Poles of rotation

One of the concepts introduced into the theory of plate tectonics is that of poles of rotation. It is supposed that the relative motion between two rigid plates can be described by the rotation of one of the plates about a pole of rotation, which shows the direction of the rotation vector. It has been pointed out that in general poles of rotation will not remain fixed. The consequences of poles of rotation which move slowly with time are examined. In particular, it is shown that fracture zones may lie very close to small circles, but that the centers of these small circles may lie as much as 20° away from the average pole of rotation. The establishment of poles of rotation from individual fracture zones is thus an inaccurate method unless it can also be shown that the pole of rotation has remained fixed.