Rock interface strength influences fluid-filled fracture propagation pathways in the crust

Rock interface strengths have often been assumed to be zero in numerical and analogue models of fracture propagation and magma intrusion in the crust. Rock strength tests were performed to explore the role that rock interfaces have on the geometry and propagation dynamics of fluid-filled fractures in the crust. We used a 1 kN test machine to study 5 mm thick cuboidal specimens cut from a sandstone-siltstone rock core, where the strata were known to host magma intrusions and the rock interface between the units was intact. By measuring the load required to grow a crack running along the lithological contact between the layers we calculate its fracture toughness Kc. The siltstone had an average Kc of 0.56 ± 0.03 MPa m1/2 compared to the sandstone at 0.42 ± 0.02 MPa m1/2. The rock interface had intermediate average fracture toughness to the parent units at 0.45 ± 0.03 MPa m1/2. These results have important implications on fracture propagation pathways through rocks, as well as for the geometry and propagation dynamics of magma intrusions in the crust.