Abstract
Sill emplacement is typically associated with horizontally mechanically layered host rocks in a near-hydrostatic far-field stress state, where contrasting mechanical properties across the layers promote transitions from dykes, or inclined sheets, to sills. We used detailed field observations from the Loch Scridain Sill Complex (Isle of Mull, UK), and mechanical models to show that layering is not always the dominant control on sill emplacement. The studied sills have consistently shallow dips (1°–25°) and cut vertically bedded and foliated metamorphic basement rocks, and horizontally bedded cover sedimentary rocks and lavas. Horizontal and shallowly-dipping fractures in the host rock were intruded with vertical opening in all cases, whilst steeply-dipping discontinuities within the sequence (i.e. vertical fractures and foliation in the basement, and vertical polygonal joints in the lavas) were not intruded during sill emplacement. Mechanical models of slip tendency, dilation tendency, and fracture susceptibility for local and overall sill geometry data, support a radial horizontal compression during sill emplacement. Our models show that dykes and sills across Mull were emplaced during NW–SE horizontal shortening, related to a far-field tectonic stress state. The dykes generally accommodated phases of NE–SW horizontal tectonic extension, whereas the sills record the superposition of the far-field stress with a near-field stress state, imposed by emplacement of the Mull Central Volcano. We show that through detailed geometric characterisation coupled with mechanical modelling, sills may be used as an indication of fluctuations in the paleostress state.
Highlights
Sheet intrusions represent an important and directional volumetric addition to the crust, with dykes accommodating horizontal extension, and sills commonly accommodating vertical thickening
To consider sill emplacement controls, we present a structural characterisation for sills that cut basement and cover sequences in the Loch Scridain area of western Isle of Mull, Scotland (Fig. 1a): The Loch Scridain Sill Complex
Sheet intrusions that fall between these angular ranges we describe as an inclined sheet
Summary
Sheet intrusions represent an important and directional volumetric addition to the crust, with dykes accommodating horizontal extension, and sills commonly accommodating vertical thickening. Several studies of intrusive systems that include sills have identified the potential for a far-field (tectonic) stress state control on intrusion geometry England, 1988; Chaussard and Amelung, 2012; Walker, 2016) This relationship between intrusion geometry and stress state is important for several key reasons: (1) far-field horizontal compression and shortening during intrusion may serve to inhibit vertical ascent of magma (via dyking) toward the surface 2, 3c), whereas the Mesozoic sedimentary, and Paleogene volcanic, cover sequences are horizontally bedded (Fig. 3a, b), presenting a rare opportunity to investigate the role of mechanical layering in controlling sill geometry. State fluctuations arising from inflation of the Mull Central Complex
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