T11E-01 INVITED
Jurassic to Present Evolution of the Arctic Ocean Region: Questions for IPY
In 1955, Carey first suggested a rotational opening of the Arctic Ocean based on his theories concerning oroclinal bending. It was only with the plate tectonic revolution in the next decade that there were a number of seminal abstracts and papers concerning the tectonic evolution of the Canada Basin. Grantz in 1966, Hamilton in 1967 and Tailleur in 1969 formulated the idea that the Canada Basin may have opened about a pivot point in the Mackenzie Delta. In 1968, Karasik first published his revolutionary idea that aeromagnetic anomalies collected over the Gakkel Ridge in the Eurasian Basin suggested that seafloor spreading during the Tertiary accounted for most of the oceanic crust between the Lomonosov Ridge and the Barents and Kara shelves. From the results of the 1979 Lomonosov Ridge experiment by Forsyth and Mair, it was established that the deep crustal velocities of the Lomonosov Ridge are remarkably similar to those of the Kara Sea shelf at 82°N, making it a continental sliver that rifted off the Barents and Kara Sea shelves. Consequently, the Arctic Ocean can be divided into a mostly Tertiary to present Eurasian Basin, and the older Mesozoic Amerasian Basin. Even with additional data from the Amerasian Basin including extensive airborne, ship-based, and satellite magnetics and gravity, the tectonic evolution of the Canada and Makarov components of the Amerasian Basin are still controversial. While the final phase of opening of the Amerasian Basin may have been rotational about a pivot point near or south of the Mackenzie Delta, the earlier phases and even the directions of the initial basin formation are still in doubt. Vogt and others first suggested a possible hotspot origin for the Alpha and Mendeleev Ridges and related them to the Iceland hotspot. Age versus depth and heat flow versus depth give a tentative age for the Amerasian Basin of latest Jurassic to early Cretaceous with seafloor spreading ending prior to the beginning of the Cretaceous Normal Superchron around 120 Ma. A brief review of the tectonic history of Arctic Ocean will be used to formulate a list of unknowns that IPY and subsequent work might investigate. We see a three or more stage opening of the Amerasian Basin. First stage is rifting between the Arctic Alaska/Chukokta block from the Canadian Arctic Islands with rotation of the Chukchi Borderland/Northwind Ridge out of the North Chukchi Basin similar to present motion in the Afar region and rotation of the Danakil block. This first stage was followed by a reorganization and the final stage is a rotational opening about a pole east of Fairbanks, AK. The Northwind Ridge was originally along the Sverdrup Basin margin of Canada. Timing of the stages is inexact due to a lack of correlatable magnetics but the final stage had to end prior to the start of the Cretaceous Normal Superchron because the gravity anomaly that marks the abandoned spreading center is flanked by magnetic anomalies.
T11E-02
Tectonic History of the Amerasia Basin, Arctic Ocean
Seismic reflection, refraction and potential field data from Amerasia Basin in conjunction with piston cores from Northwind Ridge suggest that the basin was formed by four rotational extensions. The first event stretched and thinned Pangea on a westerly-dipping crustal scale detachment fault system of Sinemurian to no later than Early Hauterivian age that day-lighted on the east along the continental margin of Northwestern Canada. This event rotated Eastern Siberia about 50° anticlockwise from Northwest Canada about a pole in the lower Mackenzie Valley and created transitional crust, which lacks seafloor spreading magnetic anomalies, beneath the marginal areas of the present Amerasia Basin. The second event, 9° or 10° of anticlockwise rotational seafloor spreading, split the earlier-formed transitional crust and emplaced MORB (mid-ocean-ridge basalt) along a northerly trending symmetry axis in the center of the Amerasia Basin. The resultant fan of magnetic anomalies, estimated to be of Late Hauterivian to Late Barremian age (136-125 Ma), is geometrically symmetrical with the first spreading event and likewise converges toward a pole of rotation in the lower Mackenzie Valley. Approximately 45° of clockwise rotation of Chukchi Microplate out of the East Siberian shelf about a pole near 72.5° N, 170° W constitutes the third rotational event, which probably occurred during or shortly following the Late Barremian. This event thrust the northeastern corner of the Chukchi Microplate across the boundary between event 1 and event 2 crusts in the western Canada Basin and created North Chukchi Basin in its wake. North Chukchi Basin is partially filled with post-Barremian to Early Campanian oceanic basalts of the Alpha-Mendeleev Large Igneous Province (125-80 Ma). In the absence of well-defined aeromagnetic anomalies or crustal-scale reflection data we can only speculate, on the basis of morphology, that North Chukchi Basin is a product of localized rotational seafloor spreading The fourth event, mildly rotational Paleocene extension, created basin and range structural morphology and the northerly-trending Northwind Basin in the axial region of the Chukchi Microplate. This extension thinned the continental crust of the microplate beneath the Northwind Basin by about 35 percent and created accommodation space for >2,000 m of water and >4,500 m of clastic sediment within the basin. The pole of this rotation was apparently located on the central Chukchi shelf. Following the four extensional events the southeastern margin of the Amerasia Basin was subject to far-field convergence of Middle Eocene to Quaternary age that appears to have originated at the Pacific Rim and created large thrust-related detachment folds that may be significant for hydrocarbon exploration.
T11E-03
Arctic Crustal Thickness and Ocean-Continent Transition from Gravity Inversion Incorporating a Lithosphere Thermal Correction
Crustal thickness and continental lithosphere thinning factors have been determined for the High Arctic using a gravity inversion method which incorporates a lithosphere thermal gravity anomaly correction. Continental lithosphere thinning factor maps, determined by the inversion of the NGA (U) Arctic Gravity Project data have been used to predict the distribution of oceanic lithosphere and ocean-continent transition (OCT) location for the Amerasia Basin. Thin crust and high lithosphere thinning factors are predicted in the Makarov, Podvodnikov and Canada Basins consistent with these basins being oceanic. Larger crustal thicknesses, in the range 20 – 30 km, are predicted for the Lomonosov, Alpha and Mendeleev Ridges. Moho depths predicted by gravity inversion have been compared with seismic estimates for the TransArctica and Arctica profiles with seismically observed sediment thickness included in the gravity inversion. Agreement between gravity and seismic Moho depths is generally good. The largest differences between gravity and seismic Moho depths occur where lower crustal seismic velocities, Vp, are in excess of ~ 7.3km/s. Gravity inversion to determine Moho depth and crustal thickness variation is carried out in the 3D spectral domain. A correction for the large negative residual thermal gravity anomaly within oceanic and stretched continental margin lithosphere is made and requires a lithosphere thermal model to predict the present day lithosphere thermal anomaly. For continental margin lithosphere, the lithosphere thermal perturbation is calculated from the lithosphere thinning factor (1-1/beta) obtained from crustal thinning determined by gravity inversion and breakup age for thermal re-equilibration time. A correction is made for crustal volcanic addition due to decompression melting during breakup and sea-floor spreading. For the Amerasia Basin, where ocean isochrons are uncertain, all lithosphere is assumed to be initially continental, and a lithosphere thinning and thermal perturbation age corresponding to the time of continental breakup is used. For the Eurasia Basin and the N Atlantic, oceanic magnetic isochron ages are used to condition the lithosphere thermal model. The new gravity inversion method, incorporating the lithosphere thermal gravity anomaly correction, provides an isochron independent prediction of OCT location for the Amerasia Basin. Predicted continental lithosphere thinning factors and crustal thickness for the Amerasia Basin are sensitive to continental breakup age and volcanic addition. Comparison of gravity and seismic Moho depths for the Makarov/Podvodnikov Basins supports a Cretaceous age for their formation. We thank Statoil and NFR for their support.
T11E-04
Upper-crustal velocity structure along 150 km of the Mendeleev Ridge from tomographic inversion of long-offset refraction data collected during HLY0602
In the summer of 2006 we acquired a unique seismic refraction data set on the Chukchi Borderlands and Mendeleev Ridge utilizing USCGC Healy and two helicopters. The array on the Mendeleev Ridge consisted of 14 instrument sites with 12 km spacing between instruments. On every site we deployed a Sea-Ice Seismometer (S- IS) especially designed for this experiment in the ice-covered part of the Arctic Ocean. Each S-IS contained a vertical component geophone that was buried in the ice and a hydrophone that was hanging from the ice edge in the water. From the 14 instrument sites, 10 contained useful data with refracted crustal arrivals up to offsets of 40 km. Because of extensive drifting of the receivers (40 km in 5 days and containing numerous loops), and because of the irregular geometry of airgun shots due to the problems of sailing through ice-covered seas, a 3D ray-shooting code was developed to calculate ray paths within a 3D velocity model that extends along 150 km in the X- direction and along 35 km in the Y-direction. Using the velocity model proposed by Lebedeva-Ivanova et al. (2006) we observe that the maximum depth of our calculated ray paths is 11 km below sealevel. Using all the available data, the Root Mean Square (RMS) difference between observed and calculated travel-times is of the order of 500 ms. Initially a simple 1D travel-time inversion was developed to constrain the velocity structure of the basement underneath a layer of water (3D) and a layer of sediment (1D). This inversion was carried out on 2 pairs of receivers: one pair in the NNE and one more towards the SSW part of the line. Inversion of S-IS 45N-42 (NNE) results in a model with a velocity of 5.5 km s-1 at the top of the basement, slowly increasing to a velocity of 5.7 km s-1 at 3 km below the top of the basement (RMS = 117 ms). Inversion of S-IS 49-45S (SSW) results in a model with a velocity of 4.8 km s-1 at the top of the basement, increasing to a velocity of 5.9 km s-1 at 3 km below the top of the basement (RMS = 67 ms). The two resulting crustal velocity models suggest that there is a significant change in velocity along the Mendeleev Ridge: >0.5 km s-1 difference between the final models from S-IS 45N-42 and S-IS 49-45S. Rays in both models penetrate to a depth of 6.5 km. These results indicate that the Mendeleev Ridge has different crustal velocity structure in the northern and southern parts. It might also provide first proof that the Arlis plateau has a different origin and is separate from the Mendeleev Ridge. A 2D inversion is being developed which will allow inverting all the data along the Mendeleev Ridge simultaneously and which will provide us with a 2D upper-crustal velocity model along 150 km of the Mendeleev Ridge. Comparison of the resulting velocity model with the velocity structure of continental crust, thinned continental crust, oceanic crust, and other oceanic ridges will allow us to interpret the upper-crustal velocities along the Mendeleev Ridge in terms of crustal lithology and tectonic history. N.N. Lebedeva-Ivanova, Y.Y.Zamansky, A.E. Langinen, M.Y. Sorokin (2006). Seismic profiling across the Mendeleev Ridge at 82oN: evidence of continental crust. Geophys. J. Int. 165(10), 527-544.
T11E-05
Kinematic History and Tectonic Evolution of the Amerasian Basin: Investigating Palaeo-Plate Boundaries around the Chukchi Borderlands
The multi-stage opening of the Arctic Ocean's Amerasian Basin is only partially understood due to the difficulty of utilizing traditional marine geologic and geophysical techniques in ice-covered waters. While the kinematic development of the Eurasian Basin is well-understood to be the northernmost extension of the Mid-Atlantic Ridge, the history of the morphologically complex Amerasian Basin may be due to multiple events, significantly complicating interpretation of its history. Any detailed model for the opening of the Amerasian Basin must both incorporate structures that accommodate spreading as well as explain the tectonic mechanisms that drove basin development. Cretaceous-age tholeiitic flood basalts and associated radiating dike swarms of the High Arctic Large Igneous Province (HALIP), found along the basin margin, provide a tectonic mechanism and geometry to substantiate sound reconstruction. Detailed models need also consider pre-existing zones of weakness such as the deformation front of the Devonian Caledonides, which may underlie Barents Shelf sediments (Gee and Bogolepova, 2003). Reactivation of these ancient structural trends along this suture zone may explain the motion of Mendeleev Ridge as it rifted from Lomonosov Ridge and created the rectangular pull-apart basin between them. We propose a revised plate model for the development of the Amerasian Basin. A Cretaceous magmatic source localized under the Alpha Ridge accompanied the onset of rifting. This generated the HALIP radiating dike swarms and tholeiitic flood basalts found on the DeLong Islands, Svalbard, Franz Joseph Land, Greenland, Sverdrup Basin and, possibly, the Alpha and Mendeleev Ridges. New bathymetric and sub-bottom profiling data also suggests the existence of igneous dikes on Chukchi Cap. The subsequent development of a triple junction resulted in dilational opening of the Canada Basin. Spreading was accommodated by the migration of the southern edges of the northeastern Siberian shelf along large right lateral transform faults in the South Anyui Suture Zone allowing for crustal "escape" toward the Pacific subduction zone. Bathymetric, aeromagnetic and gravity data support the triple junction idea as well as a complimentary zone of left lateral transform motion along the northern Alaska margin and southern edge of the Chukchi Borderlands which also accommodated spreading.
T11E-06
Chemo- and litho-stratigraphy of the Neoproterozoic Katakturuk Dolomite, northeastern Brooks Range, Alaska, and implications for the origin of the Arctic Alaska-Chukotka Plate
The Katakturk Dolomite is a 2.2 km thick Neoproterozoic carbonate succession exposed in the northeastern Brooks Range of Alaska. A diamictite at the base of the Katakturuk is capped by a black limestone with peculiar roll-up structures. Carbon-isotope chemo-stratigraphy suggests this is a Sturtian-age glacial deposit. Approximately 500 meters above the diamictite, textures characteristic of a basal Ediacaran cap carbonate, such as tubestone stromatolites, giant wave ripples, and decameters of pseudomorphosed former aragonite crystal fans rest on a silicified surface. This basal Ediacaran cap carbonate (herein refered to as the Nularuvik cap carbonate) is followed by an additional 1.7 km of unfossiliferous, Ediacaran-age shallow water dolomites. The ca. 580 Ma Gaskiers glaciation is tentatively correlated with a major exposure surface of vadose pisoids, approximately 500 meters above the Nularvik cap carbonate. This surface is capped by another aragonite crystal fan horizon that harbors depleted carbon isotope values (- 4.5 per mil). Deposition of the Katakturuk ceases before the ca. 555 Ma Shuram-Wonoka negative carbon-isotope anomaly. The Nanook Limestone rests unconformably on the Kataktruk Dolomite and begins deposition in the Botomian with on-lap from the south. The age and style of Neoproterozoic carbonate sedimentation on the Arctic Alaska-Chukotka Plate (AACP) are both inconsistent with a Laurentian origin of the AACP and with the simple "windshield wiper" model for the opening of the Arctic Ocean. We suggest instead that the Katakturuk Dolomite was deposited as the AACP shared a margin either with Siberia or a peri-Siberian terrane. In our model the AACP rifted away in the Early Cambrian, accommodating the deposition of the Nanook Limestone, whereupon it traveled alone throughout much of the Early Paleozoic. In the Late Silurian-Early Devonian, the AACP experienced a northern extension of the Caledonian orogeny. The AACP arrived at its present position by way of Late Paleozoic-Mesozoic synstral shearing along the Canadian Arctic margin. This model is consistent with detrital zircon geochronology, biogeographic affinities of Paleozoic fauna, the geological record of both Arctic Canada and Alaska, and the morphology of the Arctic Ocean.
T11E-07
Wrangel Island, a Jewel of the Arctic
The opening of the Amerasia Basin in the Arctic is generally considered to have occurred during Cretaceous time. The tectonic development of the Basin, however, remains problematic because associated sea floor magnetic anomalies are difficult to interpret and the origins of features in and around the Basin are poorly understood. Land-based geologic investigations of the surrounding Amerasia Basin region provide a means of testing mechanisms for the opening of the Basin. Such studies are valuable and necessary, but the vast and mostly hidden intervening Russian Arctic shelf region may make it difficult to actually integrate these studies into a viable model. Therefore, the Arctic islands in this shelf region are crucial for linking on-shore and shelf geology and for testing any model for the development of the Amerasia Basin. Wrangel Island occupies a critical location in this vast shelf region and as such represents a jewel of the Arctic. In 2006, an international team of geologists (S. Sokolov, M. Tuchkova, V. Verzhbitsky, E. Miller and V. Pease) visited the island. New field studies and sampling programs were undertaken. U-Pb ion microprobe detrital zircon ages from Paleozoic mylonitic clastic rocks will be presented and should characterize 1) the provenance of basement being eroded to form these deposits, and 2) the timing of mylonitic deformation.
T11E-08
Geology of Wrangel Island, Arctic Russia, Revisited
It has long been suggested that Wrangel Island represents the western continuation of the Brooks Range fold and thrust belt of northern Alaska. It is thus a unique exposure to test for the continuity of structures, lithologies and facies from Alaska to Russia across the Chukchi Sea, however no new structural and geochrononologic data has emerged since the thorough overview of Kos'ko et al. (1993, Geol. Surv.Canada Bull. 461). In 2006, an international team of geologists (S.Sokolov, M.Tuchkova, V.Verzhbitsky, E.Miller and V.Pease) visited the island with the help and logistic support of the director and scientific staff of the Wrangel Island Wildlife Preserve. Strata on Wrangel Island are highly deformed and metamorphosed, but may match part of the section described for the Hannah Trough, Alaska (Sherwood et al., 2002 GSA Spec.Paper 360): Coarse clastic strata overlie late Precambrian basement (630-700 Ma Kos'ko et al. (1993)), 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 sequence of basinal Triassic clastics constitutes the upper part of the section. Comparison of single grain U-Pb ages of detrital zircons from the Triassic of Wrangel to the Russian Arctic mainland and to the Lisburne Hills, Alaska, suggests basin continuity and similar source regions between these three regions (but not the N. Slope) in the Triassic. Single grain ages as young as 215 Ma validate the inferred Triassic age of these sediments on Wrangel Island. Penetrative deformation, increasing in strain and metamorphic grade with depth in the section, is defined 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 but its evidence has been largely obliterated. The main foliation is axial planar to tight to isoclinal folds in all units; these folds (and associated thrust 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 (biotite) 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; they are not highly asymmetric. The excellent preservation of deformation-related structures in quartz suggests 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 (1 sigma errors) and have relatively long (14 micron) unimodal track lengths, supporting this possibility.