Mechanical models to estimate the paleostress state from igneous intrusions

Tara L Stephens,Alodie Bubeck,Richard J Walker,Richard W England,David Healy

doi:10.5194/se-9-847-2018

Tara L Stephens, Alodie Bubeck + Show 3 more

Open Access

https://doi.org/10.5194/se-9-847-2018

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Journal: Solid Earth	Publication Date: Jul 9, 2018
Citations: 21	License type: CC BY 4.0

Affiliation: University of Leicester, University of Aberdeen

Abstract

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.

Highlights

Sills and dikes are traditionally treated as extension fractures with a dilation vector normal to the fracture wall; i.e. they are extension fractures (Mode I; e.g. Anderson, 1951)
Intrusions that demonstrate shear offset of markers across their margins indicate that during emplacement the dilation vector was inclined from plane normal (Muirhead et al, 2014; Stephens et al, 2017; Walker et al, 2017); this obliquity of opening can be characterised by the opening angle (Fig. 1)
The method is applicable to any dilated fracture; here we focus on the stress state associated with sill emplacement in the San Rafael subvolcanic field (SRSVF), Utah, and compare these results to fault data in the same area to demonstrate the particular importance of subhorizontal igneous intrusions as records of paleostress

Summary

Introduction

Sills and dikes are traditionally treated as extension fractures with a dilation vector normal to the fracture wall; i.e. they are extension fractures (Mode I; e.g. Anderson, 1951). This assumption has important implications for the use of sheet intrusions in constraining tectonic stress states because the minimum compressive stress (σ3) is perpendicular to the extension fracture walls; local deflections of the intrusion attitude are commonly inferred to represent local rotations of the stress axes. This is most commonly attributed to mechanical layering and the presence of pre-existing structures The method is applicable to any dilated fracture; here we focus on the stress state associated with sill emplacement in the San Rafael subvolcanic field (SRSVF), Utah, and compare these results to fault data in the same area to demonstrate the particular importance of subhorizontal igneous intrusions as records of paleostress

Dilation of pre-existing fractures

Mechanical models for fracture dilation and opening angle

Stress state model for sill emplacement

Sill geometry as a record of far-field stress

Conclusions