The Chukchi Borderland, Arctic Alaska

The Chukchi Borderland is an unusually shaped rectangular region that juts northward into the deep Arctic Ocean from Arctic Alaska's Beaufort and Chukchi Shelf (Figs 1, 2). The borderland is transected by north-south trending normal faults giving it a ridge-like nature with flat plateau-like crests (Fig. 2).

Figure 1. Index map showing basement rock affinities of the circum-Arctic region: Cratons (North America, Baltica and Siberia) to orogenic belts and displaced terranes. Modified after Miller et al. (2011 and 2017). CB, Chukchi Borderland; D, Mount Doonerak region of the Brooks Range; P, Pearya terrane of the Canadian Arctic, SV, Svalbard. Dotted lines show inferred connection of dredged rocks and contacts in the Chukchi Borderlands across the rifted Arctic Ocean to Canada.

The Convention on the Law of the Sea defines the conditions under which a coastal state may extend its continental shelf over regions beyond the 200 nautical mile limit, the Extended Economic Zone (EEZ) (UN, 1982). These conditions involve the definition of a legal “continental shelf” where a key element of this definition is the demonstration that the extended area is a “natural prolongation” of the nation’s landmass. The determination must be based on a general knowledge and interpretation of the bathymetry, geology, and nature of the seafloor in a region. One of the potential regions for an extended continental shelf beyond the current 200 nautical mile limit is found in the area of the Chukchi Borderland. For detailed information please visit: Law of the Sea and U.S. Extended Continental Shelf.

Rock samples were retrieved by dredging from the Chukchi Borderland during part of the U.S. campaign to map Alaska’s continental slope and determine the nature of the bathymetric highs that form prolongations of the Alaska continental shelf (Fig. 3). The dredging and simultaneous bathymetric mapping were carried out in a series of cruises using the U.S Coast Guard icebreaker Healy by the Center for Coastal and Ocean Mapping of University of New Hampshire, P.I. Larry A. Mayer. To ensure rocks were sampled from actual seafloor exposures of bedrock, steep slopes with little or no evidence for sediment drape were targeted for dredging. The tension on the dredge was also monitored in order to tell if it had “hung up” and then broke free on rock outcrops. When the dredges were recovered, the rocks obtained were separated into probable bedrock by the presence of clean broken fresh surfaces as opposed to cobbles and pebbles of ice rafted debris (common in the Arctic) based on a number of criteria described in Brumley et. al (2014). Rocks of inferred metamorphic and igneous origin were studied at Stanford University funded by NSF Award 0948673, “Origin and Evolution of the Amerasia Basin of the Arctic” (E.L. Miller, P.I.) The results of petrographic, geochemical, isotopic, geochronologic and thermochronologic studies of these rocks and the original data can be accessed in the Ph.D. theses and journal articles listed below with the general results summarized here.

Bathymetric mapping data of Chukchi Borderland — Figure 2. View looking south of the elevated Chukchi Borderland and its north-south trending ridges showing locus of swath bathymetric mapping along its borders with the deeper Canada Basin (left, east) and Amerasia Basin (right, west). From NOAA

Bedrock samples dredged from the easternmost edge of the Chukchi Borderland (the Northwind Ridge; Fig. 2,3) include deformed and weakly metamorphosed calcareous sandstone, slate and phyllite. Zircon crystals, present as sedimentary detritus in sandstones were dated by the U-Pb method and the resulting ages statistically compared to the detrital zircon signatures of sedimentary sequences in on-land exposures. The detrital zircon signatures of the dredged rocks are typical of those from late Precambrian to Cambrian strata deposited along the ancient North American continental margin of Arctic Canada. Dredged rock from the fault scarps more centrally located in the Chukchi Borderland included high-grade metamorphic rocks and metamorphosed sedimentary and igneous rocks. Their history is complex, but they represent parts of a Cambrian to Silurian subduction-related arc system and younger associated Silurian sedimentary rocks, which are all distinctly different in terms of age and tectonic setting from strata representing the North American continental margin. The two distinct groups of rocks are inferred to lie in fault (thrust or strike-slip) contact with one another. A very similar juxtaposition is present in the Canadian Arctic islands where volcanic arc and associated sedimentary rocks of the same age as those dredged from the Chukchi Borderland lie north of, in thrust or strike-slip contact with, deformed sediments deposited along the Canadian Arctic portion of the North American continental margin (Fig. 1). The presence of this contact in two places in the circum-Arctic, separated by a deep-water ocean basin suggests that the Chukchi Borderland represents a southern continuation of this contact that was rifted away from Arctic Canada during formation of the Arctic Ocean basins. This conclusion is supported by other recent studies of Arctic Alaska that document that it once adjoined the Canadian Arctic margin (e.g. Gottlieb et al., 2014). Our discoveries shed light on a puzzling aspect of the geology of Arctic Alaska: In the southern Brooks Range, there appears to be a juxtaposition of North American sedimentary sequences in the eastern Brooks Range with Cambro-Ordovician volcanic arc assemblages in the Doonerak Mountain region (D in Fig. 1) (Strauss et al., 2016). Our discoveries about the nature of rocks on the Chukchi Borderland help us link together these different areas based on contacts between distinctive rock units and help explain why such exotic rocks might be found in Arctic Alaska. More speculatively, these exotic arc rocks may represent an along strike continuation of the Caledonian orogen of Norway and Greenland and may have been the source for displaced rock units in the Alexander, Klamath and Northern Sierra regions of the Cordillera (Fig. 1).

Map of collection of rock samples by dredging from the Chukchi Borderland — Figure 3. Location of rock samples retrieved by dredging from the Chukchi Borderland and environs in the Amerasia Basin of the Arctic Ocean, from Brumley (2014). Rock samples were collected as part of a U.S. effort to map Alaska’s continental slope and determine the nature of the bathymetric highs that form prolongations of the Alaska continental shelf. The dredging and simultaneous bathymetric mapping were carried out with a series of cruises using the U.S Coast Guard icebreaker Healy by the Center for Coastal and Ocean Mapping of University of New Hampshire, P.I. Larry A. Mayer. See also: U.S. Extended Continental Shelf Cruise reports - Arctic

Map of reconstruction of the Chukchi Borderland prior to rift formation of the Arctic Ocean — Figure 4. Approximate reconstruction of the Chukchi Borderland to Pearya in the Canadian Arctic prior to rift formation of the Arctic Ocean from O'Brien et al. (2016)

Publications

Miller, E. L., N. Kuznetsov, A. Soboleva, O. Udoratina, M. J. Grove, et al. 2011. “Baltica in the Cordillera?” GEOLOGY 39 (8). GEOLOGICAL SOC AMER, INC: 791–94. DOI: 10.1130/G31910.1

O'Brien, Tim M., and Elizabeth L. Miller. 2014. “Continuous Zircon Growth during Long-Lived Granulite Facies Metamorphism: a Microtextural, U-Pb, Lu-Hf and Trace Element Study of Caledonian Rocks from the Arctic.” CONTRIBUTIONS TO MINERALOGY AND PETROLOGY 168 (4). SPRINGER. DOI: 10.1007/s00410-014-1071-x

O'Brien, Tim M., Elizabeth L. Miller, Jeffery P. Benowitz, Kristian E. Meisling, and Trevor A. Dumitru. 2016. “DREDGE SAMPLES FROM THE CHUKCHI BORDERLAND: IMPLICATIONS FOR PALEOGEOGRAPHIC RECONSTRUCTION AND TECTONIC EVOLUTION OF THE AMERASIA BASIN OF THE ARCTIC.” AMERICAN JOURNAL OF SCIENCE 316 (9). AMER JOURNAL SCIENCE: 873–924. DOI: 10.2475/09.2016.03

Brumley, Kelley, Elizabeth L. Miller, Alexandros Konstantinou, Marty Grove, Kristian E. Meisling, et al. 2015. “First Bedrock Samples Dredged from Submarine Outcrops in the Chukchi Borderland, Arctic Ocean.” GEOSPHERE 11 (1). GEOLOGICAL SOC AMER, INC: 76–92. DOI: 10.1130/GES01044.1