Influence of Time-dependent Healing on Reactivation of Granular Shear Zones in analogue models: A Community Benchmark

Inverted structures are some of the economically most important geological features worldwide. Besides their most common manifestation as traps for hydrocarbons, they are also interesting for the storage of CO₂ and extraction of other resources such as heat, minerals or hydrogen. Analogue modelling is frequently used to understand the long-term geological evolution of basins and basin inversion as an addition to numerical and mathematical models. Most analogue models use granular materials, like sands and glass beads, to simulate the brittle-plastic rheology of the crust. The main driving mechanism for basin inversion, both in nature and analogue models is the reactivation of pre-existing structures. This is due to strain-dependent weakening which leads to a reduced strength of a fault or shear zone in comparison with the surrounding bulk material. If the structure comes to a rest, several mechanisms lead to a time-dependent restrengthening of the structure. Therefore, older structures are usually more resistant to reactivation than younger ones, in the same material. In this study we use an annular shear tester to quantify the healing of granular materials commonly used for analogue models. We take advantage of a large collection of analogue material samples at the Helmholtz Laboratory for Tectonic Modelling, coming from many laboratories worldwide. To estimate granular healing, we employ slide-hold-slide tests with hold times comparable to typical analogue models of basin inversion. We show that all materials tested exhibit healing which follows a power-law relation quantified by with a healing rate. For example, fused glass microbeads showed a healing rate of 0.025 per decade in hold time. This means that for a tenfold increase in hold time the strength required to reactivate the given fault increases by 2.5%. Consequently, if a fault is inactive for a longer period of time, it is slightly stronger in comparison with a fault with shorter inactivity. Comparing the healing exponent for several materials reveals that some materials show a stronger healing than others. Glass beads have a stronger healing than sands, with quartz sands having lower healing rates than garnet or feldspar sands. Geomechanical tests on natural materials (quartz and gypsum fault gouges) and measurements of seismic velocities across fault zones suggest that healing obeys a similar power law. The healing rates in real rocks are roughly equal or higher depending on the temperature and water saturation of the fault. Albeit small, this change in reactivation strength for analogue materials might have a strong influence on the structural style of inversion if the models are run with different timespans between extensional phase and compressional phase. With a typical range of experimental time-spans of a view seconds to several hours this may result in up to 10% difference in reactivation strength similar to the difference between static and dynamic friction. This becomes especially relevant, if the angles of the formed pre-existing structures are close to the angle of internal friction of the bulk material which is the default in models where reactivated structures have been formed self-consistently in a pre-inversion phase.