Middle Jurassic exhumation along the western flank of the Selkirk fan structure: Thermobarometric and thermochronometric constraints from the Illecillewaet synclinorium, southeastern British Columbia

摘要

New thermobarometric and thermochronometric data from the Illecillewaet synclinorium, a broad regional southwest-verging structure in the western flank of the Selkirk fan structure, indicate that accretion of the Intermontane superterrane resulted in deep burial (20–25 km) of the outer margin of Ancestral North America and that subsequent southwest-verging folding and thrusting were accompanied by rapid exhumation. Southwest-verging folding of lower Paleozoic rocks in the Illecillewaet synclinorium was initiated during regional peak-metamorphic conditions at 6–7 kbar. Crystallization of late-synkinematic granitoid plutons that intrude the Illecillewaet synclinorium also occurred at pressures of 6–7 kbar; however, equilibration of the contact aureoles of the plutons occurred at 3–3.5 kbar, during the latter stages of the southwest-verging deformation. Evidently, a decompression on the order of 3 kbar occurred concurrently with southwest-verging folding and thrusting in the Illecillewaet synclinorium. Thermochronometric analyses further indicate that plutonic rocks from the Illecillewaet synclinorium cooled rapidly between times of crystallization and times of Ar closure in biotite. Regional relationships indicate that the accretion of the Intermontane superterrane to the edge of the North American margin in southeastern British Columbia occurred between 187 and 173 Ma (Toarcian–Bajocian). Subsequent southwest-verging deformation occurred approximately between 173 and 168 Ma (Bajocian–Bathonian). Thus, the removal of at least 10 km of rocks from above the Illecillewaet synclinorium, inferred from thermobarometric data, occurred approximately between 173 and 168 Ma. Rapid denudation of the Selkirk fan structure was probably the result of synorogenic extension (or extensional collapse) contemporaneous with the development of southwest-verging structures at deeper levels of the orogen. The data presented here indicate that all significant deformation and most of the exhumation along the western flank of the Selkirk fan structure were complete by late Middle Jurassic time. After the Middle Jurassic, the Selkirk fan structure must have been transported passively northeastward along the basal decollement of the Cordilleran foreland thrust-and-fold belt, together with the Intermontane superterrane, as the fold-and-thrust belt tectonically overrode the margin of the North American craton.