The Proterozoic Nagssugtoqidian and Ketilidian mobile belts are comparable in scale with those of the Phanerozoic rather than those of the Archaean. These two Proterozoic belts differ from one another both in the tectonic displacements which gave rise to them, and in their thermal activities as expressed by igneous and metamorphic characteristics. Similar differences between modern tectonic belts have been interpreted in terms of plate tectonics. The Nagssugtoqidian is characterized by considerable crustal shortening, very limited igneous activity, and high-pressure regional metamorphism which may be related to crustal thickening resulting from both ductile and brittle overthrusting of the Nagssugtoqidian rocks over the Archaean foreland. Evidence of crustal shortening in the Ketilidian is limited, but vertical and transcurrent movements are important. Widespread igneous activity throughout the active history of the belt resulted in the formation of mainly acid volcanic supracrustal rocks and widespread granite intrusion. The appinite suite is also well represented. Metamorphism is mainly of low-pressure type. A tentative comparison can be made between the Alpine and Nagssugtoqidian belts on the one hand, and Andean and Ketilidian belts on the other.