STRUCTURE AND EVOLUTION OF THE CONTINENTAL LITHOSPHERE

The relative longevity of continental lithosphere in comparison with oceanic lithosphere depends on the buoyancy and strength of continents. This fact has prompted the comparison of continents with icebergs, with deep continental roots extending into the mantle. However, since icebergs are simple, homogeneous bodies, this analogy is an oversimplification. Here we review the evidence for a complex, heterogeneous structure of continental roots, and provide a new model for their formation and evolution since the Neo-Archean. Our evidence comes primarily from geochemical and geophysical measurements. Seismological models from Precambrian cratons show relatively high seismic S-wave anomalies that persist to depths of 250 km and greater. The S-wave anomalies are strongest at a depth of about 140 km, and then diminish. In contrast, Phanerozoic lithosphere often shows only weak anomalies that extend to as little as 100 km. A consideration of all available information leads us to suggest the following scenario. (1) Archean and Early Proterozoic lithospheric roots formed as the residuum of high-temperature melting in the upper mantle during the extraction of juvenile crust. (2) Middle/Late Proterozoic lithosphere commonly involved the tectonic stacking, and later melting, of oceanic lithosphere. (3) Phanerozoic lithosphere has formed in a variety of environments, ranging from the mantle wedge above Andean-type subduction zones to Basin and Range-type extended regions, where the sub-crustal lithosphere forms by basaltic melt extraction followed by chilling. Thus, we can envision a geologic map of the top of the mantle lithosphere that is just as complex as a map of the exposed Canadian shield, rather than as the interior of an iceberg.