WHAT IS WRONG WITH OUR UNDERSTANDING OF CORDILLERAN PALEOGEOGRAPHY?

The ancient west margin of the Cordillera is characterized by an outer high-standing continental platform that lies west of abyssal chert, argillite and deep water limestone deposits of Selwyn basin – Kechika Trough, and which is referred to, from north to south, as the McIvoy, Cassiar, Montania platform (MCMP), respectively. Aeromagnetic anomalies attributable to the 1.8 Ga and older cratonic basement to North America end at or east of the abyssal sedimentary deposits, and mantle xenoliths and isotopic studies of young basalts indicate that the mantle underpinning the North American cratonic basement similarly terminates east of the MCMP. The MCMP is characterized by a younger mantle and an unknown basement that is in part Grenvillian (~1.1 Ga). It was overthrust from the west and tectonically buried by oceanic Intermontane and pericratonic terranes in or prior to the Pleinsbachian (~187 Ma), and subsequently exhumed by the end of the Middle Jurassic. In contrast, the North American platform, lying east of the abyssal sedimentary deposits, was characterized by a passive margin into Kimmeridgian (155 Ma) time. The North American platform was, from Devonian to Middle Jurassic time, phosphatic, indicating that it faced west to a broad ocean characterized by coastal upwelling (the southwest African and western South American margins providing modern analogues). These relationships establish that the MCMP is not an autochthonous portion of the ancient western margin of North America, and that it was far-removed from North American during the Pleinsbachian collision with the Intermontane terranes. The abyssal sedimentary deposits of Selwyn Basin – Kechika Trough are a cryptic suture that separate an accreted composite ribbon continent that includes the MCMP and which has previously been referred to as SAYBIA, from autochthonous North America.