One question frequently posed since the advent of plate tectonic theory is ``When did it begin?''. Correlations of zones of enhanced electrical conductivity associated with orogenic belts dating to half the age of the Earth suggest that plate tectonic processes have been in operation since the Early Proterozoic. The causes for such zones are examined, and, whereas anomalies associated with modern and recent subduction/collision systems can be easily explained, an explanation for Palaeozoic and Proterozoic ones is more difficult. Correlation of the geophysical characteristics of the Trans-Hudson and Svecofennian orogenic zones, and consideration of the paleopole positions of those zones and the Superior and Slave provinces during the period 1900-1825 Ma, leads to the suggestion that a Pan-Scandamerican orogenic zone was in existence at that time.
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