Precambrian tectonics: Some constraints from paleomagnetic studies in North America and East Africa.

Abstract

I report new paleomagnetic results from Precambrian age rocks in North America and East Africa in an effort to characterize the plate tectonic development of the pre-Paleozoic Earth. The paleomagnetic results are combined with attempts to constrain the age of magnetization so that tests of cratonic coherence, proposed supercontinental configurations, plate dynamics and paleoclimatic models can be developed for the early Earth's history. Previously proposed tectonic models for the interval 2.8-0.55 Ga relied on results of early paleomagnetic studies which appeared compatible with tectonic coherence of all continental blocks during that interval. A test of the long-lived supercontinental model is provided by the 2.68 Ga Nyanzian System of western Kenya. The paleomagnetic pole is well determined and well dated. A similar-age pole from the Kaapvaal Craton of southern Africa indicates that the supercontinental theory envisioned by Piper (1987) is not valid at 2.68 Ga. The formation of a supercontinent (Rodinia) during Middle to Late Proterozoic time (1.3-1.0 Ga) is based partly on geologic similarities across contemporaneous Grenvillian-Kibaran orogenic belts. A paleomagnetic study of Late Kibaran intrusives in Burundi (1.24 $\pm$ 0.02 Ga) indicates that the Congo craton was not in its hypothesized Rodinia position at 1.25 Ga. Previous paleomagnetic studies placed Laurentia near the equator at 0.60 Ga. Recent paleomagnetic studies question this conclusion and instead favor a south pole position. A paleomagnetic study of the Catoctin Formation (0.6-0.57 Ga) in Virginia, demonstrates that a recently posited high latitude position (at 0.60 Ga) for Laurentia appears valid. The subsequent equatorward Cambrian drift of Laurentia from its polar position requires a minimal plate velocity of 16 cm yr$\sp{-1}.$ This velocity is higher than a theoretical speed limit for large continental plates but I show that other large continents have moved at speeds of $\ge$20 cm yr$\sp{-1}$ for short intervals. The poleward drift of Laurentia during the interval 0.70-0.60 Ga hinted that ideas about equatorial Neoproterozoic glaciations might be explained in a more conventional manner. New paleomagnetic data argue against low elevation equatorial glaciations and reconcile the anomalous Neoproterozoic glaciations with recently developed climate models.