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Orographic precipitation, erosional unloading, and tectonic style

¹ School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia V8W 2Y2, Canada
² Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

Comparative tectonics of the Thelon (1.97 Ga) and Wopmay (1.88 Ga) orogens of north-west Canada and the Grenville (1.1 Ga) and Alleghany (0.3 Ga) orogens of eastern North America support geophysical models linking orographic precipitation and erosion to tectonic style. Paleomagnetic data imply that the Thelon and Grenville orogenic fronts were favorably situated to face trade winds and experience monsoonal precipitation. High rates of uplift could have been balanced by erosional unloading. Consequently, the orogenic fronts are deeply eroded and lack thin-skinned foreland thrust-and-fold belts. Tectonic progradation was achieved instead by thermal activation of the orogenic footwall. Foreland basins, where preserved, are overfilled with sediments that are dominantly fluvial to shallow marine, compositionally mature, and progradationally stacked. The Wopmay and Alleghany orogenic fronts developed in rain shadows. Erosional unloading could not offset even modest rates of tectonically driven uplift, resulting in mass flow onto the foreland manifested by thin-skinned thrust-and-fold belts and associated foreland basins. Foreland basins are underfilled and well preserved, containing sediments that are dominantly deep water, compositionally immature, and aggradationally stacked. The nature of the Mauritanide orogenic front is consistent with it being the windward complement of the Alleghany front.

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