Control of pore fluid pressure on depth of emplacement of magmatic sills: An experimental approach

Abstract

In sedimentary basins, the emplacement of magmatic sills tends to occur within rock of low mechanical competence and permeability, such as shale. This often contains pore fluids at abnormally high pressure. We first theoretically show that, in anisotropic media, the higher the pore fluid pressure, the deeper the sill emplacement. Then we introduce a new technique for analogue modelling of such intrusive bodies under conditions of fluid overpressure (greater than hydrostatic), which corroborate the theoretical analysis. As an analogue of brittle sediment, we use a diatomite powder. This material is a dry, fine-grained, frictional, cohesive and permeable material. As an analogue for magma, we take a silicone putty (RTV silicone), which is at first Newtonian, but then solidifies at room temperature. We use compressed air as a pore fluid. Under these experimental conditions, we investigate the intrusion of magma into the host powder under various fluid overpressures. In homogeneous diatomite powder, having uniform cohesion, intrusive bodies are segmented dykes. These become feeders to sills, if the fluid overpressure exceeds the weight of overburden. Where the sedimentary column has a horizontal discontinuity in strength, the transition from dyke to sill occurs at a smaller overpressure (hydrostatic < λ ex < 1). As a possible illustration of these results, we consider sills within source rocks of the Neuquén Basin and of the Parana Basin, both in South America, where overpressure may have resulted from maturation of organic material.