Evidence supports the hypothesis that the Laurentian and East Antarctic-Australian cratons were continuous in the late Precambrian and that their Pacific margins formed as a conjugate rift pair. Both margins extend for approximately 40° of latitude. They have a similar rift history throughout their length—i.e., Late Proterozoic rifting and Early Cambrian carbonate platform development. A geometrically acceptable computer-generated reconstruction for the latest Precambrian juxtaposes and aligns the Grenville front that is truncated at the Pacific margin of Laurentia and a closely comparable tectonic boundary in East Antarctica that is truncated along the Weddell Sea margin. These may prove to be critical, perhaps even unique, "piercing points" for relating the northern and southern continents. Geologic and paleomagnetic evidence also suggests that the Atlantic margin of Laurentia rifted from the proto-Andean margin of South America in earliest Cambrian time. Early Phanerozoic sea-floor spreading that isolated Laurentia from South America and East Antarctica-Australia in an Eocambrian supercontinent appears to balance convergence along the Mozambique suture which resulted in final amalgamation of the smaller Gondwana supercontinent at ∼500 Ma.

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