EVALUATING FLOW LAWS FOR DISLOCATION CREEP IN QUARTZ: A CRITICAL COMPARISON TO NATURAL SHEAR ZONES

Quartz deforming by dislocation creep has a power law relationship between stress and strain-rate. This relation is described in the following general form:

ε = A 𝜎ⁿ f_H2O^r e^-H/RT

where ε is the strain-rate, A is a material prefactor, 𝜎 is the flow stress, n is the stress exponent, f_H2O is the water fugacity, with exponent r, H is the activation enthalpy, T is the absolute temperature, and R is the molar gas constant. These flow laws are typically calibrated from rock deformation experiments or derived from theoretical principles and must be extrapolated over orders of magnitude in stress and strain-rate to match deformation conditions observed in natural shear zones.

To test the applicability of extrapolated flow laws to naturally-deformed rocks, we have independently estimated deformation conditions, including stress, strain-rate, temperature and pressure, of rocks from two crustal-scale shear zones. The stress – strain-rate relationship in rocks deformed throughout the Scandian thrust zone of northwest Scotland, and in the lower temperature/shallower levels of the Simplon shear zone of the central Alps, indicates that these rocks are weaker than predicted by many flow laws. The moderate discrepancies observed are thought to reflect small errors in the flow laws, which, with extrapolation, result in significant variations in predicted strain-rate. Rocks deformed deeper and at higher temperatures in the Simplon shear zone are closer to the predicted stress – strain-rate relationship; this may be due to a more complex rheology controlled by a polyphase quartz-feldspar mixture.

We have found that the stress – strain-rate relation predicted by several experimentally- and theoretically-determined flow laws generally do not coincide with independent estimations. Discrepancies between observed and predicted values may be incurred by the extrapolation of small errors in the flow laws, or the occurrence of multiple deformation mechanisms, such as grain size-sensitive processes. The role of fluids and secondary phases may also cause natural shear zones to exhibit behavior that deviates from that predicted by flow laws for quartz dislocation creep. These observations exemplify the necessity to incorporate natural rock data to better quantify crustal rheology.