UPLIFT AND DENUDATION OF THE CENTRAL ALASKA RANGE - A CASE-STUDY IN THE USE OF APATITE FISSION-TRACK THERMOCHRONOLOGY TO DETERMINE ABSOLUTE UPLIFT PARAMETERS

Apatite fission track thermochronology (AFTT) on granitic samples collected in the central Alaska Range in conjunction with geologic constraints from basins to the north (Nenana Basin) and south (Cook Inlet) of the range is used to constrain the timing, amount, rate, and pattern of surface uplift, rock uplift, and denudation since the late Miocene. The conversion from a thermal frame of reference (apatite fission track data) to an absolute frame of reference (with respect to mean sea level), which requires constraining the paleoland surface elevation, the paleomean annual temperature, and the paleogeothermal gradient, is evaluated and shown to be viable in the context of an exhumed apatite partial annealing zone (PAZ). Apatite ages at Denali (Mount McKinley) range from 16 Ma near the summit (similar to 6 km elevation) to 4 Ma at similar to 2 km elevation. A distinctive break in slope in the apatite age profile at an elevation of 4.5 km, also marked by a change fn confined track length distributions, marks the base of an exhumed apatite PAZ. Rack uplift and denudation are greatest at Denali, decreasing southward away from the McKinley strand of the Denali fault system as shown by progressively older apatite ages (7-35 Ma) from a shite of samples along the Kahiltna Glacier. A correlative decrease in topography occurs southward from the fault. The central Alaska Range lies within an are defined by the Denali fault, with the highest peaks (including Denali) concentrated at the are apex. Patterns of rock uplift and denudation within the central Alaska Range mimic topography. Between early and late Miocene, and possibly earlier, the central Alaska Range was most likely an area of relative tectonic and thermal stability. Rock uplift, denudation, and mean surface uplift of the Denali region began by the Late Miocene (similar to 5-6 Ma), being similar to 8.5 km, similar to 5.7 Km, and similar to 2.8 km, respectively, at average rates of similar to 1.5 km/m.y., similar to 1 km/m.y., and similar to 0.5 km/m.y. The amount of rock uplift, denudation, and surface uplift decreases to similar to 3 km, similar to 2 km, and similar to 1 km at Little Switzerland, some 45 km south of the Denali fault. We conclude that the topographic and rock uplift patterns of the central Alaska Range, the shape and proximity of the McKinley strand of the Denali Fault to these patterns, the timing of the onset of rock uplift and denudation at similar to 5-6 Ma, and a significant change in relative plate motion between North America and the Pacific plates circa 5.6 Ma are all inherently related.