Abstract
Sills could potentially form as a result of dykes modifying their trajectory in response to remote tectonic compression. Here, we use analogue experiments to investigate how a buoyant vertical dyke adjusts its trajectory to a compressive remote stress to form a sill, and over which vertical distance this sill formation does occur. Our investigation is restricted to an intrusion propagating through a homogeneous solid, which enables us to identify the characteristic length‐scale over which a dyke responds to remote stress compression, independently of the presence of crustal layers. The experiments involve the injection of air in a gelatine solid that experiences lateral deviatoric compression. The response of the buoyant air crack to the compressive stress in not instantaneous but operates over some distance. An important observation is that some cracks reach the surface despite the compressive environment. Dyke‐to‐sill rotation occurs only for large compressive stress or small effective buoyancy. Dimensional analysis shows that the length‐scale over which this rotation takes place increases exponentially with the ratio of crack effective buoyancy to horizontal compressive stress. Up‐scaled to geological conditions, our analysis indicates that a dyke‐to‐sill transition in response to tectonic compression in homogeneous rocks cannot occur over less than two hundred meters and would need several kilometers in most cases. This is typically greater than the average thickness of lithological units, which supports the idea that crustal heterogeneities play an important role in determining the fate of dykes and in controlling where sills could form.
Highlights
[2] The transport of magma from its source regions up to the Earth’s surface occurs essentially via vertical or sub‐ vertical dykes, which feed volcanoes
As illustrated by the laboratory experiments of Lister and Kerr [1991], magma accumulation at levels of neutral buoyancy is more likely to involve lateral dyke propagation rather than sill formation because the latter case requires an additional rotation of the intrusion or the presence of a lithological plane of weakness that could be intruded
An initially vertical dyke would be expected to turn into a horizontal sill if it were to become subjected to a horizontal compressive stress field where the least compressive stress is vertical [e.g., Gretener, 1969; Gudmundsson and Phillip, 2006]
Summary
[2] The transport of magma from its source regions up to the Earth’s surface occurs essentially via vertical or sub‐ vertical dykes, which feed volcanoes. As illustrated by the laboratory experiments of Lister and Kerr [1991], magma accumulation at levels of neutral buoyancy is more likely to involve lateral dyke propagation rather than sill formation because the latter case requires an additional rotation of the intrusion or the presence of a lithological plane of weakness that could be intruded. Whether propagating buoyant dykes could reach the Earth’s surface or instead stall in the crust and lead to magma accumulation depends on the relative importance of their effective buoyancy and the ambient deviatoric stress [Watanabe et al, 1999; Dahm, 2000; Pinel and Jaupart, 2000, 2004].
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