CRUSTAL GROWTH MODELS AND PALEOMAGNETIC METHODS FOR DEEP-TIME (ARCHEAN-PROTEROZOIC) CRATONIC RECONSTRUCTIONS

As the temporal frontier of paleogeography progresses farther back into deep time, paleogeographers must account for inevitable gaps in our knowledge of Earth history due to vagaries of geological preservation. The present distributions of rock ages and areas of cratonic preservation show trends of apparent episodically increasing growth through the past two billion years. If this trend represents actual net continental areal growth, then paleogeographers may produce maps of the ancient world that infer all “blank” areas as former oceanic lithosphere that have been subducted. If, however, the continental inventory has remained nearly constant through the past few billion years, a model named after R.L. Armstrong, then substantial continental crust has been recycled into the mantle – and blank areas on deep-time paleogeographic reconstructions may have been occupied by such vanished continental domains.

Reconstructions based on paleomagnetic data are hampered by the freedom of paleolongitude that arises from symmetry of Earth’s time-averaged magnetic field about its spin axis. In an effort to reconfigure ancient supercontinental landmasses, many paleomagnetists have utilized such paleolongitudinal freedom to follow a “closest-approach” method of cratonic juxtapositions, leaving no empty spaces between the continental blocks. This method results in ancient supercontinents substantially smaller than Pangea, surrounded by an equally larger oceanic tract than Panthalassa; it would be consistent with models of continuous continental growth. If paleomagnetic data allow for matching of apparent polar wander path segments during supercontinental tenure, however, then cratons can be arranged in both relative paleolatitude and paleolongitude; such a method would permit quantifiable separations between cratons within supercontinental assemblages. Those “holes” could have been filled by oceanic lithosphere in remnant basins like today’s Black Sea, or they may have been filled by continental lithosphere that subsequently recycled into the mantle. As such, the latter method can accommodate the Armstrong model of near-constant continental inventory through the past few billion years.