THE INFLUENCE OF MECHANICAL ANISOTROPY ON THE BEHAVIOR OF THE LOWER CRUST

The aim of this paper is to determine what can be deduced about the rh eology of the lower crust undergoing extension, the state of stress ex isting there and how the finite strain pattern is developed, from a co nsideration of the theoretical analyses of layer-normal compression, a nalogue models and outcrops of rocks from the lower crust. It is concl uded that the extended lower crust contains units with a significant c ompetence contrast, has a high mechanical anisotropy and experiences b ulk rheological changes during deformation. Based on a consideration o f hydraulic fracturing in anisotropic materials it is argued that, dep ending on the magnitude differential stress, fluids will move through and out of the lower crust by fluid fracturing along the planar, horiz ontal stretching fabric and gently inclined normal kink-bands (extensi onal shears) or by the formation of vertical fractures.