Why Some Things May Have Looked Different in the Archean

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

30. 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.

33. ~2.8 Ga Volcanic-dominated rift margin, western Superior Province

36. 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