APATITE FISSION TRACK EVIDENCE OF WIDESPREAD EOCENE EXHUMATION IN THE FORTYMILE DISTRICT OF EAST-CENTRAL ALASKA

Crystalline rocks of the Yukon–Tanana Upland (YTU) are cut by steeply-dipping, NE-trending sinistral and normal faults related to the Tintina and Denali dextral fault systems that border the YTU. Fault blocks in the western Fortymile mining district are, from W to E: 1) Mount Harper, ~105 Ma granitic batholith; 2) Middle Fork, ~70 Ma caldera; 3) Mount Veta, metamorphics and ~190 and ~68 Ma intrusions, and 4) Kechumstuk, metamorphics and a ~210 Ma batholith and ~68 Ma rhyolite dikes.

We obtained apatite fission track (AFT) analyses from 33 igneous samples within the 2300 km² western Fortymile district. Outflow tuff of the Middle Fork caldera in blocks 2 and 4 yielded ~70 Ma AFT ages that overlap the U–Pb and Ar/Ar age of the tuff, consistent with little exhumation since 70 Ma. Most AFT ages from all blocks cluster at ~56–47 Ma (early Eocene) and ~44–36 Ma (middle Eocene); some apatites have shortened track lengths and modeled t-T paths consistent with slow cooling from ~110 to 60°C. Other samples with Eocene AFT ages have long track lengths and modeled t-T paths indicating rapid cooling at ~50–40 Ma. Samples with the youngest AFT ages (~33–27, 19 ± 2, and 10 ± 1 Ma) occur along the southern end of the Kechumstuk fault zone that separates blocks 3 and 4.

We propose that most sampled rocks remained at >~110 °C from mid-Cretaceous to Eocene time. Eocene AFT cooling ages also occur in the YTU outside of the study area (Dusel-Bacon and Murphy, CJES 2001). The regional extent of these ages suggests the primary cause of cooling was regional denudation of a pre-existing elevated plateau due to accelerated erosion in the warm, wet Eocene climate. Preservation of ~70 Ma outflow tuff in a valley and the location of many samples with younger AFT ages at higher elevations suggest that the tuff was emplaced in a canyon that cut through a higher relief topography. Removal of ~1.7 km of interfluvial overburden could explain most Tertiary AFT ages. Some Tertiary exhumation was likely due to slip on faults related to large-scale tectonics (CCW rotation of SW Alaska, dextral motion on the Tintina and Denali faults, subduction of a spreading ridge). Juxtaposition of early and middle Eocene AFT ages and of rapidly and slowly cooled rocks, and younger AFT ages along the southern Kechumstuk fault zone suggests variable resetting by hydrothermal fluids or uplift along local faults.