PALEOMAGNETIC CONSTRAINTS ON AVALONIA-GONDWANA-LAURENTIA CONFIGURATIONS FOR THE NEOPROTEROZOIC

Studies on the configuration and breakup history of the late Precambrian supercontinent Rodinia emphasize the evolution of Grenville-aged orogenic belts and Neoproterozoic passive margins. However, these events also effect continental margins that faced oceans as the supercontinent assembled and dispersed. This evolution is recorded by peri-Gondwanan terranes that are thought to have evolved along the active northern margin of Neoproterozoic Gondwana. Some of these terranes probably originated from ca. 1.2 to 1.0 Ga juvenile crust within a Panthalassa-type ocean that surrounded Rodinia and became accreted to the Gondwanan margin by 650 Ma. Other terranes formed along this margin by recycling ancient Gondwanan crust. These interpretations require specific relationships with Gondwana that can be tested against the paleomagnetically constrained movements of Laurentia and Gondwana from ca. 800-500 Ma. However, there is a lack of reliable paleomagnetic data from Laurentia between 720 and 615 Ma. Poles for 720 and 615 Ma are similar, and may by explained either by the small movement of Laurentia, or by a loop-like movement similar to the older Grenvillian Loop. Similarly the location of Amazonia is unconstrained paleomagnetically. We apply two approaches. In the first, we assign the minimum movement to Laurentia and Gondwana required to satisfy the paleomagnetic data and examine the relationship between this motion and the tectonothermal evolution of the peri-Gondwanan terranes. In the second, we again satisfy the paleomagnetic data but in time periods where there is no data we allow Laurentia and Gondwana to migrate in a fashion that would make them compatible with the tectonothermal history of peri-Gondwanan terranes. The available paleomagnetic data from both West and East Avalon show systematically lower paleolatitudes than predicted by these models. An inescapable conclusion from both approaches is that either Laurentia had a more complicated movement history between 720 and 615 Ma than is currently constrained by the available data, or the configuration of Laurentia-W. Gondwana-Avalonia on many reconstructions is incorrect. The latter is borne out by the failed test of the correlation between Dalradian Scotland and the Peruvian Arequipa massif, and by the discovery of the Neoproterozoic Marañon belt in the northern Andes.