Multiple episodes of Cenozoic denudation in the northeastern Brooks Range: fission-track data from the Okpilak batholith, Alaska

Abstract

The northeastern Brooks Range of Alaska is a complex Mesozoic to Cenozoic northward-verging fold and thrust belt. In response to regional compression, shortening in the upper crust has occurred through the duplexing of thrust sheets and formation of associated fault-bend folds. Apatite and zircon fission-track data from the Okpilak batholith and adjacent sedimentary rocks exposed within the northeastern Brooks Range provide new constraints on the timing, magnitude, and rate of cooling of these thrust sheets as they were rapidly denuded in response to uplift during Cenozoic time. Fission-track results indicate that a previously recognized episode of Paleocene cooling was followed by at least two younger episodes of rapid cooling during Middle Eocene and Late Oligocene time. The two younger episodes of rapid cooling are interpreted to reflect denudation in response to uplift resulting from Cenozoic thrusting and related folding. As a result of structural thickening, up to 8 km of material was eroded from the top of the batholith between ~41–45 Ma (Middle Eocene). Renewed shortening and emplacement of an underlying thrust sheet at ~25 Ma (Late Oligocene) resulted in at least 2 km of uplift and erosion of sedimentary rocks immediately north of the batholith. These results suggest that, even though Paleocene uplift and erosion may have occurred across the northeastern Brooks Range, the major episode of thrust faulting, responsible for structural emplacement of the batholith into its present position and kilometre-scale denudation, most likely occurred during Middle Eocene time.