Structure, metamorphism, and mica thermochronology across the interface between the Purcell Anticlinorium and the Kootenay Arc, Omineca Belt, southeastern British Columbia
AdvisorPattison, David R.M.
AuthorRioseco, Nicole Alexandra
Committee MemberEnkelmann, Eva
MetadataShow full item record
AbstractThe Purcell Anticlinorium (PA) and the Kootenay Arc (KA) represent two major tectonic domains within the southern Omineca belt of the Canadian Cordillera, that are characterized by contrasting structural, metamorphic, and thermochronological histories. Rocks in the core of the PA preserve a Proterozoic phyllitic cleavage (S1). The KA is a polydeformed region in which the dominant fabric (S2) developed during the Early Cretaceous. Both S1 in the PA and S2 in the KA are overprinted by a D3 spaced cleavage and associated crenulation lineations ascribed to later stages of the Cordilleran orogeny. The structural boundary between the KA and the PA is taken as the easternmost occurrence of S2. Peak regional metamorphic conditions in the PA reached biotite grade in the Proterozoic. The KA experienced Early Cretaceous Barrovian metamorphism, with metamorphic grade decreasing eastward from sillimanite + K-feldspar zone to biotite zone towards the PA. The two diachronous domains of biotite grade metamorphism converge in the vicinity of the structural interface. The PA preserves K-Ar and 40Ar/39Ar dates in mica between 1318 and 214 Ma and 837 and 240 Ma, respectively. These dates result from disturbed age spectra, and it is unlikely that they reflect true cooling ages. The temporal and spatial scatter of these dates is interpreted to reflect partial resetting following initial cooling in the Proterozoic, due to some combination of Neoproterozoic and Paleozoic sedimentation, Cordilleran tectonic thickening, and Cordilleran magmatism. In the KA, 40Ar/39Ar dates that range from 139 to 55 Ma are interpreted as cooling ages. They span the interval between the time of metamorphism, magmatism, and final exhumation. The Proterozoic through Mesozoic K-Ar and 40Ar/39Ar cooling dates in the PA indicate that since the Proterozoic, the region was never sufficiently buried to uniformly reset mica cooling dates. It is proposed that the interface between the PA and KA is localized along a deep-seated crustal ramp, probably close to the structural and metamorphic interface. The rocks of the PA may have acted as a rigid buttress against which the rocks of the KA piled up during Cordilleran crustal thickening.
CitationRioseco, N. A. (2020). Structure, metamorphism, and mica thermochronology across the interface between the Purcell Anticlinorium and the Kootenay Arc, Omineca Belt, southeastern British Columbia (Unpublished master's thesis). University of Calgary, Calgary, AB.
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