New Insights into the Tectonic History of the Central Brooks Range, Alaska: Application of 40Ar/39Ar Dating of Encapsulated Illite-rich Shales and Clays

The central Brooks Range is a major fold and thrust belt that extends across northern Alaska. Middle Jurassic to Cretaceous south dipping subduction resulted in the stacking of seven allochthons within the Brooks Range. Shales from the lowermost allochthon (the Endicott Mountains allochthon) were dated by the 40Ar/39Ar method to better constrain their burial and uplift history. Encapsulation and vacuum crushing systems were constructed to date aggregates of clay particles (<0.2 to 10 microns), and to measure the recoil fraction prior to laser step heating. Step-heating proved to be effective in identifying and dating neoformed and detrital illite components. XRD was used to determine clay compositions and illite crystallinities.

A significant age contrast is revealed across a major east-west striking fault zone (the Toyuk thrust) within the central Brooks Range. The thrust separates the Endicott Mountains allochthon into northern and southern parts with different stratigraphies. Our analysis suggests that this thrust also separates rocks with differing tectonic or burial histories. Devonian shales from south of the thrust yield relatively flat age spectra (indicative of neoformed illite), 8 to 23% 39Ar recoil loss, and argon retention ages around 225 Ma (older than zircon fission track ages). Samples from Mississippian to Cretaceous shales north of the thrust yield staircase age spectra (indicative of a detrital illite component), 7 to 27% recoil loss, and older retention ages (233-391 Ma). Spectra from both sides of the Toyuk also provide evidence of uplift at about 140 Ma. Argon retention ages may reflect differences in depths of tectonic burial, and levels of heating, of rocks that are now separated by the Toyuk thrust. Exhumation that followed burial would have preserved these differences in the geochronologic record. However, the 40Ar/39Ar ages (Late Triassic) predate the onset of thrust deformation and consequent exhumation (Jurassic-Cretaceous) as determined from other geologic evidence. Alternatively, the age spectra might reflect differing ages of crystallization of neoformed illite within a passive margin setting. In this model, this geochronologic record of burial was not reset during later deformation.