Sill Intrusion and Associated Compressive Regimes: Examples from Basaltic Sills of the Faroe Islands and their Ambient Basaltic Host-rocks

Abstract

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 like those of North Atlantic islands such as 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 some variations with respect to mechanical properties/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 largely irrespective of mechanical properties in layered host-rocks. 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 prevailing pressure conditions. The model presented in this contribution indicates that emplacement of saucer-shaped sills in the lava successions of the Faroe Islands not only caused uplifts of their respective overburdens following ultimate inflation, but did in some instances also impose sub-horizontal compression/shortening of 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 sill inflation, likely triggered post-magmatic faulting within outer parts of the actual solidified sills themselves.