DETRITAL ZIRCON U-PB AGES FROM FORELAND BASIN DEPOSITS OF THE WESTERN BROOKS RANGE, ALASKA

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

The main phase of the Brooks Range orogenic belt involved north-directed, thin-skinned deformation produced by collision of the Koyukuk arc terrane with the south-facing continental Arctic-Alaska passive margin in the latest Jurassic to Early Cretaceous. The orogenic belt consists of allochthons of Jurassic ophiolite at high structural levels and distal Devonian to Jurassic passive-margin deposits with imbricated Neocomian foreland basin deposits (Okpikruak Fm.) at lower structural levels. Post-collisional extensional exhumation of the hinterland in the Albian and Cenomanian exposed pre-Mississippian basement at 103-96 Ma and shed voluminous clastic deposits into the foreland (Nanushuk Fm.) and hinterland. Foreland basin strata deposited in the Aptian-Albian near the end of the deformation are part of the Fortress Mountain Formation and informally named "lower Brookian" deposits. Laser ablation ICPMS was used to date 100 zircon grains from each of five foreland basin samples from a variety of structural and stratigraphic positions in the western Brooks Range. From oldest to youngest, these include structurally higher and lower samples of the Okpikruak Fm., samples from the "lower Brookian" and overlying Fortress Mountain Formation, and one from the Nanushuk north of the Neocomian deformation front. All samples yielded generally similar results, with zircons of a variety of ages ranging to at least as old as 2.4 Ga. The largest concentration of ages is 250-360 Ma with a subordinate grouping at 1.8-2.1 Ga. Although Early Cretaceous zircons occur in the three youngest samples, only two samples contain sparse zircons that approximate the ~160-170 Ma age of the ophiolite and associated arc, and the structurally highest sample yielded no zircons younger than 280 Ma. Although the data were expected to reveal differences in provenance based on the inferred exhumation history of the orogen, the similarity of the results indicate a similar provenance for all samples. For stratigraphic reasons, the Permo-Carboniferous detritus could not have been derived from the Arctic Alaska, but the results are broadly similar to detrital zircon ages from Triassic rocks in Chukotka (Miller et al., 2006, Tectonics). This suggests the possibility that detritus was shed eastward from Chukotka laterally along the foreland basin axis.