Tertiary Cooling and Deformation in the South‐Central Brooks Range: Evidence From Zircon and Apatite Fission‐Track Analyses

In the south‐central Brooks Range, Alaska, amphibolite‐facies metamorphic rocks of the Arrigetch and Igikpak orthogneisses crop out between the upper greenschist‐facies schist belt and more northern fold‐and‐thrust belt. The timing and mechanisms by which these amphibolite‐facies rocks were emplaced to the north of the lower‐grade schist belt rocks are poorly understood. We have obtained seven zircon and 24 apatite fission‐track analyses from samples collected in the Igikpak and Arrigetch orthogneisses and schist belt in the Walker Lake region. Zircon and apatite fission‐track analyses from the schist belt samples are consistent with cooling (and exhumation of the rocks to within a few kilometers of the surface) between 80 and 70 Ma. The data from the Arrigetch and Igikpak orthogneisses to the north, however, indicate that they were at much deeper crustal levels during this time period. Zircon and apatite fission‐track analyses are consistent with significant cooling in the Arrigetch and IgIkpak orthogneisses between ∼42 and 21 Ma. The deformational event responsible for this cooling is unclear, although the presence of a down‐to‐the‐north normal fault (the Takahula fault) on the northeastern side of the plutons suggests that exhumation on the footwall of a normal fault is a possibility. The data also document significant down‐to‐the‐west offset from 42 to 20 Ma along a NNW‐trending vertical fault bisecting the Arrigetch and Igikpak orthogneisses. Based on the fission‐track analyses from this study, we suggest that the southern Brooks Range has undergone episodic cooling (and deformation) during Cretaceous and Tertiary time.