The mechanics and dynamics of sills in layered elastic rocks and their implications for the growth of laccoliths and other igneous complexes

Abstract

The recent experimental work by Kavanagh et al. [Kavanagh, J.L., Menand, T. and Sparks, R.S.J. (2006). An experimental investigation of sill formation and propagation in layered elastic media. Earth Planet. Sci. Lett. 245, 799–813.] shows that lithological discontinuities and rigidity contrasts can control the formation and dynamics of sills at interfaces separating upper, rigid strata from lower, weaker strata. The present paper extends this work and focuses on its implications in terms of the length- and time-scales associated with the development of laccoliths and other igneous complexes. Sill formation controlled by rigidity contrasts is shown to provide a growth mechanism for laccoliths. The formation of a sill provides favourable rigidity anisotropy for the emplacement of subsequent sills so that laccoliths grow by over-accretion, under-accretion or even mid-accretion of successive sills. In accord with field data, this model predicts that laccoliths grow mainly by vertical expansion, representing the cumulative thickness of their internal sills, while maintaining a comparatively constant lateral extend. The model also predicts that the time-scale over which laccoliths form is essentially determined by the cumulative time between successive sill intrusions. Also, sill dynamics are controlled by viscous dissipation of the fluid along their length, which have consequences for the size and shape of sills. Viscously-controlled dynamics would enable sills to propagate further and thus to grow thicker than dykes of similar magmas. These dynamics would also enable sills to propagate faster and thus to induce non-elastic deformations in surrounding rocks that could deviate them from the interface they originally follow. This would allow them to feed new sills along other interfaces and could assist in the formation of the step structures and saucer-shapes that are commonly observed in sill complexes.