Abstract

We present an apatite fission-track (AFT) study of five plutonic rocks and seven metamorphic rocks across 310 km of the Yukon–Tanana Upland in east-central Alaska. Samples yielding ∼40 Ma AFT ages and mean confined track lengths > 14 μm with low standard deviations cooled rapidly from >120°C to <50°C during a 3–5 Ma period, beginning at about 40 Ma. Data from samples yielding AFT ages >40 Ma suggest partial annealing and, therefore, lower maximum temperatures (∼90-105°C). A few samples with single-grain ages of ∼20 Ma apparently remained above ∼50°C after initial cooling. Although the present geothermal gradient in the western Yukon–Tanana Upland is ∼32°C/km, it could have been as high as 45°C/km during a widespread Eocene intraplate magmatic episode. Prior to rapid exhumation, samples with ∼40 Ma AFT ages were >3.8-2.7 km deep and samples with >50 Ma AFT ages were >3.3-2.0 km deep. We calculate a 440-320 m/Ma minimum rate for exhumation of all samples during rapid cooling. Our AFT data, and data from rocks north of Fairbanks and from the Eielson deep test hole, indicate up to 3 km of post -40 Ma vertical displacement along known and inferred northeast-trending high-angle faults. The predominance of 40-50 Ma AFT ages throughout the Yukon–Tanana Upland indicates that, prior to the post-40 Ma relative uplift along some northeasttrending faults, rapid regional cooling and exhumation closely followed the Eocene extensional magmatism. We propose that Eocene magmatism and exhumation were somehow related to plate movements that produced regional-scale oroclinal rotation, northward translation of outboard terranes, major dextral strike-slip faulting, and subduction of an oceanic spreading ridge along the southern margin of Alaska.

You do not currently have access to this article.