INSIGHT INTO THE RATE OF DEVELOPMENT OF A MAJOR OROGENIC STRUCTURE AND EVIDENCE FOR A LONG, HOT OROGEN WITHIN THE SOUTHERN CANADIAN CORDILLERA

Widespread Tertiary extension in the southern Canadian Cordillera exhumed deep levels of the orogen, providing excellent opportunity to examine rocks affected by deep-seated tectonic processes. Investigations of the northern Selkirk and Monashee Mountains within southeastern British Columbia provide evidence for an orogen whose lower crust remained deeply buried (>25 km), hot (>600-800°C), and laden with melt for ~90 Myr prior to widespread Tertiary extension. Based on the absence of data from other orogens supportive of similar durations for sustained deep burial associated with orogenesis, this feature appears to be unique to the Canadian Cordillera, or is it?

In this region, the Selkirk fan represents a zone of structural divergence where structures change from SW- to NE-verging as they merge eastward from the core zone of the southern Omineca belt into the “thin-skinned” deformation of the Foreland belt. The fan initially developed in the Middle Jurassic, but was in part, substantially reworked and overprinted during the Cretaceous to Tertiary as it was translated ~300 km northeastward during Mesozoic contraction. More specifically, the SW-vergent structures and relatively lower grade metamorphic assemblages in the fan's west flank developed and were quickly exhumed to upper crustal levels in the Middle Jurassic (ca. 172-167 Ma). At this time, NE-vergent structures and high-grade assemblages exposed in the east flank of the fan were progressively buried to deep crustal levels (>25 km) beneath a plateau, where they remained and were tectonically reworked until the Tertiary. The degree of tectono-metamorphic overprinting was a function of structural level, and most intense in the deepest levels of the east flank.

Other important implications of this study include: 1) although there is significant temporal disparity (>100 Myr) for the thermo-structural development of the Selkirk fan, there is generally uniform consistency of geologic relationships across the fan between phases of deformation and metamorphism; 2) 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.