Abstract

Every judgement in science stands on the edge of error and is personal. –J. Bronowski

Field evidence on the nature of the basement of early and middle Proterozoic domains is incomplete. With minor exceptions, few data are at hand for existence of simatic regimes 2.6 to 1.0 b.y. ago. Geochemical and isotopic parameters and paleomagnetic constraints require an essentially continuous sialic crust consisting of stable cratons and mobile belts during this period. However, considerations of crustal volume and structure suggest that the area of the Precambrian sialic crust could not have exceeded about one-fourth of the present-day Earth surface; this leaves the nature of the remaining three-fourths enigmatic. Four alternative models are tested, but only two are consistent with the early and middle Proterozoic crustal record: (1) a sialic megacontinent and a paleo-Pacific simatic regime; (2) a smaller radius of the Earth. The first interpretation is difficult to reconcile with plate tectonics; inherent in the latter is the production of large volumes of two-stage mantle-melting materials, as around the Phanerozoic Pacific, but corresponding volcanic-sedimentary assemblages are very rare in 2.6- to 1.0-b.y.-old terranes. An existence during this time of a tectonically inert simatic regime is difficult to reconcile with the intense tectonic and thermal activity in contemporaneous mobile belts. The difficulties require a re-examination of the question of the Precambrian Earth radius.

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