Geophysical imaging of the shape of granitic intrusions at depth: a review

Abstract

Abstract This review deals with direct evidence (field statements and geochemistry) and indirect observations (modelling experiments, analogical models, and geophysics) on granite plutons to model their shape at depth. 3D modelling of granite plutons can be achieved using geophysical tools. Amongst these tools, heat-flow and heat-generation data used earlier to estimate the thickness of granitic plutons appear inadequate. Electrical methods are strongly influenced by near-surface heterogeneities and temperature, which minimize their effectiveness at depth. Magnetic surveys provide information on contacts between pluton and country rocks, since magnetic halos are appropriate to delineate surface contours, but the technique lacks the resolution to reveal deep boundaries. Anisotropy of magnetic susceptibility is particularly well adapted to determine the internal structures of plutons. Seismic profiles at usual frequencies (30–80 Hz) define the layered structure of the floor of several bodies but fail to show the rock-type variations and their internal fabrics, because of their low impedance contrasts. Conversely, high-resolution seismic reflection profiles reveal fault structures in granites but the pluton’s floor remains transparent. Gravity measurements have been widely applied in granites and owing to the 3D inversion of data, the shape of the pluton at depth and depth of its floor may be derived with confidence from density contrasts.