Mesozoic tectonic evolution of Northeastern Asia

291

Tectonophysics, 127 (1986) 291-304 Elsevier Science Publishers

MESOZOIC

B.V.. Amsterdam

TECTONIC

L.M. PARFENOV

- Printed

EVOLUTION

1 and B.A. NATAL’IN

in The Netherlands

OF NORTHEASTERN

ASIA

2

’ Geological Institute, Siberian Branch Academy of Sciences, 39 Lenin Au., Yakutsk (U.S.S.R.) 2 Institute of Tectonics and Geophysics, Academy of Sciences, 65 Kim-Yu-Chen, Khabarovsk (U.S.S.R.) (Received

April 15, 1985; accepted

June 20, 1985)

ABSTRACT Parfenov,

L.M. and Natal’in,

Zonenshain

(Editor),

The tectonic terranes

evolution

Mesozoic

continent

the following

(Siberian

platform)

well as the inferred Paleozoic

oceanic

continent

are delineated

crust

arcs, respectively, mainly

passive

ophiolites continent

that

by basins

continental

these

with

through

masses

Siberian margin

occurred

Siberian

the Okhotomorsk

Ocean

Mesozoic

of the Eastern

terrane

folding

resulted

continent

and beginning

in shape

belts.

from

the Eastern

from

the convergence

of

Siberian and

of the Late Jurassic.

to its modem

Cretaceous

the Okhotsk-Chukotsk

had

the presence

microcontinental

margins

movement

island

terranes

The

terrane

time. By the end of Neocomian

of the Bureinsk

to the northward

Siberian

and Oloysk and

as the

with the Bureinsk-Khankaisk

of the Chukotsk

similar

volcano-plutonic

towards

separated

terranes,

of the Eastern

terranes

In

Siberian

arc overlying

The microcontinental

active continental

after the attachment

and are related

microcontinental

were

Siberian

Asia had become by Andean-type

volcanic

The margins margin.

in Late Neocomian

Sikhote-Alinsk developed

continent

terranes

the Eastern

by the Udsko-Murgal’sk

at the end of the Middle

occurred

of northeastern and

Early

In: L.P.

of tectonostratigraphic

microcontinental

of the tectonostratigraphic

ocean basin and the attachment

continent

from the Pacific

of Sikhote-Alin

crust.

Asia.

(active and passive) affinities.

The Alazeya

as a microcontinent.

microcontinental

oceanic

recognition

margin

can be recognized:

terrane.

and in the north

zonations

of Northeastern

and Bureinsk-Khankaisk

microcontinental

The collision

terrane

Okhotsk-Chukotsk Eastern

Lateral

of the South-Anyui

separated

continental

is also regarded

of sialic blocks.

the Eastern

large

in the southeast

margins.

microcontinental closing

Asia is outlined

and in the south by the active continental

indicate

collision

evolution

of island arc and continental

and the Chukotsk

Okhotomorsk

tectonic

Fold Belts. Tectonophysics, 127: 291-304.

of the Eurasian

of Northeastern

and their lateral zonations

the Early

faults

B.A., 1986. Mesozoic

Tectonics

to

time, the

margins.

It was

which are marked

by the

sublongitudinal

microcontinental

strike-slip terrane

of the Kula plate together

to the with

belt in the Cretaceous.

INTRODUCTION

Mesozoic fold belts of two ages are recognized in Northeast Asia (Fig. 1). The Verkhoyano-Chukotsk fold belt lying northeast of the Siberian platform and the Mongolo-Okbotsk fold belt located southeast of the Siberian platform are generally regarded as Early Mesozoic in age. Oceanward, to the east, they pass into the Late

0040-1951/86/$03.50

0 1986 Elsevier Science Publishers

B.V.

292

Fig. 1. The fold belts of Northeast terranes

(median

massifs):

Okm -0khotomorsk; Brooks-Wrangel 7 = Cenozoic

Asia and Alaska.

Ok -Okhotsk,

3 = Late Archean-Early fold belt;

fold belts,

1 = Siberian

Proterozoic

5 = Late Jurassic-Neocomian

including

island

platform;

Om -Omolonsk,

arcs and

adjacent

Stanovoy

fold belt;

fold belts; ocean

2 = microcontinental

6 = latest floor;

sialic

Kh -Khankaisk,

Bu -Bureinsk,

4 = Middle Cretaceous

8 = trenches;

Proterozoic fold belts;

9 = structural

trends.

Mesozoic Koryaksk the Sea of Okhotsk. of Sakhalin

and Sikhote-Alinsk fold belts, apparently joined in the north of Further to the east they are truncated by the Cenozoic fold belts

and Kamchatka.

The modem

Kamchatka-Kuril

island

arc is traceable

along the Pacific boundary. The Mesozoic fold belts consist of tectonostratigraphic terranes of island-arc and continental margin (convergent and passive) affinity. These terranes provide information about fossil convergent boundaries of Late Precambrian, Paleozoic and Mesozoic age (Zonenshain et al., 1976; Natapov et al., 1977; Mazarovich, 1982; Parfenov et al., 1979; Parfenov, 1984; Natal’in, 1984) and outline the tectonic evolution of all of Northeast Asia. The pre-Mesozoic tectonic development of the region, however, is rather poorly understood because most of the knowledge of Precambrian and Paleozoic continental margins is derived from isolated fragments within Mesozoic fold belts. For this reason, the pre-Mesozoic tectonic evolution of this area is not discussed in this report.

293

The structure of the region is characterized by a mosaic of mainly Early Precambrian sialic terranes (“median massifs” in the terminology of many Russian geologists) which collided with each other and with the larger continental mass of the Siberian platform during the Mesozoic. These sialic terranes include the Okhotsk, Omolonsk, Bureinsk and Khankaisk median massifs which are as much as several hundred kilometers across. The sialic terranes are characterized by an Early Precambrian basement overlain by a variety of younger formations of variable thickness. The terranes are separated from one another by fold belts which are interpreted as the result of the interaction of island arcs and continental margins with the Kula plate, as it moved northward with respect to Eurasia and North America in the Jurassic and Cretaceous. EARLY MESOZOIC TERRANES

In the early Mesozoic several continental masses can be recognized. These are: the Eastern Siberian continent (Siberian platform); the Chukotsk and BureinskKhankaisk microcontinental terranes characterized by an Early Precambrian basement of metamorphic rocks; the Alazeya volcanic arc overlying the Paleozoic oceanic crust; and also the inferred Okhotomorsk microcontinental terrane (Fig. 2). The Eastern Siberian continent and microcontinental terranes were separated by basins underlain by oceanic crust which occurs as ophiolites within the accretionary wedge complexes of erogenic units of Mesozoic age. The margin of the Eastern Siberian continent is delineated on the southeast by the Udsko-Murgal’sk arc. The volcanic arc is composed of volcanogenic and sedimentary sequences of the Upper Permian to Hauterivian inclusion (3-7 km). Fore-arc basin complexes are largely made of Carboniferous-Neocomian sedimentary rocks which are exposed along the southeast banks of the Penzhinsk and Markovsk basins in Koryakia. Accretionary wedge complexes of the arc are within the imbricated thrust structure of the Talovski-Mainsk zone containing Precambrian metabasites, Early to Middle Paleozoic and Late Jurassic-Neocomian ophiolites and lawsonite- and glaucophane-bearing schists (Fig. 3). Back-arc basin complexes are represented by thick (10 km) marine graywacke and shale sequences of Late Permian, Triassic and Jurassic age. In the south, the margin of the Eastern Siberian continent is delineated by the Stanovoy plutonic belt located on the southwest extension of the Udsko-Murgal’sk volcanic arc. The belt is composed of multiphase massifs, made of gabbro, diorites and late granodiorites, granites and granosyenites. Their K-Ar age determinations range from 70 to 140 m.y., even reaching 200 m.y. Intrusive magmatism was accompanied by volcanic activity, the relics of which have not been preserved. Effusive rock fragments are found in Jurassic coal-bearing deposits in the south of the Aldan Shield. To the south of the plutonic belt, within the Mongolo-Okhotsk fold system, thick Upper Triassic to Middle Jurassic turbidite series interpreted as

294

Fig. 2. Late Triassic-Middle terranes; belts:

2 = oceanic crust;

7 = granitoid

batholitic

If = position

of subduction

intermontane

troughs

f -4-microcontinents: -volcanic

arcs:

belt of granodioritic

Jurassic

paleotectonic

3 = transform

faults;

belts: 8 = back-arc zones;

and foredeeps;

12 = passive

basins;

6-Alazeya,

I = continents 5 = volcanic

9 = fore-arc

continental

15 = epicollisional

I -Bureinsk-Khankaisk, 5-Udsko-Murgaf’sk,

reconstruction. 4 = rift zones;

basins;

margins;

T-Kankaren,

10 = accretionary

13 = Cretaceous

volcano-plutonic

2-Okhotomorsk,

and microcontinental

arcs; 6 = volcano-plutonic

belts;

J-Omolonsk,

wedges;

fold belts;

I-Chukotsk;

B-Yukon-Koyukuk;

14 =

Encircled:

16 = thrusts.

5- 8

9 ---Stanov

batholiths.

fore-arc basin complexes are common. They are superimposed on dislocated zoic and, possibly, Late Precambrian volcanogenic chert and gra~a~ke-shale as well as gabbroids and ultrabasites. The northeast margin of the Early Mesozoic Eastern Siberian continent

Paleoseries estab-

lished within the Verkhoyansk Range is passive and is made of CarboniferousJurassic terrigenous deposits of the Ver~oyansk complex. LithoIogicaI-facial zonation of the Verkhoyansk complex is represented by the replacement of continental and coastal marine deposits, characteristic of the eastern margin of the Siberian platform, by wide shelf accumulations and further on by thick argillo-siltstone sequences with graded bedding and slump horizons (Bulgakova et al., 1976; Dagis et al., 1979) which formed on the continental slope and rise. The southern margin of the Chukotsk microcontinental terrane is passive, similar to the northeastern margin of the Eastern Siberian continent.

295

In the Middle Paleozoic-Early Mesozoic the northern margin of the BureinskKhankaisk ~cr~ontinent~ terrane was passive. The eastern border of the microcontinental terrane developed as a convergent boundary in Paleozoic time. Late Paleozoic granitoids compose the bulk of the northern part of the microcontinental terrane and a volcano-plutonic belt characteristic of Andean-type margins extends along its southern part. No significant manifestations of granitoid or talc-alkalic volcanic activity of Early Mesozoic age have been established for the east of the Bureinsk-Khankaisk microcontinental terrane. From this we infer that its margin was a passive one. At the same time, in the Sikhote-Alinsk system there were widespread tectonostratigraphic terranes, typical of accretionary wedges of Early Mesozoic age, i.e. imb~~ated thrusts responsible for the alternation of turbidites, deep-sea sediments, and basic volcanics, as well as pre-Aptian multistage fold structures and flow melange. In the area where earlier the Kolymsky median massif was traditionally shown, the Alazeya island arc is outlined. Within the Alazeya uplift a Carboniferous-Jurassic volcanogenic and sedimentary complex of the volcanic arc is recognized showing complexly alternating intermediate, basic and acid tuffs, volcaniclastic sandstones, gritstones and conglomerates (Bulgakova, 1982). The Momsko-Polousnensk uplift system located on the periphery of the Kolymsky block separates the volcanic arc from the deep basins to the northwest and southwest and is interpreted to be an accretionary wedge. Besides typical Upper Paleozoic-Early Mesozoic terrigenous chert deposits of small thickness, the accretionary wedge includes allochthonous Late Precambrian, Early and Middle Paleozoic terrigenous carbonate sequences. The latter were separated from the continental margin to the west by the Devonian rifting. The basins (In’yali-Debin and Polousnensk synclinoria) are filled with thick Triassic and Jurassic olistostrome-bearing terrigenous flysch series. In the total lateral zonation the basins correspond to the fore-arc basins. In the modern structure of the region the boundary between the Alazeya arc and the passive margin of the Eastern Siberian continent coincides with the Adycha-Taryn high-angled fault zone. We suppose here the presence of basins underlain by oceanic crust in Late Paleozoic and Early Mesozoic time. This oceanic crust is probably represented by those Paleozoic ophiolites recently found in some areas of the Momsko-Polousnensk uplift system (Arkhipov and Volkodav, 1983). Northeast of the Alazeya arc, in the Aluchin-Oloy interfluve, there are Upper Triassic-Middle Jurassic volcanogenic and sedimentary sequences similar to the volcanic island arc complexes. The near-oceanic elements of the arc have practically disappeared in the modem structure of the region. We suppose them to occur in the eugeosynclinal sequences of the South-Anyisk fold system. LATE JURASSIC-NEOCOMIAN

ACCRETION

Early Mesozoic folding resulted from the convergence and collision of continents with microcontinental terranes. At the end of the Middle and beginning of the Late

296

0

297

Fig. 4. Late Jurassic-Neocomian paleotectonic reconstruction. Symbols are the same as for Fig. 2. l-Uyandinsko-Yasachnensk volcanic belt; t-Kolymo-Omolonsk massif: 3-Umlekano-Ogodzhinsk volcanic belt.

the Alazeya island arc collided with the passive margin of the Eastern Siberian continent (Fig. 4). This initiated the folding process producing the YanoKolymsk fold system. The earliest structural forms were low-angle thrusts along the

Jurassic

Fig. 3. Generalized geological sections across the T~ovsko-Mainsk accretionary wedge on the west of the Koryaksk upland, The accretionary wedge associated with the Udsko-Murgal’sk arc was not significantly modified by later deformational events. a. Pekul’nei Range (after Nekrasev, 1978). b. Ust-Belsk mountains (after Aleksandrov, 1978). c. Talovsk mountains (after Alekseev, 1981). I = ultramafic rock, 2 = gabbroic rock; 3 = serpentinite melange; 4 = gabbro and anorthosites; 5 = amp~bole-pyrox~e schists; 6 = schist; 7 = chert and basalt, 8 = chert, tuff, shale and sandstone; 9 = sandstone; 10 = siltstone and argillite; II = sheeted dikes; I2 = granite; I3 = silicic volcanic rocks; I4 = intermediate and mafic volcanic rocks; 2.5= faults. N - Neogene; N, -Miocene; P, - Oligocene; K, -Upper Cretaceous; Q -Quaternary; K,t -Turonian; K, - Lower Cretaceous; I(, Al- Albian; K,ap -Aptian; K,b -Barremian; K,h-Hauterivian; ~,~-V~an~~an; &-Upper Jurassic; Tsn-Norian.

298

southwestern limb of the In’yali-Debin synclinorium (Arkhipov et al., 1981). Later folding progressively migrated towards the Siberian platform and ended on the eastern margin of the Preverkhoyansk foredeep as late as the Late Cretaceous. Collision of the Eastern Siberian continent with the Alazeya arc resulted in the formation of the Late Jurassic Uy~dinsko-Yasachnensk volcanic belt and Late Jurassic-Neocomian batholitic belts of granites. The volcanic belt unconformably overlies the accretionary wedge complexes of various types and ages. The batholitic belts are somewhat displaced towards the adjacent basins with respect to the former one. The Late Jurassic was marked by the collision of the Bureinsk-~ankaisk microcontinental terrane with the Eastern Siberian continent, producing a complex nappe structure of the Mongolo-Okhotsk fold system. Structural analysis of the folds suggests that thrusting of the active continental margin elements under the Bureinsk-~ankaisk ~crocontinental terrane was related to the early collision stages. It is only later (in the Neocomian) that the Eastern-Siberian continent began thrusting over the Bureinsk-Khankaisk microcontinental terrane. Increasing thickness of the Upper Jurassic continental coal-bearing deposits in the south of the Aldan Shield relative to the Lower and Middle Jurassic ones indicates greater elevation amplitude in the Stanovoy arc in Late Jurassic time. The end of the Neocomian here saw the development of thrusts (with a northward displacement of up to 15 km) along which Archean deposits overlie Mesozoic ones. The collision of the Eastern Siberian continent with the Bureinsk-~ankaisk microcontinental terrane resulted in the formation of the Early Cretaceous Umlekano-Ogodzhinsk volcanic belt superimposed on the northern margin of the Bureinsk massif. Continued northward migration of the Kula plate after the coilision of the Eastern Siberian continent with the Bureinsk-~ankaisk microcontinental terrane, produced echelon-like left-lateral strike-slip faults along the eastern margin of the terrane. The collision of the Chukotsk microcontinental terrane with the Eastern Siberian continent occurred in Late Neocomian time and closed the South Anyui oceanic basin. The fold systems formed are characterized by a uniquely complicated structure, isoclinal folds, regionally developed schistosity, transportation structures, and thrusts (Natal’in, 1984). POST-NEOCOMIAN

RECONSTRUCTION

As a result of the Alazeya arc, Bureinsk-Khankaisk and Chukotsk microcontinental terranes being attached to the Eastern Siberian continent, by the end of the Neocomian the continental margins of northeastern Asia had become similar to its modern outlines (Fig. 5). The northern margin of the region under consideration is delineated by the Okhotsko-Chukotsk volcano-plutonic belt marking an Andean-

299

Fig. 5. Paleotectonic r~ons~ction 1 --Siote-Alinslc

volcanic arc;

foredeep; 4 -Zyryansk

of Apti~-~norn~~ Z-Okhotsk-Chukotsk

age. !Symbols are the same as for Fig. 2. volcano-plutonic

belt;

3--Preverkhoyansk

basin.

type continental margin. The southwestern flank of the belt was initiated by the beginning of the Neocomian, the northeastern one by the beginning of the Albian. In the Penzhinsk basin adjacent to the northeastern flank of the belt, the contemporaneous formations are represented by thick, gently dislocated marine, coastal marine and continental terrigenous sequences. Seismic evidence suggests the presence of a similar basin on the southwestern continuation of the Penzhin basin within the northern part of the Sea of Okhotsk. These are fore-arc basins with the Penzhin basin being superimposed on a similar basin of the Udsko-Murgalsk arc. The imbricated thrust structures of the Koryaksk upland correspond to the accretionary wedge. To the south, the Asian continent is truncated by Cretaceous island arcs recognized in Sikhote-Alin and on Sakhalin island. The volcanic arc is discerned by the exposures of the B~e~~-Turo~~ dislocated coastal marine and continental volcanogenic and sedimentary sequences from beneath the gently sloping volcanics of the Eastern Sikhote-Alinsk volcano-plutonic belt. The back-arc basin complexes made of thick terrigenous sequences make up most of Sikhote-Alin. The fore-arc basin can be traced on Western Sakhalin. Here turbidites are widespread in the lower part of the Upper Cretaceous graywacke shale sequences of the Western

Fig. 6. Paleotectonic

reconstruction

I -Eastern

Sikhote-Alinsk

3 -Bowers

arc; 4 -Aleutian

of Senonian-Paleogene

volcano-plutonic

belt;

age. Symbols

are the same as for Fig. 2.

2 -Penzhinsk-West

Kamchatka

volcanic

belt;

arc.

Sakhalin basin. Sedimentary rocks of the basin conformably overlie Albian volcanics and cherts, which implies the initiation of the fore-arc basin upon oceanic crust. In the Eastern Sakhalin mountains there are abundant argillite-chert and spilite-carbonate-chert rock assemblages, as well as eclogites, glaucophane-bearing schists, serpentinite melange, basic and ultrabasic rocks. Large thrusts are common too. Here an accretionary wedge of the island arc can be discerned. In the Senonian, the considered island-arc system was transformed into an Andean-type active continental margin (Fig. 6). Just like in the north of the Asian continent, continental

the transformation was preceded by a folding event that affected adjacent areas. The active margin included the Eastern Sikhote-Alinsk

marginal-continental volcano-plutonic belt, fore-arc basin and accretionary wedge. The Eastern Sikhote-Alinsk belt is composed of Senonian-Paleogenic volcanic and igneous complexes similar to those of the Okhotsk-Chukotsk belt. Petrochemical data

suggest

that

the dip

angle

of the associated

Benioff

zone

is about

20”

(Zonenshain et al., 1976). The fore-arc basin corresponds to a similar basin of the former island arc. The structure of rock sequences within the basin indicates its successive filling and progressing shallowing. The accretionary wedge embraces Eastern Sakhalin. The development of the island arc and later of the active continental margin was accompanied by the formation of large left-lateral strike-slip faults in the back-arc areas, within Sikhote-Alin. Displacements along the faults are estimated to be between several tens of kilometers and 150-200 km (Utkin, 1980). These strike-slip faults displaced the earlier ones confined to the eastern margin of the Bureinsk-Khankaisk microcontinental terrane. They also were related to the Kula plate. The Kula plate moved at an acute angle to the island arc trend and later to

Fig. 7. Paleotectonic reconstruction of Oligocene-Miocene age. Symbols are the same as for Fig. 2. I -Aleutian arc; 2 -Central Kamchatka arc; 3 -Kurd arc; 4 -rift-like basin system in Siiote-Alin.

that of an Andean-type margin. This back accounts for the poor development of the Late Mesozoic island arc, lesser width and smaller amounts of magmatic formations within the Eastern Sikhote-Alinsk volcano-plutonic belt relative to the Okhotsk-Chukotsk one. The collision of the Okhotomorsk microcontinental terrane with the Eastern Siberian continent by the end of the Cretaceous resulted in the wedging of the Benioff zone associated with the Okhotsk-Chukotsk belt, as well as in the cessation of magmatic activity. This caused a substantial transformation of the active continental margins and island arcs in Northeast Asia. By the end of the Cretaceous and the beginning of the Paleogene a new Benioff zone had developed in places displaced considerably to the east of this position. The margin of the Asian continent within the area under consideration is similar to an Andean-type one. Its position is delineated by the volcano-plutonic belts. In the Oligocene and Miocene the continental margin was displaced further to the east oceanward (Fig. 7). At that time the Kuril island arc which extends in the north along the central areas of Kamchatka was initiated. The development of the rift-like basin system in Sikhote-Alin accompanied by the Miocene and Paleocene-Early Quaternary alkaline-basaltic volcanism was closely related to the formation of the Sea of Japan. CONCLUSIVE REMARKS

When discussing the problem of the Mesozoic tectonic evolution of northeastern Asia, it is very important to estimate the amounts of horizontal displacement of

302

blocks and microcontinental To solve the problem

terranes

most part we lack at the moment. only a provisional paleontological

width data

estimations. The South-Anyui

with respect

to the Eastern

we would need first of all paleomagnetic

are

Therefore

of the basins quite

valuable,

the paleotectonic

underlain but

by oceanic they

do

not

Siberian

continent.

data which for the maps above can give crust.

The available

provide

quantitative

fold system serves as a sharp paleobiogeographical

boundary

in

Northeast Asia. The system is no more than a few dozen kilometers wide. According to aeromagnetic evidence it is traceable up to Bolshoi Lyakhov Island in the northwest, while in the east it extends along the South Chukotka ophiolites up to Alaska (Voyevodin et al., 1978). The South-Anyui system marks the southern boundary of the Tethys fusulinid fauna of Carboniferous age typical of Chukotka and Wrangel Island (Solovieva, 1975; Ustritsky 1971). South and southwest of the boundary within the greater part of northeastern Asia up to the Mongolo-Okhotsk fold system inclusive, Late Paleozoic and Mesozoic deposits are characterized by typical boreal fauna assemblages. The Tethys fauna assemblages are found in Late Paleozoic deposits to the south of the Mongolo-Okhotsk fold system within Sikhote-Alin and the Bureinsk-Khankaisk blocks. The data given above indicate the presence of wide marine

ocean-like

basins

in

areas of the South Anyui and Mongolo-Okhotsk fold zones during the Late Paleozoic. Unfortunately, the Triassic fossils of Chukotka are poorly understood at the moment. Therefore it is rather difficult to establish relations between Chukotka and the rest of the Verkhoyano-Chukotsk region in the Early Mesozoic. According to A.S. Dagis (pers. commun., 1985) the Bureinsk massif and the Mongolo-Okhotsk fold system are characterized by the Triassic boreal fauna, while in Sikhote-Alin, Tethys fossils of the same age are predominantly abundant. Triassic Tethys organic remains have been found in recent years in the Kankaren Range, northeast Koryakia (Bychkov and Dagis, 1984). Other authors have ‘expressed ideas concerning the former positions of many tectonostratigraphic terranes within the Verkhoyano-Chukotsk region, particularly the Omolonsk, Okhotsk and other massifs in the central areas of the Pacific Ocean (Churkin and Trexler, 1980; Fujita and Newberry, 1982; Jones et al., 1982). In the case of the Omolonsk massif, paleomagnetic data by A.N. Khramov are often referred to. In this connection it should be noted that Late Paleozoic and Mesozoic deposits within the Siberian platform, Verkhoyanye, Novosibirsk, Okhotsk and Omolonsk massifs are characterized by similar typical boreal fauna assemblages. In interpreting their own data Khramov et al. (1982) discard a long-range drift of the determinations of the Omolonsk Omolonsk massif. They write: “Paleomagnetic massif suggest its counter-clockwise rotation during the Late Permian to Late Cretaceous with small amounts of horizontal displacement” (Khramov et al., 1982, p. 249). Marine basins underlain by oceanic crust, which we suppose were present to the southwest of the South-Anyuisk zone in Early Mesozoic time, were apparently

303

limited in size. The entire area was within the boreal paleobiogeographic which at the time was significantly less in size than at present.

province

ACKNOWLEDGMENTS

The problems touched upon in the paper have been discussed by the authors with many Soviet and overseas colleagues during the last ten years. Of special importance were exchanges of views with Dr. Yu.V. Arkhipov, Dr. M.M. Churkin, Dr. G.S. Gusev, Dr. V.A. Legler, Dr. M.S. Markov, Dr. L.M. Natapov, Dr. W. Patton, Prof. S.A. Ushakov, and Prof. L.P. Zonenshain. The authors are deeply indebted to all of them. Though not all share the authors’ opinions, some of the views revealed in the paper have resulted from these fruitful discussions. The authors would like to thank Mrs. E.V. Anuchina for taking the trouble to translate the paper into English and Miss I. Morova for typing it. REFERENCES

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