The structure and tectonics of the intraplate deformation area in the Indian Ocean

Tectonophysics, 156 (1988) 89-106 Elsevier Science Publishers

The structure

YURI

89

B.V.. Amsterdam

- Printed

in The Netherlands

and tectonics of the intraplate in the Indian Ocean

P. NEPROCHNOV,

OLEG

V. LEVCHENKO,

and VLADIMIR

deformation

LEV

area

R. MERKLIN

V. SEDOV

P.P. Shirshov Institute of Oceanology, Moscow 117218 (U.S.S.R.) (Received

February

2, 1987; revised version accepted

April 6, 1988)

Abstract Neprochnov,

Y.P., Levchenko,

deformation Intense

tectonic

formed

of sediments

reflection

by the P.P. Shirshov

NE-trending

area

profiles

Institute

of crustal

and basement

from

unusual

the northern

of Oceanology

of these deformations.

by a mosaic

L.R. and Sedov, V.V., 1988. The structure

and tectonics

of the intraplate

Tectonophysics, 156:89-106

Ocean.

deformations

can be seen on seismic collected

O.V., Merklin,

area in the Indian

for the interior

Central

Indian

of the USSR

The intraplate

Academy

deformation

area

blocks which have been severely deformed

of the oceanic

Basin.

lithosphere

plates

lO,OOO-mile long CSP profiles

of Sciences

allow

has a complicated

or tilted alternating

delineation

tectonic

of a

framework,

with less deformed

parts

of the sea floor. The results of a detailed two genetic indicate Ocean

types:

an anomalous Bottom

CSP grid survey reveal that these uplifted

old nearly

meridional

structure

Seismographs

fracture

of the crust

have proved

zones, and young

and upper

mantle

faulted

within

that there is high-level

blocks are bounded

NE-striking

faults.

these blocks.

intraplate

by tectonic

Seismological

seismicity

faults of

The seismic refraction

results

observations

in the northern

from

Central

Indian

Basin. The intraplate

deformation

area is supposed

to correspond

result of the stress difference

in the Indo-Australian

Asia

in the Central

along

continental

with

part continuously plate, NE-SW the sediments the system near-latitudinal deformation

spreading

collision

led to an increase

subducted trending

beneath

shearing

and basement, of ancient scarps

failures

spreading

and

normal

In the complicated

on seismic reflection

in the

zone of shearing collision

subduction

stress in the northernmost

apparently

to the mid-oceanic

area might be partly

Ridge

the Sunda Trench.

observed

parallel

Indian

in compressional

stress abated,

to a large-scale

plate due to the continued

oceanic ridge

profiles. crust

NE-SW

in the Sunda

as a

of India and

Island

zone between

Arc.

This

these parts of the

as a result of folding

trending

(near-meridional

axis). The mosaic

that formed

part of the plate, while its southern

transitional

in the Late Miocene,

strains

of the continents

fault-fold

wrench-fault transform

and faulting

tectonics

faults

framework

and,

of

affected perhaps,

of the intraplate

a result of this interaction.

Introduction

are normally recorded only at their boundaries, while the Central Indian Basin is located within

High present-day seismicity, abnormally high heat flow, intense tectonic deformations of sediments and basement, as well as other unusual geophysical characteristics (see e.g., Weissel et al., 1980) make the Central Indian Basin unique among oceanic basins. These phenomena are not typical of the interiors of lithospheric plates and

the one-piece Indo-Australian plate (see, e.g., Sclater and Fisher, 1974; Zonenshain and Savostin, 1979). These seemingly contradictory facts aronsed our interest in the area. Until recently, the tectonic aspects of the peculiarity of the Central Indian Basin were mainly based on studies of the high intraplate seismicity.

004O-1951/88/$03.50

0 1988 Elsevier

Science Publishers

B.V.

90

Sykes (1970) proposed a convergent plate boundary and a nascent island arc between Sri Lanka and the Cocos Islands. Later, a boundary consisting of left-lateral strike-slip motion between the Indian and Australian plates along the

northern Ninetyeast Ridge was suggested (Stein and Okal, 1978). At present, there are more than ten different explanations of the nature of the largest free-air gravity low and the most prominent low of the total gravimetric geoid in the

/ \ 2c I

-

i \

1cI-

c)-

/

/ 1c)-‘

< .~ 1

./ ‘

2c I-

I,/\ ,_ 70

80

Fig. 1. Continuous single-channel

J

90

1Of3

seismic reflection profiles carried out by the P.P. Shirshov Institute of Oceanology of the USSR

Academy of Sciences in the Eastern Indian Ocean. I = profiles and detailed survey areas of R/V “Dmitry (heavy lines AA’ and BB’

show the location of the profiles from Fig. 4); 2 = the same for 1976-1981 4 = isobaths in km.

Mendeleev” cruise 31

cruises;

3 = DSDP

sites;

91

Indian

Ocean south of Sri Lanka (Chekunov

1984)

none

accepted.

of

which

The mechanism

heat flow recorded spheric

seems

to

be

and source

generally

of the high

there, which is related

deformation,

to litho-

unclear

(Geller

et al., 1983). This paper deals with intense

tectonic

deformation

of sediments

are pronounced which

also remain

et al.,

and

basement,

on seismic reflection

are the most conspicuous

which

profiles

anomalous

and phe-

nomena. The mid-plate

deformation

of sediments

and

basement in the Central Indian Basin were first reported by Eittreim and Ewing (1972) who suggested that their existence was the result creasing compressional stress in the interior

of inof the

Indo-Australian plate due to the collision between the Indian subcontinent and Asia. That hypothesis still holds true. The beginning of these deformations is believed to be connected with the Miocene Himalayan erogenic phase of this collision (Weisse1 et al., 1980). The first investigations of the intraplate deformations in the Central Indian Basin were carried out by Soviet scientists of the P.P. Shirshov Institute of Oceanology in 1976 on the R/V “Vityaz” cruise 58 (Neprochnov et al., 1979) and continued in 1980 on the R/V ‘Dmitry Mendeleev” cruise 25 and in 1981 on the R/V “Academician Kurchatov” cruise 32 (Levchenko et al., 1985). In 1984 on the R/V “Dmitry Mendeleev” cruise 31, investigations connected with these intraplate deformations out, including two highly detailed physical

were carried geological-geo-

surveys in the areas shown in Fig. 1. New

important data on the structure and seismicity of the region were obtained. We hope that the seismic reflection profiling data collected during our cruises and recent research into the tectonic framework of the intraplate deformation area will throw some light on the problem Results of continuous

of its genesis.

seismic profiling of the re-

(Weissel

et al., 1980)

to delineate

the unique

there (Levchenko (azimuth

900-1000

and

deformation

62-75”)

1600

km

are located

between

1’s

4 o N and 4 o S of the western

eastern

(near

long

and

to 89“ E (Fig. and

9’S

and

(near 76 o E)

89” E) sides of the delineated

intraplate

deformation

framework

of this area is inhomogeneous

characterized

area

were traced in a NE-trending

km wide from 77”30’E

2). They between

intraplate

et al., 1985).

The deformations band

they have made it possible

area

by extremely

respectively.

deformed

The and is

crustal blocks

that are conspicuous against the weaker and more monotonous deformations of sediments and basement.

About

15 of these large tectonized

blocks,

which are either symmetrically upwarped (Fig. 3) or asymmetrically tilted (Fig. 4), have been identified up to now. They appear in the cross section as gently sloping anticlinal basement rises from 50 to 200-300 km wide and l-2 km high, corresponding conformably to sea-floor swells from 100 to 600 m high. On the rises, the l-2 km thick sediment cover is crumpled into asymmetric folds (mostly with high-angle sloping southern limbs) up to several hundred meters high. The sediments and basement between the tectonized fault blocks are less deformed and are unconformably by the undisturbed sediments, 100-400 which form the flat basin’s

bottom.

overlaid m thick,

In the north-

ern part of the area, some of the fault blocks totally buried km thick.

under

flat-lying

sediments

are

up to 0.8

Four regional seismic reflection profiles obtained on the R/V “Dmitry Mendeleev” cruise 31 have supplemented the grid of profiles covering the intraplate deformation area in the Central Indian Basin (Fig. 1). The appearance of the fault-fold deformations on these profiles coincides with the predicted boundaries intraplate deformation area

of the identified (Fig. 2) but the

gion

seismic reflection records reveal that there is a great variety in the fault blocks and that the area

The seismic reflection data we collected during the 1976-1981 cruises have given us a clear picture of the morphology of the deformations in sediments and basement in the Central Indian Basin. Together with the published American data

has a more complex tectonic framework than we believed earlier (Levchenko et al., 1985). Two large tilted fault blocks, 100 and 200 km long, were traversed with seismic reflection profiles near 2.5”S, 81” E and 1” N, 83”E respectively (Fig. 4). In contrast to most of the folds already studied,

92

a0

Fig. 2. Tectonic

scheme

of the northern

I = continuous

single-channel

detailed

carried

survey

Oceanology

of profiles

Soviet

and American

2 = DSDP fault

expeditions

4 = intensely

is shown

folds;

6 = monotonous

deformations

7 = block-ridges:

CR -Comorin

Ridge,

A.N.-Afanazy

earthquake (after the

epicenters;

Geller 31 cruise

Nikitin

of the

Ridge,

9 = abnormally R/V

transform

0E

8 = intraplate

high heat

“Dmitry

and base-

EFER -85

group.

indexes

crustal

asymmetric

of sediments

seamount

et al., 1983), numerical

and

lines.

deforma-

individual

upthrusts

ment.

of

out by other

by dashed

in km. The intraplate

spaced

and

Institute

and BB’ show the

folded and fractured

5 = closely

Basin.

profiling

from Fig. 4); the same carried

sites, 3 = Isobaths

blocks;

Indian

out by the P.P. Shirshov

are shown by solid lines (AA’

location

tion area:

Central

seismic reflection

flow points

are values

Mendeleev”;

from

10 = old

faults.

the asymmetrical folds here are mainly southward-verging with the rises’ northern slopes more intensely folded and fractured. The anticli-

9':~ , - 9 8

A

0°

0.5 °

I

1°

I

1 ,S °

I

2.5 °

I

3°

35 °

I

...'C~'%.... -

7 . . . .

.

8~

,

:?:-?,

/

~

-~

c+$s,

~1~-. ~

' . ~

2"

:

.]'~

'~-

"

':,

>~"i ',<

V E = 20.1

# o

0.5°S

0

0.5 ° I

1 ,S °

2°N

150 °

1

I

7

?V' A : I: :! ': ~,- kk ,:

:

. "

,< -, >, ~;

,7;

7:;?

:?,;7; ]" =.

sec

:i[-X-~

~, .'" .~-

6

9

~,. b ~ Z ~ ~

i.

I

'- 7 : "

Y

. . . . .

(:1

B

~

<':,[

j'

9sec

,~,~.~-~:c.~,:~

",:3

"

.

~i'

b

Fig. 4 Regional si~gle-cbarmel seisr~fic reflection profiIes carried ou~ o~ R / V " D m i t r ' v Me~deleev '' cruise 31 w i t h i a d e f o r m a t i o n area. a. Profile AA'_ b, Profile B B ' m Figs. 1 and 2.

the i n t r a p t a t e

+

V.E.=

20.1

B

99

nal arches of these basement by local depressions formed Figure

4a shows a deep graben-like

where the sediments’ compared floor.

rises are complicated by step faults.

thickness

One can see that

block

side of the depression is similarly fault-fold basement rises undoubtedly

stress;

mainly

under

(2) shearing

by tension

conditions strain

and compression

of

is always strains.

2.5 km as

parts of the sea

the fault

form

accompanied

depression

reaches

with 1.5 km in adjacent

wrench-faults compressional

on each

Results of the detailed survey

tilted. These formed as a

R/V

“Dmitry

Mendeleev”

cruise

31 continu-

result of compressional stress. It appears, however, that these two tilted fault blocks are characterized

ous seismic reflection profiling in the survey area 2 X 2.5 o near 4”s and 80”E (Figs. 1 and 2)

by counter-vergence

which

the depression

of tectonic

itself should

deformations,

be regarded

and

was covered

in great

with the most important

as a ten-

sion pattern. The available seismological data offer evidence of the existence of near-meridional compressional stress in the northern Central Indian Basin (Fitch et al., 1973; Stein and Okal, 1978); it is reasonable to explain the observed tectonic fabric as being due to shearing strain. That supposition agrees with the results of tectonophysical analysis of the mechanism for wrench-faults generation (Sherman, 1981): (1) in the lithosphere,

detail,

provided

us

data on the structure

of

the sediments and the relief of the consolidated crust within the previously identified large upwarped fault block (Fig. 3). This feature is a broad basement rise more than 1 km high. The sediment cover overlying the basement feature is l-l.2 km thick and is completely deformed and uplifted above the flat basin’s bottom, forming a swell 500 m high. The crustal block is bounded by nearmeridional fracture zones and NE-striking faults.

old transform fault N

180" d

E

90" c

25 km Fig. 5. Single-channel

seismic reflection fracture

profile

from R/V

zone. The location

“Academician

VF=lhl Kurchatov”

cruise

32 illustrating

is shown in Figs. 6 and 7 by the heavy line b.

an old near-meridional

102

The seismic reflection

configuration

fill suggests that the near-meridional are very old. The pattern tween

reflectors

within

of the onlap fracture

zones

of the relationship

the sedimentary

be-

sequence

elements.

is bounded zones

ional

orientation

ments

On

en-echelon

other

hand,

of these

the NE-striking

lent correlation

between

faults.

faults

display

displacement

the

excel-

in the base-

ment and the whole sedimentary sequence (Figs. 3 and 4). These faults are marked by a series of near-latitudinal high-angle fault scarps of the basement’s surface up’to 500 m high on the northern and southern flanks of the rise. The asymmetric folds within the sedimentary cover correspond to these high-angle faults which are considered to be reverse faults (Eittreim

and Ewing, 1972; Weis-

that

old fracture

(63-74”).

origin

block

in size and

faults:

for the

pre-sedimentary

fault

100 X 140 km

clearly

evidence

upwarped

faults.

was

in detail has the form of a parallelogram

parallel

and the buried basement scarps 500-700 m high along which the basement’s surface is displaced, seen in Fig. 5, provides

The

surveyed

and

young

The thrusts system

along

The morphological

tectonized

fault blocks

seismic reflection

NE-striking

and folds within

are of near-latitudinal

strike

and

faults the sediform an

the major

NE-striking

similarity

between

as revealed

profiles

by near

of near-merid-

the

by the regional

allows us to extrapolate

the survey results from the area covered in great detail to the whole of the intraplate deformation area. Although we do not have enough seismic reflection profiles to draw up an exact tectonic scheme of the region, it is reasonable to suppose that its interior is controlled by the afore-mentioned

faults, and that the whole intraplate

defor-

se1 et al., 1980; Geller et al., 1983). The folds are southward-verging on the northern flank and

mation area contains similar tectonized blocks. These fault blocks are sporadically distributed,

northward-verging on the southern one. They are lo-20 km long and 5-10 km wide. The intensity of the deformations decreases away from the NEstriking faults towards both the rise crest and the basin: the folds there are smaller-5-10 km long and 225 km wide-and the amplitude of the

separated by less deformed parts of the sea floor, suggesting a fault-fold mosaic framework for this part of the Indian Ocean (Fig. 2).

thrusts falls to 100 m. The form and height (100-300 m) of each fold is constant throughout the whole cross section of the deformed part of the sedimentary

cover.

Piston cores obtained on R/V “Dmitry Mendeleev” cruise 31 indicate that the uppermost deformed

sediments

of the swell consist

of slightly

lithified terrigenous pelagic clays interbedded with turbidites. The age of these is Late Miocene (5-5.5 m.y.B.P.) as dated by Radiolaria (E.M. Emelyanov and S.B. Kruglikova, pers. commun., 1984). This age is the same as that given by Weissel et al. (1980) and indicates the time post-sedimentary deformation.

of the intraplate Consedimentary

folding in the upper part of the sediments at the foot of the rise suggests that the process of folding may have continued for some time. The basement surface relief (Fig. 6) and sediment thickness (Fig. 7) of an upwarped fault block were mapped. The maps cover a section 3 X 3.5” in size in the vicinity of the area surveyed in detail. Examination of these maps indicates the strike of the main tectonic

Seismic refraction studies and observation earthquakes by Ocean Bottom Seismograph The crustal structure using an Ocean Bottom

of

of the region was studied Seismograph (OBS) at two

sites during R/V “Dmitry Mendeleev” cruise 31. Seismological observations were carried out simultaneously with the same OBS array. According the structure Indian Basin

to previous seismic refraction data, of the Earth’s crust in the Central is a typical three-layered one (Fig.

8a) (Neprochnov et al., 1981). Sediments in its northern part are up to 2.7 km thick. The average thickness of the second layer, with a seismic velocity of 4.4-5.1 the oceanic

km/s,

is 1.5 km. The third layer of

crust, with a seismic velocity of 6.2-6.8

km/s, is 3.5-6.7 km thick. Average thickness of the crust in the basin is approximately 6 km. New seismic refraction data have revealed anomalous features in the crustal structure of the Central Indian Basin (Neprochnov et al., 1986). According to these new data, the weakly deformed sections of the basin with a smooth sea floor are characterized by a typical oceanic crustal structure

103 c

b

The northern belongs

to

seismicity

part of the Central

an

(Sykes,

of M 2 4.5 and

there

observations

seismic

(Neprochnov formed and

of the Earth’s

refraction et al., 1981).

b. Model

area (80 o E and 3.5’S).

fractured

sediments;

anticlinal

basement

2 = the second

unit;

upper mantle;

crust

and upper a.

mantle

Average

model

for monotonously

c. Model

for intensely

defolded

rise (80 o E and 4” S). I = 3 = the third unit;

5 = unconsolidated

were

during

10 days of continuous

ing (Neprochnov

measurements.

4 = normal

activity

to other ocean basins.

earthquakes

The

1907

in this region More than 100

registered

at two

seismological

collision

of the

has been

proposed

continents

of Asia

in the literature

formations

has not

been

sufficiently

to develop

this reasoning

(sediments), 5.6 km/s and 6.6 km/s, and thicknesses of 1.6 km, 1.6 km and 4.6 km, respectively.

possible

mechanism

for the origination

traplate

deformations.

tinuity

is 8.2 km/s.

discon-

The whole crust in these parts

by the continental

We suggested

studied;

it

they may have recompressive stress

tempted

of the MohoroviEiC

and

as the

cause of the intraplate deformations of sediments and basement in the northern Central Indian Basin (Eittreim and Ewing, 1972; Weissel et al., 1980). However, the mechanism that generates these de-

caused

velocity

record-

tectonic evolution

(Fig. 8b). It is made up of three layers which are characterized by seismic velocities of 1.8-2.0 km/s

The seismic

sites

et al., 1986).

has only been suggested that sulted from the N-S horizontal

upper mantle.

1983.

with the OBS indicate

high seismic

small

India from

et al., 1973; Stein

between

registered

Discussion:

Fig. 8. Deep structure

Fitch

Basin

intraplate

were

as compared

I

1970;

Indian

anomalous

40 earthquakes

Seismological

82

of

and Okal, 1978). About

extraordinarily

E3 I

area

collision.

on the grounds

We have atand propose

a

of the in-

of the results

of

the first stage of our investigations (1976-1981) that the recent deformations in the oceanic crust

of the basin is about 8 km thick. The structure of the crust and upper mantle within the folded and fractured basement rise (Fig. 3) exhibits anoma-

in the northern Central Indian Basin might have been caused by a set of left-lateral strike-slip

lous features:

faults

the layers with seismic

velocities

of

of

NE-SW

orientation

2.0, 6.1, 6.9 and 7.6 km/s are 1.1, 1.3, 4.2 and 6.6 km thick, respectively (Fig. 8c), where 6.6 km is

north

the thickness

al., 1985). The strike

(unconsolidated

of the crust up to the discontinuity upper

mantle

(?)).

The generalized seismic profile between sites 215 and 218 near 2-2.5”N displays two seismic columns with similar results (Curray et al., 1982). The surface of the layer characterized by seismic velocities of 7.7 and 7.6 km/s was found at a depth of 7-9 km below the sea-floor surface. Above it, layers with velocities of 7.0 and 6.9 km/s are 2.5 and 4 km thick, respectively. The anomalous seismic refraction data suggest that intraplate deformations are not confined to the crust but extend to greater depths.

themselves

as a large

of the Indo-Australian

deformation

area

that

wrench-fault

manifested zone

in the

plate (Levchenko

of the identified

approximately

et

intraplate

coincided

with

the orientation of transform faults in the Central Indian Ridge (off the west side of the area) and with the orientation of wrench-faults which broke the northern Ninetyeast Ridge into a series of en-echelon blocks (off the east side of the area) (Fig. 9). Thus, it would be reasonable to suggest the presence of large continuous faults that perhaps cross the whole of the Central Indian Basin. None of the observed morphological features of the deformations contradicts the main concepts of wrench-fault tectonics (Moody and Hill, 1956);

104 110

90

70

Central cm/yr

Indian

also

increases

from

3-4

to 6-7.5 cm/yr in the south 1979). We consider that the relative

(Sorokhtin, displacement

20

Ridge

in the north

of the above mentioned

parts of the

plate, probably in the Late Miocene, may have been caused by the difference in the rates and

0

directions

of

boundary

(Himalaya)

northeastern less, we noted istence

plate

boundary

collision and (Sunda

the necessity

of strike-slip

faults

at

its

northern

subduction

at

Trench).

Neverthe-

to check

its

for the ex-

in the Central

Indian

Basin. 20

The results of seismic reflection profiling carried out on R/V “ Dmitry Mendeleev” cruise 31 (1984) have added two further bits of information to our former picture of the interior of the intraplate deformation area in the Central Indian Basin:

40

Fig. 9. Schematic wrench-faulting

diagram

illustrating

in the Central

rates of plate subduction

the tectonic

Indian

Basin.

model of the

Directions

are from Sorokhtin

and

(1979).

maximum-stress direction, strike of the supposed shear planes and second-order strain directions agree with the geometry of a wrench-fault tectonic system. It should be mentioned here that largescale wrench-faults are proposed as a dominant type of failure in the Earth’s crust by us and many other authors (e.g., Sherman, 1981). According to seismological data, the horizontal compressional stress within the Indo-Australian plate

off the Sunda

Island

Arc is much

weaker

(1) Individual crustal fault blocks observed in this area, which appear in the cross section as anticlinal basement rises characterized by an intensely broken surface, are of isometric quadrangular form. (2). In the intraplate deformation area these extremely deformed fault blocks alternate with less deformed parts of the sea floor; this alternation gives the area a distinctive mosaic framework. Both of these conclusions agree with the results of the tectonophysical analysis of the origin of wrench-fault zones based on a viscoelastic model carried out by Bornyakov (1981). The following are some of the typical features of wrench-fault zones revealed as a result of tectonophysical elling: (1). The development of shear

modstrain,

than the stress that is transmitted from the northern convergent plate boundary into the adjacent

accompanied by causes disruption

oceanic regions (Fitch et al., 1973). In the northernmost part of the plate, the compressional stress increased as a result of the continental collision, while its southern part subducted normally beneath the Sunda Trench. The wrenchfaulting in the Central Indian Basin may have been caused by the different resistance of the Himalayan and the Sunda Trench plate boundaries

redundant volume of sediments manifests itself in swelling and the formation of a rise characterized by a complicated morphology of the surface and by an isometric form. (2). A complicated and inhomogeneous differentiated strain field exists in wrench-fault zones. It is this inhomogeneity that

in the presence of continued spreading in the Central Indian Ridge (Fig. 9). The near meridional subduction rate of the Indo-Australian plate in the Himalaya is 4-5 cm/yr, while its NE subduction rate beneath the Sunda Trench is 6.5-7.5 cm/yr; the NE spreading rate in the

Thus, the results of geophysical observations in the field and tectonophysical modelling seem to agree. Here, we would like to refer you once more to the seismic reflection profile in Fig. 4a, showing compression and tension structural features side by side in the intraplate deformation area. We

dilatation and of sediments.

metamorphism, The resultant

brought about the existence of blocks of a different tectonic native in the zone under study.

105

believe

that the observed

tectonic

as the system of near-latitudinal and

asymmetric

have formed

folds

pattern,

in sediments,

the intraplate

to suggest

thrusts

might

only

in the Central

Basin

has been

not simply

sional

stress as such but by the relaxation

of

Indian

by compresof this

However,

we have to admit

are not enough

to prove

phase of wrench-faulting Australian

plate

true because

took

of

This

is particularly trending strike-slip

fracfaults

in the northdata can not

It is still possible framework controlled

suggest that real “fault

numerous as echelon

discrete

segments,

arrays”

(Segall and

to suggest

that

the tectonic

of the northern Central Indian Basin is by tectonic elements of two genetic

types. The old structural pattern seems been affected by the largest near-meridional form faults formed the Late

Cretaceous.

during

crustal

Failures

to have trans-

construction

in and

Therefore,

it is likely

collision

India

then

and Asia

and

that

besides

and

nonbeen

stressed

fault

stress increased

of the continents

relaxed

tectonic

unevenly,

framework.

of thus

This ob-

the environmendeformations.

It seems to be more correct to speak about a zone of shearing strains characterized by a complicated structure

which

NE-SW

trending

system

formed

of ancient

rather

due to the interaction

wrench-fault failures

than just about

We

would

like

tectonics

in the oceanic

a wrench-fault

to note,

of

with

a

crust

zone.

however,

that

it is

premature as yet to make any positive claims about the mechanism which generated these deformations within the Indo-Australian plate.

Acknowledgements

The authors

wish to express

their appreciation

to the scientific staff and crew of the R/V “Dmitry Mendeleev”, who contributed to the successful implementation of the seismic investigations during cruise 31.

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