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.
Igneous sills can facilitate significant lateral magma transport in the crust, therefore it is important to constrain controls on their formation and propagation. Close spatial association between sills and dikes in layered (sedimentary) host rocks has led to a number of sill emplacement mechanisms that involve stress rotation related to layering; from horizontal extension and dike emplacement, to horizontal compression and sill emplacement. Here we use field observations in the San Rafael subvolcanic field (Utah, USA), on the Colorado Plateau, supported by mechanical modelling, to show that layering is not the dominant control in all cases of sill formation. We found no compelling evidence of large sills fed by dikes; all observed cases show that either dikes cut sills, or vice versa. Local sill contacts activate and follow host layer interfaces, but regionally, sills cut the stratigraphy at a low angle. The sills cut and are cut by reverse faults (1-3 m
Objective – What do students need and want from library subject guides? Options such as Web 2.0 enhancement are now available to librarians creating subject-specific web pages. Librarians may be eager to implement these new tools, but are such add-ons a priority for students? This paper aims to start a dialogue on this issue by presenting the findings of the University of British Columbia (UBC) Library’s Subject Guides Working Group (SGWG), which was tasked with assessing current library subject guides in order to make recommendations for the update and future development of UBC Library subject guides. Methods – The working group solicited feedback through a questionnaire that was distributed to undergraduate and graduate students from a variety of disciplines at UBC. The questionnaire included an evaluation of UBC subject guides, as well as guides from other academic libraries that used various platforms such as LibGuides and SubjectsPlus. Results – Respondents to the student questionnaire indicated that a simple and clean layout was of primary importance. Students also desired succinct annotations to resources and limited page scrolling. Meanwhile, few students identified Web 2.0 features such as rating systems and discussion forums as being important for their needs. The working group used the questionnaire data to create a “Top Ten” list of student recommendations. Conclusions – The “Top Ten” list of student recommendations was combined with stakeholder feedback from faculty, liaison librarians and Library Systems and Information Technology representatives to create the SGWG’s final recommendation for subject guide revision and enhancement. For the SGWG these findings called into question the necessity of Web 2.0 technologies within subject guide pages and highlighted the need for further research on the topic of subject guide usability and effectiveness.
Abstract. Dikes and sills represent an important component of the deformation history in volcanic systems, but unlike dikes, sills are typically omitted from traditional paleostress analyses in tectonic studies. The emplacement of sheet intrusions is commonly associated with Mode I fracturing in a low deviatoric stress state, in which dilation is perpendicular to the fracture plane. Many natural examples of sills and dikes, however, are observed to accommodate minor shear offsets, in addition to a component of dilation. Here we present mechanical models for sills in the San Rafael subvolcanic field, Utah, which use field-based measurements of intrusion attitude and opening angles to constrain the tectonic stress axes during emplacement and the relative magma pressure for that stress state. The sills display bimodal dips to the NE and SW and consistent vertical opening directions, despite variable sill dips. Based on sill attitude and opening angles, we find that the sills were emplaced during a phase of NE-SW horizontal shortening. Calculated principal stress axes are consistent (within ∼ 4 • ) with paleostress results for penecontemporaneous thrust faults in the area. The models presented here can be applied to any set of dilational structures, including dikes, sills, or hydrous veins, and represent a robust method for characterising the paleostress state in areas where other brittle deformation structures (e.g. faults) are not present.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.