Modeling the growth of laccoliths and large mafic sills: Role of magma body forces

Abstract

[1] The use of elastic plate theory to model the emplacement of laccoliths and large mafic sills has been debated for nearly 40 years. These intrusions typically attain a horizontal width that is large relative to the emplacement depth. Provided that large-scale plasticity and/or heterogeneity is not observed in the overlying host rock, it should then be valid to approximate its deformation based on analysis of a thin elastic plate with effective properties that are the consequence of interaction among heterogeneities that are small relative to the size of the intrusion. But the predictions that are usually cited from elastic plate theory are characterized by bell-shaped geometry, in contrast to the flat-topped, steep-sided geometry typical of many laccoliths or the nearly uniform thickness typical of large mafic sills. This fact has motivated several alternate explanations of laccolith and large mafic sill emplacement. Nonetheless, elastic plate theory should be revisited in light of the fact that previous elastic plate-based predictions have, in general, not taken into account an appropriate fracture propagation condition, fluid flow in the growing intrusion, and, importantly, the influence of the weight of the magma on intrusion growth. We present a model for the growth of circular intrusions that accounts for all of these factors. The model predicts the appropriate geometry for both laccoliths and large mafic sills. The predicted thickness to length relationships are also consistent with field data. Hence, while it may sometimes be appropriate, there is, in general, no fundamental need to appeal to large-scale rock plasticity in order to explain observed intrusion geometries, and it may, in fact, be appropriate to understand the growth of laccoliths and large sills in light of a single underlying mechanical model. Citation: Bunger, A. P., and A. R. Cruden (2011), Modeling the growth of laccoliths and large mafic sills: Role of magma body forces,

Syn-Emplacement Fracturing in the Sandfell Laccolith, Eastern Iceland—Implications for Rhyolite Intrusion Growth and Volcanic Hazards

“…Moreover, if the host rock exerted the dominant control on intrusion emplacement, most of the deformation should be accommodated in the host rock Jackson and Pollard, 1988;Kerr and Pollard, 1998;Bunger and Cruden, 2011). However, extensive fracturing and faulting occurred within the Sandfell laccolith despite the fault-aided doming of the host rock.…”

“…Keywords: laccolith, cryptodome, magma flow, intrusion emplacement, strain localization, magma degassing, volcanic hazards INTRODUCTION Deformation related to laccolith emplacement has generally been considered to be focused in the host rock, and therefore the host-rock properties are assumed to control laccolith growth (e.g., Pollard and Johnson, 1973;Jackson and Pollard, 1988;Kerr and Pollard, 1998;Bunger and Cruden, 2011). However, relict cryptodomes display extensive concentric internal deformation, including brecciated carapace and strong flow-banding and partially brecciated rims (Snyder and Fraser, 1963;Goto and McPhie, 1998;Goto et al, 2000Goto et al, , 2004Stewart and McPhie, 2003).…”

Mechanisms of overburden deformation associated with the emplacement of the Tulipan sill, mid-Norwegian margin

“…In these models, doming occurs during the sill emplacement, and the dome geometry is directly controlled by that of the underlying intrusion (e.g., Gilbert, 1877;Pollard and Johnson, 1973;Corry, 1988;Goulty and Schofield, 2008;Galland and Scheibert, 2013). Therefore, the dome amplitude f equals the sill thickness t (Figure 1a) (Pollard and Johnson, 1973;Dixon and Simpson, 1987;Goulty and Schofield, 2008;Bunger and Cruden, 2011;Galland and Scheibert, 2013); f and t can be used as a proxy for the volume of the dome and intrusion, respectively. This active doming is often referred to as forced folding (e.g., du Toit, 1920;Stearns, 1978;Trude et al, 2003).…”

“…Classic models of doming involve overburden uplift in response to elastic bending above an intruding sill or laccolith ( Figure 1a) (Gilbert, 1877;Pollard and Johnson, 1973;Dixon and Simpson, 1987;Goulty and Schofield, 2008;Bunger and Cruden, 2011;Galland and Scheibert, 2013). In these models, doming occurs during the sill emplacement, and the dome geometry is directly controlled by that of the underlying intrusion (e.g., Gilbert, 1877;Pollard and Johnson, 1973;Corry, 1988;Goulty and Schofield, 2008;Galland and Scheibert, 2013).…”

“…In recent years, distinct mechanisms controlling the formation of sill-associated doming have been proposed. The most common mechanism involves synemplacement uplift accommodating sill intrusion (e.g., Pollard and Johnson, 1973;Roman-Berdiel et al, 1995;Malthe-Sørenssen et al, 2004;Hansen and Cartwright, 2006;Kavanagh et al, 2006;Menand, 2008;Bunger and Cruden, 2011;Galerne et al, 2011;Galland, 2012;Galland et al, 2014Galland et al, , 2015. These models typically assume that uplift occurs via elastic bending, and sometimes failure, of the overburden, causing forced folding and fracturing due to the intrusion.…”