Investigating localized exhumation of rocks on the north side of the Denali fault along the eastern Alaska Range

The active, dextral-slip Denali fault in central Alaska accommodates deformation from the collision of the Yakutat microplate into the North American plate along the southern Alaska active margin 500 km to the south. Following the curve of the Denali fault is the Alaska Range, a ~650 km long mountain range characterized by high elevation peaks separated by broad expanses of much lower elevation. Within the eastern Alaska Range, part of which has been exhuming relatively rapidly since ~24 Ma, the regions of greatest exhumation are localized along strike on the north side of the Denali fault. While exhumation is expected along such a significant fault system, it is unknown why it is localized and heterogeneous within the eastern Alaska Range. Mechanisms to explain this pattern of exhumation include investigating near-field boundary conditions, such as fault dip and fault shape, to partition strain into regions of focused and unfocused exhumation along the strike of the Denali fault. We observe changes in the style of deformation from east to west within the eastern Alaska Range, which correspond with an older exhumation in the east, based on thermochronologic ages (AFT, K-feldspar and 40Ar/39Ar biotite) and a younger exhumation to west over a distance of ~29 km. Orthogneiss from the eastern region of greatest total exhumation, based on Ar/Ar mica ages and petrological analysis, have been ductiley deformed at amphibolite facies indicating deformation at depths of ~15-18 km. Kinematic indicators in these rocks suggest oblique dextral thrusting. Approximately 15 km to the west orthogneiss has a temperature of deformation of ~300-350°C, suggesting exhumation from ~11 km near the brittle-ductile transition zone. Kinematic indicators there show dominantly dextral slip with only a minor thrust component, based on the rake of the mineral elongation lineation. Five kilometers west of this site in the region of shallow, young exhumation with an AFT age of ~1 Ma, the mid-Tertiary Nenana pluton has no plastic fabric and is only locally brittley deformed. That the thermochronology ages are youngest in the region of brittle deformation suggests that this exhumation is more recent and these rocks have been exhumed from shallower depths as compared to the east, where rocks are ductiley deformed and the exhumation is older and greater. The location of the active strand of the Denali fault at this locale is well constrained by the epicenter of the 2002 6.7M Nenana Mountain earthquake. Just north of that epicenter, slickenlines from several distinct fault zones within the pluton yield a dominantly normal solution, striking approximately north-south, which is perpendicular to the curve of the Denali fault. These faults, which are mid-Tertiary or younger as constrained by the age of the Nenana pluton, may indicate that some of the exhumation in the west is partially driven by the stretching of brittle crust. These dramatic, along-strike changes in both exhumation depth and style of deformation suggest complex spatial and temporal variations in the kinematic behavior of the Denali fault zone.