Evolution and morphology of saucer-shaped sills in analogue experiments
Andrew P. Bunger, Robert G. Jeffrey, Emmanuel Detournay, 2008. "Evolution and morphology of saucer-shaped sills in analogue experiments", Structure and Emplacement of High-Level Magmatic Systems, K. Thomson, N. Petford
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The growth of shallow sills is studied in analogue experiments performed in polymethyl methacrylate (PMMA) and glass. The experimental fractures curve towards the surface to become saucer-shaped, which is consistent with many field observations of dolerite sills. The curvature of the saucer is shown to decrease as the in situ stress acting parallel to the surface increases relative to an estimate of the strength of the fracture-induced stress field. The initially circular fractures also elongate in plan view to become egg-shaped, a tendency that decreases with increasing importance of viscous dissipation in the growth process. Sill emplacement is further examined mathematically by considering a shallow, circular, fluid-driven fracture propagating in a homogeneous brittle elastic material. The fractures are shown to undergo three transitions related to the mechanics of sill growth. Each transition is associated with a characteristic time that is derived from analysis of the governing equations using scaling methods. These characteristic times provide an estimate of how long viscous flow is the dominant energy dissipation mechanism, how long significant lag between the fluid and fracture fronts is expected to persist, and how long the sill will take to attain an extent that is of the same order as its depth.
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Structure and Emplacement of High-Level Magmatic Systems
There are continual rounds of annual conferences, special sessions and other symposia that provide ample opportunity for researchers to convene and discuss igneous processes. However, the origins of laccoliths and sills continue to inspire and confound geologists.
In one sense, this is surprising. After all, don’t we know all we need to know about these rocks by now? As testified by the diverse range of topics covered in this volume, the answer is clearly ‘no’.
This book contains contributions on physical geology, igneous petrology, volcanology, structural geology, crustal mechanics and geophysics that cover the entire gambit of geological processes associated with the shallow emplacement of magma. High-level intrusions in sedimentary basins can also act as hydrocarbon reservoirs and as sources for thermal maturation.
In drawing together a diversity of perspectives on the emplacement of sills, laccoliths and dykes we hope to advance further our understanding of their behaviour.