Igneous sills record far-field and near-field stress interactions during volcano construction: Isle of Mull, Scotland

Abstract

Sill emplacement is typically associated with horizontally mechanically layered host rocks in a nearhydrostatic 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.

Noneruptive Unrest at the Caldera of Alcedo Volcano (Galápagos Islands) Revealed by InSAR Data and Geodetic Modeling

“…As for the RD fault + RD sill model, we obtained convergence for a rectangular normal fault with uniform dip‐slip of 0.54 ± 0.20 m. The center of the fault is at 0.89 ± 0.08 km below the caldera floor (Figures o–r and Table ), the RD sill has 0.6 ± 0.07 m of opening, and its center is at 2.20 ± 0.14 km below the southern caldera floor. This RD sheet is less inclined (<30°) than in solutions without the fault and can be properly classified as a sill (Stephens et al, ). The estimated Δ V is 7.4 ± 0.7 × 10 −3 km 3 (injection rate of 2.55 ± 0.35 × 10 −3 km 3 /year), similar to that estimated without the fault and ~12% lower than that estimated by the model RD fault + CDM sill.…”

Mesozoic‐Cenozoic Regional Stress Field Evolution in Svalbard

“…While sills dominate in the Svalbard area, infrequent dikes also occur (Senger, Tveranger, et al, ) and provide insight into crustal stresses. Stephens et al () argue that the geometry of sill‐dike complexes can reflect the interplay between far‐field stresses and local emplacement‐related stresses. They also note that sills, inclined sheets, and dykes can be hybrid shear‐dilation fractures.…”

Tectonic stress controls saucer-shaped sill geometry

“…Mohr--Coulomb failure becomes important in regime III of our simulations, and in regime IV the intrusion propagates as a mixed mode I--II extensional shear fracture. Previous studies have highlighted the potential for sills to resolve a component of shear within the plane (Walker, 2016;Walker et al, 2017;Stephens et al, 2017Stephens et al, , 2018Coetzee et al, 2019) as part of an extensional--shear conjugate array, in cases where the tectonic stress is relatively large. Shear across the modelled intrusions here is largely controlled by their incline.…”