Late Cretaceous-Cenozoic Exhumation of the Western Brooks Range, Alaska, Revealed From Apatite and Zircon Fission Track Data

Abstract

We report data for 112 apatite and 31 zircon fission track (AFT and ZFT) outcrop sandstone samples along a transect that spans the western Brooks Range. Sampling targeted structures that modify the Middle Jurassic-Early Cretaceous early Brookian orogen. The AFT samples record latest Cretaceous to Eocene in situ exhumational cooling and resolve two kinematic phases. The first phase was focused at 65–60 Ma. To the north, cooling age patterns at this time are attributable to wide-spaced fault-related folding. Farther south, within the allochthon belt, exhumation was related to uplift of a broad region, likely in the hanging wall of deep-seated faults that extend into basement rocks. The second kinematic phase occurred around ~45 Ma. It was characterized by north and east directed thrusting to the north, and coeval extension in the allochthon belt to the south. The ZFT cooling ages are all Early Cretaceous or older and put an upper limit on the magnitude of Cenozoic exhumation across the western Brooks Range. Synthesis of exhumation patterns and structural styles show that Paleocene rejuvenation of contraction was roughly contemporaneous along the entire ~1,000-km orogen. Later, around ~45 Ma in the Eocene, contraction in the frontal parts of the orogen was contemporaneous with extension interior to the orogen. Following previous authors, we suggest that the Paleocene rejuvenation was a far-field response to subduction of a mid-ocean ridge in southern Alaska. However, by the Eocene, strain patterns in the western Brooks Range changed, possibly to accommodate rotations of fault blocks in southwestern Alaska.