EFFICIENCY OF DUCTILE SHEAR ZONE LOCALIZATION BY GRAIN SIZE REDUCTION NASA grant NNX10AG41G Abstract #2439 ON EARTH, VENUS, AND MARS Location #558 Laurent G.J. Montési1, Frédéric Gueydan2, Jacques Précigout3 1University of Maryland, College Park, 2Géosciences Montpellier, 3Institut des Sciences de la Terre d'Orléans 24 10 104 Mantle shear zone The Challenge of Ductile Shear Zones Local Approach Viscosity Exponential creep Grain size 3 1000 10 Numerical models are used to simulate Grain size (µm) Geology and geophysics have long documented that 1023 disGBS 600 °C shear zones development at specific 700 °C deformation is localized to some extend at every depth in 700 °C 2 the lithosphere A characteristic feature of even the global depths in the lithosphere. Grain size 600 °C 10 brittle fault with 800 °C Re tectonic regime of the Earth, localization is at the very reduction produces shear zones in the cryst cohesive alliz 22 800 °C e Brittle regime cataclasite mantle as long as the dis-GBS regime 10 100 1 d GSI definition of plate boundaries. However, we have as yet 10 g ra 1000 °C in (weak phases) is encountered, i.e., at temperature Stress/strength (MPa) only a very limited understadning of the mechanics of (Pa.s) Viscosity Effective GSS s (fabric) ductile shear zones and their implications for tectonics. less than 800°C. 100 1021 101 102 103 104 The fundamental difficulty behind modeling ductile shear Considering a terrestrial continental 10 brittle fault with Grain size (μm) Transition regime lithosphere, shear localization is 0 2.42.221.81.61.41.210.80.60.40.2 pseudotachylytes zones is that the plastic rheology active in these shear (mixing rheology) Overall Shear strain γ zones is fundamentally strain-rate hardening. Increased observed: Strain 1 10 100 100 km (fabric) strain rate is associated with increased stress. Therefore, it In the brittle crust: as the coefficient MANTLE SHEAR ZONES (γ=2.3) of friction decreases from 0.6 to 0.1 2.5 1021 2.5 1022 2.5 1023 is energetically favorable to deform a large region at a slow Strain T=600°C T=700°C Viscosity (Pa.s) T=800°C rate, the opposite of shear localizaiton. In the middle crust: as phyllosilicates narrow ductile become interconnected shear zone with To generate shear zones, it is necessary to appeal to the Viscosity : as grain size Plastic mylonite evolution of a state variable that is at a different value in In the upper mantle regime the shear zone than in the surrounding rocks. The shear decreases (grain size) zone materials must be intrinsically weak. As the only layer that does not undergo Strength (MPa) Strength (MPa) Strength (MPa) (fabric) weakening, the lower crust 4002000 4002000 4002000 wide ductile shear Microstructural changes such as the development of a changes from strength minimum to zone with striped fabric in the middle crust, and grain size reduction in the strength maximum. Strain rate rocks mantle, are good candidates to explain localization increases (here. arbitrary) to 30 (Montési, J. Struct. Geol., 2013). This structural changes compensate for the oveall loss of Depth (km) . -17 -1 . -13 -1 for the strength profile of the Earth’s continental strength ε=10 s ε=10-15 s-1 ε=10 s Modified from Passchier and Trow (2005) Microtectonics lithosphere (Gueydan et al., Tectonophysics, 2014). ε=0 ε=1 ε=2 Gueydan et al. (2014), Tectonophysics A B Lithospheres Scale Venus, 10 K/km Earth, 20 K/km Mars, 10 K/km 0 0 0 As grain size decreases, the total Brittle Brittle Brittle Dislocation creep (H&K03) Dislocation creep (H&K03) Dislocation creep (H&K03) strength of the lithosphere 10 Diffusion creep (H&K03) 10 Diffusion creep (H&K03) 10 Diffusion creep (H&K03) decreases too. To keep the Dis-GBS (H&K03) Dis-GBS (H&K03) Dis-GBS (H&K03) same energy dissipation, 20 20 20 10 mm grain size, 10-18 s-1 strain rate increases. The 10 μm grain size, 10-12 s-1 30 30 30 Karato and Wu (1993) strength profiles to the right piezometer, 10-12 s-1 C show that an initial strain rate 40 40 40 of 10-18 s-1 transforms roughly Examples of shear zones at all scale. A) Hirth, 2006). B) Outcrop-scale amphibolite 2010) C) Oblique view of the 25 km wide into 10-12 s-1 after grain size 50 50 50 Microscopic view of a harzburgite-hosted facies shear zone from Dogleg Island in the Antanimora shear zone in southern Depth (km) Depth (km) Depth (km) reduction and that the 60 60 60 mylonite, showing the intense grain size Canadian Grenville Province showing phase Madagascar (Google Earth-GeoEye 2011; deformation regime changes reduction and the development of layers in a segregation (following cracking and Martelat et al., 1999; Vauchez et al., 2012). from dislocation creep to grain 70 70 70 highly deformed mantle rock (Warren and metasomatism) and layering (Gerbi et al., size sensitive creep (diffusion 80 80 80 creep or dis-GBS). 10 mm grain size, 10-18 s-1 10 mm grain size, 10-18 s-1 10 μm grain size, 10-12 s-1 10 μm grain size, 10-12 s-1 Olivine flow law parameters from 90 Karato and Wu (1993) 90 Karato and Wu (1993) 90 piezometer, 10-12 s-1 piezometer, 10-12 s-1 Hirth and Kohlstedt (2003) (dry 100 100 100 Dry Olivine (Hirth and Kohlstedt, 2003) for Venus). 0 100 200 300 400 500 0 100 200 300 400 500 0 100 200 300 400 500 Dislocation creep + diffusion creep Stress (MPa) Stress (MPa) Stress (MPa) Dislocation creep + diffusion creep + dis-GBS Energetics of localization Dislocation creep + diffusion creep + dis-GBS from Hansen et al. (2011) The final strain rate is linked to the −6 10 initial strain rate by the constant Initial grain size 10,000 micron Piezometer Piezometer As a state variable evolves, the stress −10 Grain size reduction −10 Grain size reduction Main idea: −7 Final grain size 1.0 micron 10 10 microns 10 10 microns stress/constant velocity (see and/or the strain rate across the shear zone evolve so 10 van der Wal et al., 1993 piezometer 10 mm 10 mm ) ) diagrams) or constant width (black Venus 10 K/km Earth 20 K/km, dry −1 −8 −12 −1 −12 that the energy dissipation rate remains the same 10 10 10 lines) assumption. Strain rate A) Reference state: Wide deformation zone (H), increase is often a factor of 105 to −9 ) −14 −14 Constant stress/ stress σ , corresponding strain rate ε , velocity 10 10 10 6 r r −1 10 for constant stress, so that ε σ Constant velocity V=H r, and Energy E=V r −10 mantle circulation would localize on Strain Rate (s 10 Strain Rate (s −16 −16 B) Localized state: narrow deformation zone (h), 10 10 meter-wide shear zone! For −11 Constant width stress σl and strain rate εr. State variable or 10 constant width, the strain rate, and −18 −18 temperature are different from reference state 10 10 0 1 2 3 2 3 4 5 therefore the overal shear zone −12 10 10 10 10 10 10 10 10 Strain rate (s 10 Average Stress (MPa) Integrated Stress (MPa*km) velocity, increase by less than a A B −13 10 factor of 10. Piezometer Piezometer −10 Grain size reduction −10 Grain size reduction 10 10 microns 10 10 microns −14 10 10 mm 10 mm Surprisingly, strain rate increases ) Mars 10 K/km ) Earth 20 K/km −1 −12 −1 −12 −15 10 10 more on Venus than on Earth and for 10 400 500 600 700 800 900 1000 1100 1200 dry conditions. This would imply that Temperature (C) −14 −14 10 10 shear localization is more efficient in Venus’ mantle than in Earth’s. This is H Strain Rate (s Strain rate in a olivine shear zone after grain size reduction as a function of Strain Rate (s −16 −16 E˙ = ˙ dy = ˙ H = V 10 10 because, for low geotherm expected 0 temperature (constant velocity assumption). Initial grain size of 10 mm and from stagnant lid convection, the top Two alternative assumption make it possible to strain rate of 10-15 s-1. −18 −18 10 of the ductile mantle is cool enough 0 1 2 3 10 0 1 2 3 determine the finals state: Blue: grain size is reduced to 1 micron. Localization is observed at high 10 10 10 10 10 10 10 10 for dis-GBS creep. A) Fixed velocity: The width of the deformation temperature, where grain size reduction leads to a transition to diffusion Average Stress (MPa) Average Stress (MPa) zone decreases from H to h. Accordingly, strain rate creep. However, stress is so low that grain size reduction is unlikely under 1.00 strength 1.00 strength −10 Brittle strength reduction −10 Brittle strength reduction For Mars and the Earth localization increases by a factor L=H/h but stress remains the these conditions. At low temperature, a grain size sensitive regime is 10 0.50 strength 10 0.50 strength 0.25 strength 0.25 strength is also possible by reducing brittle 0.10 strength 0.10 strength same (same energy dissipation). encountered only if dis-GBS is active. ) ) −1 −12 −1 −12 strength. Although the uppermost B) Fixed width: Assuming that the structural Red: grain size reduction to the predictions from the van der Wal et al., 10 Mars, 10 K/km 10 Earth 20 K/km ductile mantle of Venus may be change takes place in a shear zone of (1993) piezometer.