THE GREAT FALLS TECTONIC ZONE - SUTURE OR INTRACONTINENTAL SHEAR ZONE

The Great Falls Tectonic Zone is generally considered to be the bounda ry between the Archean Hearne and Wyoming provinces. Although complete ly 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 g eophysical remote sensing methods. While tectonically active ca. 1.8 G a and clearly truncating the potential field fabrics of Wyoming Provin ce and Medicine Hat Block, the Great Falls Tectonic Zone lacks a colin ear magmatic are, suggesting that the Hearne-Wyoming juxtaposition did not involve subduction of oceanic lithosphere. Furthermore, electroma gnetic 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 a ppears at depths exceeding 20 km, well below the top of the Proterozoi c basement. Taken together, these observations suggest the Great Falls Tectonic Zone may be better interpreted as a reactivated Archean(?) i ntracontinental shear zone rather than a Proterozoic age suture betwee n Archean provinces.