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Why Some Things May Have Looked Different in the Archean

This article discusses the geological features of Archean passive margins, including thinner sedimentary sequences, mantle-derived melts, thick lithospheric roots, and unique plate motion dynamics. It also explores the formation of thick lithospheric keels and the presence of diamonds in Witwatersrand conglomerates.

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Why Some Things May Have Looked Different in the Archean

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

  2. 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.

  3. ~2.8 Ga Volcanic-dominated rift margin, western Superior Province

  4. 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

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