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 km
depth. Their age constrains
timing of formation of thick
lithospheric keels
Kaapvaal diamond inclusions
yield ages of 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 Ga passive margin formation