800-625 Ma

In the four animations there is little variation in our reconstructions between 800 and 625 Ma, and so they are described together. Isotopic data suggest that Avalonia and Carolina originated as juvenile crust (proto-Avalonia - Carolina) in the peri-Rodinian ocean. Accordingly, at 800 Ma, these terranes are positioned well outboard of the Gondwanan margin. The distance, however, is unconstrained. In contrast, isotopic data indicate that Cadomia and Iberia have ancient, West African basement. These terranes are consequently positioned adjacent to the West Africa craton. Between 800 and 650 Ma, all animations show the distance between Avalonia-Carolina and Laurentia-West Gondwana progressively decreasing. This convergence is accommodated in large part by the south-southeastward drift of Laurentia-West Gondwana, and is held to be responsible for the early arc stage of the peri-Gondwanan terranes. Given that there is little evidence of coeval arc-related activity along the cratonic Gondwanan margin between 800 and 700 Ma, closure of the intervening tract of oceanic crust requires either (a) a subduction zone angled away from the Gondwanan margin or (b) the development of a Western Pacific type margin in which Gondwana is separated from the early Avalonian arc by a back arc basin.

The collision of Avalonia-Carolina with the Gondwanan margin at ca. 650 Ma brings the so-called "Avalonian-Cadomian belt" into alignment for the first time and broadly coincides with a brief hiatus in arc magmatism. The birth of the Avalonian-Cadomian belt at this time is analogous to the Mesozoic-Cenozoic evolution of western North America in that proximal and exotic terranes were incorporated into a single belt that shared a similar subsequent history.

By ca. 635 Ma, the occurrence of abundant ensialic arc-related magmatism in all peri-Gondwanan terranes, together with the presence of Gondwanan detrital zircons, indicates that a subduction zone had been established outboard of the peri-Gondwanan terranes and was angled beneath these accreted terranes and the cratonic margin of Gondwana.

625-570 Ma

The critical interval of 625 to 570 Ma is the time of greatest uncertainty because of the paucity and controversial nature of the paleomagnetic data base. As a result, there is considerable uncertainty about the paleolatitude of Laurentia. Since many reconstructions imply a connection between Laurentia and West Gondwana, resolution of this issue has fundamental implications for the interpretation of the geodynamic significance of peri-Gondwanan tectonothermal events.

Approach One: In a high latitude configuration, Laurentia, and by implication, Amazonia, drifts rapidly southward, between 615 to 580 Ma, and would imply oblique sinistral convergence across the arc along the northern Gondwanan margin. This is consistent with field data in Avalonia (Nance and Murphy, 1990, Murphy et al., 2001; Nance et al., in press) where such a style of convergence has been ascribed to the opening of intra-arc basins related to sinistral strike-slip activity (Murphy et al., 2000) and provides a geodynamic explanation for the onset of the main phase magmatic episode that characterizes the terranes along the northern Gondwanan margin. This scenario would be analogous to the modern relationship between the westward drift of North America and South America and the style of tectonic activity along the eastern margin of the Pacific Ocean.

If on the other hand, Laurentia remains at low latitudes during this time interval, the main phase tectonothermal events along the northern Gondwanan margin would require a different explanation. In this scenario, the most probable explanation is the re-establishment of subduction along the margin following accretion of outboard terranes such as Avalonia and Carolina, and the geodynamic relationship between this event and global-scale plate motions is unclear.

The potentially profound influence of the separation of Baltica from Laurentia at about 600 Ma (Meert et al., 1996) is apparent in both high and low-latitude models. This separation implies the existence of a spreading ridge between these two continents. According to Murphy and Nance, (1989) and Murphy et al. (1999), it was the collision of a spreading ridge with the northern Gondwanan margin that was responsible for the diachronous cessation of arc-related magmatism and the onset of strike-slip tectonics. Both reconstructions suggest that the colliding ridge may have been the spreading ridge between Baltica and Laurentia.Such a collision would additionally explain the change from sinistral to dextral motion along basin-bounding faults within Avalonia that occurs at about this time (Nance and Murphy, 1990; Murphy et al. 2001). The orientation of the spreading ridge between Laurentia and Amazonia would have been highly oblique to the peri-Gondwanan subduction zone, resulting the termination of arc-related magmatism (except in localized areas such as Anglesey; Gibbons and Horak, 1996) and the eastward drift of Gondwana-peri-Gondwana relative to Laurentia.

Approach Two: In these animations, we force the peri-Gondwanan terranes to remain in a general position along the Amazonian-West African margin. We feel this position best matches the geology recorded in these terranes, including the evidence of detritus derived from the adjacent cratons. If so, the style of subduction outboard of these terranes would be profoundly influenced by the drift of these continental blocks.

Once again, the drift of Laurentia-Amazonia to high latitudes during this time interval provides an explanation for the eastern Pacific-style of subducion recorded in these terranes. In this scenario, the propagation into the Avalonian-Cadomian belt of a spreading ridge associated with the separation of Baltica from Laurentia would again account for the diachronous termination of subduction.

On the other hand, arc magmatism would be expected to continue along the leading edge of Baltica (NE Norway and NW Russia), which is consistent with recent geochronological data from drill-holes beneath the Pechora Basin (Roberts and Siedlecka, 1999).

570-495 Ma

In this time interval, there is very little difference between the continental configurations derived from approaches one and two. However, there are fundamental differences in implication between the high and low latitude models for Laurentia. A high latitude position for Laurentia at 570 Ma implies a subsequent a northward drift relative to Amazonia between 570 and 550 Ma, associated with the opening of this portion of the Iapetus Ocean. On the other hand, if Laurentia maintains a low latitude position, the opening of the Iapetus Ocean between these blocks requires rapid southward drift of Amazonia and, by implication, the attached peri-Gondwanan terranes. This setting would be analogous to the modern relationship between spreading in the South Atlantic, the westward drift of South America, and the style of magmatism along the Andean margin. The peri-Gondwanan terranes are dominated by wrench-related tectonics during this interval. The relationship between this tectonothermal activity and the rapid movement of Laurentia is not immediately obvious, unless the vector of plate motion was at a low angle to the peri-Gondwanan portion of the continental margin. Such a direction would have been at a high angle to East Gondwana, and consistent with near-orthogonal collision which resulted in the formation of Gondwanaland at ca. 530 Ma (e.g. Hoffman, et al. 1998).

By 540 Ma, both models for Laurentia show the essentially the same configuration, with Laurentia at low latitudes. However, they also suggest that the spreading between Laurentia and Baltica after 600 Ma would be coincident with subduction under the leading edge of Baltica. The growing evidence for protracted deformation and metamorphism along the present northern margin of Baltica between 600 and 550 Ma (e.g. Roberts and Siedlecka, 1999) suggests that this may well have been the leading edge. This observation may shed light on controversies concerning the orientation of Baltica relative to Laurentia in the late Neoproterozoic and Early Paleozoic (e.g. Torsvik et al., 1996; Dalziel, 1997).

At 535 Ma, Baltic had reached its maximum latitudinal separation from northern Gondwana. Although subsequent convergence may be reflected in the Pampean orogeny of South America, the peri-Gondwanan terranes are dominated by stable platformal assemblages and localized rift-related magmatism at this time.