RAPID QUATERNARY EXHUMATION OF THE EASTERN ALASKA RANGE

The Denali Fault is the dominant structural feature of the Alaska Range in central Alaska. The southern Alaska block, located south of the Denali Fault, is moving north and west as a result of subduction of the Yakutat microplate 400 km to the south. The Denali Fault cuts across the Alaska Range (AR), which we separate into central and eastern regions near Broad Pass. The eastern AR was the site of the M7.9 earthquake on the Denali Fault in 2002. Most of the high topography in the central AR, including the highest peak in North America (Denali – 6188m), is located south of the Denali Fault. In contrast, peak elevations in the eastern AR near Mt. Hayes and Deborah are lower (highest at 4215m), and most of the higher elevation is on the north side of the Denali Fault. Published apatite fission-track ages from Denali show an exhumed partial annealing zone that indicates rapid exhumation beginning 5-6 Ma. We produced new apatite fission-track ages from the Hayes range in the eastern AR. The samples were collected just north of the Denali Fault, span 800 m of elevation, and the ages increase with elevation from 2.8 to 5.1 Ma. Same-sample apatite (U-Th)/He ages increase with elevation from 1.4 to 3.7 Ma. The age-elevation slopes for the AFT and He ages are similar to the exhumed partial annealing zone slope for the Denali data, but are offset to lower elevations. South of the Denali Fault in the Hayes range, He and AFT ages for one sample are three and five times higher, respectively, than same elevation samples north of the Denali Fault, implying higher exhumation rates north of the Denali Fault in the Quaternary. AFT and He age profiles from north of the Denali Fault suggest exhumation rates of ~1.5-2.0 mm/yr beginning ca 1.4 Ma in the Hayes range, which is more recent and more rapid than in the Denali area. This region of rapid rock uplift and exhumation is coincident with the edge of a tomographically identified NE-trending zone of thick crust in the southern Alaska block that projects north of the Denali fault and under the Hayes range. An interpretation of this rapid Quaternary exhumation in the Hayes range is that it was caused by impingement of the thick Wrangellian crust against thinner Yukon-Tanana crust, located north of the Denali and Hines Creek fault systems, as the southern Alaska block moved NW as a result of Yakutat microplate subduction.