VARIABLE EXHUMATION ALONG THE STRIKE OF THE ALASKA RANGE

The Alaska Range is a significant ~650 km long, topographic barrier, that impacts the climate of the interior (northern) portion of the state. As revealed by low-temperature thermochronology, the timing of formation of the central Alaska Range (Mt. McKinley or Denali region) is relatively well constrained, but the timing of formation along the rest of the range is poorly constrained. Published biotite and K-spar 40Ar/39Ar thermal histories from Denali are interpreted to reflect pluton emplacement cooling starting at ~60 Ma and do not reflect cooling related to exhumation. Apatite fission track (AFT) data records the onset of rapid denudation associated with uplift and formation of the central Alaska Range ~5-6 Ma (Fitzgerald et al., JGR v.100, 1995). Denali (6194 m) and its close neighbor Mt. Foraker (5304 m) are the only peaks above 4450 m in the entire Alaska Range and may therefore record a thermal evolution specific to that area but not the range as a whole. Preliminary biotite and K-spar 40Ar/39Ar step-heat data from plutons in the eastern Alaska Range suggest a different cooling history compared to Denali. Mt. Nenana (2377 m elevation, 167 km east of Denali) has a biotite closure age of 37 Ma, with k-spar closing between 19 Ma and 9 Ma. Mt. Balchen (3396 m, 75 km east of Mt. Nenana) has a biotite closure age of 29 Ma and a similar k-spar closing history between 19 Ma and 9 Ma. The large lag between biotite and K-spar ages and the similarity in the overall K-spar cooling histories for both plutons imply a protracted period of exhumation along the eastern Alaska Range not revealed at Denali. We are undertaking (U-Th)/He and AFT analyses on these eastern Alaska Range samples to constrain the thermal history since 9 Ma. Located north of the eastern Alaska Range is the dominantly Miocene Usibelli Group, with ~800 m of deposits reflecting fluvial, lacustrine, and peat bog environments plus extensive coal horizons. Ridgway et al. (GSA Spec. Pub. 431, 2007) suggested that the eastern Usibelli group represents the development of a regional Miocene transpressional foreland basin related to the initiation of regional shortening south of the basin. Preliminary low-temperature thermochronology data suggesting Miocene cooling are consistent with this interpretation and indicates a much more protracted exhumation history for this part of the range.