GEOCHRONOLOGICAL CONSTRAINTS ON THE EOCENE TO PRESENT SLIP RATE HISTORY OF THE EASTERN DENALI FAULT SYSTEM

We apply a broad range of modern high throughput geochronology geochronological techniques to detrital and bedrock samples to better constrain the Eocene to present offset history of the eastern Denali Fault System of Alaska. Previous researchers, based mostly on petrological observations, have suggested the Maclaren Terrane (eastern Alaska Range) and the Cottonwood Terrane (Nutzotin Mts) located along the Denali Fault are displaced fragments of the Kluane Schist/Ruby Range Batholith located north of the Denali Fault in the central Yukon (~400 km to the east of the Maclaren terrane , ~100 km to the east of the Cottonwood Terrane). Others have disputed the central evidence for these claims based on variations in degree of metamorphism between the Maclaren and Kluane metamorphic belts.

We link schist and plutons (~35 Ma to ~45 Ma) from the Maclaren Terrane region to schist and orthogneiss from the Cottonwood Terrane (~300 km to the east) by new detrital U-Pb zircon dating. Through U-Pb zircon dating of the Cottonwood Terrane orthogneiss protolith (~25 Ma peak) and dating detrital U-Pb modern glacial outwash sands between the piercing points we link the McCallum basin (~25 Ma zircon peak) to the Cottonwood Terrane. In addition we link the Miocene McCallum basin, located proximal to the Maclaren Terrane, via detrital U-Pb zircon and detrital ⁴⁰Ar/³⁹Ar muscovite dating (~51 Ma) and muscovite composition analysis (metamorphic) to the existing geochronological signature of the Maclaren Terrane. We acknowledge this link may not be unique, but offers further support for a Maclaren Terrane/Kluane Terrane offset.

We constrain a syntectonic magmatic, metamorphic and rapid exhumation event of the Cottonwood Terrane based on U-Pb zircon ages (~25 Ma), ⁴⁰Ar/³⁹Ar Hornblende (~19 Ma), Muscovite (~18 Ma), Biotite (~17 Ma) ages, and K-spar analysis (~14 Ma), AFT analysis (~10 Ma), and petrological interpretations. We correlate the Denali Fault Cottonwood metamorphic, magmatic, and exhumation event starting at ~25 Ma with the initiation of flat-slab subduction of the Yakutat microplate. Using the correlations above, and assuming only right lateral slip, we find that from ~51 Ma to ~25 Ma total slip on the Denali Fault system is at least ~100 km. Since ~25 Ma there has been at least ~300 km of horizontal offset on the eastern Denali Fault System.