Tara Stephens

University of Leicester


Are sills a record of applied horizontal compression?


Dr. Richard Walker (University of Leicester) Prof. Richard England (University of Leicester) Dr David Healy (University of Aberdeen) Prof. Ken McCaffrey (Durham University)

PhD Summary

Igneous sill complexes represent a significant volumetric contribution to upper crustal magma systems, and can play an important role in petroleum system maturation and gas generation in sedimentary basins. Recent studies have shown that sills can act as major magma transport networks. Despite their significance, the causes of sill formation, particularly in terms of the transition from dikes to sills, remains ambiguous. A level of neutral buoyancy has long been argued as causing the dike to sill transition; however, natural examples show that sills can occupy broad depth/ elevation ranges within the stratigraphy. Most analogue models achieve a transition to sills using imposed mechanical layering, implying that sills are a consequence of the host rock stratigraphy. These results are used to infer that sills tend to occupy bedding planes or “weaker” layers within a stratigraphic sequence, rather than the relatively stronger layers. One problem with these models is that detailed studies of natural examples do not show a robust regional correlation between sill position and weak unit interfaces and some horizontal intrusions cut vertically-oriented host layering or foliation (e.g., the Traigh Bhan na Sgurra Sill, Isle of Mull, Scotland). Fieldwork finds that sills are low angle structures, discordant to bedding, with geometries similar to thrust faults. Sills can be found intruding thrusts in the Isle of Skye, and the Isle of Mull provides an exquisite example of un-intruded perpendicular discontinuity sets, indicative of horizontal compression during sill emplacement. Hence, this project proposes a new model for sill emplacement, whereby sills form during tectonic compression. Importantly, this model does not require host rock mechanical layering, hence can be applied to horizontal intrusions within non-layered, or vertically-layered, media.