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

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 characterisation of basaltic sill segments and their associated host rock deformation from the Little Minch Sill Complex, Isle of Skye, 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 occured at the tips both during sheet growth and arrest.