UC Davis

The Denali fault in the eastern Alaska Range has been active as a dextral strike slip fault since at least 52 Ma. The high-grade metamorphic rocks to the north of the fault—the field area of this study—offer a unique view into the middle to lower crust in a lithospheric scale strike-slip shear zone through time. Previous work constrains the depositional, plutonic, and exhumation/cooling histories of the area however this is the first study to constrain the metamorphic history. New monazite U-Pb petrochronology dates and thermobarometric estimates constrain the peak metamorphic conditions through time.

Four metapelite country rock samples within 5 km of the active trace of the Denali fault yield a continuous spread of ages between ~26 and ~65 Ma with most of the ages between ~32 and ~45 Ma. Samples farther away from the active trace of the fault yield older ages than samples closer to the fault.

Quantitative petrology was performed on the same four dated samples and the thermobarometric results are interpreted as the highest metamorphic grade the rocks reached. Generally, all samples have significant overlap and are within the amphibolite facies between 590-656°C and 4-6 kbar. The samples farthest from the fault broadly record the highest pressures and temperatures.

This is the first study to constrain the metamorphic history of the high-grade rocks on the north side of the Denali fault in the eastern Alaska Range. The monazite ages in combination with kinematic field observations are interpreted as recording the earliest stages of dextral motion at ~52 Ma that continued until ~26 Ma. The younging of monazite ages toward the active trace of the fault suggests that strain was localized over time to a narrower area.