STRUCTURAL EVOLUTION OF THE SELKIRK FAN STRUCTURE, NORTHERN SELKIRK MOUNTAINS, SOUTHEASTERN CANADIAN CORDILLERA

The northern Selkirk Mountains are situated within the metamorphic hinterland of the southeastern Canadian Cordillera. Three generations of structures have been recognized. Of particular interest are the second (F2) and third (F3) generation folds that verge to the SW and NE, respectively, producing a structural fan termed the Selkirk Fan Structure (SFS). F2 folds dominate the west side of the fan where they are tight to isoclinal, and strongly overturned toward the SW. F3 folds have steep to moderately dipping axial surfaces that become increasingly overturned towards the northeast. The SFS is orogen parallel striking SE-NW for more than 120 km across the northern Selkirk Mountains of British Columbia. Similar zones of structural divergence extend southward into the US Cordillera and northward to Alaska. The kinematic significance of the SFS is controversial, and its correct interpretation is essential for construction of tectonic models of the Cordillera.

New data provide U-[Th]-Pb geochronologic constraints for deformation and metamorphism associated with the development of the SFS via small-fraction conventional isotope-dilution (ID-TIMS) and Sensitive High Resolution Ion Microprobe (SHRIMP) techniques. Dated monazite, zircon, and xenotime from pelitic schists, variably deformed leucocratic dykes and monzonitic plutons indicate that the thermo-structural development of the west flank of the SFS occurred principally in the Middle Jurassic. In contrast, east of the fan axis the data strongly suggest diachroneity - deformation, metamorphism, and exhumation occurred primarily in the Cretaceous. As such, these data indicate a complex and protracted origin for the SFS, and significant revision of previous models is required.