Structural and thermal evolution of the northern Selkirk Mountains, southeastern Canadian Cordillera: Tectonic development of a regional-scale composite structural fan

In the southern Canadian Cordillera, the transition from the penetrative ductile deformation, medium- to high-grade metamorphism and plutonism of the hinterland, to the “thin-skinned” style of deformation in the foreland is marked by a zone of structural divergence. In the northern Selkirk Mountains of southern British Columbia, within the southern Omineca belt, part of the zone coincides with a regional-scale structure termed the Selkirk fan. The fan is composed of low- to high-grade metamorphic rocks, and comprises at least three generations of superposed structures.; The data demonstrate that the thermo-structural development and exhumation of the west flank of the fan occurred principally in the Middle Jurassic (ca. 172–167 Ma). In contrast, east of the fan axis significant Cretaceous deformation (104–84 Ma) and Cretaceous to Paleocene metamorphism (144–56 Ma) were superimposed on an early transposition fabric. This was followed by or partly concomitant with Late Cretaceous to Early Tertiary exhumation.; Refinement of the metamorphic age constraints was also facilitated by chemical mapping for Y, Th and U coupled with the in situ U-Th-Pb SHRIMP analyses. This revealed the link between age domains and zones of relative yttrium (Y) depletion or enrichment within monazite that were correlated with metamorphic reactions involving garnet. These data clearly illustrate the cause of age dispersion within the analyzed monazites, and ubiquity of multiple age domains in metamorphic monazite.; Based on the data produced in this study a revised tectonic model is proposed in which the Selkirk fan developed within a critically tapered orogenic wedge that evolved diachronously in response to changing boundary conditions associated with periods of terrane accretion on the western margin of North America. During the Early to Middle Jurassic accretion of the Intermontane Superterrane, a proto-F1–2 fan developed above a singularity where oceanic or marginal basin lithosphere was subducted eastward beneath continental lithosphere. Subsequently, the fan decoupled along a basal décollement system and was transferred northeastward, as rocks to the east were progressively incorporated into the orogenic wedge. The mid-Cretaceous accretion of the Insular Superterrane resulted in rejuvenation of compressional forces. This gave rise to out-of-sequence deformation that thickened the tectonic pile to reestablish critical taper and the continued eastward propagation of folding and faulting within the foreland to the east. Thus, the Selkirk fan may be thought of as a composite structure of juxtaposed Middle Jurassic and Cretaceous structures and metamorphism, rather than a singular fan that developed during one progressive event. (Abstract shortened by UMI.)...