The role of water on lithospheric strength

1. The role of water on lithospheric strength • Chester et al., 1995, A rheologic model for wet crust applied to strike-slip faults • Hirth et al., 2001. An evaluation of quartzite flow laws based on comparisons between experimentally and naturally deformed rocks.

2. What is the strength of the mid-lower crust?? • 2. Are brittle frictional and ductile creep adequate?? • 3. How does H2O and additional friction mechanisms modify our understanding of lithospheric strength?? Turns out….significantly!!!

3. Multi-mechanism frictional model model experiment A-B = steady state friction at velocity, V. >0, stable; <0, unstable

5. H2O = weak

6. weakening strengthening strengthening

7. ST LS CF At intermediate T and slip rates rate weakening (LS mechanism) dominates At very large, or very small T and slip rates, CF and ST mechanisms become dominant Confirms that the transition from rate weakening to rate strengthening is a function of T and slip rate

8. No LS!!! Model assumptions: 3 cm/yr slip rate 25 Mpa/km pressure gradient 20°C geotherm i.e., no rate weakening which is inconsistent with large earthquakes Atypical???

9. Given a strain rate and shear zone thickness, deformation mode can be predicted. To have the LS mechanism with 10 m shear zone = need a higher strain rate.

11. Fletcher and Guetteri

12. Hirth et al., 2001 Motivation • Qtz is an abundant in continents. • Determine depth of seismogenic zone • Crustal strength profiles have large uncertainties on differential stress that exceed 400 Mpa at 15 km depth Approach • Three dislocation regimes observed in experiments also occur in nature • Extrapolate experimental flow laws to naturally deformed rocks that experienced a simple tectonic history. • External state variables are reasonably constrained using thermochronology, microstructure, and structural geology Results Determined reasonable values of Q and water fugacity

13. Schematic geometry of antiformal stacks

14. Regime 1: structurally lowest Dislocation climb is difficult, deformation Lamellae, undulose extinction, fine recrystallized grains Preserved along grain boundaries Regime 2: intermediate flattening, subgrains, subgrain rotation recrystallization Regime 3: structurally highest complete recrystalization, and foliation development; dislocation climb and grain boundary migration is a dominant process

15. Use field observations to constrain T, s, e, fh2o. So, So, what is Q?? T= between 250-330°C using Ar/Ar thermochronology of white mica [Dunlap, 1997] Differential stress = piezometry of recrystallized grains and quartz mylonites between regimes 2-3 is ~20-40mm consistent with a diff stress of 80-60Mpa Strain rate = thrusting at 1.5 km/m.y ~10-14 - 5x10-14/sec using tectonic reconstructions Fh2o = estimated assuming h20 present at 300°C and a pressure of 400 Mpa -> Lithostatic pressure at 15km and 20°C geotherm

16. Extrapolate experimentally derived flow laws to natural strain rates at 100 Mpa Assume: differences in studies of LP & GT is an effect of h2o fugacity Same flow law applies to LP, GT, and RGD. eLP = eGT| fh20(LP) RGD= ruby gap duplex

17. Illustrates effect of T on deformation mechanism • Extrapolation of high T experiments to low T highly underestimates strength • Quartz at lower T deforms by a semi brittle flow