Tectono-thermal evolution of the Brooks Range collisional orogen, Alaska: Insights from a structural, metamorphic, and argon-40/argon-39 thermochronology study of the Arrigetch Peaks region

Many questions remain regarding the late-orogenic tectonic, thermal, and exhumational history of metamorphic rocks in the Brooks Range collisional orogen. To address these questions, I have undertaken a field based study across a metamorphic culmination that exposes a range of crustal levels in the Arrigetch Peaks region of the south-central Brooks Range. The study integrates field mapping, structural analysis, metamorphic petrology, and 40Ar/ 39Ar thermochronology.; Observations on the north flank of the culmination indicate that following Aptian and older N-directed D₁ deformation, low-strain D₂ backfolds and N-dipping crenulation cleavages (S₂) formed in the early to mid-Albian, near the metamorphic peak. D₂ was immediately followed by D₃ extension, which produced small-displacement N-side-down normal faults in greenschist-grade rocks and more penetrative vertical thinning at deeper levels. I infer that folds in epidote-amphibolite faces rocks initiated during D₂ backfolding, but that much of the strain recorded by gently dipping S₂ crenulation cleavages and tight recumbent folds accumulated during D₃ extension.; Thermobarometry shows that the highest grade rocks (560–600°C, 8–9.5 kb) coincide with the apex of a structural culmination defined by oppositely dipping S₂ crenulation cleavages. The southern flank of the culmination underwent a high-P/low-T metamorphism not experienced by northern flank, and was buried more deeply, suggesting that the epidote-amphibolite fades core was not thrust northward over the southern flank as previously proposed. Instead, the southern flank underwent partial exhumation during northward thrusting, enabling the formerly more deeply buried rocks to escape thermal overprinting.; 40Ar/39Ar thermochronologic data and exhumation modeling indicate that much of the cooling and exhumation of metamorphic rocks in the southern Brooks Range occurred during two events. A mid-Albian to earliest Cenomanian exhumation event, delineated by hornblende and mica data, is attributed to extension and rapid erosion. A southward increase in cooling/exhumation rates is inferred to result from unroofing below normal faults on the south flank of the range. A second cooling/exhumation event, delineated by thermal modeling of 40Ar/39Ar K-feldspar data, occurred in the Paleocene to Eocene, and was probably caused by renewed contraction accompanied by surface uplift and an increase in erosion rates. Apparently, little or no cooling occurred in the Late Cretaceous between these two events.