LATE PROTEROZOIC PLATE TECTONIC AND PALEO-ENVIRONMENTAL RECONSTRUCTIONS: IMPLICATIONS FOR THE SNOWBALL EARTH HYPOTHESIS

Plate tectonic and paleo-environmental reconstructions have been produced for 5 time intervals during the Late Proterozoic (750 - 550 Ma). During this time interval the supercontinent of Rodinia rifted apart and reassembled to form a new supercontinent, Pannotia. These reconstructions show the inferred location of active plate boundaries, the sequence of Pan-African collisions that formed Gondwana - the core continent of Pannotia, as well as the approximate distribution of deep ocean basins, shallow seas, lowlands, and mountainous areas. The plate tectonic reconstructions were produced by working backwards from well-constrained early Cambrian reconstructions, using sparse but internally consistent paleomagnetic results from North America, Africa, and Australia, carefully considering the timing of tectonic events, employing the principle of “minimum surprise” and eschewing true polar wander. The paleo-environmental interpretations are based on a global compilation of chronologic, stratigraphic, lithologic, environmental, and paleoclimatic information from over 350 late Proterozoic localities. Particular attention was paid to the occurrence of lithologic indicators of climate such as tillites, evaporites, and carbonates.

Form our study we conclude that Rodinia, much like the late Paleozoic Pangea, stretched from the North Pole to the South Pole. Pannotia, on the other hand, was primarily a southern hemisphere supercontinent, centered on the South Pole. Speculative, late Proterozoic reconstructions that show the continents arrayed along the equator are untenable. The late Proterozoic was an Ice House world. The low latitude, Himalayan-sized Grenvillian and Pan-African mountain ranges that crossed the centers of Rodinia and Pannotia, respectively, may have promoted these Ice House conditions. Regarding the “Snowball Earth” hypothesis, we find that the distribution of tillites indicates that there were distinct tropical and temperate zones throughout the entire 200 million year interval. Using the method of Scotese and Barrett (1988), it was determined that these zones are statistically significant. Though some tillites are found in tropical latitudes, the majority are found in temperate and polar latitudes. Cap carbonates, on the other hand, appear to occur at all latitudes.