Time-dependent frictional properties of granular materials used in analogue modelling: implications for mimicking fault healing during reactivation and inversion

Abstract

Abstract. Analogue models are often used to model long-term geological processes such as mountain building or basin inversion. Most of these models use granular materials such as sand or glass beads to simulate the brittle behaviour of the crust. In granular material, deformation is localised in shear bands, which act as an analogue to natural fault zones and detachments. Shear bands, also known as faults, are permanent anomalies in the granular package and are often reactivated during a test run. This is due to their lower strength compared to the undeformed material. When the fault movement stops, time-dependent healing immediately begins to increase the strength of the fault. Faults that have been inactive for a long time therefore have a higher strength than younger faults. This time-dependent healing, also called time consolidation, can therefore affect the structure of an analogue model as the strength of the fault changes over time. Time consolidation is a well-known mechanism in granular mechanics, but it is poorly described for analogue materials and on the timescales of typical analogue models. In this study, we estimate the healing rate of different analogue materials and evaluate the impact on the reactivation potential of analogue faults. We find that healing rates are generally less than 3 % per 10-fold increase in holding time, which is comparable to natural fault zones. We qualitatively compare the frictional properties of the materials with grain characteristics and find a weak correlation of healing rates with sphericity and friction with an average quality score. Accordingly, in models where there are predefined faults or reactivation is forced by blocks, the stability range of the fault angles that can be reactivated can decrease by up to 7∘ over the duration of 12 h. The stress required to reactivate an existing fault can double in the same time, which can favour the development of new faults. In a basin inversion scenario, normal faults cannot be inverted because of the strong misorientation, so time consolidation plays little additional role for such models.

Fault Networks in Triaxial Tectonic Settings: Analog Modeling of Distributed Continental Extension With Lateral Shortening

“…The mixed quartz sand respects Mohr‐Coulomb behavior with cohesion C in the order of a few tens of Pa. Static and dynamic friction coefficients are μ stat = 0.69 and μ dyn = 0.55, respectively (Rosenau et al., 2018), similar to other sands used in analog modeling (Klinkmüller et al., 2016). Healing rates are low at <1% per ten‐fold increase in hold time (Rudolf et al., 2023) as is the rate dependency of friction which is <1% per ten‐fold change in shear rate (Rosenau et al., 2018). This is consistent with time‐independency of brittle deformation in nature at the long time‐scales of interest here.…”

“…This mixture provides an appropriate visual contrast pattern that allows for the digital image correlation. The sand is of fluvial origin and as such well sorted, well rounded, rather spherical and has a rather low surface roughness (Rudolf et al., 2023). The grain size is 100–400 μm, with an average of 240 μm.…”

Along-strike seismotectonic segmentation reflecting megathrust seismogenic behavior

“…Four parameters, including the shear velocity, shear stress, normal stress, and lid's vertical displacement, are measured (more information can be found in Schulz (1994), Ritter et al (2016), andRudolf et al (2023)). The velocity stepping test (e.g., Pohlenz et al, 2020) carries out under constant normal load simulating coseismic and interseismic shear-stress drops and increases to estimate the friction parameter (a-b).…”