GEOLOGIC SYNTHESIS OF THE METAMORPHIC CORE OF THE BROOKS RANGE OROGEN, ALASKA

A new geologic map of the metamorphic core of the Brooks Range orogen has been constructed at a scale of 1:500,000, and is the first compilation detailed enough to illustrate specific stratigraphic, structural and metamorphic components of the core.

Two major metamorphic belts occupy the core. The southern of the two is a 600-km long structurally and metamorphically coherent blueschist belt. Protoliths range in age from Late Proterozoic to Devonian. This belt, known as the Schist belt, is remnant of a Jurassic subduction system that predated crustal thickening and foreland basin formation.

The second belt, known as the Hammond terrane or Central belt, was deformed during the period of crustal thickening and foreland basin sedimentation. It sits along the northern boundary of the Schist belt, and contains rock units ranging from Late Proterozoic to Jurassic in age. The most striking result of the new compilation is the lack of structural and metamorphic continuity within the Central belt. Inhomogeneous deformation – ductile shear zones bounding weakly deformed to undeformed thrust slices – is typical of much of the belt, and may have been a product of several events. At least three Cretaceous metamorphic events are recognized within the belt, but their areal extents are not known. Ductile shear zones formed at blueschist-facies define an antiformal duplex in the western part of the belt (Nanielik antiform). Elsewhere, ductile shear zones appear to have formed at greenschist facies. Structures associated with an albite-epidote amphibolite-facies culmination in the central part of the belt are entirely ductile (Arrigetch-Igikpak area). Brittle Cenozoic structures define an antiformal culmination in the eastern part of the belt (Doonerak) and are known elsewhere.

The oldest known rocks in the Brooks Range (971 Ma) and lower Paleozoic sedimentary succession(s) critical to paleogeographic reconstructions are contained in the Central belt. The deeper crustal response to Mesozoic and Cenozoic crustal thickening is recorded in the Central belt. Construction of accurate tectonic models of northern Alaska depends on improved understanding of rocks and relationships exposed in the Central belt.