WHAT MADE DENALI SO TALL? STRUCTURAL GEOLOGY OF THE HIGH PEAKS OF THE ALASKA RANGE

The two tallest peaks of the Alaska Range, Mt. McKinley (or Denali) and Mt. Foraker, rise 2-3 km above the surrounding peaks and lie adjacent to the Denali fault system. A 22 degree bend in the right-lateral Denali fault system is north of the summit of Mt. McKinley. This geometry caused contractional deformation on the inside, or south side, of the bend, resulting in uplift of Mt. Foraker (17,400 feet) and Mt. McKinley (20,320 feet). Contractional deformation is focused on a thrust fault beneath the Foraker massif, and on the Pittock Pass-Rooster Comb series of thrust faults between the McKinley pluton and the adjacent Ruth pluton. Both major thrust fault systems strike northeasterly, are southwest vergent, and have gouge zones more than 100 m thick. The summit of Mt. Foraker and the ~38 Ma Foraker pluton are in the hanging wall of the Foraker thrust. This thrust extends northeasterly and connects with the Denali fault. Southeast of Mt. McKinley between the ~56 Ma McKinley and Ruth plutons, the McKinley pluton is thrust over Jura-Cretaceous country rocks along the Rooster Comb thrust.

Mt. McKinley and Mt. Foraker have different positions with respect to the subjacent thrust faults. The summit of Mt. McKinley is 16 km northwest of the toe of the Rooster Comb thrust, whereas the summit of Mt. Foraker is 6 km northwest of the Foraker thrust. It appears there are no large faults within the McKinley pluton. Shallowly dipping fractures are common in the northern half of the McKinley pluton and may reflect deformation of the pluton over a thrust-fault ramp. There is a prominent series of northwest-striking conjugate fractures within the southern half of the McKinley pluton and the adjacent rocks to the south. These fractures are perpendicular to the northeast-striking thrust faults and taken together, constitute an orthorhombic fault set. These faults allow for non-coaxial brittle deformation and uplift of the region inside the bend of the Denali fault. The age of the master thrust faults is difficult to constrain, but they are likely young and possibly active. The Rooster Comb thrust correlates with an inflection point in the topography of the range. A fission track study, which showed that rapid exhumation of Mt. McKinley began around 6 Ma and continues to the present, implies there must be young (post 6 Ma) deformation.