Fracture Network Variability in Granite: Insights from Wilsons Promontory, Southeast Australia

Permeability in crystalline rocks, considered for use as geothermal reservoirs or deep geological repositories, is controlled by networks of well-connected fractures. Fracture connectivity depends on fracture orientation and density, which are influenced by tectonic and non-tectonic stresses, pre-existing foliations, and fracture zones. This study investigates fracture variability in the Devonian Wilsons Promontory granitic batholith of southeast Australia, which intruded Devonian metasediments that are unconformably overlain by Cretaceous rift-related sedimentary rocks.

Outcrop analogues allow 2D and 3D observation of fracture networks at scales from centimeters to hundreds of meters, complementing sub-meter-scale borehole data and regional lineament mapping. Additionally, digital outcrop models from uncrewed aerial vehicle (UAV) surveys enable fracture characterization in outcrops that are difficult to physically access, such as the granites in the study area. In this study, over 2,500 fractures were mapped and characterized from a UAV-derived point cloud. Most fractures strike NNW-SSE to N-S; they are are interpreted as extensional and to have formed coevally with NNW-SSE striking joints in outcropping Cretaceous rocks during regional uplift under NNW-SSE horizontal compression. Domains characterized by distinct fracture patterns are separated by meter-scale fracture zones, suggesting structural segmentation within the granite.

Future work will investigate the geometry and origin of these domain-bounding fracture zones and their links to mechanical heterogeneities in the granite. These insights will inform discrete fracture network (DFN) and hydrological models of granite reservoirs and repositories for spent nuclear waste. They will also support comparisons of brittle deformation in granitic versus siliciclastic rocks under shared tectonic regimes, relevant to energy projects involving multi-level, multi-lithology reservoirs.