The emplacement and growth of laccoliths

Vita.I have reexamined the treatise by Gilbert (1877) on laccoliths, using an interplay between current geophysical methods of investigating geologic structures, and presently available finite element methods of modeling structural problems. My results support every major aspect of Gilbert's work. Thus, my work is remarkable only in that nearly all subsequent workers in the intervening century have differed substantially with Gilbert's interpretation. Gravity surveys of large laccoliths in Texas, Utah, Wyoming, South Dakota, California, and Montana reveal no. detectable density contrast between the laccoliths and the country rock in which they are emplaced. This result strongly supports Gilbert's (1877) hypothesis that control of the level of intrusion is by the density contrast between the rising magma and the weighted mean density of the overburden. The emplacement of laccoliths can be modeled using dislocation theory. The rising magma is treated as dislocation climb using a model given by Weertman (1971a, 1971b). The climb force is developed by the density contrast between the magma and country rock at depth. Weertman's model has been extended, together with field observations of laccolith feeder dikes, to obtain the magma driving pressure. Development of an emplacement model allows determination of the boundary conditions, and the growth of laccoliths is treated as a boundary value problem in continuum mechanics. I have treated the diverse shapes of laccoliths described in the geologic literature as a continuous series of growth modes with two distinct end members. One end member, type A, results from the intrusion of a single dominant sill which loads the roof like a punch, resulting in sheared margins and peripheral faults.