AEROMAGNETIC SURVEY OF THE ARCTIC OCEAN: NED A. OSTENSO. Office of Naval Research, Arlington, Virginia 22217, U.S.A. and RICHARD J.

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1 AEROMAGNETC SURVEY OF THE ARCTC OCEAN: TECHNQUES AND NTERPRETATONS NED A. OSTENSO Office of Naval Research, Arlington, Virginia 22217, U.S.A. and RCHARD J. WOLD University of Wisconsin at Milwaukee, 53201, U.S.A. (Received 11 June, 1970; revised 2 November, 1970) Abstract. Approximately km of low-level (450 m) aeromagnetic tracks were flown over the Arctic Ocean and adjacent Greenland and Norwegian Seas, for the greater part with a digitally recording nuclear precession magnetometer desi gned and built by Wold (1964). The digital recording feature of the system facilitated numerous data processing and analytical techniques which are described herein. These include: noise filtering coordinate conversion, removal of the regional field, second derivatives, downward continuations, polynomial fits of varying degrees to profiles and surfaces, numerical approximations, and depth to source calculations. Using these data and interpretative techniques some inferences could be made about the geologic structure and evolution of the Arctic Ocean Basin. Salient amongst these are: both gravity and magnetic data suggest that there is a 289 km basement uplift in the eastern Chukchi Shelf associated with the Tigara structure which truncates the western end of Lisburne Peninsula. A km wide basement root encircles the Chukchi Rise and extends over 30 km into the mantle. Within the Canda Basin there is a thickening of sediments from the Asian continental margin toward the Canadian Arctic Archipelago. Sediment thickness in the Makarov Basin is km. There appears to be only about a 89 km sediment cover in the Fram and Nautilus Basins. The absence of large amplitude magnetic anomalies over these basins is attributed to a 10 km elevation of the Curie isotherm. The Alpha and Nansen ridges produce magnetic profiles that show axial symmetry and correlate with profiles in the North Atlantic. A quantitative attempt has been made to verify these correlations, which infer that the Alpha Cordillera became inactive 40 mybp when the locus of rifting shifted to the Nansen Cordillera. The absence of significant magnetic anomalies over the Lomonosov Ridge reinforces the hypothesis that it is a section of the former Eurasian continental margin that was translated into the Arctic Basin by sea-floor spreading along the Nansen Cordillera axis. 1. ntroduction From cursory examination it is apparent that several factors characterize the Arctic Ocean Basin. Unlike the earth's other oceans it is nearly isolated by encompassing continental land masses. The great Precambrian heartlands of the continents directly face the polar ocean and tectonic belts, such as the Verkhoyan, Ural and Richardson Mountains, are truncated at the northern coast. The continental shelf, too, is unusual. Whereas continental shelves are normally tens of kilometers wide, along over half the circumference of the Arctic Ocean the shelf is hundreds of kilometers wide. n addition to their abnormal width, the continental shelves are frequently incised by submarine canyons, and have at least one known massive outlier of apparently con- tinental crust. More detailed investigations of the past decade have further confirmed the uni- queness of the Arctic Basin. Soundings from ice islands, airlifted expeditions, and Marine Geophysical Researches 1 (1971) All Rights Reserved Copyright by D. Reidel Publishing Company, Dordrecht-Holland

2 AEROMAGNETC SURVEY OF THE ARCTC OCEAN 179 nuclear submarines have gradually unveiled its complex physiognomy. Rather than occupying a single deep depression, as originally depicted by Nansen and believed as late as 1949, the Arctic Ocean is divided by three mutually parallel ridges. The central and highest of these is the Lomonosov Ridge which, from geological and geophysical evidence, has been interpreted to be a northward extension of the Verkhoyan fold belt. The Alpha Cordillera was discovered in 1957 and the rugged topography of its flanks suggested a fault block origin, although this was later shown to be inconsistent with observed heat flow data. The Nansen Cordillera is a trans-arctic Ocean extension of the Mid-Atlantic Ridge. ts existence was first postulated from seismic evidence and later confirmed by a limited number of submarine sounding profiles and magnetic profiles. These three ridges have added to the enigma of the Arctic Basin; one is a typically oceanic feature, one appeared to be an oceanward extension of a continental structure and one was yet to be clearly defined tectonically. Two of the four basins formed by the trans-oceanic ridges have been partly studied in detail and contain at least two abyssal plains each. Although the basins reach depths in excess of 4 km they contain sedimentary deposits much thicker than ever recorded in any other ocean. U. S. S. R.... ~,~....',~,~'.....# ~'..,~.., ~!,.:..,.o..~.;~.;... ",~..""L,-L :*l~.~ ~':'l "..." t..: 9."~ <..." ~: ~..~,. ~<.~,,~ ~'... _... "%., "~,~7 "'..5.'%- " "~ '-. %, "',, ~ ';~}"... ~,.....:'.-v/;~7~ -, +:"'... o,~,'~.~,~+ 'r,~?'-. g +o* 9 "~: "... %,... :... g '% "... ',!,>%,,',>i Fig. 1. Sketch map of the major physiographic features of the Arctic Basin.

3 180 NED A. OSTENSO AND RCHARD J. WOLD A sketch map of the major physiographic features thus far discovered in the Arctic Basin is shown in Figure 1. A generalized bathymetric chart is shown in Figure 2. A review of exploration and interpretation of the geologic structure of the Arctic Basin up to 1962 has been given by Ostenso (1962, 1963). This has been updated by King et al (1966) and Demenitskaya and Hunkins (in press). Basically, two schools of thought exist regarding the structure of the basin. Some believe it to be truly oceanic in structure, whereas others believe it to be formed by a foundered continental block or blocks. There is geologic and geophysical evidence to support both theories. As a rapid and economically feasible means of hopefully resolving this ambiguity, 140* 120" O0* 80 ~ 80" Fig. 2. Generalized bathymetric chart of the Arctic Basin. sobaths in meters, Compiled from Soviet and United States Sources. a systematic aeromagnetic survey over the Arctic Ocean was planned in Because of the short season of good flying weather (April and May) it was anticipated that three or more years would be required to complete a regional survey with reasonable density of coverage. The first series of ten flights was completed in 1961 and reported with other geophysical data (Ostenso, 1962). An aircraft was not available in 1962 but flights were resumed in 1963 and The approximately km of aeromagnetic

4 AEROMAGNETC SURVEY OF THE ARCTC OCEAN 181 data obtained during these three survey seasons are reported here. The flight lines are shown in Figure Survey Procedures A P2V- 5 (Neptune) patrol airplane was used for all flights. This aircraft is designed to produce a minimum possible magnetic field and has a long fiberglass fuselage extension that houses a magnetic submarine detector. This configuration plus the aircraft's long-range cruising capability make it ideally suited for surveying in the remote Fig. 3. AE~OMAGNETC FLGHT TRACKS ~ Aeromagnetic flight tracks completed by the University of Wisconsin. north polar region. The sensing heads were mounted inside the fiberglass tail cone and the accuracy of the installed magnetometer systems was _ 4 7- Normal flight elevation was 450 m above the ice pack and was closely controlled by radio altimeter. The greatest source of potential error in the survey was that of navigation. Positioning facilities available included radio direction finders, gyro and magnetic compasses, radar, APN-122 doppler navigator, celestial fixes and dead reckoning. Flying conditions throughout the project were generally good. n early June the cloud cover rose, and the weather at Thule and Barrow deteriorated as the ice pack began to break up. However, even under the worst weather conditions encountered, the Sun was usually sufficiently visible for navigational purposes. Although the accuracy of navigation was variable, it is believed to generally be within _ 15 kin. The close correlation of magnetic profiles at the many track crossings suggests even greater positioning accuracy (Kutschale, 1966). Further details of the flight operations have been given by Kelly (1961, 1963). The responsibility for operational support of the 1961 flights was delegated to the

5 182 NED A. OSTENSO AND RCHARD J. WOLD Naval Air Development Unit based at South Weymouth, Mass. The aircraft was instrumented with a Varian proton precession magnetometer which had a cycling period of 0.7 sec. At an average ground speed of 330 km/hr readings were obtained approximately every 65 m of distance travelled. Recording was on an analog strip chart which was later digitized for computer processing. This survey consisted of ten sorties originating from Thule, Greenland and Barrow and Eislson AFB, Alaska. Aircraft support for the 1963 survey was from the U.S. Naval Air Test Center, Patuxent River, Maryland. Both the Varian magnetometer and an experimental digital system (Wold, 1964) were used. Cycling time was again 0.7 sec. This survey was accomplished in ten sorties originating from Barrow, Alaska and Thule and Nord, Greenland. The Naval Air Test Center also provided an aircraft for the 1964 survey. On these flights the Wold digital recording magnetometer was used exclusively and the cycling period was lengthened to approximately 5 sec. The operation consisted of 16 sorties originating from Barrow, Alaska, Thule and Sondre Strom0ord, Greenland, Keflavik, celand and Bodo, Norway. The Wold digital recording proton precession magnetometer, developed at the Geophysical and Polar Research Center, University of Wisconsin (Wold, 1964), has the advantage that recorded time and magnetic data can be fed almost directly into a high-speed digital computer without the tedious job of converting analog records. The computer analysis plotting and contouring techniques used in processing these data are described in a following section. No corrections have been applied to the aeromagnetic data for temporal changes in the earth's magnetic field. An attempt to correlate such changes over the great distances involved might introduce more errors than corrections. However, a degree of credence for the anomalies along any profile can be obtained by noting the magnetic activity at the geomagnetic observations at College (Fairbanks) and Barrow, Alaska. TABLE Average K-indices for the intervals of aeromagnetic survey flights Barrow College The average K-indices recorded at the two observatories during the intervals of the survey flights are shown in Table. Weekly radio propagation forecasts from the North Pacific Radio Warning Service, N.B.S. Anchorage, Alaska and the North Atlantic Radio Warning Service, Fort Belvoir, Virginia were received by teletype at Thule and Barrow. n so far as was possible, flights were scheduled for days when ionospheric disturbances were predicted

6 AEROMAGNETC SURVEY OF THE ARCTC OCEAN 183 to be at a minimum. The low average K-indices shown in Table attests to the effec- tiveness of the forecasts. No flights were made during magnetic storms. Further, because of the high speed of the aircraft, temporal disturbances appear on the mag- netic profiles as perturbations of the regional gradient rather than major distortions of individual anomalies. 3. Aeromagnetic Data The flight tracks and aeromagnetic profiles, without residuals removed, are shown in Figures 4-9. The residual magnetic profiles are plotted over the Arctic Basin bathy- metry in Figures 10a and 10b. To avoid hopeless clutter half of the profiles are shown in each of the two figures. Although lacking in detail, these figures give a graphic presentation of the relationship of magnetic character to physiographic features and should be referred to while reading the following discussion. Another method of data presentation, adapted from King et a. (1966) is shown in Figure 11. This presentation is essentially contouring regions of similar magnetic signature and has the advantage of preserving an indication of character that cannot otherwise be shown by contours on a regional survey. Unlike King et al. we felt that our data were inadequate to contour in as many as seven different profile patterns. Rather, Figure 11 is divided into only four different types of regions. These regions encompass (1) profiles with anomaly amplitudes near or exceeding ; (2) profiles with anomaly amplitudes ranging near 500 7; (3) profiles with anomalies in the range; and (4) profiles that are undisturbed or contain anomalies of less than amplitude. 4. Computer Techniques of Data Analysis The necessity for computer analysis became apparent soon after the survey was started. The first year data alone required several hundred man hours just to pick the analog records. t was at this time that work was started on a digital recording magnetometer system. This system was completed for preliminary use in the second year of the survey and for the entire survey in the third year. The output from the digital system is punched paper tape which can be processed directly by the computer. Noise Filter: The magnetic data often contain erroneous observations or noise. These spurious data are due mainly to electrical noise, causing the gates in the counters to trigger at the wrong time. Erroneous data may also result from malfunctions in the tape punch, stepping switches, and other associated punching circuitry. A good discussion of these types of errors is given by Bullard (1960). Sporadic errors are normally isolated points at least from the expected value in the profile. Less frequently, several such readings will occur in a short interval, although usually mixed with some valid readings. Most bad readings have values higher (in gammas) than the correct version. The problem then is to extrapolate at every point along a profile an expected value and compare this with the recorded value. Significant deviations will then indicate erroneous readings. There are many methods for extrapolating data from surrounding values, including interpolational and curve fitting techniques. n order to make these sufficiently sophisticated to differentiate between sharp peaks of valid anomalies and erratic readings, however, they must be quite complex and hence costly for use on large volumes of data. Thus another attack, using filtering techniques, was developed. To illustrate the method, a particularly bad set of data was chosen which contained many missing and erroneous observations. Figure 12 shows the original data before filtering and the filtered data. The effectiveness of the method is self-evident. Coordinate Conversion: The next step in data processing is to correlate observations with geographic

7 184 NED A. OSTENSO AND RCHARD J. WOLD O O e..) r L~

8 AEROMAGNETC SURVEY OF THE ARCTC OCEAN " Fig. 4b. Aeromagnetic profiles , , , and Times indicated along profiles are keyed to tracks in Figure 4a.

9 186 NED A. OSTENSO AND RCHARD J. WOLD 2 8 O c~ C~ 4 c~ c'4 [ i.?.o q.) c~

10 AEROMAGNETC SURVEY OF THE ARCTC OCEAN 187 i i i i! ' : ~ i ~, - 5seoo ~4eoo 7~-; /1! ~ /! / - /! Fig. 5b. Aeromagnetic profiles , , , , 63~424, and Times indicated along profiles are keyed to tracks in Figure 5a.

11 188 NED A. OSTENSO AND RCHARD J. WOLD ~3 C) L) O r,-2 r 03 ~r oo ~ o ~D r ',D

12 AEROMAGNETC SURVEY OF THE ARCTC OCEAN ~200 57~00 ' -~,,TT- i ~ - T... m-w ~ ~ ~ ~ ~:~ ~ ~ ~ ~ ~ ~ ~ o:~ ~ ~ ~-~ ~ ~g 5660o" 63-4~4 55soo /a ~.~,.A~ Fig. 6b. Aeromagnetic profiles , , , 63414, and Times indicated along profiles are keyed to tracks in Figure 6a.

13 190 NED A. OSTENSO AND RCHARD J. WOLD 0 o o g d 0 d t"- d N V m [... t~

14 AEROMAGNETC SURVEY OF THE ARCTC OCEAN 191 6]-521 ~E3oo ~ o o ~ooo Fig. 7b. Aeromagnetic profiles , , , , , , and Times indicated along profiles are keyed to tracks in Figure 7a.

15 192 NED A. OSTENSO AND RCHARD J. WOLD.9 O O o~ C, ',D O,,:ft. ',.O '7 '4D tt3 b,0 O

16 AEROMAGNETC SURVEY OF THE ARCTC OCEAN G?O0 ~STO ~000 ~22g ~ [ ] ~ ~ ~. ] i.._j 5BOO0 5? ,,6 --l~ ~ A ~,.. ~,,,F ~ ~ i i l t" -! 5B2OO ~57200 Fig. 8b. Aeromagnetic profiles , , , , , and Times along profiles are keyed to tracks in Figure 8a.

17 194 NED A. OSTENSO AND RCHARD J. WOLD..= ~D t~ O O E"

18 AEROMAGNETC SURVEY OF THE ARCTC OCEAN ~B400 ~7400. r~f~l..... ~:~--'~'.'~L~ ~ L~. ~ ' ~... ~ ~ oo moo i ~ ' ~ ' ] ] ~ ~ " ' ~ ' ~ " Fig. 9b. Aeromagnetic profiles , , , , and Times indicated on profiles are keyed to tracks in Figure 9a.

19 196 NED A. OSTENSO AND RCHARD J. WOLD RESDUAL MAGNETC PROFLES Fig. 10a. Residual aeromagnetic profiles. Bathymetry in meters. Earthquake epicentres after Sykes (1965). RESDUAL MAGNETC PROFLES Fig. 10b. Residual aeromagnetic profiles. Bathymetry in meters. Earthquake epicentres after Sykes (1965).

20 AEROMAGNETC SURVEY OF THE ARCTC OCEAN 197 o _o V o N o o.~ o to ~ "r ~ o o ~ o

21 198 NED A. OSTENSO AND RCHARD J. WOLD ORGNAL DATA iifit ~... f... 1 o so 50o tso 2oo 25o ~oo 3so 4oo 4so 2Go sso eo0 6so too 7so ego aso ~oo 950 moo io50 110o.s~ 120o ~so 63'~~ l READNG NUMBERS 61~S0O 6O,5OO Sg,~OO 5e,~o0 Fig. 12. ~ FERE ' F~L D DATA 2O0 2S0 300 a~ S0 S00 SS0 ~a0 6S ab0 8~Q ~0 S0 READNG NUMBERS Magnetic Data before and after processing by Noise Filter. 10~ ~ i~aa 12S0 position and indicate the beginning and end of flight lines. This information is transferred to punched cards along with their associated times. The computer then correlates these times with the time readings punched on the data tape from the digital clock. The aircraft positions are plotted on maps based on any given projection (in this study the transverse mercator projection is used) and latitudes and longitudes are measured from these charts. For the computer to replot these positions it is much easier at high latitudes to work in an X-Y grid coordinate system. Thus a program was developed to convert latitude and longitude in geographic coordinates to X-Y grid coordinates at any map scale. Three programs have been written to handle the Lambert Conformal Conic projection, the Transverse Mercator projection, and the Universal Transverse Mercator projection. Regional Field: The regional field can be most easily determined by computer with a program developed by J. C. Cain at NASA, Goddard Space Flight Center. The accuracy of this computation of the main geomagnetic field is dependent upon the spherical harmonic coefficients used. The total intensity, vertical, north, or east component of the magnetic field may be computed for any given latitude, longitude, altitude, and time. A complete description of this program, including a program listing, is given in Cain et al. (1964; cf. also Cain et al., 1965). The advantages of using a computer determined regional field over the standard magnetic charts is its speed, immunity from personal bias, and its applicability to any geographical locality at any time. Profiling: The next step in the analysis was to have the computer draw the flight lines and their associated magnetic profiles. The computer was also programmed to print out in profile form the following information: total intensity, second derivatives, downward continuations of 5000 ft. (1525 m) and ft. (3050 m) below flight level, and two dimensional least square fits to polynomials of orders varying between 10 and 60. With this system of machine printouts a variety of mathematical tools, in addition to the original data, are displayed on a continuous strip of paper to assist in evaluating and interpreting the aeromagnetic data. Numerical Approximation: The technique used was adopted from a program package developed by BM (BM 1620 Numerical Surface Techniques and Contour Map, Plotting [1620-CX-05X]). The program takes irregularly spaced data that define a surface, and converts the irregular distribution to a square grid system of points. t is then possible to process the digital magnetic data from the paper tape to a finished contour map of the data regardless of the line spacing or density of the data. n order to analyze the Arctic aeromagnetics it was necessary to divide the area into 30 equal squares each with a 50 ~o overlap. The numerical approximation technique was then applied to each square and the square contoured. The contours of the squares do not necessarily coincide on the overlapping positions, although large trends and features are similar. This would be expected since

22 AEROMAGNETC SURVEY OF THE ARCTC OCEAN ~ of each square is composed of data not contained in any other square. Using a weighting function the several overlapping surfaces can be combined into one continuous surface reasonably consistent with each of the smaller ones. Examples of the numerical approximation are shown in Figure 13 at various grid spacings and data density. The number of data points used and grid spacing are related in that the amount of data should equal or exceed the total number of prisms created by the grid network. There are several 22 X 22 GRD <,50 PONTS 22 X 22 GRD 900 POfNT$ --to00 GAMMA CONTOUR NTERVAL GAMMA CONTOUR NTERVAL / \? L ~d;5 30 X 30 GRD 900 PONTS 40 X 40 g~ln 1600 PONTS Fig. 13. Effects on numerical approximation program of varying grid spacing and number of data points. factors to be considered in choosing the grid for a given area. One of the most important is cost. For example a 22 x 22 grid requires 70 sec to compute and contour whereas a grid requires over 1200 sec on the CDC Another factor is the filtering effect of using fewer grid points, and therefore, fewer and more widely spaced points. This results in a contour map of the larger features. Figure 14 shows the entire Arctic contoured at a 25 7 interval with each of the 30 squares divided into a 30 x 30 grid. The large scale features are quite obvious and are discussed in a later section. The numerical approximation has several good features. t fills in areas of no data with interpolated or extrapolated values, which is useful for the polynomial surface fitting and it puts the data into a grid form, which is useful for contouring or model studies.

23 200 NED A. OSTENSO AND RCHARD J. WOLD Fig. 14. Total Magnetic ntensity contour map of Numerical Approximation Surface on a 30 x 30 grid - Contour interval is 250 ~,. Principal physiographic features are outlined by dashed lines. Polynomial Surface Analysis: One of the primary problems in interpreting magnetic data is to delineate the true areal extent and relief of the anomalies. To isolate desired anomalies a regional field, which is the product of neighboring anomalous bodies and disturbances of great areal extent, must be removed. Least square polynomial surface fits eliminate many of the difficulties of the graphic, derivative, and continuation methods of removing regional fields. This method fits a two dimensional power series to the actual magnetic data and eliminates having to interpolate values into a grid and gives results which are reproducible and not subject to individual bias. Least square polynomial fits were applied to the magnetic data over the Arctic using a method described by Coons et al. (1964). n order to give a uniform unbiased approximation to the data, this analysis requires that the data points cover a square area. The method was, therefore, applied to each of the 30 squares described in the section on numerical approximation. The degree of polynomial surface selected to represent the regional is dependent on several factors: the size of the area, the density of stations, and the size and magnitude of the geologic features being investigated. Generally, the higher the degree of polynomial surface the closer such a surface will approach the actual surface whereas a lower degree polynomial surface will more nearly represent regional trends. However, when computing high degree polynomial surfaces, there is always a danger of going too high which will result in fictitious anomalies, especially in areas of insufficient data. Varying the degree of fit causes a filtering effect thereby allowing the interpreter to choose the appropriate fit to best bring out the anomalies associated with the particular features of interest. Figure 15 shows examples of surface fittings of various degree polynomials to data in the area of the Chukchi Rise. Calculations of Depth to Anomaly Source: One of the most useful quantitative tools of magnetic data analysis is the 'half slope' method of Peters (1949) for calculating the depth to the top of an anomaly source. Peter's method rests on some simplifying assumptions regarding the magnetic field vector and the shape, orientation, and susceptibility of the anomaly producing body. n practice nearly all the normally occurring deviations from the constraints assumed for Peter's method will result in a greater than true calculated depth to the top of the anomaly sources. The most notable exception is the effect of neighboring anomalies. f such effects are not apparent in the anomaly itself it should be detected in the downward continuations. Thus, limiting the analysis to only those anomalies which preserve a simple bell-shaped curve after downward continuation should heavily weigh in favor of

24 AEROMAGNETC SURVEY OF THE ARCTC OCEAN 201 \ jl / /" ~lh" polynomal SL~FAC Z / \,jz t) ~ j / ~. / h. FOLYNOMLAL SURFACE Fig th" polynomial SURFACE Effects of using different degree polynomial surfaces. reasonable estimates for depths to anomaly sources. n fact, of the thousands of anomalies on the nearly km of flight tracks only 315 stood the test to be deemed suitable for depth calculations These are plotted as histograms for the various physiographic provinces in Figure', 16. n the following discussion references to depth of anomaly sources will be in kilometers below sea level and are calculated by Peter's graphical method. 5. Discussion From inspection of Figures 10 and 11 some salient relationships between magnetic zones and physiographic features are obvious. These include: Canadian Arctic Archipelago: There is magnetic quiescence over the Canadian Arctic Archipelago, except for the Boothia Arch on Prince of Wales and ',Somerset islands.

25 202 NED A. OSTENSO AND RCHARD J. WOLD FR.M...,. lot-lena TROUGH AND 8[- / NANSEN 61- rrl', CORDJLL- 1~ H CHUKCH 8 SHELF , t ,o F 2,---i-- E 61 -MAKAROV BASN CORDLLERA,o r CANAOA BAS,N 4 LOMONOSOV RDGE 0 FCHUKCH u C 8 /o 6 CHUKCH PLAN 2 ~ - GREENLAND AND 2O 4 NORWEGAN SEAS ,o KANE BASN ~ ~-~ ] DEPTH TO TOP OF ANOMALY SOURCES BELOW SEA LEVEL N KLOMETERS Fig. 16. Histograms of the depths to the top of anomaly sources below sea level in kilometers for various physiographic provinces. The solid vertical line represents water depths of shelves or basins and average elevation of ridge crests. The vertical dashed line represents the mean elevation of ridges and rises. The vertical dotted line represents the depth of the rift valley in the Nansen Cordillera. Numbers on left side of each histogram show total number of calculated depths.

26 AEROMAGNETC SURVEY OF THE ARCTC OCEAN 203 Small isolated regions of magnetic disturbance, some of high intensity, do exist in the archipelago which Gregory et al. (1961) attribute to gabbroic dykes and sills and to basic flows which have been deformed by folding, faulting, or diapirism of gypsum. The Boothia Arch is a horst structure (Ostenso, 1962) which is located 3 km above the general basement level (Gregory et al., 1961). Chukchi Shelf." There is general magnetic quiescence over the Chukchi Shelf, except for the northeastern section. Ostenso and Parks (1964) showed that the anomalies here trended roughly north-south. Bassinger (1968), from a more detailled marine survey of the northeastern Chukchi Sea, showed that these anomalies are parts of a single magnetic high extending from Cape Lisburne to nearly 72 ~ The arctic slope of northern Alaska is a sedimentary basin containing principally Cretaceous material. Extensive geological and geophysicalinvestigations of the northern foothills of the Brooks Range and adjacent Arctic slope (Dana, 1951) showed basement rocks dipping from a 900 m under Barrow to 6700 m at approximately 69~ These rocks were identified as argillite and slate which transmitted seismic compressional waves at 5.2 km/sec. n the vicinity of Barrow and Cape Simpson P-wave velocities of 6.4 km/sec were recorded in what was believed to be granite underlying the argillite and slate sequence. Using gravity data and geological inferences, Ostenso (1968a) concluded that the Brooks Range structure strikes westward across the Chukchi Sea. Consequently, it is reasonable to suppose that 'magnetic basement' lies at a depth of some few kilometers beneath the Chukchi Shelf. The western end of Lisburne Peninsula is deformed by the roughly north-south striking Tigara uplift (Payne, 1955) which is interpreted to be formed by upfaulted Paleozoic and Triassic rocks lying adjacent to a thick Mesozoic sequence. Bassinger's (1968) magnetic data, supported by a seismic reflection profile (Moore, 1964), shows a fault paralleling the eastern margin of the magnetic anomaly with the displacement of the anomalous region being upward. From his data, Bassinger calculates the average depth of the uplifting basement surface to be 2.7 kin. Therefore, he interprets the magnetic high to reflect basement uplift related to northward extension of the Tigara structure. The calculated depths to magnetic basement of km over the Chukchi Shelf (Figure 16) all occur in the vicinity of 71 ~ 169 ~ that is, just west of Bassinger's magnetic anomaly and immediately outside the area of his survey. Approximately +20 milligals of gravity relief (Ostenso, 1968a) is associated with the magnetic anomaly. Using a density contrast of 0.2 gm/cc, which is consistent with observed seismic velocities under the Chukchi Shelf(D'Andrea et al., 1962), the gravity anomaly could represent basement shoaling of 2.5 kin. Thus both gravity and magnetic data suggest that, at least in the vicinity of 71 ~ a basement uplift of approximately 2.5 km, (from a depth of 5 km to 2.5 kin), is associated with the Tigara structure. Further north on the shelf at 74.5~ 165~ a seismic refraction survey by Kutschale et al. (1963) showed basement depth to be 6 kin. Evidence is not adequate to determine whether the western margin of the uplift is formed by faulting or sharp flexure.

27 204 NED A. OSTENSO AND RCHARD J. WOLD On the larger group of shallower source determinations shown in Figure 16, nine occur over Bassinger's anomaly. Five of these indicate a 2.5 km depth to source. The remaining anomalies over the Chukchi Shelf may be caused by near-surface intrusions. Chukchi Rise: The central Chukchi Rise is magnetically quiescent, but its margin to the west, north, and east produce large anomalies. To fit their observed gravity data Shaver and Hunkins (1964) inferred a sediment thickness of 12 km under the rise. Using magnetic profiles from drifting ice stations and available aeromagnetic data (Ostenso, 1963), Shaver and Hunkins also showed that a prominent magnetic anomaly paralleled the western and northern margin of Chukchi Rise. They interpret the source of this anomaly to be a basement ridge which is considered analogous to that off the east coast of the United States (Drake et al., 1963). As a consequence of this analogy, the absence of a marginal anomaly off the north coast of Alaska, and the abnormal narrowness of the continental shelf here, Shaver and Hunkins suggest that the rise was torn from Alaska and rotated to its present position about a pivot at 75~ 165 ~ The more recent aeromagnetic data presented here include four crossings of the eastern margin of the Chukchi Rise. Segments of all available magnetic profiles over the edge of the rise (Figure 17) are shown in Figure 18. These re-confirm the presence of a magnetic anomaly bordering the western and northern flanks of the Chukchi Rise and show, furthermore, that the anomaly occurs along the eastern margin as well. Correlation between the profiles in Figure 18 is quite striking. Occurrence of the eastern marginal anomaly obviates one of Shaver and Hunkins' strongest arguments for evoking continental drift as a mechanism for formation of the rise. Canada Basin: Within the Canada Basin there is a decrease in magnetic activity eastward from the Chukchi Rise and southward from the Alpha Cordillera toward the Canadian Arctic Archipelago. From the limited aeromagnetic profiles then available and from geologic and drill-hole data Ostenso (1963) suggested that, as in the Gulf of Mexico, a thick wedge of sediments has been spread over an oceanic crust which has been depressed due to loading. The more abundant aeromagnetic data now available continue to support this hypothesis. For instance, see the eastward attenuation of profiles across the Canada Basin in Figure 10. Wold et al. (1970) give evidence that the major source of sediments into the Canada Basin is from the Canadian coastline. The eastern boundary between the basin and the Canadian continental rise is gradational with evidence of extensive sediment transport. Contrastingly, the western contact of the Canada Basin with the Chukchi Rise and northern contact with the Alpha Cordillera are sharp scarps. Most of the anomalies available for depth computations shown in Figure 16 were from the western half of the Canada Basin. There was a tendency for the deeper calculated depth to be toward the east which further supports the concept of crustal depression by sediment loading. The very shallow depths occur adjacent to the western

28 AEROMAGN~TC SURVEY OF THE ARCTC OCEAN 20~ O O O O r,.) x~ '~ O ~. ~.~ ~ O 8~

29 206 NED A. OSTENSO AND RCHARD J. WOLD ~ '~ "J q ,q: L.../ G o8 Qz s... / ~ 6:5-422 D L3 :.-..., J "-... A Fig. 18. ~FK, (~t KLOMETERS u~ o Magnetic profile sections aross the margin of the Chukchi Rise. Dotted profiles are from Shaver and Hunkins (1964). Track locations are shown on Figure 17. and northern margin of the basin and may reflect continuity of the bordering scarps below the floor of the plain as has been observed by Hunkins (1968). Contained within the Canada Basin is the perched (at a depth of 2230 m) Chukchi Plain that lies between the Chukchi Rise and the East Siberian Shelf. An interesting group of anomalies, whose calculated depths to sources are about 8 km (Figure 16), are clustered over the plain. Under the Wrangel Plain in the Makarov Basin, which is a perched plain similar to the Chukchi Plain, Kutschale's (1966) seismic profiles showed sediment thickness to be at least 3.5 km. That is, basement lies at a depth greater than 6 km. The Chukchi Plain is closer to major sources of sediments than is the Wrangel Plain (Figure 1). t is nourished by two submarine valleys in the Chukchi Shelf and possibly others that incise the East Siberian Shelf (Figure 2). Consequently it is reasonable to believe that the calculated depths to anomaly sources are a good indication of the basement surface. Thus, sediment fill in the Chukchi Plain would be about 6 km thick. f this feature is in isostatic adjustment, as available gravity data

30 AEROMAGNETC SURVEY OF THE ARCTC OCEAN 207 suggest (Ostenso et al., 1968) the ocean crust has been depressed approximately 4 km by the overlying sediments. Thus the elevation of the sea floor prior to sedimentary loading would be about 4 km or equal to that of the western Canada Plain. Makarov Basin: Because of its small size, only 8 anomalies suitable for depth calculations are available over the Makarov Basin. Six of these gave depths to sources of km. This is in good agreement with Kutschale's (1966) seismically determined depth to basement in the southern end of the basin. As with the Chukchi Plain in the Canada Basin, the perched Wrangel Plain appears to have the same underlying basement depth as the remainder of the basin. The perched plains appear to owe their existence to their confined locations near sources of sediments. Within the Wrangel Plain sediment confinement is also aided by a basement ridge which forms a natural dam to the north (Kutschale, 1966). Sediment thickness beneath the deep basin appears to average kin. Fram and Nansen Basin: Fram Basin, lying between the Lomonosov Ridge and the Nansen Cordillera, and Nansen Basin, lying between the Nansen Cordillera and the Eurasian continental shelf, are marked by magnetic quiescence. Only four anomalies over the Fram Basin were of sufficient amplitude and shape for depth determinations (Figure 1). These indicate a sediment thickness of about 0.5 kin. The limited aeromagnetic data available over the Nansen Basin provided no anomalies suitable for such calculations. The apparent enigma of an undisturbed magnetic field over ocean basins but thinly covered by sediments may be explained by an elevation of the Curie isotherm, for which evidence will be given later. Alpha Cordillera and Lomonosov Ridge: Magnetically, the Alpha Cordillera and Lomonosov Ridge are interesting studies in contrast. Segments of all magnetic profiles that cross the two ridges (Figure 3) are shown in Figures 19 and 20. The topography of the ridges, shown as hachured lines, were scaled from the generalized bathymetric chart shown in Figures 4-9. Consequently they are without detail. Further, the bottom profiles were scaled along the strike of the aeromagnetic flight tracks which causes appreciable distortion where the ridges are crossed at a steep oblique, which is often the case. The magnetic quiescence of the Lomonosov Ridge relative to the intense disturbance associated with the Alpha Cordillera is striking and has also been observed by Galkin (1968). Over the Lomonosov Ridge anomalies are either absent or no greater than ambient disturbance on either side of the ridge. Only on the Greenland side of 180 ~ longitude (profiles and in Figure 20) does there appear to be an anomaly or anomalies related to the Lomonosov Ridge. This is the section of the ridge included in the study of King et al. (1966) who suggested that the anomalies were caused by volcanic rocks with a magnetic susceptibility of cgs units. Their calculated susceptibility is an order of magnitude greater than observed for oceanic basalts (Vogt

31 - - ~ - - ~ 208 NED A. OSTENSO AND RCHARD J. WOLD ~ - - ~ 4 ~ r ~ c ~ m. ~2000 m [ 64_408 ~ ~ ~ ~-424 ~ --.Tm~'~.rr Fig. 19. Segments of magnetic profiles across the Alpha Cordillera. Numbers refer to flight tracks in Figures 4-9. Bottom topography (hachured line) is scaled from generalized bathymetric chart (Figures 4-9) along flight tracks. Horizontal lines indicate 2000 m isobath. Profiles are arranged in sequence from Eurasia (upper left) to North America (lower right). and Ostenso, 1966) and somewhat greater than the observed apparent susceptibility (induced plus remanent magnetization). On the other hand their value is about the mean for continental gabbros. The bulk of anomalies usable for depth determinations (Figure 16) appear to originate at or near the surface of the Lomonosov Ridges. However, four anomalies indicate sources of origin at depths of 6-7 km. Topographic profiles across the Alpha Cordillera obtained from submarine transits (Dietz and Shumway, 1961 ; Beal, 1968) show it to be a region of rugged topography rather than a discrete ridge like the Lomonosov Ridge. Because of the extreme magnetic disturbance over the cordillera, the deeper indicated sources may be artifacts caused by undetected coalesced anomalies. However, it is interesting to note that

32 AEROMAGNETC SURVEY OF THE ARCTC OCEAN ~ -- "~-" [ ~ -. ~ ~ m ~" 6[~ ~ m. ~ 2000,r m. a:: s i Fig. 20. Segments of magnetic profiles across the Lomonosov Ridge. Numbers refer to flight tracks in Figures 49. Bottom topography (hachured line) is scaled from generalized bathymetric chart (Figures 4-9) along flight tracks. Horizontal lines indicate 2000 m isobath. Profiles are arranged in sequence from Eurasia (upper left) to North America (lower right). most of the sources deeper than 3.5 km occurred along the margins of the Cordillera Vogt and Ostenso (1970) showed apparent correlation between profiles that cross the Alpha Cordillera and furthermore, that there may be axial symmetry. dentification of an axial magnetic signature from the aeromagnetic profiles was made difficult by the fact that the flight tracks of this regional survey were widely spaced, not mutually parallel, and generally did not cross the strike of the ridge at right angles. n addition, the Alpha Cordillera is still poorly sounded and the position of a tectonic axis, in the absence of epicenters and an identified median rift valley, could conceivably be in error by 100 km. The possible presence of, as yet undiscovered, transverse fractures would also complicate the location of a tectonic axis. Despite these obvious uncertainties, the magnetic profiles that cross the cordillera most nearly at right angles are shown in the lower third of Figure 21. These profiles have been horizontally contracted by cosx, where x is the angle between the strikes of the flight lines and the tectonic axis. Consequently, the amplitude scale varies by about 20% between profiles and occasionally along a profile, when there Jis a dog-leg in the

33 210 NED A. OSTENSO AND RCHARD J. WOLD Lu,5 % _i L~ ~/t',, S N ~ 6 [ 525S o Fig. 21. Magnetic profiles across the Reykjanes and Mohns ridges and Alpha Cordillera. Ridge axes are to the left at distances indicated by vertical lines. Profiles have been contracted by cosx where x is the angle between the strike of the flight track and the ridge axes. Consequently the vertical scale varies between profiles and along some profiles. Profile numbers refer to Figures 4-9. Alpha Cordillera profiles connected by heavy arrows have been 'folded' at the topographic axis to show bilateral symmetry. Dashed lines connect salient probable inter profile anomaly correlations. ] flight track. Apparent correlations between anomalies are shown by dashed lines. The cordillera axis is to the extreme left. Profiles , and one leg of traversed the cordillera in a sufficiently long and straight line that they could be 'folded over' to demonstrate symmetry. These are designated as the N and S segments of a

34 AEROMAGNETC SURVEY OF THE ARCTC OCEAN 211 common profile and are connected by a heavy vertical arrow. Considering the uncertainties involved, the correlations are quite good. King et al. (1966)using independent data (except for the profiles used by Ostenso (1962) which are common to both studies) also show mutually parallel bands of anomalies extending for hundreds of kilometers parallel to and on the North America side of the Alpha Cordillera. Further, Rassokho et al. (1967) show a linear pattern of parallel anomalies on the Eurasian side of the cordillera. Nansen Cordillera: Apparent correlation of anomalies between aeromagnetic profiles crossing the Nansen Cordillera was first observed by Demenitskaia et al. (1962) and was later identified as a locus of ocean floor spreading by Ostenso and Wold (1967). Vogt et al. (1970) elaborated on this study and concluded that the Nansen Cordillera was spreading at a rate of about 1 cm/yr in each direction. There appears to be no decrease in spreading rates between the northern Mid-Atlantic Ridge and the southern Nansen Cordillera. None of the anomalies over the Nansen Cordillera were suitable for depth determinations. Greenland and Norwegian Seas: The aeromagnetic profiles over the Greenland and Norwegian seas, along with some ship-towed magnetometer data, were analyzed by Vogt et al. (1970) who arrived at the following conclusions: By comparison with published profiles over other mid-ocean ridges, the celand-jan Mayen Ridge and Mohns Ridge are symmetrically spreading at a rate of approximately 1 cm/yr. The Atka Ridge and Spitsbergen fracture zone connect the Mohns Ridge with the Nansen Cordillera to the north. The Atka Ridge incises the continental rise west of Svalbard. Although a magnetic signature typical of mid-ocean ridges is not associated with this structure, a well-developed rift valley and uplift with normal faulting on Svalbard suggest dilatational as well as strikeslip movement. f the direction of crustal spreading has been perpendicular to Mohns Ridge and parallel to the Spitsbergen fracture zone, then the Atka Ridge may be intermediate between a transform fault and a typical mid-ocean ridge segment. n this case the rate of crustal spreading at right angles to the ridge may be low. A schematic illustration of floor spreading in this complex portion of the ocean is given in Figure 22. The Spitsbergen fracture zone itself appears to be a more complicated feature than shown in this figure (see Vogt et al., 1970). All calculated anomaly sources of 3.5 km and deeper are associated with either (1) the central axis of the celand-jan Mayen Ridge, (2) the Jan Mayen fracture zone, or (3) the deep (~ 4 km) basin of the central Greenland Sea (~ 72~ ~ The shallow depths (<2 km) are nearly all associated with shoal waters near Jan Mayen sland or celand. Of 23 depth determinations over regions where bathymetry is well known, 14 coincide with indicated water depths, 4 on the Greenland continental margin are 1-3 km deeper than indicated water depths (these may represent a sediment wedge at the continental margin or they may be artifacts of uncertain navigation over an area

35 212 NED A. OSTENSO AND RCHARD J. WOLD or 0 SPTSBERGEN F.Z. qs* RDGE O ~ o, RDGE \ KM Fig. 22. Schematic illustration of ocean floor spreading in the Arctic Ocean and Greenland and Norwegian Seas. Arrows indicate the direction of motion. where bottom topography is changing abruptly), one is 0.5 km shallower than the indicated water depth, and 3 are 0.5-t.5 km deeper than indicated ocean floor. These few data suggest that the sediment cover of the Greenland Sea floor is thin or absent. This conclusion is consistent with the seismic refraction studies of Ewing and Ewing

36 AEROMAGNETC SURVEY OF THE ARCTC OCEAN ; profiles F 4, 5 and 6) and scheme of ocean floor spreading in this area, described by Vogt et al. (1970). 6. Deep Magnetic Structure Figure 14 shows the Arctic Basin contoured by the process of numerical approximations at an interval of This map is a composite of 30 squares each of which was divided into a grid as described in Section 4 of this paper. The most salient feature of this map is a change in the total magnetic field from nearly 0.58 G over the North American side of the Lomonosov Ridge (Amerasia Basin) to 0.54 G over the Eurasian side of the Lomonosov Ridge (Eurasia Basin). This change is greatest over and parallels the Lomonosov Ridge. We interpret this gross field change to reflect shoaling of the Curie isotherm from under the dormant Alpha Cordillera towards the active Nansen Cordillera. Assuming simple dipole theory, the 4000 ~ difference in field represents an elevation of the Curie isotherm of the order of l0 km. Such a shoaling of the Curie isotherm could explain the generally reduced amplitude of anomalies over the Eurasian Basin. A series of high anomaly closures occur along the strike of the Nansen Cordillera. These are caused by the elevated high susceptibility rocks of the mid-ocean ridge and the anomalies tend to disappear when coarser filtering techniques are used. Similarly, many small closures occur over the relatively elevated field of the American Basin. These anomalies result from the high susceptibility rocks of the upper crust and their contour configuration changes markedly between different grid-spacing computations. Contrastingly the gross field characteristics are little affected by varying grid sizes. n addition to the regional gradient discussed above, two other i~teresting anomaly patterns persist despite mathematical manipulations, suggesting that they have deep origins. One of these is an anomaly of a few hundred gammas amplitude that wraps around the Chukchi Rise. This anomaly supports the crustal model of the western and northern margins of the rise constructed from gravity data by Shaver and Hunkins (1964). n this model a km wide basement root extends 32 km into the mantle. The width of this marginal root is comparable to the width of the magnetic anomaly. Of particular interest is the fact that the anomaly almost completely encircles the rise, including traversing its connections with the Chukchi shelf. The encircling occurrence of this anomaly would not favor Shaver's and Hunkin's suggestion that the rise formed by rotation of a segment of the Alaskan continental shelf. 7. Morphology of the Arctic Basin f, structurally, the Alpha Cordillera is a mid-ocean ridge it should exhibit evidence of spreading that has been demonstrated with other oceanic ridges, and an attempt should be made to establish its place in the history of global tectonics. n principle the history of the cordillera can be reconstructed if its signature of geomagnetic field polarity reversals can be correlated with the well-established chronology of other mid-ocean ridges: particularly the well documented Mid-Atlantic Ridge. n 1966 two magnetic

37 214 NED A. OSTENSO AND RCHARD J. WOLD profiles were obtained across the North Atlantic Ocean from the icebreaker U.S.S. ATKA (Vogt, 1967). The profiles extend from the British slands to the Greenland shelf. The upper profile ran nearly tangent to the southern coast of celand and crosses a long section of the shelf. The lower profile lies approximately 100 km further south and crosses the Reykjanes Ridge before it intersects the celand shelf. This profile passes just north of the area of detailed survey by Heirtzler et al. (1966). An attempt to correlate the Alpha ridge profiles with the 'celand' (upper) and 'Reykjanes' (lower) profiles plus aeromagnetic profiles across Mohns Ridge north of celand (Vogt et al., 1970) is made in Figure 23. The axial anomalies over the Reykjanes [ ~- 2 /- V ~63-422S i N:O N.=50 N:O0 N=50 M:O ~ Fig. 23. Seven profiles from Figure 21 that were used for quantitative correlation. To the right the profiles have been numbered according to table. The N scale indicates the number of digitized points. The M scale indicates the number of points correlated after some profiles have been linearly contracted (profiles 3, 4 and 5). Ridge are of much higher frequency than those over the Alpha Cordillera axis. The best possible correlation of the Alpha ridge axial anomalies was with the celand and Reykjanes profiles at a distance of approximately 260 km from the axis of the Reykjanes Ridge. At the top of Figure 23 the celand and Reykjanes Ridge profiles are shown at distances originating about 260 km from the Reykjanes Ridge axis. n this figure the horizontal scales of individual profiles have been appropriately contracted to 'normal-

38 AEROMAGNETC SURVEY OF THE ARCTC OCEAN 215 ize' them to the strike of the ridges. The labelled vertical lines indicate distances to the ridge axis from that point in the profile. To demonstrate axial symmetry the West legs of the celand and Reykjanes profiles are plotted as mirror images. That is, the two profiles are 'folded' at the ridge axis and then the 520 km central section is deleted. The Mohns Ridge aeromagnetic profiles were not sufficiently long to do this. Those aeromagnetic profiles that are of good quality and most nearly normal to the strike of the Alpha Cordillera are shown at the bottom of Figure 23. n this set of profiles the cordillera axis is to the immediate left. Profile was sufficiently long that it could be 'folded' into North and South segments to indicate their degree of biaxial symmetry. n order to estimate the goodness of fit of the correlations shown in Figure 23 in some objective way, the celand and Reykjanes ship profiles and aeromagnetic profiles S and N & S, were digitized at 3 km intervals by Vogt (1967) and compared as follows: Designate the matrix of digitized magnetic field values by A~, where i, the profile index, runs from 1-7 and j, the point on each profile, runs from -N, where N= 150 except for profile celand-2w, for which N= 100. A regional was determined graphically for each profile and subtracted by the computer. The program then subtracts the average value, 1N ~= 1 Aij, from each point of a profile. A linear contraction factor was determined by inspection in such a way as to maximize the fit between the profiles. This variation in horizontal scale is justified on three grounds; (1) The profile may not be perpendicular to the strike of the magnetic anomalies, (2) the spreading rate may have been different between the two flanks of the Mid- Atlantic Ridge or the Alpha Cordillera and (3) the spreading rate on the Mid-Atlantic Ridge may have been different from the Alpha ridge. Of the seven profiles compared in Figure 23, the horizontal scale of profiles Reykjanes-E and S was reduced by 24~, and that of profile N by 31~o, before all seven profiles were correlated. For brevity of identification the ends of the profiles are numbered 1-7. The goodness of fit was estimated in two ways for each pair of profiles (m, n). The cross-correlation M Amj Anj Cm,, = 100 J" = 1 Z am~'l Zn; j=l can range between and t is if the two profiles have the same sign at every point j, and if they have opposite signs at every point. This method is insensitive to differences in amplitude. Therefore, the root-mean square difference lm,, was also computed according to the formula ~t tm n = j = 1 (Amj -- Aml) 2 M lm,= 0 implies that the profiles are everywhere identical; the larger the value the poorer the correlation.

39 216 NED A. OSTENSO AND RCHARD J. WOLD Because some profiles were contracted, M had to be made less than N, the number of digitized field values for each profile. The value M is 110 except for pairs (m, n) where m or n = 1. M= 100 for profile 1 because of its short length. The results of the computation are shown in Table. Because both Cm, and l,,, are TABLE Matrix of cross-correlations and rms differences Profile No Location 1 celand, West Flank 2 Reykjanes, West Flank 3 Reykjanes, East Flank 4 celand, East Flank S N S M t- 37 q- 64 -k 69 -/- 70 Cross correlation t t- 55 Cmn rms differences in gammas lmn symmetrical matrixes, the two are tabulated in a single square array. Cm,, in ~, is shown above the diagonal and lmn, in gammas, below the diagonal. The values of Cm, range from - 22 to + 71, whereas lm, ranges A plot of lmn against Cmn shows that, in general, the highest rms differences also correspond to the poorest cross-correlations. The values of l,,, are shifted upward wherever profile 7 is involved. This results from the exceptionally high amplitudes of that profile. The values of C,,, should be compared with C34, the correlation between profiles 3 and 4. These profiles are nearly parallel and km apart and presumably cross crustal material generated at the axis of Reykjanes Ridge where the linearity and symmetry of the magnetic pattern has been demonstrated out to a distance of-t- 100 km from the ridge axis. C34 is + 45 and/34 is The correlation between two profiles on the western side of Reykjanes Ridge is equally good (C12 = + 47 and 112 = 297). On the other hand, correlations between profiles located on opposite sides of Reykjanes Ridge are considerably lower, even after these correlations were optimized by contracting profiles 3 and 4 by 28~o. (f this contraction is significant, the spreading rate may have been somewhat greater in an eastward direction.) Yet this is a classic

40 AEROMAGNETC SURVEY OF THE ARCTC OCEAN 217 area of biaxial symmetry described in the detailed survey of Heirtzler et al. (1966). The inter-profile correlations for the Alpha ridge data are all high. C67, which is a measure of the degree of symmetry of the magnetic signature in that area, has the highest value (+ 71) out of the present sample, although the two halves of the signature differ markedly in amplitude. Profile 5 was contracted 32% to obtain a better fit. The twelve correlations between axial profiles over the Alpha Cordillera and flank profiles over Reykjanes Ridge range between - 22 and + 70, and C,,, is + 40 or greater for half the correlations. Therefore, the fit between two profiles on different ridges is on the whole as good as that between two profiles on the same ridge. n summary, the computed correlations do support the hypothesis that the Alpha Cordillera has a magnetic signature which is symmetrical about the topographic axis and whose axial anomalies correlate with anomalies now found approximately 260 km east and west of the axis of Reykjanes Ridge. Clearly many more profiles should be obtained and then cross-correlated before a high degree of confidence can be placed on these conclusions particularly, profiles that are not randomly oriented to the strike of the tectonic axis are needed. f the correlations in Figure 23 are real, as suspected, then the Alpha Cordillera has a symmetric magnetic signature and became inactive at a time when crust now approximately 260 km from the axis of Reykjanes Ridge was created. This segment of crust was determined by Avery et al., (1969) to have been formed mybp. From these correlations Vogt and Ostenso (1970) suggest that tile Eurasian Basin was actively spreading at least 60 mybp, but abruptly ceased to spread 40 mybp when the locus of rifting shifted from the Alpha to the Nansen Cordillera (Vogt et al., 1970) as is evidenced by the good correlation between the axial anomalies of the Reykjanes, Mohns and Nansen Ridges and their similar seismicity. From magnetic evidence presented here plus other geophysical data and geological inferences (Ostenso, 1962; Wilson, 1963) Vogt and Ostenso (1970) further concluded that the Lomonosov Ridge was a section of the former Eurasian continental margin that has been torn from the edge of the continent as a consequence of the change in spreading locus. Such a scheme of basin morphology is consistent with the abrupt shoaling of the Curie isotherm inferred from Figure 14 and discussed in a preceding section. Acknowledgements We are indebted to the South Weymouth Naval Air Station and the Patuxent River Naval Air Station for aircraft support in flying the aeromagnetic surveys as Project Arctic Basin. Particular thanks go to the project officers Lt. Gordon Petri and the late Lt. Charles Hall. The many details of financial and logistic support were handled by Dr. Max Britton and his staffofthe Arctic Branch, Office of Naval Research. Computer programming assistance was ably provided by Thomas Wolfe and Franklin Crow. Extensive support by the University of Wisconsin Computing Center and the University Research Committee is gratefully acknowledged. The profile correlation section is abstracted from a Ph.D. dissertation by Peter R. Vogt.

41 218 NED A. OSTENSO AND RCHARD J. WOLD This research was sponsored under the Office of Naval Research contracts Nonr- 1202(16) and Nonr-1202(25) at The Geophysical and Polar Research Center, Univ. of Wisc. The opinions expressed in this paper do not reflect those of the Navy Department nor the U.S. Government. References Avery, O. E., Vogt, P. R., and Higgs, R. H.: 1969, 'Morphology, Magnetic Anomalies and Evolution of the Northeast Atlantic and Labrador Sea, Part : Magnetic Anomalies', Trans. Am. Geophys. Union (abs.) 50, 184. Bassinger, B. G.: 1968, 'A Marine Magnetic Study in the Northeast Chukchi Sea', J. Geophys. Res. 73, Beal, M. A.: 1968, 'Bathymetry and Structure of the Arctic Ocean', Ph.D. Dissertation, Univ. of Oregon. Bullard, E. C. : 1960, 'The Automatic Reduction of Geophysical Data', Geophys. J. Roy. Astr. Soc. 3, Cain, J. C., Hendricks, S., Daniels, W. E., and Jensen, D. C." 1964, 'Computation of the Main Geomagnetic Field from Spherical Harmonic Expansions', NASA Rpt. X Cain, J. C., Daniels, W. E., Hendricks, S., and Jensen, D. C.: 1965, 'An Evaluation of the Main Geomagnetic Field, ', J. Geophys. Res. 70, Coons, R. L., Mach, J. W., and Strange, W. : 1964, 'Least-Square Polynomial Fitting of Gravity Data and Case Histories', Computers in the Mineral ndustries, p Dana~ S. W.: 1951, 'Geology of the Arctic Slope of Alaska', U.S. Geol. Survey Map O M 126, sheet 2. D'Andrea, D., Thiel, E., and Ostenso, N.: 1962, 'Seismic Crustal Studies in the Chukchi Sea', Univ. Minn. Dept. Geol. Geophys., Vol. 1, 8 pp. Demenitskaya, R. M., Karasik, A. M., and Kiselev, Yu. Yu. G.: 1962, 'Results of the Study of the Geological Structure of the Earth's Crust in the Central Arctic by Geophysical Methods', Problems of the Arctic and Antarctic, n. 11 (in Russian). Transl. by Arctic nst. of N. America, p. k-1 to k-10. Demenitskaya, R. M. and Hunkins, K. L.: in press 'Shape and Structure of the Arctic Ocean', in The Sea, Vol. 4. Dietz, R. S. and Shumway, G.: 1961, 'Arctic Basin Geomorphology', Bull. Geol. Soc. Amer. 72, Drake, C. L., Heirtzler, J., and Hirshman, J.: 1963, 'Magnetic Anomalies off Eastern North America', J. Geophys. Res. 68, Ewing, J. and Ewing, M.: 1959, 'Seismic Measurements in the Atlantic Ocean Basins, in the Mediterranean Sea, on the Mid-Atlantic Ridge, and in the Norwegian Sea', Bull. Geol. Soc. Am. 70, Galkin, R. M.: 1968, 'Variations of the Main Geomagnetic Field in the Drift Region of Station "North Pole 13" in ', Problems of the Arctic and Antarctic, n. 28, pp (in Russian). DRB Canada Trans. T. 534 R. Gregory, A. F., Morley, L. W., and Bowers, M. E." 1961, 'Airborne Geophysical Reconnaissance in the Canadian Arctic Archipelago', Geophysics 26, Heirtzler, J. R., Le Pichon, X., and Baron, J. G.: 1966, 'Magnetic Anomalies over Reykjanes Ridge', Deep-Sea Res. 13, Hunkins, K.: 1968, 'Geomorphic Provinces of the Arctic Ocean', in Arctic Drifting Stations, Arctic nst. of North Amer., Wash. D.C., pp Kelly, T. : 1961, 'Neptunes Probe Arctic Basin', Naval Aviation News, Nov., pp Kelly, T.: 1963, 'The Flights of Arctic Basin ', Naval Aviation News, Oct., pp King, E. R., Zietz,., and Alldredge, L. R.: 1966, 'Magnetic Data on the Structure of the Central Arctic Region', Bull. Geol. Soc. Am. 77, Kutschale, H." 1966, 'Artic Ocean Geophysical Studies: The Southern Half of the Siberia Basin', Geophysics 31, Kutschale, H., Thiel, E., D'Andrea, D., Hunkins, K., and Ostenso, N. : 1963, 'A Long Refraction Profile on the Arctic Continental Shelf', Abstracts of papers,, X General Assembly UGG, Berkeley.

42 AEROMAGNETC SURVEY OF THE ARCTC OCEAN 219 Moore, D. G.: 1964, 'Acoustic-Reflection Reconnaissance of Continental Shelves: Eastern Bering and Chukchi Seas', inpapers in Marine Geology, (ed. by R. L. Miller), Macmillan Co., pp Ostenso, N. A. : 1962, 'Geophysical nvestigations of the Arctic Ocean Basin', Univ. Wisc. Geophys. and Polar Research Center, Research Rept. 62-4, p Ostenso, N. A. : 1963, 'Geomagnetism and Gravity of the Arctic Basin', in Proc. Arctic Basin Syrup., Oct. 1962, Arctic nst. N. Amer., Washington D.C., pp Ostenso, N. A.: 1968a, 'A Gravity Survey of the Chukchi Sea Region, and its Bearing on Westward Extension of Structure in Northern Alaska', Bull. Geol. Soe. Am. 79, Ostenso, N. A. : 1968b, 'Geophysical Studies in the Greenland Sea', Bull. Geol.,Soe. Am. 79, Ostenso, N.A., den Hartog, S.L., and Black, D. J. : 1968, 'Gravity and Magnetic Observations from ce sland Arlis- off the Chukchi Shelf', in Arctic Drifting Stations, Arctic nst. of N. Amer., Wash. D.C., pp Ostenso, N. A. and Parks, P. E., Jr. : 1964, 'Seaborne Magnetic Measurements in the Chukchi Sea', Univ. Wisc. Geophys. and Polar Research Center Research Rpt. 64-5, p. 31. Ostenso, N. A. and Wold, R. J. : 1967, 'Aeromagnetic Survey of the Arctic Basin, (abs.)', LA.G.A. Bull. 24: 67. Payne, T. G. ". 1955, 'Mesozoic and Cenozoic Tectonic Elements of Alaska', U.S. Geol. Survey Misc. Geol. nvest. Map Peters, L. : 1949, 'The Direct Approach to Magnetic nterpretation and ts Practical Application', Geophysics 14, Rassokho, A.., Senchura, L.., Demenitskaya, R. M., Karasik, A. M., Kicelev, Yu. G., and Timashenko, N. K." 1967, 'Podovodyni Shedinnyi arkticheskii khrebet i yego mesto b sisteme shrebtov cevernogo ledovitogo okeana', Dokl. Akad. Nauk S.S.S.R. 172, Shaver, R. and Hunkins, K.: 1964, 'Arctic Ocean Geophysical Studies: Chukchi Cap and Chukchi Abyssal Plain', Deep-Sea Res. 11, Sykes, L. R.: 1965, 'The Seismicity of the Arctic', Seism. Soe. Am. Bull. 55, Vogt, P. R. and Ostenso, N. A. : 1966, 'Magnetic Survey over the Mid-Atlantic Ridge between 42~ and 46~ ', J. Geophys. Res. 71, Vogt, P. R.: 1967, 'A Reconnaissance Geophysical Survey of the North, Norwegian, Greenland, Kara and Barents Seas and the Arctic Ocean', Ph.D. dissertation, Univ. of Wisconsin at Madison, p Vogt, P. R., Ostenso, N. A., and Johnson, G. L.: 1970, 'Magnetic and Bathymetric Data Bearing on Sea-Floor Spreading North of Reykjanes Ridge', J. Geophys. Res. 75, Vogt, P. R. and Ostenso, N. A. : 1970, 'Magnetic and Gravity Profiles Across the Alpha Cordillera and their Relation to Arctic Sea-Floor Spreading', J. Geophys. Res. 75, Wilson, J. T. : 1963, 'Continental Drift', Sei. Am. 208, Wold, R. J.: 1964, 'The Elsec-Wisconsin Digital Recording Proton Magnetometer System', Univ. Wisc. Geophys. and Polar Research Center Research Rpt. 64-6, p. 83. Wold, R. J., Woodzick, T. L., and Ostenso, N. A.: 1970, 'Structure of the Beaufort Sea Continental Margin', Geophysics 35,