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.
ICDP Deep drilling and geophysical exploration of the Bushveld Complex, South Africa
Summary Geophysical data have been instrumental in the exploration and exploitation of the Bushveld Complex, South Africa. Surprising new economic discoveries are still being made, for example in the Northern Lobe, and major advances in exploration and analytical tools promise many more. We are still in the first-order characterization phase of understanding large igneous provinces and there is much to be learned on a wide range of topics. Little work has been published on linking borehole geophysical measurements with extensive geophysical data sets.
The effect of gradients in regional stress and magma pressure on the form of sheet intrusions in cross section
Elasticity theory predicts that a slit in homogeneous and isotropic material subjected to uniform internal pressure and uniform regional stresses will have a symmetric, elliptical form. The form in cross section of a dike from the Spanish Peaks, Colorado, and a sill from the Henry Mountains, Utah, is asymmetrical and similar to an elongate teardrop. The asymmetry of the dike and sill could be a function of gradients in the regional stress and magma pressure at the time of intrusion. An elasticity theory analysis for a slit subjected to linear gradients in regional stress and magma pressure predicts an asymmetrical teardrop form. The amount of asymmetry is controlled by the slit length, the difference between pressure and stress gradients (effective stress gradient), and the difference between the average pressure and regional stress (average driving pressure). A least squares regression curve for the theoretical form of the slit shows a good correlation with the measured form of the sheet intrusions. For the dike, a stress gradient sufficient to produce the asymmetrical form could have been generated by a ground surface slope of less than 5° or by bending the host rock into the La Veta syncline, which is cut by the dike. A pressure gradient of sufficient magnitude could have been generated in static magma with a strength of at least 104 dyn/cm2. Because the sill dilated in part by bending the relatively thin overburden, our model is inappropriate.