SYNCHRONOUS EXHUMATION OF THE TORDRILLO MOUNTAINS AND MT. MCKINLEY (DENALI), ALASKA, AROUND 6 MA

There are three high sections of the Alaska Range. The tallest is near Mt. McKinley (or Denali, height: 6194 m), with a 95 km length of the range above 2500 m. The Hayes range, east of Denali, has a length of 55 km above 2500 m (Mt. Hayes is 4216 m), and the Tordrillo Mountains, south of Denali, has a 43 km length above 2500 m (Mt. Gerdine, the tallest peak is 3,431 m). In order to understand the timing of mountain building in the Tordrillo Mountains, we collected a preliminary suite of 10 samples from Paleocene granite in a vertical transect between 295 and 3231 m for apatite fission track thermochronology. This approach is particularly powerful for reconstructing thermotectonic histories of the upper continental crust of crystalline terranes, especially where the use of traditional stratigraphic and structural evidence is severely limited.

Apatite fission-track ages range between 5.8 and 35.5 Ma. Kinetic modeling of the single-grain age and track-length data, utilizing the Dpar parameter for each grain from which age and/or length data was acquired, indicate early slow cooling, followed by an initial phase of rapid cooling around 25 Ma, followed by a period of relatively slow cooling until at least 10 Ma. The data and kinetic models record a second period of rapid cooling sometime between 10 and 5 Ma; the 5.8 Ma apatite age represents a minimum recorded age for the timing of this event. Differences in cooling history between some samples near the crest of the range, in combination with geologic mapping, indicate west-side-up reverse faults active after 5-10 Ma. Previously published apatite fission-track thermochronology from samples in the Denali area indicates rapid cooling after 6 Ma. Thus both the Tordrillo Mountains and the Denali area experienced rapid cooling at approximately the same time. We infer this also represents surface uplift, because voluminous Pliocene sediments of the Sterling Formation fill the Cook Inlet and Susitna basins. The onset of Sterling sedimentation predates global cooling in mid-Pliocene time, and therefore, the initiation of sedimentation is not related to climate change. We suggest synchronous uplift of the central and western and Alaska Range occurred as a result of counterclockwise rotation of southern Alaska south of the Denali Fault as a far-field effect of the Yakutat microplate collision.