U–Th–Pb geochronologic constraints on the structural evolution of the Selkirk fan, northern Selkirk Mountains, southern Canadian Cordillera

In the southern Canadian Cordillera a zone of structural divergence marks the eastward transition from penetrative ductile deformation and metamorphism in the Omineca belt to the more brittle ‘thin-skinned’ style of deformation typical of the Foreland belt. In the Selkirk Mountains of southern British Columbia, this zone includes a regional-scale structure termed the Selkirk fan. The fan trends northwest, consists primarily of medium- to high-grade metamorphic rocks, and comprises at least three generations of superposed structures. IDTIMS and SHRIMP analyses provide new U–Th–Pb age constraints for the structural evolution of the Selkirk fan. The data demonstrate that the thermo-structural development of the fanʹs west flank occurred principally in the Middle Jurassic (ca. 172–167 Ma), whereas in the east flank significant Cretaceous (ca. 104–84 Ma) deformation was superimposed on an early transposition fabric. These data require revision of previous models that concluded fan formation occurred primarily during Middle Jurassic time. Rather, the Selkirk fan is a composite structure comprising Middle Jurassic and Cretaceous deformation. Development of the fan during the Early–Middle Jurassic accretion of the Intermontane Superterrane was followed by extensive reworking and tightening of structures in the fanʹs east flank during the Cretaceous accretion of the Insular Superterrane.