Centrifuge modelling of laccolith intrusion

Laccolith intrusion has been investigated by centrifuge modelling. Silicone putty, representing magma with Bingham rheology, is intruded by overburden pressure through a circular conduit into a stack of paraffin wax layers, which represent sedimentary strata. The model intrusions evolve through a sequence of geometric forms similar to those exhibited by natural laccoliths; the results are also in accord with published mathematical analyses of laccolith formation. The model magma first flows laterally between overburden layers to form a sill. As the sill increases in area, the cover strata are arched upwards into a sinusoidal, ‘bending’ form. Subsequently, the overburden fails abruptly by kinking above the periphery of the intrusion, and the form changes to a cupola separated from surrounding horizontal strata by a sharp inflection in the layering. Finally, strata on the flanks of the cupola undergo a second episode of localized failure. The top of the dome becomes flat, and the dipping flanks become a monoclinal flexure surrounding this ‘kink’ form.A number of variables influence this evolution. Increased overburden strength, thickness of overburden strata, and total thickness of overburden all prolong the bending form to larger intrusion size, delay the localized failure which marks the cupola and kink stages, and reduce the aspect ratio of the intrusion at all stages. Effectively bonded overburden interfaces have a similar effect, but interfaces with sharply defined yield strengths enhance development of the monoclinal flexure. Magmatic driving pressure affects the rate of intrusion, but not the evolution of its form. Intrusion rate is also influenced by magma flow properties, and probably by the form of the feeder conduit. The range of final forms preserved by natural laccoliths probably results from competition between rate of intrusion, controlled by driving pressure and conduit geometry, and rate of chilling of magma in the laccoliths. The model laccoliths form in times that scale to between one and a few tens of years, in agreement with available estimates for natural laccoliths.