Structure, metamorphism, and mica 40Ar/39Ar thermochronology of the southern Purcell anticlinorium and its transition into the central Kootenay arc, Omineca belt, southeastern British Columbia

Abstract

The Purcell anticlinorium (PA) and the Kootenay arc (KA) are two adjacent Mesozoic tectonic domains in the southeastern Omineca belt of the Canadian Cordillera. Mesoproterozoic rocks of the Belt-Purcell Supergroup (BPSG) core the PA and have open, upward-facing structures. Deeper levels of the BPSG (Aldridge Formation) preserve a Mesoproterozoic phyllitic cleavage (S1) and biotite zone mineral assemblages. The KA, on the west flank of the PA, has a younger Neoproterozoic through Paleozoic stratigraphy. The central part of the KA, the focus of this study, experienced Jurassic recumbent folding and greenschist facies metamorphism that was overprinted by Early Cretaceous ductile deformation (S2 schistosity) and Barrovian metamorphism. The interface between the PA and KA is taken as the easternmost occurrence of the S2 schistosity. K–Ar and 40Ar/39Ar mica dates in the PA range from 1318 to 240 Ma and show no consistent spatial distribution. They are interpreted to reflect partial resetting of Ar following initial cooling in the Mesoproterozoic, indicating that the PA was never sufficiently buried or heated during younger tectonism to uniformly reset the mica cooling ages. In the KA, 40Ar/39Ar dates range from 139 to 55 Ma and reflect previously documented periods of Mesozoic and Cenozoic metamorphism, magmatism, and exhumation. The PA–KA interface occurs above a west-dipping ramp in the Laurentian basement. The thick Mesoproterozoic sequence of turbidites and sills in the PA may have acted as a rigid buttress against which the more ductile rocks of the central KA “piled up” during Early Cretaceous compressional orogenesis.