Granite Pluton Geometry and Emplacement Mode Inferred from Combined Fabric and Gravity Data

Abstract

This review is based on several years of joint structural and geophysical surveys on granites. Fabric measurements include lineations and foliations, the former being the more useful in understanding the magma strain pattern in a pluton. Through the inversion of gravity data, which is particularly sensitive to density contrasts, the shape at depth of the pluton, and depth of its floor, may be derived with good confidence. Vertical lineations, when associated with a deepening of the pluton’s floor, are interpreted as feeder zones. Correlation between the inner-pluton lineation pattern and the regional deformation field helps in inferring the emplacement mode of the pluton. Combined geophysical and structural data therefore allow us to describe two main types of plutons. Flat-floored plutons are rather thin (a few kilometres), extend largely in every horizontal direction, and have several feeder zones. They are emplaced as sills in the upper crust, or within rather ductile environments in extensionnal tectonic contexts. These contrast with the thick (more than 10 kilometres) wedge-shaped plutons, extending largely in one direction along which a few root zones appear, which are V-shaped in transverse section, and correspond to the infilling of dilatant volumes of the brittle crust during transcurrent tectonics.