CONTROL OF GRANITE EMPLACEMENT BY REGIONAL DEFORMATION

A combination of structural measurements and gravity data is used to investigate the mode of granite emplacement in various tectonic settings. The deepest zones revealed by gravity data, provided those zones also show vertical lineations in outcrop, are interpreted as magma feeder channels. These structures, or root zones, reflect the flow pattern of the upwelling magma while being emplaced. They can be interpreted within the regional stress field active during that time. Granites intruded during transcurrent shearing have only a few feeder channels. These roots are not connected to the main shear zone. All of them lie within locally extensional areas of the regional stress field. This situation is also observed in granites emplaced during a compressional phase of deformation. When emplaced during a regionally extensional phase, the granitic plutons are very thin, with many small deeper zones in which variously evolved granite types may be intrusive. These regions, vertically differentiated, are also interpreted as feeder channels. The different morphology at depth, as well as the difference in the number of root zones suggest that deformation controls the emplacement of granitic plutons. The root zones are oriented at a high angle to the direction of maximum compression when magma is emplaced in a plastic crust. If the pluton is emplaced higher in a brittle crust, then the roots are aligned with the maximum compression, according to the Andersonian theory of fracture. The orientation of the root zone along the direction of extension in a plastic crust may be explained by shearing which acts as a valve mechanism for segregating magma. In all the examples, the geometry of the root zone relative to the major fault plane implies that the shear zone does not induce magma generation and that it does not serve as a guide for magma emplacement.