Aspects of precambrian cyclic growth in the Atlantic region

Abstract

Recently compiled sub-Phanerozoic geologic maps provide the areal proportions by Precambrian era/eon (Archean eon, Early-, Mid-, and Late Proterozoic eras) of exposed and buried Preccambrian crust in the four Precambrian platforms of the Atlantic region. Because a tectonic-based time-rock classification is used, areal proportions of Precambrian crust by era/eon do not reflect original growth rates of continental crust, but rather accumulated orogenic histories by era, with younger eras gaining at the expense of older. Notable differences by continent include the high proportions of Late Proterozoic crust in Africa Proterozoic crust in North America, manifestations of the relative abundances of Pan-African and Hudsonian fold belts respectively in the southern and northern continents. The four Atlantic continents are considered to have developed during at least six horizontal crustal oscillations or cycles, each cycle of about 400 Ma duration. The three younger cycles, back to 1000 Ma, can be readily interpreted in terms of the Wilson Cycle; and the two preceding cycles, back to 1800 Ma, less readily so. In the still older cycles, the nature of the operating plate tectonic process is highly controversial and uncertain. The present state of paleomagnetic studies, so important to continental paleoreconstructions, does not provide unequivocal answers to many critical problems. Models for continental cyclicity advocate a thermal instability mechanism involving excessive heating of the upper mantle due to the presence of an insulating supercontinent (Pangea), leading to supercontinental dispersal and eventual reassemblage. The complete cycle from one supercontinent to the next takes about 400 Ma.