Early Proterozoic Foredeeps, Foredeep Magmatism, and Superior-Type Iron-Formations of the Canadian Shield

Paul F. Hoffman

Paul F. Hoffman

Precambrian Geology Division, Geological Survey of Canada, Ottawa, Ontario K1A 0E4

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First published: 01 January 1987
Citations: 71
Book Series:Geodynamics Series

Summary

Foredeeps (foreland basins) are linear asymmetric basins that migrate in front of, and become incorporated within, foreland fold-and-thrust belts. They develop as a flexural response to loading of the continental lithosphere by thrust sheets. Many foredeeps evolve from oceanic trenches when rifted continental margins are drawn into subduction zones, but they may also result from intracontinental thrusting. Six 2.2–1.8 Ga foredeep sequences are described in association with Proterozoic fold-and-thrust belts bordering the >2.5 Ga Superior, Slave and North Atlantic cratons of the Canadian Shield. The Proterozoic foredeeps differed from Phanerozoic examples in that mafic magmatism occurred in their axial zones and iron-formations were deposited on their outer ramps. Foredeep magmatism, not ophiolite obduction nor initial rifting, produced most of the volcanic rocks in the Proterozoic foreland belts described. Foredeep migration produces diachronous deposition of facies. Consequently, axial-zone volcanic rocks occur stratigraphically above outer-ramp iron-formations. Therefore, the absence of volcanic rocks within the iron-formation itself does not rule out contemporaneous down-dip volcanism. Such volcanism may have played an active role in the origin of “Superior-type” early Proterozoic iron-formations. For paleogeographic and geodynamic interpretations, it is critical to distinguish foredeep sequences from underlying passive-margin and initial-rift sequences.