NEOPROTEROZOIC ROCKS: HISTORY OF NEOPROTEROZOIC CONTINENTS AND INDICATIONS OF A PRECURSOR MESOPROTEROZOIC SUPERCONTINENT

A world map showing the distribution and lithic types of Neoproterozoic rocks reveals the history of continents and their margins. The margins are characterized by: (1) single or multiple rifts; (2) rifted tectonic slivers or "ribbon continents"; (3) passive margin deposits; (4) convergent structures formed by continental collision or subduction; (5) accreted terranes or microcontinents; (6) Wilson cycles of opening and closing of ocean basins; (7) magmatic arcs; and (8) subduction erosion. Some margins formed in the Mesoproterozic were still in existence in the Neoproterozic and Phanerozoic. The record of Neoproterozoic continental margin events is difficult to decipher and in places the record is largely destroyed by erosion or tectonic denudation.

Most Neoproterozoic continents are fairly tectonically cohesive, but central Africa and South America are commonly divided into several independent cratons or continents, each considered to have different plate tectonic movements. We present an alternate view, namely that central Africa and South America were joined together in the Mesoproterozoic, rifted apart into two separate coherent continents in the middle Neoproterozoic, and reassembled in the late Neoproterozoic during a time of widespread magmatic activity. Our view is supported by middle Neoproterozoic continental margin deposits that form belts along the east side of South America and the west side of Africa. The deposits lie unconformably on older rocks and reveal, at least in part, the extent and shape of the underlying Neoproterozic continents. The continents contain intracontinental rifts (aulacogens) that extend inland from the continental margins.

Our configuration, which is based on geologic and paleomagnetic constraints, places Laurentia and Baltica close together and Siberia on the west side, in present coordinates, of Laurentia. Assembly of Gondwana continents is more difficult due to ambiguities concerning paleomagnetic poles. However, geologic evidence suggests that Gondwana continents were largely assembled in the Mesoproterozoic, rifted apart in the middle Neoproterozoic, and reassembled in the late Neoproterozic.