A 1.9 Ga continental margin in the northwest of the Canadian Shield evolved in less than 15 m.y. from a zone of crustal stretching, through a phase of passive-margin subsidence, to attempted subduction beneath an exotic(?) microcontinent. Reversal of subduction polarity and generation of a major episutural magmatic arc occurred within 15 m.y. following microcontinental accretion. Terminal collision occurred 90 m.y., at most, after initial rifting. Certain differences in detail between this collided margin and those of Phanerozoic age may be related to subduction of newly rifted lithosphere. Rapid evolution of the orogen, the contracted passive-margin phase in particular, is consistent with (but does not prove) more rapid recycling of oceanic plates at that time.

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