Use and misuse of geophysical data to determine the shape at depth of granitic intrusions

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Abstract
Geological data are seldom reliable enough to quantify the shape of granitic bodies at depth. Remote sensing measurements and geophysical methods may allow hidden features inside plutons to be recognized. Seismic methods can be employed to display the interface between the granite and its sedimentary cover. Unfortunately, impedance contrasts are very low inside the intrusive body resulting in a transparency to seismic waves. In addition, the high cost of seismic lines with sufficient resolution excludes full 3D shaping. The use of magnetic methods is limited by the low content of magnetic minerals in acidic rocks and the observed magnetic field is often flat over plutons. Some high amplitude magnetic anomalies are restricted to the contact metamorphic aureoles surrounding granites. Acidic rocks generally have a lower density than the surrounding country rocks. A negative gravity signature is therefore associated with the intrusion. To assist with geological interpretation, a detailed gravity survey should be made over the pluton with a coverage density of about 1 measurement per km2. This is necessary to register the effects of facies variations. Interpretation should be restricted to 3D methods rather than 2D modelling on profiles because granitic bodies are not uniform along any one direction. However, 3D inversion of data must take into account the regional gravity field and variations of density in and outside the pluton. With these assumptions, models are presented which can be correlated with the structures acquired during the magmatic stage of pluton emplacement. The zones where magmatic foliations are vertical correlate with deep zones identified from gravity data. They are interpreted as magma feeding zones. The information provided by these methods represents a valuable contribution to determining the mode of emplacement of granitic intrusions.