Oceanic Affinities of the Alpha Ridge, Arctic Ocean

Marine Geology, 73 (1986) 237-261 237 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

OCEANIC AFFINITIES OF THE ALPHA RIDGE, ARCTIC OCEAN

H. RUTH JACKSON', D.A. FORSYTH 2 and G. LEONARD JOHNSON 3 'Atlantic Geoscience Centre, Geological Survey of Canada, Bedford Institute of Oceanography, Dartmouth, N.S. B2Y 4A2 (Canada) 2Earth Physics Branch, 1 Observatory Crescent, Ottawa, Ont. KIA OY3 (Canada) 3Office of Naval Research, Arlington, VA 22217 (U.S.A.) (Received November 5, 1985; revised and accepted April 9, 1986)

ABSTRACT

Jackson, H.R., Forsyth, D.A. and Johnson, G.L., 1986. Oceanic affinities of the Alpha Ridge, Arctic Ocean. Mar. Geol., 73:237 261.

Geological and geophysical information collected on the ice station CESAR have been integrated to develop an origin and history for the Alpha Ridge. The fossil assemblage recovered limits its age to no younger than Late Cretaceous. Fragmental alkaline volcanic rocks were dredged from the escarpments of the trough system. A high-velocity lower crust and depth of nearly 40 km to mantle is revealed by the refraction experiment. Cretaceous aseismic oceanic plateaus of the Pacific exhibit striking similarities to the Alpha Ridge. Plate reconstructions are presented that rotate the Arctic-Alaska plate away from the North American plate, with the Alpha-Mendeleev Ridge complex on a spreading centre being fed by a hot spot forming a feature similar to the Iceland-Faeroe Plateau.

INTRODUCTION

CESAR is an acronym for the Canadian Expedition to Study the Alpha Ridge. Ice station CESAR was set up on the drifting Arctic sea ice in the vicinity of the Alpha Ridge (Fig.l) and experiments were conducted from 31 March to 23 May 1983. In this paper the geological and geophysical information that relate to the tectonic origin and development of the ridge are discussed. The geographic setting of the Alpha Ridge is described to provide a framework to interpret the CESAR results. The Arctic Ocean is composed of two major basins: the Amerasia (Canada and Makarov Basin) and the Eurasia, which are separated by the Lomonosov Ridge (Fig.l). The Eurasia Basin is a typical oceanic feature formed by axial accretion along the Arctic segment of the Mid-Ocean Ridge system, the Nansen Ridge, and thus can be readily explained by plate tectonic concepts. Magnetic anomalies indicate the Nansen Ridge became active by anomaly 24 (53 Ma; Vogt et al., 1979; Harland et al., 1982). The boundary between the two basins, the Lomonosov Ridge, is generally accepted to be a continental sliver split off from the Barents

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Fig.1. The major physiographic provinces of the Arctic. The square box is the location of the CESAR experiment. The lettered lines are the position of bathymetric profiles. The North Pole is marked by the heavy cross. The dashed area in the bathymetry survey is shown in Fig.9. continental margin by accretion along the Arctic Mid-Ocean Ridge (Sweeney et al., 1982). Unlike the Eurasia Basin, the Amerasia basin is much more complex and has resisted a large number of attempts to reconstruct its structural development. This basin is dominated in the north by the Alpha-Mendeleev Ridge complex which is the most prominent and least understood of the Arctic submarine ridges. Five prime hypotheses concerning its origin have been advanced: (1) subsided or allochthonous continental crust (King et al., 1966; Coles et al., 1978; Johnson et al., 1978); (2) former spreading center (Vogt and Ostenso, 1970; Hall, 1970, 1973; Ostenso and Wold, 1977); (3) fossil subduction zone (Herron et al., 1974); (4) oceanic aseismic ridge (Jackson et al., 1984); and (5) hot-spot trail (Vogt et al., 1982). The southern Canada Basin may have formed by a relative rotation (Grantz et al., 1979) between Alaska and the continental margin of northern Canada. Taylor et al. (1981) suggest that this motion occurred from 153 Ma (anomaly M-25 time) to 127 Ma (M-12) at an opening rate of 2.6 cm yr -1, while recent investigation by Sweeney (1985) suggests the opening of the Canada Basin occurred 120-80 Ma ago. 239

PHYSIOGRAPHY

The physiography of the Amerasia Basin is described here because of the constraints it places on its origin.

Makarov Basin

The smallest bathymetrically distinct basin in the Amerasia Basin is the Makarov Basin. Its long axis is 830 km, and as defined by the 3000 m isobath, the basin is 330 km wide (Johnson et al., 1978). The Lomonosov and Alpha Ridges establish the boundaries (Fig. 1). As can be readily observed in topographic profiles, the Makarov Basin narrows toward Ellesmere Island (see Fig.2, profiles A, C, D and E). The CESAR expedition at 120°W found that the Basin resembled a graben and was between 25 and 60 km wide (Weber and Sweeney, 1985). Based on the 2500 m isobath, the basin widens to about 400 km on the Siberian edge.

Alpha-Mendeleev Ridge complex

The Alpha-Mendeleev Ridge complex is a broad, rugged arch, which lies between the Lomonosov Ridge and the Canada Basin (Figs.l, 2). The 2000 m isobath delineates its crestal region. It is a broader feature than the Lomono- sov Ridge, ranging from 250 to 800 km in width. In cross-section (Fig.2) it is roughly symmetrical with greatest elevation at the centre. The Ridge strikes east-west from the Canadian continental margin to approximately 165°E longitude. At this point it joins, or is continuous with, the southward striking Mendeleev Ridge (Fig.l). The 2500 m isobath includes both the Alpha and Mendeleev Ridges. The Mendeleev Ridge is also a broad fractured arch (Fig.2, profiles A, B), although apparently not as rough in the crestal region as Alpha Ridge. It should be noted that at the intersection of this ridge and the Siberian margin, there are grabens in a geometry similar to that found at the junction of the Nansen Ridge with the shelf (Anonymous, 1984). Morphologically, a section of the north-flank Alpha Ridge has been defined by Weber and Sweeney (1985), based on both ARLIS II and LOREX data. In the CESAR region the ridge is characterized by WNW by ESE discontinuous bathymetric depths (2000-2500 m deep) with flanking topographic highs less than 1500 m. Orthogonal features to these trends are also present. Based on T-3 data, Hall (1973) suggests that the WNW by ESE trends are an indication of the spreading axis and that the orthogonal trends are fracture zones.

Canada Basin

The Canada Basin extends in a north-south direction from 1330 km from the Alpha Ridge to the north slope of Alaska. At its maximum, this basin extends about 890 km in an east-west direction at 74°N and covers approximately 240

0 500 I000 KILOMETRES Fig.2. Bathymetric profiles across Alpha-Mendeleev Ridge complex. A-E are the profiles indicated on Fig.1 by lettered lines.

1 x 106 km z (Fig.l). Depths range to over 3900 m; the average ranging from 3500 to 3800 m.

Chukchi Plateau

The Chukchi Plateau, 400 km wide and 600 km long, extends northward from the Chukchi Sea between 160°N and 170°W longitude (Hunkins et al., 1962; Beal, 1969; Figs.1 and 2, profile A). The plateau consists of a cluster of shallow flat-topped plateaus and ridges with an intervening deep basin. Chukchi Cap, the largest high-standing plateau, rises locally to 264 m below sea level (Grantz et al., 1981). The eastern margin of the plateau is the Northwind Ridge, 241 with a steep escarpment that abuts the deep Canada Basin at depths near 3900m. Bathymetry and geophysical characteristics (Shaver and Hunkins, 1964) suggest that the high-standing parts of the Plateau consist of continental crust (Grantz et al., 1981).

GEOLOGICAL AND GEOPHYSICAL DATA

In order to evaluate the origin of the Alpha Ridge, a compilation of the CESAR data is presented. The single-channel airgun seismic reflection record collected from the ice island must be interpreted with the navigational data because ice drift direction and speed are highly variable. The firing rate is time controlled so that horizontal distances are not constant along the record. The complete seismic profile is published in Jackson (1985a). Figure 3 illustrates the track of the ice station and the bathymetric variations in the area. The sedimentary reflectors are generally flat-lying and/or conformable to seismic basement, and range in thickness up to 0.5 s. In localised regions the sedimentary horizons are obscured by a bottom simulating reflector (BSR) (Fig.4). The temperature depth range of BSR is within the zone where gas hydrates can form (Stoll, 1974). The transparent characteristic of the record immediately above the BSR is typical of regions where gas hydrates have been identified (Sheridan and Gradstein, 1983). BSRs that underlie much of the continental shelf and slope of Alaska are thought to be caused by gas hydrates (Grantz et al., 1981), and it is possible that they are the cause of the BSR in this case. In the major valley traversed by the ice island, the sedimentary layers exhibit a different character (Fig.5). A rugged basement reflector is mapped at between 0.3 and 0.5 s below the seabed and overlying this horizon is a layer that mimics its shape but is smoother. Evenly distributed pelagic sedimentation could drape basement topography in this manner. Widely correlateable glacial marine pelagic sedimentation is identified in the Amerasia Basin in the Pleistocene (Clark, 1981) and pelagic sedimentation could have occurred at an earlier time interval. On the steeper north wall, basement is exposed. The shape of the valley is that of a half graben. In the course of its movements, the ice island crossed the north wall of this valley four times (Fig.6). In Fig.6a the sedimentary horizons can be seen to terminate abruptly against seismic basement suggesting the sedimentary fill postdates the valley formation. Seismic sections across the north wall showing more detail (Fig.6c) illustrate that the sedimentary layers terminate against basement in a normal fault configuration. Core material recovered from this location was Late Cretaceous in age (Barron, 1985; Bukry, 1985; Mudie, 1985) rather than the more recent material identified in other regions, confirming the uplifting of the sedimentary section in this region. Another transect of the north wall (Fig.6d), illustrates a steep scarp with exposed basement. Across this scarp basement was dredged. The dredged rock is a highly-altered alkalic volcanic rock (Van Wagoner and Robinson, 1985). 242

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Textural characteristics suggest the volcanic fragments were erupted in shallow water. The exposed basement-rims of the trough resemble features on the Manihiki Plateau (Fig.7; Winterer et al., 1974). On the Manihiki Plateau it is not known whether these troughs are caused by either strike-slip or tensional tectonics. The fossils recovered from the area shown in Fig.6c, are well preserved siliceous forms that include silicoflagellates, dinoflagellates and diatoms (Barron, 1985; Bukry, 1985; Mudie and Blasco, 1985) of Campanian to Maas- trichtian age. The silica in their tests is in the form of primary opal-A which is rare for material of this age. This well-preserved state indicates the sediment was not subjected to prolonged temperatures of greater than 35°C or depth of burial of greater than 400 m (Mudie and Blasco, 1985); this suggests the Alpha Ridge has been thermally inactive since their deposition and restricts its age to > 80 Ma. The fossil assemblage also provides information on the environ- 243

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Fig.4. Seismic reflection section illustrating the bottom simulating reflector (BSR) that occurs 0.15 s after the initial reflection. ment of the region. Within the assemblage many cysts of diatoms are found, a resting stage typically formed in deep shelf environment (Barron, 1985). The dinoflagelates from the base of the core are similar to those onshore in the Arctic Islands in the shale member of the Kanguk Formation, Upper Cretace- ous in age, which was deposited in a marine shelf region with continental influences (Plauchut and Jutard, 1976). The crustal structure of the Alpha Ridge (Forsyth et al., in press) is illustrated in Fig.7. A refraction line, 210 km long, was shot along the crest of the Ridge. The salient feature is nearly 40 km-thick crust (Forsyth and 244

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Jackson, 1984). The sedimentary layer overlying the crust is less than 1 km thick. A velocity of 5.1km s -~ with an increasing velocity gradient is measured immediately beneath the sedimentary rocks. The dredged samples indicate a basaltic basement and the velocity associated with it is similar to that commonly associated with oceanic layer-2. Lateral and vertical variations occur in the crust in the next zone. A velocity of 6.45 km s-~ with a steep gradient is measured below layer-2 at the ends of the line and in the central portion, an additional velocity of 6.8 km s- 1 is measured. Beneath this, a high- velocity layer of 7.3 km s- ~ of between 10 and 16 km thick occurs along the entire line. All plateaus in the ocean believed to be formed of oceanic crust have this diagnostic velocity (Carlson et al., 1980). The layer-2 and -3 velocities are typical of oceanic crust but several times thicker than in ocean basins. Similarities between the velocity structure of the Alpha Ridge and Iceland suggest it may be a current tectonic analog (Forsyth et al., in press). "WO WAY TRAVEL TIME (sec) "WO WAY TRAVEL TIME (sec)

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Fig.& Four seismic sections across the northern wall of the major valley in the region. The bar at the base of each section is 1.5 km long. (a) shows a steep scarp with sediments truncated along it. The arrow in (c) is the location of the core containing the Late Cretaceous fossils; (d) displays a basement rim. 248

COMPARISON WITH OTHER OCEANIC RIDGES

The characteristics of the Alpha Ridge can be combined to suggest a crustal type and origin for the feature. The information is consistent with the Ridge being a large oceanic plateau with many features in common with the Ontong- Java Plateau, Manihiki Plateau, the Hess Rise, the Shatsky Rise, and Iceland (Fig.8).

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In plan view the configuration of the valleys on the Alpha Ridge is en echelon (Fig.9). The valleys, with their graben shape and distinctive basement 249

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Fig.9. Bathymetry data from T3 and CESAR, combined to show regional trends. rims, run roughly parallel to the north side of the Ridge. Additionally, orthogonal to these depressions are a series of lineations identified by Hall (1970) as fracture zones. The Manihiki Plateau also has a system of deep internal troughs with basement rims that are nearly parallel to the boundary of the Plateau (Winterer et al., 1974). These valleys also have an en echelon pattern and a second system of troughs similar in cross-sectional shape but narrower and orthogonal than the first (Winterer et al., 1974). In cross-section, the flanks of the Alpha Ridge exhibit rugged topography with seamounts of up to I km relief. The crestal regions have extensive areas of low relief cut by troughs (Fig.2). On the Manihiki, the acoustic basement is smooth on the High Plateau but in other regions the relief is much greater than on typical oceanic crust (Winterer et al., 1974).

Magnetics

On the Alpha Ridge, magnetic anomalies of up to 1500 nt are observed that are related to basement topography (Hall, 1970). On the Manihiki Plateau, anomalies of 1000 nt are present over the regions of high basement-relief (Winterer et al., 1974). The higher magneti'c latitude of the Alpha Ridge compared to the Manihiki Plateau at the time of formation, could account for some of the difference in amplitude. The magnetic anomaly pattern on the Alpha Ridge that correlates with basement topography (Hall, 1970, 1973) is probably a function of the time of formation of the Plateau during the Cretaceous positive polarity chron from 118 to 83 Ma (Harland et al., 1982). The Manihiki Plateau, Ontong-Java Plateau, the Shatsky and Hess Rise are all Cretaceous in age (Schlanger et al., 1981). The Hess Rise was formed 250 entirely during the Cretaceous magnetic non-reversal period (Kroenke and Nemato, 1982) when most of the Manihiki Plateau and its environs were also generated (Winterer et al., 1974).

Rock type

The dredged material from the Alpha Ridge contained a suite of highly altered fragmental volcanics that had alkaline chemistries (Van Wagoner and Robinson, 1985). The igneous rocks recovered on Pacific plateaus: Manihiki, Magellan and Ontong-Java, have a range of chemistries similar to mid-ocean ridge basalts but are highly vesicular and do not exhibit pillow lava structures. Igneous rocks from the other plateaus such as Shatsky and parts of the Iceland (Palmason, 1971) are alkalic basalts. Scott (1979) suggests that the genesis of these features includes an initial period where theoleiitic magmas are extruded followed by ocean island type basalt and finally a high-alkalic veneer is produced.

Crustal structure

The crustal structure of the Alpha Ridge (Forsyth and Jackson, 1984) is compared and contrasted to adjacent features and to plateaus whose tectonic origin has been determined (Fig.8). For example, the Lomonosov Ridge profile demonstrates two conspicuous differences from the Alpha Ridge data; the crust is 10 km thinner and no high-velocity deep crustal layer is observed. The refraction techniques and data processing techniques were similar to those used here (Forsyth and Mair, 1984). The Lomonosov Ridge is believed to be continental crust (Sweeney et al., 1982). There is little evidence of high- velocity (7.0-7.7 km s- 1) deep crustal layers within the continents in adjacent regions. Refraction data in the Sverdrup basin (Forsyth et al., 1979), the Kara Sea, and an average profile of Canadian Shield do not show this layer. However, the velocity distribution and crustal thickness on the Ontong-Java Plateau (Furmuto et al., 1976) and the Alpha Ridge are comparable (Fig.8). Both regions have sedimentary cover up to 1 km thick underlain by layer-2 velocities in the 4.5-6.45 km s-1 range. A thick interval of layer-3 velocities 6.8-7.05 km s- 1 is measured in both areas. The uppermost mantle is overlain by about 10-16 km of a 7.3-7.7 km s- 1 layer. The depth to mantle is about 40 km in both areas compared to average oceanic crustal thickness of 6.5 km (Christen- sen and Salisbury, 1975). Iceland is composed of thick oceanic crust (Palmason, 1971). The velocity structure of Iceland and the Alpha Ridge are similar (Fig.8) (Forsyth et al., in press).

Gravity

The free-air gravity anomaly measured over the Alpha Ridge is generally positive (Bowin et al., 1981) similar to that of the other large oceanic plateaus 251

(Watts and Leeds, 1977). A model of the gravity field predicts a free-air gravity anomaly of -200 mGal (Furmuto et al., 1976). A similar model across the Shatsky Rise predicts -86mGal free-air anomaly. An excess mass in the mantle is thus required to compensate for the thick crust (Gettrust et al., 1980). Where hot-spot traces have traversed previously formed oceanic crust as in the Hawaiian Ridge, a distinct free-air gravity pattern with negative values on either side of a 200 mGal positive (Watts and Leeds, 1977) is formed which is not observed in this region.

Subsidence

De Laurier (1978) and Herron et al. (1974) concluded that the Alpha Ridge could not be a Cretaceous spreading centre because its present sediment- compensated depth is 2900 m _+ 500. They assumed the ridge height was initially similar to currently active ridges and that the relief of the Alpha Ridge relative to the seafloor should now be about 500 m. If the Alpha Ridge was formed near sea level about 100 Ma, thermal subsidence curves predict a present sediment-corrected depth of about 3000 m (Detrick et al., 1977) similar to its present values. In the Maastrichtian, the water depths were probably deep shelf (Barron, 1985). The highly vesicular nature of the rocks dredged from the Alpha Ridge support shallow water conditions. The Manihiki Plateau is also believed to have formed at sea level based on the presence of coral atolls (Winterer et al., 1974) and the large vesicule size and abundance in the cored rocks (Jackson and Schlanger, 1975).

Heat flow

Heat flow measurements on the Alpha Ridge at CESAR average 33 W m-2 (1.39 HFU) with a range of 28-40 W m -2 (1.2-1.67 HFU; Judge et al., 1984). Heat flow values in the adjacent Canada Basin have a mean of 34 W m 2 (1.41 HFU; Lachenbruch and Marshall, 1966). The best age estimate for the Canada Basin region is Mid-Cretaceous (about 120-80 Ma; Sweeney, 1985). Mean heat flow for Late Cretaceous crust in the North Pacific and South Atlantic is 34.1 and 33.9 W m -2 (1.43 and 1.42 HFU), respectively. The age and heat flow measurements of the Alpha Ridge are compatible with other oceanic regions of the similar age.

Formation

The large Pacific plateaus including the Manihiki, Hess, Kerguelen, Magel- lan and Shatsky are thought to have formed at, or near, triple junctions (Winterer et al., 1974; Moberley and Larson, 1975; Sharman and Feelay, 1979) during the Mesozoic. Furthermore, these plateaus may originate as thick volcanic piles at mid-ocean ridges where spreading rate has not kept up with magma production (McKenzie and Sclater, 1971), or at plate boundaries when 252 major changes in relative motion between plates occur (Goslin and Patriat, 1984). The shared characteristics of the Alpha Ridge and the Pacific plateaus suggest a similar origin; that is, a Mesozoic spreading centre at which thick accumulations of magma were produced.

GEOLOGICALAND GEOPHYSICAL CONSTRAINTS ON THE DEVELOPMENT OF THE AMERASIA BASIN

From the landmasses adjacent to the Amerasia Basin, constraints exist that limit the timing and style of its formation. The polar margins of North America exhibit characteristics of a passive continental margin developed in the Mid- Cretaceous. Along the Alaskan margin, seismic reflection records demonstrate structures typical of rifted, not transform, regions (Grantz and May, 1983). Three major geologic features can be correlated from the Canadian Arctic to Alaska. The Canadian Sverdrup Basin sequence, whose age range is Mississip- pian to Late Mesozoic, has a counterpart in northern Alaska; in addition, two pre-Mississippian assemblages are shared (Tailleur, 1973; Hamilton, 1983). The Alaskan and Canadian polar shelves appear to have formed at the same time (Sweeney, 1985) based on sedimentary and structural sequences observed at the edge of the continental blocks. The geological correlations also provide support for the concept that the Amerasia Basin was formed by the counter- clockwise rotation of Alaska (Tailleur, 1973). Thus the Alaskan and Canadian polar shelves would be conjugate margins. From Ellesmere Island, the landmass to the south of the Alpha Ridge westward to Brock Island, continental break up based on stratigraphic control is associated with a 150 Ma-long hiatus. On Ellesmere Island and the adjacent Ellef Ringnes Island, there are extensive mantle-derived basalts of Early to Mid-Cretaceous age (Thorsteinsson and Tozer, 1970; Ricketts et al., 1985). Ricketts et al. (1985) interpret the Strand Fiord volcanics as an extension of the Alpha Ridge structure. Westward of Brock Island (Fig.l), little volcanic activity is mapped. The continental break up in this region, based on the stratigraphic sequence, can be bracketed between Valanginian and Albian age (135-100 Ma; Sweeney, 1985). Along the Alaskan continental margin, the break-up unconformity indicates seafloor spreading began in the Canada Basin at the beginning of Hauterivian time, about 125 Ma ago (Grantz and May, 1983). From the seafloor of the Canada Basin, depth-versus-age curves indicate an age of formation of 125-90 Ma (Lawver and Baggeroer, 1983). In addition, heat flow values (Lachenbruch and Marshall, 1966) plotted on heat flow-versus-age curves suggest ocean formation between 110 and 80 Ma. The subdued magnetic anomaly pattern of the southern Canadian Basin is consistent with ocean development during the Cretaceous non-reversal period 118-83 Ma (Harland et al., 1982). In northeastern Siberia in the Mesozoic there is evidence for crustal shortening near Chukotka, the western edge of the Arctic-Alaska plate (Fujita 253

Fig.10. The pre-drift position of the plates in the Arctic. The extent of GR, EU and AA are shown in the Mid.Mesozoic.The LomonosovRidge (LR) is fitted adjacent to the EU. The Sverdrup Basin (SB) is drawn overlapped by the Chukchi Plateau (CB). and Newberry, 1982) along the South Anyui Suture zone (Fig.10). The suturing occurred in Berriasian to Hauterivian time (144-131 Ma; Harland et al., 1982) with the emplacement of ophiolite complexes. The lithology and sequence of events suggest the closing of an ocean in this region (Fujita and Newberry, 1982) perhaps in response to the opening of the Amerasia Basin. In addition, ophiolites of Mid-Jurassic age are mapped on the Arctic-Alaska plate on its leading edge in the Brooks Range in Alaska and also in Chukotka. These complexes, typical of crustal shortening, pre-date the continental rifting along the southern Canada Basin margin. However, most of the compression in the Brooks Range is due to folding of Upper Cretaceous rocks (Dutro, 1981). In addition, the rotation hypothesis requires increasing crustal shortening away from the pole of closure. Tailleur (1973), shows some evidence for variable thrusting along the Brooks Range orogen. There are plate reconstructions that require strike-slip motion of Alaska along the Canadian polar margin (Jones, 1982). As Jones (1982) indicates, the principle geologic argument against this theory is the northern source needed 254 to supply the Permo-Triassic rocks of Alaska. The rotation hypothesis allows the Sverdrup Basin to be the source. The drainage pattern for the Arctic archipelago at this time is in an appropriate direction; south-north. The structures in northeastern Siberia during the equivalent time frame are related to crustal shortening. This margin is not the conjugate of the Canadian polar region.

PLATE RECONSTRUCTIONS

Once an age and crustal origin of the Alpha Ridge are determined and the constraints from adjacent regions are considered, it is possible to reconstruct the plate motions that produced the Amerasia Basin. Because the magnetic anomalies are difficult to identify, a unique solution is not possible but insight can be gained into the basic processes that shaped the regions and the date of occurrence. The situation of the North American (NA) and Eurasian Plates (EU) during the Mesozoic in the Arctic is considered here to be as follows. Prior to initiation of seafloor spreading in the Eurasia Basin in the Cenozoic, the Lomonosov Ridge was an extension of the present Barents- and Kara Sea- shelves (Sweeney et al., 1982; Forsyth and Mair, 1984; Srivastava, 1985). The present position of Ellesmere Island and perhaps the Lomonosov Ridge has been influenced by the Late Cretaceous to Mid-Tertiary Eureken Orogeny. Forces directed from the southeast have compressed the crust of Ellesmere Island (Jackson, 1985b). The northeastern section of the island has received 340 km of shortening based on the plate reconstructions of Srivastava (1985). This motion occurred after the rotation of Arctic-Alaska (AA) away from the Canadian Arctic Islands. The Lomonosov Ridge is shown on Fig.10, 200 km to the east of its present position with respect to Ellesmere Island next to the shelf of the rotated EU plate. Table 1 contains the pole positions used to develop this pre-drift reconstruction. The plate boundaries for the AA plate are based on available geological limitations. One margin of the plate is shown in Fig.10 and includes the Seward Peninsula (Fujita and Newberry, 1982). The plate probably extends

TABLE 1

Poles of total opening with respect to North America

Latitude Longitude Rotation

Pre-drift Greenland 73.68 - 113.80 - 11.21" Eurasia 79.83 143.02 - 25.90* Arctic Alaska 71.0 - 131.0 - 80.0

Rotations to the East are positive. Latitude -- North positive; Longitude -- East positive. *Srivastava (in press). 255

Fig.11. Position of AA as it rotates away from NA (A-D). The motion occurred during the Cretaceous positive-polarity chron. The lineations are drawn to show the manner in which seafloor was developed. The short solid lines are the position of a magnetic anomaly and a gravity low (G) possibly associated with the extinct ridge. from Alaska to Chukotka based on the continuation of geologic and geophysical structures (Churkin and Trexler, 1981; Grantz et al., 1981). Also similar outcrops of Devonian, Mississippian, Permian and Triassic sequences occur on Wrangel Island and on northern Alaska (Tailleur, 1973). The western limit of the plate is the South Anyui suture where ophiolite complexes are clearly identified (Shilo and Tilman, 1981). The pre-drift reconstruction of AA adjacent to NA is shown in Fig.10. The AA plate in its described configuration rotates into a space between the Lomonosov Ridge and Ellesmere Island. Figure 11 illustrates the proposed development of the Amerasia Basin. The growth of the Amerasia Basin can be described by the rotation of the AA plate away from NA. In this reconstruction continental breakup is initiated in the region adjacent to Ellesmere Island and the island to the west of it, where localized tholeiitic volcanism in an intraplate setting occurs in the early Late Cretaceous (Ricketts et al., 1985). This volcanism is adjacent to the landward termination of the Alpha Ridge. The breakup of the plates in this region took place in continental crust. Preferential rifting of continents resulting in the formation of new ocean basins is commonly observed (McKenzie and Sclater, 1971; Vink et al., 1984). A history for the Alpha-Mendeleev Ridge complex similar to the development of the Iceland-Faeroe Plateau (Vink, 1984; Forsyth 256

TABLE 2

Poles of total opening with respect to North America

Latitude Longitude Rotation

Arctic-Alaska 71.0 - 131.0 - 80.0 70.5 - 132.5 - 60.0 69.1 - 135.0 - 30.0

Rotations to the East are positive. Latitude -- North positive; Longitude -- East positive. et al., in prep.) is suggested: a hot spot located close to a spreading centre so that a lot of magma and therefore thick crust are produced, forming a plateau. The age progression of the plateau is synchronous with seafloor spreading and the plateau is symmetric with respect to the ridge crest. Thus, the Alpha- Mendeleev Ridge is symmetric and perpendicular to the spreading direction. The pole of opening for the counter-clockwise rotation of Alaska and creation of oceanic crust was modified from Grantz et al. (1979). Here the pivot is chosen initially several hundred kilometers to the north and migrated to the same final pole (Table 2). The rotational motion with southward migration of the pole places the South Anyui Suture zone in the position shown in Fig.10 and leaves it in its present position. As seafloor spreading created crust in the Amerasia Basin during the Cretaceous quiet period, magnetic anomaly trends are probably due to basement ridges. In the southern Canada Basin where the anomalies are of low intensity ( ~< 250 nt) and difficult to correlate, Vogt et al. (1982) have identified a pattern of fan-shaped anomalies similar to that expected from the model presented here. In the Canada Basin, no topographic expression of the extinct spreading centre exists. This lack of a ridge is consistent with thermal decay over an 80-Ma time period, and thick sediment accumulation in the basin. Extinct spreading centres in the Labrador Sea and Baffin Bay have no topographic expression but are associated with a gravity low and a magnetic lineation (Srivastava, 1978; Jackson et al., 1979). Figure lld shows the position of a gravity-low and magnetic-high that could be associated with an extinct spreading centre in the Canada Basin (Taylor et al., 1981; Vogt et al., 1982). The offset relative to our model is probably due to the complications of the spreading process relative to the simple model presented here. The dramatic change in amplitude intensity between the Alpha-Mendeleev Ridge and the southern Canada Basin (Vogt et al., 1979) from 1500 to < 250 nt may be related to change in crustal thickness. The unusually thick crust of the Alpha Ridge, up to 40 km, contrasts sharply to the average crustal thickness of 10 km measured by Baggeroer and Falconer (1982) and Mair and Lyons (1981). Crustal thickness and magnetic amplitudes have been shown to be positively correlatable (Jackson and Reid, 1983). In this scheme for the development of the Arctic Ocean, the Chukchi Plateau 257 overlaps on the Canadian polar margin. Balkwill (1978) describes the Sverdrup Basin as resembling a small ocean basin with a basaltic crust. Grantz and May (1983) believe that the high-standing flat-topped blocks that form the Chukchi Borderland, are fragments of the continental Arctic platform moved northward and outward from the present shelf by a minor spreading axis. A.M. Karasik (pers. commun., 1984), indicates that magnetic anomalies are present on the plateau and a magnetic edge anomaly exists similar to that associated with the ocean crust of the Voring Plateau. Summarizing, there are several possible crustal configurations that would reduce the apparent overlap to the Chukchi Plateau on the North American polar shelf and no final solution is possible until more information on the region is available.

CONCLUSIONS

The Alpha Ridge is a distinctive and major topographic feature of the Amerasia Basin. In cross section the shallowest depths are located at its centre. The deeply incised valleys are associated with peculiar basement rims similar to those observed on the Manihiki Plateau. Along the valley walls faulting is apparent and the major valley crossed during CESAR appears to be a half graben. Fossils recovered from the faulted zone indicate a minimum age of Campanian for the Ridge. The heat flow measurements are consistent with this age. The fossil tests provide further limitations on the thermal events on the Ridge; these tests in the form of opal-A suggest little heating since their deposition. In the Late Cretaceous the Ridge was shallower than at present. The shallow depth is suggested by the highly vesicular nature of the dredged rocks and several of the fossil groups are typical of shallow water. A shallow bank distant from any major sediment source is the most probable paleo- environment. Crustal refraction, seismic reflection and the dredged rock are compatible with a layer-2 oceanic crust for seismic basement. The refraction measurements describe a thick, high-density crust more typical of the oceanic plateaus and Iceland than the adjacent continental crust. The large Pacific plateaus such as the Manihiki, Hess, Magellan and Shatsky exhibit similar morphology, crustal structure, age, rock type, heat flow and gravity values. Plate reconstructions for the AA can be accomplished so that the Alpha Ridge formed in a manner similar to the Iceland-Faeroe Plateau; that is, a ridge crest being fed by a hot spot. The initial opening may have occurred at the position where the Alpha Ridge abuts the Canadian polar margin. The reconstructions are consistent with the regional geology. The Alpha Ridge is believed to be an oceanic plateau formed prior to the Late Cretaceous by the process of seafloor spreading that separated AA from NA. 258

ACKNOWLEDGEMENTS

Any scientific program carried out on the Arctic Sea ice is indebted to many individuals and groups for its success. Polar Continental Shelf Project provided the logistic support and expertise to establish the camp. The Canadian Armed Forces built the runway necessary for the Hercules C-130 aircraft that enable bulky and heavy equipment to be used on CESAR. Technical and engineering support for the scientific programs was provided by the Atlantic Geoscience Centre, Earth Physics Branch and Lamont-Doherty Geological Observatory. The authors held many valuable discussions with CESAR workshop participants.

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