Framework of the Ancestral Brooks Range from Detrital Zircon Populations in Early Brookian Foreland Basin Deposits

Moore, Thomas E. and O’Sullivan, Paul B.
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Early foreland basin deposits of the north-vergent Brooks Range orogen consist of Upper Jurassic to Albian foredeep and wedgetop deposits of the Colville basin and predecessor foreland deposits that were imbricated during construction of the Brookian fold-and-thrust belt. In the western Brooks Range, detrital zircon U-Pb age populations from these deposits fall into three groups: 140-160 Ma (peak at 155 Ma), 220-500 Ma (peaks between 230 and 330 Ma), and a lesser population at 1.80-2.0 Ga (peaks at ~1.8 and ~2.0 Ga). The Late Jurassic to Early Cretaceous (140-160 Ma) population can be ascribed to sources in the subduction, ophiolite, and island-arc assemblages of the Angayucham and Koyukuk terranes, which compose klippen forming the upper plate of the modern Brooks Range orogen, but the Paleozoic to Triassic (220-500 Ma) and the Paleoproterozoic (1.8-2.0 Ga) populations do not correspond to zircon populations in Paleozoic and older rocks of the Arctic-Alaska-Chukotka microplate (AACM), which forms the lower plate of the orogen. Partially reset to unannealed fission-track ages from the dated zircon populations point to derivation instead from terranes and allochthons that must have once formed the structurally highest parts of the orogen but were completely eroded away during the Brookian structural event. Zircon populations derived from the AACM to not appear in the foreland basin deposits until mid-Albian time, coincident with the first appearance of glaucophane in the sediments, suggesting that prior to the mid-Albian the ancestral Brooks Range exposed thrust sheets of far different origin than those that compose the range today.

The Triassic and older zircon populations in the early Brookian sandstones closely resemble zircon populations in Triassic deep-marine siltstone and sandstone in the Russian Far East that are thought to have been derived from ultimate source areas in the northern Urals and Taimyr. These sandstones rest unconformably on the AACM in Chukotka and are structurally interleaved with the Upper Paleozoic and Lower Mesozoic Alazeya-Oloy arc terrane and paleo-oceanic terranes of the South Anyui zone, which in part have been thrust northward onto the AACM. Sediment pathways are uncertain but may have involved long distance axial transport of zircons down the foreland basin into the western Brooks Range from source areas in Chukotka and(or) direct sourcing and multicycling into wedgetop and foredeep basins from thrust-imbricated Triassic sandstones that once extended across large parts of the AACM.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013