THERMOTECTONIC HISTORY OF MT LOGAN, YUKON-TERRITORY, CANADA - IMPLICATIONS OF MULTIPLE EPISODES OF MIDDLE TO LATE CENOZOIC DENUDATION

The extreme relief of the St. Elias Mountains, including Mt. Logan, is suggestive of a young mountain belt. New apatite fission track data i ndicate that the Mt. Logan massif experienced rapid low-temperature co oling during three distinct periods: during the middle Eocene, Middle Miocene and Pliocene. Each cooling episode, the magnitude of which var ies due to temporal variations in recorded paleogeothermal gradients, can be tentatively linked to a tectonic cause. Eocene cooling may be d ue to re-equilibration of isotherms following normal faulting, caused by a combination of thermal weakening of the crust and a decrease in r egional compressional stress, due to the similar to 43 Ma change in re lative motions between the Pacific and North American plates. The caus e of Miocene cooling is problematic, and could reflect denudation in r esponse to initial underplating of the Yakutat terrane, or a recorded change in heat flow unrelated to denudation. Pliocene cooling reflects erosion due to surface uplift that produced the spectacular present-d ay topography of the St. Elias Mountains. This surface uplift is proba bly related to the coeval significant change in relative motion betwee n the North American and Pacific plates and/or resistance to subductio n by the Yakutat terrane.