Sill intrusion and compressive regimes: Examples of intrusion-induced compression in host-rocks during sill emplacement in the Faroe Islands

Sills of predominantly mafic compositions are ubiquitous in many onshore and offshore extension-related sedimentary basins worldwide and do in some instances also appear in extension-related volcanic settings such as those in North Atlantic islands like NW Britain, Iceland and the Faroe Islands. Both of these settings are typically composed of sub-horizontal layered strata, in which individual layers may possess variable mechanical properties and strengths. Previous models on sill intrusion have commonly invoked sill development, where intruding melts advance sub-horizontally through relatively weak layers and cut upwards through stronger ones at relatively steep angles (ramp-flat geometries), while other models point to gradual and continuous sill climbing irrespective of mechanical properties in layered host-rocks. Precursor melts to most known mafic sills worldwide stem from extension-related melting within the upper mantle. As long as extensional environments are maintained within the upper crust, ascending melts will advance to the Earth’s surface via sub-vertical dykes. Once extension cease to affect areas of intrusion, ascending melts may pool in various magma chambers, or they may be intruded as sills within the uppermost crust. Various researchers have previously attributed intrusion of inclined and/or saucer-shaped sills to regional far-field shortening/compression within actual intrusion areas, while others point to sill emplacement in more neutral environments, when it comes to principal stress axes. The model presented in this contribution strongly suggests that emplacement of saucer-shaped sills of the Faroe Islands not only caused uplifts of their overburdens following ultimate inflation, but did also impose compression/shortening on surrounding host rocks so as to trigger faulting and displacement within some of these. In turn, compression/stresses built up in surrounding host-rocks in response to ultimate sill inflation did not un-commonly trigger post-magmatic faulting within the actual solidified sills themselves.