Erosional denudation of the British Columbia Coast Ranges as determined from fission-track ages of detrital zircon from the Tofino basin, Olympic Peninsula, Washington
In this study, we use up-section changes in fission-track (FT) ages of detrital zircons to infer the tectonic evolution of the source region that fed sediments to the Tofino basin, a Tertiary forearc basin in northwestern Washington State and southwestern British Columbia. We have dated 50 grains from each of eight samples from a 6-km-thick section that ranges in age from 40 to 19 Ma. The detrital zircons have not been reset and thus preserve information about the thermal and denudation history of the source region from which the sediments were derived. Each of our eight samples contains a distinct set of grain-age populations or peaks, indicating that the source region contained several FT source terrains. The number of peaks and their lag time (peak age minus depositional age) contain thermochronologic information about these source terrains. In this study, there is little evidence of active volcanism in the source region; thus the FT ages reflect cooling during denudation of the source regions. The oldest sample in our suite, from the upper Eocene Lyre Formation, shows a dominant peak at ca. 40 Ma, coincident with cooling ages of denuded metamorphic rocks of the Leech River Schist on southern Vancouver Island. This interpretation is supported by paleocurrent directions indicating a source in the vicinity of Vancouver Island, the development of a late Eocene unconformity across much of southern Vancouver Island and western Washington, and the predominance of lithic detritus in this part of the section. Samples from strata above the Lyre Formation are feldspathic, and they presumably reflect erosional denudation of a plutonic source terrain. Peaks in samples from the lower part of this feldspathic interval have widely scattered Jurassic to mid-Cretaceous FT ages; young zircons are notably absent. Up-section, peak ages in samples from the upper part of the feldspathic section form a dominant peak that gets progressively younger with time. While the age of the dominant peak becomes younger in younger samples, the lag time for this peak remains constant at ∼40 m.y. This peak can be recognized in a compilation of zircon FT ages from modern exposures of the Canadian Coast Mountains. We infer that zircons in our samples were derived from progressive erosional denudation of the Coast Plutonic Complex, which had, based on our results, a constant rate of denudation of ∼250 m/m.y. The scatter of ages in the older samples from the feldspathic interval may indicate the unroofing of a high-level sedimentary basin (containing the Cretaceous Nanaimo and Burrard Groups) that used to overlie parts of the Coast Plutonic Complex. Our study, along with others, shows that up-section studies of FT ages of detrital zircons can be effective in resolving the long-term denudation history of continental source regions.
The Eocene Deltaic System of West-Central Washington
The Puget deltaic system is one of the larger and more extensive Eocene deltaic sequences exposed in the Pacific Northwest. The Puget sequence is a large fluvial-deltaic system that extended from a marine embayment in western Washington, eastward across a broad continental platform to a large granitic highland. Mississippi-type deltaic sedimentation was initiated during the early Eocene on the eastern margin of a large marine embayment in the vicinity of Mount Rainier. In late Eocene time, patterns of Puget deltaic deposition were significantly modified by extensive local volcanism and an apparent abrupt shallowing of the marine environment. In the late Eocene, normal deltaic sedimentation resumed, followed by Oligocene volcanism which buried and terminated Puget deltaic sedimentation.
LITHOPROBE: Exploring the subduction zone of Western Canada
Lithoprobe is a multidisciplinary geoscientific research program that is investigating fundamental questions concerning the nature and evolution of the lithosphere in Canada. It integrates modern geophysical, geological, and geochemical concepts and technology to extend knowledge of the lithosphere in various key areas in Canada into the third dimension — depth. Collaboration among geoscientists from universities, government and industry is a key link to the program. Financial support is provided by the Natural Sciences and Engineering Research Council of Canada and the federal Department of Energy, Mines and Resources. Industry has contributed data where applicable and valuable advice through an ad hoc committee of the Canadian Society of Exploration Geophysicists. Representatives of the mineral and petroleum industries sit as members of the lithoprobe Steering Committee.