Use of vegetable oil and silica powder for scale modelling of magmatic intrusion in a deforming brittle crust

In the upper crust, intrusive bodies adopt different configurations, depending on the tectonic setting. This paper describes a new technique for analogue modelling of such intrusions, specifically of low-viscosity magma into a deforming brittle crust. Proper dynamic scaling is an important consideration. If the crust has a Coulomb failure envelope, the model material should have a cohesion C of 40 Pa to 7500 Pa. If the intruding magma is highly mobile, the model fluid should have a viscosity of 4 × 10− 9 Pa s to 75 Pa s.For the model crust, we have used crystalline silica powder (SI-CRYSTAL), siliceous microspheres (SI-SPHERE) of grain size < 30 μm, and a mixture (SI-MIX) of both materials. The mechanical properties of these powders have been obtained by shear and tension–shear tests. SI-CRYSTAL powder is cohesive (C ≈ 300 Pa, angle of internal friction ϕ ≈ 45°) and represents competent rock. SI-SPHERE powder is much less cohesive (C ≈ 10 Pa, ϕ ≈ 25°) and represents incompetent rock. SI-MIX has intermediate properties. The model magma is a vegetable oil, which solidifies at room temperature. Its viscosity when molten is η = 2 × 10− 2 Pa s at 50 °C.Using these materials, we have done some preliminary experiments, to investigate the intrusion of low-viscosity magma into sedimentary basins. In non-deformed settings, intrusions were saucer-shaped cone sheets. In horizontal extension, they were steep dykes. Finally, in horizontal shortening, intrusions were basal sills that branched into thrust faults.