Fault zones are key elements in plate tectonics and important pathways for fluid flow within the Earth's crust, which makes their presence or absence a governing factor for the occurrence of mineral deposits and petroleum reservoirs. Furthermore, seismic activity on fault zones is a hazard to nearby settlements. Mechanically, fault zones can be regarded as narrow, weak zones within a strong host material that form through the processes of strain localisation and strain weakening. Understanding fault system evolution in detail, however, is hindered by their long lifetime and the limited resolution of available geological data. Analogue experiments using loose quartz sand are a viable tool to circumvent these problems and to shed light on the processes controlling fault zone evolution. So far these experiments have mostly considered the kinematic evolution. In this study I develop a new analogue sandbox experiment that combines high-resolution measurements of deformation and strength in order to deepen our understanding of fault system evolution.
In a first step new scaling relations are derived that take into account the transient strength evolution of both, analogue material and natural prototype. Through detailed mechanical tests a previously unrecognised scale dependence of strain weakening is detected that restricts the applicability of the tested analogue material to models with a certain length scale. This length scale is determined by comparison with natural data, considering different common assumptions for the strength of the brittle crust. The scaling values thus obtained can be transferred to other tectonic sandbox experiments.
In a second step a new experimental set-up is developed that is capable of simultaneously measuring deformation and strength evolution at the required resolution and in various tectonic settings. A detailed description of the set-up is given and two standard experiments, a convergent wedge and a Riedel-type strike-slip experiment, are analysed with respect to the temporal relations between deformation and strength evolution. This analysis reveals neither of the two tectonic settings to be particularly well suited to the problem under consideration.
Therefore, in a third step, the new set-up is applied in a different strike-slip setting akin to a transfer fault zone connecting two dip-slip faults. The work required to grow a fault is measured as a function of fault system size. It is found to increase in an approximately quadratic relation with fault length. This is caused by a corresponding increase of diffuse deformation around the fault, which can be interpreted as reflecting sub-seismic deformation in nature. The observed dependence is in accordance with theoretical predictions for natural fault zones; and the numerical values are similar to estimates from measured earthquake energy release rates in nature.
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