Are sills a record of applied horizontal compression?

Igneous sills represent an important component of the deformation history in volcanic settings, yet palaeostress analyses in volcanic regions focus only on dykes. The close spatial relationship between sills and dykes in layered host rocks has led many models to propose that sills are fed by dykes, and that dyke-to-sill transitions are driven by layering. Such models require a local stress rotation – from horizontal extension for dykes, to horizontal compression for sills – suggesting low-deviatoric stress, facilitating the dilation and intrusion of pre-existing discontinuities. These models, however, do not account for sill geometries that cut across layered host rocks.

To constrain the controls on the emplacement, geometry, and distribution of sills in layered media, this thesis presents a detailed structural characterization of three sill complexes: The Loch Scridain Sills (Isle of Mull, UK); the Little Minch Sills (Isle of Skye, UK); and the San Rafael Sills (Utah, USA). To constrain the stress state during sill emplacement I fit mechanical models for slip tendency, dilation tendency, and fracture susceptibility to local and overall sill attitudes. I also present a new depth-independent mechanical model to constrain the paleostress ratio and driving fluid pressure ratio during emplacement, using sill attitude and opening angle.

This thesis demonstrates that host rock layering is not always the primary control on the location and distribution of sills in layered media. Sills in Mull and Utah represent magma-filled thrust faults, which were emplaced contemporaneously with a previously unrecognized applied horizontal compression; sills in Skye reflect components of layering control, overprinted by stress state control. I show that structural field analyses are an important, and often overlooked, component of sill-complex studies. The results presented here have critical implications to the development, final architecture, and tectonic significance of sill complexes, and also to the scale of observation required to properly assess the dyke to sill transition.