Intergranular pressure solution creep is an important deformation mechanism in the Earth’s crust. The phenomenon has been frequently studied and several analytical models have been proposed that describe its constitutive behavior. These models require assumptions regarding the geometry of the aggregate and the grain size distribution in order to solve for the contact stresses, and often neglect shear tractions. Furthermore, analytical models tend to overestimate experimental compaction rates at low porosities, an observation for which the underlying mechanisms remain to be elucidated. Here we present a conceptually simple, 3D Discrete Element Method (DEM) approach for simulating intergranular pressure solution creep that explicitly models individual grains, relaxing many of the assumptions that are required by analytical models. The DEM model is validated against experiments by direct comparison of macroscopic sample compaction rates. Furthermore, the sensitivity of the overall DEM compaction rate to the grain size and applied stress is tested. The effects of the interparticle friction and of a distributed grain size on macroscopic strain rates are subsequently investigated. Overall, we find that the DEM model is capable of reproducing realistic compaction behavior, and that the strain rates produced by the model are in good agreement with uniaxial compaction experiments. Characteristic features, such as the dependence of the strain rate on grain size and applied stress, as predicted by analytical models, are also observed in the simulations. DEM results show that interparticle friction and a distributed grain size affect the compaction rates by less than half an order of magnitude.
The zip-file Van-den-Ende_2017.018.zip contains several folders with raw data from the laboratory experiments, output data from Discrete Element Method simulations, and Python 2.7 script files that read and process these data. All data are stored in ASCII format.
Phyllosilicate-bearing faults are characterized by an anastomosing foliation with intervening hard clasts and are believed to be long-term weak structures. Here, I present results of sliding experiments on gouges of 80 wt% quartz and 20 wt% muscovite, sheared under hydrothermal conditions at constant velocity. The results show that significant strengthening occurs over a narrow range of sliding velocities (0.03-1* m-6/s). At the lowest velocity investigated, weakness is achieved after a considerable sliding distance of over 20 mm with friction reaching a value of 0.3. Microstructural observations and the application of existing models point to the operation of frictional-viscous flow (FVF), through the serial operation of frictional sliding over a weak foliation and pressure solution of intervening clasts, resulting in low frictional strength and pronounced velocity-strengthening. At higher velocities, grain size reduction becomes dominant in a localized zone, which results in disruption of the foliation and the cessation of the FVF mechanism. In natural settings, earthquakes originating elsewhere on the fault would be rapidly arrested when encountering a foliated part of the fault deforming via FVF. Furthermore, pulses of elevated slip velocity would lead to grain size reduction which would destroy the foliation and cause a long-term strengthening of the fault.