Structure of the Bushveld Complex from resistivity measurements
Knowledge of the deep structure of the arc-like bodies constituting the Bushveld Complex (South Africa) is essential not only to understand the emplacement history of this huge layered intrusion, but also for success in the exploration for the platiniferous Merensky Reef and chromitite layers occurring near its base. Despite many studies the structure of the complex has remained obscure. Here we describe new insights into the structure and additional geophysical boundary conditions, based on a geoelectrical study. This has provided a geoelectrical stratigraphy according to which a conductive layer below the granitic rocks, of both the eastern and western compartments of the Complex, is correlated with sedimentary rocks of the Transvaal Sequence over large areas, rather than with the mafic sequence as conventionally assumed. A new feature of this model is that the mafic sequence in both compartments occurs as segments of an inward-dipping sheet transgressing the layering of the sedimentary rocks into which it intruded. The geoelectrical model supports earlier gravity models in that mafic rocks are absent from the central part of the Complex, but shows that here the Bushveld granite directly overlies Archaean basement rather than Transvaal Sequence rocks as previously accepted.
V s structure of the crust containing the Bushveld Complex
The Bushveld Complex (BC) (Fig. 1, end of paper) was emplaced at ca. 2.06 Ga in the northern part of the Kaapvaal Craton, South Africa and is one of the largest mafic layered igneous intrusions in the world. It covers an area of ~66,000 km 2 within the craton (e.g., Walraven, 1997; Eglington and Armstrong, 2004; Webb et al., 2004). It has an abundance of platiniferous group metals, as well as large reserves of chromium, titanium, vanadium, nickel and gold (e.g., Cawthorn et al., 2006). It intruded into the intracratonic sedimentary sequences of the Transvaal Supergroup and is stratigraphically divided into the Rooiberg Group (RG), the Rashoop Granophyre Suite (RGS), the Lebowa Granite Suite (LGS) and the Rustenburg Layered Suite (RLS; e.g., South African Committee for Stratigraphy, 1980; Hatton and Schweitzer, 1995; Cawthorn and Webb, 2001; Cawthorn et al., 2006).
Mechanics of growth of some laccolithic intrusions in the Henry mountains, Utah, II: Bending and failure of overburden layers and sill formation
Deformation of host rocks during growth of a laccolithic intrusion is analyzed using the theory of bending a stack of thin elastic plates. The theoretical model suggests that magma spreading laterally in the form of a sill will eventually gain sufficient leverage on the overlying strata to deflect them upward and form a laccolith. The amount of bending increases as the fourth power of the distance the magma spreads, whereas the overburden resists bending as the third power of its effective thickness. Effective thickness is the thickness of a single layer which has the same resistance to bending as a multilayer of similar length and elastic modulus. The effective thickness of overburden in the Henry Mountains is estimated as between 17 and 23 of the actual thickness. The form of bending is similar for Newtonian, pseudoplastic, and Bingham magmas. The magnitude of the bending depends upon the total upward force and its distribution and is not simply related to magma viscosity as has been suggested by several previous investigators.

After elastic bending strata should fail over the periphery of an intrusion, the site of maximum bending strain and differential stress predicted by the theory. Field observations described in Part I correlate well with these predictions. Because bending strains are proportional to layer thickness, strata of comparable strength but different thicknesses fail at different stages of laccolith development. This leads to the different cross-sectional forms of laccoliths observed in the field.

The effect of host rocks on sill form and growth is analyzed using the elastic solution for an elliptical hole under uniform pressure. The theory suggests that sill thickness increases in proportion to length. The concentration of high stresses near the sill termination should induce permanent deformation and account for the blunt terminations described in Part I. This blunting is most likely to occur in relatively ductile rocks whereas sills simply split brittle rocks and maintain sharp terminations. The driving pressure in sills can be calculated from measurements of length and termination radius of curvature, if the yield strength of the host rocks can be estimated. This driving pressure must be greater than the overburden pressure, but sills apparently do not form or propagate by lifting their overburdens. Instead they propagate by locally deforming the host rock. After spreading over a distance about three times the effective overburden thickness, the overlying layers begin to bend upward significantly. This stage marks the transition from a sill to a laccolithic intrusion.