EXTENSIONAL UNROOFING OF THE BROOKS RANGE REVEALED BY U-PB DETRITAL ZIRCON SUITES FROM YUKON-KOYUKUK BASIN STRATA

Early Brookian orogenesis resulted from the collision of an island arc with the continental margin of Arctic Alaska during the Middle Jurassic-E. Cretaceous. N-directed obduction of oceanic allochthons was followed by N-vergent imbrication of passive continental margin strata. The Yukon-Koyukuk (YK) Basin developed to the south of the Jura-Cretaceous Brooks Range (BR) orogenic belt, in response to extension collapse. Deposits of the YK Basin consist of Aptian-Albian marine turbidites and proximal basin margin conglomerates and sandstones. The evolution in clast compositions of the marginal conglomerates led Dillon and Smiley (1984) to interpret that basin fill was due to extensional unroofing of the BR. We present new U-Pb detrital zircon (DZ) results from samples collected along a N-S transect of the John River. Results reveal three distinct types of relative probability patterns (RPP). Type 1 RPP (stratigraphically youngest) is dominated by 330-580 Ma zircons, with peaks at 370 and 435 Ma and by a broad distribution of Precambrian age zircons. This DZ signature is observed in conglomerates that contain a high proportion of quartzites, metamorphic lithics and metamorphic minerals (e.g. Ms, Chl, Cld). Type 2 (stratigraphically oldest) is dominated by 160-220 Ma zircons, with minor peaks around 103 and 360-420 Ma. This RPP is observed in conglomerates dominated by cobbles of chert and gabbro. Petrographic observations indicated that these sediments are composed of volcanic rock fragments, clasts of coarse-grained gabbro, chert and mafic minerals (e.g. Am and Ep) and are interpreted to represent the erosion of an ophiolitic sequence or juvenile island arc. Type 3 (stratigraphically intermediate) displays a RPP reflecting a mixture of type 1 and 2 but are primarily type 1 dominant with zircons between 330-500 Ma, with secondary peaks of 180-250, 530-700 and 900-2100 Ma. The composition of the sediments reflects the erosion of an ophiolite and island arc sequence (e.g. mafic minerals). These new results supports the interpretation that the sequence represents progressive unroofing of the BR. Uplift of the BR first led to erosion of the uppermost oceanic and arc derived allochthons. Continued erosion of structurally deeper metamorphic core deposited younger sequences containing second cycle DZ populations.