Evolution of dilatant fracture networks in a normal fault — Evidence from 4D model experiments

Dilatant fractures in normal fault zones are widely recognized as major pathways of fluid flow in the upper crust where the ratio of rock strength and effective stress is suitable for their formation, but the structure of these fracture networks in 3D, their connectivity and their temporal evolution is poorly known.Here we build on 2D studies of scaled models of fracture networks in dilatant normal fault zones, using a series of X-ray computer tomographic scans of a physical model. We show how the dilatant fracture network evolves in 3D, as a complex self-organizing system with self-similar geometry.We processed the CT-scan data using a threshold filter to identify the open fracture volume, to allow visual and quantitative analysis of the evolving fracture system in 3D. Dilatant jogs initiated along the evolving fault plane coalesce into a self-similar percolating volume (Fd = 1.91). The fracture volume increases non-linearly with progressive displacement as the velocity of the fault blocks diverges from the master fault orientation and we infer that the normal stress on the fault decreases correspondingly. This process continues until the system triggers the formation of antithetic faults, with a corresponding increase in normal stress on the master fault and a decrease in the rate of fracture volume creation.We infer that although parameters like the width of the fractures are not scaled with the same ratio as length and stress, the processes and evolution of fracture geometries in our model are robust and apply to a wide range of normal fault zones in nature. Since our physical model does not involve chemical processes such as cementation or fault healing, the experiment suggests that fault systems can show a non-linear change of fracture network properties caused by a geometric evolution only.

Research highlights
► Non-linear fracture growth is produced in an experiment using dry hemihydrates powder.
► Computer tomography analysis allowed non-destructive 3D visualization and analysis.
► Quantitative analyses of the 4D fracture volumes show complex self-similar geometries.
► Dilatant fracture planes may have a complex, self similar geometry.
► Non-linear fracture growth is caused by adaptation of the system to the master fault.