Quartz rheology constrained from constant-load experiments: Consequences for the strength of the continental crust

Abstract

The mechanical properties of quartz are fundamental to control the plastic behaviour of the continental crust. Our understanding of quartz rheology is still limited in the following respects: i) the large variability of flow law parameters in the earlier literature (stress exponent n = 4 to ≤ 2 and activation energy Q = 120 to 242 kJ/mol), and ii) the difficulty to identify the rate-limiting deformation mechanism, if several mechanisms are operating simultaneously. These two issues are connected and cannot be resolved separately. The present study has carried out constant-load experiments to constrain the flow law parameters of quartz. A new generation hydraulically-driven Griggs-type apparatus has been employed, resulting in reproducible mechanical data, even at very low strain rates (10⁻⁸ to10⁻⁹ s⁻¹; so far, closest to the natural ones). Furthermore, the Q-value in constant load experiments can be estimated without prior knowledge of the n value. Our new n (= 2) and Q values (= 110 kJ/mol) are fairly low. We calculated an A-value of 1.56 × 10⁻⁹ /MPa/sec. Microstructural analysis suggests that the bulk sample strain in our experiments is achieved by crystal plasticity, i.e., dislocation glide with minor recovery by sub-grain rotation, accompanied by grain boundary migration. Micro-cracking helps to nucleate new grains. It is inferred that strain incompatibilities induced by dislocation glide are accommodated by grain boundary processes, including dissolution-precipitation creep and grain boundary sliding. These grain boundary processes are responsible for the n-value that is lower than expected for dislocation creep ( 3). The new flow law can consistently estimate strain rates (especially at low stresses) in excellent agreement with documented natural case studies and predicts a rapid drop in strength of quartz-bearing rocks in the continental crust below a depth of ∼10 km or at a temperature of ∼300 °C and higher.