On the origin of recent and modern vertical movements in the island systems of northeastern Asia

Tectonophysics,

283

122 (1986) 283-305

Elsevier Science Publishers

B.V., Amsterdam

- Printed

in The Netherlands

ON THE ORIGIN OF RECENT AND MODERN VERTICAL THE ISLAND SYSTEMS OF NORTHEASTERN ASIA

P.M. SYCHEV, Institute USSR,

V.K. ZAKHAROV

of Marine Yurhno

(Received

March

IN

and V.P. SEMAKIN

Geology and Geophysics,

-Sakholinsk,

MOVEMENTS

For East Science Center of the Academy

of Sciences of the

693002 (U.S.S.R.)

2, 1984; revised version accepted

July, 1985)

ABSTRACT

Sychev,

P.M., Zakharov.

movements

V.K. and Semakin,

in the island systems

The paper

deals

(Kamchatka,

with recent

Sakhalin

V.P., 1986. On the origin

of northeastern

and modern

and Japan).

of recent

Asia. Tecronophysics,

vertical

movements

in the island

It is shown that recent and modern

vertical

systems

deformations

of East Asia

of the earth’s surface

are often correlated

with geological

structures

(secular)

in reality change

their sign within short periods of time and are similar in this respect

movements

to rapid (impulsive) the crustal

movements.

deformations

by mass supply

Often inversion

and gravity

into the earth’s

often accompanied

anomalies

crust.

by electrically

of the vertical

vertical

deformations,

allow us to conclude

mantle

and its intrusion

where zones of partial

occurs in the areas of neotectonic

melting

that both’modern

layers that are present

movements,

at predominantly

from more ancient

together

two levels: one in the upper are formed,

and the other at depths

levels, causing

wavelengths.

uplift seems to be related

formation

of tension

The conclusion intra-plate

fractures.

is drawn

of magma

Some phenomena

that the above

a complex

accompanying

mechanism

of vertical

inflow from the

near the lower crustal

of lo-15

to its excess pressure, magmatism

movements

are

of lo-15

of the sources of

magma

km in the crust.

range of vertical movements crustal

uplifts

at depths

estimations mantle

of

are accompanied

and neotectonic

with quantitative

Slow

uplifts. Comparison

in some regions

that they are caused by intermittent

They occur both at deeper and shallower The mechanism

movements.

suggests that positive vertical movements

It is also revealed

conductive

km. All these features

boundary,

and are inherited

and modern

122: 283-305.

of various

leading

to the

are briefly discussed.

may be widespread

in the

regions at least.

INTRODUCTION

Discovery of physical causes of modern and recent crustal movements and also their relations with some other geophysical phenomena has long been one of the most important objectives of the earth sciences. Despite ‘numerous investigations concerning this problem, the nature of crustal movements still remains debatable. At present there are well documented examples of both horizontal movements of up to some dozens of kilometers and vertical ones with amplitudes of several kilometers.

0040-1951/86/$03.50

0 1986 Elsevier Science Publishers

B.V.

284

Recently, widespread ideas of “new global tectonics” have contributed to revealing the greater role of horizontal movements of “mobile plates” in explaining various

types

of movements,

including

those

which

cause

mountain

formation

(Smith, 1976; Helwig, 1976). Meanwhile, the relation of recent and modern vertical movements with tectonic structures, the duration and discontinuity of the development of the latter, structural peculiarities of the crust and upper mantle and some other phenomena in the considered regions (Kamchatka, Kuril Islands, Sakhalin, Japan) testify to the fact that vertical, rather than horizontal movements correlate better with the available data. It has been already noted (Sychev, 1976a, 1979) that the most probable causes of at least some of the intra-plate crustal deformations are crustal and subcrustal magmatism. Similar conclusions were drawn for some regions of North America (Brown et al., 1978). In general, magmatism as a factor determining the occurrence

of various

types of displacements

of the earth’s surface seems to

be a widespread phenomenon. It will be shown in this paper that available geological and geophysical data for the region considered really allow us to explain most of the crustal deformations as caused by magmatic activity. Magma movement mechanism and the associated phenomena RECENT

are briefly discussed.

AND

MODERN

VERTICAL

MOVEMENTS

The modern relief of the region under consideration was formed mainly in the Quaternary period. Quaternary movements were ubiquitous and differentiated. Maximal amplitudes of neotectonic uplifts were 1500-1700 m in Kamchatka, 1300-1400 m in Sakhalin and about 2000 m in the Japanese Islands. At the same time, maximal subsidences did not exceed 200-500 m, and reached 1200 m only in the Japanese Islands (Erlich, 1968; Iosikawa et al., 1969; Soloviev, 1970). Definite ment

inheritance

is characteristic

of a recent stage from the earlier history of tectonic of the territories

of these regions.

Here,

develop-

large neotectonic

dislocations generally coincide, according to the strike and direction of formation, with older tectonic forms of anticlinorial and synclinorial type. Although for structural forms of higher order full coincidence is not always ascertained, it is observed rather distinctly in some regions. For example, in North Sakhalin, neo-tectonic uplifts generally correspond to anticlinical zones, and modern troughs to synclinal zones (Fig. 1). The same is observed in Kamchatka (Vlasov and Klenov, 1964) and in some regions of Japan (Kobayashi, 1957). Recent movements within the region were not stable in time. Discontinuity of the movements is noted while observing terraces, which mark separate stable stages. Besides the above-noted features of neo-tectonic movements, general uplifts and subsidences, covering vast territories, should also be mentioned. Again one can make reference to Sakhalin where in the Quaternary both regions of general uplift and some regions of general subsidence were formed (Alexandrov, 1973).

285

SAKHALIN

--PI

TERPENYA

Fig. 1. A scheme of neo-tectonic troughs,

3 = arched

Northern

Sakhalin,

absohtte

neo-tectonic

conventional

uplifts

of Sakhalin

of Northern

5 = axes of local relative downwarpings

The numbers

stand

2 -Western

Schmidt,

IO-Lunsk-Vengeriysky,

movements

neo-tectonic

neo-tectonic

6-Tonin-Aniva.

\

Island. Sakhalin,

neo-tectonic

of Northern

Sakhalin,

I = neo-tectonic

uplifts,

2 = neo-tectonic

4 = axes of local neotectonic

troughs

of Northern

7 = disjunctive

Sakhalin,

neo-tectonic

6

uplifts

of

= zones of

boundary,

8=

boundary.

for neo-tectonic

uplifts

3 -Eastern

Neo-tectonic

and

Sakhalin, trot&s:

II -Pogranichny,

troughs.

Neo-tectonic

4 -Western

7-Central 12-Susunai,

Schmidt,

uplifts:

Sakhalin, 8--Nabilsky,

13-Muraviev.

i-Eastern

Schmidt,

5 -Susunai-Korsakov, 9--Tym-Poronai,

286

The knowledge of modern (secular) movements within the region under consideration is extremely fragmentary. Nevertheless, the available data testify to the fact that

many

of the above

regularities

particular,

in some regions

structures

(Fig. 2) while subsidences

(Kobayashi,

maximal

1957). But in general,

are also uplifts

true

coincide

more often modern

for modern striking

correspond

movements

short periods of time they often have inverse character

movements.

In

with axes of anticlinal to synclinal

troughs

are more complex relative to tectonic

and for

structures.

For example, vertical movements measured in Japan for the periods from 1898 to 1930 and from 1930 to 1950 (Miyamura, 1969) differ greatly, indicating rather rapid change of sign of the earth’s surface dislocations. Results of repeated levelling in South Sakhalin are very indicative in this respect. As is shown in Fig. 3, sharp changes of the intensity of movements and reversals in sign are observed along the being more distinctly expressed for the profile Lovetskoye-Okhotskoye, Yuzhno-Kamyshoviy Range, which is a part of the large West Sakhalin neo-tectonic uplift. Three periods are distinguished here. The first period (1959-1969) is characterized by intensive subsidence (Zakharov et al., 1975): the second (1968-1971/72) is characterized by a relatively stable regime and the third (1971/72-1974) by intensive uplift (up to 14 mm/yr). If in the first period the sign of dislocations was inverse relative to the neo-tectonic uplift, in the third one inheritance of recent deformations was clearly manifested. Although the summarized value of modern movements turned out to be negative (Karta.. . , 1981) at the place of the neo-tectonic uplift, the last two periods point out to the inheritance of movements. The above situation is rather important for understanding the regularities of modern deformations. It is revealed that in most cases inversion of modern movements turns out to be characteristic of neo-tectonic uplift regions, subsidence being mainly inherited. This tendency is a general characteristic of Sakhalin, where for the period of the last 15-20 years modern movements within the region of neo-tectonic uplifts often have an inverse nature. When comparing modern movements in Japan (Dambara, 1975) with the Map of Quaternary Tectonics (Hatory et al., 1969) one

Fig. 2. Vertical displacements 40 km eastward, 1957).

based

of the earth’s surface along the profile:

on the 1892-1938

levelling

data.

from Noshiro

A-anticlines

(N) harbour

of Tertiary

to about

folds (Kobayashi,

287

k

Okhotekoe

Lovetskoe

Fig. 3. Newest structures

and modern

profile.

A. Newest

structures:

Uplift,

d-Muraviev

Trough.

planation,

2 = newest uplifts,

fault established coast

networks,

levelling

along the Lovetskoe-Okhotskoe

Uplift,

I = modem

location

of an assumed

3 = newest troughs,

B. Velocity

of repeated

3 = planned

movements Sakhalin

based on the zone of high gravity

of Sakhalin.

Location

vertical

a-Western

Basin,

gradients,

of modem

vertical

lines:

I = starting

levelling

(Sakhalin

6 = a terrace movements benchmarks,

Isl.)

c-Susunai-Korsakov

Neogene-Quaternary

4 = newest and rejuvenated

curves

4 = sea level measuring

b-Susunai

faults, complex

surface

5 = an ancient

of

(Cr?)

on the southwestern

for different 2 = repeated

time periods. levelling

C.

lines,

points.

can see that most of them are also inverse relative to neotectonic uplifts. Sign-changing movements are characteristic of the Avacha Volcano region in Kamchatka, although upwelling is characteristic of the entire region of Kamchatka (Gorelchik and Dmitriev, 1974). Thus the pattern of modern movements in this or that region reveals that for some relatively short period of time it can appear chaotic. But while estimating movements for longer periods of time, it occurs that there are regions of predominant uplift or subsidence, i.e. the movements have, as a rule, an inherited character. Therefore, if the rate of modern movements for short periods of time can reach 10 and more mm/yr, average rates of uplift or subsidence for long periods of time are usually several millimeters per year. One more factor complicating the character of regularities in the distribution of modern movements is the fact that uplifts or subsidences have different wavelengths and oscillations seem to be superimposed on each other (for details see the corresponding section below.)

288

Rapid (impulsive) often

associated

displacements

vertical

movements

with volcanic during

studied

eruptions

such events

and

both in Kamchatka earthquakes.

is very high, reaching

and Japan

The value several

are

of vertical

meters

and even

more per year. During eruption

volcanic

eruptions

the volcanic

are observed

rather

structure

at a distance.

regular

deformations

and its neighbourhood

When ‘volcanic

activity

are observed:

uplifts, ceases,

whereas

before

subsidences

the sign of movements

changes. The observed surface deformations allow us to determine precisely the depths of secondary magmatic sources within the crust (Yokoyama, 1974). Rapid displacements of the surface related with some earthquakes also have regularities similar to those observed during volcanic phenomena. Vertical displacements are the greatest in those places where strong earthquakes occur (Tsubokawa, 1972). In Japan, a relation between modern vertical movements and continued folding on the one hand and between modern vertical movements and crustal earthquakes, on the other, has been revealed for some regions (Mizoue, 1969). In some cases, rapid movements accompanying earthquakes can be considered as a continuation (or separate stages) of slow movements. During crustal earthquakes an uplift within the epicentral area is often observed, which does not always coincide with the sign of preceding slow (secular) movements (Mizoue, 1969; Rikitake, 1979). After earthquakes, further uplift is sometimes observed, but opposite displacements are more common. The deeper an earthquake focus is, the greater are the lateral dimensions of uplifted areas and of subsidences of approximately the same volume at a distance from the epicenter. It is noteworthy that, similarly to volcanic eruptions, focal depths of earthquakes that are in good agreement with seismological estimates may, in some cases, be determined from the observed surface deformations (Okada, 1962). Thus, one of the main regularities in the distribution of recent and modern movements in the region is that they are mostly connected with continued structure formation, inherited from earlier stages of geological history. In some cases, impulsive vertical surface displacements occurring during crustal earthquakes are connected definite

with the development similarity

VERTICAL

between

MOVEMENTS

AND

of fold structures.

secular and impulsive GRAVITY

ANOMALY

It also turns modern

out that

there

is a

movements.

CHANGES

An important aspect associated with the nature of vertical movements is its correlation with gravity anomalies. It is natural that displacements of the earth’s surface lead to gravity anomaly changes. When surface height is simply decreased or increased, corresponding changes in free-air gravity anomalies will be equal to 0.3086 mGal/m. In reality, the displacements associated with volcanic eruptions are usually accompanied by gravity anomaly changes of about 0.2 mGal/m (Jachens, 1978) i.e. they approach the Bouguer gradient which indicates the appearance of

289 WATION

Fig. 4. Temporal

gravity

changes

CNANGE

IN

in the Matsushiro

CM

region (Fujita

and Fujii, 1974).

local excess masses distributed beneath the summit of a volcano. In other words, parameter Ag/Ah is indicative of the fact that a supply of excess mass into the crust in the form of magma causes vertical displacements. Gravity anomaly changes of 0.2 mGal/m also correspond to vertical surface displacements accompanying earthquakes in those rare cases when gravity observations have been made. Such values were observed during the Alaskan earthquakes of 1964 (Barnes, 1966) New Zealand earthquakes of 1968 (Hunt, 1970) and some others (Jachens, 1978). But sometimes gravity field changes are larger as compared to vertical surface displacements, which directly indicates some rapid mass transport within

the crust and upper

Continuous during

observations

the Matsushiro

mantle

(Fujita

et al., 1975; Chen Yun-tai

of surface displacements

earthquake

et al., 1979).

and gravity changes carried out

swarm of 1965-1966

are of great interest

for the

region under consideration. During seismic activity, an uplift of about 90 cm was formed in the epicentral part. Gravity changes during and after seismic activity are subdivided into three stages (Fujita and Fujii, 1974; Fig. 4): (1) decrease of anomalies equal to free-air gradients in the initial period of the surface uplift; (2) decrease of anomalies near to Bouguer reduction; (3) increase of gravity corresponding to the free-air gradient with subsidence of the uplift. The first stage is a simple expansion of rocks of indefinite volume without change of mass; the second stage is caused by the appearance of excess mass; the third stage indicates simple compression (volume decrease) without change of mass. Matsushiro is one of few regions of the globe where a great many geological and geophysical observations have been carried out. Nevertheless, the available data did

290

not allow one to draw a conclusion as to the nature of the observed phenomena. At first it was supposed that these phenomena were caused by water injection from deep sources (Hagiwara, 1967) or by regional compression in latitudinal direction (Kasahara, 1970). However, a subsequent analysis (Stuart and Johnston, 1975; Sychev, 1979) and some additional

data on higher helium content

in soil (Wakita

et

UOGLIKI

, (4

E

B YUZHNO

2 LA

VW ar

PEROUZE

-SAKHALINSK

?

PTRA/7

KUNASHIRO

0

k”’

Fig. 5. Schematic structures gravity

or buried

minima.

80

:

i20

:

location

180

1

rem

of axial

uplifts (Sychev,

lines of positive

anomalous

1966). I = axial lines of gravity

zones

that

anomaly

correspond maxima.

to anticlinal

2 = axial lines of

291

al., 1978) supported about

the idea that,

3-5 km was the most probable

ments. It is still very difficult because

they

undoubted. gravity

have

anomalies

magma

low values.

of modern

shows that gravity

intrusion

cause of seismic activity

to reveal gravity anomaly

extremely

Comparison

in Matsushiro,

At

and recent maxima

changes the

same

movements

at depths

of

and surface displace-

during time,

slow movements, their

presence

with the distribution

in free air and Bouguer

is of

reductions

usually correspond to uplift areas. And at least for Sakhalin, based sometimes on magnetic data and sometimes on indirect data (such as the metamorphism of coals and oil), it is assumed that anticlinal structures or buried uplifts associated with the gravity maxima are accompanied by large magmatic intrusions at some depth (Sychev, 1966). In Sakhalin, the regions of neo-tectonic or buried uplifts correspond, excluding some regions, to the linear zones of gravity maxima, whose values reach some dozens of mGals (Fig. 5). In some regions of Japan, positive gravity anomalies are well correlated with modern uplifts (Fig. 6). All these data show that first of all vertical movements occur, despite isostasy as it is generally understood and, secondly, positive displacements of the surface are accompanied by supply of excess mass into

8 HORIZONTAL

A 50

0I

HORIZONTAL

J

c c!

INNER

150 1

100 I

1

WAVE

ZONE

OF TOHOKU

LENGTH

20-

0 r

WAVE LENGTH 20- (00 KM 150 50 100 I I

IMKM

,?j DIST- 5 &XT

2

KM Fig. 6. Secular crustal movements relative to large-scale geostructural Japan (Mizoue,

1969).

elements in two areas (A and B) of

292

the crust, gravity

reaching

maxima

rather

may formally

earth’s crust, the character North Sakhalin,

large amounts

for instance,

for long periods

be accounted

of the deformations

of time. Although

for by horizontal

compression

that form anticlinal

of the

structures,

is to a great extent related to the vertical movements

the basement blocks (Alexeichik et al., 1963; cf. also Fig. 7). As a whole, the general regularities of the gravity field variation

the as in of

turn out to be

similar for both rapid and slow movements. In many cases recent uplifts and subsidences can be identified with anticlinal or synclinal crustal structures, respectively. Wavelengths (distance between the axes of anticlinal or synclinal linear zones) of the latter vary from IO-20 to 100 km, being predominantly 60-70 km (Figs. 5 and 6). For the purpose of comparison it should be noted that the diameters of volcanic uplifts usually are not larger than lo-20 km. SOME REGULARITIES

OF CRUSTAL

STRUCTURE

Continental structure is characteristic of the earth’s crust of \he region under consideration. Within the upper crustal horizons, basement uplifts, which are either revealed by seismic methods or derived from gravity data, usually correspond to anticlinal structures. An example of interrelations between sedimentary layers and the underlying basement is shown in Fig. 7. In the seismic cross-section a basement block uplift is clearly distinguished along fractures. The overlying sedimentary layers were affected by predominantly plastic deformations, forming an antielinal structure. A maximal value of magnetic anomaly is attributed to a near-arch portion of the anticline. According to calculations, the upper margin of rocks with increased magnetic susceptibility, that can be caused by a large intrusion, occurs at a depth of less than 4.0 km. The cross-section

in Fig. 7 is an exception

because

more often

magnetic

anomalies are not observed in areas of basement uplift. If the basement uplift and the anticline formation are caused by the hypothesized intrusion, then the active phase of the process is dated as the end of the Tertiary, since all the sedimentary layers of this age have been deformed. But in many cases, large anticlinal zones in Sakhalin developed during a longer period of time. The decrease in thickness of the Neogene deposits within the anticline arches as compared to syncline troughs testifies to this fact (Alexeichik et al., 1963). Physical properties of the basement rocks are variable. After subtraction of the gravity effect of Tertiary sediments in North Sakhalin, the residual gravity anomalies can be explained by deficit of mass of the basement rocks corresponding to the uplifts (Sychev, 1966). This conclusion is to a certain degree consistent with seismic data. Namely, the crustal average velocities at depths of 15-20 km beneath the Susunai Ridge in South Sakhalin are lower than beneath the neighbouring trough of the same name (Bikkenina et al., 1976). Crustal inhomogeneities are especially clearly distinguished by magnetotelluric

293

m

t Fig. 7. A seismic cross-section

of an anticlinal

given in the upper part of the Figure in gammas

structure

in Northern

Sakhaiin.

Magnetic

anomaiy

AT is

(v).

sounding data in Sakhalin, where observations were carried out at more than 350 points (Alperovich et al., 1979,198O). The periods for measurements were within the range of 0.1-1500 s. Minima of resistivity are found along two long longitudinal (north-south) zones (Fig. 8) in the period intervals from 40 to 50 s. Results of mathematical modelling have shown that these minima of resistivity reflect the zones of higher electric conductivity within the crust at depths of lo-15 km. Their nature is supposed

to be associated

with temperature

increase

at those depths and possible

partial melting or with the presence of highly mineralized solutions (Alperovich et al., 1979). The eastern zone is well correlated with neo-tectonic uplifts, which coincide with anticlinal structures in the east of Sakhalin and the western zone is traced along the eastern part of the West-Sakhalin neo-tectonic uplift. A zone of higher conductivity is also revealed within the mid-volcanic belt of Kamchatka at depths of lo-15 km (Moroz and Pospeev, 1975; Moroz, 1976). It should be added that in Northern Sakhalin the data from magnetotelluric sounding carried out within the period range of lo’-lo4 s allow one to distinguish a highly conductive layer within the upper mantle, the resistivity minima of which is at a depth of about 80-90 km (Vanyan et al., 1983). As compared to the geoelectric section in stable geotectonic zones, the resistivity in the above anomalous zones of the crust and upper mantle of Sakhalin is 2-3 orders lower, being approximately 15-20 Om.

V ‘A Fig.

8.

1979).

PEROUZE

STRAIT

Location of anomalously I = anomalously

conducting zones in Sakhalin at depths of IO-15

km (Alperovich

et al..

conducting zones.

Lastly, great heterogeneities

are revealed within the transition

zone from the crust

to the upper mantle. Deep seismic sounding in Kamchatka (Anosov et al., 1978) revealed a crustal lens-like layer of decreased velocity (VP = 7.5 km/s) with maximal thickness of about 15 km, occurring beneath the East-Kamchatka anticlinorium

295

within a depth range of about 30-45

km. A layer of increased

electric conductivity

observed

all places in Kamchatka

(Moroz

at these depths

1975). There are reasons

in almost to suppose

that the transition

is

and Pospeev,

from the crust to the mantle

has a similar structure in many other regions too. For example, taking the analysis of converted waves from the earthquakes as a basis, it is noted that the transition from the crust

to the upper

mantle

in Kamchatka

as well as in the Kuril

Islands

and

Sakhalin is expressed in the form of a transition zone with a thickness of about 10 km (Bulin, 1976). In general, it should be noted that in most tectonically active and volcanic regions, the upper mantle shows anomalous features to depths of 150-200 km (Sychev, 1979). WAVELENGTH

OF VERTICAL

MOVEMENTS

AND CRUSTAL

STRUCTURES

In many cases vertical movements of the surface and formation of anticlinal structures can be considered a result of crustal and upper mantle vertical displacements. Similarly, the formation of structures with relatively small cross-sections is often caused by vertical displacements of the basement (Figs. 6 and 7). Vertical movements during volcanic eruptions are caused by hydrostatic pressure change (or equally by change in volume) within a magmatic source. If the shape of the source is spherical, it is possible to calculate the depths of its center (Mogi, 1958; Eaton, 1962). If the formation of anticlinal structures is considered as the combined result of deformation of crustal upper layers and the corresponding surface displacements caused by hydrostatic pressure change, one can, according to cross-sections, proximately estimate the depths of the sources of structure-forming forces.

ap-

The wavelength of the main Sakhalin anticlinal zones (see Fig. 5) is about 60 km. As these zones are characterized by a clearly pronounced linearity, it is reasonable to represent the dilatational sources not in the form of spheres, but in the form of horizontal cylinders. The calculations performed show that the depths of the axes of the horizontal cylinders are about 13-15 km for this wavelength (Sychev, 1979). The wavelength of anticlinal structures and vertical movements varies widely, however. For instance, in Japan (Mizoue, 1967) vertical movements with wavelengths less than 20 (4-20), 20-100 and more than 100 km (100-200 km and more) were distinguished. The same subdivision can be made in other regions also. Structures with short wavelength (4-20 km) reflect small depths of structure-forming forces (approximately l-4 km). The second group of structures is caused by processes occurring at depths of 5-20 km; and the third by those occurring at depths of 30-40 km and more. Not only structures of small cross-section, but also vertical movements caused by volcanic activity can be referred to the first group. For a wavelength range of 20-100 km, 60-70 km is the most characteristic value of the region that corresponds to cross-sections of anticlinal zones. General uplift of Sakhalin in the Quaternary may belong to the third group mentioned above. Within the movements for the last

296

70-year period, there is a distinguished uplift in the central high-mountain area of Honshu, the northern part of which corresponds to a volcanic chain, while subsidence is revealed in the eastern coast of Hokkaido and Northern Honshu (Dambara, 1975). Generally, vertical movements with different wavelengths are superimposed. PROBABLE

CAUSES

OF VERTICAL

MOVEMENTS

The above data allow us to summarize the main features of intra-plate movements (with wavelengths up to 100 km) and associated events:

vertical

(1) Not only rapid but also secular modern movements have a pulse character. (2) Despite the pulse character of modern secular movements, there exist zones of predominant uplifts and subsidences which often spatially coincide with neo-tectonic uplifts and troughs. The latter, in their turn, usually correspond to anticlinal and synclinal tectonic dislocations of the same order, i.e. modern movements inherit recent and older movements connected with the formation of tectonic structures. (3) Change of gravity anomalies during vertical displacements seems to be similar for all types of movements and caused by additional mass supply. (4) Zones of deficit of mass, decrease of P-wave velocities or increase of crustal electric conductivity, especially at depths of lo-15 km, correspond to the crustal uplifts in some cases. Besides, there are also reasons to suppose that in some regions the M-discontinuity is a rather thick transition zone (up to 15 km) and the upper mantle has low seismic wave velocities. However, the correlation of the upper-mantle, low-velocity zones with crustal uplifts is not really simple. There are regions with pronounced low-velocity zones in the upper mantle, showing predominant crustal subsidence, as is observed in northwestern Honshu (Sugi et al., 1983). (5) Sources causing vertical displacements of the earth’s surface (assuming that they are caused by change of hydrostatic pressure) are probably within the crust. For anticlinal zones with wavelengths of 60-70 km, their depth does not appear to exceed 13-15 km. Items 1, 2 and 3 suggest that there is no basic difference in their accompanying phenomena between rapid and secular movements, as was also observed earlier (e.g., Nikolaev, 1975). Based on the above data, the authors consider that there is no basic difference between the causes of those movements either. That is, similarly to the case of rapid movements caused by volcanism, the cases of slow (secular) movements, as well as folding, are mainly related to magmatism in one way or another. This fact is obvious during volcanic activity, but crustal magmatism is latent and unobservable during slow movements. Item 4 shows that there are specific changes of crustal and upper-mantle properties beneath uplifts which directly or indirectly testify to magmatic activity taking place in one form or another. This process will be discussed later. Now we only note that the good coincidence of depths of the crustal electrically conductive layers with estimated source depths causing formation of vast anticlinal zones is of great

297

interest,

giving additional

arguments

in favour of the magmatic

tions. The wide spectrum

of vertical

movements

the depths of formation the level of lo-15 Vertical these depths

nature

of deforma-

forms indicates

sources can be quite different,

that

although

km seems to be the main one.

movements

reflect processes

of crustal magmatic

and structural

with wavelengths

at depths

of 30-40

of more than

100 km (200-300

km and more. There

and the zones of low velocities

is a correlation

and high electric

conductivity

km) can between which,

probably, are zones of partial melting (Sychev, 1979). Thus, the long-wave crustal movements may also be caused by magmatic activity that takes place at greater depths. So, taken together, all available data, in our view, show that vertical movements and structure formation reflect the crustal and upper mantle magmatic activity in its various forms. MAGMA

Within formation

MOVEMENT

MECHANISM

the crust and upper mantle the magmatic process responsible for the of anticlines may be represented by the following scheme (Fig. 9).

Fig. 9. Approximate

scheme of magmatic process within the crust. I = sediments,

2 = partial melting

zones within the upper mantle and during short periods of time within the crust, 3 = zones of rapid magma crystallization

(see explanation

in the text).

29X

As it is known, one of the sharpest Mohorovicic discontinuity, where density

boundaries of physical properties is the changes are equal to 0.3-0.4 g/cm. In a

case when rising melts are very close to ultrabasic into

the crust

horizontally,

is impossible

intruding

because

the surrounding

nism (Peck, 1968). This means

of high

in composition,

density

rocks according

that magma

will intrude

and

they

their intrusion will

to a hydro-rupture

distribute mecha-

along a system of interre-

lated, predominantly horizontal fractures forming sill-like zones of partial melting. Formation of such zones will cause general crustal uplift. This is an explanation for vertical movements with wavelengths of more than 100 km. Interaction of high-temperature magma with rocks and (or) continued differentiation of melts will result in fusion of more acid rocks. It is easy to show that even if the fluid phase has a comparatively small volume of, for example, basalts and is distributed within a system of fractures, excess pressure rises sufficiently for the formation of tension fractures within the overlying layers and the secondary melts rise into the earth’s crust along them. The density of the lower crust being equal to 2.9-3.0 g/cm’ and that of basaltic magma 2.6-2.7 g/cm3, the density difference will be not less than 0.2 g/cm3. Then, if the thickness of a layer containing light fractions is 2-5 km, excess pressures will be equal to AP = gApH, gravity; Ap is the density

i.e. 4410.10” N/m2 difference; H-layer

(where R is the acceleration of thickness). The tensional stresses

generated will be approximately of the same magnitude. Although estimations of the real rupture strength of crustal rocks are not exactly known, their approximate limits can be assumed to be 2-5 . 10h N/m* (Magnitsky, 1965). Hence, it follows that with a layer thickness of 2-5 km the excess stresses are sufficient for the formation of rupture fractures along which secondary differentiates will rise up to levels where their density will be equal to the density of the surrounding rocks. Of course, the above process is complicated by many factors, such as separation of volatile components as magma rises and pressure decreases, etc. But first of all this should be a multi-action process: periods of intrusion of separate portions of secondary differentiates into the crust will be replaced by periods of their accumulation within the partial melting layer, i.e., the process will be of an intermittent character. Due to the intermittent rise of secondary differentiates into the crust, correlation of vertical positive movements with anticlinal structural forms will be broken for some periods of time. The average amplitude of vertical movements per year allows us to estimate the voiume of material intruded into the crust. If the secondary source is in the form of a sphere at a depth of 10 km, and the average rate of vertical positive movements is I-10 mm/yr, the rate of material supply will be 0.1-1.0 km/yr. From the above process one can suppose that the shorter the wavelength of vertical movements, the shorter and clearer the pulsation periods are. The formation of secondary crustal sources at depths of lo-15 km is probably accompanied by relatively rapid magma crystallization and separation, in particular, of mineralized

299

water. However, and

other

temperatures. confirm

effects

under

The available

that long-wave

DISCUSSION

the conditions

will not

of the lower crust and upper mantle,

be so rapidly

variable

data for Japan (Mizoue,

movements

due

to higher

1967; Dambara,

thermal

pressures

and

1975) seem to

are more stable in sign than short-wave

ones.

OF THE RESULTS

Ideas about the leading role of magmatism in the process of crustal deformations are not new. Possible connections between surface displacements and earthquakes with magma movements were partly noted for Japan (e.g. Inoue, 1960). The large anticlinal structures in Sakhalin (Sychev, 1966) and volcano-tectonic belts in Kamchatka (Svyatlovsky, 1967) were also related to crustal intrusions of magma, but these are not exceptions. Vertical movements and their successive development from the more ancient stages of the geological history were observed in places over the entire eastern part of the Asiatic continent (Denisov, 1965; Nikolaev and Neimark, 1978). As a matter of fact, there is no region within this vast territory which could be considered for some

as a rigid plate. At any rate, the available geological and geophysical data regions of this territory, for instance, the Far East of the U.S.S.R.

(Lishnevsky, 1965; Khudyakov, 1967) and the Kolymo-Chukotsk folded belt (Snyatkov and Snyatkov, 1964) indicate that structural formation can be caused mainly by magma intrusion, either as melts or as ascending movements of intrusive bodies. Recently in the Far East region, dome-circle structures, distributed predominantly along axial zones of mobile belts have been found. The formation of these structures seems to be caused by material movements from the upper mantle forming secondary crustal sources (Glukhovskoy, 1978; Masurenkov and Komkov, 1978; and others). In general, both subcrustal and crustal magmatism seems to be a widespread phenomenon which is confirmed by its role in crustal formation (Smirnov, 1969; Naumov, 1975; Smithson, 1978; and others). If it is assumed to be the main

agent

determining

the development

of near-surface

geological

processes,

“it

opens prospects for construction of a theoretical model providing a satisfactory description of the character and general trend of the evolution of the known forms of tectonic and magmatic activity” (Rozinov and Kolesnikov, 1975, p. 152). As a matter of fact, mantle magmatism seems to determine (Sychev and Sharaskin, 1984; Sychev, 1985).

many forms of deep processes

The above geological and geophysical data, taken together, are difficult to fit with some suppositions as to the nature of vertical movements (horizontal movements of lithospheric plates, convective currents, phase transitions etc.). The available examples of horizontal surface movements and thrusts may also find a reasonable explanation within the limits of the adopted model. The formation of uplifts during magma intrusion implies that there appears to be tension in their arches and compression in their wings. When the intrusion process ceases, the stress sign changes to the opposite one. Besides, the forms of melt intrusion can be varied and

300

secondary inclination forms

sources are most probably represented by dykes. Depending on the of dykes and feeding channels, surface uplifts may have asymmetric

with large

movements

horizontal

accompanying

It should

probably

crust are partially

components volcanic

be expected

molten

of movements

eruptions that

as is often

observed

(e.g., Fiske and Kinoshita,

the secondary

sources

formed

and cause high heat flow at the surface.

for

1969). within

Although

the such

phenomena are not excluded from the above estimations of average velocity of magma supply into the crust, it is more probable that a fluid magmatic phase exists for a very short period of time. As magma rises discontinuously. it is rapidly crystallized with water discharge in places of the formation of secondary sources, due to the relatively low temperatures of the environment. Fracture zones formed during this process and filled with mineralized water can be recognized as layers of low velocities and high electric conductivity. From the estimates (e.g. Rikitake, 1959; Eremin, 1982) intrusions at depths of more than 5 km, having volume of less than 1 km, will not essentially affect the surface heat flow. Besides, as was noted above, the relief-forming movements in the region under consideration were activated recently, i.e. in the Quaternary. This accounts for the normal heat flow in most of the regions in Sakhalin, for instance. At the same time, the existence of hot springs and especially the ratio of helium isotopes show that in Sakhalin, as well as in Kamchatka and the Kuril islands, heat-mass flux (“probable impulse”) from the mantle does exist and is compared

in value with other tectonically

active regions

(Polyak

et al.,

1979). Crustal magmatic processes are of course more compfex and varied than the schematic mechanism described above. In particular, in final stages, gradual up-flow of more acid and viscous magmas will take place. Magmatic activity in almost ail its forms will be accompanied by formation of stresses and earthquakes. Xntra-plate earthquakes can be caused by various factors: formation of tension fractures during upward movement of magmas by excess pressure (Robson et al., 1968) or formation of dykes and silts (earthquake swarms) (Stuart and Johnston, 1975); rapid formation of secondary sources, resulting in volume expansion and displacement of rocks along fractures

(Takagi,

1972; Hayakawa

and

Iizuka,

1976), and

so on. Although

this

question is to be discussed separately, it should be noted that magmatic activity and associated thermal effects (including compression) can explain many features of intra-plate earthquakes (e.g. Mogi, 1963; Matuzawa, 1964; Balakina and Golubeva, 1979). We have discussed above mainly movements in areas of predominant uplifts. However, formation of large or small uplifts within plates is accompanied by downwarping of neighbouring areas. This observation is to a large extent explained by the fact that rise of rock mass or melted magma is accompanied by subsidence of an appro~mately equaf volume of material (Ramberg, 1970) The predominance of uplifting regions over troughs, however, cannot be explained by such a simple model.

301 CONCLUSION

Recent systems

and

of Eastern

attributed ated

modern

movements,

Asia,

to some definite

events

and

as well as structure

are characterized linear

structural

by similar

zones. Similarity

features

of the crust

formation regularities

of crustal and

in the island and

are often

deformations,

upper

mantle

associ-

allow

us to

conclude that at least some of the vertical movements are caused by crustal and subcrustal magmatic activity. The latter may be expressed as a step-like and discontinuous rise of magmatic melts, with formation of partial melting zones in the crustal base and upper mantle. Final products of differentiation of melts reach the middle and upper crust, where magma rapidly crystallizes. Intrusion of melts is accompanied by hydromechanical effects on enclosing rocks, causing deformations of overlying layers. The complex character of observed vertical movements is determined by the combined effect of several deformation sources distributed within both the crust and upper mantle. But the predominent influence seems to be caused by those at two levels located approximately at depths of lo-15 km and near the base of the crust. The proposed mechanism of magmatic processes also some of the horizontal displacements of the portantly, it explains their discontinuity as caused by melt rise. Crustal and subcrustal magmatism as a

explains not only vertical but earth’s surface and, more imthe irregular pulse character of cause of surface deformations

seems to be widespread and is not the peculiarity of the region considered. At the same time, it should be noted that the relation of crustal deformations to magmatism does not always manifest itself clearly. In fact, this relation seems to take various forms and, therefore, the problem needs further study. ACKNOWLEDGEMENTS

We are very greatful to Prof. S. Uyeda for critical reading of our manuscript. We also acknowledge Mr. M.S. Fedorishin and Mrs. Z.S. Polyakova for the translation of the paper from Russian into English and Mrs. T.F. Eliseeva for her very useful assistance. REFERENCES Alexandrov,

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