Mechanics of emplacement of basic intrusions

Abstract

The sunken nature of many basic intrusions can be explained by simple models for flexure of the lithosphere subjected to loading by a magma. The lithosphere is divided into the strata above the magma which deforms as a stack of elastic plates, and the substratum below the magma which is modelled as an elastic plate overlying a weak fluid asthenosphere. Plane-strain plate theory is used to calculate shape and size of plutons. Significant mechanical parameters controlling the shape are: (1) depth of emplacement; (2) width of intrusion; (3) total lithospheric thickness; (4) magma density; (5) effective thickness of the overburden; and (6) magmatic pressure. In areas with lithospheric thickness of 50–100 km, basic intrusions emplaced in the upper crust (< 10 km) will be laccolithic whereas bowl-shape intrusions form in a thin lithosphere (< 50 km) or result from viscoelastic deformation subsequent to emplacement. For a thick lithosphere (100 km) emplacement of magma in the lower crust should result in thin sill-like plutons. If lithospheric thickness was 25 km and magma pressure about 1 kbar, emplacement of the Freetown basic complex at 15 km depth results in a maximum magma thickness of 10 km. The thickness of the model compares well with that determined from Bouguer gravity models. Calculated thickness from the elastic bending model for Umfraville, Thanet and Tudor gabbros, Ontario, yield values smaller than thickness determined by gravity. Progressive thickening of these plutons subsequent to emplacement may have caused the greater thickness, where the underburden, subjected to high temperatures, reacted viscoelastically. Dip of igneous layering towards the center of some intrusions may result from viscoelastic depression of the floor while the magma crystallized.