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.
Transport of the Yakutat Terrane, Southern Alaska: Evidence from Sediment Petrology and Detrital Zircon Fission-Track and U/Pb Double Dating
Two hypotheses have been offered to account for the transport and accretion history of the Yakutat terrane in southern Alaska. To investigate these two options, we deconvolved fission-track (FT) and U/Pb ages of detrital zircons from stratigraphically coordinated samples collected in the northern Robinson Mountains into component populations. The strata of the Yakutat terrane include the Middle Eocene Kulthieth Formation, the Lower Oligocene to Lower Miocene Poul Creek Formation, and the Miocene-Pleistocene Yakataga Formation. The Kulthieth and Poul Creek formations record erosion of a simple, uniform, long-lived, nonvolcanic source terrain that crystallized from ∼50 to 220 Ma and cooled from ∼40 to 110 Ma. Miocene cooling episodes recorded in the source to the Kulthieth and Poul Creek formations are likely associated with plutons in the northern Coast Plutonic Complex and the Kuiu-Etoilin belt. The Upper Miocene to Pleistocene Yakataga Formation records erosion of rocks that crystallized from ∼50 to 53 Ma and cooled below the zircon FT closure at ∼70–20 Ma. Upper Miocene strata are likely derived from erosion of the Chugach–Prince William terranes and the superimposed Sanak-Baranof plutonic belt. The uniform provenance of the Kulthieth and Poul Creek formations, the overall FT grain age distribution, and the distinct lack of volcanic zircons favor a northern position of the Yakutat terrane since the Eocene. However, a far-traveled southern option for the basement rocks cannot be ruled out, but it is unlikely that the Eocene and younger cover strata were deposited far to the south.
Discordant paleomagnetic poles from the Canadian Coast Plutonic Complex: Regional tilt rather than large-scale displacement?
Stratigraphic, petrologic, and isotopic data indicate that parts of the Coast Plutonic Complex and the North Cascade Range have been tilted northeast-side-up by angles of {approximately}30{degree} about north-northwest-trending axes. These tilts can account for discordant paleomagnetic directions observed in mid-Cretaceous plutons from these regions without large-scale displacement relative to North America.