HR: 17:15h
AN: V34C-06    [Abstracts]
TI: Constraints on the Extent of Phanerozoic Sedimentary Sequences in the Western Canadian Shield From Radiation Damage Control on (U-Th)/He Apatite Dates
AU: * Flowers, R
EM: flowersr@colorado.edu
AF: University of Colorado, Dept of Geol Sciences, Boulder, CO 80309,
AB: Dynamic topography and plate margin tectonism, in combination with global sea level rise, have been invoked to explain the distribution of Phanerozoic sedimentary sequences extending deep into the cratonic interior of North America. This interpretation suggests a more dynamic history of vertical motions than once believed in regions traditionally considered "stable". Constraints on the timing and magnitude of continental interior flooding are required to resolve low amplitude episodes of burial and unroofing linked to such epeirogenic events. (U-Th)/He apatite thermochronometry provides a means to establish the thermal imprint of strata for which geological evidence has been erased by subsequent unroofing. Recognition of radiation damage control on apatite He retentivity now allows additional insights into thermal histories by investigation of apatites with a range of closure temperatures from the same crustal level in these low relief cratonic landscapes. The East Lake Athabasca region in northern Saskatchewan, now devoid of Phanerozoic cover, is located within the cratonic western Canadian Shield. Previously published (U-Th)/He apatite dates from this area range from 0.95 to 0.55 Ga. Low [eU] apatites were targeted for additional analysis following recognition of the radiation damage effect on apatite dates. The new dates are Phanerozoic in age and significantly younger than previously published Proterozoic dates for moderate to high [eU] apatites from the same area (and even the same outcrop). The distributions of dates correlated with [eU] can be explained by application of the radiation damage trapping model, and using a thermal history characterized by Phanerozoic reburial of this region by > 1 km of strata that have since been denuded. Apatite fission-track dates for the same samples are Proterozoic regardless of [eU], and similarly appear to require a Phanerozoic phase of unroofing. Together these results point toward significant deposition of Phanerozoic strata substantially further eastward in the Canadian cratonic interior than previously recognized, most likely during inundation of the continent during the Devonian or Cretaceous sea level high- stands.
DE: 1140 Thermochronology
DE: 8103 Continental cratons
DE: 8110 Continental tectonics: general (0905)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting