Cordilleran-Brooks Range Connection from U-Pb Detrital Zircon Dating
Thomas E. Moore1, Kenneth J. Bird1, and Paul B. O'Sullivan2
1U.S. Geological Survey, Menlo Park, CA.
2Apatite to Zircon, Inc, Viola, ID.
Because crystalline basement rocks are rare in the Arctic Alaska terrane (AAT), its tectonic affinity is poorly understood with most current speculations being based on the paleogeographic affinity of its fossils. To address the question of its origin, we conducted detrital zircon U-Pb geochronology on more than 30 sandstones from various parts of the terrane in northern Alaska. The grain populations of these rocks provide insights into the ages of the basement rocks in their provenance areas and thus information about the origin of AAT.
Several units containing distinct age populations appear to characterize significant parts of AAT. These include: (1) the Neoproterozoic Neruokpuk Quartzite in the Romanzof Mountains, marked by numerous 1.8 and 2.3-2.7 Ga grain ages; (2) Neoproterozoic strata in the Sadlerochit and Shublik Mountains, distinguished by grain populations similar to the Neruokpuk but with additional peaks between 0.9 and 1.7 Ga; (3) Lower Paleozoic sandstones in the Lisburne Peninsula and in the Endicott Mountains allochthon in the southern Brooks Range, delineated by abundant 550 to 700 Ma grains and a broad distribution of grains between 0.9 and 1.6 Ga; and (4) Lower and Middle Devonian sandstones in the subsurface of the North Slope, the Mt. Doonerak structural window, and the northeast Brooks Range, all characterized by abundant 400 to 470 Ma grains and sometimes with a broad distribution of grains between 0.9 and 2.0 Ga. Analyses of latest Devonian to Triassic sandstones of the Ellesmerian sequence, which overlies and links together all of these units by the Mississippian, indicate derivation from local sources consisting of one or more of the population groups.
Together with field and subsurface observations, the data suggest that AAT consists of at least two crustal components of Neoproterozoic and early Paleozoic age. An eastern component, represented by the Neruokpuk Quartzite population, displays similar isotopic and stratigraphic characteristics to strata in northwestern North America, suggesting that the eastern part of AAT may have formed as a marginal part of Laurentia. A western component, represented by the samples from the Lisburne Peninsula and parts of the southern Brooks Range, suggest these areas may have formed as part of Arctida, a continental terrane composed of northern Baltica and Chukotka. The Sadlerochit-Shublik Mountains population suggests this area could have affinity with either North America or Baltica. A fourth provenance consists of Devonian deposits shed from an Ordovician to Devonian source region that included debris derived from Baltica. These results suggest that AAT is a composite terrane built by the merger of components of North America and Baltica in the Devonian. Subsequent Devonian contractional structures overprint the dated units and suggest the merger may have occurred as part of the closure of the early Paleozoic Iapetus Ocean during the Caledonian orogenic event.
AAPG Search and Discovery Article #90130©2011 3P Arctic, The Polar Petroleum Potential Conference & Exhibition, Halifax, Nova Scotia, Canada, 30 August-2 September, 2011.����������������������������������������������������