The late Archean record: a puzzle in ca. 35 pieces

Abstract

The global Archean record preserves ca. 35 large cratonic fragments and a less well defined number of smaller slivers. Most Archean cratons display rifted margins of Proterozoic age and therefore are mere fragments of supercratons, which are defined herein as large ancestral landmasses of Archean age with a stabilized core that on break-up spawned several independently drifting cratons. The tectonic evolution of individual Archean cratons, such as the Slave craton of North America or the Kaapvaal craton of southern Africa, should therefore always be considered in the context of their ancestral supercratons. This is particularly true for many of the smaller cratons, which are too limited in size to preserve the complete tectonic systems that led to their formation. These limitations not only apply to the crustal geology of Archean cratons but also to their underlying lithospheric mantle keels. If these keels are Archean in age, as their broad correlation with ancient surface rocks suggests, they also are rifted and drifted remains of larger keels that initially formed below ancestral supercratons. The study of Archean cratons and their lithospheric keels should thus be global in scope. In the search for late Archean supercratons, a craton like the Slave, with three to four rifted margins, has a ca. 10% maximum probability of correlating with any of the ca. 35 remaining cratons around the globe, assuming wholesale recycling of Archean cratons has been limited since ca. 2.0 Ga. Alternatively, if the original number of independently drifting cratons was significantly larger, the probability of successful correlations is much less. Due to repeated cycles of break-up and plate tectonic dispersal since ca. 2.0 Ga, the probability of correlation is probably independent of present-day proximity, unless there is independent evidence that two neighbouring cratons were only separated by a narrow ocean. Nevertheless, most previously proposed craton correlations rely (erroneously?) on present-day proximity, implicitly extrapolating relatively recent paleogeography back to the Archean. Considering the fundamental differences between some of the better known cratons such as the Slave, Superior, and Kaapvaal, it seems likely that these cratons originated from independent supercratons (Sclavia, Superia, Vaalbara) with distinct amalgamation and break-up histories. This is contrary to widely held opinion, based largely on an idealized view of the supercontinent cycle, that all cratons shared a common history in a single late Archean supercontinent. In contrast to the Superior and Kaapvaal cratons, which are radically different from the Slave craton and each other, the Dharwar craton of peninsular India, the Zimbabwe craton of southern Africa, and the Wyoming craton of North America all show significant similarities to the Slave craton. It thus seems likely that at least some of these were nearest neighbours to the Slave craton in the ca. 2.6–2.2 Ga Sclavia supercraton.