Tara L. Stephens - A new perspective on the emplacement of igneous sills

Dear colleagues,
 
From the 23-25th of April, we are running a workshop at the department on igneous intrusions in sedimentary basins. Some of the participants will give presentations at the department during this time. 
 
The second presentation, for Tuesday April 24th, will be given by Tara L. Stephens from The University of Leicester in “Kontinentalsokkelrommet”.
 
See details below.
 
Best regards
Christian Haug Eide
 
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Title:
A new perspective on the emplacement of igneous sills
 
Presenter:
Tara L. Stephens
PhD student, final year
School of Geography, Geology and the Environment
University of Leicester 
 
When:
Tuesday 24th of April
12:15
 
Where:
Kontinentalsokkelrommet, 2nd floor
 
Abstract:
Igneous sills are commonly considered to result from the deflection of dykes at bedding or layer interfaces, or as a result of contrasting material layer properties. Such models assume that the dominant control on emplacement is the local stress – related to layer weakness – and typically in the absence of a tectonic stress: A low-deviatoric stress state (i.e. σ1 = σ2 = σ3), which allows low-cohesion structures of all attitudes to dilate via opening mode (plane-normal) dilation. Here we present field observations from two sill complexes that challenge the requirement for layering to promote sill emplacement: (1) The Loch Scridain Sill Complex, Scotland, where sills cut vertically layered Moine basement, and horizontal cover units with consistent low-angle dips; and (2) The San Rafael Sub-volcanic field, Utah, where sills cut across horizontal bedding and formation boundaries at a consistent low angle. Detailed field observations are input into mechanical models for slip and dilation tendency, fracture susceptibility, and opening angles, to show that sills in these two regions were emplaced during horizontal shortening, rather than as a result of host rock layering. Although both localities are host to dykes, no feeder relationships are observed at the level of exposure. Our paleostress inversion of intrusions from both areas shows that dykes are compatible with a horizontal maximum compressive stress, and that the transition from dykes to sills represents a switching of the minimum and intermediate principal stresses. For the Loch Scridain Sills, these fluctuations can be linked, spatially and temporally, to near-field effects of volcanic centre construction, imposed on the far-field tectonic stress state; in Utah, the sills appear to relate to horizontal contraction imposed by the reactivation of ancient basement structures.
 
 
Biography
Tara Stephens is a final year PhD student at the University of Leicester, UK, working in the fields of physical volcanology and the emplacement of magmatic intrusions, structural geology, and rock mechanics. Prior to starting her PhD, Tara completed a combined Bachelor and Masters Geology degree (MGeol) at Leicester, which focussed on experimental rock deformation of igneous rocks. Tara’s PhD project aims to investigate whether igneous sills can be used as a record of horizontal tectonic contraction, using a combination of traditional field techniques, mechanical modelling, and experimental rock mechanics testing. Tara has completed approximately 3-months of fieldwork in total during her PhD, at three different sill complexes: the Loch Scridain Sill Complex, Isle of Mull, Scotland; The Little Minch Sill Complex, Isle of Skye, Scotland; and in the San Rafael Subvolcanic Field, Utah. Her talk will highlight the key findings of her PhD research.