Precambrian sial-sima relations: evidence for earth expansion

Abstract

It is suggested that the early and middle Proterozoic crustal record is not amenable to explanations in terms of present-day Earth surface dimensions. With few exceptions, lithostratigraphic, geochemical and isotopic data for the 2.6−1.0 aeons time span indicate intrasialic crustal environments — with little evidence for the existence of contemporaneous sima and operation of two-stage mantle melting processes. Unlike Archaean history, which was dominated by sima-sial transformation processes, the majority of cratonic and mobile Proterozoic domains show evidence for sialic basement. Principal examples for this observation are brought from the Australian, South African and Canadian shields. The relations between Proterozoic mobile belts and the Archaean crust allow a three-fold classification in terms of intercratonic, marginal and external mobile belts. Marginal mobile belts flanked on one side by Archaean crust display some features analogous to Alpine geosynclines but fail to yield evidence for simatic crust. In external mobile belts, whose relation to the Archaean crust is large unknown, geochemical and isotopic evidence commonly suggests an underlying sialic substratum. Ophiolites are unknown and ocean-floor or arc-trench type volcanic assemblages are rare, though not unknown, in these terrains. With few exceptions, Proterozoic palaeomagnetic data suggest that little horizontal relative movement has occurred between cratons across intercratonic belts. The essentially intact nature of the early and middle Proterozoic sialic crust suggests that, had simatic crust existed — on an Earth of present-day surface area — it must have occupied a vast, hemispheric regime. It is calculated that sea-floor spreading, subduction and partial melting processes within and along the margins of such oceanic regime should have given rise to at least 500 · 10 6 km 3 of sima-derived materials, for which no record is observed. The Sr isotopic evolution of carbonates and K/Na ratios in clastic sediments argues against extensive distribution of simatic crust and its fusion products during 2.6−0.6 aeons ago. Unless all evidence for sima was inexplicably eliminated, a global sial must have existed during the early and middle Proterozoic. However, considerations of the Proterozoic sial budget do not allow a global sial of continental thickness on an Earth of present-day dimensions. Nor is a thin sialic crust consistent with the evidence, as its subsequent accretion into thicker crust is disallowed by palaeomagnetic data and as evidence from mineral PT indicators suggests continental thicknesses of the Archaean sial. The enigma regarding the nature of the unrecorded two-thirds to three-quarters of the Proterozoic crust on a globe of present-day dimensions is explained had Earth's surface area grown with time — in agreement with the expanding Earth hypothesis. The increasing importance of ophiolites and calc-alkaline suites in several regions about 1.0 aeon ago may correspond to onset of expansion and related horizontal plate tectonics.