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AN ABSTRACT OF THE THESIS OF

MIAH ALLAN BEAL for the Doctor of Philosophy (Name) (Degree) in presented on 12.1968 (Major) (Date) Title:Batymety and_Strictuof_thp..4rctic_Ocean Redacted for Privacy Abstract approved: John V.

The history of the explordtion of the Central is reviewed.It has been only within the last 15 years that any signifi- cant number of depth-sounding data have been collected.The present study uses seven million echo soundings collected by U. S. Navy nuclear along nearly 40, 000 km of track to construct, for the first time, a reasonably complete picture of the physiography of the basin of the .The use of nuclear submarines as under-ice survey ships is discussed. The physiography of the entire and of each of the major features in the basin are described, illustrated and named. The dominant ocean floor features are three mountain ranges, generally paralleling each other and the 40°E. 140°W. . From the Pacific- side of the Arctic basin toward the Atlantic, they are: The Alpha Cordillera; The ; andThe Nansen Cordillera. The Alpha Cordillera is the widest of the three mountain ranges. It abuts the continental slopes off the Canadian and off across more than550of on each slope.Its minimum width of about 300 km is located midway between and Asia.In section, the Alpha Cordillera is a broad arch rising about two km, above the floor of the basin.The arch is marked by volcanoes and of "high fractured plateau,

and by scarps500to 1000 meters high.The small number of data from , heat flow, magnetics and gravity studies are reviewed.The Alpha Cordillera is interpreted to be an inactive mid-ocean ridge which has undergone some subsidence. The Lomonosov Ridge is a narrow mountain range which extends from the continental slope off the Lincoln to the Eurasian slope off the .In the middle of the basin, the Lomonosov Ridge is only 40 to 75 km wide.It becomes broader at the continental margins, where it merges with the Alpha Cordillera. In cross section, the Ridge has side slopes convex upward to a rounded crest.The minimum water depth on the Lomonosov Ridge is about 750 meters.It has a relief of about 3,200 meters above the floor of the basin.Aeromagnetic data indicates that the Ridge is not of volcanic rock and distribution data show that it is aseismic.The Lomonosov Ridge is interpreted to be a continental outlier in the Arctic basin. The Nansen Cordillera is an extension of the Mid-Atlantic Ridge across the Arctic basin.It extends from the passage between and (The Trough) to the Eurasian continental slope off the Lena River.Its axis lies on a great circle between these two points.In cross section, the Nansen Cordillera is about 200 km wide.It is made up of volcanoes or ridges standing about 1500 meters above the basin floor.There are V-shaped rifts in the Cordillera which are 1500 meters deeper than the basin floor (to 5500 meters water depth).Earthquake study shows that essentially all epicenters in the Arctic basin are in the Nansen Cordillera. Aeromagnetic data indicate that its rocks are only weakly magnetic. The Nansen Cordillera is interpreted to be an'embryonic" mid- ocean ridge.The basin floor is being rifted, but no uplift has occurred as yet. The three mountain ranges divide the Arctic basin into four sub-basins, each floored by a plain. From the Pacific-side of the Arctic basin these are:1) The Basin, bounded by the conti- nental slopes of and and the Alpha Cordillera and floored by Canada Plain; 2) The Makarov Basin, bounded by the Alpha Cordillera and the Lomonosov Ridge and floored by Fletcher Plain; 3) The Basin, bounded by the Lomonosov Ridge and the Nansen Cordillera and floored by Hakkel' Plain; and, 4) the Nautilus Basin, bounded by the Nansen Cordillera and the continental slope of and floored by the Sverdrup Plain.Each of these plains is close to 4000 meters deep.Echograms show that they have slopes of only about 1:4000, but minor 'bumps" and swales of some ten meters relief are also shown. There are three regions of a few hundred meters depth on the margin of the Arctic basin.These are called the Arctic Rises.The largest of these is the Chukchi Rise, which appears to be an exten- sion of the between Alaska and Siberia.The other two rises flank the Lena Trough.The Morris Jesup Rise, north of Greenland, appears to be a part of the Lomonosov Ridge.The Rise is an extension of the north of Svalbard. Two plains at intermediate depth (2000 to 3000 meters) are located on the flanks of the Alpha Cordillera at its intersection with the continental slope of .These are named the Chukchi Plain and the Wrangel Plain.Each appears to be ponded . The Lomonosov Ridge is taken as a boundary to divide the Arctic basin into two major sub-basins.The Fram and Nautilus Basins together make up the Eurasia Basin.The Makarov and Canada Basins together form the Amerasia Basin. The continental slopes and rises around the margins of the Arctic basin are described and illustrated.The slopes generally are steep and broken and the rise poorlydeveloped except for the section off the Mackenzie River and north of Alaska. Geophysical data from the Arctic basin are few and do not provide good geographical coverage.The data are reviewed and related to the physiography of the basin.It is hypothesized that the entire floor of the Arctic basin formed on the Alpha Cordillera following fracturing of a continent by mantle upwelling.Following formation of the basin by sea floor spreading, the cell centered beneath the Alpha Cordillera ceased to convect and is now in a cooling phase. A new phase of mantle is now thought to be begin- ning beneath the Nansen Cordillera. The geographical arrangement of the ocean floor features in the Arctic basin is discussed.It is suggested that the Arctic Rises and the Lomonosov Ridge are composed of fragments of the Arctic Fold Belts.This in turn indicates that the continental block was fractured in the Paleozoic Fold Belts rather than in the Shields. Possible former extensions of the mantle convection cell, which is thought to have existed beneath the Arctic basin, are discussed.Evidence is presented for a "circular" termination of the cell at the Eurasian end of the Alpha Cordillera.Toward the Atlantic, the axis of the former convection cell is traced through and into the Sea. A farther extension to an intersection with the Mid-Atlantic Ridge at 28° N. Lat.is suggested.The possible size and extent of the former convection cell is discussed and the name Wegener Rise proposed for its manifestation in the 's crust.The time of the formation of the Arctic basin is considered, although there are no unique data for age dating avail- able from the basin itself. and Structure of the Arctic Ocean

by Miah Allan Beal

A THESIS submitted to Oregon State University

in partial fulfillment of the requirements for the degree of Doctor of Philosophy

June 1969

APPROVED:

Redacted for Privacy

Professo\ of Oceanography \ in charge of mjOr

Redacted for Privacy

Chairm n of the Departmeof Ocean?py

Redacted for Privacy

Dean clf graduate School

Date thesis is presented August 12, 1968 Typed by Marion F. Palmateer for Miah Allan Beal AC KNOW LE DGEME NT

I am deeply indebted to many people for assistance in the col- lection of the data upon which this study is based.Above all, great credit must be given to the officers and ratings of the nuclear sub- marines: The men who probed into the unknown regions beneath the ice pack to gather knowledge where none had existed before. A debt to Dr. Waldo K. Lyon, Civilian Research Director for the Arctic expeditions is gratefully acknowledged.It was due to his careful planning that a reonaissance of all parts of the Arctic basin was accomplished.Mr. Arthur Molloy participated in most of the submarine cruises and in the processing of the echo- grams.His contribution to this study cannot be over emphasized. I have profited greatly from discussions with Dr. Ned A. Ostenso, Dr. Arthur Lachenbruch and Dr. Kenneth Hunkins, three of the foremost students of the Arctic basin.Each has been generous with his thoughts and with the unpublished data referenced in this study.The interpretation of these data is, of course, entirely my responsibility. Dr. John V. Byrne has guided this research and has had con- siderable influence on its content.Under his guidance, I have been encouraged to avoid the easy way and to enter controversial regions at the forefront of geological knowledge in the data interpretation. This freedom and encouragement are much appreciated, but it is even more incumbent than usual on the writer to stress that he is solely responsible for any errors in judgement in the study. Finally, I am privileged to acknowledge the debt that I owe to my wife, Phyllis, for her support and encouragement.She has as cheerfully lived with me on the shore of the Arctic Ocean as in San Diego, and has struggled alone with home and hearth while I was on board and submarines for months at a time in carrying out my studies. TABLE OF CONTENTS Page

INTRODUCTION Exploration of the Central Arctic

METHODOF STUDY 18

PHYSIOGRAPHY OF THE FLOOR OF THE ARCTIC OCEAN 29

Geographic Names 29 Geography 31 The Alpha Cordillera 32 Geophysical Data 39 The Lomonosov Ridge 47 The Nansen Cordillera 57 The Plains 69 The Continental Margins 75 Summary of Geophysical Data 97 Physiography Related to Possible Structure of the Crust and Upper Mantle 109 AN HYPOTHESIS FOR THE ORIGIN OF THE ARCTIC OCEAN 132

BIBLIOGRAPHY 175

APPENDIX 188 LIST O FIGURES Figure Page

1 Geographic names, the floor of the Arctic Ocean 7

2 Locations of the echograms used in this study 19

3 Average acoustic velocities in the Arctic Ocean and water depth as a functioi of travel time 23

4 Bathymetry 28

5 The Alpha Cordillera 34

6 The Lomonosov Ridge 48

7 The Nanseti Cordillera 59

8 The Lena Trough 60

9A The Arctic Rises 77

9B The Arctic Rises 78

10 The Continental Slopes and Rises 86

11 Summary of geophysical data 98

12 gradients after convection ceased 121

13 Paleozoic Orogenic Belts 134

14 Schematic of the formation of the Arctic basin A.Initial fractures in the continental block 141 B. An early stage of sea floor spreading 141 C.The Arctic basin just prior to cessation of mantle convection and sea floor spreading 143 D.Present configuration of the Arctic basin 143

1 5 Bathymetric features and islands in the Chukchi and northern Bering 147 LIST OF FIGURES (continued) Figure Page

16 The Wegener Rise 149

17 Evidence for former extension of the Alpha Cordillera into the North Atlantic 1 50

Appendix Figures

18 Track of the NAUTILUS 1957 expedition 189

19 U.S.S. NAUTILUS 1957 190

20 Track chart of the NAUTILUS 1958 expedition 191

21 TJ.S.S. NAUTILUS 1958 192

22 Track chart of the SKATE 1958 expedition 193

23 U.S. S. SKATE 1958 194

24 Track of SKATE 1959 expedition 195

25 U.S.S. SKATE 1959 196

26 Track of SARGO 1960 expedition 197

27 U.S.S. SARGO 1960 198

28 Track of SEADRAGON 1960 expedition 199

29 U.S.S. SEADRAGON 1960 200

30 Track of SKATE 1962 expedition 201

31 U.S.S. SKATE 1962 202

32 Track of SEADRAGON 1962 expedition 203

33 U.S.S. SEADRAGON 1962 204 Page

)c ean LIST OF PLATES

Plate Page

1 The floor of the Arctic Ocean 205

2 The three-dimensional fence model of depth soundings in the Arctic basin 27

3 Sea-floor photographs on the Alaskan continental rise 88

4 Location of faults and the margin of the convection cell 145

5 The islands in and the location of the line discussed in the text 172 BATHYMETRY AND STRUCTURE OF THE ARCTIC OCEAN

INTRODUCTION

Exploration of the Central Arctic

The Arctic regions of the Earth have long held a fascination for explorers and scientists of many lands.In addition to being men of daring, the explorers who discovered the Arctic were prolific writers.Arctic Bibliography (Tremaine, 1953 et seq. ) now includes more than 30, 000 separate titles in 13 volumes.Despite the numerous voyages of exploration, the nature of the Central Arctic was completely unknown only 70 years ago.As late as 1897, 3. W. Gregory writing in "Natur& hypothesized that the Central Arctic must be a shallow sea dotted with islands.This shallow sea idea was held by so many distinguished explorers of the time that, as we shall see later, it convinced even the great . It is not possible, and not necessary, to detail the hundreds of expeditions to the Arctic regions carried out in historic times.The permanent cover of drifting ice in the Central Arctic is even today impenetrable by the most powerful surface ships.The sailing vessels and primitive steamers of the last century were capable of only the most limited exploration in the fringes of the Arctic ice pack. Thus nearly all of the voyages of were concerned with the a

Arctic coastlines and islands and none shed any light on the nature of the Central Arctic. By the last quarter of the 19th century, the coasts of Greenland, North America and Eurasia had been charted and men turned their attention to the conquest of the .By the turn of the century, two expeditions had been accomplished which were of im- portance to an understanding of the geography of the Central Arctic. The first of these was the ill-fated voyage of the JEANNETTE

under the command of Lieutenant George DeLong.The JEANNETTE entered the Arctic by way of Bering Strait in the summer,

1879.She was beset in the ice at 7135' N. Lat. ,17506'W. Long. near and drifted westward and northward for two years. In early summer, 1881, the JEANNETTE was crushed in the ice near the New Siberian Islands.DeLong and his men abandoned the wreck and trekked over the ice pack to open water where three boats were launched.One boat was swamped and all hands lost.The re- maining two boats became separated, but both reached the Lena Delta. DeLong and all but two men of one boat perished of starvation, while the others were saved by natives.The narrative of the expedition and the record of its discoveries were saved by DeLong and found with his body when a rescue party located their camp (see DeLong,1883). The eminent German geographer, Dr. Petermann, had supposed that "Wrangel Land" stretched across the Pole and was continuous with Greenland.It was one of the purposes of the JEANNETTE expe- dition to explore this coastline (DeLong, 1883 p. 49).The drift of the JEANNETTE (op.cit. map packet vol.1) proved that "Wrangel Land" was an island.The discovery of three islands of the New Siberian Islands and the wire soundings taken during the drift

(op.cit., p. 566) demonstrated the great width of the Siberian conti- nental shelf.These were important findings. In 1884, three years after the JEANNETTE had been abandoned, debris from a ship was found on the southwest coast of Greenland. Seamen's clothing with name tags left no doubt that this flotsam was from the JEANNETTE. The famous Norwegian scientist, Professor H. Mohn, hypothesized in a newspaper report that the debris had been carried across the Arctic by a sea-current (see Sverdrup, 1950 and Nansen, 1897, p.14).This hypothesis led the Norwegian explorer Fridtjof Nansen to suggest that a small ship designed to withstand external pressures could be frozen into the icepack and used to, investigate the great unknown that surrounds the Pole,

(Nansen, 1897, p.37). Nansen carried out his plan and the polar ship FRAM was frozen into the ice north of the New Siberian Islands in 1893.The FRAM expedition was well equipped for its time, but the "shallow-sea" con- cept was so firmly believed that Nansen had provided wire rope for sounding to only 1900 meters (Nansen, 1897, p. 464).Shortly after 4 being beset in the ice FRAM drifted off the continental shelf and the sounding line could no longer reach bottom. A wire rope forestay from FRAM's rigging was unlayed and a long sounding line fabricated on a rope walk set up on the ice.This line was so fragile and difficult to handle that only 38 deep soundings were attempted during the three year drift and only seven of these reached bottom.However, the discovery of the Arctic Ocean dates from these soundings, since they showed at least part of the Central Arctic to be 4000 meters deep. Nansen's map of the bathymetry of the Arctic Ocean (Nansen, 1904, map pocket vol. 4) showed a single basin of 4000 meters depth.This map was still being published as recently as 1958.In fact, until the 1930s few new data were collected in the Central Arctic Nansen's map gave an alternate concept to the previously held shallow-sea idea, but Harris (1904 and 1911), studying the decay of the North Atlantic tidal wave along the Arctic coastlines, concluded that there must be land or shallow water in the Central Arctic. Peary reached the North Pole in 1909 without finding land. Amundsen and Lincoln Ellesworth flew across the Central Arctic in 1926 in the dirigible from Spitzbergen to Alaska without finding islands. Two years later, Sverdrup (reported in Marmer, 1928) concluded that the Arctic tidal data could be explained by frictional effects on the floor and slopes of a single deep basin.There the matter rested until 1945 when I.V. Maksimov (1945) suggested on the basis of 5 water , and tidal phenomena that 1000 meter shallows must exist in the Central Arctic.Maksimov's work was not readily available at that time to researchers outside the . Worthington (1953) came to the conclusion independently that a ridge extending from to the New Siberian Islands must exist.North of Alaska, Worthington obtained deep water tempera- tures which were as much as 0. 5C warmer than those reported for the same depths on the other side of the Arctic Ocean.This tempera- ture contrast was most easily explained by a barrier to the deep circulation of the Arctic Ocean. A few soundings were obtained in the Arctic Ocean by various expeditions during the first half of the twentieth century.Peary - tamed a few wire soundings north of Ellesmere Island in shallow water and one ttno bottomTt sounding on the dash to the Pole in 1909. Stefans son carried out several expeditions in the region north of Alaska and west of the Canadian Archipelago from 1913 to 1918.In 1913 his ship KARLUK made nine wire soundings north of Alaska. These soundings did not reach bottom, but proved that the area was not shallow.Stefansson developed techniques for traveling over the ice pack and in 1914 a number of "no bottom" wire soundings were obtained west of Banks Islands (Stefansson, 19Z1, map p.140). George H. Wilkins (later Sir Hubert) pioneered the use of air- planes for Arctic Ocean exploration. On Z9 1927, with Carl B. Eielson as , Wilkins flew 550 miles northwest of Point , Alaska and landed on the ice pack. A sonic depth sounding was made in this unexplored region.The sounding apparatus con- sisted of a hydrophone, battery powered amplifier and headphones, and a stop-watch. A small detonator was exploded and the travel time for the sound to echo from the sea floor was measured with the stop-watch (Wilkins, 1931, p.122).One sounding was made while the plane's engine was running and a doubtful travel time of about 7. 3 seconds recorded. A second sounding was made with engine stopped and a travel time of 7.25 seconds recorded (op. cit.,p.135). A loca- tion 550 statute miles northwest of Barrow is over the Chukchi Plain (Figure 1) with a depth of about 2250 meters.Wilkins' measurement gave a depth of 5400 meters.He must have missed the first echo, recorded the second echo and had some one-half second delay in operation of the stop-watch.Unfortunately, this erroneous sounding was not disproved for 25 years and had a considerable impact on maps of the Arctic basin (for example, see Emery, 1949). In 1928, Wilkins and Eielson flew from Point Barrow to Svalbard.The route was selected to explore for islands in previously unseen regions of the Central Arctic.No islands were seen and, as no landing was made, no soundings were taken (Wilkins, 1931, p. 168). In 1931, Wilkins turned his attention to the use of a submarine for Arctic Ocean research.Using a modified U. S. Navy Submarine u_NI.. F . 7 a E Ni AND S E A !% 0 a n d Figure 1. THE FLOO1 OF THE Al NAME S OCEAN I / GEOGRAPHIC i , ' I N .T.. II if 4 1 , 4Li 14 / / //' I'IU C a H ¶ IN- C 1 \\ ¼J 1 f A I , )., - I .1:;tz,, E Ah'JS7 . I EMF' R -' _ ., .7 r. . I, , * .\ \\ " 1 H .':: : I . kAIfl I HUIF I N 0 N I N TI 0 U '\\-___- - j.. - S d I ) / / - i\ '-c A A 41 C - I - 'T // , : N A D A B - A, S I N / "N11-',- - I " r-.-.. 41 - N 'I 4 - I' U , ,1 - \ I AN 0 E L P - * --,/ E - - - t- - U -'U- -. I' '\ '.. - - - '10 - UN')' St U 4, I' - - - -.- --S / I b I - * N I I C N K c N U '0 A I * I I of World War I vintage, he planned an under-ice crossing of the Arctic Ocean (Wilkins, 1931).His submarine, which was named NAUTILUS, was beset with misfortune from the start.Her overaged engines failed during the Atlantic crossing and she was towed to England for repairs.Later, when the NAUTILUS arrived at the ice edge north of Svalbard, it was found that the stern diving planes had been sheared off.An under-ice journey to the Pole was impossible because of this mishap, but H. U. Sverdrup, chief scientist, and his party, obtained valuable oceanographic data in this previously unex- plored region (Sverdrup, 1933). An echo sounder was carried by NAUTILUS, but it could only record to about 3600 meters.Never- theless the soundings proved the existence of deep water in the region even if the exact depths could not be recorded. In the Soviet Union, research in the Arctic Seas and Ocean be- came national policy after the 1917-1918 Revolution.According to Kienova (1960), founded a bureau to develop an Arctic fishery, and Stalin emphasized development of the across the northern coasts of Eurasia.These efforts required the support of an extensive research organization.The Arctic Scientific Investi- gational Institute in Leningrad has provided this research activity. In 1937, established a camp on an ice floe near the North Pole, using heavy aircraft for transportation.The Papanin station drifted across the Eurasia Basin and along the coast of East Greenland where the men and equipment were taken aboard an (Papanin, 1939).Wire soundings were taken during the drift and many were included in the report (op.cit. ,frontispiece). In 1941, heavy aircraft of the Soviet Union made at least three landings on the ice at 'the Pole of Relative Inaccessibility" in the Arctic Ocean.(That is, the region most distant from land in all directions. )Scientists of the Soviet Union resumed large scale air expeditions into the Arctic Ocean after World War II.By. 1956 more than 500 temporary stations had been occupied in all parts of the Arctic Ocean.Laktionov and Shamonte'v (1q57) have reported on the techniques used and the kinds of observations taken on these "High

Latitude Aerial Expeditions(HLAE). The discovery of a submarine mountain range extending across the Arctic Ocean from Ellesmere Island to the New Siberian Islands was apparently made by the HLAE of 1948 and 1949.Although the reference is obscure, this range is said to have been reported in a Soviet journal in 1954 and named the Lornonosov Ridge.Saks, Belov and Lapina (1955) reported that airborne magnetic surveys indicate that the Verkhoyansk Mountains (Mesozoic age) extend northward under the to the New Siberian Islands and connect with the Lomonosov Ridge. At the time the Soviet Union was establishing the High Aerial Expeditions, Emery (1949) in the constructed a 10 bathymetric chart of the Arctic Ocean using all available soundings deeper than 500 fathoms; a total of 152.Emery's map shows a single basin and, in fact, is little different from Nansen's bathy- metry published in 1904.Analyzing these scanty data, Emery pre- sented several hypotheses which are of historical interest.Because of the belt of earthquake epicenters shown, Emery wrote: The fact that the belt extends from an irregular submarine mountain range [the Mid-Atlantic Ridge] across the poorly known Arctic Basin suggests that the topography of the part of the basin through which the belt passes also be irregular ...The basin is oval shaped, perhaps deepest near Alaska and eastern Siberia, and it may be divided by a structural ridge extending northward from a position between Wrangel Island and the New $iberian Islands...The floor of the basin may be fairly irregular because of the present slow rate of deposition of by all agents. Emery was wrong in his hypothesized region of maximum depth and the shape of the basin because of the 5400 meter sounding recorded by Wilkins in 1927 (discussed previously).He was correct in his other speculations. At this point in reviewing the exploration of the Central Arctic, let us consider the ideas on the original geologic structure which were available up to 1950.The following is summarized from Eardley's (1961) paper on "History of Geologic Thought on the Origin of the Arctic Basin. 11

1860--1910 Permanency of Ocean Basins

The geologists of this era, beginning with J. D. Dana considered the and ocean basins to be permanent features of the earth's crust.Schuchert (1916) expressed the opinion that the conti- nents on a shrinking earth were rising, but that the ocean bottoms subsided by an equal amount.Thus Nansen's soundings from the FRAM expedition showing a deep basin in the Arctic did not force any revision of geologic thought.

1910--1930

The hypothesis of continental drift advanced by Taylor and during this era focused attention on the Arctic. Taylor (see Eardley, 1961) specifically showed the Arctic Ocean to be a "disjunctive basin" formed when Eurasia and North America separated.

1930-- 1950 Continental Subsidence

One school of Arctic researchers in the Soviet Union, notably N. S. Shatski (1935) and his followers, considered the basin of the Arctic Ocean to be a submerged part of the continent.On Soviet tectonic maps of the Arctic it was called the Hyperborean Platform. Eardley (1948) summarized the geology of the land masses around 12 the Arctic Ocean and agreed with the Soviet school that the basin was a sunken region which in Precambrian and perhaps early Paleozoic had been land.Eardley (1961) explained his 1948 position as follows: The broad shelves and relatively small size of the basin, the facing Pre-Cambrian shields (Canadian, Greenland, Russian-Baltic, and Angara), the Paleozoic orogenic belts that project to and under (?) the Arctic Ocean (UralNovaya Zemlya, and Spitzbergen, East Greenland, Canadian , North- land and New Siberian) suggested to him that the region was once land and beginning in Paleozoic time has foundered. Thus by 1950, three alternative hypotheses for the origin of the basin of the Arctic Ocean had been stated:(1) the basin is permanent, dating from the earth's formation, (2) the basin was formed when Eurasia and North America drifted apart, and (3) the basin is a foundered continental region.No acceptable mechanism for any of these proposed origins had been stated and the bathymetry of the basin was so poorly known that it did not impose constraints on any of the models. Scientists of the United States began research in the Central Arctic after 1950, while the research efforts of the Soviet Union con- tinued and expanded to become perhaps the most comprehensive study ever conducted in any part of the World Ocean.The HLAE were con- tinued and, in 1950, a manned camp was established on the ice in the northernmost (76° 03' N. Lat. ,166° 36' W. Long). A scientific program was carried out for more than a year as the station 13 drifted north over the Chukchi Rise. (See Figure 1 for location.) The station commander published a complete report of this expedition, including depth soundings (Somov, 1954).This station was called North Pole - 2 and Papanin's 1937 ice floe station was renamed North Pole -1 at this time.During 1954 drift stations North Pole 3 and North Pole 4 were established and one or more drift stations have beein in operation at all times since then.In 1967, North Pole -.16 was established and, with North Pole -15, is carrying out re- search as this is being written. In April 1952 a U. S. Air Force weather station was established on drifting Ice Island T-3, then located 125 miles from the North Pole. Ice islands are tabular fragments of glacier ice that have broken away from the Ward Hunt Ice Shelf on the northern coast of Ellesmere Island.The ice islands were first discovered by U. S. Air Force weather reconnaissance flights when they appeared on the plane's radar scopes.They were referred to as T'Targets' which became shortened toT 1,T-. 2, nT-. 3, etc.Ice Island T-3 was nearly 15 km in length and more than 60 meters thick when first occupied as a research station. Between April, 1952 and May, 1954, T-3 served principally as a weather station although some seismic reflection and oceanographic studies were carried out.The station was abandoned in May,1954 near Ellesmere Island, but was reoccupied for five months in 1955. 14

The geophysical data collected during these periods have been col- lected in a single volume edited by Bushnell (1959).T-3 later be- came one of the United States' IGY Arctic stations and has been manned almost continuously since then. Research workers of the United States have made limited use of ski-equipped airplanes for landing on the ice pack to collect geo- physical data.These studies have taken place in the segment of the Arctic bounded by 100° W. Long., the 180° meridian and the North Pole.Crary, Cotell and Oliver (1952) obtained a few echo soundings north of Alaska by landing on the ice.Worthington (1953) reported three deep soundings from the Navy's Project SKIJUMP.These two projects used ski-equipped twin engine aircraft.Later use of air- planes has been limited to small, single engine types with a range of only a few hundred miles.These have been based along the Alaskan coast or on the United States drifting stations. During the IGY, the United States established two drifting sta- tions using the Air Force for logistics and support.Ice Island T-3 became station Bravo! and a second station was established on an ice floe at 8051' N. Lat., 160' 17' W. Long. named "Alpha'. "Alpha" lasted for 17 months (May 1957 to 1958) before breakup of the floe forced evacuation of the personnel.The Office of Naval Research and the United States Air Force set up a drifting 15 station, "Charlie, in April, 1959 to continue the programs begun during the IGY."Charlie' drifted in the region of the Chukchi Rise until 1960, when the station was evacuated.Station Bravo was occupied until Fall, 1961.However, for the previous year the station had been aground in the Chukchi Sea 150 km northwest of Point Barrow.Stations Alpha, Bravo and Charlie were supported by Air Force personnel with civilians carrying out the scientific re- search.After station Charlie was evacuated, the United States Office of Naval Research took over the task of supporting Arctic drifting stations.The Arctic Research Laboratory (ARL) at Point Barrow, Alaska has provided logistic support for three stations while civilian scientists have carried out the programs from the stations. ARL set up its first station, ARLIS-I (for ARL Ice Station) on 25 1960 at 74° 40' N. Lat., 141° 06' W. Long.The sta- tion drifted generally westward until 18 March 1961, when it was abandoned at 74° 59. 2' N. Lat., 169° SOt W. Long. ARLIS - II was established on an ice island in May, 1961 at 730 10' N. Lat. 1560 05, W. Long.This station drifted across the entire Arctic Ocean; the first United States station to do so.ARLI5-II was evacu- ated by a Navy icebreaker in May, 1965 west of . In 1962 Ice Island T-3 (IGY station Bravo) was lo- cated north of Point Barrow, having gone adrift again. A small party from ARL occupied the station and a limited scientific program 16 was begun. When ARLIS-Il was evacuated, the research effort on T-3 was increased.During the period 1965 to 1968, T-3 has been the only United States drifting station in the Arctic Ocean.In addi- tion to the research efforts carried out on the drifting stations, depth soundings and gravity data have been obtained by landing small air- craft on the ice.This program was started in 1960 and has been continued to the present time.The aircraft have been based either at locations along the north Alaskan coast or on the drifting stations. The small planes used are range and weather limited and can operate in the ice pack only from March through May, as daylight is required for the operation and the surface of the ice becomes too soft and puddled for safe landings in summer. The United States has collected aeromagnetic data over the Arctic regions since 1950.The Office of Naval Research sponsored low level flights in 1961, 1963 and 1964, and the United States Air Force and United States Coast and Geodetic Survey obtained high alti- tude profiles in a cooperative study from 1950 to 1952. It is from these research efforts on the drifting stations and by aircraft, that the limited amount of gravity, seismic, magnetic and heat flow data which are available from the Arctic Ocean have been obtained.The data are of great value and will be discussed in detail in the appropriate sections. From 1957 to 1962, the operated nuclear 17 submarines in the Arctic Ocean.These vessels obtained echograms of the ocean floor in all areas of the CentralArctic.Use of these data allow for the first time development of areasonably complete picture of the physiography of the floor of theArctic Ocean.It is the purpose of this study to presentthese data and to discuss the possible origin of the Arctic Ocean basin in light of theconstraints imposed by the new information. METHOD OF STUDY

The available depth soundings from the Central Arctic collected by the United States drifting stations and North Pole -2 (Somov, 1954) together with echograms taken by nuclear submarines have been used in the present study.Figure 2 shows the tracks of the submarines in the Arctic from 1957 to 1962.The few wire soundings taken in the Arctic before World War II have been neglected as trivial in number and of dubious location and reliability. To date, use of nuclear submarines as survey vessels is unique to the Arctic Ocean and their advantages and disadvantages have not been reported in the scientific literature.It is necessary to discuss these ships and their use, as most of the data used in this study were collected from submarines. The submarine NAUTILUS made the first under-ice penetration of the Arctic Ocean in the summer of 1957.She entered the Arctic Ocean from the , went half way to the North Pole and returned.Following this northern penetration, echograms were ob- tamed in the Greenland-Svalbard passage.During August,1958, NAUTILUS made her famous transit from the Pacific to the Atlantic by way of the North Pole and provided the first echogram across the Arctic Ocean. (1959) has written a popular narrative about the NAUTILUS's Arctic voyages and Dietz and Shumway (1961) 19

a . I a. 900w+

S.

Figure 2.Locations of the echograms used in this study. published a preliminary report based primarily on the 1958 echo- gram. In August,1958 the USS SKATE carried out a reconnaissance of the Eurasia Basin at the same time NAUTILUS was making her transit.In March,1959 SKATE returned to the Arctic and obtained valuable echograms along the Lomonosov Ridge.Both of these cruises have been described in a popular book by Calvert (1960). During January and February,1960, the submarine SARGO carried out a survey in the Amerasia Basin.This survey was supplemented the next summer by echograms from the SEADRAGON.The SEADRAGON made the through the Canadian Archipelago and then ran sounding lines in the Canada Basin, the Makarov Basin, and along the continental slope off Siberia. A general narrative of this expedition has been written by Steele (1962). The next Arctic cruise by United States submarines took place in and August1962. SKATE entered the Arctic Ocean by way of (the narrow passage between Greenland and Ellesmere Island) and surveyed the Nansen Cordillera and the continental slope north of the Barents and Kara Seas. SEADRAGON entered the Arctic the same summer by way of Bering Strait.She obtained soundings in the Canada Basin, crossed the Alpha Cordillera, and investigated the continental slope of the Laptev Sea, concentrating on the region where the Nansen Cordillera abuts the continental slope.SKATE and SEADRAGON rendezvoused north of Severnaya Zemlaya and ran in company to the Pole and thence across the Lomonosov Ridge and the Canada Basin to the continental slope north of the Mackenzie River.From there, SKATE returned to the via the Northwest Passage, and SEADRAGON to the by way of the Bering Strait. The cruises outlined above have provided more than seven million echo soundings along 37, 000 km of track in the Arctic Ocean.Hakkelt(196Z)estimated that about 10, 000 depth soundings had been collected in the Arctic Ocean.If this number is correct, the nuclear submarines have increased the number of soundings available by three orders of magnitude.

The submarine NAUTILUS on her1957and1958Arctic cruises did not use a Precision Depth Recorder (PDR).The echograms from these surveys were made with the UQN-.1 echo sounder; the uEdo sounder in common use on United States Navy and civilian research vessels.The data obtained are not as precise as those taken from the other submarines, each of which was equipped with a Precision

Depth Recorder (see Luskin and Israel,1956,for description of this unit). The 'qualityof the echograms taken by submarines under ice is excellent as these vessels are very quiet.Running several hundred feet deep in the ice covered ocean, there is almost no water 22 noise to obscure the bottom return.The submarine is a stable plat- form when submerged so the vertical axis of the echo sounder trans- ducer remains vertical (in contrast to the case of the surface survey ship in rough weather).Finally, acoustic scattering layers are almost nonexistent in the Arctic.This leads to uncluttered echo- grams with well defined bottom returns. The UQN echo sounder and the PDR use a speed of sound in sea water of 1463 rn/sec (4800 ft/sec) in converting travel time to depth recorded on the chart paper.If the sound speed through the water column under the ship does not average 1463 rn/sec over its length, the sounding recorded will be in error.Figure 3 shows sound velocity plotted against depth. A value of 1495 rn/sec (4900ft/sec) is a more realistic figure for the sound velocity path in the Arctic Basins at depths of about 4, 000 meters.The basin depths reported are thus too shallow by about 100 meters.Conversely, the depths over the crests of the Alpha Cordillera and LornonosovRidge (with a water depth of about 1000 meters) are toodeep by about 50 meters. In addition to precise measurement of water depths, it is necessary to know the location of each measurement before an ac- curate bathymetric representation can be created.Errors in posi- tioning are more common and more serious in survey work than are errors in measuring water depth.This results from the advanced state of the art of echo sounding machines and the still relatively T>I.71>T>3.5sec. Il sec. sec. ______U) 7 145C AVERAGE ACOUSTIC VELOCITIES 5000 142 7 ____ IN THE ARCTIC OCEAN AND WATER ______140C .PDR-- DEPTH AS A FUNCTION OF TRAVEL TIME (AFTER OSTENSO, 1962) 1.0 ao TRAVEL TIME .10 (seC.) 4.0 5.0 b.0 4000 5000 0CD a000 ;CD

U) II.'.'.-0 Figure 3. Averageas a function acoustic of travelvelocities time. in the Arctic Ocean and water depth 7.0 (J.) 24 inaccurate methods of positioning a survey ship on the high seas.For example, all of the bathymetric charts of the Pacific Ocean, the and the South Atlantic Ocean are based on soundings located by sextant and chronometer.-essentially the method used during the famous research voyage of the HMS Challenger in the last century.Without an independent check on the celestial , it is even difficult to estimate probable errors in the positions used. However, these errors in positioning are probably not serious for reconnaissance surveys of large oceanic areas.If a volcano is 'misplaced' ten miles on a map of the ocean floor, this in itself is not apt to lead to serious mistakes in interpretation of geological history. The soundings collected by the nuclear submarines in the Arctic Ocean have been positioned by concurrent use of celestial navigation, dead reckoning, and Ships Inertial Navigation System

(SINS).Inertial navigation systems have been developed only during the last decade.The theory and principles have been described in the open literature (for example, Draper, Wrigley and Hovorka, 1960), but engineering details and data on positioning accuracy ob- tamed with SINS on submarines are classified.One estimate was published by the navigator of NAUTILUS during her Arctic Ocean crossing.The ship left the Barrow Canyon on 1 August 1958 and surfaced in the Greenland Sea on the morning of 5 August where Z5 lines were taken for a celestial "fix' The ship's position from SINS and the dead reckoning position were within ten nauticalmiles of the celestial 'fix" (Jenks, 1958). In reducing the echograms to a form suitablefor publication, it has been possible to evaluate the position accuracyof the sub- marines.This has been accomplished by comparing depthsmeasured by two submarines where their tracks crosseach other.Over steep slopes this is a sensitive test of whether or not the twoships were actually at the same location.An analysis of 50 track crossings shows that the positioning of the Arctic Oceansoundings used in this study is at least as good as one expects in the open oceanusing care- ful celestial navigation. The disadvantages of using a nuclear submarinefor bathymetric surveys are of two kinds: First,the survey ship is not operating on the sea level datum sothe soundings must be correctedfor keel

depth.This is easy enough to do; however, it is important tokeep in mind that a submarine can "create" interesting canyons, scarps or peaks on the sea floor bychanging depth.For example, if the ship is running at 400 feet, then ascends to 100 feet toobtain a water temperature trace and returns immediately to400 feet, the result will be a "V' shaped canyon some 100 meters deep onthe echogram. Any such interesting features as deep sea channels,distributaries or small hills on abyssalplains must be carefully checked against the shipts log books to insure that they are not the creation of the submarine itself. The second kind of problem arises from the fact that the per- formance of nuclear submarines is classified.The echograms are annotated with speeds and keel depths and cannot be released in the original form. For the present study, the echograms were examined and water depth scaled every 20 minutes, at every change in slope and at every maximum and minimum depth of small scale features. These data were plotted on a time versus distance scale to construct profiles of the sea floor along each submarine track.Reduced copies of all of the profiles at a scale of one inch equals one degree of latitude and with a 50 X vertical exaggeration are shown in Appendix 1 together with a track chart for each submarine cruise. The profiles were used to construct a fence model of Arctic depth soundings at a scale of 1:1, 000, 000.Available soundings from drifting stations were added.The model was photographed and an enlargement was used as the basis for a physiographicdiagram of the floor of the Arctic Ocean (Plate 1).Two photographs of the fence model are shown in Plate 2. The depth soundings scaled from the echograms were plotted on a polar projection at a scale of 1:2, 000, 000.This plot was used to construct a bathymetric map of the Arctic Ocean (Figure 4).

S A - -. p I 1 -1 : - I 4 1 4 I j$\i ,. 4 -),lvJ, Ji\*1TJp \ : c 4. S j L ,j' ,r 1! f ;yJ -I \ '.. 1 :2 -, '- 5' -. -. lp '4 '' \ -S + 29

PHYSIOGRAPHY OF THE FLOOR OF THE ARCTIC OCEAN

Geographic Names

The basins, mountains and rises of the floor of the Arctic Ocean are so newly discovered that nomenclature has only recently been standardized.Consequently, researchers have been hampered by a confusion of names applied to the same features, the boundaries of which are often only surmised.

In January1966a group of marine geologists and geophysicists from the few institutions in the United States actively working with Arctic data met in San Diego with an advisor to the U. S. Board on Geographic Names.In the three day meeting, agreement was reached on names to be used for the major features as they were known.The results of the meeting have been published (Beal. etal.,1966)and the names suggested have been approved by the Board on Geographic Names. In this study, the approved nomenclature will be used without particular reference to older names,The reader is referred to

Bealetal.(1966)for information on previous nomenclature. Figure 1 shows the approved names and the names of three additional fea- tures which were not clearly defined at the time of the San Diego meeting.At the time of the meeting, itwas not certain that the Nansen 30

Cordillera was continuous across the Eurasia Basin.Now that the submarine-collected echograms show this to be the case, the Eurasia Basin is divided into two smaller elongate basins, each con- taming a plain.

The name Fram Basin (Beal etaL,1966) hasbeen retained for the depression bounded by the Nansen Cordillra and the Lomonosov Ridge.The basin bounded by the Yermak Rise, the Eurasian conti- nental slope and the Nansen Cordillera will be called Nautilus Basin in honor of Wilkins'193Zsubmarine which carried out oceanographic studies in the region north of Svalbard. It was the desire of the U. S. Board on Geographic Names to perpetuate the name of Joseph Fletcher, who initiated the scientific program on drift stationT-3,in the region of the Pole.To this end, the Board approved the name Fletcher Plain for the floor of the Makarov Basin.In the same spirit, the plain in the Nautilus Basin will be named Sverdrup Plain in honor of the late Hareld U. Sverdrup, chief scientist of NAUTILUS.The plain in Fram Basin will be called Hakkel' Plain to honor the late Ya. Ya. Hakkel', leader of the Soviet

HLAE of1948and1949,discoverer of the Lomonosov Ridge and a leading student of the floor of the Arctic Ocean.These names have been submitted to the Board on Geographic Names for approval. 31

Geography

The Arctic Ocean has an area of about 9. 5 millionkm2,not including the passages of the Canadian Archipelago or Baffin Bay.If these are included, the area is slightly more than 11. 5 millionkm2. The Arctic Ocean contains 13 millionkm3of sea water.The total area and volume of the World's Ocean is 362 millionkm2and 1349 millionkm3(Menard and Smith, 1966).Thus the Arctic Ocean makes up only about 3% of the area and 1% of the volume of the World's

Ocean. The most striking feature of the Arctic Ocean is its broad conti- nental shelves.Excluding the passages of the Canadian Archipelago, the shelves underlie about 62% of the area.Including the Archipelago, this figure approaches 70%.Thus most of the area of the Arctic Ocean is underlaid by the northern margins of the continental blocks of Eurasia and, to a much lesser extent, North America.Figure 4 illustrates this point. The extent and bathymetry of the continental shelves in the Arctic have been well known in contrast to the basin of the Central Arctic. (Klenova, 1960; Hunkins and Kaplin,1966; lonin, 1966a andb; Fairbridge, 1966 have published summary studies on the Arctic conti- nental shelves.) The nuclear submarine expeditions have provided significant new data on the Arctic continental shelves only in the 32

LincolnSea0 The present study will not be concerned with the shelves even though they make up the greatest area of the Arctic Ocean. The basin of the Arctic Ocean includes an area of about 3. 6 millionkm2.This is slightly more than half the area of the Conti- nental United States.The basin is made up of four sub-basins separated by three mountain ranges (see Figure 4 and Plate 1).The mountain ranges are completely different from each other in physiog- raphy and each will be treated separately in the following sections. The basins will be considered in one section, as they are all close to the same 4 km depth.The continental slopes, rises and plains at intermediate depths and the marginal escarpments of the basin of the Arctic Ocean will be treated in a separate section.Finally, all the data will be considered in an attempt to construct an hypothesis for the origin and geologic history of the Arctic Ocean.

The Alpha Cordillera

A map published by Hakkel' (1958) titled, 'TSchematic chart of volcanism in the Arctic Ocean basinushows by wavy contours the base of an unnamed high extending from a broad shelf off Ellesrnere Island toward Wrangel Island and the East .Hope (1959) reported that this map was first published in 1955.Drift station Alpha reported this range in August,1957, (IGY Bulletin Number 6, ,1957) and it was named by Hope (1959) the Alpha Range. 33 In later Soviet publications, these mountains are called the Mendeleyev Ridge (Gordienko and Laktionov, 1960 and Treshnikov,

1961).Dietz and Shumway (1961), reporting on the NAUTILUS 1958 echograms, called this mountain range the Central Arctic Rise. The nuclear submarine cruises provide nine profiles in the area of the Alpha Cordillera.These are shown in Figure 5 with locations. The two profiles marked M-N-O were taken by two submarines running in company about 4 km apart, from the North Pole south along the 135th meridian.These profiles are normal to the axis of the Cordillera and well away from the continental margins.These cross sections show the Alpha Cordillera to be an arcuate "rise"

600 km wide with side slopes of about0023'.The rise is sym- metrical about the axis with 2000 meters of relief.This basic shape is modified by scarps up to 1000 meters high with slopes of 05' to 100, peaks and valleys--especially near the crest--and ex- tensive, rather level areas with superimposed minor relief.The minor relief as seen on the echograms consists of many small hy- perbolas and peaks and resembles the "high fractured plateau" of the Mid-Atlantic Ridge described by Heezen, Tharp and Ewing (1959, see especially Plate 18).Three of the large peaks north of the axis of the "rise" on profiles M-N-O are concave with craters in the tops and cones of ZOO to 300 meters relief on the flanks.These FATHOMS 0METERS 34 B B Figure 5 A 20003000 THE VE R TI ALPHA CORDI LI ERA F 4000 (NP NON- PRE CI SI ON DEPTH R ECORDICMETERS AL5OX NG) HOR I ZONT AL D J F I H 4000 M 5000 01 4Km x U M w NP r /W WRANGEL1PLAIN 35 are certainly volcanos.The north flank of the Cordillera is marked by scarps about 500 meters high rising from the Fletcher Plain, while the southern margin slopes gently under the Canada Plain.The NAUTILUS 1958 crossing (profile P-Q) shows a peak rising abruptly from the Canada Plain at the southern margin of the Alpha Cordillera, however.Hunkins' (1961) profiles of the southern margin of the Cordillera between 150° and 165° W. Long., show steep scarps, which led him to suggest that this mountain range is a fault block structure. Hunkins carried out seismic reflection studies and analyzed data from 600 explosions for bottom slope and water depth. Minimum depth recorded was 1426 meters at 85° 03' N. Lat., 171° 00' W. Long.Bottom slopes ranged from 0° to 22°, with an average of 2. 7°.Most dips were between 0° 30' and 1° 30'.Hunkins (1961, Figure 3) shows the frequency of these dips. In plan, the Alpha Cordillera has an "hourglassT' shape with the narrowest part located in the region of the 150° - 160° West meridians (see Figure 4).Here, the Cordillera is some 400 km wide. The cross section, R-S-T in Figure 5, becomes unsymmetrical with the steeper side slope on the north. The north flank rises abruptly from Fletcher Plain in a two step scarp 1500 meters high with slopes of 05° to 10°.The crest is at 1200 meters, giving the Cordillera about 2800 meters of relief above the plains to the north and south. Profile R-S-T in Figure 5 shows a series of volcanoes and one region 36 of high fractured plateau south of the crest.The flank of the southernmost volcano, at point T on the profile, dips under the northernmost margin of the Canada Plain. The Alpha Cordillera widens in both directions, toward the Canadian Archipelago and toward Eurasia, away from Profile R-S-T, Figure 5, discussed above.The Cordillera becomes part of the con- tinental slopes of both Eurasia and the Canadian Archipelago.The continental slope from northern Greenland along Ellesmere Island and the Archipelago is broken and irregular (see Plate 1).Off northern Greenland, is the Morris Jesup Rise (discussed later, but see Figure 9) with depths of1000 to 2000 meters. On the Ellesmere Island slope are peaks and valleys and areas of high fractured plateau where the crest of the Alpha Cordillera becomes the continental slope.Farther south along the slope of the Archi- pelago, the upper slope exhibits small scale roughness and the large peaks and valleys occur on the deeper parts of the slope. Toward the North Pole the irregular, blocky slope separates into two mountain ranges.To the north, the Lomonosov Ridge (described later) is separated from the Alpha Cordillera by a narrow, flat floored valley.Figure 6 shows profiles across this valley. Along the 125° West meridian the valley is 25 km wide at the bottom with the flat floor at 3900 meters.The north side of the valley is the southern flank of the Lomonosov Ridge which rises, with a slope 37 of 05°, to the crest at 1100 meters.The south flank of the valley slopes upward at about 15° to a crest at 2900 meters.This crest is the northernmost line of peaks of the Alpha Cordillera.It was dis- covered by Crary (1954) from drift station T-3 and named the Marvin Ridge (Figure 1, Marvin Spur). The valley separating the Lomonosov Ridge from the Alpha Cordiliera opens rapidly until at the 175° W. meridian it is 140 km in width.The flat floor of this valley is the Fletcher Plain.There are no nuclear submarine echograms from Fletcher Plain on the Eurasian side of the Pole (Figure 2).The shape of the Plain shown in Plate 1 is taken from a bathymetric chart by Hakkelt (1958). The Alpha Cordillera abuts the Eurasian continental slope inca series of steps topped by two plains, or flat areas, separated by the crest (see Plate 1 and Figure 4).The western plain (i. e. toward Europe) is called the Wrangel Plain.Kutschale (1966) reported re- suits of seismic reflection studies on the Wrangel Plain and the adjacent edge of Fletcher Plain (Siberia in his paper).

He reported,"...3. 5 km of subhorizontal, stratified sediments underlie Wrangel Abyssal Plain... . The sediments of Wrangel Plain are dammed behind a basement ridge which separates Wrangel Plain from Fletcher Plain and appears to connect with the Alpha Cordillera.According to Kutschale, the sediment overflow from Wrangel Plain moves to the Fletcher Plain through a gap in the basement ridge which he named Arlis Gap. A submarine collected echogram, Figure 5 prOfile W-X.-Y, shows the floor of the Plain to be rough in the region near point X.There may be several, or many, gaps through which sediment moves to the deeperPlain. Profile R-V in Figure 5 is from an echogram taken closer to Siberia. The flat area on both sides of point U is Wrangel Plain.The floor here shows a few broad swales, but generally is smoother than it is farther north (profile W-X-Y).The Wrangel Plain is at about 2700- 2800 meters depth. The eastern plain is smaller than the Wrangel Plain (see

Figure4)and shallower at about 2200 meters.It was described by

Shaver and Hunkins(1964),who named it the Chukchi Abyssal Plain. (The name was changed to Chukchi Plain by the U. S. Board on

Geographic Names. )Hunkins(1966)estimates its area to be no more than 5000km2.None of the submarines crossed this Plain, but profile W-X-Y, Figure 5 shows the gap through which sediment may flow from Chukchi Plain to the Canada Plain below.This feature was named Ch3rlie Gap (Figure 1) by Shaver and Hunkins

(1964)after drift station Charlie which crossed it several times. Charlie Gap is the deepest point on profile W-X-Y. The crest of the Alpha Cordillera separates the Wrangel Plain from the Chukchi Plain in the region of the .The crest of the Cordillera is shown on profiles W-X-Y, between point X 39 and the Charlie Gap, and on R-V, between points U and V (Figure 5). Farther east, toward Alaska, is the Chukchi Rise.This feature will be discussed in a later section and is mentioned here only because it appears to be related to the Alpha Cordillera.If this is the case, then the Cordillera intersects the continental slope of Eurasia across 55 degrees of longitude.Along the North American slope, it is difficult to distinguish the northern margin of the Alpha Cordillera from the Lomonosov Ridge and the Morris Jesup Rise. However, the Alpha Cordillera intersects the continental slope across at least 55 degrees of longitude.

Geophysical Data

There are more non-Soviet geophysical data available for the Alpha Cordillera than for any other part of the Arctic basin.Even so, there is not a wealth of information and much of the data col- lected from drifting stations is from a small area on the southern margin.The geophysical data are reviewed below.

Earthquake Seismology

The Alpha Cordillera is almost completely aseismic.Sykes (1965) carried out detailed analysis of epicentral locations in the Arctic.Only one of the 281 epicenters he relocated by least squares fit from records of ten or more Arctic seismic stations is near the 40 Alpha Cordillera.The epicenter was at 80° N. Lat. ,119° W. Long. a location where the southern margin of the Cordillera has disap- peared beneath the Canada Plain.

Explosion Seismology

Hunkins(1961),working from drift station Alpha, made five short, unreversed refraction profiles over the Alpha Cordillera. These profiles show a layer of unconsolidatedsediment 0. 38 km thick; a. 80 kmthick layer with a velocity of 4. 70 km/sec and a

layer below this with a velocity of6.44 km/sec.The thickness of this lower 'oceanic layer" was not determined.

Bottom Samples

Schwaracher and Hunkins(1961)reported on material obtained in nine dredge trawis, 14 cores and observed in ZOO sea floor photo- graphs over the Alpha Cordillera.The photographs show the bottom to be littered with gravel and the dredge obtained many gravel sam- ples which were predominantly .More than half the pebbles obtained showed glacial striations.The authors con- cluded that the pebbles are all ice rafted debris.No sample of the rocks of the Alpha Cordillera itself are available. 41

Magnetic Data

Two aeromagnetic surveys have been reported for the Arctic Ocean.King, Zietz and Alldredge (1966) reported on a high altitude survey flownat 6100 meters.Ostenso (1962) reported on a low alti- tude survey flown at 450 meters above sea level. Both surveys show that the rocks of the Alpha Cordillera are intensely magnetic.Ostenso stated that, 'High amplitude magnetic anomalies over the Alpha Ridge [Cordillera] indicate that whatever its orogenesis may have been, the result was an uplifting of magnetic basement material. " Anomalies of 1000 to 1500 gammas are corn- mon.The Canada Plain and the Fletcher Plain adjacent to the Alpha Cordillera show magnetic anomalies greater than 500 gammas. Shaver and Hunkins (1964)reported that the western and northern margins of the Chukchi Rise are marked by a "prominent magnetic anomaly" which they interpret, with the aid of gravity data, as a "basement ridge".The Chukchi Rise is interpreted as a 12 km thick wedge of sediment. A model showing a crustal thickness of 32 km beneath the "basement ridge' and 18. 5 km beneath the Chukchi Rise was derived.Their model indicates a 21 km crustal thickness beneath the Chukchi Plain. Kutschale (1966) reported on the Wrangel Plain.By use of magnetic and gravity data collected from drift station ARLIS-Il and 42 Ostensots 1962 aeromagnetic profiles in the region, he derived a model yielding a crustal thickness of 22 km beneath the spur of the Alpha Cordillera which dams the sediments of the Plain against the continental slope.The model gives a crustal thickness of 15 km beneath the Wrangel Plain.

Heat Flow Data

Lachenbruch and Marshall (1966) reported heat flow data from the southern margin of the Alpha Cordillera and the adjacent Canada Plain.Their data indicate:(1) that the heat flow through the floor of the Canada Plain is,"...within about 5% of 1. 4 microcalories/ cm2sec over an area of about 8 x km2.. !;(2) T!Near the southern edge of the Alpha rise the heat flow falls from about 1. 4 to about 0. 8[microcalories/cm2 sec]over a horizontal distance of less than 25 km; (3)'...the cause of the anomaly is believed to be a sharp lateral discontinuity in rock properties involving the entire crust.It evidently cannot be explained by downfaulting on the basin side of a laterally uniform crust... Lachenbruch and Marshall examined a number of possible models, varying both geometry and thermal conductivities, and concluded that a contrast in rock types between the Alpha Cordillera and the adjacent Plain is the most reasonable solution to the heat flow data.The thermal conductivities of the rocks must contrast by 43 a factor of two, with the Alpha Cordillera having the lowerconduc- tivity.The geometry of the suggested model allows projection of the lower conductivity rocks of the Cordillera under the basement rocks of the Canada Plain.It should also be noted that the heat flow meas- ured on the Plain is about the world average as it is now known, while the heat flow on the Cordillera is one-half the world average.

Summary

The Alpha Cordillera is an aseismic rise marked by scarps, vol- canoes and regions and high fractured plateau.The highest peaks along its crest are about 1ZOO meters below sea level, giving the Cordillera80O meters of relief above the adjacent plains.In plan, the Alpha Cordillera varies from a width of more than 1000 km at the continental slopes to a minimum width of about 400 km midway between Eurasia and North America.Its crest axis is generally parallel to the 400 West, 140° East meridian, but is arcuate.This will be discussed later in relation to the other mountains on the floor of the Arctic Ocean. ljnreversed seismic refraction data from a small area on the Alpha Cordillera show a layer of sediment some 0. 3 or 0. 4 km thick over a layer Z. 8 km thick which has a velocity of 4. 7km/sec.Be- neath this is an "oceanic layerof undetermined thickness with a velocity of 6. 4 km/sec.No rock samples have been obtained and 44 the crustal thickness has not been measured. A model designed from gravity and magnetic data for a spur on the northern flank of the

Cordillera yields a crustal thickness of22km, including the water

column. Aeromagnetic flights over the Alpha Cordillera show that its rocks are intensely 'magnetic ".I-feat flow measurements on the Cordillera give a value one-half the world average while the adjacent Canada Plain shows heat flow values equal to the world average. The geologic history of the Alpha Cordillera has been hy- pothesized by various authors, some ofwhom have been referencedinthe paragraphs above.Hunkins(1961)and Dietz and Shumway(1961) proposed block faulting as the origin; Ostenso(1962)suggested that it may represent an upfaulted mass of basement material to explain

the intense magnetic anomalies.King, Zietz, and Alldredge(1966) suggest that the Amerasia Basin (see Figure 1)is a down-dropped continental block) presumably a Pre-Cambrian complex like the .In this interpretation, the Alpha Cordillera would be a block which has not subsided to the depth of the basins. A number of researchers in the Soviet Union have long con- sidered the Amerasia Basin (see Figure 1) a down-dropped continental area and apparently still hold to thishypothesis. A recent Soviet

tectonic chart by Atlasovetal.(1964)showed the Alpha Cordillera and the Lomonosov Ridge to b e Caledonides with "fields of basic

46 of the Alpha Cordillera under the Canada Plain. Mime (1966) re- ported results of an unreversed refraction profile from the southern margin of the Canada Plain adjacent to the north Alaskan Coast.His crustal section showed 4 km of water, 2. 5 km of sediment with a velocity of 2. 4 km/sec over 8. 7 km of material with a velocity of 4. 4 km/sec.Directly below this was Hmantle with a velocity of 7. 5 km/sec.He explained the lower than normal mantle velocity by suggesting that the M discontinuity dips toward the continent in the region of the refraction profile.Milne reported, T1No evidence existed for the presence of the normally encountered 6. 5 km/sec oceanic layer. The 15 km thick (including water) crustal section is two or three km greater than is usually found in oceanic areas. However, the sequence of sediment, 2. 5 km thick, is greater than usually encountered and the 15 km crustal thickness is much less than would be expected for a foundered continental block. The 1000 to 1500 gamma magnetic anomalies of the Alpha

Cordillera are consistent with the mid-ocean ridge hypothesis.The Cordillera is wider than was previously known and appears to dip under the Canada Plain and the Fletcher Plain.The interpretation of the Alpha Cordillera as a mid-ocean ridge could explain the mag- netic activity encountered over nearly all of the Amerasia Basin. The absence of seismic activity in the Cordillera indicates that it is inactive at the present time.The bathymetry suggest that this 47 mid-ocean ridge has undergone subsidence; the central rift closed or was flooded with lavas and faulting has occurred.Further evi- dence for the inactive, or udeadfl, mid-ocean ridge hypothesis will be discussed in a later section where it will be shown that this con- cept can explain the physiography of the Arctic Basin.

The Lomonosov Ridge

The discovery by the Soviet Union's HLAE of a mountain range extending across the Arctic basin from the New Siberian Islands to the northwest Greenland-Ellesmere Island continental slope has been reviewed in a previous section.This range, the Lomonosov Ridge, is an enigma in that it does not resemble any other mountain range on the floor of the Ocean (see Plate I).Heezen and Ewing (1961, Figure 11) compared the Lomonosov Ridge with the Walvis Ridge of the South Atlantic.However, they had only a limited number of spot soundings available and included the Marvin Spur as part of the

Lomonosov Ridge.

Bathymetry

The nuclear submarines made seven crossings of the Lomonosov Ridge and zig-zag surveys along its entire length.The profiles constructed from the echograms are shown in Figure 6.The gaps in several of the profiles indicate timeswhen the echo sounder rERS H I A E'IrA I14L A

Figure 6 48 THE LOMONOSOV RIDGEVERT I CALSO X HORI ZONT AL $ I 0 N D E P T H I £ C 0 1 D I N 0) A (N P = NO N- P 1 E C I FATHOMS J K K L 111 'F

NAUTICAL LES 100 KNUC9NA 1k KLOWtT 5 ilH was shut down or had malfunctioned.Each of the submarines visited the North Pole and each of the profiles originates there (point A in Figure 6).The seven profiles of normal, or nearly normal, cross- ings of the Lomonosov Ridge are extended south across the Marvin Spur to show the valley which separates the Lomonosov Ridge from the Alpha Cordillera. The profiles of the Lomonosov Ridge have slopes which are convex upward. From North America to the North Pole the crest of the Ridge occurs between 1100 and 1ZOO meters.The coverage is not as good from the Pole to the New Siberian Islands, but the zig- zag profile (A-R, Figure 6) shows a number of interesting features. First, the echograrn confirms that the Lomonosov Ridge is indeed continuous from North America to Eurasia.Profile A-R (Figure 6), the zig-zag from the Pole to the Eurasian Shelf, indicates that the crest along this line has two saddles.The turning-points along the zig-zag profile are not far enough north or south to reach the Plains adjacent to the Ridge.However, each of the legs or runs close to the region of the crest.Section A-N crosses the crest of the Ridge with a least depth of 1700 meters.Section N-O is at a low angle to the Ridge axis along the north flank, coming to, or near, the crest at point 0.The smooth, gently sloping section starting at point N is real.To the south (toward the right on the profile), the ??saw_tooth! section indicates gullies on the flank of the Ridge. 50 Point 0 is the crest at 1000 meters.Section 0-P indicates the pos- sibility of a second saddle with a depth of 2800 meters.However, the track is along the flank so the evidence is not conclusive.Again a gullied area between smooth sections is indicated.According to Soviet bathymetric maps, points P and Q should be on the crest of the

Ridge.South of point Q, the profile lies along the north flank.The slopes and steps on the right hand end of the profile are the Eurasian continental slope.Point R is the shelf break at 200 meters. Profile B-A is a straight run over the crest and along the south flank of the Lomonosov Ridge to the Hakkel' Plain and the Pole. This profile shows smooth areas and gullied sections with the gullies becoming wider on the lower slope.The zig-zag, profile A-C, crosses the crest of the Lomonosov three times.Section A-F is along the north flank and F-E crosses the crest which is broken, wider than the more northern crossings, but shows about the same minimum depth.Crossings E-D and D-.0 show a smooth crest at a shallower minimum depth--750 meters on profile D-C. It is difficult to select the location of the axis of the Lomonosov Ridge in the region off Greenland and Ellesmere Island.The entire slope is a confused area where the Morris Jesup Rise, the Lomono- soy Ridge and the Alpha Cordillera all abut the continental slope (see Plate 1).North of 85° N. Lat. ,the axis lies about on the 60° West meridian to 88° N. Lat., from which point the axis lies between the 51

140and 150° East meridians to the Eurasian slope.(See Plate 1 and Figure 4.) The Lomonosov Ridge is narrow.The seven normal crossings in Figure 6 show a width of 75 km on the Greenland side of the North Pole (profile A-H) and less than 40 km along the 175° W. meridian. According to Soviet maps, the Ridge becomes somewhat wider as it approaches Eurasia.

Geophysical Data

There are few geophysical data available for the Lomonosov

Ridge.Earthquake seismology shows that the Ridge is aseismic (Sykes, 1965).There are no data from explosion seismology. There are no published gravity data for this part of the Arctic

Ocean.The Soviet researchers, Deminitskaya, Karasik, and Kiselev (1962) have published a small scale, schematic map of crustal thickness down to the Mohorovicic discontinuity.The authors w rote, In crustal structure the Lomonosov and Mendeleev Ridges [Alpha Cordillera] stand out prominently and have trootst like other submarine ridges, e. g. the mid- Atlantic and mid-Indian ridges.Generally the Lomonosov Ridge does not have very distinct 'roots', and several parts of the ridge are represented only by a projection into the ultravasic layer. And, In the area of greatest uplift of the Lomonosov Ridge, the crust probably attains a thickness of 15-20 km. 52 The above is said to be based on all available geophysical and bathy- metric data.The extent or quality of these data are not revealed, so it is impossible to evaluate the conclusions. Two sets of aeromagnetic profiles crossing the Lomonosov

Ridge have been reported by Ostenso(1962)and King, Zietz and

Alidredge(1966). The conclusions about the nature of the Ridge drawn from the two sets of data differ. The aeromagnetic profiles reported by King, Zietz and

Alidredge(1966)show three high altitude(6096meters above sea level) crossings of the Lomonosov Ridge between the North Pole and

Greenland.Positioning of theB-29aircraft was by celestial navi-

gation and the authors suggested a precision of ± 5 miles(± 9.1 km) for the flight lines.King, Zietz and Alldredge concluded, The Lomonosov Ridge is marked by a persistent anomaly of moderate size (i. e. 250-350 gammas) which indicates that it is composed at least in part of magnetic rocks. Assuming that it is caused entirely by topographic relief, the rocks have a fairly high magnetic susceptibility of 0. 007 cgs units indicating possible volcanic rock, but the ridge probably formed tectonically by folding and faulting.

Ostenso(1962)reported aeromagnetic profiles over the Lomonosov flown at an altitude of 450 meters above sea level.Navi- gation was by sun-lines, but a doppler navigation radar was used to give true ground speed and drift.Ostenso estimated position ac- curacy to be ± 15 km.The magnetic data were analyzed by corn- puter to allow an estimation of source depths of anomalies.(See 53

Wold and Wolfe,1966for a description of the methods used.) Ostenso reported, In contrast to the Alpha Ridge, the Lomonosov Ridge is practically devoid of shallow anomalies along its entire length.On nine crossings of the ridge, only one near surface (2 i/z km) anomaly was observed with the exception of a few on the North American extremity of the ridge near Ellesmere Island.The Lomonosov Ridge must be comprised of folded sediments with few mafic crystalline elements. Because of the paucity of geophysical data from the Arctic, Ostenso computed the depth of deep-source magnetic anomalies as an aid to estimating crustal thickness.Several assumptions are in- volved, including the assumption that the anomaly sources are re- lated to the Mohorovicic discontinuity.This analysis showed that the crust beneath the Lomonosov Ridge and the Alpha Cordillera averages 14. 25 km thick, with a range of values from6.25 to 24 km. Crustal thickness beneath the plains, including the Fletcher Plain, averaged 10. 75 km.Ostenso concluded, TtThere can be no doubt of a greater crustal thickness in this region[Lomonosov Ridge, Alpha Cordillera] than is found under the adjoining, and enclosed, oceanic deeps.

Ostenso (personal communication, December,1966)has ob- tamed additional aeromagnetic profiles over the Arctic since his

1962report was published.The new data support the1962conclu- sion that the Lomonosov Ridge is of sedimentary rock. 54 The conflict between the conclusion drawn by King, Zietz and Alldredge that the Lomonoov Ridge is probably volcanic rock and Ostensots conclusion that it must be sedimentary rock may be more apparent than real.The three aeromagnetic profiles reported by King, Zietz and Alidredge cross the Lomonosov Ridge in the where the Ridge and the intensely magnetic Alpha Cordillera converge.In contrast, Ostenso 's flight lines provide coverage along most of the length of the Ridge.The King, Zietz and Alldredge profiles were flown with the magnetometer 7 km above the crest of the Ridge while Ostenso's instrument was 1 i/z km above the crest.There may be several reasons for the conflicting conclusions drawn from the aeromagnetic data, but without rock samples from the Lomonosov Ridge, the conflict cannot be resolved. The bathymetry (Plate 1 and Figure 4) indicates that the Alpha Cordillera may plunge beneath the Lomonosov Ridge.That is, the Lomonosov Ridge may be a ridge of sedimentary rock perched on the flank of the intensely magnetic Alpha Cordillera.At both the Eurasian and North American ends, the Alpha Cordillera and the Lomonosov Ridge merge.It is possible that a magnetometer meas- uring intensity along every 65 meters of flight path at low altitude would show the Lomonosov to be nonmagneticUwhile an instrument 6 km higher would indicate a small anomaly due to the volcanic rocks of the Alpha Cordillera adjacent to and beneath the sedimentary ridge. 55 For completeness, mention should be made ofUHakkePs volcano" in connection with the Lomonosov Ridge.Hakkel' (1958) reported that while station North-Pole 3 was drifting over the

Lomonosov Ridge on Zi November 1954 a severe shock was felt,On the5th, four more shocks occurred and the ice floe split across.

HakkeP wrote,". .a strong smell of hydrogen sulfide and pos- sibly sulfur dioxide was noted. " He concluded that the station was over an active volcano.The position shown on Hakkel's map is 88° 15N. Lat., 70° W. Long.According to the bathymetry in the present study (Plate 1 and Figure 4), the reported location of HakkePs volcano is on the margin of the Alpha Cordillera rather than on the Lomonosov Ridge.There are no confirming data of present- day volcanism in the area and no earthquake epicenters have been reported there.The Soviet Union's eminent Arctic geophysicist, Madam Deminitskaya, was asked about Hakkel's volcano during the 1966 International Oceanographic Congress in Moscow.She stated that the deformation of the ice pack in the region north of Greenland is usually severe and the shocks and splitting of North Pole-3 may have been caused by this.(Personal communication to the writer by Dr. Robert Dietz, July, 1966.) Apparently Hakkel's report of an active volcano in the Arctic basin is not completely accepted in the U.S.S.R. 56

Summary

The Lomonosov Ridge is a narrow mountain range that crosses the entire Arctic Ocean basin from the continental slope off Greenland to the Eurasian slope north of the New Siberian Islands.In the Central Arctic, the Ridge varies from 40 to 75 km in width.Slopes are generally convex upward leading to a rounded crest.Echo- grams along the flanks of the Ridge show both smooth and gullied regions and some normal crossings are smooth while others show one or more peaks and steps suggesting valleys running along the flanks.Near the Greenland slope, the Lomonosov merges with the Alpha Cordillera and the Morris Jesup Rise and farther offshore the two ranges are separated only by a 25 km wide valley.Maximum separation is near the 180th meridian where the Fletcher Plain is 140 km wide.The Lomonosov Ridge is aseismic and its rocks are "non-magnetic.The Ridge forms the boundary between the mag- netically active Amerasia Basin and the magnetically quiet Eurasia Basin.Sketchy data suggest crustal thickening beneath the Lomono- soy Ridge.

1- t' 1 1 Ci C-r

The Lomonosov is a ridge of sedimentary rock perched on, and perhaps 'welded" to, the Alpha Cordillera.An hypothesis to 57 explain the origin of the Lomonosov Ridge will be presented in a later section.

The Nansen Cordillera

Although the nuclear submarine echograms are valuable for showing the physiography of the features in the Arnerasia Basin, the existence, at least, of all the major features had been known before the submarine voyages.In the Eurasia Basin, the echograms re- vealed a mountain range which has not appeared on previous maps. A recent Soviet bathymetric map (Academy of Sciences USSR, 1963) showed a few scattered peaks in an otherwise featureless Eurasia

B a s in.

Bathymetry

The existence of an extension of the Mid-Atlantic Ridge through the Eurasia Basin was suggested by Emery (1949) and Heezen and Ewing (1961) among others, on the basis of the seismicity of the region.Before the IGY, seismograph stations were remote from the Arctic regions, and a plot of earthquake epicenters covered nearly all of the Eurasia Basin.For the IGY, new stations were in- stalled in Canada, Greenland, Iceland and the Soviet Union.These, plus new computational techniques, allowed determination of epi- centers to about 10 km.Sykes' (1965) paper on seismicity of the Arctic showed that essentially all of the on the floor of the Arctic basin fall in a narrow band running from the northeast tip of Greenland to the Eurasian Coast between the Lena Delta and the New Siberian Islands.Sykes reported,

..the epicenters presented here indicate that the seismic belt between Greenland and Siberia is re- markably narrow and straight for a distance of more than ZO°.This is one of the longest, linear features of the entire mid-oceanic ridge system. The submarine echograms show that the authors who postulated an extension of the Mid-Atlantic Ridge across the Eurasia Basin were correct.Profiles across the Eurasia Basin are shown in Figure 7 with locations.Thirteen profiles are available: eight between the Greenland-Spitzbergen passage and 30E; four between 90 E and 1ZO° E, and one zig-zag crossing of the area where the belt of earth- quake epicenters enters the Eurasian continental slope. An inter- pretation of the extension of the Mid-Atlantic Ridge, the Nansen Cordillera, is shown in Plate 1 with earthquake epicenters from Sykes (1965) shown in red.All of the epicenters located in the floor of the Eurasia Basin are in the Nansen Cordillera.The epicenters shown by red "posts'T beyond the Yermak Rise and Svalbard are in- cluded to show the change in trend of the epicenter belt as it enters the Arctic Basin from the Greenland Sea.It was not possible on the drawing to show the sea floor in this area.Figure 8 is an interpre- tation of the sea floor in the region between Svalbard and Greenland

it ci 7 - 7 N.: .\ / O"f/' r' \ I' A:. = Ti 1 , i3 'Ti \ I; / \ 'S / \i_ ' / / 7 I II C, v /i/' Figure 8. The1*/ Lena Trough - / / -7 _5 61 showing the Yermak Rise, the Lena Trough and the Morris Jesup Rise.This drawing is based on photographs of the same fence model used to prepare Plate 1. An examination of the profiles and Plate 1, reveals that the Nansen Cordillera is remarkably straight.There is some indication that the axis bows slightly toward the Lomonosov Ridge near the 60th East meridian, but it will take a crossing in the area to confirm this. The profiles show occasional isolated peaks on the floor of the Hakkel' Plain.The large seamount shown at the Atlantic end of the Sverdrup Plain is taken from a Soviet bathymetric map and justified by one submarine echogram showing the flank of a mountain at the same location. Lines A-D and E-F of Figure 7 show that the entire floor of the Lena Trough from the western flank of the Yermak Rise to the east side of the Morris Jesup Rise is occupied by peaks and valleys. Between points A and B, one of the peaks shows what may be a crater in the crest.Between points B and C is a flat, smooth area, but note that this section of the profile trends north-south.This may indicate that some, or many, of the "peaks" on normal cross- ings are actually ridges elongate along the axis of the Cordillera.

Point C at 4400 meters depth may represent a central rift.On profile E-F, the deepest point is the easternmost valley.If one wished to choose a rift valley in the Lena Trough it would lie at the 62 base of the Yermak Rise. Profile G-H shows three mountain peaks rising above Hakkelt Plain.The following profile, K-J-I, shows that the Nansen Cordil- lera is about 200 km wide.Point K shows an isolated peak and point I is on the Morris Jesup Rise.No echogram exists for the section between 3 andI. The profile was constructed from soundings re- corded in the shipts log book every 15 minutes.The smoothness is therefore not real. The three profiles, L-M, N-O, and P-Q in Figure 7 cross the axis of the Nansen Cordillera at about 450 and are sub-parallel to each other.The Cordillera is about 200 km wide.The southern ends of the profiles are on the north flank of the Yermak Rise.The flat floor of Sverdrup Plain at the base of the Yermak Rise is shown by profiles N-O and P-Q.The greatest depth found in the Arctic to date, 5250 meters uncorrected, is shown in profile N-O.If this sounding is corrected, assuming simple adiabatic warming of the water column because there are no data between 4000 and 5000 km, it shows a depth of slightly more than 5, 500 meters.Profile R-U is from the NAUTILUS 1958 transpolar crossing.It is from the "region of seamounts' of Dietz and Shumway (1961).The echogram is from the UQN sounder operating on the 0-6000 fathom scale, so detail was not recorded.There is a deep valley near point T with an uncorrected depth of nearly 5100 meters.If a PDR had been in use, 63 this valley could well have been recorded at 5500 meters, corrected. There is a 500 km section along the axis of the Cordillera where no crossings have been made.Continuity of the Cordillera is assumed because the belt of earthquakes is continuous. Profile V-X shows several interesting features.Section V-W is along the 90th East meridian. An isolated peak projecting through Hakkel' Plain is crossed near point V.The north flank of the Cordillera is a scarp about 800 meters high with a slope of 20°. Point W should be on or south of the "crest" and W-X is along the flank of the Cordillera.The smooth area, broken by a few peaks, slopes upward toward Eurasia.This may be a section of a plain or the flank of a ridge.Profiles Z-Y, which were collected by two sub- marines running in company 4 km apart, cross this area and so does A'-B'.The profiles normal to the axis are more "broken" than is the profile along the flank.If the smooth section east of point W is the eastern end of the Sverdrup Plain, the Plain slopes from 3500 meters north of Severnaya Zemlaya to 4, 000 meters at the Atlantic end.In Plate I,this region may be depicted as more broken than it actually is.However, echograms along the continental slope in this region, (discussed later) show that the slope is far from smooth. The 5000 meter deep valley in the southern half of profile V-X appears to be a low-angle crossing of a rift valley.There is a 20 km section of "high fractured plateau" type topography in the floor of the 64 valley.Three of the earthquake epicenters reported by Sykes (1965) for a nine year period are located in this section.There is another section of Mhigh fractured plateauu, about 40 km long, north of point X at 3500 meters depth.This is close to the Eurasian continental slope.The echograms for profiles Y-Z were collected by two sub- marines on parallel courses 4 km apart.They show that the Nansen Cordillera intersects the continental slope north of Severnaya Zemlaya and is about ZOO km in width. Profile D' - H' in Figure 7 is a zig-zag along the upper conti- nental slope in the region where the earthquake belt enters the conti- nental block.The flat sections at D' and H' are the continental shelf. The profiles show that the upper continental slope, 150 to 2000 meters, has a slope of about 150.Point F is at the shelf break and the section of profile, F' - G', indicates a rather broken upper slope. Profile D'- H' provides coverage in the region where Soviet bathymetric maps show a large canyon cutting the shelf and slope. According to a map published by Hakkel' (in Hope, 1959), the axis of this feature, named the Trough, lies along the 1 30° E. meridian.Hakkel' interpreted the Sadko Trough to be a fault trough. The four legs of profile DLH' are aligned either 45° (D' - E' and E' - F') or 900 (F' - G' and G' - H') to the shelf edge; the profile cannot provide information on the width of the Sadko Trough or even 65 confirm its existence.

Sykes (1965)reported, 'One of the largest earthquakes in the Arctic during the last ten years't occurred near 78° 1Z' N. Lat.,

1Z6° 361E. Long. This location is about 80 km due north of point G' on profile D' - H'.The Sakdo Trough itself is the location of six or more of the epicenters reported by Sykes.

Geophysical Data

The data from earthquake seismology have been reviewed above and the epicenters are shown in Plate 1.The Nansen Cordil- lera is the site of essentially all of the earthquakes in the basin of the Arctic Ocean. No seismic refraction or reflection profiles have been shot in the area by U. S. investigators and none have been reported by foreign scientists.There are essentially no gravity data available from the Nansen Cordillera.Ostenso(1966)reported on gravity measurements taken from drift station ARLIS-Il.Unfortunately, the track of the station was over the upper continental slope north and west of Greenland and over the shelf in the Greenland Sea.The data indicate that the Greenland shelf is of continental thickness, while the Greenland Sea is probably oceanic. There are low altitude aeromagnetic profiles available for the

Nansen Cordillera.These are shown in Ostenso's1962and1966 papers.King, Zietz and Alidredge(1966),on the basis of both high altitude flights and Ostenso's(1962)low altitude profiles, wrote, "There is no magnetic evidence of a mid-oceanic ridge through the or in the Lena Trough between Greenland and Spitzbergen". A more recent map of magnetic anomalies in the Arctic, including unpublished low altitude measurements, has been shown to the writer by Ostenso (personal communication, December,

1966). The Eurasia Basin is generally characterized by magnetic anomalies of less than 100 gammas.However, the Nansen Cordillera is marked by a band of anomalies in the range of 100-300 gammas. In contrast, the Mid-Atlantic Ridge south of Jan Mayan Island is intensely magnetic, especially over the axis of the ridge.(See

Heirtzler and Le Pichon,1965,and Vogt and Ostenso,1966.)The "axial magnetic zone is of the order of 1000 km wide and is centered on the ridge axis", according to Heirtzler and Le Pichon.Anomalies on the order of 1000 gammas are typical of the Mid-Atlantic Ridge south of Jan Mayan Island. There are at least three explanations for the low level of mag- netic activity over the Nansen Cordillera as compared with the Mid- Atlantic Ridge farther south.(1) The Curie-point isotherm is near the surface; (2) the rock types are different (as suggested by Vine,

1966);(3) the magnetic anomalies over mid-ocean ridges are due to narrow intrusive bodies, and these do not occur in the Nansen 67 Cordillera.No doubt there are other, more sophisticated hypotheses which could be formulated.For reasons which will be discussed later, the first alternative is favored here.

Summary

The Mid-Atlantic Ridge, itself a part of a worldwide system of mid-ocean ridges, continues across the Eurasia Basin of the Arctic

Ocean.It has been named the Nansen Cordillera (Beal etal. ,1966). The Cordillera fills the floor of the passage between the Greenland Sea and the Arctic Ocean from the continental slope of Europe (Yermak Rise) to the slope off northeast Greenland.The maximum depth of 4300 meters in this region is found on the Greenwich meridian.(This passage is named the Lena Trough after the Soviet icebreaker which first proved the non-existence of the so-called Nansen Sill between Svalbard and Greenland.The name is somewhat unfortunate, because the opposite end of the Nansen Cordillera abuts the Eurasian Continent near the Lena River and there is a possibility of confusion. The Nansen Cordillera is strikingly different from any other part of the mid-ocean ridge system reported to date.First, dis- counting a few isolated peaks, it is only about 200 km wide while the other parts of the ridge system are more than 900 km in width; usually much more.Second, the other parts of the ridge system are always elevations above the adjacent sea floor while the Nansen Cordillera has nearly as much relief below the level of the Plains as it does above.That is, the individual mountains (or ridges) stand 1000 to 1500 meters above the Plains, while the deep rifts are 1000 to 1500meters deeper than the Plains.There is no detectable sedi- mentary fill (flat floor) in the V-shaped valleys or rifts on the PDR echogram s. The axis of the Nansen Cordillera appears to be slightly arcu- ate as shown in Plate 1, but this is no more thana 30 km departure from a perfect great circle. A 'Tstraight lineudrawn from the north end of the Lena Trough to the Sadko Trough, a distance of 2200 km, would lie entirely within the Cordillera.The Cordillera appears to become narrower in the eastern half of the Eurasia Basin, but this may be an illusion because the southern margin becomes involved with the continental slope north of Severnaya Zemlaya.There seem to be fewer peaks and valleys in this part of the Basin, but one im- portant exception to this is a 5000 meter deep rift at about 82N. Lat.However, in general, the eastern end of the Cordillera ap- pears to be less developed than does the western end.The eastern- most end of the Cordillera butts against the Eurasian continental slope along the 1 30th meridian where the Sadko Trough is shown on Soviet charts.In this region, the upper continental slope is steep, 15eand irregular. The Cordillera is seismically active with all recorded epi- centers falling in the central part of the mountains.The earthquake belt continues into the Eurasian continent, but becomes much broader there (Sykes, 1965). The Nansen Cordillera is not marked by the high amplitude magnetic anomalies which are characteristic of mid-ocean ridges. However, enough aeromagnetic data have been collected to show a band of 100-300 gamma anomalies over the Cordillera.The adjacent Plains show anomalies less than 100 gammas in amplitude.

Conclusion

The Nansen Cordillera is a youthful extension of the Mid- Atlantic Ridge.While it is all in a youthful stage of development,

the eastern end appears to be younger than the Atlantic end.(A possible age will be considered in the last section. )It is suggested that the Nansen Cordillera is an example of the first stage of develop- ment of a mid-ocean ridge.The sea floor is rifted in places and volcanoes and ridges are forming, but the floor has not yet been arched upward.

T1 1D1;,ir

There are six plains in the basin of the Arctic Ocean.Four of these plains are in the four sub-basins of the Arctic.All are close 70 to 4000 meters deep; and they will be referred to as Hthe deep plains ".From Alaska toward the Atlantic they are: the Canada Plain in Canada Basin, Fletcher Plain in the Makarov Basin, Hakkelt Plain in the Fram Basin, and Sverdrup Plain in the Nautilus Basin (Figure 1). The remaining two plains lie on the flanks of the Alpha Cordil- lera at its intersection with the Eurasian continental slope.These are the Chukchi Plain at about 2200 meters between the crest of the Alpha Cordillera and the Chukchi Gap, and the Wrangel Plain between the crest of the Alpha Cordillera and the Lomonosov Ridge at a depth of about 2700 meters.These plains will be referred to as "the plains at intermediate depth!.Submarine-collected sounding data for these plains have been presented in Figure 5 and discussed in a previous section (The Alpha Cordillera).

The Canada Plain

The Canada Plain forms the floor of the southeastern part of the Canada Basin.This plain is the best known of the deep plains since U. S. drifting stations, aircraft and icebreakers have collected at least some geophysical data in the area. The Canada Plain has a depth of about 3850 meters (3940 meters, corrected).The western margin of the plain has relief of a few tens of meters.This relief is especially evident along the 71 155° W. meridian between the Alaskan continental rise and the Chukchi Rise and between the north end of Chukchi Rise and the Alpha Cordillera. A recent Soviet bathymetricmap (Academy of Sciences, USSR, 1963) shows a spur extending north from the Chukchi Rise toward the Alpha Cordillera.The NAUTILUS, 1958 echogram showed a peak in the same region,so several peaks are shown here in Plate 1. Submarine echograms crossing the Canada Plain show broad "swells" and "scarps" with slopes of 1° to 2° and 10 to 15 meters of relief.It is suggested that these are real features on the sea floor, but the nature of the survey ship must be considered.The sub- marines could have created these shapes on the echograms by gradual depth changes.However, for several reasons, this is con- sidered unlikely.First, experience with under-ice submarines sug- gests that even a one meter departure from ordered keel depth would not be tolerated by the Conning Officer.Second, the speed of the nuclear submarine is high and control surfaces so large that any possible density change in the water could not change the keel depth by more than a fraction of a meter.Third, the submarines have each been equipped with an upward-beam echo sounder for measuring profiles of the lower ice surface. A change in keel depth of more than two or three meters would be obvious to the operator of this sonar.The only real possiblity is that of changes in the water 72 column, and thus in sound velocity, along the track.Available oceanographic stations are not spaced closely enough in time to evaluate possible changes in the sound velocity profile along the tracks.However, the submarine transiting under ice maintains a keel depth of more than 100 meters, so the echo sounder transducer is below the surface layer where extreme changes in water charac- teristics might be expected.The echograms are not closely spaced enough to evaluate the horizontal extent of these bumps and scarps on the floor of the Canada Plain. There are some geophysical data from the Canada Plain. Milne (1966), reporting on an unreversed seismic refraction line at the base of the continental rise north of Alaska, recorded 2. 5 km of sediment over the 4.40 km/sec velocity "basement' Ostenso (1962) reported a gravity survey in the region north of Alaska and con- cluded, "The Chukchi Shelf and Canada Deep (Plain) are in iso static equilibrium. " and Ostenso (1962), working from drifting sta- tion Alpha between the Chukchi Rise and the Alpha Cordillera, con- firmed isostatic equilibrium for this part of the Canada Plain as well. Ostenso's (1962) aeromagnetic studies suggest that sediment thickness in the Canada Plain is greatest near the continent and thins toward the Alpha Cordillera.Heat flow data have been discussed in a previous section (The Alpha Cordillera) where it was noted that the values of

1. 4rnicrocalories/cm2sec through Canada Plain are close to the 73 world average.

The Fletcher Plain

The Fletcher Plain occupies the deepest part of the Makarov

Basin.It is bounded on the north by the Lomonosov Ridge and on the east, wouth and west by the Alpha Cordillera.This Plain has a maximum width (along a line normal to the axis of the Lornonosov) of about ZOO km and a length of about 300 km, not including the val- ley extending toward Greenland between the Lomonosov Ridge and Marvin Spur.Crossings of this valley are shown in Figure 6.There is only one echogram available from the main area of this Plain, and it is in the extreme northern part.(See SARGO, 1960, Profile 5 in Figure 27).This crossing gives about 200 km of profile, but en- counters the Alpha Cordillera at both ends.The flat floor is at 3900 meters depth (3990 meters, corrected); or 50 meters deeper than the Canada Plain.There are no echograms placed well enough to show whether or not the apparent swales and scarps found on the Canada Plain are on this Plain also.

The HakkeP Plain

The Hakkel' Plain is bounded on the north by the Lornonosov Ridge, on the south by the Nansen Cordillera and by the Morris Jesup Rise and the Eurasian Slope at the ends.As mentioned in a previous 74 section, the limits of the Plain on the Eurasian end is taken from a map by Hakkel' (1958).There are seven submarine tracks crossing the Hakkel' Plain (Figure 2) which run along meridians, and one track which cuts diagonally across the Nansen Cordillera.-Eurasian slope corner.(This track is the source of the 3910 meter depth shown on the Eurasian end of the Plain in Plate 1. ) This Plain is second only to the Canada Plain in size.Its maximum width (across a line normal to the Lornonosov) is about 230 km near the Pole, narrowing to about 200 km near Eurasia (as- suming Hakkel's chart is correct).In length, the Plain is about 1650 km. The soundings are extensive enough to show that the Hakkel' Plain slopes from the Lomonosov Ridge downward to the Nansen Cordillera, and from Eurasia toward Greenland.The floor of the plain shows minor relief, ubumpst and gentle scarps which are too large to explain by submarine depth gage errors.The slope across the width of the plain is seen on all meridonal profiles and amounts to about 50 meters depth difference in 200 km of distance; a slope of 1:4000.The depth of the Hakkelt Plain, away from the continental rise of Eurasia, is from 3990 to 4320 meters, uncorrected.It is the deepest of the plains in the Arctic Ocean basin.. 75

The Sverdup Plain

The Sverdrup Plain in Nautilus Basin is the smallest of the four deep plains in the basin of the Arctic Ocean.Three submarines have crossed this small plain, so its extent and depth are reasonably well known (Plate 1 and Figure 4).The surface of the plain slopes down- ward from east to west and shows minor bumps and distributaries of less than ten meters relief on its surface.The adjacent continental slopes are broken and irregular.The depth of Sverdrup Plain ranges between 3825 meters and 3980 meters, uncorrected.Thus it is a little shallower than Hakkel' Plain and spans the depths of the Canada and Fletcher Plains.

The Continental Margins

The nuclear submarine expeditions have concentrated surveys in the deep basin of the Arctic Ocean.Except for one crossing of the , several transits of the Chukchi Sea and two through the Canadian Archipelago, no data have been obtained from the conti- nental shelves which make up most of the underwater area of the Arctic.Echograms have been collected, however, over the conti- nental slopes and rises which bound the basin of the Arctic Ocean. The profiles constructed from the echograms will be discussed in this section. 76 There are three areas of intermediate depth (i. e. 200 to 2000 meters) in the Arctic Ocean which have both oceanic and continental aspects to their physiography: these are the Yermak Rise, the Morris Jesup Rise and the Chukchi Rise shown in Figure 9A and 9B.These will be considered in turn.

The Yermak Rise

The Yermak Rise is a projection of the continental block north of Vest Spitzbergen (the largest island of Svalbard).There are seven submarine crossings available and the profiles are shown in Figure

9A.The interpretation of this feature is shown in Plate 1 and Figure

4. The Rise takes its name from the Russian icebreaker, YERMAK, which explored this shallow area.(Reported by Laktinov, 1959.) The Yermak Rise is T!separatedu from the shelf north of Vest Spitzbergen by a unotchu having a depth of 1500-2000 meters with shallower depths both south and north. There are no east-west crossings available, but the western flank must plunge steeply to the Lena Trough of the Nansen Cordillera. (See Figure 8. ) Minimum depths on the Rise are 500-650 meters. The north flank has gentle slopes in the west, steepening with depth and becoming steeper toward the east.The minimum depth found on the echograms, 500 meters, is near the northern edge of the Rise on the 10° E. meridian.This region is shown by profile D-E, Figure9A. 77

METERS

10 IR si 00

0 eS NP 000 E

3000

4000 ti Q I C I I F . IRI a. I 1 I

o [r G

U I______FATSOIS METERS 0 0 \111 'P H G FI

1000 NP 0 NP I - I00O- 2000 E

3000 ii

0000

4000 METERS

A B °°° K C 000 s000 MiiIIIIIIi_AA t NP

3000

VERTICAL EXAGGERATION 50 I 4000

0000 NP = NONPRECISION DEPTHYERMAK RECORDING RISE

Figure 9A.The Arctic Rises. METERS 000 A VERTICAL EXAGGERATiON 50:1______LOMIV 2000 B - - HA 9O°-- +

3000 D E METERS E D A 5000 2000 p C +85° MORRIS R I S E J ESUP LINCOLN SHELF p8 0° 80°

FATHOMS I000- 2000

F CHUKCHI RISESHEL F

Ostenso(1962)reported on aeromagnetic data over the Yermak Rise, "North of Spitzbergen on the northeastern corner of the Nansen Swell [Yerrnak Rise], there is a cluster of prominent anomalies from which it was possible to obtain four depth estimations ranging from

1 1/2km in the center to 3 and 4 km on either side. "This is in con- trast to the general magnetic quiescence of the Eurasia Basin, but note that the area specified is in the "high fractured plateau" part of the Rise. A more recent paper (Ostenso,1966)reporting aeromag- netic studies in the Greenland Sea showed that the western and central Yermak Rise is not marked by magnetic anomalies.These data are consistent with the bathymetry; smooth undulating surface and a gentle north slope on the western part of the Rise with steeper slopes and more broken surface on the eastern flanks. The Morris Jesup Rise

The Morris Jesup Rise, which takes its name from Cape Morris Jesup, Greenland, is the least known of the three Arctic Rises.There are only two nuclear submarine crossings of the feature (Figure 9B).Profile C-E in Figure 9B shows the physiog-. raphy of the Rise.Section C-D of this profile shows the floor of Hakkel' Plain at 3900 meters on the left and a slope which is the north flank of the Rise.The slope is about 5° for the lower two- thirds and increases to about 10° on the upper one-third.The mini- mum depth on the Rise is 700 meters.Following the profile toward the Lincoln Sea, the Rise deepens to 1700 meters, shoals to 900 meters and then shows a broad saddle with a maximum depth of 2200 meters.This saddle separates the Rise from the Lincoln Shelf. The Lincoln Shelf will be mentioned here because it is the most inaccessible continental shelf in the Arctic.The one echogram avail- able is therefore of particular interest.The upper continental slope has a steepness of 5° to 70 and is steeper just over the shelf break than in the region from 1000 to 2000 meters.The shelf break is at 250 meters.The minimum depth crossed is just under 100 meters. The shelf deepens shoreward of this minimum, reaching 750 meters in the narrow passage between Greenland and Ellesmere Island.The northwestern coast of Greenland is not ice covered and does not 31 appear to have been glaciated at any time during the . (Flint, 1947).This is substantiated by the echogram.During the maximum developments of the North American ice sheet, this part of the Lincoln Shelf would have been emergent.If the ice sheet had extended northwest of Greenland, the Lincoln Shelf would certainly have been deepened by ice scour. The other profile available, A-B in Figure 9B, crosses the Morris Jesup Rise from east to west.It shows a 3000 meter depth east of point A (point A is on the Alpha Cordillera) which may be a part of the valley which separates the Lomonosov Ridge from the Alpha Cordillera.The Morris Jesup Rise shoals in a series of scarps to a broken crest with a minimum depth of 500 meters.East of the crest, the flank of the Rise has a slope of 10° to a depth of 3100 meters.The section of the profile between the 3100 meter maximum and point B is the lower slope north of Greenland.The slope off the northeast corner of Greenland is shown to be extremely steep in Plate 1 and Figure 8.A word of caution is in order, how- ever.The Canadian cartographer, M. M. DeLeeuw has reported (personal communication, December, 1966) that, according to informa- tion he received from the Danish Government, the location of the coast of northeast Greenland is not known with any precision.Exist- ing maps may be in error by 25 km or more.The location of the lower continental slope is known from two submarine crossings to a better precision than this, but the steepness of the slope was deter- mined by the mapped location of the coastline.If the coast is actually 25 km farther south, the slope may be more gentle than shown. Five aeromagnetic profiles have been reported by Ostenso (1962 and 1966) and King, Zietz and Alldredge (1966) for the region north of Greenland.The Morris Jesup Rise is magnetically quiet and so is the northernmost coast of Greenland adjacent to the Rise.There are no other geophysical data available on which to base a hypothesis, but if the Rise is considered to be a thick sequence of sediments, it is in keeping with the magnetic information.

The Chukchi Rise

The Chukchi Rise is a northward extension of the Chukchi Shelf. It is the best known of the three Arctic Rises thanks to two submarine crossings and data from Soviet drift station North Pole 2 (Somov, 1955) and Ti. S. drifting stations (Shaver and Hunkins, 1964 and Ostenso, Den Hartog and Black, 1966).The two submarine collected profiles are shown in Figure 9B with locations.The interpretation of the physiography of Chukchi Rise shown in Plate 1 is based on all available soundings.The southeastern quadrant of the Rise is based on contours published by Fisher, Carsola and Shumway (1958). Various names have been given to parts of the Chukchi Rise as they have been discovered.The northern end was thought to be separated from the Chukchi Shelf by deep water and was named the Chukchi Cap (see Shaver and Hunkins, 1964, for example).In keeping with the rules for naming undersea features of the U. S. Board on Geographic Names (Edvalson, 1964), this part was renamed the Chukchi Plateau

(Bea1etal. ,1966).Fisher, Carsola and Shumway (1958) called the shallow area in the southeast the Northwind Seahigh.The name Chukchi Rise includes all of these features. Profile H-J in Figure 9B shows that the Chukchi Rise and Chukchi Shelf are not separated by any significant hmnotcht? or saddle. Along this profile, the Rise is continuous with the Shelf.The surface of the Rise is fairly smooth along this profile.There is one valley with about 300 meters of relief, and a number of small peaks of a few tens of meters relief toward the northern end of the Rise.Be- tween points I and J, the profile shows the Charlie Gap which has been discussed previously.The depth at point I is 250 meters.The other submarine collected profile, F-G, is from the NAUTILUS, 1958 cruise.This is a non-precision echogram, so the minor rough- ness shown on profile H-J would not have been recorded even if it were present on this part of the Rise.This profile is along the upper surface of the eastern flank of the Rise and shows more relief than does the echogram along the crest.Between the two profiles, drift station soundings show a deep valley trending north-south. A recent bathymetric map by Ostenso (unpublished, December 1966) which includes new data from aircraft landings and spot soundings, mdi- cated that the northern margin of the Chukchi Rise extends farther east than was previously known--to 148° W. Longitude. The U. S. drifting stations have provided gravity and magnetic data over the Chukchi Rise.No seismic data have been taken. Shaver and Hunkins (1964) have computed a model of the structure of the Rise which fits the gravity and magnetic data.This model calls for a basement ridge beneath the western and northern margin of the Rise and a 12 km thickness of sediment beneath the northern end of the Rise.Kutschaleetal. (1963) reported 6 km of sediment on the Chukchi Shelf south of the Rise (74° 31' N. Lat., 165° W. Long. ) based on a seismic refraction profile. The Chukchi Rise is much larger than the Yermak Rise and both are larger than the Morris Jesup Rise.However, all three of these features may be wedges of sedimentary rock adjacent to the continental shelves.Based on magnetic data, the Yermak Rise and Chukchi Rise appear to have dikes or buried seamounts (volcanoes) along at least the parts of their margins which border the basins, while the Morris Jesup Rise does not.

Continental Rises and Slopes

The physiography of the continental rises and slopes surround- ing the basin of the Arctic Ocean is interpreted in Plate 1.In general, the slopes are steep and broken and the continental rise poorly developed except along the margin of the Canada Basin between 120° and 150° W. Long.This is surprising since the continental area supplying sediment to the great rivers which empty into the Arctic Ocean is as great as the area of the ocean. A number of the profiles showing the continental slopes have been discussed in connection with the mountain ranges and the Arctic

Rises (see Figures 5,6,7, and 9).The remainder of the margins of the Arctic Ocean basin will be considered in this section beginning at the 120° W. meridian and proceeding clockwise around the basin. The profiles are shown in Figure 10. The continental margin off the southwestern Canadian Archi- pelago and eastern Alaska is shown in Figure 10, profiles 9,10,11, and 12.This part of the continental slope of the Canada Basin is quite smooth. From the shelf edge to a depth of 2000 meters, the slope is about 2°.Deeper than 2000 meters, the slope is less than 1° to the Canada Plain.West of the Mackenzie River (134° W. Long.), the continental slope becomes steeper, 4° to 10°, on the upper 2000 meters.(Carsola etal., 1961 have published icebreaker-collected profiles from the continental shelf and slope north of Alaska. )The continental rise off the Barrow Canyon (155° W. Long.) is broader and more gently sloping than it is farther east, but all of the rise north of Alaska has topography which may be due to slump structure PROFILE NO. b PROFILE NO. 2 b a PROFILE NO b U 86 I oo IIh PROFILE NO 4 a b 1.00 PROFILE NO 3 TILE NO 6 PROFILE NO.9 b METERS a 5j! S 6 . PROFILE NO. I 2 - IL CONTINENTAL SLOPES AND Figure 10 RISES and deformation. During August 1965, about 5000 bottom photographs of the lower slope and continental rise north of Point Barrow, Alaska were ob- tamed.Plate 3 shows two of these photos from 2800 meters.Note the scarps on the surface of the sediment.Dozens of the photos taken on the lower continental slope show linear scarps and cracks in the sediment.Sediment being actively deposited from suspension does not have the mechanical strength to support "faults" on the surface. The photographs suggest that the lower slope has not been the site of rapid sedimentation for some time.It is probably much as it was when sea level rose some 10, 000 years ago and flooded the broad Chukchi Shelf.Sediment which was formerly carried to the shelf edge by the Alaskan rivers is now probably trapped on the shelf. One other interesting finding from the bottom photos is that only five of the 5000 showed ice rafted material.This contrasts sharply with Schwarzacher and Hunkins' (1961) photographs of the Alpha Cordillera which showed great amounts of ice rafted debris on the bottom.This suggests that ice rafting was an important mechanism during the Wisconsin glaciation when the ice pack "grounded" at the shelf edge during and melted over the slope and basins during summer.The winter now grounds along the coasts and islands well away from the basins.Ice which has picked up gravel apparently melts in summer over the shelf rather

than over the basins and slopes.Thus the picture of sedimentation in the Arctic Ocean was changed completely by the rise in sea level. (as well as in the rest of the world ocean).The largest river of the , the Mackenzie, must have poured great quantities of sediment directly into the Canada Basin when sea level was lower.Now the river is building its delta 200 km from the slope. One enigma presents itself in the photographs of the continental rise north of Point Barrow.While the sediments appear to be 'told" and partially consolidated, an extensive bottom-dwelling fauna was photographed.These animals must have a source of organic detritus from the surface, yet the rise itself appears to be a region of slow or non-deposition at the present time. A strong surface current moves north past Point Barrow over this part of the slopeand rise. Casual observations made with a dip-net from a small boat in the waters off Barrow during the early of 1957 and 1958 showed a profusion of animal life in this current.(Pteropods and arrow worms were especially abundant.) Possibly theseanimals perish quickly when the warm, low water in their current mixes with the cold water farther north.If this is the case, there would be a considerable amount of organic matter raining down on the lower slope and rise. The Alaskan continental slope between Barrow Canyon and the Chukchi Rise is steep and broken (Figure 10, profile 11).The shelf break occurs between ZOO and 250 meters, but the slope is gently rounded to 500 meters.Below 500 meters, the upper slope is 15° to a broken shelf at about 1500 meters. This shelf is some 20 km wide and shows several peaks and valleys of more than 200 meters relief.The lower slope is irregular and there is a second shelf at 3100 meters depth. Before leaving this part of the margins of the basin, mention will be made of the "Beaufort Terrace" (also called the "Beaufort Plateau").This submarine feature was first shown on a Soviet bathymetric map dated 1954 (see Hope, l956. On this map, the "terrace" has minimum depths of less than 1000 meters as an elongate, east-west crest between 138° and 145° W. Long. at 73° N. Lat.The 2000 meter isobath extends east to the continental slope off Banks Island. Two submarine expeditions (Figure 10, profiles 11 and 12) have crossed the region of the t'Beaufort Terrace" and show that it does not exist.In addition to the profiles shown data were collected by two submarines which carried out operational exercises in the area for ten days during summer 1962.Course and speed changes were frequent during the exercises and the tracks cannot be recon- structed.However, the echograms collected show only the conti- nental rise and Canada Plain. 91 The source of the "Beaufort Terrace" may well be a map published by Stefansson (19Z1).In 1914 Stefansson and his men trekked across the sea ice from the of Alaska to about 740 N. Lat., 1400 W. Long. and thence eastward to Banks Island. Wire soundings were taken, but Stefansson wrote (op.cit. page 170), "We took soundings in most of these leads but were never able to get bottom with the amount of wire we had, so we are able to say only that the depth was in excess of 4500 feet (1386 meters). We had had more wire than this when this when we left shore, but we had been breaking and losing it at various soundings. " Yet Stefans son's map of the ice journey (fold-out, following page 140) showed spot sound- ings in meters with no indication that they are "no bottom at" wire lengths.Several of these soundings are less than 1000 meters in the area later shown as the crest of the "Beaufort Terrace". It appears likely that the compilers of the 1954 Soviet bathy- metric map took Stefansson's "no bottom" soundings as actual depths and thereby created the "Beaufort Terrace".Since no U. S.ship or drifting station ever crossed the area, the "Beaufort Terrace" has been copied on later bathymetric maps of the Arctic Ocean.(For examples, see Ostenso, 196Z; Hunkins, 1966 and Academy of

Sciences U. S. S. R., 1964. )However, Ostenso has said (personal communication, December, 1966) that the spot soundings taken in connection with his gravity survey in the Canada Basin have led him 92 to revise his 1962 map and contour the IBeaufort Terrace! as a much smaller feature. Between 155° W. Long. and 900 E, Long. the continental slope is marked by the intersection of the Chukchi Rise and the three mountain ranges with the continental block.Most of the available profiles have been discussed in previous sections (see Figures 5,6, 7 and 9), and the rest will be discussed here.Profile 1 in Figure 10 is across the upper slope and shelf edge inshore from the Chukchi Plain and the crest of the Alpha Cordillera.The upper slope above the Chukchi Plain is quite smooth.The shelf break in this region is well inshore and two submarine canyons cut the shelf.Creager and McManus (1965) showed that the Pleistocene drainage pattern on the then emergent Chukchi Shelf emptied into these canyons.Ice- breaker echograms taken by Beal (unpublished map, 1962) showed that the canyons lead into the slope shown in profile 1.Thus this slope and the Chukchi Plain below no doubt received a considerable amount of sediment when the shelf was above sea level.The right half of profile 1 shows that the region near the shelf break above the crest of the Alpha Cordillera is marked by gullies of 20 to 40 meters relief. Profile 2 in Figure 10 is generally along the axis of the Alpha Cordillera where it meets the East Siberian shelf.This profile shows a gentle slope from the shelf edge to a broad step at about 93 1200 meters.The outer edge of this step is shallower than its inner edge.Seaward of the step, a saddle separates the step from the peaks of the crest of the Alpha Cordillera, which show a least depth of 750 meters on the profile. The left sides of profile 3 and profile 4 in Figure 10 are along the continental slope north of the New Siberian Islands and the Laptev

Sea.Both profiles originate on the upper slope where the Lomonosov Ridge intersects the slope, and continue to the HakkeP Plain. (Profile 3 extends beyond the Plain across the Nansen Cordillera and the slope off .) Profile 3 shows an upper slope of less than10(but the profile is not normal to the slope) to a sequence of peaks on the slope between 2000 and 3000 meters.Hakkel' Plain then slopes downward to the Nansen Cordillera.Profile 4 shows the continental slope to be marked by steep sections separated by more gently sloping steps and saddles.These are taken as evidence of faulting on the slope. A well developed continental rise is evident on these profiles, but it slopes upward toward the Lomonosov Ridge. In profile 4, about half the width of Hakkel' Plain, 70 to 80 km, adjacent to the Nansen Cordillera is marked on the echogram by small hyperbolas less than five meters high.In texture, this section resembles the TThigh fractured plateau" type of echogram encountered on the Alpha Cordillera and in one of the rifts in the Nansen Cordil- lera near this section of profile 4.This may be an extensive lava 94 plain adjacent to the flank of the Nansen Cordillera.

Profiles 5,6,7 and the western end of profile 3 show the con- tinental slope of the Eurasia Basin from the to the Yermak Rise north of Svalbard.Profile 3 shows large blocks of some 300 meters relief, 30 to 60 km in length between the NansenCordillera and the lower continental slope off Severnaya Zemlya.These might be consolidated blocks which have slipped down slope (this seems un- likely to the writer), or fractured and tilted blocks of the seafloor. The slope above the blocks is smooth, but there is a ZO km section of "high fractured plateau" texture between 800 and1000 meters on the upper slope. Profile 5 is inshore of profile 3 discussed above and, like profile 3,is at a shallow angle to the trend of the slope.This pro- file shows that the edge of the shelf has minor relief of ZO to50 meters.The slope is a series of peaks and valleys to a depth of 3000 meters. Profile 6 is along the continental slope from Severnaya Zemlya to the Yermak Rise.Profile 7 is a nearly normal crossing of the slope off (see location map, Figure 10).These profiles indicate that the slope off the Kara and Barents Seas is smoother than is the slope farther east, closer to the regionwhere the Nansen Cordillera abuts the continental block.Profile 7 shows an irregular continental risedeeper than 3000 meters sloping down 95 downward to Sverdrup Plain.The lower continental slope above the rise is about 15° up to 1000 meters and is marked by three steps or peaks.Above this, the slope is about50 toa depth of 400 meters and more gentle toward FranzJosepf Land. The break in slope at 400 meters may be the shelf break in this region, or thisdepth may be related to the Svataya Anna and Voronin Troughs which are found on the floor of the Kara Sea (see Figure 4). The continental slopes north of Svalbard and north of Greenland are marked by the Yermak and MorrisJesup Rises.They have been discussed and the profiles presented in a previous section.The slope from the Lincoln Sea to 120° W. Long. is formedby the inter- section of the Lomonosov Ridge and the Alpha Cordillera withthe continent and these data have been presented and discussedin the appropriate sections. Profiles 8 and 9 in Figure 10 show some details of the upper continental slope and shelf off the Canadian Archipelago.The sec- tion of profile 8 north of the turning point shows thepeaks of the Alpha Cordillera.Southwest of the turning point, there is a scarp with a slope of about 10, from 2000 meters to 475 meters.This scarp is the upper continentalslope north of Ellesmere Island.The 475 meter deep level area may be the shelf in this area, orthe pro- file may run seaward of the shelf break along the slope.The rest of profile 8 and all of profile 9 are near the shelf edge offthe Archipelago.Nearly all of the profiles show a gullied shelf edge. Off Nansen Fijord, Sverdrup ChannLl, and McLure Strait, the profiles show depths of 500 to 600 meters.These 'troughs" may be evidence of glacial action on the continental shelf. In summary, more than half the perimeter of the Arctic basin has continental slopes marked by the inter section of three mountain ranges and three rises with the continental blocks.Along the rest of the perimeter, the continental slopes have slopes between 1° and 150.

The slope off eastern Alaska is the gentlest: 10to 2°.All other slopes appear to range between 10° and 15°.(Note in Figure 10 that most of the profiles are at a small angle to the trend of the slope.

Of course, only a normal crossing can show the true steepness ofa slope.) Many of the profiles show an irregular or blocky continental slope.No Arctic Ocean slopes of the type where the angle increases with depth have been found.According to Shepard (1964, Chapter 10), this type of slope is known only in the Pacific Ocean to date.There is a continental rise north of Alaska and in parts of the Eurasia Basin off the Barents and Kara Seas.The rise north of Alaska shows evidence of slumping and those off Eurasia are poorly developed and show considerable large scale roughness.

The continental are supplying sediment to the great rivers which empty into the Artic Ocean is about equal to the area of the Artic Ocean itself. 97 The finding that the continental slopes around the Arctic basin show signs of smoothing only across about5° of Longitude north of Alaska is surprising.If the Arctic basin has been in existence for long periods of geologic time, gentle, smooth slopes should be the rule rather than the exception.

Summary of Geophysical Data

The limited number of geophysical data from the Arctic Ocean basin have been reviewed in previous sections as they have applied to each of the ocean floor features discussed.In this section, geo- physical data will be summarized and considered in the context of the physiography of the entire basin.Data most pertinent to this study are listed in Table I and illustrated in Figure 11.

Earthquake Data

(1)Distribution: Sykes (1965) showed that essentially all of the earthquakes in the Arctic basin are located in the Nansen Cordil- lera.(Z)Study of surface waves: By studying the dispersion of earthquake surface waves, it is possible to judge whether the crust through which the waves travel is oceanic (about 10 km thick) or continental (about 35 km thick).Oliver (1962) has published emperi- cal dispersion curves for Love and Rayleigh waves propagated through oceanic and continental crusts. 0

510

V 15 Magnetic anomalies mostly ( 200 gammas Magnetic anomalies 1500parallel -2000 gammas, to cordillera's axis may be°banded" C t 0 0 V eq NansenEarthquake Cord. belt - i$Lomonosov Ridge Alpha Cordillera km/sec 6A44.70 , f q Chukchi Rise Shelf iL. km/sec 6V ,e' \' 4.40km/sec 10 E Figure Il e0' ) 7.56km/sec (model study) 15 Summary Of Geophysical Data V 0 25 "schematic"scale only, approximate horizontal (model study,Wrong.1 Plain) 35 M C M- 25 S0E (model study) 7.56 km/sec 35 Table I.Geophysical data from the Arctic Ocean Location! Reference kind of observation Result/Conclusion

D'Andrea. Thiel Chukchi Shell-seismic Crustal thickness, 30 km mantle and Ostenso, 1962 refraction line, 320 ion velocity 7. 56 km/sec other layers doubtful Mime. 1966 Southern margin of Canada Crustal thickness, 15 Ian, mantle Plain-seismic refraction velocity 7. 56 km/sec, no oceanic line, 50 kin layer present, 2. 5 ion of sediment Ostenso, 1962 Chukchi Shelf, Southern Isostatic equilibrium Canada Basin-gravity meter

Shaver and Chukchi Rise-Plain Isostatic equilibrium Hunkins, 1964 gravity meter, magneto- "basement ridge", crust 30-35 km meter thick, 12 km of sediment Black and Chukchi Rise and adjacent Isostatic equilibrium Ostenso, 1964 Canada Plain-gravity Hunkins, 1961 Southern flank of Alpha 0. 38 km of sediment, Cordillera-3 short, tin- 2. 80 km layer with 4. 70 km/sec reversed refraction lines velocity, over an oceanic layer of 6.44 km/sec Kutchale, Wrangel Plain on the 3. 5 km of sediment 1966 Alpha Cordillera crust 22 1cm thick under spur of Alpha reflection lines, gravity Cordillera, 15 1cm under Plain meter, magnetometer Lachenbruch South flank of Alpha Heat flow is world average in the and Marshall, Cordillera and adjacent Plain, one-half as muth in the 1966 Canada Plain-Héatflow Cordillera. Cont'ast in rock types required. Crary and Northeast flank of Alpha little if any sediment on Cordillera, Goldstein. 1957 Cordillera--seismic reflec- 1.0 km in valley with 5. 96 km/sec tion lines, short refraction below.High density material below lines, some gravity readings valley floor.

Ostenso, 1962 Arctic Ocean-aeromag- Amerasia basin and Alpha Cordillera netic survey, low altitude intensely "magnetic", Eurasia Basin and Lomonosov Ridge show low mag- netic activity, Shelves and Rises show low activity. King, Zietz Western hemisphere of Amerasia Basin and Alpha Cordillera and Alidredge, Arctic Ocean-aeromagnetic intensely "magnetic", Eurasia Basin 1966 survey, high altitude very low activity, Lomonosov shows 200-300 gammas anomaly.

Sykes, 1965 Arctic Ocean-Greenland Nansen Cordillera is site of Sea -- earthquake seis- essentially all earthquakes in the mology Arctic Ocean. 100 Surface wave studies have been carried out specifically for the Arctic basin by Oliver, Ewing and Press (1955) and by Hunkins (1963). Bath (1959) and Santo (1960) studied surface wave dispersion on a world wide scale, but each made special reference to the Arctic basin.All of the surface wave studies agree that the Arctic basin is underlaid by oceanic crust of about 10 to 15 km thickness.Love waves are filtered out in crossing the basin and Rayleigh waves show oceanic type dispersion.

Kings Zietz and Alldredge (1966) reviewed the Arctic surface wave data and suggested that if the Eurasia Basin only were inter- preted as oceanic, the data interpretation would be satisfied.This arises from the fact that all the Arctic earthquakes studied occurred in the Eurasia Basin,Some part of each propagation path to the re- cording station always crossed the Eurasia Basin.Because of this, King, Zietz and Alldredge (1966) believed that the surface wave data cannot rule out the possibility that the Amerasia Basin is underlaid by a crust of continental thickness,The writer does not agree with

this conclusions because Rayleigh wave dispersion of one earthquake reported by Hunkins (1963) did give information on the crustal thick- ness of the Amerasia Basin.The epicenter of that earthquake was in the Laptev Sea (76. 7o)N. Lat. . 131,10E) and the recording station at Resolute in the Canadian Archipelago.Nearly 50% of the propaga- tion path was in the continental shelves of the Laptev Sea and the 101 Archipelago, but the deep water part was entirely within the

Amerasia Basin (Hunkins,1963,Figure 1, path RB4).The Rayleigh wave dispersion indicated a crust of oceanic thickness for the

Amerasia Basin (op. cit. Figure2). Love waves were not studied for this earthquake. Because of the wide continental shelves bordering the deep basin, studies of surface wave dispersion from Arctic earthquakes all suffer from the fact that a large part of the paths to the recorders is continental.Nevertheless, each of the surface wave studies re- ported indicated a crust of much less than continental thickness under the deep Arctic basin.

Aeromanetic Data

Recordings of the total field by airborne magnetometers are perhaps the most useful of the geophysical data from the Arctic basin, because of reasonably good geographical coverage.Flight lines are widely spaced, but information is available from all parts of the basin.This is not the case for the other kinds of geophysical data.(Information on earthquake distribution, etc., is available for the entire world, including the Arctic.

Ostenso(1962and1963)and King, Zietz and Alldredge(1966) are the principal references for Arctic aeromagnetic data.The major conclusion reported by these writers is that the Amerasia 102 Basin shows very diUerent magnetic patterns from the Eurasia Basin.The Eurasia Basin is magnetically quiescent with anomalies generally less than 200 gammas.In contrast, the Amerasia Basin is marked by anomalies of from 1500 to 2000 gammas (Figure 11). Ostenso (telephone conversation, July, 1967) has told the writer that he finds it possible to connect anomalies across the Arctic basin flight lines, even though the lines are widely spaced.When this is done, he finds that the anomalies are banded generally parallel to the axies of the Nansen Cordillera and the Alpha Cordillera.This is the pattern which appears to be typical of mid-ocean ridges and is corroborating evidence that the Alpha and Nansen Cordilleras are mid-ocean ridges.

Seismic Reflection-refraction Studies and Models Based on Gravity Data

Crustal layering and thickness interpreted from explosion seismology are important data for attempting to understand the structure and geologic history of the .Unfortunately such data are very limited for the Arctic basin, and the few which are available are mostly from the Canada Basin (see Figure 11).The almost complete lack of seismic refraction studies is the most serious lack in our knowledge of the structure of the Arctic basin. Without seismic velocities for guidance, structure models based on 103 gravity surveys alone will be ambiguous.(Several gravity surveys have been accomplished, but are as yet unpublished.) There have been three refraction profiles shot on the conti- nental shelves of the North American Arctic. ie, reported by DtAndrea, Thiel and Ostenso (1962), on the Chukchi Shelf showed a depth of 30 km to the M discontinuity (Figure 11).The mantle velocity oi 7. 56 km/sec reported is lower than the 8. 00km/sec con- sidered normal'.The authors reported that crustal layering or velocities could not be determined from the shot records. l'wo refraction profiles have been shot in the Canadian Archi- pelago off the northern end of in water depths oil less than 500 meters.The crustal thickness was determined to be 30 km.(These results were mentioned by Weber, 1963 Figure 2, in a published discussion, but the writer has not been able to find a more complete report in the literature. Mime(1966) reported results of an unreversed refraction pro- file from the margin of Canada Basin on the Alaskan continental rise.Mantle velocity of 7. 56 km/sec was found 15 km below sea level.Mime suggested that the low mantle velocity is a result of the dip of the M discontinuity downward toward the continent.Above the M discontinuity, Mime (1966) reported 2. 5 km of sediment laying directly on basement 8. 68 km thick with velocity of 4. 40km/sec. The oceanic layer was not present.(This crustal section is shown 104 schematically in Figure 11under Canada Plain.It should be on the Alaskan edge of the Plain; but the cross section was drawn to show the Chukchi Rise. )Mime's refraction profile is the only one reported to date which obtained a crustal thickness in the Arctic basin. The three determinations of crustal thickness of the continental shelves and the single measurement in the basin show that the shelves are of continental crustal thickness while the basinis underlaid by oceanic crust.This was obviously to be expected, but in view of the suggestions (discussed previously) that the Amerasia Basin may he a foundered continental block, these few seismic measurements of crustal thickness are especially significant.The results cor- rohoratetHeearthquake surface wave data and show that the basin of the Arctic Ocean is, despite its small area and location between the great continental blocks, a true ocean h sin. Shaver and }-Iunkins (1 964) have reported gravity data for the Chukchi Rise and Plain; west of the location of Mime's (1966) seismic refraction line in the basin, and north of D'Andrea, Thiel and Ostenso's (196Z) line on the Chuckchi Shelf.A density model presented calls for 15 km of sedimentary rock and a total crustal thickness of 30 km under the Chukchi Rise.There are no seismic velocities for guidance, so this is only one of many possible density models that would explain the gravity data.At best,it can be said that the gravity data indicates that the Rise is an extension of the 105

Chuckchi Shelf. Farther west around the margin of the Amerasia Basin, Kutschale (1966) obtained gravity and seismic reflection data from Wrangel Plain.With guidance from the reflection data for sediment thickness, Kutschale reported a crustal model 25 km thick to fit the observed gravity.This is not, of course, an exclusive model, but, considering the location of Wrangel Plain on the continental slope, it appears to be reasonable. One seismic refraction study has been reported from the Alpha Cordillera (Hunkins, 1961) but mantle velocity was not obtained.The results indicated a thin veneer of sediment; 2. 8 km of material having a velocity of 4. 70 km/sec over an "oceanic layer" of undetermined thickness and 6.44 km/sec velocity.(This section is shown in Figure 11. Crary and Goldstein (1957) reported two, "one shot' seismic refraction measurements from the region of the Marvin Spur in the Alpha Cordillera (see Figure 2).Shot point to receiver spacing was only 12. 7 km and, as the radio "shot time" signals were not re- ceived, this distance was obtained by surveying (both shot point and receiving hydrophones were adjacent to ice island T-3).One of the measurements was made on the north flank of Marvin Spur in about 2600 meters of water.The slope of the ridge was computed to be about 2.50and the refracted wave showed material with a velocity 106 of 3. 77 km/sec directly under the sea floor.This implies that there is little or no sediment on the flank of Marvin Spur, but the velocity is low for volcanic rock.Possibly the rock is heavily fractured, if volcanic, or it is conceivable that it is sedimentary rock.(Under the interpretation, discussed later, that the Lomonosov Ridge is a conti- nental fragment welded to the north flank of Alpha Cordillera, this would not be unexpected. The second refraction measurement was made near the crest of Marvin Spur in 1670 meters of water.The records showed about 1300 meters of sediment with a velocity of Z. 44 km/sec over base- ment with a velocity of 5. 96 km/sec.The slope of the bottom was 0.50.This thickness of sediment and the velocity of the underlaying rock are both greater than Hunkins (1961) reported for the south flank of Alpha Cordillera (300 meters of sediment over 4. 70km/sec basement).However, the 5. 96 km/sec velocity is within the range of volcanic rock velocities.Furthermore,acomparison of two short, unreversed seismic refraction measurements on the opposite flanks of a cordillera is not likely to be meaningful. The geophysical data outlined in the paragraphs above are all that have been published for the Arctic basin and shelves.Ostenso (1963) has published some gravity data from the Alaskan continental shelf and adjacent margin of the Canada Basin and has more exten- sive coverage of the Amerasia Basin in process.Weber (1963) and 107

Weber and Sobczak (1962) have mentioned gravity data from the Canadian Archipelago.There are two tentative conclusions which may be drawn from these gravity data:(1) The Canada Basin is in isostatic equilibrium; (2) The margin of the continental shelf from Alaska to in the Canadian Archipelago is marked by a positive free air gravity anomaly of 80 to120 millegals.The anomaly is similar to those described by Worzel and Shurbert(1955) for the Atlantic continental shelf.This is interpreted to be the result of the thinning of the crust at the edge of the continent.This cor- roborates the seismic refraction measurements; the North American continent is of continental thickness at the margin of the Arctic, while the Canada Basin is underlaid by oceanic crust. There are no seismic refraction or gravity data available from the Eurasia Basin.

Heat Flow Data

The limited number of heat flow measurements from the Arctic basin are grouped in a small area on the southern flank of Alpha Cordillera and the adjacent margin of Canada Plain (Lachenbruch and Marshall,1966).The results are summarized in Figure 11.The heat flow of 1. 4cal/cm2secthrough Canada Plain is about the world average as it is presently known.Within about 25 km, on the flank of the Cordillera, the heat flow has fallen to 0. 8cal/cm2sec.The implications of the low heat flow through the flank of the Cordillera will be discussed in a following section.Heat flows which are higher than average can easily be explained by a convenient dike or magma chamber.Low heat flows are more difficult to understand. The possibility that the 1. 4 cal/cm2sec heat flow through the plain could result from unusual amounts of radioactive minerals in the sediments should be considered.The concentrations of radio- active materials in deep sea sediments is generally less than 4 ppm (Koczy, 1954).However, a principal source of sediment for Canada Plain is the Mackenzie River system which drains a Precambrian terrain.The regions around Great Bear Lake and Great Slave Lake especially contain commercial deposits of uranium minerals. Lachenbruch and Marshall (1966) calculated that 40 ppm of uranium distributed in a 1 km thickness of sediment would provide enough radioactive heat to account for the observed heat flow through Canada Plain.(The sharp boundary in heat flow values between the Cordillera and the Plain requires that the sediment thickness be only about 1 km. A thickness of 5 km, for example, would allow lower concentration of uranium, but would 1blurT the boundary. ) Lachenbruch and Marshall (1966) reported results of gamma ray spectroscopy on two short cores; one from the Canada Plain and one from the Alpha Cordillera.These tests indicated a concentra- tion of only about 2 ppm of radioactive material in each of the cores. 109

Because of this, the thermal model of contrastingrock conductivities was accepted as the mostlikely explanation of the heat flow data. However, the writer believes that the possibilityof radioactive heat- ing in the sediments of the Canada Plain cannotbe ruled out entirely until longer cores have been analyzed and data onseismic velocities in the crust of the Cordillera and Plain are available.

Physiography Related to Possible Structure of the Crust and Upper Mantle

In view of the paucity of geophysical data, it maybe useful to discuss the possible structure of the Arctic basin based onphysiog- raphy, with such guidance as the geophysical data canprovide. Further, the concept of sea floor spreading frommid-ocean ridges will be taken as a working hypothesis and thephysiography of the basin evaluated in this light. A hypsographic diagram of the elevations ofthe earthvs sur- faces (Menard and Smith,1966, for example) showsthat the general level of the ocean basins (not including mid-oceanridges) is about 5 km below sea level.According to contemporary hypothesis, this level is determined by the surface of the mantle.The ocean floor is the lowest temperature condition of the mantlerocks; a ITrindit rather than a crust (Hess, 1962Dietz, 196Z).If this hypothesis is correct, the four deep basins of the Arctic areeach at about 4 km 110 depth because they are floored by the rind of the mantle overlaid by 1 km or less of sediment.Taking this as a starting point, the gross physiography will be discussed from the viewpoint of departures from the 5 km level. The Amerasia Basin is dominated by the Alpha Cordillera. The main arch" or uplift of the Cordillera has a relief of about 2 km above the Canada and Fletcher Plains (not including the volcanoes on the Cordillera).At the southernmost margin of Canada Plain, some 1500 km from the crest of the Cordillera and adjacent to the conti- nent, the basement rocks are found about 6. 5 km below sea level beneath 2.5 km of sediment (Milne,1966).Thus, for the Amerasia Basin, some 4. 5 km of relief (not including volcanoes on the Cordil- lera or the sediments) exists.About 3 km of the relief is above the 5 km "reference level" discussed above, and some 1. 5 km is below that level.

Onecouldtabulate a numberofrock columns with different densities extending to various depths which could account for the 4. 5 km of relief mentioned above.However, considering the com- plete absence of rock samples and lack of seismic velocity data at depth, this sort of computation will not be very meaningful unless some reasonable working hypothesis is taken.Mantle convection of the type suggested by Orowan (1965) is assumed: the viscosity of the mantle is plastic-Andradean and particle motion is by hot-creep. 111 Under these conditions, the rising limb of the convection cell is a broad plugor dike the width of the ridge which it forms.Material must move horizontally away from the ridge, but as Orowan(1965, 1966) showed, particle motion is not restricted to motion away from the ridge axis and along the flanks with linear velocity.(In fact, this sort of discontinuous motion from the vertically rising dike is im- possible in hot-creep. )The motion is continuously diverging as shown by Menard (1964) for the East Pacific Rise and by Orowan (1966) as a general hypothesis.The consequences of this model are that the rocks of the crest of a mid-ocean ridge may be older than the rocks of the flanks (i. e., new oceanic crust is formed by differ- entiation of mantle material on the flanks of the ridge) and that the rate of spreading is not necessarily linear from ridge axis tothe point where the cell descends (Orowan,1966 and Vogt and Ostenso,

1967). The phenomenon which causes mantle convection to begin along some particular line of upwelling is unknown.If the temperature gradient of the upper mantle is super-adiabatic, which it certainly is (Elsasser, 1966, for example), then the mantle is unstable in a thermal sense.Hess (1965) assumed the vertical dimension of a convection cell to be about 750 km, because this is the maximum depth of earthquakes.Below this depth, radiative heat transfer is effective and dissipates stresses.According to this hypothesis, heat 112 from the liquid core warms the lower mantle by radiation.(Crystal- line material such as olivine is somewhat transparent to heat.At temperatures above 10000 C, radiative transfer is significant; at higher temperatures it becomes overwhelming.See Elsasser,1966, for discussion. )Radiative transfer is less efficient in the upper mantle.Therefore, the temperature gradient becomes steeper and dike-type upwelling occurs.The place where the upwelling corn- mences could be related to minor inhomogeneities at the core-mantle boundary or in the mantle itself.Orowan(1966)has put forth the intriguing suggestion that the upwelling dike may occur preferentially under continents, because the continental plate itself is heated by radioactivity.The temperature of the mantle under the continent in- creases and upwelling starts there.

Orowans(1966)model of dike convection is here combined with the petrography of mid-ocean ridges and sea floor spreading of Hess

(1965). The mantle is periodotite which differentiates into and serpentine as the upwelling material enters a regine of lower pres- sure and temperature beneath the ridge .The water for serpentini- zation is in the upwelling mantle material itself and the reaction occurs when the temperature drops to about 500° C.Serpentinization of the peridotite releases some 100 calories of heat/gm and this re- action may provide a part of the heat flow on the crest and flanks of an active mid-ocean ridge.The serpentine layer formed (perhaps 113 more correctly, the serpentinizedperidotite layer) is bounded below by the5000 Cisotherm while its upper surface is coated withbasalt.

According to the Hess(1965)model, this strip of oceanic crust or 1rind moves down the flanks of the ridge andbecomes the floor of the ocean.According to Hess (1962) and Dietz (1962), the oceanic crust is disposable at the downward movinglimb of the convection cell.As the serpentine and basalt sinks (usually in atrench or under a continent) and heats to morethan500°C, the layer reverts to peridotite with a release of water to the oceans.This reaction re- quires about 100 calories/gm and therefore servesto depress the

500*C isotherm in the descending limb ofthe convection cell.The rate of motion of sea floor spreading fromseveral sections of the

mid-ocean, ridge has been investigated by Vine(1966)on the basis of reversals of the earth's magnetic fieldrecorded in the rocks of the sea floor.This study demonstrated spreading rates offrom 1 to4. 5 cm/yr; which is an indication of the speed of the convectivemotion. It is not proposed to discuss here thediverse evidence relating to mantle convection, the growth ofmid-ocean ridges, continental drift, and sea floor spreading.The model outlined above will be accepted and applied to the physiography of the Arcticbasin. The case for an active mid-ocean ridgehas been described by

Hess(1966). The rising dike of peridotite is assumed tobe 100°C warmer than the surroundingmantle.The coefficient of thermal 114 expansion of peridotite is 0.0037% per 100°C at1000° C.Therefore, the 750 km dike expands in length by someZ. 8 km.The thermal situation in the descending limb of the convectioncell has not been considered in a similar way by Hess, but it appearsreasonable to consider it to be a 'shrinking' dike.If the descending limb is 100° C cooler than the surrounding mantle, the length of thedike will de- crease by Z. 8 km.The surface expression of the different lengths of the upwelling and shrinking dikes will be about 5.6 km. As de- scribed above, the relief of the Alpha Cordillera isabout 4. 5 km from the crest, not including volcanoes, to thebasement rocks be- neath the Alaskan continental rise.Therefore, the Alpha Cordillera exhibits relief of the right order to be considered amid-ocean ridge and the relief could be explained by the mantleconvection model out- lined above. The Alpha Cordiflera is presently aseismicand does not show rifts on any of the echograms.Further, there are scarps of 500 to 700 meters on the flanks of the rise and the width ofthe arch, or rise, varies from some 350 km to 1000 km.These lines of evidence suggest that the convection cellwhich, according tothe hypothesis accepted previously, formed this mid-ocean ridgeis no longer active and that subsidence has occurred.It is therefore of interest to con- sider what conditions might have prevailed inthe oceanic crust and upper mantle when convectionceased. 115 Assuming that convection commences due to an increase in the temperature gradient in the lower part of the upper mantle, convec- tion would cease when the gradient had been reduced to the adiabatic gradient or less.This would be a simple overturn of the upper mantle.At the end of convection, the temperature would show a de- crease with depth and the mantle material immediately beneath the crust would be much hotter in the region of overturn than elsewhere on the earth.This condition would persist until radiative heating warmed the lower part, thereby restoring a super adiabatic gradient; perhaps promoting another cycle of convection. The other extreme case to be considered is that the convection cell, once initiated, would not cease.That is, the motion would be in balance with the thermal forces: heating of the lower parts of the cell by radiative transfer from the lower mantle-core, and cooling of the upper surface by heat flow through the crust would maintain a super adiabatic gradient.Aslong as the heating and cooling were in balance, convection would continue.However, the evidence of geology indicates that convection cells have not occupied the same positions in the mantle throughout all of geologic time.(The reader is reminded that the existence of mantle convection is assumed here. Various qualitative modifications to this model could be made. For example, taking the petrography from Hess's model, the upper horizontal limb of the cell creates a serpentine layer; the main 116 oceanic crust.The thermal conductivity of serpentine (or serpen- tinized peridotite) is considerably lower than the conductivity of peridotite (about 6 x 1O cal/cm sec vs about 9 x 10 cal/cm sec °C).Therefore, the cell becomes "capped with an insulating layer.Present knowledge of seismically active mid-ocean ridges shows that high heat flow, volcanic activity, earthquakes, hot springs (Iceland) and rifting are confined to the crest and/or flanks of the ridge.There is no evidence of such activity under the adjacent plains.(The descending limb, or shrinking dike, is the site of earthquakes, however.) On the crest and flanks of the ridge, mantle heat is lost rapidly and the serpentine layer forms.Mantle material, carrying the insulating oceanic crust, moves away from the flanks of the ridge.This upper limb of the cell undergoes heating from below; by molecular conduction and some radiative transfer, but especially by addition of material from below as the speed of flow increases away from the ridge crest.Loss of heat is reduced because of the low conductivity of the oceanic crust.The wider the cell becomes, the more warming of the upper limb has taken place.That is, the uppermost layer farthest from the ridge is warmer than the material at the same level adjacent to the ridge.Note that a high rate of heat loss through the surface of the ridge and horizontal inflow of hot material beneath the upwelling dike serves to maintain, or even in- crease, a super adiabatic temperature gradient in the upwelling dike. 117 However, in the shrinking dike, cooler material is being moved down- ward and, as the cell becomes broader, the material of the upper limb entering from the side becomes warmer.Therefore the tem- perature gradient in the shrinking dike decreases. As the gradient approaches and then achieves the adiabat, convection slows and then ceases, even though the temperature gradient under the ridge re- mains super adiabatic and the total heat content of the column is greater than in any column under the adjacent sea floor or in the (previously) shrinking dike. One of the constraints of the upwelling dike-hot creep convec- tion model is that material must move away horizontally from above the dike.Thus an adiabatic temperature gradient in the downward moving material will either stop the convection; cause the location of the shrinking dike to shift toward the upwelling dike, or allow a new convection cell to form somewhere else as the old cell ceases to convect. The model of convection outlined above offers (again, assuming mantle convection does occur) the interesting possibility that the maximum horizontal dimension of the cell may be limited by the thickness and thermal conductivity of the oceanic layer of the crust. This model differs from the simple overturn model in that the gradients are not adiabatic (or less) throughout the former cell, but only under one part of the cell.Thus the mid-ocean ridge above the 118 previously upwelling dike can persist even though convective motion has ceased. The model of mid-ocean ridge formation and sea floor spread- ing discussed above has been developed in an attempt to reconcile the data from the Arctic basin with a mantle convection model. There are several lines of data which almost preclude presently active convection or a simple mantle turn-over model.First, the heat flow measurements are low on the flank of the Cordillera and normal on the adjacent plain.Second, the main oceanic crustal layer (seismic layer 3) exists in the Alpha Cordillera, but is missing immediately adjacent to Alaska.And, as has been discussed in a previous section, the bathymetry suggests a mid-ocean ridge which has undergone subsidence. Reasoning from the model suggested above, consider the con- ditions existing in the crust and mantle of the Arctic basin at the geo- logical moment when convection ceased. The width of the mid- ocean ridge, the Alpha Cordillera, was perhaps 1000 km wide, and the sea floor strips adjacent to the Cordillera each about 1000 km wide.The Cordillera contained the oldest rocks on the crest; per- haps Z km of basalt overlaying serpentine 3 to 4 km, or more, thick. The base of the serpentine layer was at 500° C and the temperature gradient was steep below the serpentine.Because mantle convection and volcanism had ceased, transfer of heat upward in the upper 119 mantle and crust was by molecular conduction; an extremely slow process.The basalt, serpentine layer (the oceanic crust) was younger and thinner on the lower flanks of the Cordillera and the adjacent sea floor.At the site of the shrinking dike, the 500° C iso- therm and the oceanic crust were depressed, either under the Alaskan-Canadian continental block, or at the base of the slope. (The seismically determined depth of 6. 5 km below sea level to the basement rocks (Milne,1966) at the base of the Alaskan slope sug- gests a filled trench similar to the one described by Fisher and Shor (unpublished) west of Cedros Island off Baja . )The tern- perature gradient in the dike was adiabatic and its average tempera- ture somezoo°cooler than the dike under the Alpha Cordillera. The shrinking dike at the other margin of the convection cell was located under the continental shelf of Eurasia (or at the margin of the continent) between Svalbard and the point where the shelf break changes direction west of the Lena River.There are no data avail- able to show whether or not a filled trench now exists beneath the continental rise, but the shelf itself in this region is marked by troughs and closed basins suggesting subsidence. The cooling of the former convection cell will now be con- sidered.The temperature of the interior of the earth is poorly known.There are only two data which may set limits on the possible temperatures of the interior.The first of these is the heat flow 120 measured at the surface; the second is the fairly well established solid inner core of the earth, which appears to be iron.The heat flow averages some 1, 2 to 1. 4cal/cm2sec through the earth's sur- face.Taking a conductivity of 6 x 10 cal/cm2sec °C for the crust, the average heat flow shows a thermal gradient of about 20°C/km.The temperature of the liquid-solid core boundary must be no more than about 5000°C; otherwise the solid core would be liquid.(This assumes that the core is not in some unknown state of matter.The melting point of iron has been investigated at pres- sures up to 60, 000 bars (Strong, 1962), but the pressureat the liquid-solid core is about 1. 3 x106bars. )From these two data,it is obvious that the 20°C/km temperature gradient cannot continue deep into the earth.If it did, the temperature at the base of the mantle would be 58, 000°C! Thus at some shallow depth, the tern- perature gradient must become much less than 20°C/km. Aside from the measured heat flow, the estimate of core temperaturede- duced from density considerations and earthquake waves, and the reasonable inference that the temperature gradient must have an in- flection at some shallow depth, the thermal conditions of the mantle are unknown.For that reason it is not particularly useful to specu- late on the actual temperatures that might have occurred in the former convection cell.The cooling of the cell will be considered in terms of changing gradients (see Figure 12).Two data which may ALPHA CORDILLERA 100 C/km Low heat flow t..upwellingthke Boundary of Normal" heat flow Boundarydownwellingdike.) of ALASKA Cool _._._._._._._.._P!t ._._._._._._._._. \ \Hot (layer upper I- mantle _._._._.) I _____._i-.-.--.-.--. 3. I ) Isotherms Comparatively hot Cool ReducedFigureradiant heat 12. transfer Temperature gradients after convection ceased. (See text for discussion. No scale intended.) Increasedradiant heat transfer 'za be useful are noted: (1) Thermal conductivities of rocks and minerals appear to converge to a common value at temperatures above a few hundred degrees (Clark,1967); (Z) The base of the main oceanic crust, the serpentine layer of the model, can never be hotter than 500° C because of the deserpentinization reaction.Therefore, it is not necessary to consider changes in material in the mantle af- fecting the temperature gradient; below a few tens of kilometers the conductivities would be the same.Datum number Z, above, rules out the possibility of temperatures higher than 500° C in the oceanic crust (or in the Cordillera), unless the serpentine layer has been entirely removed by 'thermal erosion' (deserpentinization). As discussed above, at the end of the convective cycle, the temperature gradient under the mid-ocean ridge was steep (Figure la). The lower levels in the upper mantle were at the same tem- perature as the underlaying lower mantle, so little or no heat would be gained either by molecular conduction or radiative heat transfer. The uppermost levels of the mantle were cool and, because of the strong gradient, transfer of heat upward occurred more rapidly than in other parts of the former cell. (Without convection, however, heat transfer would be extremely slow; even in a geological time sense. ) Heat was also lost through the surface.The net effect of these processes would be to:(1) reduce the heat content of the upper mantle under the ridge; and (2) tend to reduce the steepness of the 123 temperature gradient. In the former shrinking dike, the temperature gradient was adiabatic, or less, and the upper mantle relatively warm in the upper layers and relatively cool in the lower layers.Thus, radiative transfer of heat from the lower mantle would be effective.This, coupled with heat loss through the surface, would tend to:(1) steepen the temperature gradient with time; and (2) because radiative trans- fer of heat to the lower layers is more rapid than heat loss by molecular conduction through the surface, increase the heat content of the column with time. If these processes continued long enough, the result would be the almost complete subsidence of the mid-ocean ridge, and the uplift of the crust at the site of the former shrinking dike.In short, these processes would tend to eventually equalize the heat content and temperature gradients across the former convection cell.

If a difference of 2000 Cinthe average temperature between the upwelling and shrinking dikes in the convecting mantle led to a relief of 5. 6 km, then the present relief of the Arctic .basin of 4. 5 km (discussed above) indicates about 1 km of subsidence of the Alpha Cordillera and/or uplift of the basin's margins since convection ceased.This could result from a change in the average temperature difference of 40°C (from 200° C to 160°C) between the former upwell- ing and shrinking dikes.If the age of the Alpha Cordillera and 124 adjacent sea floor can eventually be dated by fitting magnetic re- versals into the time scale of Vine (1966), at least a crude indication of the length of time required for subsidence of a mid-ocean ridge can be obtained. Assuming that the change in heat contentis the re- sult of processes occurring equally in each limb of the former con- vection cell, the Alpha Cordillera has subsided about 500 meters and the sea floor adjacent to the margins of the continents has up- lifted some 500 meters.This is the right order of subsidence to explain the fault scarps on the Cordillera.There are two few data from the sea floor adjacent to Alaska to indicate whether or not a filled trench actually exists and whether it might have been uplifted 500 meters or more.However, the absence of the main oceanic layer under the Alaskan continental rise (Milne, 1966) may be sig- nificant.At the time mantle convection ceased, the temperature under the floor of the trench may have been high enough at a depth of a few kilometers to result in deserpentinization of themain oceanic crust.Subsequent uplift, due to the processes described above, has perhaps resulted in a thickened basalt layer laying directly on 'warmt' mantle with abnormally low seismic velocity, as shown by Milne's refraction study. The existing heat flow data from the flank of the Alpha Cordil- lera and Canada Basin (Lachenbruch and Marshall,1967) can be ex- plained by the processes hypothesized in the paragraphs above.The 125 contrast in heat flow is the result of temperature differences in the upper mantle.Beneath the Cordillera the upper mantle directly be- neath the crust is comparatively cool, while beneath the Canada Plain the upper mantle is comparatively warm.The sharp contrast in heat flow over a distance of some 25 km, which is also related to a 700 meter scarp from the Plain to the Cordillera, can be explained in terms of the location of the former upwelling dike.According to hotcreep theory, the lateral boundaries of the upwelling dike must be sharp.Both the fault scarp and sudden drop in heat flow, reflect the former boundary of the upwelling dike.The steep temperature gradient, comparatively low temperature of the upper mantle and subsequent subsidence of the Cordillera are related to events in the upwelling dike; such as a very high rate of heat loss due to volcanism during convection on the flanks of the ridge.Under the adjacent strips of sea floor the uppermost mantle layer could regain its heat content by addition of material from greater depth, as required by the continuous divergence constraint of the hot-creep model.The 'coolT' upper mantle beneath the Cordillera must limit the tempera- ture gradient in the crustal rocks to about 10°C/km to account for the measured heat flow.Beneath the Canada Plain, the gradient must be close to 20°C/km.The heat flow under the Alaskan conti- nental rise should be 'normal' (i. e.,1. 2 - 1.4cal/cm2sec) or greater.The upper mantle should be slightly warmer at the top of 126 the former shrinking dike than under the Plain.And, in fact, the one seismic mantle velocity datum available(Milne,1966) does mdi-. cate anomalously low velocity mantle beneath the margin of the Arctic basin. If the hypothesis discussed above has any merit in understand- ing the relief of the Arctic basin, it predicts at least two unusual geophysical data which will be obtained.First, the Alpha Cordillera will show low heat flow across the entire tarch.If future studies show that the heat flow is low on the flanks, but increases to normal on the crest, it will indicate that the presentlyestablished contrast is the result of an edge effect of a low conductivity layer which is present in the Cordillera, but not under the adjacent Plain.Second, gravity study will show a deficiency of mass beneath the Cordillera, but seismic refraction study will show a normal (8km/sec) or greater than normal velocity for the upper mantle. The hypothesis formulated in the preceeding paragraphs ascribes the formation of the entire Arctic basin to sea floor spread- ing associated with the formation of the Alpha Cordillera.The other major features in the basin will now be discussed in the light of this hypothesis.

The Lomonosov Ridge

The physiography of the Lomonosov Ridge was described in a 1Z7 previous section (see especially Figure 6).The shape of the Ridge and its lack of magnetic signature (Ostenso, 1963) indicates that the Lomonosov Ridge is not basic volcanic rock; that is, not a line of volcanoes or a welt of basalt.In keeping with the magnetic data, the Lonionosov Ridge is here interpreted to be a fragment of the conti- nental block which was split and moved apart by the mantle convec- tion cell which formed the Alpha Cordillera and the Arctic basin. Wilson (1963), Johnson and Heezen (1967) and Vogt (1968) have con- sidered the Lomonosov Ridge to be the former edge of the Eurasian Continental Shelf which was split-off and translated to its present position by sea floor spreading froin the Nansen Cordillera.This interpretation is not accepted here for the following reasons:

1. If a convection cell forms under and splits a continental plate and sea floor spreading forms an ocean basin, the ridge formed by the upwelling dike will be centered in the basin. he Mid- Atlantic Ridge, for example, is on the mid-line of the Atlantic Ocean. The Alpha Cordillera is centered in the Arctic basin.But, if the formation of the Eurasia Basin is ascribed to sea floor spreading from the Nansen Cordillera, and only the Amerasia Basin to spread- ing from the Alpha Cordillera (Vogt,l968, for example), then the Alpha Cordillera is not on the mid-line of the basin it supposedly formed.It is then necessary to postulate very different spreading rates in each direction away from the mid-ocean ridge.Considering 128 the tenuous nature of the data on mantle convection, this can be done. However, it contradicts the knowledge gained from study of the Mid- Atlantic Ridge, and is not necessary if sea floor spreading from the Alpha Cordillera formed the entire Arctic basin.

2. The physiography of the Nansen Cordillera (Figure 7 and Plate 1) shows that there has been, as yet, no "arching" of the sea floor in the Eurasia Basin.Half of the relief of Nansen Cordillera is below the level of the adjacent sea floor.Therefore, it is not likely that 200 km of sea floor spreading on each side of the Cordil- lera has occurred. When the mantle convection cell first formed under the conti- nental plate and the hot dike began to upwell, as described previously, the plate fractured along two sub-parallel lines near the mid-line of the dike.As Orowan (1967) pointed out, the most likely "flow pat- tern' in mantle convection is hot-creep with continuous divergence at the top of the upwelling dike.This allows for non-linear sea floor spreading; the strip of oceanic crust formed on the flanks of the ridge can accelerate as it moves away from the ridge.If this is so, the fragment of continental crust (the Lomonosov Ridge) moved down the flank of the mid-ocean ridge some 400 to 500 km while the continental plates were each being translated about 1500 km.In moving down the flank of the Alpha Cordillera, the Lomonosov Ridge subsided about

1 km.Because, according to this hypothesis, the Lomonosov Ridge 129 initially occupied a position directly over the upwefling dike, the lower part of its Troot' was perhaps melted and absorbed by the hot mantle material. If the continental plate were 35 km thick and its surface near sea level when fracturing occurred, some 7 km of the thickness of the fragment which became the Lomonosov Ridge must have been re- moved to permit subsidence to its present depth.(Assuming a conti- nental crust 35 km thick stands 5 km above the general level of the ocean floor, then the thickness must be reduced to 28 km if it is to stand 4 km above the ocean floor, or 1 km below sea level.This assumes that the Lomonosov Ridge is in isostatic equilibrium, but there are no gravity data available to indicate whether that is true or not.

The Nansen Cordillera

The physiography of the Nansen Cordillera has been described in a previous section (see especially Figure 7), where it was shown that this range consists of ridges or volcanoes standing as much as 1500 meters above the sea floor and deep rifts 1500 meters below the sea floor.The Nansen Cordillera is seismically active.The only other geophysical data available are aeromagnetic profiles, which show anomalies of ZOO gammas or less. The Nansen Cordillera is here interpreted to be the surface 130 manifestation of the earliest phase of mantle convection of the kind discussed previously. A dike some hundreds of kilometers wide and extending at least from the Lena Trough to the Eurasian Shelf (and probably beyond in both directions) in the upper mantle has begun to move upward.The comparatively brittle crust and uppermost mantle, the Coulomb layer, is being fractured and, perhaps, moved aside or the blocks tilted into ridges (Orowan, 1966).This is in keeping with the seisrnicity of the Cordillera.If the upwelling dike is nearly as broad as the Eurasia Basin, about 600 km, warming of the crustal rocks could account for the low amplitude of the magnetic anomalies. Vogt (1968) has pointed out that the anomalies recorded over the Eurasia Basin may be parallel to the NansenCordillera. If so, they are also parallel to the Alpha Cordillera, because both Cordilleras are parallel to each other.Under the hypothesis of the formation of the Arctic basin presented here, the magnetic anomalies in the oceanic crust of the Eurasia Basin were imprinted as the crust formed on the Alpha Cordillera.

The Arctic Rises

The Chukchi, Morris Jesup and Yermak Rises are illustrated in Figure 9.These are interpreted to be continental fragments. The Morris Jesup Rise appears to be associated with the Lomonosov Ridge and is here hypothesized to have originated in the same manner 132

AN HYPOTHESIS FOR THE ORIGIN OF THE ARCTIC OCEAN False facts are highly injurious to the progress of science, for they often endure long; but false views if supported by some evidence do little harm, for every one takes a salutary pleasure in proving their falseness: and when this is done, one path toward error is closed and the road to truth is often at the same time opened. Charles R. Darwin (The Descent of Man, Chap. XXI)

The facts of the present study are the submarine-collected echograms and the profiles of the floor of the Arctic basin constructed from them.The geophysical and sounding data (referenced throughout this study) reported by other students of the Arctic basin are also highly useful facts.Throughout the text of this work, the writer has attempted to keep a clear distinction between fact and hypothesis. The profiles of the ocean floor in this ice-covered region will be of continuing use, even if the hypotheses formulated here become un- tenable in the light of future research. The preceding section dealt with the possible structure of the crust and upper mantle based on the physiography of the Arctic basin and available geophysical data.Consideration was given to a two dimensional model of mantle convection and sea floor spreading to account for the relief of the ocean-floor features.In this section, the third dimension, the geographical arrangement of the major 133 features in the Arctic basin, will be considered. A review of the ideas on the formation of the Arctic Ocean has been given in a previous section.The concept of permanency of the basin throughout all of geological time does not appear acceptable, because the trend of the structures in the continents surrounding the basin (see Baadsgaard et al. ,1961, for example and Figure 13) terminate at the coasts and face each other across the Arctic basin. Continuation of the ancient structures across the floor of the basin, or drifting of continental blocks appears to be required: in brief, either vertical or horizontal movements of the continents.It has been primarily researchers in the U. S. S. R. who have advocated subsidence of a continental block to explain the formation of the Arctic Ocean.(Eardley (1948) in the Ti. S.stated this argument, but later (1961) abandoned the concept as better sounding data became available.) The most recent statement of the vertical movement hypothesis known to this writer, was written by the eminent Soviet marine geologist, the late Ya. Ya.Hakkel4(1962).He illustrated nets of radial cracks in the floor of the Arctic basin and suggested that they were the result of movement of the crust over the spheroid into the region of polar flattening; because of this, the crust subsided and the basin was formed.The present study has not confirmed the existence of these cracks in the floor of the basin. The formation of a part or all of the basin of the Arctic Ocean 134

Figure 13.Paleozoic Orogenic Belts (After Baadsgaardetal. 1961) 1 35 by fracturing of a continent and drift of the two sections,Eurasia and North America, away from each other, has been stated in contem- porary terms by Carey (1958), Heezenand Ewing (1961), Wilson (1963, 1966), Johnson and Heezen (1967) and Vogt (1968).Carey proposed that the Arctic basin is a tension rift formed by ascissors- like drifting apart of Eurasia and North America.The pivot of the

IscissorsUwas (or is) in south central Alaska andthe European continental slope from Svalbard to the region off the Lena Delta is a strike-slip fault.The Lomonosov Ridge was the only mountain range known in the basin at that time.According to Carey's hypothesis, the Lomonosov is a thread of sialic material which formed because the isotherm at which fracture passes into flow was above the Mohorovicic discontinuity at that location.Carey (1958, page 195) uses the analogy of a slab of toffeewhich is, ...cold and brittle except for one warm spot, the slab will break cleanly exceptat the hot spot where a thread of toffeewill bedrawn out across the rift. The hypothesis postulated regions of distributed tension and great shear zones and is quite complex. Carey's rifting hypothesis is intriguing.However, the dis- covery of two additional mountain rangesin the Arctic basin parallel to the Lomonosov Ridge, but dissimilar toit, and the extremes in magnetic activity on either side of the Lomonosovintroduce addi- tional complexity.Furthermore, the hypothesis calls for flexure of 136 the Alaska Range (the hinge point) and distribution of tension across all of central and northern Alaska.The geology of Alaska is quite well known and great tensional distortions have not been reported.

Heezen and Ewing(1961)postulated the extension of the Mid'- Atlantic Ridge through the Arctic Ocean along the belt of earthquake epicenters.The authors did not present an hypothesis for the origin of the Arctic basin, but they considered that the Eurasia Basin is growing wider.Heezen called upon internal expansion of the earth as the mechanism, while Ewing suggested mantle convection under the ridge.The Lomonosov Ridge was compared with the Walvis Ridge of the South Atlantic, but no particular origin was suggested. The authors pointed out that the physiography of the Arctic basin ap- pears to be oceanic and rejected the foundered continent hypothesis.

Wilson's(1966)paper treated continental drift in general; the Arctic Ocean is mentioned only in passing.Wilson wrote, "As is well known, if the two sides of the Atlantic Ocean separated by rifting, the movements involved a rotation through 15° about a ful- crum at the New Siberian Islands.Compression on the opposite side of the fulcrum has been held to have caused the compression of the

Verkhoyansk Mountains.(Wilson,1963)." Wilson(1966,Figure 2) showed the Mid-Atlantic Ridge to bifurcate well south of Greenland. One branch enters Baffin Bay and terminates against a strike-slip fault which extends from northern northeastward 137 between Ellesmere Island and Greenland to the Morris Jesup Rise. The eastern branch is shown extending through Iceland to a transform fault south of Svalbard.This fault trends northwest through Lena Trough to the Morris Jesup Rise.Motion along this fault has offset the Mid-Atlantic Ridge and the Nansen Cordillera and separated Svalbard from a former position north of Ellesmere Island.Sea floor spreading has formed Baffin Bay, the Eurasia Basin and the Greenland-Norwegian Seas. Johnson and Heezen (1967) ascribed the formation of the Eurasia Basin to sea floor spreading from the Nansen Cordillera. Vogt (1968) suggested sea floor spreading from the Alpha Cordillera to explain the formation of the Amerasia Basin; and from the Nansen Cordillera to explain the Eurasia Basin.King, Zietz and Alidredge (1966) combined continental drift (the Eurasia Basin) and continental subsidence (the Amerasia Basin) to account for the formation of the Arctic Ocean. The geology of lands around the basin of the Arctic Ocean in- dude, of course, the geology of all of northern Eurasia and North America. A detailed review of the available data from such an ex- tensive area is far beyond the scope of the present study.The excel- lent symposium volume, !Geology of the Arctic" (Raasch 1961) and the geologic map compiled for the symposium (Link,, etal..,1960) provide the latest data summary for the region.Nalivkin's (1960) 1 38 summary of the geology of the U. S. S. R. is especially valuable, be- cause it includes a detailed geological map and has been published in an English edition. In general, the basin of the Arctic Ocean separates two of the earth's great stable platforms; the Canadian-Greenland Precambrian Shield, the Russian-Baltic-Angara Shield, and the Paleozoic rocks along the margins of these Shields.For the present study, the Paleo- zoic orogenic belts are of particular interst.Figure 13 (after Baadsgaardetal., 1961) shows the locations of these belts. A glance at Figure 13 shows why geologists familiar with the Arctic regions have, for decades, called for continental drift or a sunken continental block in the Arctic basin.As a further generali- zation, to which there are exceptions, the rocks are younger (Meso- zoic and Cenozoic) in the Asian sector--from the Lena River across eastern Siberia and Alaska to the Mackenzie River- -than in the Canada, Greenland, European sector.The part of Siberia from the Lena Delta to Bering Strait is called the Verkoyansk-Chukotski Folded Region by Soviet geologists (Fotiadi, et al., 1965).This re- gion was deformed and subjected to volcanic activity in the Mesozoic and Tertiary.South of the Folded Region is the ; the scene of Cenozoic volcanism and a part of the present day circum -Pacific ". Northern Alaska can be simplified into two geological 139 provinces: The Coastal Plain and Brooks Mountain Range.The basement of the Coastal Plain appears to be Precambrian argillite and slate overlying ugranite(Payne,1951)and thus may be a part of the stable platform of North America.The Brooks Range is thought to be an extension and analogy of the with similar structure (Tailleur and Snelson,1966). The Brooks Range has been extensively deformed and subjected to thrusting, which displaced blocks at least 200 km and perhaps as much as300km.In the eastern part of the Brooks Range, Tailleur and Snelson(1966)re- ported resting on sediment with both involved with a "one and one-half mile thickness' of arkose of Mississippian to Permian age.This intense thrusting is thought to have taken place at the end of the Mesozoic and early Cenozoic.

Ostenso(1966)reported on the basis of gravity studies that the Brooks Range may be structurally continuous with the Chukchi- Anadar fold belt across the Chukchi Sea, but is not a continuation of the Paleozoic belt of the northern Canadian Archipelago.The geology of the Brooks Range and the Chukchi-Anadar fold belt is similar.Thus, the Verkoyansk-Chukotski Folded Region may con- tinue into northern Alaska. The possible formation of the Arctic basin by sea floor spread- ing from the Alpha Cordillera is illustrated in Figure 14.The four parts of the illustration show; (A) the continental plate, with Figure 14.Schematic of the formation of the Arctic basin.

A.Initial fractures in the continental block.Stipple indicates the possible extent of the Paleozoic orogenic belts.(The indications of present shore- lines are for orientation only.)

B.An early stage of sea floor spreading.Mantle upwelling is forming the Alpha Cordillera. Arrows indicate the movement of the continental blocks. The Paleozoic belts are omitted for clarity. 141 Figure 14.(Continued)

C. The Arctic basin just prior to cessation of mantle convection and sea floor spreading.(See text for discussion.)

D. Present configuration of the Arctic basin.The Alpha Cordillera has undergone some subsidence and faulting.The Nansen Cordillera is forming. Stipple indicates the known exposures of the Paleozoic orogenic belts on land.(See text for discussion. -4 144,

Precambrian Shields and Paleozoic Belts; (B) and (C) intermediate stages of sea floor spreading; and, (D) the present configuration of the basin.This illustration was constructed by ITreversingir sea floor spreading.That is, moving the continental blocks of Eurasia and North America inward toward, and along lines normal to, the crest of the Alpha Cordillera.The present-day shorelines of Eurasia and North America are shown in Figure 14 for orientation.The fit of the continental slopes, the Paleozoic Fold Belts and the continental fragments (the Lornonosov Ridge and the Arctic Rises) appears to be good considering the facts that:(1) The bathymetry of these features is known only on a TTreconaissance scaleIT, and (2) The exact location of the former crest of the Alpha Cordillera may now be masked by differential subsidence.Taking the regions of least depth of the Alpha Cordillera at the continental slopes of North America and Eurasia and connecting them with a straight line (which is a great circle on the polar projection used) yielded the line shown in Plate 4 as the crest of the Alpha Cordillera during theconvection phase. This line was used to determine the azimuth of the great circle, the axis of convection, shown in Figure 16 and discussed below.This line appears to be the mid-line of the Arctic basin to within 50 km. Plate 4 shows that the basin is wider, on a line parallel to the crest of the Cordillera, off the Eurasian continent than it is off Alaska; and wider along the line from the Mackenzie Delta to - - - - FaultBoundary lines of the Arctic basin somewhat simplified iiMIIDhIIIIlUuiii AxisRegion of upwellingof the slope during convection phase notched by the present seismic belt Plate 4. ContinentalPlainsPresent at crest fragments23 kmof AlphaLocations depth in theCordillera basin of faults and the margin of the convection cell. --, - - - - Axes of troughs and basins of continentalAxisMargin borderlandof theof the Nansen convection Cordillera cell 146 Svalbard than along the Siberian continental slope. A reason for the shape of the basin will be offered when the possible continuations of the axis of the Alpha Cordillera are discussed. The reconstruction of the "original' continental block that existed prior to the growth of the Alpha Cordillera (Figure 14 A) brings the belts of Paleozoic rocks into juxtaposition, if the assump- tion is made that the belts extend across the broad continental shelves of Eurasia.This appears to be reasonable in view of the exposures of Paleozoic rocks on the islands on the shelf.(Franz Josef Land is an exception, because the exposed rocks are and younger.However, most of the land area is ice covered and the ex- posures are limited to a few coastal areas. )As discussed in the previous section, the Lomonosov Ridge fits into the pattern of sea floor spreading as a continental fragment.Figure 14 indicates that it may be composed of a part of the Paleozoic Belts.The Arctic Rises may also be outliers of Paleozoic rocks, fragmented by the initial mantle upwelling under the Alpha Cordillera and translated to their present locations by the spreading of the ocean floor.This point is worth stressing, because, to date, no rocks older than Cretaceous have ever been dredged from the ocean floor.If the hypothesis outlined in the present study is correct, Paleozoic rocks may be dredged from the Lomonosov Ridge and the Arctic Rises in the future.Should this occur, it would not mean that the Arctic basin 147

SUBMARINE CANYONS 72° HERALD IS. 7-SEA VALLEY

BA SINS AL A S K A 68°

DIOMEDE ISLANDS

FAIRWAY ROCK

KING \ BASINS ST. LAWRENCE \d.;..O__2ol64O YUKON_i kilometers DELTA

Iou.

Figure 15.Bathymetric features and islands in the Chukchi and northern Bering Seas.The fault line discussed in the text is shown by the dashed line. (Bathymetry simplified after Creager and McManus 1967.) 148 is Paleozoic in age, or that it formed by subsidence of an ancient continental block. The possible extensions of the axis of the Alpha Cordillera, and the shape of the Arctic basin will now be considered.The present mid-ocean ridge is a world wide phenomenon.If the Alpha Cordillera is a now inactive mid-ocean ridge, it is likely that it too was a part of a much more extensive ridge system.In fact, if such an extension cannot be traced, the interpretation ofthe genesis of the Alpha Cordillera presented here will be weakened considerably. Figure 16 shows a prolongation of the "least depth axis" of the crest of the Alpha Cordillera, on a globe, toward the Atlantic Ocean. This line runs exactly into the great fijords of Ellesmere Island; along the mid-line of Baffin Bay, and into the .Con- tinuing the great circle, the line passes between the Grand Banks and Flemish Cap and intersects the crest of the Mid-Atlantic Ridge at Z8° N. Lat.If ocean-floor magnetic and other studies should suggest the continuation of the Alpha Cordillera beyond the Labrador Sea, it would have important implications for the formation of the North Atlantic by sea floor spreading and for the "fitting' of the slopes of North America and Europe.Drake etal. (1963) presented evidence for an old, partially buried mid-ocean ridge in the Labrador Sea. This is substantiating evidence that the Alpha Cordillera is, or was, a part of a more extensive mid-ocean ridge (seeFigures 16 and 17). 149

'--7r- IL 3

S S S S T S

I.

_ I2 -;----

Great circle extension of the Alpha Cordillera Crest of the Mid-Atlantic Ridge

Margins of convection cell Region of the Mid-Atlantic ridge offset by faults fMirror-image of the northern end of the convection cell --- Simplified boundary of the Arctic Basin

Figure 16. The Wegener Rise. 1 50

52°W 49°W 1uuufin

0 100 200 300 400 500 Miles

48°W 42°W 2000 2500

Two eastwest profiles of Southeast Ridge

$ Bermuda Rise Corner Rise Mid Atlantic Ridge Azores

(Upper two profiles after Drakeet at.1963 Lower three profiles after Heezenet at.1959)

Figure 17. Evidence for former extension of the Alpha Cordillera into the North Atlantic. Arrows show the location of the great circle discussed in the text. 151 The continuation of the axis of the Alpha Cordillera into the Eureka Fijord system, through Baffin Bay and into the Labrador Sea is, to the writer at least, fairly persuasive. A continuation of the axial line of the Cordillera toward the Pacific, however, is not con- vincing.The great circle crosses the shallow, smooth shelf of the ; continues across the Verkhoyansk-Chukotski Folded Region and the Okhotsk Sea and intercepts Hokkaido, Japan. There is no evidence of great tensional distortion of the shelf (see Plate 1), and the is characterized by compressional features, as its name implies.It would be possible to suggest that sedimentation on the continental shelf may have masked any rifting of the continental block; and that the was formed by the mantle convection cell beneath a continuation of the Alpha Cordillera.However, the Verkhoyansk-Chukotski Folded Region cannot be readily fitted into this concept, even though it was the scene of intense volcanismduringlate Cretaceous-Tertiary time (Nalivkin,1960).The hypothesis of mantle convection beneath mid- ocean ridges and present knowledge of the location of the axis of the world-wide ridge system, strongly indicates that wherever the con- vection cell occurs beneath a continentai hicck, the block is rifted: The and the are two presently active examples; the Canadian Archipelago and Greenland is, in the present study, a presently inactive example.The Sea of Okhotsk is 152 separated from the Pacific by the Kamchatka Peninsula, which is the site of intensive volcanism.It appears that the Sea of Okhotsk may be related more to the circum-Pacific "ring of fire" than to aformer convection cell beneath the Alpha Cordillera and its probable exten- sions.For these reasons, especially the absence of rifting in the Verkhoyansk-Chukotski Folded Region, the writer believes that the upwelling dike which formerly existed beneath the Alpha Cordillera did not extend far into the Eurasian Continent. Wilson (1966) wrote, "mid-ocean ridges usually end in large transcurrent faults. " Hess (1965) suggested that the patternof "downwelling" of world-wide convection cells can be pictured as re- sembling the stitching on a tennis ; the upwelling limbs, of course, would be on the mid-lines of the patternof stitching.The present writer suggests that the convection cellwhich formed the Alpha Cordillera (and the Arctic basin) terminated at the Eurasian end of the Cordillera.The continental slope of Eurasia from the Lena Delta to the base of the Chukchi Rise would then mark a great transcurrent fault at the termination of the former mantleconvection cell.The "tennis ball" analogy is also useful here, because it im- plies a circular (in plan view) termination of the downwellinglimb (the shrinking dike) of the cell.If this is correct, it can explain the "tapering" of the Arctic basin from North America toward theAsian

continent.Measurement on a globe shows that the length of the fault, 153 from the Chukchi Rise to the sharp change in azimuth of the continental slope north of the Lena Delta, is 1600 km.The great fault marking the North American boundary of the basin, from the slope off the

Mackenzie Delta to Svalbard, is 1200 km longer.(See Plate 4. )The fault boundary on the Asian margin is slightly arcuate, closed toward the Pacific, and so is the coast line of the mainland (Plate 4).These facts can be explained by a "tennis ball" pattern of downwelling at the end of the former convection cell. The possible pattern of the shrinking dike, which is thought to have been associated with convection beneath the Alpha Cordillera and the Arctic basin, will be considered further.It was suggested above that the Alpha Cordillera is a now abandoned branch of the Mid- Atlantic Ridge which can be traced at least as far south as the Labrador Sea.The Atlantic Ocean is 5000 km or more wide over most of its length (Wilson,1966, Table II).The maximum width of the Arctic basin, along the North American Slope,isslightly less than 3000 km. As explained above, this may be the result of the "tapering" of the convection cell toward its terminus.But, this leads to a consideration of the question, was the former convection cell 5000 km, or more, wide south of the Arctic basin? Examination of a globe reveals some interesting facts, which are illustrated in Figure 16.Note that a continuation of the line of the continental slope of Alaska through the Mackenzie Delta extends 154 (as a line which is equidistant from the axis on the globe) through the large lakes of the Canadian Shield and into the Great Lakes.By taking the axis of the Alpha Cordillera and its great circle prolonga- tion into the Labrador Sea as the mid-line of a former convectioncell and drawing great circles at 90° to this line, the following distances were obtained:

1.From the western shore of Great Bear Lake to the crest line in Eureka Fijord is 2000 km. A distance of 2000 km east along the same great circle falls on the continental borderland in the .

2.From the western shore of Great Slave Lake to the crest line is 2500 km. A prolongation of the great circle 2500 km to the east falls on the continental slope west of Tromso, Norway.

3. From the western end of Lake Athabaska to the crest line in Baffin Bay is 2500 km.The prolongation 2500 km east falls on the continental slope west of Boda, Norway.

4.From the Narrows in Lake Winnepeg to the crest line in is 2500 kin.The 2500 km eastern prolongation falls in the Faero-Shetland Channel.

5.From a point in the western end of Lake Superior to the crest line in the Labrador Sea is 2500 km.The eastern prolongation falls on the continental slope west of Scotland, inshore from Rockall

Bank. [55

6. From a point in the narrows between Lake Huron and Lake Erie to the crest line in the Labrador Sea is 2500 km.The prolonga- tion 2500 km east is on the continental slope west of Ireland.

7.The prolongation of the line of lakes exactly crosses , which is 2500 km from the crest line between the Grand Banks and Flemish Cap.The proiongation to the east, how- ever, falls in the eastern half of the West European Basin on a north-south chain of sea mounts.Interestingly, the continuation of this great circle marks the axis of the deep water Ttarrowheadll which enters the , and the axis of the Mountains.

8.Farther south, where the great circle of the Alpha Cordil- lera extension intersects the Mid-Atlantic Ridge at about 28° N. Lat., a great circle struck 90° to the crest line marks the north.end of the Puerto Rico Trench at the 2500 km point to the west, and the continental slope west of the Santa Criiz Islands 2500 km to the east.

9.Extending the great circleothe crest line even farther south (and the writer realizes that this in a great distance along an orientation determined by a feature in theArcticbasin!) to 17° N.

Lat. ,the western 2500 km prolongationnt greatcircle normal to the crest axisfalls inthe eastern VcneiijeLi riBa sin.The eastern prolongation is or the continentalslope w'si ntMan rtana, Al rica.

10.If the crest is continued south hetween 17° N.Lat. and

8° N. Lat. ,it intersects sever;il highsonthe present Mid-Atlantic 156 Ridge system.The 2500 km great circle to the east marks the conti- nental slope of the west African Tlbulgeu as far south as Portugese , and then continues into the Sierra Leone Basin. A 2500 km prolongation to the west, however, cuts through Venezuela and the basin of the Oronoco River system. The 2500 km distances east and west from the great circle ex- tension of the axis of the Alpha Cordillera are not meant to imply that a former convection cell was exactly 5000 km in width.The lakes and the continental slopes discussed have "lengths" of several hundred kilometers.The reader is invited to strike the great circles himself on a large globe to see how convincing the pattern is. The extension of the axis of the Alpha Cordillera into the North Atlantic, discussed above, while not obvious, may be in evidence on a bathymetric map.Baffin Bay has a maximum depth of 2300 meters and a number of peaks penetrate the sediment (Pelletier, 1966, Figure 7 and 12).South of Baffin Bay, a submarine ridge extends from Baffin Island to Greenland with minimum depths of about 400 meters.Tertiary volcanics are exposed on the coast at the Baffin Island end of the ridge.On the Greenland side, Tertiary vdlcanics are exposed on the coast, although they are north of the submarine ridge.The ridge may be entirely volcanic and represent a former center of effusive lava on the Cordillera's crest. The evidence for a nearly buried mid-ocean ridge in the 157 Labrador Sea is convincing (see Figure 17), but this segment has undergone much greater subsidence than has the Alpha Cordillera, if it is a part of the same mid-ocean ridge system.The mountains on the crest are more than 3500 meters below sea level.However, the ridge is in the correct location with the proper azimuth to be, in fact, an extension of the Alpha Cordillera. Farther south, possible evidence of a ridge extension may be the gap between the Grand Banks and Flemish Cap, and the Southeast Newfoundland Ridge (see Figure 17).South of the Southeast New- foundland Ridge, the great circle crosses the easternmost end of Sohm Plain and intercepts the Mid-Atlantic Ridge near a sea mount region; the Corner Rise (Figure 17). Making the assumption that the former convection cell and its surface effects terminated in the south in the same manner as in the Arctic Ocean, a circular pattern was drawn centered on the point of intersection with the Mid-Atlantic Ridge.This is a mirror image of the northern end of the cell with the same dimensions.The hypothe- sized line of downwelling is shown in Figure 16.This line, on the ocean floor, generally follows the southwestern margin of the North American Basin, the Puerto Rico Trench, and across the Mid-. Atlantic Ridge in a region where there is no well defined crest.Con- tinuing into the eastern North Atlantic, the line marks the south- eastern margin of the Cape Verde Basin, the western margin of the 158

Cape Verde Plateau and intercepts the continental slope of west of the Canary Islands.The southernmost part of the curve lays just north of the region where the Mid-Atlantic Ridge is heavily faulted and offset in the narrowest part of the equatorial Atlantic. (See Heezenetal., 1959, Plate 1.) If the Alpha Cordillera is a part of a former, older, North Atlantic branch of the mid-ocean ridge system, the writer suggests that the southern termination of its convection cell ended in the "tennis ball stitching" pattern described above.The extensions of the margins of the cell (which are not a part of the circular pattern shown in Figure 16) could be considered reasonable along the conti- nental slope of the African bulge, but would certainly be less so in .However, an extension of the axis of the convection cell into the would cross along the Bahia Graben (see Wilson,1966). If a former mantle convection cell and its surface manifesta- tions actually existed as shown in Figure 16, the remaining "shadow" of its dimensions are impressive.The major axis spans some 1000 of latitude and the width is about one-eighth the circumference of the earth.These are almost exactly the dimensions of the Darwin Rise, as drawn by Menard (1964, Figure 6. 15).The Darwin Rise is also thought to be the surface expression of a now dormant mantle convec- tion cell (Menard,1964 Chapter 6). 1 59 The writer proposes to name the hypothesized feature dis- cussed in the paragraphs above, "the Wegener Rise" in honor of the late Alfred Wegener, whose publications on the drifting of continents more than half a century ago initiated an intellectual stimulation which is even now very much alive.Whether or not the reader is prepared to accept the existence of the Wegener Rise as a working hypothesis, the feature requires a name.Researchers whose data tend either to corroborate or to deny the existence of the Wegener Rise will need a name for the mantle convection cell and its image in the crust which did, or did not, exist. The axis of the Wegener Rise and its margins have been de- scribed briefly.Nearly all of the margins of the mantle convection cell are related to the edges of the ocean basins.(Indeed, it has determined their locations. )The exception is the section of the Wegener Rise between the Mackenzie River and Chesapeake Bay and to the northeast of that line.The Darwin Rise apparently developed entirely beneath the ocean floor of the Pacific.In this the Wegener Rise was different:it developed under a continental block and spread the fragments apart.In keeping with the concepts of sea floor spread- ing, the Arctic coastline of North America should have been displaced toward the southwest until it reached the downwelling margin of the convection cell.According to Hess (1965), the continental crust drifts on the moving surface of the mantle as does an ice floe on the 160 ocean surface; unless it meets an opposing current.South America is thought to be an example of a continental block positioned between two mantle convection cells; one beneath the eastern Pacific and the other beneath the South Atlantic.The great trench along the Pacific coast is the result of the sea floor shearing beneath the continental block.The are the result of compression of the continental crust between the two convection cells.Applying this concept, by analogy, to North America moving on the surface of the Wegener Rise, the continent moved away from Eurasia until it became locked between the Wegener Rise and a similar feature in the Pacific Ocean.(Possibly the Darwin Rise? Hess (1965) believes that the present floor of the Pacific, including the , was formed on the Darwin Rise. )If this is correct, the line of lakes along the margin of the Wegener Rise are the surface expression on a Pre- cambrian Shield of the same forces which would have formed trenches in an oceanic crust.It is of interest to note that White (1966) esti- mated Z5, 000 feet of sediments in western Lake Superior associated with a conspicuous gravity low.Eardley (1962) reported 14, 000 feet of subsidence in the Michigan Basin.It is obvious that the Precambrian shields are so strong in compression that the forces which mold the earth's crust do not have much affect on them. If this were not true, they could not have remained essentially un- marked for more than half-a-billion years.However, they would 161 have much lower strength in tension. The western margin of tIre Wegener Rise crosses the Appa- lachian Mountains between Lake Erie and Chesapeake Bay (Figure16). Evidence for its existence along this 500 km segment may be present. The line extends generally along the valley of the Susquehanna River across the Newer Appalachians; crosses the Great Valley and con- tinues eastward through the gap between the north end of the Blue Ridge Mountains and the Upland to the northeast.(See

Eardley,1962,Figures 7. 1 and 7. 3.) Eardley(1962)wrote about this region, "Several long, narrow basins of Triassic sediments rest un- conformably on the older rocks of the , and:in one place on the Blue Ridge belt.U (And it is exactly here that the margin line crosses the Blue Ridge.) "They are down faulted troughs, all ap- parently part of a major fault or rift zone. " King(1964)suggested that the Blue Ridge Province of the Appalachians is bounded by a transcurrent fault on the southeast along the Brevard Schist zone. He wrote, "Why should there be a great strike-slip fault, parallel to the grain of the rocks, in the midst of a deformed mountain belt?"

This fault crosses the margin line of the Wegener Rise at nearly90°.

King(1964)also reported a negative gravity anomaly of 100 millegals in this region. East of the Appalachians, the line marking the margin of the Rise crosses the north end of the Piedmont, and the Atlantic Coastal 162

Plain through Chesapeake Bay.Continuing, it crosses the narrow shelf off Cape Hatteras and the continental slope at Hatteras sub- marine canyon.Its southern extension on the ocean floor has been described above. The geology of both the continents and the ocean floor is ex- ceedingly complex. When dealing with the strikes of many features over a great area of the Earthts surface, it would appearinevitable that some of them would be aligned with each other purely by chance. The writer does not believe that chance (in the sense that Bullard (1965) meant when he wrote that it is pure chance that Italy resembles a boot) can explain the facts described above.To recapitulate briefly: The great circle extension of the azimuth of the Alpha Cordillera in the Arctic basin lays on the mid-line of Baffin Bay along the strike of a known, buried ridge in the Labrador Sea; between Flemish Cap and the Grand Banks; across the Southeast Newfoundland Ridge to an intersection with the Mid-Atlantic Ridge between Corner Rise and the crest, some 8000 km from the Alpha Cordillera.Note that an azi- muth for the Alpha Cordillera different from this one by only ten degrees would extend either through Greenland or northeastern North America.In both cases, the great circle would pass through the deep basins of the Atlantic; and, if the azimuth were even50 to the west, the great circle would not intercept the Mid-Atlantic Ridge at all.It appears to the writer more reasonable to suggest that this 163 axis is the result of a large scale process in the earth than to assign it to chance. The geological implications of the Wegener Rise are too di-. verse to pursue in detail in this study, which isconcerned with the Arctic basin.However, a few of the most important may be listed.

1. The North Atlantic may be older than the South Atlantic (un- less an older branch of the Mid-Atlantic Ridge is traced in the South Atlantic).In this regard, Heezen and Laughton (1963) wrote, Abyssal plains are not as well developed in the South Atlantic as they are in the North Atlantic...Whereas on either side of the North Atlantic, abyssal plains are found in all but a very few areas, in the South Atlantic there are many places where the continental rise merges with the abyssal hills without an intervening abyssal plain. The initial fracturing and drift of North America from Europe is con- sidered by some geologists to have occurred during the Triassic (Eardley,1963, for example).Yet Gaskell (1965) reported results of deep drilling in western Africa which indicated that Africa and South America were together until at least Lower Cretaceous.Wilson (1965) on the basis of the geology of the land masses, suggested that the splitting of North America from Europe commenced in the Arctic and progressed toward the south.If these indications are correct, the North Atlantic may well be older than the South Atlantic.

2.Bullard etal. (1965) achieved a convincing computer fit of Europe and North America and Africa and South America.However, 164 the result was less convincing when both the northern and southern blocks were fitted from the same mathematical point of rotation. had to be left out, and Spain folded against Europe. The recognition of a more ancient branch of the Mid-Atlantic Ridge in the North Atlantic suggests that different points of rotation should be used for the North and South Atlantic.

3.Wilson (1963) wrote that the Atlantic Ocean opened by a ro- tation of 15° about a fulcrum at the New Siberian Islands.The bathy- metry of the Arctic Ocean (Plate 1) does not indicate continental drift about a point of rotation in the Arctic.However, it is a mathematical theorem that if '1sheets" are moved on the surface of a sphere, there will be a point of rotation on the surface of the sphere.The writer suggests that for the Arctic and North Atlantic Ocean the initial point of rotation was the conjugate point of the center of the axis of the Wegener Rise.This conjugate point is about on the 160th meridian south of , and suggests that the Arctic and North Atlantic Oceans opened like a clam shell rather than by a fan-like rotation. The possible extension of a former convection cell from the Arctic basin to the equatorial Atlantic has been suggested above. The origin of the Arctic basin by sea floor spreading from the Alpha Cordillera has been illustrated in Figure 14.In the following para- graphs an attempt to date the origin of the Arctic basin will be made and indications of a possible sequence of geological events suggested. 165 Most of the previous studies (reviewed above) have called for a two stage origin of the Arctic basin.Wilson (1965, page 159) be- lieved that the Amerasia Basin and the Northwestern Pacific Ocean are floored by the oldest oceanic crust on earth.He suggested that these were continuous with each other across Asia where the Verkhoyansk Mountains now exist.For reasons which have been

discussed previously, Wilsons hypothesis is not accepted here.The writer believes that the floor of the entire Arctic basin formed at the same time and that the formation was contemporaneous with the formation of the North Atlantic Ocean.(This is not to rule out the possibility of an extension of the Pacific into Asia atan earlier time. ) Estimates of the time of initial fracturing and the beginning of the formation of the Atlantic Ocean range from Permian (Eardley, 1962) to Cretaceous (Wilson,1965).Unfortunately, there are no unique data from the Arctic basin which allow a more meaningful estimate of the time of its formation.However, the method of age dating the sea floor by means of magnetic reversals (Vine, 1966) holds promise. An adequate aeromagnetic survey of the Arctic basin might well al- low an estimate of the time of its formation. Baadsgaard, etal. (1961) showed that the Paleozoic orogenic belt of and (according to the present study) itscon- tinuation across Novaya Zemlaya and the TJrals was active until 220 million years ago.This would appear to place an upper limit on the 166 age of the Arctic basin:it did not exist in the late Paleozoic.The Verkhoyansk-Chukotski Fold Region underwent folding and volcanism during the Cretaceous and Tertiary (Nalivkin,1960).Tailleur and Snelson (1966) dated the intense thrusting in the Brooks Range as Cretaceous and Tertiary in age.Iceland was during Tertiary time, or possibly during the late Cretaceous.The volcanic effusives of eastern Baffin Island and Greenland are considered to be of Tertiary age (Link etaL, 1960).To the southwest of the Arctic basin, the Laramide Revolution occurred during Cretaceous and early Tertiary time (Eardley, 196Z).For these reasons, the writer suggests that the initial fracturing of the Arctic continent occurred during the Cretaceous and that the Arctic basin formed during the Tertiary time. Farther south along the axis of the Alpha Cordillera, the con- vection cell in the mantle abandoned the mid-ocean ridge in the Labrador Sea, and possibly farther south, and reformed along the axis of the Mid-Atlantic Ridge during early Tertiary time.This is suggested by the evidence that the Labrador Sea Ridge has undergone much greater subsidence than has the Alpha Cordillera.However, the writer does not believe that the Mid-Atlantic Ridge extended into the Eurasia Basin of the Arctic Ocean at that time. As has been shown in previous sections, the extension of the Mid-Atlantic Ridge across the Arctic basin (the Nansen Cordillera) appears toresemble 167 the initial fracturing of the ocean floor; no significant uplift has taken place as yet. The possible sequence of events, based on the ideas outlined above, might be:

1. Cretaceous: A mantle convection cell upwelled beneath the continent.Fracturing occurred in the Paleozoic Fold belts and the northern margin of the Precambrian Shield.Europe and North America drifted apart, away from the axis of the Wegener Rise. Following drift on the order of a few hundred kilometers, the North American block encountered a convection cell moving from the Pacific Ocean side.The resulting compression may have arched the Cordilleran Geanticline and deformed the Brooks Range and the Verkhoyansk-Chukotski Folded Region.The downwelling limb of the Wegener cell resulted in the subsidence of the continent to the east of the Cordilleran Geanticline.(See Eardley,1962, Plates 11 and lZ Eardley described the deformation caused by the Laramide Revolution and wrote, If the relations as depicted are correct, then only one conclusion seems warranted, namely, that the belts of deformation are due to deep-seated causes, not in- fluenced particularly by deeply filled troughs or basins, nor by the crystalline basement with athin veneer of sedi- ments.(Eardley, 196Z, p. 301)

2.Early Tertiary: The mantle convection cell beneath the growing North Atlantic changed its axis of upwelling to the present Mid-Atlantic Ridge.The segment of the ridge in the Labrador Sea, and south to 28° N. Lat., subsided as the new ridge grew.In the Arctic basin, convection may have continued beneath the Alpha Cordil- lera.However, the forming Mid-Atlantic Ridge did not enter the Arctic basin.On the continents, the Laramide Revolution continued as the continental blocks remained positioned between the convection cells beneath the Atlantic and Pacific.The compression of Eastern Siberia and Alaska continued, and the part of North America which was down warped between the Cordilleran Geanticline and the Pre- cambrian Shield extended farther to the northeast than it had in late Cretaceous time (Eardley, 1962, Plate 13).This may have resulted from movement of the continent to the southwest over the convection cell, bringing the downwelling limb relatively farther to the north- east.The development of the Mid-Atlantic Ridge, at the expense of the former ridge, resulted in a rotation of North America and a greater separation between North America and Africa than between the continental blocks farther north.

3.Middle and Late Tertiary: During this period, the conti- nental blocks achieved their present locations (see Figure 14 C) and the Arctic basin and North Atlantic their present form.The down- welling line under North America was as shown in Figure 16 and the previously subsided region southwest of that line became emergent (Eardley, 1962, Plate 15).Convection beneath the Alpha Cordillera 169 ceased.Possibly convection beneath the Mid-Atlantic Ridge ceased at the same time except, perhaps, for the segment near Iceland.

Langstreth etal.(1966)reported, on the basis of heat flow data, that there has been no mantle convection beneath the Mid-Atlantic Ridge during the late Cenezoic.The most recent major event in the geo- logical history of the Arctic basin has been the formation of the

Nansen Cordillera by the earliest stage of mantle upwelling.The writer does not believe that the beginning of this event can be older than late . The writer admits that the events and their ages outlined above are hypothetical to the point that they are suggested with considerable hesitation.However, the post-Paleozoic age for the Arctic basin is reasonably clear.The two dissimilar mid-ocean ridges exist and are certainly of very different ages.The former extension of the Alpha Cordillera into the Labrador Sea is substantiated by good evi- dence, even though a more southward extension may be considered speculative.The geological events related to the downwelling limb of the mantle convection cell are, of course, the most tenuous part of the hypothesis.Field data from mid-ocean ridges are much more extensive than are data from the great trenches, which are thought to be the most clear cut examples of mantle downwelling (Fisher and

Hess,1963). Consequently, less guidance exists for this part of the convection cell.Nevertheless, the conservation of mass demands 170 that if mantle material rises there must behorizontal motion and downwelling: although the dimensions involvedcould have a great range of values.It appears inevitable, however, that ifmantle con- vection exists, the regions of downwelling mustbe regions of subsi- dence on a continental scale. The margins of the Arctic basin will nowbe summarized with reference to transcurrent faults and the lineof downwelling.Plate 4 shows the probable locations of the transcur rentfaults along which the continental blocks moved as the Arcticbasin grew.The continu- ation of the fault line along the Eurasiancontinental slope crosses the Chukchi Plain, but then continues across one ofthe broadest, and flattest continental shelves on earth: The Chukchiand northern

Bering Seas.Nevertheless, there is good evidence for such a con- tinuation (see Figure 15).The fault line crosses the northern margin of the Chukchi Sea shelf at the location of twosubmarine canyons; continues south past Herald Island, where theshelf is marked by an elongate, closed basin; along the western marginof Herald Reef and into Bering Strait.The Strait is marked by an elongate, closedbasin.

Creager and Macmanus(1965and1967)have presented detailed bathymetry of the Chukchi Sea. Moore(1964),on the basis of sub- bottom profiling, reported a change inlithology beneath the thin blanket of sediment across the region.He believed the bedrock be- neath the northwestern to bevolcanic and intrusive rock, 171 while that in the eastern Chukchi Sea appeared to be stratified sedi- mentary rock; an extension of the Brooks Range Structure.The fault line shown in Figure 15 passes between these two regions. Farther south, the fault line extends between Saint Lawrence Island and the mainland, and between Nunivak Island and the Delta. The islands in the Chukchi Sea also may indicate the fault line. Herald Island, Island, Fairway Rock and each lays along, but slightly west of the fault.Each is a shear sided plugof crystaline rock uplifted 300 meters or more.Plate 5 shows Fairway Rock.The shelves of the Chukchi and shallow Bering Seas were emergent (but apparently not glaciated) during eachglacial lowering of sea level, and have been smoothed by repeated trans- gressions of the sea.Nevertheless, evidence of the fault line re- mains. The total movement along the fault must have equaled the length of the Eurasian continental slope, less the amount of any shortening of the continental blocks or post-faulting increase in the length of the slope.The present length of the slope is about 1600 km.Tailleur and Snelson (1966) have reported 300 km of shortening inthe Brooks Range and, as will be discussed later, the length of the slope may have been increased as much as 100 km since the Nansen Cordillera began to form.This indicates some 1ZOO km of movement along the

fault.Note that the shelf edge which separates the deep Bering Sea

173 from the shallow continental platform is about 1ZOO km in length (Plate 4). The transcurrent fault along the North American coast is well marked by its straightness for nearly 3000 km (Plate 4).South of the fault line, motion along the fault is revealed by the fracturing of the continental block which formed the islands of the Canadian Archi- pelago, by the opening of Baffin Bay and the Greenland Sea.And, of course, by the opening of the entire North Atlantic Ocean. The downwelling limb of the convection cell around the margins of the Arctic basin is shown in Plate 4.These should be regions of subsidence.The evidence (one seismic refraction profile) for a partiallyrelaxed and sediment filled trench at the base of the Alaskan continental slope has been mentioned.Along the Asian mar- gin of the Amerasia Basin, the subsided regions may be indicated by the Chukchi and Wrangel Plains.The continental slope at the inter- section with the Alpha Cordillera would not show evidence of subsi- dence, because this would have been a region of upwelling.Evidence for or against a subsided region farther to the east of Wrangél Plain does not exist.Echograrns have not been collected from the inter- section of the Lonionosov Ridge with the slope, and no useful geo- physical data are available.The segment of the continental slope at its intersection with the Nansen Cordillera would probably not retain evidence of a formerly subsided region, because, in the writer's 174 opinion, mantle upwelling is now commencing there.The bathy- metry of the continental slope between the Sadko Trough (See Figure 1) and the sharp change in azimuth of the slope shows evidence of 'notchingT1, as though the slope itself were being opened (refer to Plate 4). North of European and Europe, the line of downwelling enters the continental shelf of the Kara and Barents Seas.This broad shelf, unlike the shelf beneath the Laptev, East Siberian and Chukchi Seas, is characterized by troughs and closed basins.It can be described as a continental borderland.The concept of mantle downwelling at the margins of the Wegener Rise can perhaps account for this borderland.The dimensions and depths of the troughs are on the same scale as the large lakes of the Canadian Shield.The hypothesized downwelling line intercepts the continental slope off northern Norway.Its possible southward extension along the conti- nental slope of Europe has been described previously. 175

BIBLIOGRAPHY

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The echograms taken from the nuclear submarine cruises and the ocean floor jrofiles constructed from the echograms are the basic data of this study.In previous sections, the profiles have been presented in a geographical arrangement as they have pertained to each of the ocean floor features.In this appendix, the track chart of each of the nuclear submarine cruises is shown together with the ocean floor profiles arranged by individualcruise.The writer believes that the presentation of the profiles in this way will be useful to other researchers. APPENDIX + + 90°E

BT B

4B C

NO.4A

A

NO.7 / LB NO 8

1' ).5_--/ +8OB Ck\ NO.98

NO.6

I

Figure 18.Track of the NAUTILUS 1957 expedition. Numbers refer to profiles shown in Figure 19. 190

FAIl A FAT n A FATHOMS METERS

PROFiLE No. I PROFILE NO. 2 PROFILE NO. 3 A rAil B C A B C FATHOMS 0 E

PROFILE NO.4A PROFILE NO. 4 B FATHOMS B METERS FATHOMSA B C METERS A

PROFILE NO.4 C PROFILE NO. 5 A PROFILE NO.6 FATHOMS B C 0 £ METERSFATHOMSA B METERSFA A R Cfl

PROFILE NO. 7 PROFILE NO.8 PROFILE NO. 9 VERTICAL EXAGGERATION 50:1 NAUTICAL MILEA sSN7I) t957 v.* uru 100 Figure 19 191

.ç-NO.I

1% p.-r'a 90°w + NO.4 +

I_-_--NO5 S /

I 4a

I

II

Figure 20.Track chart of the NAUTILUS 1958 expedition. Numbers refer to profiles shown in Figure 21. 192

PROFILE NO. I PROFILE NO. 2

PROFILE NO. 3 B C

PROFILE NO 4 A B C 0 C F

PROFILE NO 5 ASS NAUTILUS (SUN 571) Figure 21 195$ 193

w 0 __I:I:II:

+ 750

,.

4-80°

+ 85° A

a NO3 -NO.2 BA 900W + 8* " 0E1 A D B\\

Figure 22.Track chart of the SKATE 1958 expedition.Numbers refer to profiles shown in Figure 23. 194

PROFILE NO I

PROFILE NO. 3

PROFILE NO. 4 PROFILE NO 5

PROFILE NO.6 195

1

.-NO.3

D

4 II3

90w-f 1o0

, S \,..-NO.5 \

,ANO.6

p

Figure 24.Track of SKATE 1959 expedition.Numbers refer to profiles shown in Figure 25.

197

+° 0.10 NO.1 B Rc NO.2 p +8 D .k A/" N 0.3... A B A NO.4 A+850 B

B90 A A + / NO.6

S I

Figure 26.Track of SARGO 1960 expedition.Numbers refer to profiles shown in Figure 27. 0 198

PROFILE NO. I PROFILE NO.2

APROFILE NO. 3 B A PROFILE #0. 4

FATHOMS APROFILE NO.5 BC PROFILE NO.6 B

PROFILE NO.7 PROFILE 60.6

PROFILE NO.8 )oont.) PROFILE NO.9

PROFILE NO. 10 PROFILEUSS NO. SARGO 1558 883)1960 II 199

1800 'W

+ 750

wp + 80° I

\_o

A *ig

F

B GA 1900E

.4 qI +850

-1-80°

+ °

Figure 28.Track of SEADRAGON 1960 expedition. Numbers refer to profiles shown in Figure 29. ITHOMSA B NETR$

PROFILE NO. I E F METERS

ThoMS A PROFILE NO. I (cont.) B C METERS

ThOMS PROFILE NO. 2 METERS FATHOMS A METERS

PROFILE NO. 3 -- PROFILE NO. 4 uss SEADRAGONVERTICAL (SSN EXAGGER4TION 584) 1960 50:1 oToo 0 NAUTICAL MILES KILOMETERS 50 60 O 00 ioo

..., .... -j---. . '0V1W .... ol

+ °

,%

-1-8 NO.7

+ 850 It]

0 ' W +

Figure 30.Track of SKATE 1962 expedition. Numbers refer to profiles shown in Figure 31. 202 FATHOMS B C METERS p

FATHOMS PROFILE NO. I MFTFRS

FATHOMS A PROFILE NO. 2 B METERS FATHOMS A METERS

FATF -- PROFILE NO. 3 PROFILE NO. 4 B C

FATHOMS PROFILE NO. 5 METERS FATHOMS B C METERS

PROFILE NO.6 PROFILE NO. 7 Uss VERTICAL EXAGGERATION 50: ISKATE(SSN578) 1962 NAUTICAL MILES 0 0 0rrm20 rIp,iI! KILOMETER40 S 50 60 eo 100 00 203

+70° 80°

\ +7°

BNQ.1 *p o.Z

+ 5o

NO

D

+ 85 it

+ Oo V

+ r50 --

Figure 32.TraCk01SEADRAGON Nurribe r s refer to prof1l shoWn Figu FATSOMS roAs * B U

FAThOMS A B-.

FAT PROFILE NO. 2

FSrSO PROFILE NO.3 -iOOO ii FATHOMS A PROFILE NO. 4 -. .------. ----.- 000 OMSA PROFILE NO. 5 N C A B C NE

PROFILE NO.6 - USO SEAORAGON (SON 5R4) (962 PROFILE NO.7

PROFILE NO.3 ii FAtHOMS A PROFILE NO. 4 - B

HMA PROFILE NO. 5 9 C A B C BE

PROFILE NO.6 - USS SEADRAGON (OSN 5R4) 1962 PROFILE NO 7 205

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