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Arctic ocean geophysical studies: Chukchi Cap and Chukchi abyssal plain
Deep-Sea Reaearc~ 1964, VoL 11, pp. 905 to 916. Pergamon Pr¢~ IAd. Printed in Great Britain.
Arctic Ocean geophysical studies: Chukchi Cap and Chukchi Abyssal Plain* RALPH SHAVER AND KENNETH HUNKINS Lamont Geological Observatory, Columbia University, Palisades, New York (Received 17 June 1964)
Abatract--A bathymetric chart of the Chukchi Cap region was compiled with soundings obtained from Fletcher's Ice Island (T-3), as well as from other ice stations and from U.S. Navy icebreakers. New details of the Chukchi Cap are shown, including two submarine troughs on the southwest side. West of the Chukchi Cap, a small abyssal plain was found with a depth of 2230 m. This abyssal plain is connected through an abyssal gap with the deeper Canada Abyssal Plain. The promifient magnetic anomaly discovered during the drift of Station Charlie was crossed more recently by T-3 and by aeromagnetic flights. The continuity of the anomaly along the western and northern sides of the Chukchi Cap was further established by the new measurements. An interpretation was made of the anomaly as an expression of induced magnetization in basement rocks. The interpretation shows a basement ridge beneath the anomaly maximum at the edge of the Chukchi Cap. The Cap itself is interpreted as being underlaid by a 12 km thickness of sediments. Both magnetic and gravity data were used for an interpretation of total crustal thickness along the same section. Crustal thickness ranges from 18--~km beneath the Chukchi Cap to 32 km beneath the large basement ridge. INTRODUCTION Tim Chukchi Cap and Chukchi Abyssal Plain are prominent submarine features located in the Arctic Ocean about 1200 k m north of the Bering Strait. The Chukchi, Cap was discovered during the drift of the Soviet ice station, NT-2, in 1950 and 1951 (SoMOV, ed., 1955). Further sounding and magnetic data over the cap were provided by U.S. Ice Station Charlie during its drift in 1959 (HuNKrNS et al., 1962). Recent sounding, gravity and magnetic data in the vicinity of the cap which were taken from Fletcher's Ice Island (T-3) during 1962 are reported here. The drift of T-3 between M a y 12 and September 23, 1962 was particularly favourable for study of the Chukchi Cap (Fig. 1). T-3 has been in nearly continuous use as a United States scientific drifting station since 1952. In M a y 1960 the ice island, which is about 50 m thick, became grounded on the Alaskan continental shelf at 71 ° 55'N, 160 ° 20'W. When it appeared unlikely that T-3 would become free again, the station was vacated in October 1961. However, sometime in late 1961 or early 1962, T-3 did float free and resumed its drift in the clockwise gyre of the Canada Basin. It was resighted on February 16, 1962 from an airplane of the Arctic Research Laboratory of Point Barrow, Alaska. On the following day, a three-man party from the Arctic Research Laboratory was landed on T-3 to reestablish it as a scientific drifting station. During the spring, T-3 was *Lamont Geological Observatory Contribution No. 752. 905
906
RALPH ShAVeR and Ir~NNt~n HUNKINS
then resupplied and reorganized. By May the ice island was in full operation as a scientific station after the authors had begun a geophysical program. Navigation, soundings, magnetics and gravity were included in the Lamont Geological Observatory program.
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50O KI LOMETERS
Fig. 1. Drift track of Fletcher's Ice Island (T-3)between May and September, 1962. Contours based on Geological Map of the Arctic (1960).
NAVIGATION
The position of T-3 was determined by solar fixes during the summer period covered here. The usual overcast conditions in the high Arctic prevailed, allowing only 91 fixes during the 135 days studied. The fixes were determined to within a few tenths of a mile. During long periods between fixes, interpolated positions may be in error by several miles. During the summer of 1962, T-3 drifted in a northerly direction along a track somewhat similar to that of Soviet NP-2 in 1950 and 1951. Between May 12 and September 23, 1962, T-3 drifted a total of 1388 km. The rate of drift was 10.35 km/day. This rapid rate may be attributed to the open ice conditions of that summer. Ice-breaker activities that fall revealed how open the pack ice was. In September 1962 the icebreaker U.S.S. Burton Island penetrated to T-3 when it was located at 78 ° 56'N. This is a record northern penetration for icebreakers in this sector of the Arctic Ocean.
Arctic Ocean geophysicalstudies : Chukchi Cap and Chukchi Abyssal Plain
907
DEPTH SOUNDINGS A total of 368 spot echo soundings were made between May 12 and September 23, 1962. The soundings were taken from the pack ice off the edge of T-3. Explosives ranging from detonator caps to 1 lb. charges were used as sound sources. The echo was received with a geophone on the ice surface and recorded with an ink-writing oscillograph operated at 125 mm/sec. The echo time can be determined to within
Fig. 2. Drift tracks and icebreakertracks providing bathymetriccontrol in the vicinity of the
Chukchi Cap.
1 millisec from the record. In reducing the data, corrections were made for the speed of sound in water but not for bottom slope (MA~t~ws, 1939). The T-3 soundings were used along with data from all other available sources to compile a bathymetric chart of the Chukchi Cap and vicinity. The track chart (Fig. 2) shows the distribution of soundings from drifting ice stations and ice breakers. The western gide of the cap is reasonably well sounded. The eastern side and the saddle
908
RALPH S~vlu~ and K E t ~ H HUNKINS
connecting the Cap and the shelf are not so well known. The bathymetric chart (Fig. 3) covers the Chukchi Cap, the Chukchi Abyssal Plain and a portion of the Chukehi continental shelf.
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Fill, 3. Balhymeiric sketch of the Chukchi Cap and Chukchi Abyssal Plain, Contour interval 500 m except for certain supplemental contours, Dotted lines indicate lack of data.
The Chukchi Cap is roughly 150 km in diameter at the 500 m depth contour. The shallowest sounding recorded was 246 m. On the southwest side, the cap is indented by two troughs. The larger of the troughs is 75 km long and 200 m deep. The top of the Cap is marked by roughness 5 to 30 m in height (HtrNKINS e t a l , , 1962). The Chukchi Abyssal Plain is bordered by the Chukchi Cap, the continental shelf and the Alpha Ridge. On the north it opens to the Canada Abyssal Plain. All available soundings within the basin are plotted in Fig. 3. They are sparse but they indicate a flat floor with a depth of about 2230 m. A final decision as to whether this is a true abyssal plain must await a crossing with a precision depth recorder. Meanwhile, the soundings, the location at the foot of the continental rise, and the gap to the north all suggest that it is a true abyssal plain. The abyssal gap on the northern edge of the abyssal plain was crossed several
Arctic Ocean geophysical st~dies : Chakchi Cap and Chvkchi Abyssal Plain
909
times by Drifting Station Charlie and precision depth records were obtained. Four profiles are shown in Fig. 4. The gap width across the flat floor varies between 0.6 and 2"7 km. The width measurement is subject to some error since the drift rate at the time of crossing is only known approximately. The floor of the gap drops 114 m between the first and last profiles of Fig. 4, a distance of 55 km. This is an average gradient of 1 : 500. In plan the gap follows closely the base of the Chukchi Cap (Fig. 3). It is likely that the location of the gap is structurally controlled and that the floor is sediment covered. Turbidity currents flowing from the Chukchi Abyssal Plain evidently pass through the gap to the deeper Canada ['/'2eW
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Fig. 4. Bathymetric profiles across the Charlie Gap. Note that profiles are taken directly from
precision depth records and that the horizontal scales are variable due to different drift rates. Dotted lines indicate proper projection onto the map scale. Abyssal Plain. The gap forms a connection between the upper 2230 m level o f the Chukchi Abyssal Plain and the lower 3790 m level of the Canada Abyssal Plain to the north. A similar case of a small abyssal plain and accompanying gap has been found by KUTSCHALE(private communication) in the Central Arctic Basin. In that case the upper Wrangell Abyssal Plain is connected through the Arlis Gap to the lower Siberia Abyssal Plain. The usual practice of naming submarine features after nearby geographical regions has been followed. The Chukchi Cap and Chukchi Abyssal Plain have been named after the Chukchi Sea which lies to the immediate south. The Chukchi are a native people of northern Siberia. The Charlie Gap has been named after
910
Rm2n SI~WR and K~NNmmHUNKINS
Ice Station Charlie in agreement with the practice of naming abyssal gaps for the discovering ship. In this case, Station Charlie was the equivalent of a discovering ship. MAGNETICS
A prominent magnetic anomaly exists along the western margin of the Chukchi Cap. This anomaly was crossed several times during the drift of Station Charlie (HuNKINS et al., 1962). More recent observations from airplanes (OSTENSO, 1963) and from T-3 bare further confirmed the existence of this feature and provided new details. All available magnetic profiles are shown in Fig. 5 and the tracks are shown in Fig. 6. The trend of the anomaly peak is also indicated in Fig. 6. The T-3 data were taken with a portable Varian proton-precision magnetometer which was usually
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Fig. 5. Total magnetic intensity profiles across the western side of the Chukchi Cap. Drifting Station Arlis II, A; Drifting Station Charlie, B, C and E; Fletcher's Ice Island (T-3) D and G;
aeromagneticprofiles(OsT~SSO,1963) F and H.
read every hour with a precision of ± I0 gammas. Daily averages were made in order to eliminate diurnal fluctuations. The aeromagnetic profiles were flown at an altitude of 450 m with a Varian proton precession magnetometer which recorded continuously. The new data confirm that the anomaly is a continuous feature closely following the depth contours along the edge of the cap. The anomaly peak occurs over the slope where water depths range between 700 and 2500 m. On profiles B to E, the
Arctic Ocean geophysical studies : Ch~tkchi Cap and Chukchi Abyssal Plain
911
peak follows the depth contours especially closely,, occurring regularly over depths between 1000 and 1500 m. The amplitude of the anomaly ranges between 500 and 1500 gammas from peak to trough.
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Trend of maximum magnetic anomaly values (triangles). Tracks refer to magnetic profiles in Fig. 5. Bathymetriccontour interval, 500 m.
The manner in which the anomaly follows the submarine topography indicates that it reflects structural trends, but the shape and position of the anomaly do not allow an explanation solely in terms of simple topographic effects in uniformly magnetized rocks. The anomaly must be caused by rocks of varying magnetization within the crust which follow the topographic trends. Although it could be caused by a belt of highly magnetic material such as a basic intrusion, it is most probably due to a combination of induced and permanent magnetization in basement rocks with high susceptibilities. Such a situation is found off the east coast of the United States where both good magnetic and seismic data are available for interpretation (DRAKe et al., 1963). The Chukchi Cap anomaly most closely resembles the eastern United States anomaly in the region north of Cape Hatteras. In that region DRAK~ et al., 1963 found it possible to make a fairly good fit to the observed magnetics by
912
RALPH SHAWa and K E ~ r H HtmK~S
assuming uniform magnetization of the basement layer as determined by the seismic method. DRAKEet al. (1963) assumed a susceptibility of zero for the sediments and 0.01 e.m.u, for the basement rocks. In the case of the Chukchi Cap, no seismic information is available. Since similarity of the Atlantic coast and Chukchi Cap anomalies suggested similar origins, an attempt was made to find the basement topography beneath the Chukchi Cap by fitting the observed magnetic anomaly. Susceptibilities were the same as those assumed by DRAKEet al. (1963), I The strike of the body, magnetic intensity and inclination were chosen to agree with actual conditions. The magnetic inclination is so dose to the vertical in this area that changes in the
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Fig. 7. Observed total magnetic intensity over the edge of the Chukchi Cap compared with total intensity calculated for the crustal model shown in cross section. Observed lines a r e C and D (Fig. 5). strike of the body have little effect on the calculated anomaly. The anomalies were calculated on a high speed digital computer with a program for polygons of arbitrary shapes (T^LwANI and I-IEmTZL~g, 1964). The inferred basement topography beneath the Chukchi Cap and both observed and calculated magnetic profiles are shown in Fig. 7. A sediment thickness of 12 km is inferred beneath the Chukchi Cap. A basement ridge is inferred beneath the foot of the slope. From the results on the Atlantic coast by DRAKE et al. (1963) it can be seen that the basement as inferred from the magnetic field, as was done here, would have greater
Arctic Ocean geophysical studies : Chukchi Cap and Chukchi Abyssal Plain
913
relief than that found by seismic methods. It is likely that remanent as well as induced magnetization is often present and that this often makes the anomalies larger than would be expected for a purely induced field. Thus the basement topography calculated for the Chukchi Cap is probably exaggerated. The actual relief is probably similar in shape but somewhat subdued. A seismic refraction survey of the Chukchi continental shelf south of the Cap found the sediment thickness one-half that deduced for the Chukchi Cap (KtrrsCHALe et al., 1963). At 74-1/2°N 165°W the 3 km/sec layer, which is presumably the sediments, was 6 km thick. Underlying the sediments at that location was a basement layer with a velocity of 4.9 km/sec. One difference between the basement configuration found on the Atlantic coast and that inferred for the Chukchi Cap is evident. Both areas have a basement ridge beneath the continental slope which separates two sedimentary basins, one beneath deep water and one beneath the shelf. On the Atlantic coast the sediment basin on the deep-water side usually contains the greater sediment thickness but on the Chukchi Cap, the sedimentary basin beneath the shallow water contains more sediments. GRAVITY
A gravity profile over the Cap was obtained from T-3 during 1962. A LaCosteRomberg Model G gravity meter was read four times daily. Ties to the University of Wisconsin ~endulum station at Point Barrow were made on 7 May, on 4 June
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Fig. 8. Observed free-air gravity section from T-3 across the edge of Chukchi Cap compared with free-air gravity calculated for the crustal model shown in cross section.
914
RALVH SHAVERand KENN~Xtt HUNKINS
and on 5 October, 1962. The difference between the ties on 4 June and 5 October, 1962 was 0.73 milligals. The gravity profile across the Chukchi Cap along the T-3 track is shown in Fig. 8. The free-air anomaly is about 4- 50 milligals over the Cap and approaches zero in the deep water regions. Near the south side of the Cap the topographic effects of the submarine troughs are apparent in the gravity values. The simple Bouguer anomaly over the Cap is between -~ 70 and -4-90 milligals except for the same topographic effects on the south side. The Bouguer anomaly is ÷ 200 milligals over the basin to the north and -~- 160 miiligals over the basin to the south. A crustal model based on both the magnetic and gravity data was calculated across the edge of the Chukchi Cap. The calculations were made with a high-speed digital computer using a program based on the method of TALWANI et al., (1959). The basement topography as calculated from the magnetic results was used in computing the two-dimensional gravity model. It was assumed that sediments with an average density of 2-4 g/co occur above the basement. This rather high average value for sediments was chosen on the basis of the large sediment thickness and the probability of compaction and consolidation at depth. The average density of the deep crust below the sediments was assumed to be 2.9 g/cc. The upper mantle density was taken as 3.4 g/cc. This mantle density coriesponds to an 8.2 km/sec compressional velocity (TALWANI et al., 1959). With these values fixed and with the ocean bottom and basement topography already determined, it was possible to adjust the shape of the Mohorovi~i6 discontinuity so that the calculated gravity agrees reasonably well with the observations. At either end, the section was assumed to have the equivalent balance of a crustal column with a thickness of 32 km and an average density of 2.87 g/cc. overlying a mantle with a density of 3.4 g/cc. The calculated crustal structure is shown in Fig. 8. The crustal thickness is 18-1/2 km beneath the Chukchi Cap, 32 km beneath the basement ridge and 21 km beneath the Chukchi Abyssal Plain. It must be realized that a number of assumptions are made in calculating this crustal model. The density values were chosen to agree reasonably well with results of other studies. The assumption that the feature may be treated as two-dimensional also seems to be in good agreement with the facts. On this basis, the model may be considered a first approximation to the crustal structure in this area with the details still subject to considerable refinement. DISCUSSION
Although the data are still fragmentary, it is tempting to speculate on the origin of the Chukchi Cap as revealed by the information available so far. The Chukchi Cap is apparently composed of a thick sequence of sedimentary rocks. This indicates that the cap was formerly a part of the continental shelf. Such a sequence of sediments is usually found on continental shelves where a source area exists inland. The present continental shelf north of Alaska is built of such a sediment sequence. In its present position, the Chukchi Cap has no readily available sediment source. If the Chukchi Cap has been a part of the continental shelf, it should be possible to point out its original site and the path by which it was moved to its present location. The most probable original site lies along the Alaskan continental shelf east of Point Barrow (Fig. 1). Along this stretch of coast the present continental
Arctic Ocean geophysical studies : Chukchi Cap and Chukchi Abyssal Plain
915
shelf is relatively narrow although in most other areas the Arctic continental shelves are extremely wide. The motion of the cap in its removal from the shelf would have been a quarter-turn counter-clockwise about a pivot where the Chukchi Cap presently joins the shelf at 75°N 165°W. The eastern side of the Chukchi Cap would formerly have been joined to the present continental shelf. The western side of the Chukchi Cap, which resembles present-day shelf structure on the Atlantic and G u l f coasts, would have been the continental margin at that earlier period. Other features also tend to support this hypothesis. N o pronounced magnetic anomalies are found across the continental shelf immediately cast of Point Barrow in the region in which the Cap would have originated. It would strengthen the hypothesis if the characteristic Chukchi Cap anomaly were found again along the coast further to the northeast off the Canadian Arctic Islands. There are some indications of such an anomaly but the results are still inconclusive (GREGORY et al., 1961). Few data are available along the eastern side of the Chukchi Cap, but it appears that no distinctive magnetic anomaly is found in that area. The magnetic field is rough but apparently does not have the unique type of anomaly found on the western side of the Cap. Also, the area near the assumed pivot point is topographically rough and could be interpreted as an area which has been subject to tension and shear. This area near the pivot has been called a "continental b o r d e r l a n d " by FISHER et al., (1958). It contains the so-called Northwind Seahigh. The history of hypotheses on the origin of the Arctic Basin has been reviewed by EARDLEY (1961). The concepts fall into two general categories, those invoking vertical subsidence and those invoking horizontal movements. The ideas presented here on the movement of the Chukchi Cap would support those hypotheses invoking horizontal movements, such as the " expanding earth " theories of CAREY and o f HEEZEN.
Acknowledgment--We are indebted to the director, MAx BP~WEa, and to the personnel of the Arctic Research Laboratory, Barrow, Alaska, for their initiative in reestablishing T-3 as a scientific drift station and for their spirit of cooperation in all aspects of the field work. JAMESALBEPdNO (Lament Geological Observatory) assisted in field operations. HENRY KtrrSCHALE (Lament Geological Observatory) furnished depth data from. Arlis II. THEOOOgEWAMnACHof the U.S. Navy Underwater Sound Laboratory made available his navigational data which were used in constructing the T-3 drift track (Fig. 2). This work was supported by the Ottic¢ of Naval Research as a part of Project Vela Uniform, sponsored by the Advanced Research Projects Agency, Department of Defenc¢. REFERENCES
DEN HARTOG S. L. and OSTENSON. (1962) Gravity and magnetic observations from Ice Island Arlis II - 1 July to 6 October 1961 (unpublished manuscript). DRAKE C. L. HEmTZLER J., and HIRSHMANJ. (1963) Magnetic anomalies off Eastern North America. J. geophys. Res. 68, 5259-5275. EARDLEY A. J. (1961) History of geologic thought on the origin of the Arctic Basin. In : Geology o f the Arctic, Prec. of the First International Symposium on Arctic Geol. Univ. of Toronto Press, Vol. 1,605-621. FISHER R., CARSOLAA., and SHUMWAY G. (1958) Deep-sea bathymetry north of Pt. Barrow. Deep-Sea Res. 5, 1-6. GREGORY A. F., MORLEY L. W., and BOWERS M. E. (1961) Airborne geophysical reconnaissance in the Canadian Arctic Archipelago. Geophysics, 26, 72%737. HUNKINS g., HERREN T., KUTSCHALE H., and PETER G. (1962) Geophysical studies of the Chukchi Cap, Arctic Ocean. J. geophys. Res. 67, 235-247.
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KUTSCHALEH., THIEL E., D'ANDREA D., HUNKINS K., and OSTENSO N. (1963) A long refraction profile on the Arctic continental shelf. Abstracts of papers, vol llI, XlII General Assembly IUGG, Berkeley. MATTHEWSD. J. (1939) Tables o f the velocity o f sound in pure water amt sea water ]or use in echo-sounding and sound-ranging. Second edition, Hydrographic Department, Admiralty, London. H.D.282, 52 pp. OSTENSO N. (1963) Geomagnetism and gravity of the Arctic Basin. In : Proc. Arctic Basin Symposium, Oct. 1962, Arctic Inst. N. Amer., Washington, D.C. 9-45. Soiov M. M. (1955) ed. Observational data of the scientific-research drifting station of 1950-1951, v. I-III, Morskoi Transport 1954-1955. (Trans. by the Amer. Met. Soc., ASTIA Doe. 117-133). TALWANI M., SUTTON G., and WORZEL J. (1959) A crustal asection across the Puerto Rico Trench. d. geophys. Res. 64, 1545-1555. TALWANIM., WORZELJ. L., and LANDISMANM. (1959) Rapid gravity computations for two-dimensional bodies with application to the Mendocino Submarine Fracture Zone. J. geophys. Res. 64, 49-59. TALWANI M., and HEIRTZLERJ. (1964) Computation of magnetic anomalies caused by two-dimensional structures of arbitrary shape. In : Computers h~ the Mineral Industries, ed. G. A. Parks, Stanford Univ., 464--479. GEOLOGICAL MAP of the Arctic. First International Symposium on Arctic Geology, Alberta Soc. Petroleum Geol., 1960.
Arctic Ocean geophysical studies: Chukchi Cap and Chukchi Abyssal Plain* RALPH SHAVER AND KENNETH HUNKINS Lamont Geological Observatory, Columbia University, Palisades, New York (Received 17 June 1964)
Abatract--A bathymetric chart of the Chukchi Cap region was compiled with soundings obtained from Fletcher's Ice Island (T-3), as well as from other ice stations and from U.S. Navy icebreakers. New details of the Chukchi Cap are shown, including two submarine troughs on the southwest side. West of the Chukchi Cap, a small abyssal plain was found with a depth of 2230 m. This abyssal plain is connected through an abyssal gap with the deeper Canada Abyssal Plain. The promifient magnetic anomaly discovered during the drift of Station Charlie was crossed more recently by T-3 and by aeromagnetic flights. The continuity of the anomaly along the western and northern sides of the Chukchi Cap was further established by the new measurements. An interpretation was made of the anomaly as an expression of induced magnetization in basement rocks. The interpretation shows a basement ridge beneath the anomaly maximum at the edge of the Chukchi Cap. The Cap itself is interpreted as being underlaid by a 12 km thickness of sediments. Both magnetic and gravity data were used for an interpretation of total crustal thickness along the same section. Crustal thickness ranges from 18--~km beneath the Chukchi Cap to 32 km beneath the large basement ridge. INTRODUCTION Tim Chukchi Cap and Chukchi Abyssal Plain are prominent submarine features located in the Arctic Ocean about 1200 k m north of the Bering Strait. The Chukchi, Cap was discovered during the drift of the Soviet ice station, NT-2, in 1950 and 1951 (SoMOV, ed., 1955). Further sounding and magnetic data over the cap were provided by U.S. Ice Station Charlie during its drift in 1959 (HuNKrNS et al., 1962). Recent sounding, gravity and magnetic data in the vicinity of the cap which were taken from Fletcher's Ice Island (T-3) during 1962 are reported here. The drift of T-3 between M a y 12 and September 23, 1962 was particularly favourable for study of the Chukchi Cap (Fig. 1). T-3 has been in nearly continuous use as a United States scientific drifting station since 1952. In M a y 1960 the ice island, which is about 50 m thick, became grounded on the Alaskan continental shelf at 71 ° 55'N, 160 ° 20'W. When it appeared unlikely that T-3 would become free again, the station was vacated in October 1961. However, sometime in late 1961 or early 1962, T-3 did float free and resumed its drift in the clockwise gyre of the Canada Basin. It was resighted on February 16, 1962 from an airplane of the Arctic Research Laboratory of Point Barrow, Alaska. On the following day, a three-man party from the Arctic Research Laboratory was landed on T-3 to reestablish it as a scientific drifting station. During the spring, T-3 was *Lamont Geological Observatory Contribution No. 752. 905
906
RALPH ShAVeR and Ir~NNt~n HUNKINS
then resupplied and reorganized. By May the ice island was in full operation as a scientific station after the authors had begun a geophysical program. Navigation, soundings, magnetics and gravity were included in the Lamont Geological Observatory program.
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50O KI LOMETERS
Fig. 1. Drift track of Fletcher's Ice Island (T-3)between May and September, 1962. Contours based on Geological Map of the Arctic (1960).
NAVIGATION
The position of T-3 was determined by solar fixes during the summer period covered here. The usual overcast conditions in the high Arctic prevailed, allowing only 91 fixes during the 135 days studied. The fixes were determined to within a few tenths of a mile. During long periods between fixes, interpolated positions may be in error by several miles. During the summer of 1962, T-3 drifted in a northerly direction along a track somewhat similar to that of Soviet NP-2 in 1950 and 1951. Between May 12 and September 23, 1962, T-3 drifted a total of 1388 km. The rate of drift was 10.35 km/day. This rapid rate may be attributed to the open ice conditions of that summer. Ice-breaker activities that fall revealed how open the pack ice was. In September 1962 the icebreaker U.S.S. Burton Island penetrated to T-3 when it was located at 78 ° 56'N. This is a record northern penetration for icebreakers in this sector of the Arctic Ocean.
Arctic Ocean geophysicalstudies : Chukchi Cap and Chukchi Abyssal Plain
907
DEPTH SOUNDINGS A total of 368 spot echo soundings were made between May 12 and September 23, 1962. The soundings were taken from the pack ice off the edge of T-3. Explosives ranging from detonator caps to 1 lb. charges were used as sound sources. The echo was received with a geophone on the ice surface and recorded with an ink-writing oscillograph operated at 125 mm/sec. The echo time can be determined to within
Fig. 2. Drift tracks and icebreakertracks providing bathymetriccontrol in the vicinity of the
Chukchi Cap.
1 millisec from the record. In reducing the data, corrections were made for the speed of sound in water but not for bottom slope (MA~t~ws, 1939). The T-3 soundings were used along with data from all other available sources to compile a bathymetric chart of the Chukchi Cap and vicinity. The track chart (Fig. 2) shows the distribution of soundings from drifting ice stations and ice breakers. The western gide of the cap is reasonably well sounded. The eastern side and the saddle
908
RALPH S~vlu~ and K E t ~ H HUNKINS
connecting the Cap and the shelf are not so well known. The bathymetric chart (Fig. 3) covers the Chukchi Cap, the Chukchi Abyssal Plain and a portion of the Chukehi continental shelf.
................
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Fill, 3. Balhymeiric sketch of the Chukchi Cap and Chukchi Abyssal Plain, Contour interval 500 m except for certain supplemental contours, Dotted lines indicate lack of data.
The Chukchi Cap is roughly 150 km in diameter at the 500 m depth contour. The shallowest sounding recorded was 246 m. On the southwest side, the cap is indented by two troughs. The larger of the troughs is 75 km long and 200 m deep. The top of the Cap is marked by roughness 5 to 30 m in height (HtrNKINS e t a l , , 1962). The Chukchi Abyssal Plain is bordered by the Chukchi Cap, the continental shelf and the Alpha Ridge. On the north it opens to the Canada Abyssal Plain. All available soundings within the basin are plotted in Fig. 3. They are sparse but they indicate a flat floor with a depth of about 2230 m. A final decision as to whether this is a true abyssal plain must await a crossing with a precision depth recorder. Meanwhile, the soundings, the location at the foot of the continental rise, and the gap to the north all suggest that it is a true abyssal plain. The abyssal gap on the northern edge of the abyssal plain was crossed several
Arctic Ocean geophysical st~dies : Chakchi Cap and Chvkchi Abyssal Plain
909
times by Drifting Station Charlie and precision depth records were obtained. Four profiles are shown in Fig. 4. The gap width across the flat floor varies between 0.6 and 2"7 km. The width measurement is subject to some error since the drift rate at the time of crossing is only known approximately. The floor of the gap drops 114 m between the first and last profiles of Fig. 4, a distance of 55 km. This is an average gradient of 1 : 500. In plan the gap follows closely the base of the Chukchi Cap (Fig. 3). It is likely that the location of the gap is structurally controlled and that the floor is sediment covered. Turbidity currents flowing from the Chukchi Abyssal Plain evidently pass through the gap to the deeper Canada ['/'2eW
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Fig. 4. Bathymetric profiles across the Charlie Gap. Note that profiles are taken directly from
precision depth records and that the horizontal scales are variable due to different drift rates. Dotted lines indicate proper projection onto the map scale. Abyssal Plain. The gap forms a connection between the upper 2230 m level o f the Chukchi Abyssal Plain and the lower 3790 m level of the Canada Abyssal Plain to the north. A similar case of a small abyssal plain and accompanying gap has been found by KUTSCHALE(private communication) in the Central Arctic Basin. In that case the upper Wrangell Abyssal Plain is connected through the Arlis Gap to the lower Siberia Abyssal Plain. The usual practice of naming submarine features after nearby geographical regions has been followed. The Chukchi Cap and Chukchi Abyssal Plain have been named after the Chukchi Sea which lies to the immediate south. The Chukchi are a native people of northern Siberia. The Charlie Gap has been named after
910
Rm2n SI~WR and K~NNmmHUNKINS
Ice Station Charlie in agreement with the practice of naming abyssal gaps for the discovering ship. In this case, Station Charlie was the equivalent of a discovering ship. MAGNETICS
A prominent magnetic anomaly exists along the western margin of the Chukchi Cap. This anomaly was crossed several times during the drift of Station Charlie (HuNKINS et al., 1962). More recent observations from airplanes (OSTENSO, 1963) and from T-3 bare further confirmed the existence of this feature and provided new details. All available magnetic profiles are shown in Fig. 5 and the tracks are shown in Fig. 6. The trend of the anomaly peak is also indicated in Fig. 6. The T-3 data were taken with a portable Varian proton-precision magnetometer which was usually
H\ / /
/l\
K ~
_j--/
TEI~:S
Fig. 5. Total magnetic intensity profiles across the western side of the Chukchi Cap. Drifting Station Arlis II, A; Drifting Station Charlie, B, C and E; Fletcher's Ice Island (T-3) D and G;
aeromagneticprofiles(OsT~SSO,1963) F and H.
read every hour with a precision of ± I0 gammas. Daily averages were made in order to eliminate diurnal fluctuations. The aeromagnetic profiles were flown at an altitude of 450 m with a Varian proton precession magnetometer which recorded continuously. The new data confirm that the anomaly is a continuous feature closely following the depth contours along the edge of the cap. The anomaly peak occurs over the slope where water depths range between 700 and 2500 m. On profiles B to E, the
Arctic Ocean geophysical studies : Ch~tkchi Cap and Chukchi Abyssal Plain
911
peak follows the depth contours especially closely,, occurring regularly over depths between 1000 and 1500 m. The amplitude of the anomaly ranges between 500 and 1500 gammas from peak to trough.
1170"W
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6.
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Trend of maximum magnetic anomaly values (triangles). Tracks refer to magnetic profiles in Fig. 5. Bathymetriccontour interval, 500 m.
The manner in which the anomaly follows the submarine topography indicates that it reflects structural trends, but the shape and position of the anomaly do not allow an explanation solely in terms of simple topographic effects in uniformly magnetized rocks. The anomaly must be caused by rocks of varying magnetization within the crust which follow the topographic trends. Although it could be caused by a belt of highly magnetic material such as a basic intrusion, it is most probably due to a combination of induced and permanent magnetization in basement rocks with high susceptibilities. Such a situation is found off the east coast of the United States where both good magnetic and seismic data are available for interpretation (DRAKe et al., 1963). The Chukchi Cap anomaly most closely resembles the eastern United States anomaly in the region north of Cape Hatteras. In that region DRAK~ et al., 1963 found it possible to make a fairly good fit to the observed magnetics by
912
RALPH SHAWa and K E ~ r H HtmK~S
assuming uniform magnetization of the basement layer as determined by the seismic method. DRAKEet al. (1963) assumed a susceptibility of zero for the sediments and 0.01 e.m.u, for the basement rocks. In the case of the Chukchi Cap, no seismic information is available. Since similarity of the Atlantic coast and Chukchi Cap anomalies suggested similar origins, an attempt was made to find the basement topography beneath the Chukchi Cap by fitting the observed magnetic anomaly. Susceptibilities were the same as those assumed by DRAKEet al. (1963), I The strike of the body, magnetic intensity and inclination were chosen to agree with actual conditions. The magnetic inclination is so dose to the vertical in this area that changes in the
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Fig. 7. Observed total magnetic intensity over the edge of the Chukchi Cap compared with total intensity calculated for the crustal model shown in cross section. Observed lines a r e C and D (Fig. 5). strike of the body have little effect on the calculated anomaly. The anomalies were calculated on a high speed digital computer with a program for polygons of arbitrary shapes (T^LwANI and I-IEmTZL~g, 1964). The inferred basement topography beneath the Chukchi Cap and both observed and calculated magnetic profiles are shown in Fig. 7. A sediment thickness of 12 km is inferred beneath the Chukchi Cap. A basement ridge is inferred beneath the foot of the slope. From the results on the Atlantic coast by DRAKE et al. (1963) it can be seen that the basement as inferred from the magnetic field, as was done here, would have greater
Arctic Ocean geophysical studies : Chukchi Cap and Chukchi Abyssal Plain
913
relief than that found by seismic methods. It is likely that remanent as well as induced magnetization is often present and that this often makes the anomalies larger than would be expected for a purely induced field. Thus the basement topography calculated for the Chukchi Cap is probably exaggerated. The actual relief is probably similar in shape but somewhat subdued. A seismic refraction survey of the Chukchi continental shelf south of the Cap found the sediment thickness one-half that deduced for the Chukchi Cap (KtrrsCHALe et al., 1963). At 74-1/2°N 165°W the 3 km/sec layer, which is presumably the sediments, was 6 km thick. Underlying the sediments at that location was a basement layer with a velocity of 4.9 km/sec. One difference between the basement configuration found on the Atlantic coast and that inferred for the Chukchi Cap is evident. Both areas have a basement ridge beneath the continental slope which separates two sedimentary basins, one beneath deep water and one beneath the shelf. On the Atlantic coast the sediment basin on the deep-water side usually contains the greater sediment thickness but on the Chukchi Cap, the sedimentary basin beneath the shallow water contains more sediments. GRAVITY
A gravity profile over the Cap was obtained from T-3 during 1962. A LaCosteRomberg Model G gravity meter was read four times daily. Ties to the University of Wisconsin ~endulum station at Point Barrow were made on 7 May, on 4 June
5O
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Fig. 8. Observed free-air gravity section from T-3 across the edge of Chukchi Cap compared with free-air gravity calculated for the crustal model shown in cross section.
914
RALVH SHAVERand KENN~Xtt HUNKINS
and on 5 October, 1962. The difference between the ties on 4 June and 5 October, 1962 was 0.73 milligals. The gravity profile across the Chukchi Cap along the T-3 track is shown in Fig. 8. The free-air anomaly is about 4- 50 milligals over the Cap and approaches zero in the deep water regions. Near the south side of the Cap the topographic effects of the submarine troughs are apparent in the gravity values. The simple Bouguer anomaly over the Cap is between -~ 70 and -4-90 milligals except for the same topographic effects on the south side. The Bouguer anomaly is ÷ 200 milligals over the basin to the north and -~- 160 miiligals over the basin to the south. A crustal model based on both the magnetic and gravity data was calculated across the edge of the Chukchi Cap. The calculations were made with a high-speed digital computer using a program based on the method of TALWANI et al., (1959). The basement topography as calculated from the magnetic results was used in computing the two-dimensional gravity model. It was assumed that sediments with an average density of 2-4 g/co occur above the basement. This rather high average value for sediments was chosen on the basis of the large sediment thickness and the probability of compaction and consolidation at depth. The average density of the deep crust below the sediments was assumed to be 2.9 g/cc. The upper mantle density was taken as 3.4 g/cc. This mantle density coriesponds to an 8.2 km/sec compressional velocity (TALWANI et al., 1959). With these values fixed and with the ocean bottom and basement topography already determined, it was possible to adjust the shape of the Mohorovi~i6 discontinuity so that the calculated gravity agrees reasonably well with the observations. At either end, the section was assumed to have the equivalent balance of a crustal column with a thickness of 32 km and an average density of 2.87 g/cc. overlying a mantle with a density of 3.4 g/cc. The calculated crustal structure is shown in Fig. 8. The crustal thickness is 18-1/2 km beneath the Chukchi Cap, 32 km beneath the basement ridge and 21 km beneath the Chukchi Abyssal Plain. It must be realized that a number of assumptions are made in calculating this crustal model. The density values were chosen to agree reasonably well with results of other studies. The assumption that the feature may be treated as two-dimensional also seems to be in good agreement with the facts. On this basis, the model may be considered a first approximation to the crustal structure in this area with the details still subject to considerable refinement. DISCUSSION
Although the data are still fragmentary, it is tempting to speculate on the origin of the Chukchi Cap as revealed by the information available so far. The Chukchi Cap is apparently composed of a thick sequence of sedimentary rocks. This indicates that the cap was formerly a part of the continental shelf. Such a sequence of sediments is usually found on continental shelves where a source area exists inland. The present continental shelf north of Alaska is built of such a sediment sequence. In its present position, the Chukchi Cap has no readily available sediment source. If the Chukchi Cap has been a part of the continental shelf, it should be possible to point out its original site and the path by which it was moved to its present location. The most probable original site lies along the Alaskan continental shelf east of Point Barrow (Fig. 1). Along this stretch of coast the present continental
Arctic Ocean geophysical studies : Chukchi Cap and Chukchi Abyssal Plain
915
shelf is relatively narrow although in most other areas the Arctic continental shelves are extremely wide. The motion of the cap in its removal from the shelf would have been a quarter-turn counter-clockwise about a pivot where the Chukchi Cap presently joins the shelf at 75°N 165°W. The eastern side of the Chukchi Cap would formerly have been joined to the present continental shelf. The western side of the Chukchi Cap, which resembles present-day shelf structure on the Atlantic and G u l f coasts, would have been the continental margin at that earlier period. Other features also tend to support this hypothesis. N o pronounced magnetic anomalies are found across the continental shelf immediately cast of Point Barrow in the region in which the Cap would have originated. It would strengthen the hypothesis if the characteristic Chukchi Cap anomaly were found again along the coast further to the northeast off the Canadian Arctic Islands. There are some indications of such an anomaly but the results are still inconclusive (GREGORY et al., 1961). Few data are available along the eastern side of the Chukchi Cap, but it appears that no distinctive magnetic anomaly is found in that area. The magnetic field is rough but apparently does not have the unique type of anomaly found on the western side of the Cap. Also, the area near the assumed pivot point is topographically rough and could be interpreted as an area which has been subject to tension and shear. This area near the pivot has been called a "continental b o r d e r l a n d " by FISHER et al., (1958). It contains the so-called Northwind Seahigh. The history of hypotheses on the origin of the Arctic Basin has been reviewed by EARDLEY (1961). The concepts fall into two general categories, those invoking vertical subsidence and those invoking horizontal movements. The ideas presented here on the movement of the Chukchi Cap would support those hypotheses invoking horizontal movements, such as the " expanding earth " theories of CAREY and o f HEEZEN.
Acknowledgment--We are indebted to the director, MAx BP~WEa, and to the personnel of the Arctic Research Laboratory, Barrow, Alaska, for their initiative in reestablishing T-3 as a scientific drift station and for their spirit of cooperation in all aspects of the field work. JAMESALBEPdNO (Lament Geological Observatory) assisted in field operations. HENRY KtrrSCHALE (Lament Geological Observatory) furnished depth data from. Arlis II. THEOOOgEWAMnACHof the U.S. Navy Underwater Sound Laboratory made available his navigational data which were used in constructing the T-3 drift track (Fig. 2). This work was supported by the Ottic¢ of Naval Research as a part of Project Vela Uniform, sponsored by the Advanced Research Projects Agency, Department of Defenc¢. REFERENCES
DEN HARTOG S. L. and OSTENSON. (1962) Gravity and magnetic observations from Ice Island Arlis II - 1 July to 6 October 1961 (unpublished manuscript). DRAKE C. L. HEmTZLER J., and HIRSHMANJ. (1963) Magnetic anomalies off Eastern North America. J. geophys. Res. 68, 5259-5275. EARDLEY A. J. (1961) History of geologic thought on the origin of the Arctic Basin. In : Geology o f the Arctic, Prec. of the First International Symposium on Arctic Geol. Univ. of Toronto Press, Vol. 1,605-621. FISHER R., CARSOLAA., and SHUMWAY G. (1958) Deep-sea bathymetry north of Pt. Barrow. Deep-Sea Res. 5, 1-6. GREGORY A. F., MORLEY L. W., and BOWERS M. E. (1961) Airborne geophysical reconnaissance in the Canadian Arctic Archipelago. Geophysics, 26, 72%737. HUNKINS g., HERREN T., KUTSCHALE H., and PETER G. (1962) Geophysical studies of the Chukchi Cap, Arctic Ocean. J. geophys. Res. 67, 235-247.
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RALPH SHAVERand KENNETHHUNKINS
KUTSCHALEH., THIEL E., D'ANDREA D., HUNKINS K., and OSTENSO N. (1963) A long refraction profile on the Arctic continental shelf. Abstracts of papers, vol llI, XlII General Assembly IUGG, Berkeley. MATTHEWSD. J. (1939) Tables o f the velocity o f sound in pure water amt sea water ]or use in echo-sounding and sound-ranging. Second edition, Hydrographic Department, Admiralty, London. H.D.282, 52 pp. OSTENSO N. (1963) Geomagnetism and gravity of the Arctic Basin. In : Proc. Arctic Basin Symposium, Oct. 1962, Arctic Inst. N. Amer., Washington, D.C. 9-45. Soiov M. M. (1955) ed. Observational data of the scientific-research drifting station of 1950-1951, v. I-III, Morskoi Transport 1954-1955. (Trans. by the Amer. Met. Soc., ASTIA Doe. 117-133). TALWANI M., SUTTON G., and WORZEL J. (1959) A crustal asection across the Puerto Rico Trench. d. geophys. Res. 64, 1545-1555. TALWANIM., WORZELJ. L., and LANDISMANM. (1959) Rapid gravity computations for two-dimensional bodies with application to the Mendocino Submarine Fracture Zone. J. geophys. Res. 64, 49-59. TALWANI M., and HEIRTZLERJ. (1964) Computation of magnetic anomalies caused by two-dimensional structures of arbitrary shape. In : Computers h~ the Mineral Industries, ed. G. A. Parks, Stanford Univ., 464--479. GEOLOGICAL MAP of the Arctic. First International Symposium on Arctic Geology, Alberta Soc. Petroleum Geol., 1960.