Rotation of domains with the upper crustal brittle layer around vertical axes has been documented in many actively deforming regions. Some domains in western Turkey and elsewhere comprise sets of blocks that are bounded by subparallel faults, are elongated and angular in plan view, coupled to the underlying plastic deformation, and rotate in the same sense at a uniform rate that may equal approximately half the vertical vorticity in the underlying plastic deformation. However, other domains in western Turkey that appear approximately circular in plan view and comprise a single block with little internal deformation appear to rotate in the opposite sense to the regional counterclockwise vertical vorticity, and are thus apparently decoupled from the underlying plastic deformation. Rotational behavior of any block in the brittle layer may be affected by torques acting both on its base and on its margins. The torque acting on the base of a circular block with radius R caused by viscous coupling with the underlying plastic deformation is likely to be proportional to R4, as well as proportional to the difference between the angular velocity of the block and that expected given the vertical vorticity in the underlying plastic deformation.

The fricitional torque acting on the margins of such a circular block is likely to be proportional to R2. Thus, provided a circular block has sufficiently small radius, the frictional torque acting its margins will exceed the viscous torque acting on its base. Frictional torques generated in this manner may cause circular blocks to rotate in the opposite sense to the regional vertical vorticity beneath the brittle layer. The circular domains identified in western Turkey comprise single blocks, are within the size range for frictional torques to dominate viscous torques, and their geometrical relationship to adjacent domains and to the regional deformation pattern allows them to rotate in the opposite sense to their surroundings. This suggests that frictional torques may cause their observed clockwise rotations. ¿American Geophysical Union 1990