NOTES

The transition zone between the Eurasian continent and the Arctic Ocean1

R. M. DEMENITSKAYA, A. M. KARASIK, Yu. G. KISELEV, I. V. LITVINENKO, AND S. A. USHAKOV Academy of Sciences of the U.S.S.R., Moscow, U.S.S.R.

Received May 7, 1968 Accepted for publication June 26, 1968

Geophysical and bathymetric results have been used for the study of the transitional zone between the Eurasian continent and the Arctic Ocean, which is being considered by the authors as a junction area of continental margins with the adjacent seaward structures, and is dependent of the actual type of the crust found beneath the ocean.

The deep Arctic Ocean comprises different types of crust: basins have an oceanic crust with increased sedimentary thickness; the Lomonosov and Mendeleev Ridges have crusts close to subcontinental; and the crust of the Gakkel Ridge is typically mid-oceanic. At least four types of transition zones may be distinguished: (1) 'normal' transition zone, where the 'granitic' layer is wedging out and the crust consists of a thinned sedimentary layer and 'basalt'; this is the junction of the continent and the oceanic basin; (2) a zone where the continental crust thins to the oceanic crust, the 'granitic' layer is absent, and the 'basalt' layer is underlain by the 7.5 km/s layer, rather than the normal mantle; this is repre- sented by the junction of the continent with the rift zone of the median ridge that juts out into the shelf and extends under the continent; (3) a zone where the continental crust thins, the 'granitic' layer is wedging out, but the crust does not thin to the oceanic crust, mainly because of an increase in thickness of sediments; this zone is a junction of the continent with a basin with subcontinental crust; (4) a zone where the continental crust thins, but does not reach oceanic thickness; this is a junction of the con- tinent with the oceanic crust. In the transition zones of the first and third types structural downwarping compensated by the sediments has been developed.

The development of the transition zone of the Arctic was intermittent in the geological past, which we see expressed by the asymmetric development of the Greenland-Canadian and Eurasian sectors. These examples of different structures of transition zones are not unique. The transition zone between the Asiatic continent and the Indian Ocean appears to be most similar in its complexity to the transition zone of the Arctic Ocean. However the 'normal' type of transition zone characteristic of much of the continents of Australia, South America, Africa, and parts of other continents frequently occurs here.

We consider the region where the continental margin adjoins the structures of the Arctic Ocean basin to be a transition zone, whatever the particular nature of the oceanic crust. The crustal structure of the deep Arctic basin is rather vari- able : within the basins it is represented by normal oceanic crust accompanied by a gradual increase of thickness of sediment towards the continental rise. The Lomonosov and Mendeleev (Alpha) Ridges have subcontinental crust, and the Kakkel Ridge is a typical mid-oceanic ridge with a rift zone, and an apparently thinned crust, and is underlain by the layer in which the compressional wave velocity is 7.5 km/s.

The crust of the Eurasian shelf in the Barents Sea, including the Baltic Shield (Fig. 1) has been investigated by deep-seismic sounding. The crustal sections of the Barents Sea and shield have about the same thickness, approximately 35-40 km, but differ considerably in structure. A shallow seismic layer with compressional wave velocity greater than 6.5 km/s (basalt) is observed. It is interesting to note that the layer with seismic

'Presented at the International Symposium on Con- tinental Margins and Island Arcs, held in Zurich, Switzerland, September 28-29, 1967.

Canadian Journal of Earth Sciences, 5, 1125 (1968)

velocity of 7.2-7.5 km/s is established above the M discontinuity, and under shield conditions it cannot be interpreted as the 'crust-mantle mix' typical of the rift zones. Detailed investigations have been made of the horizontal layering of the crust according to changing elastic properties, and of juxtaposed block structures within the crustal sections, comprising -$ of the whole thickness of the crust. Within the separate crustal blocks of the shield approximately 6 seismic boundaries have been delineated, most of them within the basalt layer.

In the crustal sections of the Barents Sea the thick sedimentary layer with compressional wave velocity of 5.5 km/s is seen everywhere. In the offshore area the surface of the consolidated crust is well traced (boundary with V = 6.0-6.1 km/s), and corresponds with the surface of the granitic layer outcropping on the shield. The thickness of the basalt layer observed locally under the ocean floor is significantly less than under the shield; it is not more than 20 km thick.

The Barents Sea possesses characteristically a complex, blockstructure presently poorly studied. Indications of this block structure of the crust are shown on Fig. 1, where sections of supposed

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1126 CANADIAN JOURNAL OF EARTH SCIENCES. VOL. 5, 1968

FIG. 1. The crustal section across the profle of the Barents Sea - Kola Peninsula (compiled by Litvinenko, I. V., Platonenkova, L. N., and Teljakova, Z. H.). 1 = The averaged position of base (a) and other @) seismic boundmes. 2 = The averaged position of base (a) and other (b) seismic boundaries from inexact data. 3 = The estimate of the average and boundary velocities in km/s. 4 = The supposed tectonic disturbances. 5 = Sedimentary deposits of the Barents Sea. 6 = Riphean sedimentary complex. 7 = Sediment-effusive Pechenga series. 8 = Schist, amphibol~tes of the permafrost (Tundra) series. 9 = Gneisses, granites, granite gneisses. 10 = Granulites (acidic and basic). 1 1 = Basic and ultrabasic intrusions. 12 = The non-uniform upper part of the granulitebasalt complex; crustal layer with compressional wave velocity of 6.3 - 6.8 kmls. 13 = The lower, more uniform part of the granulitebasalt layer. 14 = The subcrustal layer. a = Belomorides; b = granulitic complex; c = Karelides; d = Kola series; e = Riphean complex; f = Paleozoic mantle of the platform (?); g = Riphean complex (Kanin-Medvezhii) (?); and h = Paleozoic mantle of the platform (?).

Paleozoic platform regions differ from zones of Riphean complexes.

The available data about the aggregate thick- ness and internal crustal structure of the Barents Sea in the shelf region give additional reasons to extend the continental crust on the Eurasian continental slope. The differences in the crustal structure of the Arctic basin as well as separate blocks of the continental crust of the slope con- stitute characteristic features for the crustal structure of the transition zone between the Eurasian continent and oceanic crust of the deep Arctic Ocean. Taking into account these differ- ences one may subdivide the transition zones into at least four types (Fig. 2).

(1) The 'normal' transition zone where the 'granitic' layer is wedging out and the crust consists of a thin sediment layer and 'basalt', where total thickness does not exceed 7 km. (2) The transition zone where the continental

crust thins and changes into an oceanic one, the 'granitic' layer is absent, and the 'basalt' layer is underlain by the mantle with com- pressional wave velocity of 8.0-8.2 km/s. This is a junction area between the continent and a mid-oceanic ridge. (3) The transition zone where the continental crust thins and the 'granitic' layer wedges out, but the crust thickness is not as thin as an oceanic crust because of increased sediment thickness. This is a junction area of the con- tinent and a basin, characterized by a sub- continental crust. (4) The transition zone where the continental crust progressively thins, but is not as thin as the oceanic crust, for it changes into the sub- continental crust of the submarine ridges. The characteristic feature of the transition

zones of the first and third type is the existence at the continent-ocean boundary of structural

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NOTES

a b r

T T i~ T T 41 T T T T T T T T T T T T T T

0 wateR ......... nonconsoLidaied and ( '.,~con~o~,&fed sedlmnfs I++I , p a n i fic" Layeu

llbasaLtic* la ye^

mmdnfLe

1757 the rtpons of sflpsed cfus and ma L m/x L ongitudina,! wave ve+!oci fies at IatSmIc boundaries

FIG. 2. Hypothetical models of the Earth's crust in the transition zone of the Arctic Ocean (compiled by R. M. Demenitskaya, A. M. Karasik, and Yu. G. Kiselev). I. a = Baltic Shield; b = Barents Sea; c = Arctic Ocean; d = Franz-Josef Land; e = Nansen Basin; f = Gakkel Ridge; g = Valley of Hydrographers; h = Amundsen Basin. 11. a = Verkhoyansk range; b = Laptev Sea; c = Arctic Ocean; d = southern part of the Nansen-Amundsen basin; e = southern end of the Gakkel Ridge. 111. a = Polousnyi Ridge; b = East-Siberian Sea; c = New Siberian Islands; d = Arctic Ocean; e = Lomonosov range. 1v.B = East-Siberian Sea; b = Arctic Ocean; c = Makarov basin.

troughs or trenches commonly filled with sedi- bordered on the west by the Nansen Sill and on ments and consequently not expressed in the the east by the Lomonosov Ridge is referred to bottom topography. The condition of isostasy is the first type. The exception is a section near not valid for these troughs. 130" W where the mid-oceanic Gakkel Ridge

The transition zone of the Eurasian continent adjoins the Asiatic continent, under which a zone

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1128 CANADIAN JOURNAL OF EARTH SCIENCES. VOL. 5, 1968

of the 'rift' mantle connected with the ridge extends. This section of the transition zone is referred to the second type. The available geo- physical data suggest that the characteristic rift structure of the consolidated crust of the surface may occur in this section of the continental rise, but it is not represented in the bottom topography because of the thick (about 2.5 km) sequence of sediments.

The crust of the transition zone of the first type, in spite of common features-a gradual decrease in thickness and wedging out of the 'granitic' layer within each section-has characteristic structural features and, probably also those of dynamics. In particular, the pattern of the transi- tion zone between the Gakkel Ridge and Lomonosov Ridge on the whole is closer to isostasy than in any other region. The area of anomalous structure has been revealed in the transition zone east of Severnaya Zemlya, Franz Josef Land, where structural troughs covered by sediments are traced; St. Ann and Franz Josef trenches may be attributed to the world-wide system of dislocations, extended under the oceanic crust of the Eurasian basin, considerably changing at the intersections with the mid-oceanic structure of the Gakkel Ridge. At the junction of the continental crust of the Asian continent with the subcontinental crust of the Lomonosov and Mendeleev Ridges, the crustal structure appears to change less than in any other section of the transition zone of the Arctic. However, the magnetic pattern shows that continental struc- tures had developed, and if they had continued to develop, the submarine ridges would have signifi- cantly changed crustal structure; this made difficult the establishment of any similarity with adjacent parts of the continent.

Between these two segments of the transition zone, referred to the fourth type, there is a wide strip of the transition zone of the third type. The Toll 'basin' cannot be interpreted as a typical basin, for it represents a continental rise with considerable thickness of sediments, which have accumulated because of the comparatively limited de~ositional area but vast source area. The thickness of sediments in some areas exceeds 4 km. The sediments smooth the rather rugged topography of the consolidated crust beneath the continental rise. The structure of the 'basalt' layer of the crust under this part of the continental rise is rather complex. The region of wide-spread

plateau basalts is delineated from the magnetic pattern. The regions of junction of the suboceanic crust of the Toll 'basin' with the subcontinental crust of the Lomonosov and Medeleev Ridges have been revealed. A highly magnetized material was injected in the crust.

Between the Lomonosov Ridge and the Chukchi submarine plateau is a basin whose type of crust and considerable thickness of consoli- dated sediments (more than 2 km) permit us to classify them as the transition zone of the third type.

The available evidence suggests that the Chukchi plateau is a relic of the continental crust presently greatly changed-crushed and rebuilt. A

The areas of anomalous structure of the crust and the mantle intersect the Chukchi plateau in two directions : near-latitudinal and near-meridional. These are expressed in the bottom topography , and the latter probably extends to the north to the Canada basin and to the east, where it juts out into the Mendeleev Ridge.

It is not excluded that the area of anomalous upper mantle extends beneath the Chukchi plateau to the east from the side of the Alaska transition zone. The analysis of gravity data made by Ostenso (1962) has indicated that within the Alaska transition zone there was a disturbance of isostasy characteristic of the transition zone of the Pacific type. This downwarping of the base- ment filled by a thick sequence of sediments revealed by seismic methods coincides with the minimum of the isostatic anomaly, The minimum of isostatic anomaly in the northern part of the shelf and continental slope of the transition zone ofAlaskacorresponds to theconjugate maximum. A

All available geophysical data suggest that the junction of the continental slope and the rise is coincident with a major fault expressed in the lack of correlation of reflecting horizons, character- isticmagneticanomalies, and bottom topography, etc. In some places materia1 of higher magnetic susceptibility was injected along these faults, such as along the northern and western boundary of the Chukchi plateau; in other places it was weaker.

The evolution of the Arctic transition zone as well as the whole oceanic crust of the abyssal basin was intermittent on the western Eurasian and eastern America-Asia sections in the geo- logical past. This suggestion is proved by the asymmetric development of the entire region, in

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NOTES 1129

particular, the Greenland-Canadian and Eura- siatic sections. This asymmetry is a part of the general asymmetry of the eastern and western parts of the Earth, which is caused by an essential difference in the Pacific and Atlantic mantle.

Of great interest is the structure of the Earth's crust of the Eurasian shelf. The geophysical investigations by the deep seismic sounding

v (DSS) method has shown that the crustal struc- ture beneath the Baltic Shield - Kara Sea profile varies considerably over short distances. At a distance of about 100 km the 'granitic' layer has almost completely wedged out and transformed into the low seismic velocity layer, i.e. presumably the sedimentary layer up to 10-1 5 km thick.

The aggregate crustal thickness is charac- teristically the same as on the continent there. Thus, on the northern shelf of Europe the crustal structure is similar to that of the Caspian Sea. The depth of anomalous bodies suggests that the

greater thickness of sediments is associated with the northern shelf of Asia as well. because the depth of magnetic basement in some regions attains 10-12 km.

Comparing the transition zones on a world- wide scale, it is notable that the non-uniform structure of the transition zones between the continent and the ocean is not unique. The transi- tion zone from the Asiatic continent to the Indian Ocean, where at a short distance one can observe a junction of the continent with oceanic structure, is most similar in complexity to that with the Arctic Ocean.

The normal type of transition zone, charac- teristic of much of Australia, South America, Africa, and separate regions of other continents is wide-spread.

OSTENSO, M. A. 1962. Geophysical investigations of the Arctic Ocean basin. Geophys. Polar Research Center, Rept. N62-4.

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