Structural Origins of the Doonerak Anticline, Central Brooks Range, Alaska

The Doonerak anticline in the central Brooks Range, Alaska, exposes a package of Early Paleozoic metavolcanic and metasedimentary rocks overlain by Carboniferous to Triassic sedimentary rocks which have been correlated to the North Slope subsurface. The flanks of the anticline have been interpreted as portions of a single north-vergent nappe system emplaced during the Brookian orogeny. The Doonerak anticline is a key location for understanding the structure of the Brooks Range fold and thrust belt. Field mapping, microstructural analysis, Raman-spectroscopy of carbonaceous material (RSCM) paleothermometry, and detrital zircon geochronology data suggest the structure is not a tectonic window resulting from erosion of a single north-vergent nappe system. Structures in the units to the north of the anticline are south-vergent, while structures in units to the south of the anticline are north-vergent. Detrital zircon geochronology and petrographic data show that Devonian clastic units in the Endicott Mountains allochthon north of the anticline are not correlative to slates and phyllites in the Hammond assemblage south of the anticline. RSCM paleothermometry shows the southern units reached temperatures of 368+/-16°C and were buried ~12 km while the northern units reached 313+/-19°C and were buried ~10 km. A new model is proposed which accounts for observed structural and lithologic relationships and calculated paleotemperatures. In this model, the southern assemblage was emplaced by north-directed thrusting and northern assemblage was emplaced by south-directed motion along a later back-thrust during the predominantly north-vergent Brookian orogeny.