AEROMAGNETIC
TECHNIQUES
SURVEY
AND
OF THE
ARCTIC
OCEAN:
INTERPRETATIONS
N E D A. O S T E N S O
Office of Naval Research, Arlington, Virginia 22217, U.S.A.
and
R I C H A R D J. W O L D
University of Wisconsin at Milwaukee, 53201, U.S.A.
(Received 11 June, 1970; revised 2 November, 1970)
Abstract. Approximately 147000 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 289km
basement uplift in the eastern Chukchi Shelf associated with the Tigara structure which truncates
the western end of Lisburne Peninsula. A 30-40 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 1-189km. There appears to be only about a 89km 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. Introduction
F r o m cursory e x a m i n a t i o n it is a p p a r e n t that several factors characterize the Arctic
O c e a n Basin. U n l i k e the earth's other oceans it is nearly isolated by e n c o m p a s s i n g
continental land masses. The great P r e c a m b r i a n heartlands o f the continents directly
face the p o l a r ocean and tectonic belts, such as the V e r k h o y a n , U r a l and R i c h a r d s o n
M o u n t a i n s , are t r u n c a t e d at the n o r t h e r n coast. T h e continental shelf, too, is unusual.
W h e r e a s c o n t i n e n t a l shelves are n o r m a l l y tens o f kilometers wide, along over h a l f
the circumference o f the Arctic Ocean the shelf is h u n d r e d s o f kilometers wide. In
a d d i t i o n to their a b n o r m a l width, the continental shelves are f r e q u e n t l y incised by
s u b m a r i n e canyons, and have at least one k n o w n massive outlier o f apparently continental crust.
M o r e detailed investigations o f the past decade have further c o n f i r m e d the uniqueness o f the Arctic Basin. Soundings f r o m ice islands, airlifted expeditions, and
Marine Geophysical Researches 1 (1971) 178-219. All Rights Reserved
Copyright 9 1971 by D. Reidel Publishing Company, Dordrecht-Holland
179
AEROMAGNETIC SURVEY OF THE ARCTIC OCEAN
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. Its 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.
..... ~,~.
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.:..,.o..~.;~.;.... "
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Fig. 1.
Sketch map of the major physiographic features of the Arctic Basin.
"%.
"~
180
NED A. OSTENSO AND RICHARD 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"
IO0*
80~
80"
Fig. 2. Generalized bathymetric chart of the Arctic Basin. Isobaths in meters, Compiled from
Soviet and United States Sources.
a systematic aeromagnetic survey over the Arctic Ocean was planned in 1960. 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 1964. The approximately 147 000 km of aeromagnetic
AEROMAGNETIC SURVEY OF THE ARCTIC OCEAN
181
data obtained during these three survey seasons are reported here. The flight lines are
shown in Figure 3.
2. Survey Procedures
A P 2 V - 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
AE~OMAGNETIC
. . . . . .
1961
FLIGHT
TRACKS
~965 . . . . . . .
1964
Fig. 3. 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. In 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
182
NED A. OSTENSO AND RICHARD 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,
Iceland 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 I
Average K-indices for the intervals of aeromagnetic surveyflights
1961
1963
1964
Barrow
College
4
2
3
3
1
2
The average K-indices recorded at the two observatories during the intervals of the
survey flights are shown in Table I.
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. In so far as was
possible, flights were scheduled for days when ionospheric disturbances were predicted
AEROMAGNETIC SURVEY OF THE ARCTIC OCEAN
183
to be at a m i n i m u m . The low average K-indices shown in T a b l e I attests to the effectiveness o f the forecasts. N o flights were m a d e during m a g n e t i c storms. F u r t h e r ,
because o f the high speed o f the aircraft, t e m p o r a l disturbances a p p e a r on the magnetic profiles as p e r t u r b a t i o n s o f the regional g r a d i e n t rather t h a n m a j o r distortions
o f i n d i v i d u a l anomalies.
3. Aeromagnetic Data
T h e flight tracks a n d a e r o m a g n e t i c profiles, w i t h o u t residuals r e m o v e d , are shown in
Figures 4-9. T h e residual m a g n e t i c profiles are p l o t t e d over the Arctic Basin b a t h y m e t r y in Figures 10a a n d 10b. To a v o i d hopeless clutter h a l f o f the profiles are shown
in each o f the two figures. A l t h o u g h lacking in detail, these figures give a graphic
p r e s e n t a t i o n o f the relationship of m a g n e t i c character to p h y s i o g r a p h i c features a n d
should be referred to while r e a d i n g the following discussion.
A n o t h e r m e t h o d o f d a t a presentation, a d a p t e d f r o m K i n g e t aI. (1966) is shown in
F i g u r e 11. This p r e s e n t a t i o n is essentially c o n t o u r i n g regions o f similar m a g n e t i c
signature a n d has the a d v a n t a g e o f preserving an i n d i c a t i o n o f c h a r a c t e r t h a t c a n n o t
otherwise be shown by c o n t o u r s on a regional survey. U n l i k e K i n g e t al. we felt t h a t
our d a t a were i n a d e q u a t e to c o n t o u r in as m a n y as seven different profile patterns.
R a t h e r , F i g u r e 11 is divided into only four different types o f regions. These regions
e n c o m p a s s (1) profiles with a n o m a l y a m p l i t u d e s near or exceeding 1000 7 ; (2) profiles
with a n o m a l y a m p l i t u d e s r a n g i n g near 500 7; (3) profiles with a n o m a l i e s in the 300 7
r a n g e ; a n d (4) profiles t h a t are u n d i s t u r b e d or c o n t a i n a n o m a l i e s o f less t h a n 100 7
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. It 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 100 7 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. In 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
184
N E D A. OSTENSO A N D R I C H A R D J. W O L D
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r
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AEROMAGNETIC
SURVEY OF THE ARCTIC OCEAN
185
56600
61-513
63 - 4"22
63-426
Fig. 4b.
56800
55800
Aeromagnetic profiles 61-513, 63-419, 63-422, and 63-426. Times indicated along profiles
are keyed to tracks in Figure 4a.
186
NED A. OSTENSO AND RICHARD
J. W O L D
2
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187
AEROMAGNETIC SURVEY OF THE ARCTIC OCEAN
|
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,
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7~-;
Fig. 5b.
/1!
~ /
!
/
I II
I
- /
!
Aeromagnetic profiles 61-511, 61-519, 61-603, 63-417, 63~424, and 64-321. Times indicated
along profiles are keyed to tracks in Figure 5a.
188
NED A. OSTENSO
AND RICHARD
J. W O L D
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AEROMAGNETIC SURVEY OF THE ARCTIC OCEAN
189
5~200
61 - 6 0 8
57~00
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-~
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i
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.........
57500
m-W
58500
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Fig. 6b.
/a
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56700
A e r o m a g n e t i c profiles 61-608, 63-322, 63-411, 6 3 4 1 4 , a n d 6 3 4 2 1 . T i m e s indicated a l o n g
profiles are k e y e d to tracks i n F i g u r e 6a.
190
N E D A . OSTENSO A N D R I C H A R D J. W O L D
0
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d
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191
AEROMAGNETIC SURVEY OF THE ARCTIC OCEAN
6]-521
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56000
58000
57000
~
5
5
6
o
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63-415
54000
64 - 329
53000
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57000
64 -412
56000
-55000
Fig. 7b. Aeromagnetic profiles 61-521, 63-412, 63-415, 64-329, 64-412, 64-417, and 64-408. Times
indicated along profiles are keyed to tracks in Figure 7a.
192
N E D A. OSTENSO A N D R I C H A R D J. W O L D
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o~
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AEROMAGNETICSURVEYOF THE ARCTICOCEAN
193
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61-515
~STO0
61-530
57000
5~000
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51200
55500
54500
53~00
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64-414
[ . - - - ] - - - ~ ~
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,,,F..------~ ~
-I
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5BOO0
5?000
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~57200
Aeromagnetic profiles 61-515, 61-530, 64-319, 64-401, 64-406, and 64-409. Times along
profiles are keyed to tracks in Figure 8a.
194
NED A. OSTENSO AND
RICHARD
J. W O L D
..=
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t~
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AEROMAGNETIC
61
-
SURVEY OF THE ARCTIC OCEAN
195
523
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61 - 5 2 8
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.
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-
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331
64-403
i
~ ' ~ '
] ]
~ ~ " ' ~ ' ~ " - -
--
57000
-56000
Fig. 9b.
Aeromagnetic profiles 61-523, 61-526, 61-528, 61-609, and 64-330. Times indicated on
profiles are keyed to tracks in Figure 9a.
196
NED A. OSTENSO AND RICHARD J. WOLD
RESIDUAL MAGNETIC PROFILES
Fig. 10a. Residual aeromagnetic profiles. Bathymetry in meters. Earthquake epicentres
after Sykes (1965).
RESIDUAL MAGNETIC PROFILES
Fig. 10b. Residual aeromagnetic profiles. Bathymetry in meters. Earthquake epicentres
after Sykes (1965).
A E R O M A G N E T I C S U R V E Y OF T H E A R C T I C O C E A N
197
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to
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198
NED
A. OSTENSO
AND
RICHARD
J. WOLD
ORIGINALDATA
iifit
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so
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READINGNUMBERS
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D DATA
2O0 2S0 300 a~0 400 4S0 S00 SS0 ~a0 6S 700 750 ab0 8~Q ~0 S0 I
10~01100 11~ i~aa12S0
READING NUMBERS
Fig. 12.
Magnetic Data before and after processing by Noise Filter.
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
10000 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
IBM (IBM 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. It 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.
In 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
199
AEROMAGNETIC SURVEY OF THE ARCTIC OCEAN
50 ~ 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 GRID
<,50 POINTS
22 X 22 GRID
--tO00
GAMMA CONTOUR INTERVAL
--250
GAMMA CONTOUR INTERVAL
900 POfNT$
/
L
30 X 30 GRID
Fig. 13.
\
?
~d;5
900 POINTS
40 X 40
g~ln
1600 POINTS
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 40 • 40 grid requires over
1200 sec on the CDC 3600. 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. It 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.
200
N E D A. O S T E N S O A N D R I C H A R D J. W O L D
Fig. 14. Total Magnetic Intensity 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). In 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. In 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. If 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
201
AEROMAGNETIC SURVEY OF THE ARCTIC OCEAN
\
jl
/
/"
\
/
~ l h " pOLYNOMIAL SL~FAC Z
,jz
t)
~ j /
~.
/
h. FOLYNOMLAL SURFACE
13th" pOLYNOMiaL SURFACE
Fig. 15. Effects of using different degree polynomial surfaces.
reasonable estimates for depths to anomaly sources. In fact, of the thousands of anomalies on the
nearly 150000 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. In 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
F r o m inspection o f Figures 10 a n d 11 some salient relationships between m a g n e t i c
zones a n d p h y s i o g r a p h i c features are obvious. These include:
Canadian Arctic Archipelago: There is m a g n e t i c quiescence over the C a n a d i a n A r c t i c
A r c h i p e l a g o , except for the B o o t h i a A r c h on Prince o f W a l e s a n d ',Somerset islands.
202
NED A. OSTENSO AND RICHARD J. WOLD
lOt-LENA TROUGH AND
1~
8[/I
NANSEN 8
FR.M...,.
I
61-
2 545
6 789
I
,oF
61-MAKAROV BASIN
7
25
ICORDJLL4
,5
6 7 8
4
LOMONOSOV RIDGE
2
,---i--I
I
345
2
3
4
5
6
7
8
t
I0
2
5
45
6
FICHUKCH I
E
6 7
1816
2
I
rrl',
H CHUKCHI
SHELF
2
I
2 345
914-
6
CORDILLERA
12I0-
,o r CANAOA BAS,N
6
4
I
u.
IC
8
6
4
2
3
4
5
6
7
8
I
9
/o
CHUKCHI PLAIN | 2 ~ 2O
345678910
GREENLAND AND
NORWEGIAN SEAS
262345678914-12
,o
KANE BASIN
8
6
I
4112 ~ ~-~
42 I-]
II
I
I 2 5 4 5 8 7 8 9
I 2 3 4 5 6 7 8 9 I0
DEPTH TO TOP OF ANOMALY SOURCES BELOW SEA LEVEL IN KILOMETERS
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.
AEROMAGNETIC SURVEY OF THE ARCTIC 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. In 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 5.0-5.5 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.
204
NED A. OSTENSO AND RICHARD 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
AEROMAGN~TIC SURVEY OF THE ARCTIC OCEAN
20~
O O
O
O
r,.) x~
'~
O ~.
~.~
O
8~
~
206
NED A. OSTENSO AND RICHARD J. WOLD
63-414
~
61-550
64-417
65 -411
65-411
'~
"J
q.
64-521
61-530
65 - 5 2 2
61-525
,q:
....
L........./
64-
321
G
Qz
6:5 - 4 2 2
64-4o8
s
..../
D
~
: .-...,
J
65-
412
L3
u~
o
~FIK
(~t
,
" - .....
A
300
I
I
I
KILOMETERS
0
Fig. 18. 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). It 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. If this feature is in isostatic adjustment, as available gravity data
AEROMAGNETIC SURVEY OF THE ARCTIC 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
5.0-5.5 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 1-1.5 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 61-519 and 61-515 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 0.007 cgs units. Their calculated susceptibility is an order of magnitude greater than observed for oceanic basalts (Vogt
208
NED A. OSTENSO AND RICHARD J. WOLD
61-608 ~
__
- - ~ 4 ~ r ~
65-412
-
~
-
64-40 c
61-609
64-408
65-322
6 3 - 4 2 2 ~
-
-
~
61-530
61-609
--
2000m.
~2000 m
64_408 ~
__
63-41[
1 2000~
~
4-412
6~-424 ~
--.Tm~'~.rr
61-515
61-523
61-513
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 L o m o n o s o v 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 L o m o n o s o v 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
209
AEROMAGNETIC SURVEYOF THE ARCTIC OCEAN
63-412 ~
--
"~-" 64- 412
65-417
61-5[5
64-409 ~ -
. ~
63-424
~
m
61-519
~"
65 -424
I
6[~525
2000 m.
~2000 ,r
6
5
-
3
~
61-515
--
61-519
---
2000 m. sa::
i
Fig. 20. Segmentsof 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. Identification 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. In
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
210
N E D A. O S T E N S O A N D R I C H A R D J. W O L D
Lu
,5
%
L~
5000_i
2000
----
63-422 S
~/t',,
I I000
o
~
6
[
61-525N
525S
]
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. A p p a r e n t correlations between a n o m a l i e s are s h o w n by dashed lines. The
cordillera axis is to the extreme left. Profiles 63-523, 61-523 a n d one leg of 63-411
traversed the cordillera i n a sufficiently long a n d straight line that they could be 'folded
over' to d e m o n s t r a t e symmetry. These are designated as the N a n d S segments of a
AEROMAGNETIC SURVEY OF THE ARCTIC 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 Iceland-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. If 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. In 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 Iceland-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 Island
or Iceland. 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
212
NED A. OSTENSO AND RICHARD J. WOLD
or
0
SPITSBERGEN F.Z.
Iqs*
RIDGE
O ~
o,
RIDGE
\
I00
Fig. 22.
4 0 0 KM
Schematic illustration of ocean floor spreading in the Arctic Ocean and Greenland and
Norwegian Seas. Arrows indicate the direction of motion.
where b o t t o m t o p o g r a p h y 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
AEROMAGNETIC SURVEY OF THE ARCTIC OCEAN
213
0959; 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 250 7. This map is a composite of 30 squares each of which was
divided into a 22 • 22 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.
In 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). In this model a 30-40 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
If, 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. In 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. In 1966 two magnetic
214
NED A. OSTENSO AND RICHARD J. WOLD
profiles were obtained across the N o r t h Atlantic Ocean from the icebreaker U.S.S.
A T K A (Vogt, 1967). The profiles extend from the British Islands to the Greenland
shelf. The upper profile ran nearly tangent to the southern coast of Iceland 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 Iceland 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 'Iceland' (upper) and
'Reykjanes' (lower) profiles plus aeromagnetic profiles across Mohns Ridge north of
Iceland (Vogt et al., 1970) is made in Figure 23. The axial anomalies over the Reykjanes
[I
~-
2
3
/-
4
~63-422S i
~6
V
7
N:O
M:O
N.=50
N:IO0
N=I50
Fig. 23. Sevenprofiles from Figure 21 that were used for quantitative correlation. To the right the
profiles have been numbered according to table II. 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 Iceland and
Reykjanes profiles at a distance of approximately 260 k m from the axis of the Reykjanes Ridge.
At the top of Figure 23 the Iceland and Reykjanes Ridge profiles are shown at
distances originating about 260 km from the Reykjanes Ridge axis. In this figure the
horizontal scales of individual profiles have been appropriately contracted to 'normal-
AEROMAGNETIC SURVEY OF THE ARCTIC 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 Iceland 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. In this set of
profiles the cordillera axis is to the immediate left. Profile 63-411 was sufficiently long
that it could be 'folded' into North and South segments to indicate their degree of
biaxial symmetry.
In order to estimate the goodness of fit of the correlations shown in Figure 23 in
some objective way, the Iceland and Reykjanes ship profiles and aeromagnetic profiles
63-422S and 63-411 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 I-N,
where N = 150 except for profile Iceland-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, 1IN ~ = 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 MidAtlantic 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 63-422S was reduced by
24~, and that of profile 63-411N 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 Iam~'lIZn;I
j=l
can range between + 100 and - 1 0 0 .
It is + 100 if the two profiles have the same sign at every point j, and - 100 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
(Amj
tm n = j = 1
-- Aml) 2
M
lm,= 0 implies that the profiles are everywhere identical; the larger the value the poorer
the correlation.
216
NEDA.OSTENSOANDRICHARDJ. WOLD
Because some profiles were contracted, M had to be made less than N, the n u m b e r
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 o f its short length.
The results o f the c o m p u t a t i o n are shown in Table II. Because b o t h Cm, and l,,, are
TABLE II
Matrix of cross-correlations and rms differences
Profile No
Location
1
2
3
4
5
6
7
Iceland, West Flank
Reykjanes, West Flank
Reykjanes, East Flank
Iceland, East Flank
63422S
63411N
63411S
M
1
1
2
3
4
5
6
7
297
375
313
290
269
365
2
3
4
5
6
7
+ 47
-- 8
§ 22
-t- 37
+ 10
+ 45
q- 64
+ 24
-t- 29
+ 65
-k 69
+ 6
--22
§
+ 51
-/- 70 Cross
+ 23 correlation
+ 32
-t- 55 Cmn
+ 46
+ 71
322
308
298
316
402
283
322
371
409
236
285
342
269
363
330
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 f r o m - 22 to + 71, whereas lm, ranges 236-409 7. 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 u p w a r d wherever profile 7
is involved. This results f r o m the exceptionally high amplitudes of that profile.
The values o f C,,, should be c o m p a r e d with C34, the correlation between profiles
3 and 4. These profiles are nearly parallel and 50-100 k m apart and presumably cross
crustal material generated at the axis of Reykjanes Ridge where the linearity and
symmetry o f the magnetic pattern has been demonstrated out to a distance of-t- 100
k m f r o m the ridge axis. C34 is + 45 and/34 is 283 7. The correlation between two profiles on the western side of Reykjanes Ridge is equally g o o d (C12 = + 47 and 112 = 297).
On the other hand, correlations between profiles located on opposite sides o f Reykjanes Ridge are considerably lower, even after these correlations were optimized by
contracting profiles 3 and 4 by 28~o. (If this contraction is significant, the spreading
rate m a y have been somewhat greater in an eastward direction.) Yet this is a classic
AEROMAGNETIC SURVEY OF THE ARCTIC 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.
In 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.
If 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 40-60 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.
218
NEDA.OSTENSOANDRICHARDJ. WOLD
This research was sponsored u n d e r the Office of N a v a l Research contracts N o n r 1202(16) a n d Nonr-1202(25) at The Geophysical a n d Polar Research Center, Univ. of
Wisc.
The o p i n i o n s expressed in this paper do n o t reflect those of the N a v y D e p a r t m e n t
n o r the U.S. G o v e r n m e n t .
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