Abstract
The brittle-viscous transition in the lithosphere occurs in a region where many large earthquakes nucleate. To study this transition, we sheared bimineralic aggregates with varying ratio of quartz and potassium feldspar. We deformed the samples in a solid-medium deformation apparatus at temperature, T = 750 degrees C and pressure, P-c = 800 MPa under either constant displacement rate or constant load boundary conditions. Under constant displacement rate, samples reach high shear stress (tau = 0.4-1 GPa depending on mineral ratio) and then weaken. Under constant load, the strain rate shows low sensitivity to stress below tau approximate to 400 MPa, followed by a higher stress sensitivity (stress exponent, n = 9-13) at higher stresses irrespective of mineral ratio. Strain is localized along "slip zones" in a C and C ' orientation. The material in the slip zones shows extreme grain size reduction and flow features. At peak strength, 1-2 vol% of the sample is composed of slip zones that are straight and short. With increasing strain, the slip zones become anastomosing and branching and occupy up to 9 vol%; this development is concomitant with strain-weakening of the sample. Slip zones delimit larger cataclastic lenses, which develop a weak foliation. Our results suggest that strain localization leads to microstructural transformation of the rocks from a crystalline solid to a fluid-like material in the slip zones. The measured rheological response is a combination of viscous flow in the slip zones and cataclastic flow in coarser-grained lenses and can be modeled as a frictional slider coupled in parallel with a viscous dashpot.
Plain Language Summary Fault slip occurs over a vast range of rates and depths in nature. Earthquakes are generated by this fault motion and therefore we need to understand the mechanical properties of fault rocks. To study the mechanism of rock failure at pressures and temperatures corresponding to about 30 km depth, we deformed rocks in the laboratory and analyzed their mechanical behavior. We further studied the samples after deformation using electron microscopy to identify features that are responsible for the measured mechanical behavior. We find that failure occurs due to the development of nanocrystalline to amorphous zones that interconnect upon highest stress and cause sample weakening. Such zones represent a failure mechanism in its own right, distinct from brittle cracks developing at lower pressures and temperatures or crystal-plastic flow at higher pressures and temperatures. The development of these zones introduces a fluid-like behavior in small parts of the sample which results in a mixed mechanical behavior: parts of the sample are solid and break and parts of the sample are fluid and flow. This mechanical behavior and its connection to the earthquake cycle is currently only poorly understood and hence is generally not incorporated in models of fault slip.