TIMING OF BROOKS RANGE AND NORTH SLOPE UPLIFT AND DENUDATION: A SUMMARY OF THE FISSION-TRACK RESULTS

O'SULLIVAN, Paul B., Apatite to Zircon, Inc, 1075 Matson Road, Viola, ID 83872-9705, [email protected], MOORE, Thomas E., U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, WALLACE, Wesley K., Department of Geology and Geophysics, University of Alaska, Fairbanks, Fairbanks, AK 99775, and POTTER, Christopher J., U.S. Geological Survey, Mail Stop 939, Denver Federal Center, Denver, CO 80225-0046

The Brooks Range orogen has a protracted history of development occurring primarily during: 1) arc-continent collision during the Late Jurassic to Early Cretaceous Brookian orogeny, 2) crustal-scale tectonic extension in the hinterland during the middle Cretaceous, followed by 3) a resumption of north-directed thrusting in the early Tertiary. The Jurassic to Cretaceous collisional event was caused by south-directed subduction, which was replaced, by north-directed subduction in southern Alaska at about the time of the mid-Cretaceous extensional event. Tertiary events are the result of retroarc thrusting that emanated from subduction in southern Alaska, followed by collision of the Yakutat terrane with the southern Alaska continental margin in the Neogene.

Zircon and apatite fission track results from Devonian through Cretaceous granitic and sedimentary rocks help resolve the timing of uplift and denudation of these events within the northern Brooks Range orogen, the northern foothills belt, and the southern flank of the adjoining North Slope foreland basin. Zircon results from outcrop samples within the orogen record cooling below paleotemperatures >240°C at ~140 Ma, ~120 Ma, ~60 Ma, and ~45 Ma, whereas apatite results record rapid cooling below paleotemperatures >110-130°C during discrete episodes at ~100 Ma, ~60 Ma, ~45 Ma, ~35 Ma, and ~25 Ma. Within the foothills and adjoining foreland basin, zircon results record provenance cooling ages with distinct grain-age populations at ~160 Ma, ~140 Ma, and 120 Ma, whereas apatite results record rapid cooling below paleotemperatures >110-130°C at ~100 Ma, ~60 Ma, ~45 Ma, ~35 Ma, and ~25 Ma. Within the basin, apatite results from subsurface samples indicate exposure to maximum paleotemperatures in the Late Cretaceous to early Paleocene as a result of subsidence and burial by Jurassic and Cretaceous sedimentary rocks. Rapid cooling from these elevated paleotemperatures subsequently occurred at ~60 Ma, ~45 Ma, and ~25 Ma. These episodes of rapid cooling are interpreted as recording: 1) absolute timing of the aforementioned tectonic events within the Brooks Range and its foreland basin, and 2) that the elevated Brooks Range orogen seen today is a primarily a mid-Cretaceous and Cenozoic feature, unrelated to the Late Jurassic to Early Cretaceous Brookian deformation.