1 The Arctic Beaufort Sea Alaska, Yukon, NWT margin and the Amphibious Array Frank Vernon1 and Roy Hyndman2 Michael Riedel2, John Orcutt1, Michael West3 And S. Dallimore, M. Côté,, G.C. Rogers, J. Cassidy, T. Allen, J. Henton (1) University of California, San Diego, (2) Pacific Geoscience Centre, Geological Survey of Canada, (3) University of Alaska, Fairbanks Introduction The Arctic margin of the Beaufort Sea extends across the north coasts of Alaska, the Yukon, and western Northwest Territories of Canada (see Figure 1). Until recently sea ice severely limited marine operations. However, rapid arctic warming is resulting in much less ice, now allowing marine OBS and seismic reflection operation with considerably less difficulty. This margin provides an outstanding scientific opportunity for the Amphibious Array: offshore OBSs, land stations, multichannel seismic and associated surveys. There also are important applications in petroleum basin analyses and other increasing activities in the Arctic. The USArray Alaska stations now being installed are very complementary. Figure 1. Beaufort Sea Margin Amphibious Array Proposed Study Area 2 Figure 2. Geological structure of the Beaufort Sea Alaska and NW Canada, illustrating the margin depositional regime and the rifted (blue dashed) and convergent (dashed red) parts of the margin (modified from Houseknecht and Bird, 2011). There have been extensive surveys by the petroleum industry and work by the U.S. and Canadian Geological Surveys mainly on the coastal region and continental shelf. However, there has been only limited survey and study beneath the deep sea, of the deep crustal structure across the margin, and of the current and recent tectonics. The formation of the Beaufort Sea and its role in the overall Arctic Ocean assembly is still not well understood, for example, has the Beaufort Sea been formed by rotation of Alaska away from the Canadian arctic islands. A second important question is the origin of the only recently recognized active margin fold and thrust belt. 3 The Arctic Margin as a Fold and Thrust belt “Subduction Zone” and Strain Transfer from the Yakutat Terrane Collision in the Gulf of Alaska There is a passive rifted margin off western arctic Alaska and the Canadian Arctic Islands (e.g., Grantz et al., 1994; Houseknecht and Bird, 2011; see Figure 1). However, for the intervening 400 km wide coastal region extending from the sinistral Canning River Displacement Zone in eastern Alaska to the dextral Richardson-Eskimo (Husky) Lakes Fault zone at the eastern edge of the Mackenzie River Delta in westernmost Northwest Territories, there is a well- developed margin fold and thrust belt of Cretaceous to the present age (e.g., Lane, 2002; see Figures 1 and 3). This is an extraordinary type of “subduction zone”, where the continental upper crust appears to be overthrusting the oceanic lithosphere for a section of the margin. Alternatively, with a continental reference frame, there is underthrusting of the ocean crust beneath the margin. There are no arc volcanics or Wadati-Benioff seismicity probably because the convergence rate is so slow, a few mm/yr. The northerly thrust motion over the Beaufort Sea is interpreted to be driven by the Yakutat terrane collision in the Gulf of Alaska (see Steep Project) 1000 km to the southwest (e.g., Mazzotti and Hyndman, 2002) (Figure 5). Crustal temperatures are concluded to be hot enough in the northern Cordilleran mobile belt to have a detachment in the lower crust, allowing this long distance northwestward motion. The Yakutat collision is also concluded to drive the westward motion of the Mackenzie Mountains at the Cordillera mountain front to the west. These mountains are currently very seismically active with large historical thrust earthquakes. To the south of the arctic margin the Canning and Richardson fault zones are seismically active with slow but significant motion rates of a few mm/yr (e.g., Leonard et al., 2008; Figure 2, Right) with left-lateral and right-lateral earthquake mechanisms respectively. The intervening 4 crustal block is moving toward and overriding the arctic ocean lithospehre in a type of “subduction zone”. Figure 3. (Left) Model of long-distance crustal transport from the Gulf of Alaska Yakutat terrane collision (adapted from Leonard et al., 2008); (Right) Seismicity defining northwestward moving block over-riding the Arctic ocean. However, the margin thrust belt has very little seismicity, so it is not yet clear if, (1) convergence motion is aseismic, (2) the thrust belt has only large, ~M8, infrequent events with large tsunamis, similar to Cascadia. This is a critical hazard question for the Arctic margin. The thrusting is most evident for the Mackenzie Delta. The geological history of the Delta fold and thrust belt has been quite well determined, mainly using petroleum exploration wells on the continental shelf. There have been progressive thrust accretion of sediment packages since the Cretaceous (Figure 3; see Lane et al. 2002 for a summary). Only the most recent motion can be ascribed to the Yakutat collision. There must have been earlier northerly 5 Figure 4. The Arctic margin progressive fold and thrust belt sediment accretion of the Mackenzie Delta interpreted to result from slow convergence and ocean crust underthrusting, from the Cretaceous to Recent (Modified from Lane, 2002). motion of the Delta driven by terrane motions northward along the Pacific margin (e.g., Johnson, 2001). Offshore Beaufort Sea Seismicity Offshore in the Beaufort Sea there is a significant cluster of seismicity that may result from downward bending of the oceanic lithosphere as it is overthrust by the continent (or ocean underthrusts, depending on the reference frame). This seismicity is interpreted to result from flexure of the arctic ocean as it is thrust beneath and loaded by the Mackenzie Delta accreted fold and thrust belt. The seismicity in the Beaufort-Mackenzie area is poorly monitored with distant observation stations limiting the interpretation of the location and depth of current and historical earthquakes. 6 Figure 4. Distribution of ~200 earthquakes >M2.5 in the Mackenzie -Beaufort area from 1985-2012. They are interpreted to result from flexure of the oceanic crust by underthrusting beneath the margin and loading by the Mackenzie Delta The earthquake catalog for the region has more than 200 earthquakes located in the Beaufort Sea (see Figure 4) with location accuracy in the order of 50 km without depth control since the nearest station Inuvik is ~1000 km away and the next closest Dawson City ~2500 km distant. Two stations have recently been installed in Paulatuk and Sachs Harbor, and an initial USArray TA supported station was installed at Eagle Plains (Yukon Territory) making some improvement to the coverage. As the extensive USArray stations are deployed in northern Alaska and the northwestern Canadian Arctic, the deployment of the very complementary amphibious array OBSs can address the question of the origin of this seismicity and the hazard it represents. 7 Strain Transfer from the Yakutat Terrane Collision in Gulf of Alaska An important hypothesis to be tested is the long-distance strain transfer to the Arctic Ocean margin thrusting from the Yakutat Terrane Collision in the Gulf of Alaska. It has been proposed that there is a detachment in the lower crust of the northern Cordillera that allows the upper crust to move independent of the upper mantle. The estimated temperatures in the lower Figure 5. Model for transfer of strain from the Yakutat terrane collision in the Gulf of Alaska to overthrusting of the margin of the arctic Beaufort Sea. There is inferred detachment in the lower crust of the northern Cordillera (modified from Mazzotti and Hyndman, 2002). crust of this area are high enough to allow a very low strength detachment surface (e.g., Mazzotti and Hyndman, 2002). This kind of detachment may be important in a number of continental areas globally. Some Scientific Questions to be be Addressed by the Amphibious Array: 1. The origin and tectonic history of the Beaufort Sea and Canada Basin, and the formation of the arctic margins. 2. The deep structure of the Beaufort Sea margin, both passive portion and convergent portions. 8 3. Is the model of long-distance tectonic transport from the Yakutat collision zone (Figure 5) to the Beaufort Sea margin correct? How is such motion accommodated? Has there been ongoing such motion since the Cretaceous to explain the history of the arctic margin thrust belt? Is this type of detachment and long-distance crustal transport more common globally than recognized? 4. To obtaining accurate earthquake locations and depths and source mechanisms of events in the Beaufort Sea. What is the origin of this seismicity? 5. Improving the characterization of the seismic stress regime. The stress regime is important for tectonic understanding and for petroleum development. 6. Characterizing the relationship of the offshore thrusts and the onshore Canning Displacment Zone in the west and the Eskimo Lakes Fault (Husky Lakes) system in the east. 7. Is there potential for great thrust earthquake on the margin of the Beaufort Sea? Are there paleo-tsunami deposits or other great earthquake indicators along the coastal areas of the Alaska- Beaufort region? Great thrust earthquakes are an important question for the seismic hazard of the Beaufort region of northern Alaska and NW Canada. 8. Increased knowledge of ground-motion amplification and its regional variation (as function of arctic soil conditions. Little is known especially of the effect of permafrost. Some References: (1) Grantz, A., S.D. May, and P.E. Hart, 1994. Geology of the Arctic continental margin of Alaska, Geol. N. America, Vol. G-1, GSA. (2) Houseknecht, D.W. and K.J. Bird, 2011. Geology and petroleum potential of the rifted margins of the Canada Basin, Geol. Soc. London, Memoirs 35, 509-526.