This chapter contains sections titled: Overview, Theme 1: What are The Controls on Fluid-Rock Chemical Interaction in and Adjacent to Fault Zones?, Theme 2: How Does Fluid Flow Change Before, During, and After Earthquakes?, Theme 3: What are the Magnitudes of Fluid Flux Throughout the Lithosphere in Different Tectonic Environments?, Summary, References
It is widely believed that around the brittle‐ductile transition, crustal faults can be significantly weaker than predicted by conventional two‐mechanism brittle‐ductile strength envelopes. Factors contributing to this weakness include the polyphase nature of natural rocks, foliation development, and the action of fluid‐assisted processes such as pressure solution. Recently, ring shear experiments using halite/kaolinite mixtures as an analogue for phyllosilicate‐rich rocks for the first time showed frictional‐viscous behavior (i.e., both normal stress and strain rate sensitive behavior) involving the combined effects of pressure solution and phyllosilicates. This behavior was accompanied by the development of a mylonitic microstructure. A quantitative assessment of the implications of this for the strength of natural faults has hitherto been hampered by the absence of a microphysical model. In this paper, a microphysical model for shear deformation of foliated, phyllosilicate‐bearing fault rock by pressure solution‐accommodated sliding along phyllosilicate foliae is developed. The model predicts purely frictional behavior at low and high shear strain rates and frictional‐viscous behavior at intermediate shear strain rates. The mechanical data on wet halite + kaolinite gouge compare favorably with the model. When applied to crustal materials, the model predicts major weakening with respect to conventional brittle‐ductile strength envelopes, in particular, around the brittle‐ductile transition. The predicted strength profiles suggest that in numerical models of crustal deformation the strength of high‐strain regions could be approximated by an apparent friction coefficient of 0.25–0.35 down to depths of 15–20 km.
Geophysical observations as well as deformation experiments indicate that under hydrothermal conditions, crustal faults can be significantly weakened with respect to conventional brittle‐plastic strength envelopes. Pressure solution has long been proposed as a mechanism leading to fault weakness. However, pressure solution has also been proposed as contributing to interseismic fault healing, and the competition between the weakening and healing effects of pressure solution is unclear. To investigate this issue, we have conducted rotary shear experiments on synthetic faults containing granular halite (NaCl) gouge using NaCl‐saturated mixtures of water and methanol as pore fluid. The NaCl‐water‐methanol system was chosen as a rock analogue because pressure solution is known to be important in this system at ambient conditions. We explored the influence of varying pore fluid composition (hence pressure solution rate), gouge grain size, and wall rock surface roughness, as well as normal stress and sliding velocity on slip behavior. All experiments were done under drained conditions. An acoustic emission detection system allowed detection of brittle events in the gouge. The results show no evidence for steady state pressure solution‐controlled fault slip. Frictional, rate‐insensitive behavior was observed, whereas the microstructures and compaction behavior clearly demonstrated that pressure solution was active in the gouge. Our data show that fluid‐assisted healing effects dominated over weakening, causing fault strength to be controlled mainly by brittle‐frictional processes. Existing models describing pressure solution‐controlled fault creep may not be applicable to a porous gouge undergoing compaction as well as slip.
