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Abstract: Three-dimensional seismic datasets have provided unrivalled insights into magma flow within sub-volcanic systems. One of the key revelations is that sills appear to be constructed of a series of discrete magma lobes that form during the emplacement of magma into host-rock. We focus on a large sill, within the Faroe-Shetland Basin, North Atlantic, that is well imaged on seismic data, and identify the presence of 'broken bridges' within the sill, developed between elongate magma lobes, and reveal for the first time in three dimensions the development of broken bridges. Critically, by relating the imaged structures to key outcrop-scale examples we confirm that bridge and broken-bridge structures are oriented perpendicular to the magma flow direction. This work thus demonstrates a key link that can be made between seismic-scale investigation of intrusions and sub-seismic (outcrop-scale) processes, highlighting the seemingly scale-invariant nature of the magmatic emplacement process.
In recent years our knowledge of sub-volcanic magmatic plumbing systems has increased greatly from the study of hydrocarbon industry 3D seismic datasets in offshore sedimentary basins (Davies et al. 2002; Smallwood & Maresh 2002; Thomson & Hutton 2004; Archer et al. 2005; Planke et al. 2005; Thomson & Schofield 2008). A key advance in our understanding of the geometry and emplacement of sub-volcanic sheet intrusions has been the revelation that sheet intrusions are emplaced as a series of lobes, broadly analogous in morphology to lava lobes observed in extrusive settings (Thomson & Hutton 2004; Hansen & Cartwright 2006). Such magma lobes have been documented in numerous sills within basins along the NW European continental margin (NE Rockall Trough, Thomson & Hutton 2004; Faroe-Shetland Basin, Trude 2004; Norwegian margin, Miles & Cartwright 2010). However, even high-quality 3D seismic reflection datasets have a limit to what they can resolve; thus, to allow a better understanding of detailed emplacement mechanisms and to test the validity of subsurface- based interpretations, it is critical to bridge the resolution gap that exists between seismic and outcrop datasets.
This paper provides a key link between seismic-scale magma lobes and sub-seismic-scale intrusion processes. In particular, we provide the first ever seismic-based 3D analysis of the geometry of 'broken bridges', and we argue that many of the magma lobes described in previous 3D seismic studies (e.g. Hansen &...