 121, 1–46. [3] Montési, L.G.J., and Zuber, loading stress, controlled by large-scale tectonics, does M.T. (2002) JGR, 102, doi:10.1029/2001JB000465. not change. The lower crust, which does not weaken, [4] Vauchez A. et al. (2012) Tectonophysics 558-559, becomes the load-bearing layer at high strain (Figure 1-27. [5] Montési, L.G.J. (2013) JSG 50, 254–266. [6] 3). Regenauer-Lieb, K., and D. Yuen (1998) GRL 25, In that model, the increase of strain rate accompa- 2737– 2740. [7] Kaus, B.J.P., and Podladchikov, Y.Y. nying microstructural evolution is arbitrary. Two ap- (2006) JGR 111, doi:10.1029/2005JB003652. [8] Brun proaches make it possible to determine the strain rate J.-P., and P.R. Cobbold (1980) JSG 2, 149–158. [9] self-consistently. 1) The stress at each depth remains Camacho A. et al. (2001). JSG 23, 1007– 1013. [10] unchanged and the strain rate is that corresponding to Collettini, C. (2009) Nature 462, 907-911. [11] Nie- that stress. The increase in strain is regarded as a meijer, A. GRL 37, doi:10.1029/2009GL041689. [12] change in shear zone width. However, the horizontal Rutter, E.H. et al. (2013) JSG 51 shear resulting from the sloping edge of the shear zone doi:10.1016/j.jsg.2013.03.008. [13] Jefferies, S.P.,et al. is neglected. 2) The integrated strength of the litho- (2006) JSG 28, 220–235. [14] Gueydan, F., et al. sphere remains unchanged and the strain rate is uni- (2003) JGR 108, 2064 doi:10.1029/2001JB000611. form with depth, but at the value needed to match the [15] Drury, M.R.et al. (1991), Pure Appl. Geophys. integrated strength. 137, 439– 460 [16] Jin, D. et al. (1998) JSG 20, 195– In either of these models, the strength profile is not 209. [17] Precigout, J. and Gueydan, F. (2009) Geolo- much affected by grain size evolution as grain size gy 37, 147-150 [18] Platt J.P., and Behr, W.M. Behr remains at an equilibrium value close to where it does (2011) JSG 33, 537– 550. [19] Montési, L.G.J., Hirth, not affects rock rheology. However, if the brittle G. (2003) EPSL 211, 97–110 [20] Hirth, G., and strength decrease during shear, the ductile lithosphere Kohlstedt, D. (2003) Geophysical Monograph 138, becomes more and more load bearing (for assumption 83–105. [21] Hansen L.N. (2011) JGR 116, doi: 2), driving further grain size reduction. 10.1029/2011JB008220. [22] Gueydan, F. et al (2014) Tectonophysics, 631 (2014) 189–196. [23] van der Wal, D., et al. (1993) GRL, 20, 1479-1482 CRUSTAL SHEAR ZONES (=2.3) T=300°C T=400°C T=500°C Strain Viscosity Strain 1 10 100 100 km 2.5 1021 2.5 1022 2.5 1023 Figure 2: Localized shear zone formed by grain size reduction at different temperatures, assuming including diffusion creep, dislocation creep, and dis-GBS creep (from [22]). Figure 3: Evolution of the strength of the continental lithosphere as the fabric evolved according to layer development (brown layers) or grain size diminution (green layer) and as the strain rate increases (from [22]). .
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