EXHUMATION PATTERNS DURING CENOZOIC GROWTH AND GLACIATION OF THE ALASKA RANGE: INSIGHT FROM DETRITAL GEO- AND THERMO-CHRONOLOGY

Cenozoic growth of the Alaska Range has created the highest topography in North America, but the space-time pattern and drivers of exhumation are poorly constrained. We use detrital geo- and thermo-chronology from 12 catchments that span 450 km west to east along the Alaska Range to illuminate the spatial extent of exhumation during the Oligocene-Pliocene. We analyzed U/Pb and fission track double-dates of both zircon and apatite grains from modern river sand. U/Pb ages indicate a dominant Late Cretaceous-Oligocene igneous (plutonic) provenance for the detrital grains, with only a small percentage of grains recycled from the Mesozoic and Paleozoic sedimentary cover. The detrital U/Pb ages match the ages of local Late Cretaceous-Oligocene plutons. Consequently, fission track ages record the exhumation of this plutonic belt during Alaska Range growth and incision and reveal three distinctive patterns. First, initial Oligocene exhumation was focused in the central Alaska Range ca. 30 Ma and progressively expanded outward along the entire length of the range until 18 Ma. This exhumation was coeval with evidence for initial Yakutat microplate collision over 600 km to the southeast and suggests a far-field response to collision that was localized by the Denali Fault within a broad zone of weakness. Second, the variable timing of Late Miocene exhumation suggests independently-evolving histories influenced by local structural complexities. We observe time-transgressive exhumation ages west of the Denali Fault restraining bend, which suggests successive uplift of Mt. Foraker (12 Ma), Mt. Hunter (9 Ma) and Mt. McKinley (6.6 Ma) as crust was advected into the restraining bend. Third, Pliocene exhumation is synchronous (3.7-2.7 Ma) along the length of the Alaska Range, but only occurs in high relief (2.5-5.5 km), glaciated (20-80% modern area) catchments. We hypothesize that Pliocene exhumation records an acceleration in glacial incision that was coincident with the onset of northern hemisphere glaciation.