EARLY TERTIARY THERMOTECTONIC HISTORY OF THE NORTHERN YUKON AND ADJACENT NORTHWEST-TERRITORIES, ARCTIC CANADA

Apatite fission-track data from 16 sedimentary and crystalline rock sa mples indicate rapid regional Early Eocene denudation within the onsho re Beaufort-Mackenzie region of northwestern Canada. Rocks exposed in the area of the Big Fish River, Northwest Territories, cooled rapidly from paleotemperatures of >80-110 degrees C to <60 degrees C at ca. 56 +/- 2 Ma, probably in response to kilometre-scale denudation associat ed with regional structuring. The data suggest the region experienced a geothermal gradient of similar to 28 degrees C/km prior to rapid coo ling, with similar to 2.7 km of section having been removed from the t op of the exposed section in the Moose Channel Formation and similar t o 3.8 km from the top of the exposed Cuesta Creek Member. Farther to t he west, rocks exposed in the headwaters of the Blow River in the Barn Mountains, Yukon Territories, were exposed to paleotemperatures above 110 degrees C in the Late Paleocene prior to rapid cooling from these elevated paleotemperatures due to kilometre-scale denudation at ca. 5 6 +/- 2 Ma. Exposure of these samples at the surface today requires th at a minimum of similar to 3.8 km of denudation occurred since they be gan cooling below similar to 110 degrees C. The apatite analyses indic ate that rocks exposed in the northern Yukon and Northwest Territories experienced rapid cooling during the Early Eocene in response to kilo metre-scale denudation, associated with early Tertiary folding and thr usting in the northern Cordillera. Early Eocene cooling-uplift ages fo r onshore sections are slightly older than the Middle Eocene ages prev iously documented for the adjacent offshore foldbelt and suggest that the deformation progressed toward the foreland of the foldbelt through time.