EVIDENCE FOR A PROLONGED, HOT OROGEN WITHIN THE SOUTHERN CANADIAN CORDILLERA: IMPLICATIONS FOR DEEP-SEATED TECTONO-METAMORPHIC OROGENIC PROCESSES

In the southern Canadian Cordillera some of the deepest levels of the orogen have been exhumed, providing excellent opportunity to examine rocks affected by deep-seated tectono-metamorphic processes. Investigations in the northern Selkirk and Monashee Mountains of southeastern British Columbia provide evidence for an orogen whose lower crust was deeply buried (>25 km), hot (>600-800°C) and melt-laden beneath a plateau for ~90 Myr. This length of time for sustained deep burial associated with orogenesis appears to be unique to the southern Canadian Cordillera.

The region straddles the NW-SE trending Selkirk fan, a zone of structural divergence where structures change from SW- to NE-verging as one moves eastward from the core zone of the orogen into the “thin-skinned” deformation of the foreland. The fan initially developed in the Middle Jurassic, but deeper levels were substantially reworked and overprinted as it was translated ~300 km northeastward during Mesozoic contraction. Southwest-vergent structures and relatively lower grade metamorphic assemblages in the fan's west flank were developed and exhumed to upper crustal levels in the Middle Jurassic (ca. 172-167 Ma). Conversely, NE-vergent structures and lithostratigraphy in the fan’s east flank were increasingly buried to deeper crustal levels (>25 km) beneath a plateau, where they remained, were tectonically reworked and recrystallized from Middle Jurassic to Late Cretaceous time. The significant temporal disparity (>100 Myr) for the thermo-structural development of the Selkirk fan is noteworthy, because there is generally uniform consistency of geologic relationships across the fan between phases of deformation and metamorphism. Moreover, the age of the metamorphic assemblages used to establish the regional isograds vary significantly across the fan from west (Middle Jurassic) to east (Cretaceous-Tertiary), yet the isograds all trend NW-SE with no significant break or jump in grade from one isograd to the next.