Segment tip geometry of sheet intrusions, II: Field observations of tip geometries and a model for evolving emplacement mechanisms

Tara Stephens,David Healy,Alodie Bubeck,Richard Walker

doi:10.30909/vol.04.02.203225

Tara Stephens, David Healy + Show 2 more

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https://doi.org/10.30909/vol.04.02.203225

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Abstract

Igneous sheet intrusions are segmented across several orders of magnitude, with segment tip geometry commonly considered indicative of the propagation mechanism (brittle or non-brittle). Proposed propagation mechanisms are inferred to represent host rock mechanical properties during initial magma emplacement; typically, these models do not account for segment sets that show a range of tip geometries within the same lithology. We present a detailed structural characterization of basaltic sill segments and their associated host rock deformation from the Little Minch Sill Complex, Isle of Skye, UK, and a broader comparison with segment geometries in three additional intrusive suites (Utah, USA; and Mull and Orkney, UK). Each separate host lithology shows multiple tip geometries and styles of host rock deformation, from elastic-brittle fracture, to viscous indentation and fluidisation. We attribute this range of host rock deformations to evolving conditions that occur at the tips both during sheet growth and arrest.

Highlights

Magmatic sheet intrusions, such as sills and dykes, play a fundamental role in magma transport through the Earth’s crust
The conceptual models used to inform interpretations of seismicity and ground deformations are derived from field observations of exposed intrusions, studying preserved magmatic networks is critical for developing our understanding of the mechanism(s) of magma propagation, and the likely signatures of emplacement
We present examples from the Little Minch Sill Complex, Isle of Skye, UK where intrusive segments display different tip geometries in the same host rock to highlight that intrusive segment geometry and the style of host rock deformation may vary with changes to the conditions of emplacement

Summary

Introduction

Magmatic sheet intrusions, such as sills and dykes, play a fundamental role in magma transport through the Earth’s crust. Seismic surveys, and analogue models have revealed that sheet intrusions are commonly segmented across several orders of magnitude in scale, from centimetres to kilometres [e.g. Segment geometry is typically linked to a mechanism of propagation, by association with host rock deformation. The conceptual models used to inform interpretations of seismicity and ground deformations are derived from field observations of exposed intrusions, studying preserved magmatic networks is critical for developing our understanding of the mechanism(s) of magma propagation, and the likely signatures of emplacement. This section summarises the key models of intrusive segment propagation and their associated styles of host rock deformation. Several factors influence the propagation mechanism, including host rock properties such as lithology [e.g. Schofield et al 2012], cohesion [Schmiedel et al 2017], and elastic moduli Two models have been proposed: (1) the elastic-splitting model [Figure 1A; Pollard 1973], and (2) the Barenblatt-cohesive zone model [Figure 1B; Rubin 1993].

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