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Volume 9, issue 4 | Copyright
Solid Earth, 9, 847-858, 2018
https://doi.org/10.5194/se-9-847-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 09 Jul 2018

Research article | 09 Jul 2018

Mechanical models to estimate the paleostress state from igneous intrusions

Tara L. Stephens1, Richard J. Walker1, David Healy2, Alodie Bubeck1, and Richard W. England1 Tara L. Stephens et al.
  • 1School of Geography, Geology and the Environment, University of Leicester, Leicester, LE1 7RH, UK
  • 2School of Geosciences, King's College, University of Aberdeen, Aberdeen, AB24 3UE, UK

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.

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We present mechanical models that use the attitude and opening angles of igneous sills to constrain stress axes, the stress ratio, and relative magma pressure during dilation. The models can be applied to any set of dilated structures, including dikes, sills, or veins. Comparison with paleostress analysis for coeval faults and deformation bands indicates that sills can be used to characterise the paleostress state in areas where other brittle deformation structures (e.g. faults) are not present.
We present mechanical models that use the attitude and opening angles of igneous sills to...
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