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 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.

Segment tip geometry of sheet intrusions, I: Theory and numerical models for the role of tip shape in controlling propagation pathways

“…We suggest that evolving conditions at the tip, including changes to the host rock and magma properties, would lead to an evolution of the dominant emplacement mechanism as the intrusion grows and arrests [Stephens et al 2021]. This evolution may be important to the preserved geometry of sheet intrusions, and the features of the tip region, with the potential for late-stage processes and features to overprint those from earlier stages.…”

“…Modelled sill dimensions are varied to consider (a) the effect of tip shape on the magnitude and distribution of stress in the host rock, and (b) the relative effect of tip shapes for sills of different lengths as a function of sill aspect ratio R " a{b. Tip shape is Segment tip geometry of sheet intrusions, I Walker et al, 2021 varied within the models by changing the n exponent in Equation 4, in the range 2 to 100, and we consider each sill tip shape for a range of aspect ratios R " t2, 5, 10, 50, 100, 1000u. For an ellipse-a superellipse where n " 2, which separates cross-like superellipses (n ă 2) from squared-appearance superellipses (n ą 2)-the curvature is given by:…”

“…This is important for interpretation, since the features associated with the preserved segments are biased towards processes that led to an end of growth for that particular observed segment; field-observationbased interpretations of growth processes are subject to preservation bias. We explore this point in more detail in part II, using field-based observations [Stephens et al 2021].…”

“…Magma sheet growth models and emplacement mechanisms are critical to contemporary studies of active intrusions, since they provide key insights into the signatures of seismicity during active magma sheet growth [Pallister et al 2010], the role of magma sheets in volcano flank instability [McGuire 1996], and surface topographic effects of intrusion in planetary rift Segment tip geometry of sheet intrusions, I Walker et al, 2021 systems [Rubin 1992;Wilson and Head 2002;Wyrick and Smart 2009]. A number of models have been proposed for sheet intrusion propagation [Rivalta et al 2015;Galland et al 2018;Stephens et al 2021], including: (1) the splitting model [Lister 1990;Rubin 1993], related to tensile stress concentration at the tip according to linear elastic fracture mechanics (LEFM) (e.g. Figure 1); (2) the LEFM-Barenblatt cohesive zone model [Rubin 1993]; (3) host rock shear-failure models, via brittle and/or ductile tip zone faulting [Pollard 1973], or viscous indentation [Merle and Donnadieu 2000;Spacapan et al 2016] (e.g.…”

“…The purpose of the present two-part contribution (see Stephens et al [2021]) is: (1) to demonstrate the effects of sill tip geometry on propagation potential, using Finite Element (FE) models, for a range of idealised non-bladed tip shapes (e.g. Figure 2A); (2) to critically assess the inference that intrusion segments represent the early stages of the intrusion process; and (3) to propose an evolutionary model for intrusion emplacement [Stephens et al 2021].…”