Two-stage rifting of Pangea by two different mechanisms

Two types of rifting may have characterized the two-stage breakup of Pangea—passive rifting during Early Jurassic rapid supercontinental motion and active rifting during Cretaceous supercontinental stillstand. This suggestion is based primarily on the observation that global continental angular momentum in the hotspot reference frame was very high from 180 to 150 Ma and very low from 140 to 90 Ma. From 180 to 150 Ma, motion of the continents was probably too high to allow active rifting. Instead, rifting may have been due to stresses associated with the motion. The pole of rotation for the continents was very close to that for opening of the central Atlantic. Africa lagged behind the other continents in its progress over the mantle, perhaps because of the anchoring effect of a mantle slab under the Alleghenian suture. From 140 to 90 Ma, the low continental momentum is consistent with Anderson's model for continental breakup, in which stillstand of a supercontinent leads to mantle up-welling and continental dispersal through active rifting and sea-floor spreading. Major breakup of the hemispherical supercontinent occurred during the low-momentum period, and the geoid anomaly coincides with the supercontinent's position at that time. The two types of rifting have different geological and paleomagnetic signatures that may allow recognition of them for earlier supercontinents.