45432-AC8
Does the Brooks Range Fold and Thrust Belt of Alaska Continue to Wrangel Island, Arctic Russia?
Structures on Wrangel Island are believed to represent the western continuation of the Brooks Range fold and thrust belt of northern Alaska. With renewed exploration of Alaska's Chukchi Shelf, Wrangel Island represents a unique exposure to test the continuity of structures, lithologies and facies from Alaska to Russia. No significant new data have been published for Wrangel Island since the thorough study by Kos'ko et al. (1993, GSC. Bull. 461). We visited the island in 2006 with the logistic support of the director and scientific staff of Wrangel Wildlife Preserve. The trip involved four weeks of waiting for weather clearance for helicopter flights and three weeks of fieldwork, compromised at times by logistics and weather. We carried out geologic mapping, structural measurements and sampled extensively for paleontology, geochronology and thermochronology. Samples were delayed in Moscow for a year, As of this final report, 9 samples spanning the upper Paleozoic and Mesozoic stratigraphic succession of Wrangel have had their detrital zircons suites dated by the U-Pb method with the LA-ICP-MS at U. Arizona (collaboration with G. Gehrels); 4 more samples have been analyzed from the Lisburne Hills, Alaska, for comparison. Twenty-eight samples of apatite are irradiated and ready for fission track dating (with Trevor Dumitru, Stanford) to determine the low-T cooling history and place constraints on the age of deformation of the ~ 10-15 km thick structural succession exposed on the Island. Miller supported the visit of a young scientist Vladimir Verzhbitsky at Stanford during the fall of 2006, where we integrated data from the Pevek region (Miller and Verzhbitsky, in press). The first reports on Wrangel were presented at the Fall 2007 AGU meeting in (Miller et al., 2007). A paper on the stratigraphy and provenance of the Paleozoic and Triassic of Wrangel is currently being written and the data will be presented at the AGU 08 Fall Meeting. A paper outlining the structural geology of the island, the petrography and analysis of deformational and metamorphic fabrics (in collaboration with Brad Hacker at UCSB) together with thermochronologic results will constitute the last of the three major contributions based on our fieldwork. PRF funds supported the work of two undergraduates at Stanford. Sarah Aarons received summer support in form of a PRF minority fellowship. We visited the Brooks Range and North Slope together, collected samples of Alaska's slate and greywacke belt, separated and analyzed detrital zircons from these samples. Julie Padilla worked under my direction for a year on Arctic related research projects. The main conclusions from PRF-funded field and laboratory work include: 1. The Paleozoic succession of Wrangel appears to match the Hannah Trough section on Alaska's Chukchi Shelf. Dev(?)-Miss(?) conglomerates and sandstone overlie basement dated as 630-700 Ma (Kos'ko et al., 1986) and are overlain conformably by late Paleozoic limestone, shale and lesser clastic rocks deposited in a shelf basin setting. The Triassic siliciclastic section matches the Russian mainland but not northern Alaska. 2. Detrital zircon suites from Paleozoic strata on Wrangel Island are similar through the upper Paleozoic, reflecting a platformal setting with stable sources for siliciclastic detritus. Basement detritus (~750-550) is present in all sandstones but decreases up section, replaced by Ordovician zircons (~ 490-440 Ma). All Pz units contain lesser Precambrian zircons in the 1000-2000 Ma interval. A significant change in provenance occurs at the base of the Triassic where 1000-1500 Ma sources disappear in concert with the arrival of upper Paleozoic sources (320-250 Ma zircons). 3. Comparison of Pz and Mz detrital zircon populations of Wrangel to the Russian Arctic mainland and to new data from Alaska, suggests basin continuity and similar source regions between these three regions throughout the Pz-Mz, These data also constrain plate tectonic reconstructions of the Cretaceous Amerasian Basin. Restoration of Wrangel/Chukotka/N.Alaska back to the Taimyr and/or Barents Shelf to receive Caledonian as well as Baltic shield detritus in the upper Paleozoic and share its Permo-Triassic rift history is supported by the new data. 4. Strata on Wrangel Island are highly deformed and metamorphosed to greenschist facies. Deformation increases with depth in the section and is characterized by a pronounced foliation that dips south and a 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. The excellent preservation of deformation-related structures in quartz leads to the inference that the observed deformation may be related to extensional 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.
