CRATON ASSEMBLY AND FRAGMENTATION (Invited Presentation)

Cratons are domains of continental lithosphere characterized by long-term relative stability, typically >1 billion years. Originally introduced as part of the developing theory of geosynclines and orogenesis, and converging forelands of stable continental areas (“kratogens”), the term has blossomed into an essential concept for understanding and systematizing modern continental lithosphere. Some modern literature implicitly equates “cratons” with areas of exclusively Archean continental crust, and their subcontinental lithospheric roots, and discusses much of their geological characteristics in terms of how this or that craton is thought to have formed. This tends to ignore the fact that essentially all cratons are mere fragments, or even more generally fragments of fragments, of much larger geological systems of crust generation that spanned scales much larger those of the cratonic fragments presently preserved. It is therefore essential we reconstruct, to the extent possible, these larger crust formation systems, i.e. ancestral supercratons and their geodynamic setting.

The image of repeated fragmentation is a useful concept and relates to the following: a typical late Proterozoic cratonic fragment, let’s say Laurentia, has a typical length scale of ~3-5 x10³ km; embedded in it are late Archean fragments with typical length scales of ~0.5-1 x10³ km; and embedded in these Archean fragments are yet older distinct terranes, with no prior relationship to some of the surrounding terranes or domains, that have typical length scales of ~100 km. This complicated assemblage of preserved continental fragments, and fragments of fragments, presents a puzzle that can be solved using modern reconstruction techniques using integrated approaches of 1) sorting puzzle pieces by general similarities, 2) magmatic barcoding to identify multiple exact age matches of pre-breakup magmatic events, 3) comparing apparent polar wander paths, and 4) identifying robust correlations in pre-breakup cover stratigraphy.

Using these techniques we can identify “nearest neighbour” crustal fragment that must have been contiguous or nearly contiguous prior to breakup and dispersal. In total there are ~50 main crustal fragments to consider through time, back to the Mesoarchean. If the puzzle is not solvable, too many fragments have been swept under the rug (e.g., subduction erosion).