Porhyroblast rotation versus non-rotation
[quicktime]https://www.ub.edu/elle/wp-content/uploads/PorphNoRot1.mov[/quicktime]
This model shows the effects that “mica” caps can have on the kinematic behavior of a rigid object (a porphyroblast in this case). This model runs to a shear strain of 2.0. In a homogeneous, viscous material undergoing simple shear, a rigid object with circular cross section should rotate 57 degrees relative to the instantaneous stretching axes after a shear strain of 2.0. By assigning different competency contrasts between the matrix and mica caps (the porphyroblasts is assumed to be rigid), we can achieve rotations between zero and 57 degrees relative to the ISA. This movie shows zero degrees of rotation! This was done with Elle. Mica caps are continually generated around rigid objects during deformation through dissolution of soluable phases like quartz and feldspar, with or without growth of new mica. An important aspect of this movie is that I included manual “dissolution” to concentrate mica in the cap. If this is not done, the mica flows past the porphyroblast and the cap diminishes. Scott Johnson