ELECTROMAGNETIC IMAGES OF MODERN AND ANCIENT SUBDUCTION ZONES

One question frequently posed since the advent of plate tectonic theory is ''When did it begin?''. Correlations of zones of enhanced electrical conductivity with orogenic belts dated to half the age of the Earth support the suggestion that plate tectonic processes have been in operation since at least the Early Proterozoic. The causes for such zones are examined, and, whereas anomalies associated with modern and recent subduction/collision systems can be readily explained, an explanation for Palaeozoic and Proterozoic ones is more difficult. Correlation of geophysical characteristics associated with the Trans-Hudson (North America) and Svecofennian (Fennoscandia) orogenic zones, and consideration of the paleopole positions of those zones and the Superior and Slave provinces during the period 1.900-1.825 Ga, lead to the suggestion that a Pan-Scandamerican orogenic zone was in existence at that time.