PROGRESS TOWARD RECONSTRUCTING PALEOPROTEROZOIC SUPERCONTINENTS

Global abundances of isotopic ages for deformation, plutonism, and juvenile isotopic signatures, all point to periods of enhanced tectonism at 2.7–2.6 and 2.1–1.8 Ga. Conversely, widespread mafic and bimodal magmatism characterizes the interval 2.45–2.1 Ga. These observations suggest the existence of two Neoarchean to Paleoproterozoic supercontinents, Kenorland and Nuna (a.k.a. Columbia), separated by an interval of cratonic fragmentation. Confidence in Kenorland's existence is limited by relatively fragmentary preservation of earliest Proterozoic supracrustal rocks; a relatively bountiful late Paleoproterozoic geological record provides more assurance that Nuna was a true supercontinent of modern proportions. Aside from a few hypothesized connections between pairs or trios of cratons, the shape of earliest Paleoproterozoic Kenorland is ill constrained. Several postulated configurations of Nuna/Columbia, however, endeavor to include most of the world's cratons. These geologically based reconstructions, qualitative by nature, find little quantitative paleomagnetic support.

An important constraint on any supercontinental reconstruction is the need for temporal context; that is, assembly and fragmentation of the various constituent pieces must flow reasonably (according to our understanding of plate tectonics) from and toward the preceding and succeeding amalgamations. A major stumbling block for accurate composition of Paleoproterozoic supercontinents is our current inability to configure the younger Rodinia landmass. The search for pre-Rodinia supercontinents might therefore appear hopeless at present. On a more optimistic note, however, many proposed cratonic juxtapositions in Rodinia lack "Grenvillian" (late Mesoproterozoic) collisional sutures and are thus conjectured to be inherited from Paleoproterozoic time. Tectonostratigraphic and paleomagnetic data from those older intervals may then be used to test Rodinia reconstructions, which in turn can provide bases for backward extrapolation toward Nuna and Kenorland. This iterative approach may lead more efficiently to a paleogeographic framework for the increasingly acknowledged, remarkable events in Earth's Paleoproterozoic evolution.