Mechanisms of formation of deep basins on continental crust in the Verkhoyansk fold belt; miogeosynclines and cratonic basins

Tectonophysics,

217

122 (1986) 211-245

Elsevier Science Publishers

MECHANISMS

B.V.. Amsterdam

OF

- Printed

FORMATION

OF DEEP

CRUST IN THE VERKHOYANSK CRATONIC BASINS

EUGENE

FOLD

’ and MICHAEL

V. ARTYUSHKOV

in The Netherlands

BASINS

BELT;

ON

CONTINENTAL

MIOGEOSYNCLINES

AND

A. BAER 2

I Institute of Physics of the Earth, Academy of Sciences, Moscow (U.S.S.R.) ’ Ministry

of Geology of the U.S.S.R.,

Moscow (U.S.S.R.)

(Received

April 18, 1984; revised version

accepted

July 3, 1985)

ABSTRACT

Artyushkov,

E.V. and Baer, M.A., 1986. Mechanisms

the Verkhoyansk

fold belt; miogeosynclines

The Verkhoyansk lo-15

fold belt is a wide, folded

km deep, was formed

Proterozoic-Middle significant (5-15

of continental

There are no deformations

stretching

or shortening

of deep basins on continental

basins.

area in Northeastern

there by a slow subsidence

Paleozoic.

lithospheric

of formation

and cratonic

Tectonophysics

Asia. A large sedimentary crust during

in the sedimentary

during

this slow subsidence.

suggested

as a mechanism

Several continental without Mesozoic

transformation

deep-water

cover that could A number

basins - l-5

stretching

under the convergent

belts. Among

deep basins

were

formed

upwelling

or thrust

loading.

plate motions.

on continental

layer may be suggested

of hydrous

anomalous

in the Verkhoyansk

Ma in the Late Paleozoic

in a thick cratonic

basins”.

A slow

lithosphere

can be

belt

The basins

formed

by

rapid

subsidence

The subsidence

were strongly

folded

of

occurred

in the Late

There were many basins of the same type in the other fold Rapid gabbro-eclogite

as the mechanism

mantle

fold

and Early Mesozoic.

crust only those produced

folded. They can be called “miogeosynclines”. of the basaltic

crust that occurred

indicate

of deep basins

for their formation.

crust during

significant

in the lowermost

basin,

- 1000 Ma in the Late

km) of the same type exist or existed in other areas. They can be called “cratonic

gabbro-eclogite

crust in

122: 217-245.

by rapid

subsidence

transformation

of their formation.

were intensely

with the destruction

This probably

occurred

under

to the base of the crust.

INTRODUCTION

The formation of deep basins on continental crust is usually explained by stretching (McKenzie, 1978; Le Pichon and Sibuet, 1981, and others). This mechanism produces large deformations in the upper brittle part of the crust, - 10 km thick, in rift valleys (Morton and Black, 1975; Wernicke and Burchfiel, 1982). The structure of the sedimentary cover has been studied in numerous deep-water basins on continental crust in the Urals, Appalachians, Scandinavian Caledonides, and in the Alpine fold belt (Artyushkov and Baer, 1983, 1984). In most basins no deforma-

0040-1951/86/$03.50

0 1986 Elsevier Science Publishers

B.V.

218

tions typical of significant

stretching

have been found.

produced

subsidence

with a duration

subsidence Many

by a very rapid was preceded

of these

uplifts

produced by rapid strongly compressed

by slight crustal were associated

subsidence without during the epochs

uplifts with

Deep basins commonly of a few million

of about

slight

several hundred

volcanism.

Most

were

years. Such metres.

deep

basins

significant stretching were, subsequently, of orogeny. No cratonic blocks were in-

tensely shortened in the above fold belts. In this paper we present a similar analysis

for the Phanerozoic

Verkhoyansk

fold

belt in Northeastern Asia. A comparison with the results that were obtained earlier for other fold belts is also presented. This reveals a global occurrence of two main types of deep basins on continental crust: those formed by a very rapid (- l-10 Ma) or a very slow ( - 10” Ma) subsidence, without significant stretching. REVEALING

OF THE PRESENCE

OR ABSENCE

OF SIGNIFICANT

STRETCHING

Let us describe briefly the method that was used in our previous papers to reveal the presence or absence of significant stretching. In order to produce a deep basin by stretching, the stretching factor fi should be sufficiently high. Then deep-water strata that were deposited after stretching must cover a considerably wider area than shallow-water strata that were deposited just before stretching (Fig. la). For example, stretching by p - 2 is necessary to form a water-loaded basin of an initial depth strata that h,, - 2 km. In such a basin, deep-water strata will rest on shallow-water were formed immediately before the subsidence over only - 50% of the region. In the other part of the basin, deep-water strata must overlie some other rocks: much older shallow-water strata or a deep crystalline basement. This can be revealed very easily if there are numerous sections or exposures of the base of the deep-water strata that were deposited on the underlying rocks. There are practically no broad structures in fold belts with deep-water strata that were deposited directly on the sialic basement after the subsidence. Commonly, an increase in the territory due to an intense stretching is compensated by the formation of tilted blocks in the uppermost crust (Fig. lb). In this case deep-water strata rest on the side BC of the tilted block. Some normal faults that bound tilted blocks can be transformed into thrust faults in the process of folding. Then block tilting during stretching can be revealed by a large angular unconformity 8 which exists between deep-water and shallow-water strata on the top of the block. The magnitude of 8 is - 15’-35” for the initial crust in water-depth h, - l-2.5 km that was typical of deep basins on continental fold belts. Large angular unconformities between deep-water and shaIlow-water strata are very rarely observed in fold belts: i.e., in not more than in a few per cent of the cases. Deep-water strata rest conformably on shallow-water strata, that were deposited just before the subsidence in almost all long sections and field exposures

219

(a)

Fig. 1. Relationships between the strata formed before and after intense stretching. a. The areas covered with shallow-water strata deposited before stretching and deep-water strata deposited after stretching. b. Tilted blocks bounded by normal faults and angular unconformity between shallow-water and deep-water strata.

(except the cases of detachment during folding). This stretching, independently of its mode. Large normal faults existed in most deep basins indicates crustal extension during the subsidence. The blocks (several tens of kilometres or more) where the

position

precludes

significant

on continental crust, which faults bounded broad crustal crustal surface was horizontal

or only slightly tilted. It is easy to calculate that the relative extension is not more than - 5% in such basins. This is much smaller than the extension necessary to form a deep basin by stretching. FORMATION

OF THE VERKHOYANO-KOLYMSK

Slow subsidence in the Lute Proterozoic-Middle

CRATONIC

BASIN

Paleozoic

The first rapid subsidence of a large magnitude in the Verkhoyansk fold belt occurred in the middle Early Carboniferous. A shallow-water basin, however, already existed there for 700-1300 Ma, from the Late Proterozoic-Middle Paleozoic (Kosygin, 1969; Pushcharovsky, 1960; Khain, 1979). This basin can be called “the Verkhoyano-Kolymsk Basin”. It covered an area - 1500 km long and 800-1000 km wide from the western margin of the Verkhoyansk Basin on the west up to the central part of the Kolymsk Massif on the east (Fig. 2).

Fig. 2. Main tectonic data

units of the Verkhoyansk

from Mokshantsev

et al., 1964; Geology

XXX, 1970b; Abramov, 1982).

I-3

= shales

Verkhoyansk originated

and

Mesozoic

turbidites

Basin that originated in the Early

subsidences

Triassic,

in the Early Triassic shallow-water

khoyano-Kolymsk 6 = Upper

deposits

Basin:

deposits

and Lower Proterozoic

Mesozoic sedimentary

shallow-water cover

of

molasse

1971; Chekhov.

of deep-water

1976; Gusev, 1979; Korostelev.

basins

3 = of the Inyali-Debin and between

the Triassic

of the Yano-Okhotsk of the Archean

Paleozoic

shallow-water

of the Lower crystalline

Paleozoic

formed Basin

Zone and

Massif,

by rapid

that

4 = Upper

(a) thin (b) thick, Lower

Proterozoic deposits

and Upper

Mesozoic

10 = volcanic

subsidence:

was formed

terrigeneous

rocks in the eastern

using the

I = of the

Foredeep, rocks

Basin that

by two subsequent Paleozoic 5-6

and Lower

cratonic

crystalline

7 = thin

and the outcrops Platform,

9 = Mesozoic-Cenozoic of the Okhotsko-Chuckotsk

Ver-

basement,

and carbonates,

part of the Siberian

Vol.

1982; Tilman,

2 = of the Yano-Kolymsk

and Jurassic,

of the Pre-Verkhoyansk

the Kolymsk

areas (compiled

Vol. XIX, 1966; Vol. XVII, 1970a;

in the Early Carboniferous,

5 = outcrops

Proterozoic-Middle

cover of shallow-water Archean

1970; Merzlyakov.

fold belt and the surrounding of the U.S.S.R.,

of the

8 = Upper volcanicBelt.

ii = lines of geological sections.

Slow subsidence started in the western part of Proterozoic. The sedimentary cover is underlain by terozoic crystalline basement (Konstantinovsky, 1969; XVII, 1970a; Semikhatov, 1974). The subsidence in

this basin in the early Late the Archean and Lower ProGeology of the U.S.S.R., Vol. the eastern part of the basin

221

followed

in the late Late Proterozoic

cover overlies the Late Proterozoic The rate of deposition Proterozoic,

and it reached

of the sediments

(Gusev,

1979; Khain,

folded basement

of shallow-water - 70 m/Ma

that were deposited

sediments

1979). The sedimentary

in this region. was

- 10 m/Ma

in the Early Paleozoic.

during

the slow subsidence

in the Late

The total thickness varies over the area,

reaching - 15 km in the deepest part of the basin. In many places the sedimentary complex of the basin is exposed in continuous sections for a distance of up to several tens of kilometres (Fig. 3) (Gusev, 1979). The sections reveal consedimentary normal faults that separate the regions with different thicknesses of the deposits. They were later transformed into thrust faults. The distance between normal faults was 20-50 km (Konstantinovsky, 1969). The displacements along normal faults were l-2 km and in one case they reached 5 km. This gives a relative extension /3 - 1 5 5% associated with normal faulting. The minimum magnitude of relative extension that is necessary to produce sedimentary basin of the depth h, after the crust and mantle cooling is: (P, - p,)h,

p-1=

a

(1)

&-0r:-(Pm-Ps)hs where ps is the density of the sediments, thickness before stretching and p,,,--the lithosphere. Taking p, = 2.55 g/cm3, h, = hz = 40 km, pm = 3.35 g/cm3, we find for

p,-the crustal density, hz--the crustal density of the mantle in the subcrustal lo-15 km, together with p, = 2.85 g/cm3, the Verkhoyano-Kolymsk Basin:

p-1=0.7-1.5.

(2) This quantity is much larger than the above-mentioned crustal extension of 5 5% that was associated with normal faulting in the basin. In order to compensate for stretching by a factor of /3 - 1.7-2.5, the formation of narrow blocks tilted by 8 - 20”-30” is necessary. Gradual tilting during slow subsidence would have produced angular unconformities of this magnitude between the top and the base of the sedimentary complex and strong disruptions of the oldest strata. However,

all sedimentary

strata are continuous

between

large normal

faults in

the Verkhoyano-Kolymsk Basin, and they are parallel in the regions that have been not too strongly deformed by the subsequent folding (Fig. 3). This indicates no significant stretching during the formation of the basin. Slow crustal subsidence in the Verkhoyano-Kolymsk Basin was interrupted by a number of uplifts associated with short periods of erosion. The main hiatuses can be related to the global epochs of orogenythe Elsonian, Grenvillian, Baikalian, Salairian, and Late Caledonian (Acadian). Pronounced uplifts occurred in the Late Proterozoic, just before the Vendian and in the early Middle Devonian (Gusev, 1979; Korostelev, 1982). The latter uplift was associated with a regional hiatus, basic volcanism, and intrusion of ultrabasic and alkaline plutons. No significant crustal shortening occurred in the basin during the uplifts; however, intense folding occurred several times in the adjacent region of the Kolymsk Massif and east of it.

222

km

2 1 0

1

SE

NW

f

01

2345km

(b) Fig. 3. Geological The sections

sections across the southern

demonstrate

Verkhoyano-Kolymsk can be observed a. Schematic Platform 2 = upper and

Lower

across

synclinorium.

of the Late Proterozoic-Middle

basin. The strata are approximately

the Yudom~Maysk

1979) (line AA’ Proterozoic

deep-water sections

shales, across

volcanics,

Paleozoic

parallel.

Their continuity

and the southeastern

3-6

= Upper

Paleozoic

Proterozoic

shallow-water

Proterozoic

margin

of the Siberian

shallow-water argillites

and

crystalline terrigeneous carbonates,

basement, deposits 8 = Upper

9 = faults. the Sette-Daban

carbonates,

12 = basal&

Basin

in Fig. 2). I = Archean-Lower

7 = Vendian-Middle

f - 7 = shallow-water sandstones,

sedimentary

sequence

for S-10 km.

carbonates,

Paleozoic

cratonic

section

(Khain,

b. Geological

and western parts of the South-Verkhoyansk

a very thick sedimentary

Horst

8- 9 = shallow-water

Anticlinotium

(Gusev,

argillites

and carbonates,

1979) (line BB’ in Fig. 2). 10 = quartz&es,

II =

13 = faults.

Mechanism of slow subsidence

that occurred in the The low intensity of the vertical tectonic movements (up to the Middle Verkhoyano-Kolymsk Basin and the absence of volcanism Devonian) are typical of cratonic areas. The lithospheric thickness is supposed to be

223

high,

d 2 100 km, in such areas (McKenzie,

basin

with respect

to crustal

compression

1978, and others). also indicates

The stability

of the

that it was underlain

by a

thick cratonic lithosphere. The most common term for basins of this type is “a cratonic basin”. We will use it below for deep basins that were formed without significant

stretching

or compression

by a slow, compensated

subsidence

(t - lo3

Ma) of a high magnitude (13 km). The subsidence of a thick lithosphere over a wide region can be produced by downgoing convective flows in the underlying mantle or by density changes in the lithosphere. In order to produce a deep basin by downgoing flows in the mantle, these flows should exist under the region throughout the period of subsidence. Furthermore, their intensity should gradually increase in order to provide continuous increase in the sediment thickness which increases the magnitude of deviations from isostasy. If the flows cease, the lithosphere will emerge and the sediments will be eroded. Continental drift produces displacements of lithospheric plates over thousands of kilometres. Hence, it is quite improbable that downgoing flows of increasing intensity can exist under one and the same region for - 1000 Ma (even if flows of such a kind can really exist in the mantle). Therefore, this mechanism can hardly explain the formation of the Verkhoyano-Kolymsk cratonic basin. The density changes in the lithosphere can be associated with thinning of the crust or with rock compaction. In the absence of stretching at the surface, the crustal thinning may be caused by stretching of only the lower crust or by erosion of this layer by flows in the underlying mantle. Both these processes are impossible when the crust is underlain by a thick layer (>, 50 km) of solid subcrustal lithosphere. Hence, it is most probable that the formation of the Verkhoyano-Kolymsk Basin was caused by compaction of rocks in the lithosphere. Consider a possible role of thermal relaxation. This mechanism requires a much shorter

characteristic

subsidence magnitude

time,

t - 50 Ma

(Sleep,

1971)

than

the duration

of the

in the Verkhoyano-Kolymsk Basin (- 1000 Ma). Furthermore, the of the subsidence was too high to be compatible with a simple thermal

relaxation. Intense tectonic movements and volcanism did not occur in the Verkhoyano-Kolymsk Basin during the period of 2 100 Ma that preceded the beginning of slow subsidence (Semikhatov, 1974). Hence, by the beginning of the subsidence the temperature of the crust and mantle was already low and typical of cratonic areas. The maximum magnitude of lateral temperature variations, AT,, , in cratonic areas at a depth of the base of the lithosphere of d,,, can hardly exceed 200”-300°C in the coolest regions. This corresponds to lateral variations in the average temperature of the crust and mantle above this depth of the order of AT,,,.2. The maximum depth of a sediment-loaded basin that can be produced by relaxation of these thermal inhomogeneities is:

224

where ct is the thermal subcrustal g/cm3, (hy),,,

lithosphere,

expansion p,--the

cx= 3.3. 10e5 deg-‘, - 1.7-2.6

coefficient,

sediment AT,,,

p,-the

density.

- 200°-3OO’C

Taking

density

of the mantle

P,,, = 3.35 g/cm3,

and d,,,

= 125 km, we find:

km

Thus, the depth of a sediment-loaded basin cratonic area cannot exceed a few kilometres.

produced

in the

p, = 2.55

by thermal

relaxation

(4) in a

The depth of the Verkhoyano-Kolymsk Basin reached much larger values, lo-15 km. A large density increase in a thick layer is necessary for a hssediment-loaded subsidence of this magnitude. We suggest that the formation of the Verkhoyano-Kolymsk Basin was associated with a slow gabbro to eclogite phase transformation in the lowermost crust in a thick cratonic lithosphere. At the present level of experimental data, this mechanism is highly speculative: however, we do not see any other one that could produce the observed subsidence. A gabbro-eclogite transformation is associated with a large density increase from p - 2.993.0 g/cm for gabbro up to p - 3.5-3.6 g/cm3 for eclogite (Ringwood and Green, 1966; Ito and Kennedy, 1971). The extrapolation of high-T experimental data shows that dense garnet granulite or eclogite represent a stable mineral assemblage under p-T conditions typical of the lower continental crust. It was suggested on this basis that gabbro cannot exist in the lower crust with P-wave velocities V, - 7 km/set. This layer should then be composed of sialic rocks or amphibolites (Ringwood, 1975). Many kimberlitic and basaltic pipes, however, exhibit numerous xenoliths of gabbro or low-density garnet granulites that had been derived from the lower crust (Sobolev, 1977; McCulloh et al., 1982, and others). This indicates that in many regions the lower crust consists mainly of these rocks. The existence of gabbro or garnet granulites of low density in the lower crust can probably be attributed to a very low rate of the phase transformation at low temperature which is typical of cratonic areas (Kennedy and Ito, 1972; Sobolev, 1978; Artyushkov and Sobolev, 1983). If this reaction rate is really very low, at T - 400”-500°C gabbro can be preserved in the lower crust in cool cratonic areas or it can gradually transform into garnet granulite or eclogite during geological time. The rate of transformation increases strongly, by several orders of magnitude, in the presence of fluids containing water (from several per cent up to several tens of per cent of H,O) (Ringwood and Green, 1966; Ito and Kennedy, 1971; Ahrens and Schubert, 1975). It is most probable that in cool cratonic regions, even a very slow transformation is possible only in the presence of such a fluid. It can be supposed that a transformation of this kind took place under the Verkhoyano-Kolymsk Basin. Its rate was limited by a low temperature and a small water inflow from the asthenosphere into the crust. Metamorphism was mainly concentrated in the lowermost crust where the temperature was higher (Fig. 4a). Dense eclogite or garnet granulite were separated from the asthenosphere by a thick solid layer of the subcrustal lithosphere. This prevented sinking of dense rocks into the asthenosphere.

225

shallow-water

deDOSitS

basaltic

layer

_,__

--

-anomalous __-----

mantle

-

-

-

normal

_---

_

-

-

-

--

-

osthenwphere

(a)

fb)

Fig. 4. The structures mantle

upwelling

basalt-eclogite anomalous eclogitization

on continental

and the eclogite transformation

mantle

and

of the upper

crust sinking

that formed

within a thick cratonic

sinking

by slow and rapid eclogitization,

into the mantle.

of eclogite

part of the basaltic

into

a. Deep cratonic

lithosphere.

the mantle.

layer under

b. Crustal

by a low-rate

uplift from upwelling

c. Deep-water

the upwelling

the anomalous

basin formed basin

of hydrous

formed anomalous

of the

by rapid mantle.

226

The Verkhoyano-Kolymsk isostatically garnet

balanced

granulite

sediment-loaded

Basin was very wide. The lithospheric

over a wide region.

of the density

Then,

p, remains

layer must be

in the case when eclogite

in the lower

crust,

or dense

the depth

of the

basin should be:

he = &h$,

(6)

Here ps and h, are the density and initial thickness of the layer of gabbro that underwent the phase transformation, he--the thickness of the layer of eclogite or garnet granulite. Because of a high density of eclogite, the transformation produce sedimentary basins of a large depth. Let us take ps = 3.0 g/cm’ and that the transformation resulted in the formation of eclogite with the p, = 3.55 g/cm3. Then, in order to form a sedimentary basin of the depth km, a layer of gabbro of the thickness h, = 15 km should be transformed into

can assume density h, = 10 a layer

of eclogite of the thickness h, = 13 km. The development of slow subsidence interrupted by uplifts in one and the same region for a very long period of time can be explained by the existence of a long-living trap at the base of the lithosphere (Fig. 4a) (Artyushkov et al., 1980; Artyushkov, 1983). A segregation of anomalous mantle from the asthenosphere occurs in the epochs of upwelling of a hot, light material from the core-mantle boundary (Artyushkov, 1983). In this model, the anomalous mantle is considered as being different in density and composition from normal asthenosphere. The lowviscosity anomalous mantle ascends to the base of the lithosphere and rapidly spreads along this boundary. It accumulates in the regions where the lithospheric thickness is reduced. Intrusion of small portions of anomalous mantle into such regions produces a slight uplift, of the order of several hundreds of metres, at the surface. Anomalous mantle brings water to the base of the lithosphere. After a slow filtration of water through the subcrustal lithosphere to the crust, this again produces slow eclogitization and subsidence after a slight uplift. The mechanism of crustal subsidence from a gabbro-eclogite transformation with eclogite resting in the lower crust has been suggested by many authors (Joyner, 1967; Artyushkov et al., 1980, and others). One of the purposes of this paper is to show that this mechanism can operate in reality. With a high probability, it can be responsible for the formation of the Verkhoyano-Kolymsk Basin and other deep cratonic basins (see next section). Eclogite or dense garnet granulite can be preserved in the lowermost crust only in a thick lithosphere. Upwelling of low-viscosity anomalous mantle to the base of the crust decreases the lithospheric thickness, which should result in sinking of dense eclogite into the mantle (Fig. 4b). This produces isostatic uplift by: 5 = [(PC - &)/&I

h, + lo.

(7)

221

Here &, is the magnitude and

of the underlying

g/cm3,

of crustal

uplift caused by upwelling

of anomalous

normal

asthenosphere of the density p,. Taking we find: h, = 13 km, and pa = pm in the first approximation,

mantle pe = 3.55

5 - 0.8 km + lo.

(8)

As was mentioned

above, an intense

uplift, regional

volcanism

and ultrabasic

and

alkaline magmatism took place in the Verkhoyano-Kolymsk Basin in the early Middle Devonian. Upwelling of anomalous mantle to the base of the crust probably also occurred at that time. The masses of eclogite that formed during the preceding slow subsidence could have sunk into the mantle in the Middle Devonian, which increased the magnitude of the crustal uplift. Some other cratonic basins The Verkhoyano-Kolymsk Basin is not unique. There are and there were numerous deep sedimentary basins on continental crust that have been formed by slow, compensated subsidence without significant stretching or compression. These basins were amagmatic or characterized by only a very low magmatic activity. The example of the Belskian-Eletskian Basin in the Southern Urals was considered earlier (Artyushkov and Baer, 1983). Up to 15 km of shallow-water sediments were deposited there without significant stretching in the Late Proterozoic-Middle Paleozoic during - 1000 Ma. No intense shortening took place in this region during slow subsidence, despite intense folding that occurred several times in the adjacent regions. The Belskian-Eletskian Basin was intensely compressed only after rapid subsidence took place in this region in the Late Devonian. A system of deep basins on continental crust that had been formed by slow subsidence surrounds the Siberian Platform. They were developing in the Late Proterozoic-Middle Paleozoic during 600-1000 Ma. The average thickness of the shallow-water

deposits

is

5-7

km.

The

South-Taimyrsk,

Turukhansk,

Prisayansko-Eniseisk and Pribaikalsko-Patomsk basins can be taken as examples (Kosygin, 1969; Klitin et al., 1970; Votakh, 1976; Khain, 1979). Rapid subsidence took place later over a considerable part of the basins. Then the regions where rapid subsidence had occurred were strongly folded. The Hercynides of the Taimyr and the Baikalides of the western and southern margins of the Siberian Platform were formed in this way. The strata that had been deposited during slow subsidence became exposed at the surface after folding. In numerous sections, their parallelism and continuity can be traced over a long distance, which precludes significant stretching. Those parts of the above basins where rapid subsidence did not occur remain undeformed or only slightly deformed. Shallow-water strata are approximately parallel there and their continuity can be traced over a long distance, according to seismic and drilling data (see, e.g., Fig. 5a, b).

t

11

km

t

W

+

scale

+

+

vertical

horizontal

+

t

+

1I

10 ,

+

0I

0I I 1

21

10 / 20 fi

+

31

30 1

t

4KM 1

4flKM 1

t

(b)

(a>

+ rTJrrrm]z

+

AR-PRf a31","14=5

+

+

-k

t

E

229

L

E

Y

230

The Vilyuy Syncline on the eastern margin Paleozoic

and Mesozoic.

since the Late Cambrian,

- 600 km long and wide and up to 13 km deep, was formed of the Siberian Platform by slow subsidence during the No rapid subsidence

of a large magnitude

occurred

there

and the basin is still unfolded.

A system of deep basins was formed on the western and northern margins of the North American Platform by slow subsidence of continental crust in the Proterozoic and Phanerozoic (Trettin, 1973; King, 1976; Porter et al.. 1982). Most basins were folded after rapid subsidence in the Paleozoic. Parallelism of the strata and their continuity are observed over long distances in numerous sections (see, e.g.. Fig. 5~). A long system of broad and deep cratonic basins was developing on the northern margin of Gondwana during - 500 Ma in the Vendian-Mesozoic (Tschopp, 1967; Stocklin, 1968; Shah and Sinha, 1974; Brinkmann, 1976; Belov, 1981). The thickness of shallow-water deposits there reached 5-8 km. After rapid subsidence in the Late Paleozoic, Mesozoic and Cenozoic, these basins were folded and became incorporated into the Alpine fold belt in the Taurus, Zagros, Pamirs, Himalayas and other regions. The Adelaide Basin, 1500 km long and 500 km wide, was developing in eastern Australia (on the other margin of Gondwana) for 900 Ma from the Late Proterozoic till the Early Cambrian (Thomson, 1969). Up to 15 km of shallow-water sediments were deposited there. Rapid subsidence occurred between the Early and Middle Cambrian (Daily, 1963). Then an intense folding took place in the eastern part of the basin in the late Middle Cambrian (Harrington, 1974). There are many other structures of the same type; however, even the above considerations show a wide occurrence of deep cratonic basins. The Verkhoyano-Kolymsk Basin is a typical example of this broad structural type. RAPID SUBSIDENCE

Formation

IN THE LATE PALEOZOIC

of the Verkhoyansk

AND MESOZOIC

Basin in the Early Carhoniferous

Slight crustal uplift above sea level occurred on the margins of the Verkhoyano-Kolymsk shallow-water basin and in the adjacent regions of the Siberian Platform and Kolymsk Massif in the Early Carboniferous, between the Tournaisian and Visean ages. The uplift lasted for a short period of time and was in some places associated with acidic volcanism (Geology of the U.S.S.R., vol. XVII, 1970a, vol. XXX, 1970b; Korostelev, 1982). Then rapid subsidence started in the western part of the Verkhoyano-Kolymsk Basin in the Early Visean. A deep-water Vcrkhoyansk Basin was formed there (see Fig. 2); it was - 1500 km long and 300-400 km wide. The subsidence produced an abrupt change of shallow-water carbonates to cherty shales (Fig. 6-11, III). The transition is several tens of metres thick. The subsidence took 3-5 Ma. Thin beds of shaly limestones in the lower part of the deep-water

231

strata

probably

pensation

The transition is observed

indicate

that the floor of the basin

was near the carbonate

com-

strata of the Verkhoyansk

Basin

depth (CCD) in Visean time. from shallow-water

in numerous

sections

to deep-water

(Abramov,

1970). These strata are parallel

the basin (see, e.g., Fig. 7) except a small region in its southwestern of shallow-water

strata can be traced

where the transition is Parallelism of the strata plexes can be seen in the horizontal (see Fig. 7a). deposited deposited

for distances

of up to 5-6 km in the sections

approximately parallel to in thick shallow-water and sections where the beds are Very few places are known

on considerably older on the sialic basement.

strata.

all over

part. Continuity

No places

a horizontal plane (Fig. 7b). deep-water sedimentary cominclined at a high angle to the where deep-water strata were are known

where

they

were

The subsidence was preceded by only a slight volcanism and very few dikes were formed at that time. This leaves no room for the mechanism of stretching with dike injection (Royden et al., 1980). Thus, there are no indications of significant stretching during the subsidence over the major part of the Verkhoyansk Basin. Large angular unconformities exist only in a small region within the western slope (- 40 km wide) of the southern part of the Verkhoyansk Basin (Stavtsev, 1984). They are observed over a distance - 100 km on 5-6 blocks, l-2 km wide. The total width of these blocks before and after tilting is - 10 km and - 14 km, respectively. This gives the absolute value of stretching as - 4 km. East of this small stretched region on the floor of the Verkhoyansk Basin, - 200 km wide, parallelism of deep-water and shallow-water strata is observed in all sections; this precludes significant stretching. The region where an intense stretching is observed in the Verkhoyansk Basin constitutes less than 1% of its territory. Thus, the major part of this basin was formed without significant stretching. The time of stretching in the above small region is very difficult to determine. An intense erosion took place there in the early Middle Devonian, and the Carboniferous deep-water

strata

directly

overlie

the Lower

Paleozoic

strata.

Local stretching

could take place at any time between the Early Paleozoic and Carboniferous. Probably it occurred at the epoch of the crustal uplift and volcanism in the early Middle Devonian. The depth of the Verkhoyansk

Basin

The Verkhoyansk Basin was at middle and high latitudes in the Late Paleozoic (Irving, 1977). The value of the CCD is lower there than in the tropical zone. Hence, an abrupt facies change is insufficient to prove a large depth of the basin. There are, however, numerous large olistoliths in the Visean deposits on the western margin of the Verkhoyansk Basin, 30-50 km wide (Abramov, 1970, 1974; Chekhov, 1976; Gusev, 1979; Guide Book of Excursion, 1980). They indicate that the basin was

AR-PR,

Fig. 6. Stratigraphic

columns

of the main tectonic

data by Geology

of the U.S.S.R.,

1975; Chekhov,

1976; Guide

Pre-Verkhoyansk

Foredeep,

Verkhoyansk (Okhotsk

Book of Excursion, II-northern

Basin (South-Verkhoyansk

Massif), V-Inyali-Debin

units of the Verkhoyansk

Vol. XXX, 1970a; Merzlyakov, part

1980;

Korostelev,

of the Verkhoyansk

Synclinorium). Basin. VI-western

IV-southern margin

fold belt (compiled

1971; Abramov,

1982). I-eastern Basin,

using the

1974; Konstantinovsky, III-southern

margin

of the

part

of the

part of the Yano-Okhotsk

of the Kolymsk

Massif.

Zone

233

300

(0)

300

0

300

600

900m

j

‘,

nl

E 2km

(b) Fig. 7. Fragments clinorium) observed

of geological

(Abramov,

shallow-water

?

0

sections

across

to deep-water

in both sections.

Continuity

the southern

part of the Verkhoyansk

Basin (meganti-

et al., 1964) (lines CC’ and DD' in Fig. 2). The transition

1970, Mokshantsev

limestones

6 km

4

?

deposits

is indicated

of shallow-water

by arrows.

limestones

Parallelism

from

of the strata

can be

can be traced for - 10 km in the lower

section.

separated by a wide and steep slope from the adjacent Siberian Platform, implies considerable depth. A very large amount of sediments (up to 9-13 km) had been deposited Verkhoyansk Basin after the rapid subsidence in the Early Carboniferous shoaling at the end of the Paleozoic. This again indicates a large initial depth basin.

which in the till its of the

A slight crustal uplift and slight volcanism occurred in the area just before the rapid subsidence. This could be produced by upwelling of anomalous mantle at moderate magnitude

temperature to the base of the crust (see below). Designate by Ah, of the additional crustal uplift in the basin that is caused by

Fig. 6 (cont.).

Three main levels of a rapid change

seen in these columns: base of the Jurassic. cherty

Deep-water

and shaly limestones

and bedded), some places),

of shallow-water

in the middle Lower Carboniferous,

dolomites,

and deeper-water

and olistostromes.

molasses,

deposits

3 = shallow-water

sandstones,

6 = limestones,

7 = dolomites,

8 = limestones,

II = turbidites.

12 = cherty shales, cherts,

deposits

I = elastics,

4 = shallow-water argillaceous

13 = basalts.

to deep-water

the Permian

are represented

Shallow-water

coals and evaporites.

deposits

between

are represented 2 = olistostromes

argillites

14 = andesites

15= liparites (a) and their tuffs (b), 16 = tuffs of various composition.

and at the

cherts.

shales,

by limestones (probably

(u) and siltstones

(a) and sandy

ones can be

and Triassic,

by turbidites.

the the

(h), 9 = evaporites,

(reef

tillites in

(h). 5 = coals, 10 = shales,

and basalts (0) and their tuffs

(h).

234

buoyancy and depth,

force from the anomalous respectively.

Then, using the condition at any moment

mantle.

Let the density of isostatic

Designate

and thickness

balance,

by pa, h, the vjater density of the sediments

be p,, h,.

we find for the depth of the basin

h,,

of time:

hi - h,, = ~” - ” h, + Ah,, - Ah; Pa - PO

where p,, is the density of the asthenosphere under the anomalous mantle layer. The values of the parameters for the initial moment of time, just after the subsidence, are marked by the upper index “0”. Shallow-water and terrestrial conditions

(h, = 0) were reached in the Verkhoyansk Basin by the end of the Permian (see Fig. 6-11, III). Slight crustal uplift above sea level and slight volcanism again took place in the basin and in the surrounding regions at that time. This could be associated with a new upwelling of anomalous mantle. Let us take the magnitude of this uplift as equal to that of the uplift in the Then, taking p, = 3.35 Early Carboniferous (Ah, = Ah:) as a first approximation. g/cm3, p, = 2.55 g/cm3, h, - 9-13 km and pa = 1.03 g/cm3, we find from (9): hi - 3-4 km

(10)

This estimate gives the maximum possible initial depth of the basin. the subsidence took place in one short impulse. Formation

of the Yano-Kolymsk

It is valid only if

Basin in the Mesozoic

The rapid subsidence in the Late Paleozoic did not spread over the eastern part of the Proterozoic-Paleozoic Verkhoyano-Kolymsk cratonic basin. A shallow-water shelf existed there in the Late Paleozoic. This covered the Yano-Okhotsk Zone, the region of the Yano-Kolymsk

Basin and the western part of the Kolymsk

Fig. 2). A slight uplift associated place on the shelf in the late Late early Early Triassic. A deep-water km long was formed as a result in

with volcanism Permian. Then Yano-Kolymsk the central and

Massif (see

of intermediate composition took rapid subsidence occurred in the Basin 300-400 km wide and 1500 eastern parts of the area (see Fig.

2) (Chekhov, 1976; Dagis et al., 1979; Gusev, 1979). A shallow-water shelf existed in the western part of the area-in the Yano-Okhotsk Zone, 150-200 km wide. The duration of the subsidence in the Early Triassic was several million years. The subsidence resulted in a change of shallow-water elastics by deep-water carbonate-free siltstones, shales and cherts (Fig. 6-V). The thickness of the transition ranges from a few tens of metres to a few hundred metres. This is observed in a number of large anticlines. Shallow-water and deep-water strata are parallel in all the sections (see Fig. Sa), which indicates no significant stretching. The continuity of shallow-water strata can be observed over a long distance. The next regional crustal uplift took place in the late Late Triassic. This was associated with volcanism of intermediate composition. The crust was uplifted above

235 YANO-KOLYMSK YANO-OKHOTSK

(ADYCHA-DETRIN

ZONE

BASIN

INYALI-DEBIN

ANTICLINORIGM)

(INYALI-DEBIN

BASIN

SYNCLINORIUM)

SW

NE

NE

(b)

2

(Cl Fig. 8. Geological

sections

0

0!

2

t

across

2

4

2:

6

4:

8km 1

6 km

4

the Yano-Kolymsk

Megasynclinorium

(lines EE’,

FF’

and GG’ in

Fig. 2). a. Schematic

section, 200 km long, across the slightly deformed

Yano-Kolymsk illustrates

and

the different

rapid subsidence b. and

intensity

of different

c. Fragments

Inyali-Debin

Inyali-Debin

basins

(Geology

of crustal

shortening

of the geological

(Tsn) and deep-water

arrows.

Continuity

1970b).

folded

The profile

region and in deep basins formed

sections

across

the central

(b) and

et al., 1964). The beds of shallow-water

shales of the Lower Jurassic

of shallow-water

sea level on the margins

in a cratonic

Zone and intensely

Vol. XXX,

by

magnitude.

Basin (Mokshantsev

Triassic

Yano-Okhotsk

of the U.S.S.R.,

strata

can be traced

of the Yano-Kolymsk

(5, ) are, parallel. for 5-10

northern

deposits

(c) parts

of the upper

The transition

of the Upper

is indicated

by

km.

Basin. The magnitude

of this uplift

was approximately the same as that of the uplift that preceded the subsidence in the Early Triassic which permits us to take Ah, = Ah: in (9). The uplift resulted in deposition of turbidites in the central part of the basin due to erosion of the Yano-Okhotsk Zone and the Kolymsk Massif. Deposition of turbidites in wide basins takes place if the water depth is >, 1 km. 4-5 km of black shales, siltstones and sandstones had already been deposited in the Yano-Kolymsk Basin after the rapid subsidence in the Early Triassic and before the deposition of turbidites. Taking h, = 4-5 km, h, 2 1 km and Ah, = Ah: in (9) we find for the maximum initial depth of the Yano-Kolymsk Basin: ht 2 2.5 km.

(11)

236

After the uplift subsidence occurred

and volcanism in the eastern

Triassic

and Jurassic

resulted

in a considerable

in the late Late Triassic, part of the Yano-Kolymsk

(see Fig. 2) (Chekhov, deepening

1976; Gusev,

the second rapid Basin between the

1979). The subsidence

of this part of the basin with the formation

of

the deep-water Inyali-Debin Basin, - 1500 km long and 200-300 km wide. Turbidites were overlain by cherty shales there (Fig. 4-V). The duration of the subsidence was - 1 Ma. The transition is from several me&es up to several tens of metres thick. This is observed in a number of folds (see, e.g., Fig. 8b, c). Parallelism of the Triassic turbidites and the Jurassic cherty shales and continuity of beds in the turbidites can be traced over a long distance in many sections. This indicates no significant stretching during the subsidence. Cherty shales in the Inyali-Debin Basin were gradually followed by a typical terrigeneous flysch in the late Early Jurassic. The basin was filled with 5.6 km of Jurassic deposits. Volcanism of various composition (from basic to acidic) occurred in the basin in the late Middle Jurassic and in the early Late Jurassic. The initial depth of the Inyali-Debin Basin in the Early Jurassic can be estimated from (9) as hi-- 2 km. This value is in agreement with the sediment facies. The last rapid subsidence in the Verkhoyansk Belt took place in the early Late Jurassic. (Leipzig, 1963; Trushkov, 1975). The Pre-Verkhoyansk Foredeep (see Fig. 2) originated from this moderate magnitude subsidence without significant stretching. Several kilometres of sediments (up to 6 km on the eastern margin of the foredeep) were deposited in this basin before shoaling in the late Early Cretaceous.

Compression

Crustal

of deep basins formed by rapid subsidence

compression

in northeastern

Asia took place mainly

due to plate motions

in the northwestern Pacific. An island arc originated on the northeastern margin of Asia, on the eastern margin of the Kolymsk Massif, in the Middle Devonian (Khain, 1979; Tilman et al., 1982). This was a time when a pronounced uplift and volcanism occurred

in the Verkhoyano-Kolymsk

cratonic

basin.

This basin was, however,

far

from the island arc: they were separated by the cratonic Kolymsk Massif, 1000-1500 km wide. The above island arc existed till the end of the Jurassic. Folding of oceanic crust occurred to the east of this island arc between the Middle and Late Jurassic (Ruzhentsev and Sokolov, 1985). After the orogeny, the crustal surface was elevated above sea level. That was the beginning of the formation of the Koryakian folded zone to the east of the Kolymsk Massif (see Fig. 9). In the mid-Late Jurassic, sea-floor spreading, with the formation of the Koryakian marginal sea, started within the Koryakian folded zone. A new island arc- the Koryakian island arc originated on the eastern margin of the Koryakian Zone at that time.

237

Fig. 9. Map showing the main tectonic elements of the northeastern part of the U.S.S.R,

Three epochs with an increased rate of the northward motion of the Kula Plate (up to 14-18 cm/year) occurred in the late Late Jurassic-Late Cretaceous (Larson and Pitman, 1972; Lancelot and Larson 1975; Cooper et al., 1976; Nishiwaki, 1981). The first epoch (in the late Late Jurassic) was associated with folding of oceanic crust to the east of the Koryakian Arc (Ruzhentsev et al., 1982). At the same time, the Inyali-Debin and Yano-Kolymsk basins in the eastern part of the Verkhoyansk Belt (Fig. 2) became shortened (Konstantinovsky, 1975, Gromov et al., 1980). An increase in the volcanic activity took place in the Koryakian Arc during the second epoch of an increased rate of the Kula Plate motion, in the mid-Early Cretaceous. At that time, the Ver~oyansk Basin became shortened. The third epoch of increased rate of the Kula Plate motion took place during the Late Cretaceous. Two phases of folding of oceanic crust to the east of the Koryakian Arc occurred in the beginning and at the end of this period of time. In addition, gentle folding took place in the Pre-Verkhoyansk Foredeep in the early Late Cretaceous (Leipzig, 1963). The Koryakian barony sea was folded in the Paleogene. Thus, all deep basins in the Verkhoyansk Belt formed by rapid subsidence have been shortened. In contrast, not one cratonic block has been intensely deformed during the orogeny. The occurrence of all the rapid subsidences before the onset of folding precludes thrust loading (Beaumont, 1981) as a mechanism. Furthermore, no new subsidence of a large magnitude occurred near the front of folding. Deep basins in the Verkhoyansk Belt were formed within the Asian continent, far from the plate boundaries. This permits of considering them as inland seas. The crustal thickness in the Verkhoyansk Belt is h, - 40-45 km (Belyaevsky, 1974). In the Verkhoyansk and Inyali-Debin basins this was reached after a compression by - 1.5-2 times. Hence, before the compression h, there was - 20-30 km. Up to lo-12 km of sediments were deposited in these basins after the rapid

238

subsidence. sediments

Hence,

Yano-Kolymsk thickness

h, was - lo-20 km just after the subsidence. Up to 6-8 km of after the rapid subsidence in the western part of the

were deposited after

Basin, the rapid

The

intensity

subsidence

of shortening can be estimated

is

- 1.5 times. as h, - 20-30

The

crustal

km in this

region. The above basins

were formed

in a cratonic

area. It is most probable

that the

crustal thickness before the rapid subsidence was approximately the same there as in the adjacent Siberian Platform (- 35-40 km). Then, in the deepest basins, the rapid subsidence should be associated with crustal thinning by several times. The intensity of shortening reveals a direct correlation with the magnitude of the total preceding rapid subsidence and crustal thinning (see Fig. 8a). This is higher in the Verkhoyansk and Inyali Debin basins than in the western part of the Yano-Kolymsk Basin. A folding of even lower intensity took place in the PreVerkhoyansk Foredeep, where rapid subsidence had a smaller magnitude. Only a slight folding took place in the Yano-Okhotsk Zone. The thickness of the Late Paleozoic-Early Mesozoic deposits is rather high in this region (up to 4-5 km). However, no rapid subsidence took place there. A low intensity of folding probably indicates

that the lithospheric

layer was thick in this region.

Mechanism of rapid subsidence Rapid subsidence in the Verkhoyansk Belt took place in the same way as in the other fold belts studied (Artyushkov and Baer, 1983, 1984). Possible mechanisms of rapid subsidence (- l-10 Ma) without significant stretching or thrust loading have been discussed in the above papers. Thermal relaxation (Sleep, 1971) is inapplicable to this phenomenon since it has a much longer characteristic time (- 50 Ma). Comparison of the intensity of crustal shortening in young fold belts with the present crustal thickness shows that rapid subsidence is associated with a strong tinning of continents crust. Reduced crustal thickness is also typical for many unfolded basins formed by rapid subsidence, e.g.. the Pannonian Basin and Aegean Sea (Artyushkov

and Baer, 1984).

In the absence of significant stretching at the surface, crustal thinning requires the destruction of the lower continental crust. Many authors consider subcrustal erosion or nonuniform stretching as a cause (Gillufy, 1963; Sclater et al., 1980 and others). It is most probable that these phenomena can occur only when the lower crust is strongly heated and has a low viscosity. Strong heating of the crust and mantle should produce a high uplift, at least at the initial stage of erosion. Strong volcanism will take place if some normal faults form during the subsidence. The crustal material removed from beneath the basin should accumulate under the crust in the adjacent regions which will produce an intense isostatic uplift. This set of features is appropriate to the Basin and Range Province, for which the mechanism of subcrustal erosion has originally been suggested. This province is

239

highly

elevated

characterized Erosion

(-

2 km), surrounded

by a strong volcanism

or stretching

of only the lower crust, however,

the Basin and Range Province, surface (Wemicke Deep-water

and Burchfiel,

basins

by even more highly and high temperature

since an intense

elevated

regions,

and

of the crust and mantle.

cannot

stretching

be suggested,

is observed

even for

directly

at the

1982).

in the Verkhoyansk

and the other

fold belts were formed

in

cool cratonic areas after only a slight uplift and volcanism. In most cases, the subsidence was associated with no significant concomitant uplift in the adjacent regions. Rapid erosion or stretching of the lower crust by convective flows in the mantle seems to be quite improbable under such conditions. It can be expected that the lower crust in cool regions can be rapidly destroyed only if it becomes denser than the underlying low-viscosity mantle. This is why, in our previous papers, we suggested a destruction of the lower crust from gabbro-eclogite transformation as a possible cause of rapid subsidence. Rapid destruction of the lower crust can take place where this layer consists mainly of gabbro. Then upwelling of hydrous anomalous mantle at moderate temperature (T - 800°C) to the base of the basaltic layer can produce rapid transformation of gabbro into eclogite or dense garnet granulite. These dense rocks sink into the anomalous low-viscosity mantle. After the destruction of the basaltic layer, the sialic upper crust subsides, with the formation of a deep-water basin (Fig. 4c). This mechanism is even more speculative than that suggested above for slow crustal subsidence in cratonic regions. From a petrological point of view, it is very difficult to envisage a rapid formation of large masses of eclogite in the lower crust (Herzberg et al., 1983 and others). As was already mentioned, it is not certain in how many regions the lower crust may consist of gabbro. Granulite rather than eclogite may be the stable assemblage in the lower crust. Amphibolite rather than eclogite may form in the lower crust under a contact with hydrous reaction rate under low temperatures is poorly known.

anomalous

mantle.

The

Despite all these difficulties, our impression is that the other known mechanisms meet even more objections when applied to rapid subsidence. This is why we use the mechanism of gabbro-eclogite transformation as a working hypothesis. At least, this mechanism does not contradict the data on the development of rapid subsidence. It is in agreement with the slight uplift and volcanism that took place before most rapid subsidences, e.g., before the formation of deep-water basins in the Verkhoyansk Belt. These phenomena may indicate upwelling of anomalous mantle at moderate temperature. In the Verkhoyansk Belt the destruction of the basaltic layer occurrred during both slow subsidence in the Late Proterozoic-Devonian and rapid subsidences in the Carboniferous-Mesozoic. The thickness of sediment that was deposited after the rapid subsidence increases where the thickness of sediment deposited during slow subsidence reduces. The total thickness of the deposits is 15-20 km and it is rather

240

uniform

in space. A complete

form a sedimentary A large additional crustal

thickness

destruction

of a thick basaltic

layer was necessary

to

basin of such a large depth. force, Z, arises in the lithosphere

(Artyushkov,

should be overcome

from inhomogeneities

in the

1973, 1983). In order to shorten the crust, this force (Artyushkov et al., 1982; Artyushkov and

by an outer force Z,,,

Baer, 1983, 1984). The force Z is generally

proportional

to the square of the crustal

thickness, hf. After the rapid subsidence in the Verkhoyansk Belt, the crust became very thin. It remained considerably attenuated (- 20-30 km) even after the basins were filled by the sediments. This strongly reduced the force Z,,, necessary to compress the basins. At the epochs of new asthenospheric upwellings, anomalous mantle upwelled to the base of the attenuated crust, which strongly reduced the lithospheric thickness (down to d - 20-30 km). This permitted an intense crustal shortening in the basins under the convergent movement of the Eurasian and Kula plates. In addition to folding, a low lithospheric thickness under deep basins formed by rapid subsidence permits an easy break-up of the attenuated crust under divergent plate motions. In all the other fold belts studied, sea-floor spreading occurred in deep basins on continental crust. Then the attenuated continental crust was intensely folded, together with oceanic lithosphere. The Late Paleozoic-Mesozoic Verkhoyansk Belt is an exception to this rule. Deep basins on continental crust in this fold belt were not broken up and only continental crust was folded. According to a popular point of view (Churkin, 1972) rifting and drifting took place in the Verkhoyansk fold belt in the Late Paleozoic. No blocks are, however, known in this region which have the structure typical for oceanic lithosphere. Some tectonic blocks of ultramafic rocks that are bounded by fault planes have been found in the intensely dislocated Mesozoic sedimentary rocks of the Inyali-Debin Basin (Tilman et al., 1975); however, their origin is uncertain. Maybe, they are the remnants (tectonic wedges) of the Precambrian oceanic crust that had been incorporated into the basement of the eastern part of the Verkhoyansk Belt after folding and were then remobilized in the Late Jurassic. Miogeosynclines

Many authors use, after Stille (1940) the term “miogeosyncline” for deep basins on continental crust that can be strongly compressed and incorporated into fold belts. It is usually supposed, after Hall (1859) that any deep basin on continental crust can be strongly folded if it is filled with a thick pile of sediments. Our analysis shows that only the basins formed by rapid subsidence can be intensely shortened. Hence, we suggest the use of the term “miogeosyncline” for these basins. According to a paleogeographical position, miogeosynclines represented inland and marginal seas underlain by attenuated continental crust, continental rise and basement of passive margins, foredeeps and intramountain depressions. Miogeo-

241

synclines

are characterized

deep-water

deposits:

etc., although, sediments.

and cherts,

of facies. Most of them were filled by

radiolarites,

pelagic

in many basins these deposits were gradually

Some foredeeps

shallow-water sediments

by a large variety

shales

molasse

and intramountain

under

that were deposited

limestones,

changed

depressions

were entirely

a very high rate of deposition. after rapid subsidence

hundreds of metres to lo-15 km. The miogeosynclinal sediments are underlain composition. Some 5-15 km of shallow-water

turbidites

by shallow-water filled by

The thickness

until folding

of the

varies from a few

by deposits of various thickness and deposits underlie deep-water sedi-

ments in many basins. In contrast, there are some basins where deep-water deposits are separated from the basement only by a thin layer of basal elastics ( ,< 1 km). Volcanism occurred in many miogeosynclines, especially at the early stage of their existence, although about 50% of the basins were nonvolcanic. Various combinations of the above factors produce a large variety of the basins. An impression may arise that they have almost no common features and thus it is not necessary to call them by one and the same name (Coney, 1970, and others). However, the above features have no direct relevance to the mechanism of the formation of miogeosynclines. All these basins have been produced by rapid subsidence of a large magnitude without significant stretching which permits a subsequent compression. This kind of subsidence is the major specific feature which permits of relating miogeosynclines to a specific basin type. CONCLUSIONS

As follows from the above considerations, deep basins on continental crust in the Verkhoyansk fold belt can be related to two main types. The Verkhoyano-Kolymsk sedimentary basin, up to lo-15 km deep, was produced by a slow sediment-loaded subsidence without significant stretching or crustal shortening in the Late Proterozoic-Middle Paleozoic. The duration of the subsidence was - 1000 Ma. A number

of basins

of the

same

type

existed

in other

areas

in

the

Late

Proterozoic-Phanerozoic. The duration of their formation was - 500-1000 Ma. These basins, 5-15 km deep, were filled by shallow-water or terrestrial deposits. No significant shortening occurred during the slow subsidence, despite orogenies that took place in the adjacent regions. This indicates a great thickness of the lithosphere under the basins that can be called “cratonic basins”. The large magnitude of the subsidence in cool regions during a very long period of time can probably be explained by a slow gabbro-eclogite transformation in the lowermost crust. Three deep-water basins in the Verkhoyansk fold belt were formed by rapid subsidence in the Early Carboniferous, Early Triassic and between the Triassic and Jurassic. The duration of the rapid subsidence was several million years. It was associated with no significant stretching or thrust loading. The subsidence was preceded by slight crustal uplift and volcanism and was associated with a strong

thinning of the crust. The basins were gradually filled by 5-12 km of the deposits. They were intensely folded in the Mesozoic. The intensity of the crustal shortening increased

with the magnitude

No cratonic Numerous stretching

of the preceding

block was involved deep

basins

or thrust loading

rapid subsidence

in intense

folding

formed

by rapid

were

and crustal thinning.

in this area. subsidence

in the other fold belts. Among

without

deep basins

significant on continen-

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