^a Department of Earth and Planetary Sciences, GEMOC ARC Key Centre, Macquarie University, Sydney, NSW 2109, Australia

^b CSIRO Exploration and Mining, North Ryde NSW 2113, Australia

^c Kennecott Canada Exploration Inc., 200 Granville Street, Vancouver, BC, Canada V6C 1S4

^d Great Western Diamond Co., PO Box 1916, Fort Collins, CO 80522, USA

^e Kennecott Canada Exploration Inc., Thunder Bay, Ontario, Canada P7B 2Y1

^f United Institute for Geophysics and Mineralogy, Russian Academy of Science, Novosibirsk, Russia

In the Slave Province, the SCLM in most areas shows a two-layer structure with a boundary at 140–160 km depth. The upper layer shows pronounced lateral variations, whereas the lower layer, after accounting for different degrees of melt-related metasomatism, shows marked uniformity. The lower layer is interpreted as a subcreted plume head, added at ca. 3.2 Ga; this boundary between the layers rises to <100 km depth toward the northern and southern edges of the craton. Strongly layered SCLM suggests that plume subcretion may also have played a role in the construction of the lithosphere beneath Michigan and Saskatchewan.

Outside the Slave Province, most North American Archon SCLM sections are less depleted than similar sections in southern Africa and Siberia; this may reflect extensive metasomatic modification. In E. Canada, the degree of modification increases toward the craton margin, and the SCLM beneath the Kapuskasing Structural Zone is typical of that beneath Proterozoic to Phanerozoic mobile belts.

SCLM sections from several Proterozoic areas around the margin of the Laurentian continental core (W. Greenland, Colorado–Wyoming district, Arkansas) show discontinuities and gaps that are interpreted as the effects of lithosphere stacking during collisional orogeny. Some areas affected by Proterozoic orogenesis (Wyoming Craton, Alberta, W. Greenland) appear to retain buoyant, modified Archean SCLM. Possible juvenile Proterozoic SCLM beneath the Colorado Plateau is significantly less refractory. The SCLM beneath the Kansas kimberlite field is highly melt-metasomatised, reflecting its proximity to the Mid-Continent Rift System.

A traverse across the continent shows that the upper part of the cratonic SCLM is highly magnesian; the decrease in mg# with depth is interpreted as the cumulative effect of metasomatic modification through time. The relatively small variations in seismic velocity within the continental core largely reflect the thickness of this depleted layer. The larger drop in seismic velocity in the surrounding Proton and Tecton belts reflects the closely coupled changes in SCLM composition and geotherm.