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45432-AC8
Does the Brooks Range Fold and Thrust Belt of Alaska Continue to Wrangel Island, Arctic Russia?
This project capitalized on a unique opportunity to carry out geologic field work on Wrangel Island in the Russian Arctic. It has long been suggested that Wrangel Island irepresents the western continuation of the Brooks Range fold and thrust belt of northern Alaska. With recent interests in offshore exploration, Wrangel Island represents a unique exposure of strata to test for the continuity of structures, lithologies and facies from Alaska to Russia across the Chukchi Sea. Since the thorough study by Kosko et al. (1993, Geol. Surv. Canada, Bull. 461) there has been no additional structural and geochronologic data published for Wrangel Island.
In the summer of 2006, with ACS-PRF funding, we visited the island with an international team of geologists ( S.Sokolov, M. Tuchkova, V. Verzhbitsky, E. Miller and V. Pease) with the help and logistic support of the director and scientific staff of the Wrangel Island Wildlife Preserve. It was a logistically difficult trip that involved three weeks of waiting for weather clearance for helicopter flights from the town of Pevek and three weeks of work, compromised at times by logistics and weather. We achieved all of the sampling objectives outlined in our proposal. I was in addition responsible for geologic mapping (using topographic and satellite-image base maps) and structural measurements. It took our colleagues about one year to receive permission from Moscow authorities to ship our rock samples out of the country. Their analysis is now underway.
Preliminary findings have been discussed and presented informally to many interested parties. Miller will provide the first formal report on our preliminary data at the Fall 2007 AGU meeting in San Francisco. The results of our PRF-funded are briefly summarized below and are subject to modification pending forthcoming additional data.
Strata on Wrangel Island are highly deformed and metamorphosed, but may match part of the section described for the Hannah Trough, Alaska by Sherwood et al., 2002 GSA Spec.Paper 360): Coarse clastic strata overlie late Precambrian basement (630-700 Ma Kos'ko et al (1986)), followed by a succession of mid to late Paleozoic limestone, shale and lesser clastic rocks. Wrangel Island differs from the N.Slope and Brooks Range in that a thick section of basinal Triassic clastics constitutes the upper part of the section on Wrangel. Comparison of single grain U-Pb ages of detrital zircon populations from the Triassic of Wrangel (3 samples, ~ 250 zircon ages) to the Russian Arctic mainland and to the Lisburne Hills, Alaska, suggests basin continuity and similar source regions between these three regions in the Triassic. Single grain ages as young as 215 Ma validate the inferred Triassic age of these unfossiliferous sediments on Wrangel.
Penetrative deformation increasing in intensity with depth in the section is characterized by a foliation that dips south and a pronounced N-S mineral elongation or stretching lineation. The structural style of deformation is unlike the style of folding and thrusting in the external (northern) part of the Brooks Range, but similar to that of the internal (southern) zone of the Brooks Range. Limited thin section observations suggest that this deformation may have been superimposed on an earlier, lower strain event whose evidence has been largely obliterated. The main foliation is axial planar to tight to isoclinal ~ E-W trending folds in all units; these folds (and associated older on younger faults) intricately involve the sediment-basement interface (no brittle detachment). Low-angle faults and map-scale boudinage omit section and structurally thin the stack of folded rocks. Low greenschist facies metamorphism (chlorite-white mica) transitional to mid-greenschist (bitotite) accompanied deformation. Fifteen oriented thin sections show that the high strain in these rocks was accompanied by formation of mylonitic textures in quartz: subgrain formation, grain size reduction, grain boundary migration and syn-tectonic recrystallization. Fabrics indicate mostly flattening perpendicular to foliation and stretching in a NS direction and are not highly asymmetric. The excellent preservation of deformation-related structures in quartz leads to the inference that the observed deformation may be related to unroofing rather than to (thrust) burial. Apatite fission track ages from structurally deepest crystalline basement rocks (samples provided by M. Cecile, GSC) are 82.9 ± 6.7 and 82.5 ± 6.4 Ma (1sigma errors) and have relatively long (~ 14 micron) unimodal track lengths, in support of this inference.
