MICHAEL CHURKIN, JR. U.S. Geological Survey, Menlo Par{, California 94025

Western Boundary of the North American

Continental Pkte in Asia

ABSTRACT The geologic history of Yakutia indicates that it was the site of a major seaway that The mid-Atlantic Ridge and its northern ex- separated the Siberian platform from the tension across the Arctic, the Gakkel Ridge, Kolymski massif during the early Paleozoic form the eastern boundary of the North Amer- and then again in the early Mesozoic. Deposits ican continental plate. The position of the of geosynclinal proportions developed along western boundary separating North America the margins of these subcontinents and were from Asia is less certain and has been sought in deformed into foldbelts as the seaways closed the Bering Sea region and in Yakutia, North- between the leading edges of the Eurasian and east USSR. North American continental plates. The Geologic similarities between Alaska and boundary between the Cherskiy and Verkho- easternmost USSR indicate that these two con- yansk foldbelts marks the suture of the collision tinental areas have been connected since the of these plates in Early Cretaceous time. Paleozoic and probably since the Precambrian, and that the western plate boundary does not INTRODUCTION pass through the Bering Strait. Oceanographic data lend support to the old The trend of earthquake epicenters aligned idea of continental drift of North America from along the Gakkel Ridge continues inland along Eurasia and link that idea to the theory of sea the Cherskiy system of mountain ranges and floor spreading and plate tectonics. In at- depressions. High-angle faulting and volcanism tempting to explain how the drift occurred, it in the Moma-Zyryansk Basin along the east has been postulated that sea floor spreading side of the Cherskiy region seem to be a land- caused differential movement between the ward extension of the seafloor-spreadin g tecNort- h American plate on the one hand and a tonics on the Gakkel Ridge. Modern seismic Eurasian plate on the other (Blackett and activity, faulting, and volcanism in Yakutia others, 1965). may represent incipient rifting of the Eurasian The mid-Atlantic Ridge and its northern continent, a situation similar to that in the extension across the Arctic Ocean, the Gakkel Gulf of California. Ridge, are generally accepted as representing In Yakutia earlier compressional features of the eastern boundary of the North American late Paleozoic and especially Mesozoic age on plate (Fig. 1). The western boundary sepa- trend with the Arctic Ocean opening contrast rating North America from Asia is less agreed strongly with the modern tensional movements. upon and has been drawn or inferred in the Mesozoic rocks of the Verkhoyansk geo- Bering Sea region (Le Pichon, 1968; Hamilton, syncline on the west are faulted against a 1970; Ross and Ingham, 1970) and in various Paleozoic geosyncline in the Cherskiy Moun- positions in Northeast USSR—the Verkho- tains that passes eastward into platform yansk Mountains proper (Wilson, 1963; Heezen deposits of the Kolymski massif. Most of the and Tharp, 1965; Morgan, 1968) and where the faults are large thrusts that not only bring rocks of the Verkhoyansk Mountains come in Paleozoic rocks against Mesozoic rocks, in- contact with the Cherskiy Mountain system cluding synorogenic granite masses, but also (Churkin, 1971). The purpose of this paper is to dispkce sedimentary facies in the Paleozoic examine evidence for each of these boundaries rocks. High-pressure metamorphic minerals are and to give a tentative outline of the geological developed along these faults in wide zones of history of the northwestern edge of the North breccia and mylonite. American plate and its relation to Asia.

Geological Society of America Bulletin, v. 83, p. 1027-1036, 5 figs., April 1972 1027

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E, LTR A5S I A N

PACIFIC PLATE

Le Pichon. 1968 BERING SEA REGION ^=^ Hamilton. 1970 • Verkhoyansk Mountains (Wilson. 1963; Heezen and Tharp. 1965; Morgan. 1968) XXX XXBoundary separating Cherskiy and Verkhoyansk Foldbelts (Churkin, 1971; this paper) Figure 1. Major crustal plates in the northern boundary, hemisphere and various postulated positions of western

PLATE BOUNDARY IN Siberia (Fig. 2). A younger circum-Pacific fold- BERING SEA REGION belt of Cenozoic age that has folds and general Le Pichon (1968) arbitrarily drew a plate uplifts in Kamchatka and the Koryak Moun- boundary along the western margin of the tains includes a thrust belt in the Gulf of Bering Sea and through the Bering Strait on Alaska. A less obvious foldbelt of middle the basis of geometric requirements of an Paleozoic age discontinuously rims the southern earth model of rigid plates; Hamilton (1970) margin of the Canada basin and is deeply drew a boundary in the Bering Sea on the basis eroded and largely covered by flat-lying of geologic correlations between Alaska and Mesozoic and Cenozoic deposits that form the Siberia (see Fig. 1). On the basis of the dis- Arctic coastal plain (Churkin, 1969, 1970a). tribution of an Ordovician trilobite-brachiopod There is considerable evidence for long- fauna, Ross and Ingham (1970, Fig. 5), placed established connections across the Bering- the boundary in the Bering Strait so that Chukchi Seas area (Hopkins, 1967; Egiazarov, northern Japan and the Asian mainland (op- 1969; Belyi, 1969), and rock units have been posite Sakhalin Island) fit against western traced by rock dredgings and geophysics from Alaska, and the Chukotsk Peninsula is aligned the Bering Sea to the coasts of southwest with Ellesmere Island of Arctic Canada. If any Alaska and Northeast USSR (Cape Navarin- of these three Bering Sea plate boundaries are Koryak Mountains) (Scholl and others, 1966; correct, there should be major differences in Scholl and Bumngton, 1971), and rock units the geology of Northeast USSR and Alaska. in the Chukchi Sea have been traced to Arctic However, evidence of such differences is non- Alaska, Herald Shoal, and Wrangell Island existent. (Ostenso, 1968; Grantz and others, 1970). The Cordilleran foldbelt of Alaska, which Furthermore, the rocks at the western tip of had its climactic orogeny during the Mesozoic, Seward Peninsula and at the eastern tip of the correlates with the Chukotka foldbelt of Chukotsk Peninsula where the two continental

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RUSSIAN:

PLATFORM

EXPLANATION

Structural trends Active spreading center Fossil spreading center

PLATFORMS

Cenozoic and Mesozoic rocks Paleozoic rocks and Precambrian shield

r——i =^=, FOLDBELTS

Cenozoic Mesozoic Late Paleozoic Middle Paleozoic Early Paleozoic

Figure 2. Major tectonic features of the Arctic.

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landmasses come together have a clear-cut ronment (Ross and Ingham, 1970), could have similarity in geologic history (Fig. 2; Churkin, continued unbroken across the Bering Sea to 1970a, Fig. 4) and have many specific features connect with a similar fauna in the Alaskan in common such as tin deposits (Sainsbury, Cordillera. In the Chukotsk Peninsula, midway 1969, p. 4) and rare alkali-rich plutonic rocks between these two areas, there are in fact very (Miller, 1970). similar Ordovician fossils in stratigraphic sec- Finally, a paleogeographic reconstruction tions like those in the Kolymski massif area (Fig. 3) based on correlation of Paleozoic sec- (Oradovskaya, 1970) that provide another link tions (Churkin, 1970b) provides one of the for a biostratigraphic correlation between most compelling lines of evidence against a Alaska and Northeast USSR. Considered to- major discontinuity in the geology of the gether, the various different lines of evidence, Bering Sea region. A Paleozoic geosyncline summarized here, imply that Alaska and North- rims the northern Pacific, and another geo- east USSR have been connected since the syncline rims the southern margin of the Arctic Paleozoic and probably since the Precambrian basin. Separating the two geosynclines is a nar- (Churkin, 1970a). row stable area that received mainly carbonate sediments and that may have extended rather PLATE BOUNDARIES IN ASIA continuously from the North American craton to the Kolymski massif of Northeast USSR. North Atlantic-Arctic Opening and Mesozoic volcanic-terrigenous rocks of the Its Continuation into Asia Yukon-Koyukuk province break the continu- The pivot of rotation of North America ity of the carbonate area between east-central with respect to Eurasia to produce the opening Alaska and Seward Peninsula. The Yukon- of both the northern Atlantic and the Eurasian Koyukuk province, however, has been de- basins has been determined to be lat 78° N., scribed as a much younger wedge-shaped rift long 102° E., using azimuths of fracture zones feature rimmed by ophiolite masses that sug- in the northern Atlantic (Morgan, 1968; Le gest it is floored by oceanic crust (Patton, Pichon, 1968). Another estimate of the pole of 1970). If the main elements of this reconstruc- rotation, lat 73° N., long 96.5° E., is based on tion are correct, then the trilobite-brachiopod the best fit of Greenland to Europe (Bullard fauna of the Kolymski massif, which indicates a and others, 1965). These poles of rotation, Middle Ordovician continental margin envi- which lie near the apex of the Eurasian basin average out any changes in direction of spreading over the entire period of North Atlantic- Arctic opening. A more specific estimate of the pole of rotation for the Eurasian basin opening using the best fit of the continental margin of Eurasia with that of the Lomonosov submarine ridge on the opposite side of the basin gives the position lat 64° N. and long 138° E. within Yakutia, considerably south of the apex of the Eurasian basin (A. M. Karasik, 1971, oral commun.). Poles of rotation required to rotate magnetic anomalies on the west and east sides of the north mid-Atlantic Ridge into coincidence for anomalies up to 38 m.y. old also plot in Yakutia (Pitmann and Talwani, 1972). On the other hand, anomalies 63 m.y. and older in the north Atlantic require poles of rotation that lie in the north Greenland area to bring their west and east sides into coincidence. This change in the pole of rotation from the north end of the Atlantic to the south end of Shelf and land areas Succe: the Eurasian basin suggests to Pitmann and S-Sverdrup Talwani that the Eurasian basin opening Figure 3. Early Paleozoic paleogeography. started about 63 m.y. ago. Magnetic profiles

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across the Gakkel Ridge, the arctic extension Morgan, 1968). However, except for some of the mid-Atlantic Ridge (Fig. 2), suggest that minor seismic activity and broad uplifts, the the Eurasian basin began spreading about 40 area lacks the through-going faults and igneous m.y. B.P. (Vogt and others, 1970; Wold and activity that mark the major tectonic activity Ostenso, 1971). along the east side of the Cherskiy Mountain Tensional tectonics along the Gakkel spread- system. Evidence for a fossil plate boundary ing center continue south of the Eurasian basin within the Verkhoyansk foldbelt, excluding the (Fig. 4), into Yakutia (Demenitskaya and narrow synclinorium on its east edge, is also Karasik, 1969). Epicenters of shallow-depth lacking. The Verkhoyansk is a north-trending earthquakes aligned with the Gakkel Ridge Mesozoic foldbelt (Yanshin, 1966) that sepa- and Sadko trough continue into Yakutia in a rates flat'lying upper Precambrian and Pale- trend that follows the Cherskiy system of ozoic rocks of the Siberian platform from the mountain ranges and depressions (Rezanov, Paleozoic Cherskiy foldbelt and the Kolymski 1964; Sykes, 1965; Tarr, 1970; A. M. Karasik, massif or platform farther east (Fig. 4). The 1971, oral commun.). The long block uplifts west side of the Verkhoyansk region is com- and depressions associated with the Moma- posed mainly of Permian strata over 7,000 m Zyryansk basin and range system on the east thick in places. These rocks are mainly clastic side of the region seem to be controlled by and are deformed into large linear folds bordered high-angle faults; a good example is the Ulak- on the west by a thick section of Cretaceous han fault. Near the Ulakhan fault (Fig. 4) orogenic sediments derived from the Verkho- is a series of rift features along which Balagan yansk foldbelt (Yanshin, 1966). Large-scale dis- Tas, a Quaternary volcano of highly alkaline placements of geologic trends, overturned basalt, and hot springs have developed. All folds, thrust faults, melanges, great transcur- these have been related to a landward extension rent faults, and associated high-pressure meta- of the tensional tectonics of the submarine morphism and igneous activity that are to be Gakkel Ridge (Demenitskaya and Karasik, expected along compressional plate boundaries 1969; A. M. Karasik and A. F. Grachev, 1971, have not been found here. oral commun.) and thus obviate the necessity The east side of the Verkhoyansk foldbelt for the Eurasian basin spreading to be offset has an enormous synclinal zone filled largely by into the Pacific Ocean by a hypothetical Upper Triassic and, in places, Jurassic deposits. transform fault (Hamilton, 1970, p. 2570). The The western belt of the synclinal zone is char- available data, in my opinion, suggest that the acterized by very gentle folds, but, on the east tectonic conditions extending into Yakutia side of the zone, adjacent to the Paleozoic from the Gakkel Ridge may be similar to those rocks of the Cherskiy foldbelt, there is the in the Gulf of California, where faults extend Inyali-Debin synclinorium, with many kilom- inland from the East Pacific Rise (Larson and eters of Jurassic strata that are tightly folded others, 1968). and intruded by Lower Cretaceous granitic batholiths (Fig. 4). Fossil Plate Boundary in Asia If there is a major break in the geology of the Verkhoyansk Mountain Range. The west- Verkhoyansk Mountains proper, it would have ern boundary of the North American plate has to be of pre-late Paleozoic age and lie below the been assumed by some to lie in the Verkhoyansk upper Paleozoic-Mesozoic rocks of the Verk- Mountains of Yakutia (Fig. 1; Wilson, 1963). hoyansk geosynclinal complex. Magnetic anom- The Verkhoyansk Mountains, south and on alies in the Verkhoyansk region are weak and trend with the apex of the Eurasian basin of dispky small horizontal gradients that have the Arctic Ocean, were deformed in two periods been interpreted to represent a thick section of —from Late Jurassic to early Tertiary, and virtually nonmagnetic clastic sedimentary rocks from Pliocene to Holocene. According to Wil- of Mesozoic age (Belyaev, 1970, p. 241). The son, these were the major periods of spreading basement is thought to be more than 8 km in the Atlantic and Arctic. On the basis of below the surface and to represent nonmagnetic earthquake epicenters that cross Siberia on Precambrian rocks (Rezanov and Zarudnyi, trend with the Gakkel spreading center (Sykes, 1965), but I believe that it could also represent 1965), a plate boundary separating America Paleozoic material of the oceanic type that is from Eurasia was also drawn along the Verk- characterized by low-amplitude magnetic hoyansk Mountains (Heezen and Tharp, 1965; anomalies. Whatever the nature of the rela-

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EXPLANATION 80° Early Cretaceous and some Late Jurassic granitic plutons

Late Mesozoic erogenic deposits on Paleozoic and Precambrian basement

Early Mesozoic geosynclinal rocks. In western part of Verkhoyansk foldbelt includes thick Permian section

• • Paleozoic geosynclinal rocks; mainly Ordovician, Silurian, and Devonian

Mainly Paleozoic platform deposits on Precambrian basement. Cretaceous and younger cover not shown

Major fault

Fold trends # Balagan las volcano

MOMA-ZYRYANSK BASIN AND RANGE SYSTEM

CENOZOIc

SEA OF OKHOTSK

•AS E , . ••„• ~.•••> TH^uEr CRUST Figure 4. Major geologic features of Yakutia (after and Zarudnyi, 1965). Yanshin, 1966). Cross section modified (after Rezanov

lively featureless magnetic basement in the zones of breccia and mylonite (Dobretzov and Verkhoyansk foldbelt, well-developed mag- others, 1966). Also developed along this zone netic anomaly patterns in the Siberian plat- of regional faults is a belt 1,000 km long of form on one side and in the Cherskiy Mountain granitic batholiths, most of which are Early system-Kolymski massif on the other have dif- Cretaceous and some Late Jurassic. ferent trends, which suggests a major break in Blueschist rocks representing high-pressure- the magnetic character of the basement low-temperature metamorphism in thrust belts, (Belyaev, 1970, Fig. 28; Mokshantsev and associated with ultramafic rock around the Gusev, 1968, Fig. 1). Thus, the Verkhoyansk Pacific basin, are thought to be characteristic complex, although internally lacking a plate of plate boundaries or subduction zones where boundary, may conceal a pre-Permian bound- oceanic crustal plates have slid beneath con- ary. Evidence for this is discussed in the next tinental margins. In easternmost USSR section. (Koryak Mountains, Sakhalin Island, and Cherskiy-Verkhoyansk Foldbelt Juncture. Kamchatka Peninsula), according to Firsov Stratigraphic, structural, and petrological fea- and Dobretsov (1969), there is a sequence of tures in the Cherskiy Mountain system suggest blueschist belts and ultramafic rocks of middle that this mountain system, where it comes in Paleozoic, early Mesozoic, and late Mesozoic contact with the Verkhoyansk foldbelt, marks a age that had progressively developed toward fossil pkte boundary along which the Eurasian the Pacific Ocean. Probably these mark fossil and North American continental plates were subduction zones where the ancient Pacific sutured. The compressional features of Mesozo- plate interacted with the southern margin of ic age along this boundary contrast strongly with the continental plate of eastern Asia. However, the more recent, mainly tensional movements no blueschists are known to me within the along the east side of this region that have been Cherskiy area, and, except for small occur- related in the preceding section to modern plate rences, neither is a belt of ultramafic rocks. motions in the Eurasian basin part of the Arctic. Major faults in the Cherskiy Mountain sys- HISTORY OF THE WESTERN tem separate three fundamentally different MARGIN OF THE NORTH Stratigraphic sequences of Cambrian through AMERICAN PLATE early Carboniferous age (Bogdanov, 1963). To the east, the Kolymski massif has a platform The eastern Verkhoyansk-western Cherskiy sequence from 1,500 to 3,000 m thick of region marks the site of a major seaway that carbonate strata interlayered with sandstone separated the Siberian platform from the and shale of ktest Precambrian and Cambrian Kolymski massif during the latest Precambrian through Devonian age. This sequence rests (Mokshantsev and Gusev, 1968, Fig. 3) and unconformably on complexly deformed Pre- early Paleozoic (Fig. 3; Bogdanov, 1963). Some cambrian basement rocks. West of this plat- evidence suggests that this seaway closed in the form section, in the eastern ranges of the middle Paleozoic. Coarse clastic sediments in Cherskiy Mountain system, is a tightly folded, the upper parts of the Devonian and in the mainly carbonate rock section of Ordovician lower Carboniferous sections of the Cherskiy through early Carboniferous age that is as foldbelt were transported from sources to the much as 13,000 m thick. Farther west, along west, from the region now occupied by the the western edge of the Cherskiy Mountains, a Verkhoyansk foldbelt (Bogdanov, 1963, p. 155, largely clastic section of Ordovician through 162). Granitic detritus in these conglomerate Carboniferous strata, including mafic volcanic beds and radiometric age dating of small rocks, is over 8,000 m thick. These rocks are granitic plutons in the western part of the area isoclinally folded and are cut by large thrust provide evidence that the Cherskiy geo- faults dipping east. The western edge of this syncline was uplifted and folded during the major fault zone marks the boundary of the Carboniferous. Paleozoic geosyncline in the Cherskiy Moun- In the Permian, another cycle of sedimenta- tains with the lower Mesozoic rocks, mainly tion started, and thick Triassic and Early Triassic and Jurassic, of the| Verkhoyansk geo- Jurassic deposits were laid down in the Verk- synclinal complex (Fig. 4; Yanshin, 1966). hoyansk area (Fig. 5). In the Late Jurassic and A zone of high-pressure metamorphism has Early Cretaceous, this last period of geo- developed along these faults as well as wide synclinal sedimentation closed with a climactic

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tervening oceanic plate slid beneath them along subduction (Benioff) zones." On the other hand, most interpretations of the Urals do not invoke long-distance tectonic transport, but instead postulate vertically moving blocks along deep-seated faults (Yanshin, 1966). The general plan of vertical tectonics has also been applied by many Soviet geologists to Northeast USSR. Thus, the Kolymski and Okhotski massifs, besides several other outliers of Paleozoic and Precambrian rocks in more easterly parts of the region (including the block of crystalline rocks in Chukotsk Penin- sula), are considered to be remnants of an originally much larger Precambrian platform (Yanshin, 1966, p. 178) or two or three protoplatforms (Mokshantsev and Gusev, 1968, p. 74). This platform, according to many Shelf and land areas Oceanic areas with geosyncltnes authors, was broken up by deeply subsiding Successor basins B-Brooks troughs that were filled by Paleozoic and S-Sv«rdrup younger deposits, some of which reach geo- Figure 5. Late Paleozoic paleogeography. synclinal proportions. These troughs in later orogeny involving deformation, thrust faulting, Mesozoic time were, in turn, deformed into and plutonic activity. foldbelts. According to this theory of vertical In general, the Cherskiy foldbelt had a his- tectonics, the geosynclinal complexes of the tory of development that is closely parallel to Mesozoic Verkhoyansk and the Paleozoic that of the major foldbelts rimming the North Cherskiy foldbelts are floored by a down- American craton (King, 1969) : the stratigraphy dropped Precambrian crystalline basement. of the Cherskiy foldbelt resembles in consider- Unfortunately, there is little direct information able detail that of the Cordilleran foldbelt. on the basement in the critical areas with In particular, the east to west transition of shelf which to resolve this question. There is, how- carbonate rocks into thicker carbonate strata of ever, in the region, ample evidence of com- miogeosynclinal proportions, and farther west pression that requires considerable crustal into considerably thinner graptolitic shale shortening. The big question, it seems to me, strata with minor submarine basalt flows (A. V. is how much lateral transport has occurred in Kanygin, 1971, oral commun.) resembles the closing the Paleozoic and Mesozoic seaways facies changes in the Nevada- Idaho parts of the that successively separated the Siberian plat- Cordilleran foldbelt. The Cherskiy foldbelt form and the Kolymski massif? Estimates of differs from the foldbelts rimming North sea floor spreading in the northern Atlantic America in that, like the Ural foldbelt, it lies indicate a rate of about 1 cm/yr (Vine, 1966) wholly within a continental landmass: it sepa- and different amounts of Atlantic opening rates the Siberian platform or craton on the since the Triassic (Dietz and Holden, 1970). west from the Kolymski massif on the east These estimates and reconstructions indicate . that over 1,000 km of total separation had al- Much farther west, the more intensively ready occurred on the Atlantic side of North studied Ural Mountain region has a geologic America before the Early Cretaceous, the time history somewhat parallel to that of Yakutia. of climactic orogeny in Yakutia. Separation of In the Urals, a major fault system, with blue- this magnitude should be somehow expressed schists and ultramafic rocks developed along it, in a reduction of the distance between the has been interpreted as a Paleozoic plate Pacific sides of North America and Eurasia, boundary separating the Russian and Siberian assuming the sides were once separated platforms (Hamilton, 1970). According to (Churkin, 1970a). If global movements of this Hamilton, "the geology of the Ural region magnitude took place, then the closure of a accords with the concept that the two sub- major seaway in the Yakutia region between continents approached and collided as the in- the leading edges of the approaching Eurasian

and North American continental plates could Sibirsk. Otdeleni Severo-Vostoch. Kompleks. have culminated in continental collision by Nauchno-Issled. Inst., Trudy, Vyp. 29, 175 p. Early Cretaceous time. Continued spreading Blackett, P.M.S., Bullard, E., and Runcorn, S. K. in the Atlantic, especially during the Late (organizers), 1965, A symposium on conti- Cretaceous and early Tertiary after the North nental drift: Royal Soc. London Philos. Trans., American and Eurasian plates were sutured, ser. A, v. 258, no. 1088, 323 p. Bogdanov, N. A., 1963, Tektonicheskoe razvitye v produced internal deformation resulting in Paleozoe Kolymskovo Massiva i vostochnoi sharp bends in the structures of Alaska and Arktiki [Tectonic development of the Kolyma Northeast USSR (Churkin, 1970a). massif and eastern Arctic during the Paleozoic]: Akad. Nauk SSSR, Geol. Inst., Trudy, v. 99, CONCLUSIONS 237 p. Bullard, Edward, Everett, J. E., and Smith, A. G., Geologic correlations between Alaska and 1965, The fit of the continents around the easternmost Siberia indicate that these two Atlantic, in A symposium on continental continental areas have been connected since the drift: Royal Soc. London Philos. Trans., ser. Paleozoic and probably since the Precambrian, A, v. 258, no. 1088, p. 41-51. and that no plate boundary passes through the Churkin, Michael, Jr., 1969, Paleozoic tectonic Bering Sea region. history of the Arctic Basin north of Alaska: Manifestations of sea floor spreading tec- Science, v. 165, p. 549-555. tonics on the submarine Gakkel Ridge of the 1970a, Fold belts of Alaska and Siberia and drift between North America and Asia, in Arctic Ocean continue landward along the east Adkison, W. L., and Brosge, M. M., eds., side of the Cherskiy Mountain system, where Proceedings of the geological seminar on the they include seismic activity, faulting, and North Slope of Alaska: Am. Assoc. Petroleum volcanism. Geologists (Pacific Sec.), p. G1-G17. Earlier compressional features of Paleozoic 1970b, Paleozoic and Precambrian rocks of and, especially, Mesozoic age on trend with the Alaska and their role in its structural evolution: Arctic Ocean opening, contrast strongly with U.S. Geol. Survey open-file rept., 131 p. these modern tensional movements. Strati- 1971 Geologic concepts of Arctic Ocean basin graphic, structural, and petrological features [abs.] in 2d Internal, symp. arctic geology, San suggest that a fossil plate boundary separates Francisco, 1971: Am. Assoc. Petroleum Geologists Program, p. 12. the Paleozoic Cherskiy foldbelt on the east Demenitskaya, R. M., and Karasik, A. M., 1969, from the Mesozoic Verkhoyansk foldbelt on The active rift system of the Arctic Ocean: the west. Tectonophysics, v. 8, p. 345-351. Dietz, R. S., and Holden, J. C., 1970, Reconstruc- ACKNOWLEDGMENTS tion of Pangaea; breakup and dispersion of continents, Permian to present: Jour. Geophys. The critical comments of E. H. Bailey, P. B. Research, v. 75, p. 4939-4956. King, E. H. Lathram, and D. W. Scholl on an Dobretsov, N. L., Reverdatto, V. V., Sobolev, early draft of this paper and discussions with V. S., Sobolev, N. V., Ushakova, E. N., and Soviet geologists, notably A. M. Karasik, A. F. Khlestov, V. V., 1966, Karta metamorfi- Grachev, S. M. Tilman, A. L. Yanshin, N. A. cheskikh Fatziy SSSR [Metamorphic facies Bogdanov, and A. V. Kanygin, are much ap- map of the USSR]: Ministerstvo Geologii, preciated. scale 1:7,500,000. Egiazarov, B. Kh., 1969, Geologicheskoe stroenie Alyaski i Aleutskikh Ostravov [Geologic struc- REFERENCES CITED ture of Alaska and the Aleutian Islands]: Belyaev, I. V., 1970, Osnovnye geologicheskie Nauchno-Issled. Inst. Geologii Arktiki, Trudy, rezul'taty regional'nykh geofizicheskikh rabot Leningrad, Nedra, 264 p. [Major geological results of regional geo- Firsov, L. V., and Dobretsov, N. L., 1969, Age of physics], in Sidorenko, A. V., chief ed. glaucophane metamorphism at the northwest- Geologiya SSSR, Tom 30, Severo-Vostok ern fringe of the Pacific Ocean [AGI transl.]: SSSR, Geologicheskoe opisanie, Kmga 2, Mos- Akad. Nauk SSSR Doklady, v. 185, p. 883- cow, Ministry of Geology, Nedra, p. 236-246. 886. Belyi, V. F., 1969, Vulkanicheskie formatzii i Grantz, A. W., Wolf, S. C., Breslau, L., Johnson, stratigrafiya severnoy chasti Okhotsko-Chu- T. C., and Hanna, W. F., 1970, Reconnaissance kotskogo poyaca [Volcanic rocks and the geology of Chukchi Sea as determined by stratigraphy of the northern part of the acoustic and magnetic profiling, in Adkison, Okhotsk-Chukotsk belt]: Akad. Nauk SSSR W. L., and Brosge, M. M., eds., Proceedings of

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