The Great Falls Tectonic Zone: suture or intracontinental shear zone?

Abstract

The Great Falls Tectonic Zone is generally considered to be the boundary between the Archean Hearne and Wyoming provinces. Although completely buried beneath the western Canadian sedimentary basin, the zone can be studied indirectly through variations in Phanerozoic sedimentation patterns, faulting, basement geochronology, and xenoliths, and with geophysical remote sensing methods. While tectonically active ca. 1.8 Ga and clearly truncating the potential field fabrics of Wyoming Province and Medicine Hat Block, the Great Falls Tectonic Zone lacks a colinear magmatic arc, suggesting that the Hearne-Wyoming juxtaposition did not involve subduction of oceanic lithosphere. Furthermore, electromagnetic studies fail to detect a response that can be interpreted as a plate-edge foreland basin, typical of exposed Proterozoic suture zones. The only conductivity anomaly associated with the zone is weak and appears at depths exceeding 20 km, well below the top of the Proterozoic basement. Taken together, these observations suggest the Great Falls Tectonic Zone may be better interpreted as a reactivated Archean(?) intracontinental shear zone rather than a Proterozoic age suture between Archean provinces.