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Michael Niebling, Complex, University of Oslo (Norway)
Galland Olivier, Physics of Geological Processes, University of Oslo (Norway)
Planke Sverre, Volcanic Basin Petroleum Research (Norway)
Flekkøy Eirik G., Complex, University of Oslo (Norway)
Malthe-Sørenssen Anders, Physics of Geological Processes, University of Oslo (Norway)
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Saucer-shaped intrusions are fundamental intrusion geometries in sedimentary basins, resulting from the intrusion of either magma or fluidized sand. Previous studies have suggested that such saucer-shaped intrusions result from the mechanical interaction between a growing horizontal hydraulic fracture and the deforming overburden due to the near free surface. In order to quantify and understand the physical processes controlling the emplacement of saucer-shaped intrusions, we resorted to numerical simulations in 2D.
The model consists of (1) a discrete spring network representing the solid fracturing media, and (2) a continuum description of the fluid pressure using a Poisson equation. In such model, the fluid is able to flow through the porous solid and is mechanically coupled with the solid matrix. However, in our simulations, the permeability of the country rock was set to zero, so that the fluid flow was focussed into fractures.
The initial geometry of the simulations consisted of a horizontal crack (inner sill) located along a horizontal weakness. A fluid pressure was increased into the crack until it started propagating horizontally along the weakness. During its propagation, the under-pressurized fluid into the inner fracture lifted up the overburden and formed a dome, at the rim of which asymmetric stresses generated. The fracture kept propagating until a critical state where the asymmetric stresses were large enough to deflect the fracture tip upward, initiating inclined sheets. Subsequently, the inclined sheets kept propagating and exhibited a typical dip angle.
Such numerical simulations allowed us to vary independently the depth of emplacement, the rheological contrast between the horizontal weakness and the surrounding material, the mechanical properties of the overburden, the far-field tectonic state of stress, and the viscosity of the intruding fluid. The final crack shapes were consistent with saucer-shaped intrusions in nature. The results of the simulations showed that the diameter of the inner fracture and the dip angle of the inclined sheets varied systematically. Thus, we found that inner fracture diameter increased and the dip angle of the inclined sheets decreased when (1) the depth of emplacement increased, (2) the rheological contrast between the horizontal weakness increased, (3) the compressional tectonic state of stress increased, and (4) the viscosity of the intruding fluid decreased. By using a dimensional analysis, we will specify the controlling parameters of the system. Our numerical simulations are in good agreement with previous experimental results (see Galland et al., this session). Our results show that saucer-shaped intrusions typically result from the mechanical interaction between a fluid-filled crack and the Earth surface. Consequently, the saucer shape is the natural shape resulting from the evolution of large shallow flat-lying intrusions in sedimentary basins.
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