DEEP EXHUMATION, NEOTECTONICS, AND CONSTRAINTS ON THE DENALI FAULT SYSTEM LONG-TERM OFF-SET HISTORY: THE TOTSCHUNDA STRAND AND THE SOUTH-EASTERN ALASKA RANGE

The dextral, continental-scale, ~2000 km long, arcuate Denali Fault System (DFS) bisects south-central Alaska. The DFS has been a suture zone and the site of dramatic topographic development during the last ~60 Ma. The DFS is still active today with Pleistocene to modern slip-rates of ~7mm/yr to ~13 mm/yr. Controls on long-term slip rates are complicated by several factors: a) previously used offset marker plutons of the same age (~40 Ma), near Mt. McKinley, are non-unique piercing points, based on new geochronology b) oblique convergence is locally accommodated by extreme vertical tectonics, thus correlation across the fault is problematic and c) there is limited information on the timing of formation and slip rates of the Totschunda Fault strand of the DFS in the southeastern Alaska Range.

In 2011 we visited the Cottonwood metamorphic complex and the Nutzotin Mountains of the Totschunda/eastern Denali Fault intersection. The neotectonics of the region maybe linked to the rotation history of the Southern Alaska Block driven by the progression of the Yakutat microplate and to terrane dismemberment along the DFS during the Neogene. We executed a thermochronological sampling regime (horizontal transect and vertical profile) and structural analysis in the region to address timing of fault initiation and constrain total displacement.

The Cotttonwood complex is a fault-bounded sliver of orthogneiss between strands of the presumed active eastern Denali Fault. The ductilely deformed orthogneiss has K-Ar hornblende and biotite cooling ages of ~20 Ma suggesting this time records a period of exhumation from mid-crustal levels. Hence, this area is a unique locale to increase our understanding of deep exhumation patterns and processes along a strike-slip fault system. In addition, the country rock of the Cottonwood metamorphic complex has been noted (e.g., Nokelberg and Richter, GSA Special Paper 431, 2007) as similar to the Maclaren terrane located ~250 km to the west and south of the DFS, providing a potential offset marker.

To the west, Nutzotin Mountains (~2500 m) are a topographic high between the Totschunda and eastern Denali Fault strands of the DFS. The timing and patterns of exhumation from this region will provide information on the history of strain distribution between these two sub-fault systems.