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.

1. 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!!!

A-B = steady state friction at velocity, V. >0, stable; <0, unstable

Multi-mechanism frictional model

experiment

model

H2O = weak

strengthening

stre

ngth

enin

g

weakening

At intermediate T and slip ratesrate weakening (LS mechanism) dominates

At very large, or very small T and slip rates, CF and ST mechanismsbecome dominant

Confirms that the transition from rate weakening to rate strengthening is a function of T and slip rate

ST

CFLS

Model assumptions: 3 cm/yr slip rate25 Mpa/km pressure gradient20°C geotherm

Atypical???

No LS!!!i.e., no rate weakeningwhich is inconsistent with large earthquakes

Given a strain rate and shear zone thickness, deformation modecan be predicted.

To have the LS mechanism with 10 m shear zone = need a higher strain rate.

Fletcher and Guetteri

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

ResultsDetermined reasonable values of Q and water fugacity

Schematic geometry of antiformal stacks

Regime 1: structurally lowestDislocation climb is difficult, deformationLamellae, undulose extinction, fine recrystallized grains Preserved along grain boundaries

Regime 2: intermediateflattening, subgrains, subgrain rotation recrystallization

Regime 3: structurally highestcomplete recrystalization, and foliation development; dislocation climb and grain boundary migration is a dominant process

Use field observations to constrain T, , , 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 ~20-40m 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

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.

LP = GT| fh20(LP)

RGD= ruby gap duplex

• 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