Quantifying effects of laboratory-simulated diagenetic sediment lithification on frictional slip behavior

On major plate-boundary fault zones, it is generally observed that large-magnitude earthquakes tend to nucleate within a discrete depth range in the crust known as the seismogenic zone.  This is generally explained by the contrast between frictionally stable, velocity strengthening sediments at shallow depths and lithified, velocity-weakening rocks at seismogenic (10’s of km) depth. Thus, it is hypothesized that diagenetic and low-grade metamorphic processes are responsible for the development of velocity-weakening frictional behavior in sediments that make up fault gouges.  Previous laboratory studies comparing the frictional properties of intact rocks and powdered versions of the same rocks generally support this hypothesis, however controlling lithification in the laboratory and systematically quantifying frictional behavior as a function of lithification and remains a challenge.

Here, we simulate the lithification process in the laboratory by using mixtures of halite and shale powders with halite-saturated brine, which we consolidate under 10 MPa normal stress and subsequently desiccate.  The desiccation allows precipitation of halite as cement, creating synthetic rocks.  We vary the proportion of salt to shale in our samples, which we use as a proxy for degree of lithification.  We measure the frictional properties of our lithified samples, and equivalent powdered versions of these samples, with velocity-step tests in the range 10^-7 – 3x10^-5 m/s.  We quantify lithification by two methods: (1) direct measurement of cohesion, and (2) measuring the porosity reduction of lithified samples compared to powders.  Using these measurements, we systematically investigate the relationship between lithification and frictional slip behavior.

We observe that powdered samples of every halite-shale proportion exhibits predominantly velocity-strengthening friction, whereas lithified samples exhibit a combination of velocity strengthening and significant velocity weakening when halite constitutes at least 30 wt% of the sample.  Larger velocity weakening generally coincides with friction coefficients of > 0.62, cohesion of > ~1 MPa, and porosity reduction of > ~50 vol%.  Although none of our lithified samples exhibit strictly velocity-weakening friction, this is consistent with the frictional behavior of pure halite under our experimental conditions.  Scanning electron microscopy images do not show any clear characteristics attributable to velocity-weakening, but did reveal that the shear surfaces for powders tends to exhibit small cracks not seen in the lithified sample shear surfaces.  These results suggest that lithification via cementation and porosity loss may facilitate slip instability, but that microstructural indicators are subtle.