Why Some Things May Have Looked Different in the Archean Andrew Hynes, McGill University

Conclusions Stretching at Archean passive margins would have resulted in markedly thinner passive-margin sedimentary sequences. Passive margins would have been characterized by voluminous mantle-derived melts. The voluminous melts would have approximately restored crustal thicknesses to those preceding stretching. Development of thick lithospheric roots would have resulted in passive margins similar to modern ones, due to the resulting cooler geotherms. Driving forces for plate motion would have been half those today but resistive forces would have been reduced by much more. Subduction rates would have been more than twice those today, perhaps leading to universally erosional subduction zones.

~2.8 Ga Volcanic-dominated rift margin, western Superior Province

Thick Kaapvaal lithosphere at 3.3-2.9 Ga <2.88>2.76 Ga Witwatersrand conglomerates (Klerksdorp; Kositcin et al. 2001) contain detrital diamonds (Hallbauer et al. 1980) Diamonds form at 150-250 km depth. Their age constrains timing of formation of thick lithospheric keels Kaapvaal diamond inclusions yield ages of 3.3-3.2 Ga (Sm-Nd) and 2.9 Ga (Re-Os) (Richardson et al., 1984; Pearson et al. 1998) Thick (250+ km) lithospheric keel beneath Kaapvaal (James et al. 2001) was present prior to 3.0-2.9 Ga passive margin formation