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1 Seismological Laboratory, California Institute of Technology, Pasadena, California 91125
We address the question of the depth extent of mantle high-velocity zones under ancient cratons by using seismic velocity maps from recent mantle tomographic studies. We divided old continents into two age provinces: 800–1700 Ma (Middle Proterozoic) and older than 1700 Ma (Archean and Early Proterozoic). The areas included in these age provinces are cross-correlated with the tomography at different depths to quantify the global occurrence of high-velocity anomaly (HVA) extensions beneath cratons. Statistically significant HVAs underlie the oldest cratons to a depth of 250 km. There is no significant consistent correlation between tomography and younger cratons. The older cratons have correlated HVAs extending to depths varying from <100 to 450 km. There is no significant difference in the average velocity anomaly beneath the Early Proterozoic part and the Archean part of the Canadian Shield, whereas the Middle Proterozoic part does not contain any HVA. The mantle under Early Precambrian crust is distinctly different from mantle elsewhere. We propose two different hypotheses to explain these results: (1) cratons of Early Proterozoic age or older contain ancient and deep, physically attached roots, or (2) these cratons contain a small "permanent" physical root that induces cold downwellings beneath it. A root thickness of 
200 km can be explained by an FeO-poor, olivine-rich mineralogy, whereas a requirement of high strength probably demands, in addition, a volatile-poor composition. Different root depths for cratons of similar age may be related to their location in the various supercontinents in which they were involved, and to their subsequent deformational history, or they may represent different snapshots in time of transient "roots," such as thermal boundary layers.
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