The quasi-rigid premise in Precambrian tectonics

Abstract

The proposition that movements between multiple continental fragments driven by plate tectonics have taken place during the earlier eons of geological time can be simply tested. A random distribution will be derived when the collective palaeomagnetic poles are rotated into any quasi-rigid reconstruction. This elementary test surmounts the most serious limitation of Precambrian palaeomagnetic data, namely that the ages of magnetisation are mostly poorly known. In the event, the late Archaean and Proterozoic poles (2850–590 Ma) produce a non-random distribution with a single peak which is only compatible with a quasi-rigid continental configuration occupying a preferential position on the globe. This solution is not favoured because uncertainties in definition of the poles contribute to their scatter and bias the solution towards the random one. Hence confirmation of the quasi-rigid premise is derived in a debased form, but is not suppressed, by the deficiencies of the database. This palaeomagnetic constraint explains the distinctive isotopic, geochemical and lithofacies signatures of the Proterozoic eon and is independently supported by the trend and long temporal continuity of crustal lineaments. It does not preclude the opening of ocean basins between the shields, but it does predict that development of these basins was limited by the influence of controlling mantle systems. This constrained the continental crust into a form which was essentially rigid on the gross scale resolvable by palaeomagnetism.