ROTATION OF RELATIVELY LARGE RIGID OBJECTS DURING DUCTILE DEFORMATION - WELL ESTABLISHED FACT OR INTUITIVE PREJUDICE

Geologists have generally accepted the intuitively appealing concept that rigid objects, which are larger than matrix grains, rotate relative to geographic co-ordinates during non-coaxial ductile deformation. This has been reinforced by experiments in which rigid body rotation occurs in continous media deformed under bulk simple shear. Widespread acceptance of this concept, and its application to rock deformation, incorrectly implies that it has been thoroughly and successfully tested in real rocks. When data obtained from deformed rock suggest that relatively large rigid objects do not rotate, they have generally been rationalized as products of coaxial deformation and considered rarities. This degree of rationalization is no longer acceptable in the light of accumulating data, as well as more recent modelling which suggests that non-rotation of relatively large rigid objects during ductile deformation is the more common situation, and that very special circumstances are required for such objects to rotate. If these objects do not rotate relative to geographic co-ordinates during non-coaxial ductile deformation, they provide a unique tool for unraveling the kinematic history of orogenic belts.