Mechanical analyses of the emplacement of laccoliths and lopoliths

Abstract

Abstract. Elastic deformations of host rocks during the emplacement within the Earth's crust of magmatic intrusions, such as laccoliths and lopoliths, are analyzed. The present analysis is built upon semianalytic elastic solutions for a pressurized horizontal crack buried between an overburden and a semi-infinite base. The current work improves upon recent analyses by including several important features inherent to the development of laccoliths and lopoliths. First, both elongated intrusions (plane strain case) and circular intrusions (axisymmetric case) are described by the current models. Second, the effect of the difference in elastic moduli between the overburden and the substrate is considered.This difference in elastic moduli is characterized by one of the Dundurs parameters. Third, the stress intensity factor at the tip of the crack is assumed to be zero. Thus the stresses are finite, and the obtained laccolithic shapes are doubly hinged with horizontal slopes at the peripheries, as observed in the field. Fourth, unlike plate bending models, which are commonly used for major laccoliths, the current models describe a full range of ratios of intrusion width to overburden thickness. Numerical results are obtained for different combinations of the geometrical parameters, the material contrast and the driving pressure distributions. Graphs of these results confirm the plate bending model for large laccoliths, support Gilbert's concept pertaining to small laccoliths, permit the prediction of the sill-laccolith transition, allow the analysis of the asymmetry in the vertical displacements above and below the intrusion, and lead to a possible mechanism of deep lopoliths emplacements. Some of the obtained results are compared to the available field data from the Henry Mountains.

Intrusion and deformation at Campi Flegrei, southern Italy: Sills, dikes, and regional extension

“…Apparently, the sill initially spread to a given distance and then evolved primarily by thickening. This condition is consistent with observations of exhumed magmatic sills and laccoliths [Paige, 1913;Pollard and Johnson, 1973;Jackson and Pollard, 1988;Zenzri and Keer, 2001;McCaffrey and Cruden, 2002]; it may also be consistent with the swelling of aqueous sills, although this has yet to be confirmed independently. In either case, the pattern is consistent with uplift being driven by an increasing overpressure within a sill of constant radius ( Figure 5).…”

Rate of construction of the Black Mesa bysmalith, Henry Mountains, Utah

“…Hunt 1953;Kerr & Pollard 1998;Zenzri & Keer 2001), because it seems to approximate the laccolithic structural form conceived initially by Gilbert (1877). , used the BMb to validate a three-stage model for forceful emplacement: the sill phase, controlled by lateral propagation of a thin igneous sheet; followed by the laccolith phase characterized by lateral propagation plus bending of the overburden; and finally the bysmalith stage, during which lateral propagation is stopped and the …”

Determining magma flow in sills, dykes and laccoliths and their implications for sill emplacement mechanisms

“…Mechanical theories for the emplacement and growth of pressurised cracks, sills and laccoliths are well established and continue to be refined by analogue, numerical and theoretical studies (Roman-Berdiel et al 1995;Kerr and Pollard 1998;Zenzri and Kerr 2001;Malthe-Sørenssen et al 2004). By their very nature, such studies reach a consensus that the key parameters that govern intrusion shape are magma-driving pressure and the depth of emplacement.…”