Tectonothermal Evolution of the Southern Omineca Belt, Southeastern British Columbia
Committee MemberGuest, Bernard
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AbstractThis study documents the geological structure, metamorphism, U-Pb geochronology, 40Ar/39Ar thermochronology and tectonic evolution of the Omineca belt of southeastern British Columbia. In this area, three tectonic domains with different tectono-metamorphic histories spatially overlap. In the southwest of the study area, Early Jurassic deformation and greenschist facies metamorphism (D1M1) is crosscut by Middle Jurassic intrusions of the Nelson suite (174-161Ma) that were emplaced at approximately 9-15km depth. A subdomain of Jurassic 40Ar/39Ar cooling ages is centered over these intrusions, suggesting rapid cooling and exhumation to upper crustal levels, where they have remained. The Jurassic structures are progressively overprinted northwards by Early Cretaceous deformation and Barrovian metamorphism (D2M2) in a domain that is abruptly terminated to the west by the Eocene Midge Creek fault (MCF) and to the east by the Purcell Trench faults (PTF). Peak sillimanite zone metamorphism occurred at 6-7kbar and 650-700ºC. The D2M2 domain is the southerly continuation of a 144-134Ma domain of regional deformation and metamorphism to the north of the study area. Metamorphic isograds associated with M2 metamorphism are transected by intrusions of the mid-Cretaceous Bayonne suite (118-90Ma). Microstructures and cross-cutting relations reveal that D2 deformation outlasted M2 metamorphism, was ongoing as late as 118Ma, but ceased by 111Ma. South of the Barrovian metamorphic belt and in the hangingwall of the PTF and MCF, 40Ar/39Ar ages record a regional, mid-Cretaceous thermal event. The Early Cretaceous D2M2 makes a southerly transition into a domain of Late Cretaceous (ca. 80-70Ma) regional Barrovian metamorphism and deformation (D3M3) bounded by the PTF and Blazed Creek-Next Creek fault. 40Ar/39Ar cooling ages are indistinguishable from monazite U-Pb ages of peak metamorphism (ca. 70Ma), implying rapid exhumation of these rocks during the Late Cretaceous. This narrow domain of cooling ages extends across the cryptic interface between the Early and Late Cretaceous Barrovian metamorphic belts, suggesting exhumation as one continuous block. 53-45Ma 40Ar/39Ar cooling ages in the immediate footwall of the PTF suggest extension did not commence until ~53Ma. Therefore, exhumation occurs via a two-stage process: 1) rapid decompression in the Late Cretaceous (70-60Ma); 2) exhumation in the footwall of Eocene extensional structures (53-45Ma).
CitationWebster, E. (2016). Tectonothermal Evolution of the Southern Omineca Belt, Southeastern British Columbia (Unpublished doctoral thesis). University of Calgary, Calgary, AB. doi:10.11575/PRISM/27991
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