KOBUK FAULT ZONE OF THE SOUTHERN BROOKS RANGE, ALASKA PRESERVES PALEOCENE EXHUMATION OF AMPHIBOLITE GRADE ROCKS ALONG A DEXTRAL-SLIP FAULT SYSTEM

The Kobuk fault zone is an E-W striking set of low- and high-angle faults and ductile shear zones that defines a major boundary along the southern Brooks Range. The fault zone involves low-grade metamorphic rocks along most of its extent, though two small (< 20 km2) amphibolite grade metamorphic complexes occur within the fault zone in low- and high-angle fault contact with very-low-grade rocks. The amphibolite-facies metamorphic complexes have similar protoliths to low-grade rocks on either side of the fault zone, dominantly pelite and metabasite with minor silicic orthogneiss and ultramafic rocks. Previously rock fabrics within the metamorphic complexes were interpreted as inherited from the Mesozoic Brookean orogeny. We find that the fabrics in both metamorphic complexes record an intense simple-shear deformation in the proximity of faults. Mylonitic foliation strikes dominantly 050° - 090°, locally ~100° - 120°, with a low-angle dip. In close proximity to the high-angle faults the mylonitic foliation is subvertical and subparallel to their strike, ~045° -060°. All orientations of the mylonitic fabric contain well-developed stretching lineations defined by elongate quartz and hornblende, trending ~035°-070°. Microscopic kinematic indicators show dextral slip on the high-angle shear planes and the orientations of the fabrics are consistent with the whole of the complex being deformed in a high-strain dextral slip system. A muscovite Ar/Ar plateau date of 54.8 ± 1.3 Ma is interpreted as a cooling age recording exhumation from amphibolite facies conditions in the Paleocene. Paleocene exhumation of the Kobuk fault zone metamorphic complexes in a dextral-slip system coincides with a number of regional tectonic events. They are located due N of similar-sized metamorphic complexes exposed along the Tintina-Kaltag fault system, 200 km to the south, which also yield Paleocene cooling ages. The Paleocene ages may record exhumation associated with initiation of oroclinal bending of western Alaska. The exhumation of the metamorphic rocks also coincides with early Tertiary apatite fission track cooling ages in the central and eastern Brooks Range, thought to have formed during a pulse of N-S convergence.