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

摘要

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 fan's 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 fan's east flank during the Cretaceous accretion of the Insular Superterrane.