Tectonic aspects of intraplate seismicity in the northeastern Indian Ocean

?Iectonophys& 1‘70 (1989) 125-139 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

125

Tectonic aspects of intraptate seisrnicity in the northeastern Indian Ocean

OLEG V. LEVCHENKO

P. P. Shirshtw Institute of Oceonoiogv, Moscow if 7218 (U.S.S.R.)

(Received February 2,1987; revised version accepted April 6,1989)

Abstract

Levchenko, O.V., 1989. Tectonic aspects of intraplate seismicity in the northeastern Indian Ocean. Tectonophysics, 170:

125-139.

Since the Late Cenozoic, the northeastern Indian Ocean region of the Ind~Aust~~ plate has been characterized by tectonic activity that is unwual in a mid-plate setting. This activity, including deformation of the basement and the sediments and a high level of seismicity, is typical of plate boundaries. The characteristics of the seismicity and other

phenomena there bears evidence to the initial stage of diffuse, intraplate deformation of oceanic lithosphere. It would be reasonable to suggest that the intraplate deformation in the northeastern Indian Ocean is episodic in behaviour. The previous pulse of deformation seems to have occurred in the Late Miocene, when the recent deformation of the

sediments and the basement observed on the seismic reflection profiles was generated. Modem tectonic activity, manifesting itself in high se&&city, can be related to a new pulse of intraplate deformation.

A near-latitudinal equatorial seismic zone, comprising the northern segment of the Ninetyeast Ridge and northern Central Indian and Cocos Basins, was suggested on the basis of analysis of seismological data for the eastern Indian Ocean from 1907 to 1983. The intraplate earthquake epicenters are concentrated in this broad band, without distinct physiograpbic boundaries, from nearly 80 o E in the Central Indian Basin to the Sunda Trench. Northeastern trends of earthquake epicenters, that seem to be related to reactivation of some old faults, can be observed within this band. The orientation of the intraplate seismic zone is approximately similar to that of the continental collision zone at the northern Indo-Australian plate margin, the Himalaya. The possible cause of this high intraplate seismicity might be the southward migration or “jumping” of the Himalayan subduction zone after the subduction there was stopped by the Indian continental block due to continental collision. The episodic intraplate deformation in the northeastern Indian Ocean region may manifest a future site of subduction along the southern margin of the Indian subcontinent.

Introduction

Present ideas on the evolution of the Indian

Ocean are based on plate-tectonics. The seismic&y

in the oceans is confined to island arcs and mid-

ocean ridges, or to the divergent and convergent

boundaries of the lithosphere plates. Almost all the earthquakes in the Indian Ocean have been recorded in the Sunda Island Arc and the seismic belts associated with the three branches of the

mid-Indian Ridge. Many authors have noted, however, an extraordinary concentration of earth-

WO-1951/89/$03.50 0 1989 Elsevier Science publishers B.V.

quakes within the Indo-Australian plate in the northeastern Indian Ocean, to the south-east of

Sri Lanka (Stover, 1966; Sykes, 1970; Stein and

Okal, 1978; Bergman and Solomon, 1980, 1985).

This high level of intraplate earthquake activity

within the oceanic plate, which should be aseismie

according to classical plate tectonics, has been measured by seismological observations on the ocean floor by means of the Ocean Bottom Seismometers (Sedov and Rykonov, 1981; Nep-

rochnov et al., 1985b, 1986). Sykes (1970) and Stein and Okal (1978) had

126 O.V. LEVCHENKO

made an attempt at considering the “anomalous” high level of intraplate seismicity from a tectonic standpoint. Sykes (1970) considered a NE-SW trending seismic zone parallel to the Sunda Island Arc as a nascent large tectonic feature, most probably a new island arc (subduction zone), between Sri Lanka and Australia, but was confused by the impossibility in explaining the outward migration of the Sunda Island Arc within the current geody- namic model. Stein and Okal (1978) suggested that the earthquakes are concentrated in a broad band along the northern Ninetyeast Ridge. The Ridge was regarded as a complex zone of the intraplate deformation, similar to a transform fault, along which the left-lateral strike-slip motion of the western (Indian) and eastern (Australian) parts of the Indo-Australian plate occurred. The proposed explanation of the Nine- tyeast Ridge’s aseismicity south of 10 “S by the assumption that “most of the intraplate deformation takes place off the ridge” (Stein and Okal, 1978) seems unconvincing because of the following. Firstly, that assumption is not consistent with the correlation of the epicenters; secondly, no distinct intraplate deformation of the sediments and the basement has been recorded and reported south of 10°S off the Ridge. F~the~ore, it is not clear how far south this strike-slip motion extends. Despite interpretational disagreement, the quoted authors agree, that the cause of this “anomalous” intraplate seisrnicity is the NW-SE or N-S compressional stress in the Indo- Australian plate due to the active continental collision.

Besides a high rate of active seismicity, the northeastern Indian Ocean is characterized by other phenomena also unusual for the interior of oceanic lithosphere ,plates. The recent tectonic deformation of the sediments and the basement in the northern Central Indian Basin, as revealed by seismic reflection profiling (Eittreim and Ewing, 1972) is the most conspicuous anomalous phe- nomenon. This intraplate deformation includes long-wavelength undulations in the basement, coinciding with undulations in geoid and gravity free-air anomalies, and reverse faults accompanied by asymmetric folds (Weissel et al., 1980). An abnormally high rate of heat flow in this area

seems to be associated with the intraplate deformation (Geller et al., 1983). The quoted authors (Eittreim and Ewing, 1972; Weissel et al., 1980; Geller et al., 1983) suggest that the deformation was initiated in the Late Miocene from N-S horizontal compressive stress, due to the collision of the Indian subcontinent with Asia. On Leg 116 of the Ocean Drilling Program, it was discovered that the onset of the recent intraplate deformation is about 7 M.y. old (Co&ran et al., 1987).

This deformation of the basement and sediments in the northern Central Indian Basin was traced along a broad band trending nearly W-E (Weissel et al., 1980) or NE-SW (Levchenko et al., 1985; Neprochnov et al., 1985a). Thus, neither the proposed NW-SE trending seismic zone (Sykes, 1970) nor the N-E trending one (Stein and Okal, 1978) correspond to the orientation of this band. This fact is surprising, and needs to be explained, because both the modern seismicity and the recent intraplate deformation of sediments and basement appear to be generated by the same tectonic cause (continental collision along the Himalayas), as has been proposed. This, as well as the contradictions contained in both Sykes’ (1970), and Stein’s and Gkal’s (1978) hypotheses oblige to review the seismological data. The main purpose of this paper is an attempt to understand what tectonic process (or processes) may bear the responsibility for high intraplate seismicity within the oceanic part of the Indo-Australian plate.

The intraplate earthquake epicenters distribution

The spatial distribution of the intraplate earthquakes (M 2 4.5) in the eastern Indian Ocean (from 70” to 120” E and between 20°N and 60”s) was analysed for the 1907-1983 period based on the KS, ISC and NOAA seismological catalogues. Figure 1 illustrates the location of the earthquake epicenters in that part of the Indo- Australian plate. This covers an area of over 25 . 106 km’. More than 350 earthquakes were recorded there, and the distribution of their epicenters is irregular. Reliable statistical analysis of the available seismological data cannot be carried out because of the great sparseness of the intraplate earthquake epicenters and infrequent recur-

TECTONIC ASPECTS OF INTRAPLATE SEISMICITY. NE INDIAN OCEAN 127

Fig. 1. Seismicity in the Ind~A~~~~ plate, earthquake epicenters in the eastern Indian Ocean. Symbols in key: 1-4 = epicenters

of earthquakes with different magnitudes (I --M.7; 2=7>M>,6; 3=6>M>S; 4=5>Mandunknownmagnitude); 5-the

suggested anomalous intraplate seismicity zone; 6 = seismicity of the plate boundaries.

rence of these events. Therefore, the seismicity of the main tectonic features or separate areas of the Indo-Australian plate were compared only ap-

proximately. As it can be seen from Fig. 1, a higher level of

seismicity, as compared with other areas of the plate, is observed in and near Western Australia. However, it is necessary to note that 95% of all the earthquakes have been recorded there after the installation of the World Wide Standardized Seismograph Network (WWSSN) in 1961 and large events (A4 2 6) contributed less than 4%. Two factors-the regularity of the earthquakes, and relative number of large ones among them-should

be taken into account when comparing seismicity of different parts of the plate. The North

Australian Basin (Argo and Gascoyne abyssal plains) is almost aseismic, and distinct seismicity to the north of it may be as a consequence of the

volcanic activity of the ocean islands and tectonic activity of the island arc.

A large territory, about 6.5 - lo6 km2 comprising almost all the West (the Warton Basin) and South Australian basins, the Broken Ridge and the south end of the Ninetyeast Ridge appears on the map as practically aseismic (Fig. 1). An area of the Indo-Australian plate to the west and northwest of this territory, including the southern

128 O.V. LEVCHENKO

segment of the Ninetyeast Ridge and the southern half of the Central Indian Basin, is characterized by low seismic activity. The Chagos-Laccadive Ridge is at present tectonically inactive and aseismic, with the exception of the Chagos Bank (Stein, 1978). The available data have confirmed the low seismicity of South India (Sykes, 1970; Chandra, 1977). Most of the earthquakes in the Bay of Bengal have been recorded, just as in Western Australia, after the installation of the WWSSN and only one large event (M 2 6) was recorded there. The largest earthquakes (M >, 6 and M > 7) are mainly concentrated in the north- em Ninetyeast Ridge and the adjacent parts of the Central Indian and Cocos basins. Events have been recorded there regularly over a considerable period of time and the analysis shows a high seismicity rate in the time former to the WWSSN installation. This area is also characterized by high percentage (about 20%) of large events.

Thus the results of the analysis do not allow the establishment of any regularities for the main topographic features of the eastern Indian Ocean floor. Separate seismic areas in the Central Indian and Cocos basins are identified together with practically aseismic West, North and South Australian basins. That holds true for block-ridges, among which, the Broken Ridge is aseismic, the Chagos-Laccadive Ridge and the southern Nine- tyeast Ridge are characterized by low seismicity and the northern part of the latter is the most active.. The identified intraplate seismic zone has no obvious natural boundaries and does not corre- tate with any of the physiograp~c features on the eastern Indian Ocean floor.

Although the results of the analysis confirm the anomalously high intraplate seismicity in the northeastern Indian Ocean, the suggested seismic zone coincides with none of the previously identified ones by Sykes (1970) or Stein and Okal (1978). According to Sykes (1970), Western Australia and the adjacent seas belong to a NW-SE trending seismic zone. However, the presence of the continuous Sri Lanka-Australia seismic zone seems doubtful due to a large aseismic gap in the West Australian (Warton) Basin. The southeastern extension of this zone would be limited by latitude 2.5 o S, assuming that the Cocos

Island region’s seismicity is related to the local volcanic activity. Beyond Sri Lanka the northwest- em continuation of the Sri Lanka-Australia zone, as it was shown by Fitch et al. (1973), is lacking. The high intraplate seismicity can be seen not only in the northern Ninetyeast Ridge area, as it was suggested by Stein and Okal (1978), but away from it in both adjacent basins. Contrary to the previously identified NW-SE (Sykes, 1970) or N-S (Stein and Okal, 1978) trending seismic zones. It can be now suggested that the highest seismicity is concentrated in the broad near-latitudinal equatorial zone (Levchenko, 1985).

The anomalous intraplate seismicity zone

The high level of the earthquake activity within the oceanic part of the Indo-Australian plate is confined to the area comprising the northern segment of the Ninetyeast Ridge and the northern Central Indian and Cocos basins. Figure 2 illustrates the suggested near-latitudinal equatorial zone of high intraplate seismicity in the northeast- em Indian Ocean. Within the Central Indian Basin most of the intraplate earthquakes are concentrated in the sublatitudinal area between 2” N and 5 o S. In the Ninetyeast Ridge it seems to turn gradually to east-northeast and continues into the Cocos Basin north of 2.5”s as far as the Sunda Trench. About 45 earthquakes have been recorded in the suggested zone, of which there are three of M 2 7 and seven of M 2 6 magnitude.

The earthquake epicenters within this zone have an irregular dist~bution: about 20 earthquakes, including 6 large events (four M 2 6 and two M >, 7), are concentrated around l-2” S in the Ninetyeast Ridge and the adjacent areas of the basins forming a narrow NE trending band (Fig. 2). This evident trend (azimuth 50 ” ) is likely to be confined to the NE striking Paleocene-Eocene (?) wrench-fault or a fault set recognized in the Nine- tyeast Ridge area from the seismic reflection and bathymetry data (Milanovsky et al., 1981b; Milanovsky, 1984). The morphology of these dis- locations would demonstrate their recent partial reactivation.

It also seems possible that other earthquakes in the northern part of the Ridge and adjacent parts

TEmONIC ASPECTS OF INTRAPLATE SEISMICITY. NE INDIAN OCEAN

i i

!o

I 80 90 103

, .I

c * .

Y i i I UIIn -1 lzl -2

Fig. 2. Largescale fragment of the map from Fig. 1 showing the anomalous zone of intraplate seismicity in the northeastern Indian Ocean (I) and the proposed trends of the earthquake epicenters (2) confined probably to the NE-SW striking wrench-faults recognized in the Ninetyeast Ridge area from the seismic reflection and bathymetric data (Milanovsky et al., 1981b). Other symbols are the same as in Fig. 1. Some seismic reflection profiles in the northern Central Indian Basin that passed close to the earthquake epicenters are shown by dotted

lines and marked by their Figure numbers.

of the Central Indian and Cocos basins are related to the reactivation of some sections of similar NE striking faults which break the Ninetyeast Ridge into a series of en-echelon blocks. These faults of northeasterly orientation are also widespread in the southern part of the Ridge (~l~ovsky et al., 1981b; Milanovsky, 1984), but young movements have not been found along these faults south of 9 o S. The attenuation of seismic activity up to its complete absence in the southern Ninetyeast Ridge may be explained by the large distance from the plate-tectonic forces generating the intraplate compressive stress. It is as yet commonly accepted that the main compressive resistive force seems to be the continental collision between India and Asia (see e.g., Fitch et al., 1973; Stein and Okal, 1978; Bergman and Solomon, 1985). Another force (also in the north) may be related to the subduc-

129

tion of the northern part of the Ninetyeast Ridge under the Sunda Trench.

The relation of the earthquake epicenters in the

Central Indian Basin with the crustal structure is difficult to establish due to the thick sediment series masking the basement topography. Most of the earthquakes there are confined to the seamounts of volcanic origin probably connected with deep-seated faults (Fig. 3). Their relation with steep fault scarps is most clearly seen in areas with thin sediments (Fig. 4). In some places the intraplate earthquakes relation with unique folded and faulted features of the sediments and basement can also be observed (Fig. 5) (Weissel et al., 1980; Geller et al., 1983; Levchenko et al., 1985). A number of earthquakes of the above-mentioned distinct trend near 1-2”s are confined to a large feature which is shown by a seismic reflection profile made underway by the D/V “Glomar Challenger” on Leg 22 between Sites 215 and 216 (Von der Borch et al., 1974). Some earthquakes shown in these figures are not evidently associated with structures in the oceanic crust. That may be explained either by the location of the seismic reflection profiles being slightly away from the epicenters, or by the inaccuracy of the epicenter determination, because many of the events occurred rather a long time ago. Long-term seismological observations at the ocean floor, supported by a seismic reflection survey could be useful there.

It is reasonable to believe that the high seismicity in the northern Central Indian Basin is in- duced by the reactivation of some sections of old NE striking faults of unknown origin within the area of diffuse intraplate deformation. This is due to the compression stress increase as a result of the continental collision at the northern convergent plate boundary. The lack of earthquakes in the Central Indian Basin west of 80 “E may be explained by the influence of the southward extension of the Indian subcontinent into the stress field in the Indo-Austria ~thosphere.

It needs to be noted for comparison, that although the observed rate of intraplate seismic activity in the northeastern Indian Ocean is the highest in the oceans (Sykes, 1970; Fitch et al., 1973), it is still several times less than that in the

130 0.X’. LEVCHENKO

i

c


26.06. i95? M=5.7 13.OLi.1918 M= 6.5

* *

25.021956 M= 4.8

I 50 Km I

-5

-6

Fig. 4. Single-channel seismic reflection profile showing confinement of intraplate earthquakes (denoted by stars) to steep fault scarps

in the northern Central Indian Basin.

adjacent spreading centers (Central Indian and South East Indian Ridges) and ten times less than that in the adjacent subduction zone (the Sunda Trench).

Analysis of earthquake source mechanisms

The focal mechanism of earthquakes is one of the main sources of information about contemporary crustal movements and types of faulting. All the available published epicentral data in the part of the Indo-Australian plate under examina- tion are summarized here. Parameters for 25 intraplate events, including fault plane solutions of the earthquakes, are listed in Table 1, and their focal mechanisms are shown in Fig. 6.

25.09.1978 M--4.7

25.01. 4951 08.02.i938

*

The Chagos Bank region is the most intense source of oceanic seismicity apart from the con- ventionally defined plate boundaries. The cause of this activity is essentially unknown (Wiens, 1986). The focal mechanisms of the earthquakes there are characterized by normal faulting on an E-W trending plane and indicate N-S extension (SteinJ978; Wiens and Stein, 1984) which cannot be easily reconciled with the present-day tectonics of the area (Singh, 1988). Stein (1978) suggested that the earthquakes represented a still active fracture at depth, remaining from the break-up of the Chagos Bank and Mascarene Plateau. Wiens et al. (1985) included the seismicity near Chagos in a broad seismic zone stretching across the equatorial Indian Ocean, which seems to be a diffuse

6

85

Fig. 5. Single-channel seismic reflection profile showing confinement of intraplate earthquakes (denoted by stars) to unique folded

and fractured features of sediments and basement in the northern Central Indian Basin.

132 O.V. LEVCHENKO

TABLE I

Parameters of focal mechanisms for intraplate earthquakes in the northeastern Indian Ocean and the adjacent part of the Indian

Peninsula

Event Date Epicenter M Depth Nodal plane Type Ref. *

No. lat.

(“)

long.

(“)

(km) strike dip slip

(“) (“) (“)

faulting

Chagos Bank

1 Sep 12,1965

2 Nov 11,1967

3 Mar 02.1968

4 Nov 30,1983

Indian Peninsula

5 Mar 27,1967

6 Dee IO,1967

7 Apr 13,1%9

Bay of Bengal

8 Nov 24,1972

9 Au8 30,1973

10 Apr 08,1982

Central Indian Basin

11 Feb 29,1944

12 Apr 23,1967

13 Sep 14,1%8

14 act 10,197o

15 Jun 25,1974

16 Au8 03,1978

17 Au8 21,1983

Ninetyeast Ridge

18 Mar 21,1939

19 May 251964

20 Apr 07.1973

21 Dee 02,198l

Cocos Basin

22 Mar 22,1955

23 Out 31,1965

24 Nov 21,1%9

25 Jun 26,197l

6.5s 70.8 6.1 15 270 60 260 N (1) 6.1s 71.3 5.6 15 280 65 270 N (2) 6.1s 71.4 5.5 15 280 60 270 N (2) 6.9s 72.1 7.6 16 270 45 270 N (3)

15.6 N 80.1 5.4 17 47 90 S (4) 17.5N 73.7 6.0 12 26 78 s (4) 17.9N 80.6 5.3 33 60 70 S (4)

11.7N 85.3 5.2

7.15N 84.3 5.2

18.5N 86.3 5.5

27

27

254 52 76 T (5) 290 52 118 T (5)

91 54 51 T (6)

0.3N 75.4 7.2

1.6N 80.2 5.1

24.53 80.4 5.5

10

33

3

4

27

39

23

18

13

39

10

130 45

3.6s 86.2 6.3

26.0s 84.3 6.6

0.9s 84.2 5.5

3.2N 87.5 5.2

291 48 82

315 23 129

19 89 358

32 73 21

10 50 25

47 53 73

36 48 41

261 53 90

254 77 71

T (1) T (7) T (5) T (5) S (5) S+T (5) S (5) T (5) S+T (5) T (5) T (8)

0.9s 89.5 7.2

9.1s 88.9 6.0

7.ON 91.3 6.6

15.8s 88.4 5.5

10 166 82 57 S+T

17 177 87 5 S

13 34 84 356 S

6 20 87 9 S

23 222 44 78 T

8.8s 91.7 7.0

14.2s 95.3 5.4

2.ON 94.6 6.4

5.2s 96.9 6.0

24

20

29

244 80 78

165 68 9

14 67 19

N

S

S

T+S

(9)

(5)

(5)

(5)

(5)

(9)

(5)

(10)

(5)

l References: (1) Wiens (1986); (2) Stein (1978); (3) Wiens and Stain (1984); (4) Chandra; (5) Bergman and Solomon (1985); (6)

Singh (1988); (7) Sykes and Sbar (1974); (8) Dziewonski et al. (1984); (9) Stein and Okal (1978); (10) Fitch (1972).

boundary separating the Australian plate from a area (Wiens, 1986) may be the consequence of

unified Indo-Arabian plate. It should be noted, tectonic activity on the Central Indian Ridge rather

that the strike-slip faulting mechanism of some than on the Chagos-Laccadive Ridge.

earthquakes located in the west-northwest of the Fault-plane solutions of earthquakes in the In-

TECTONtt ASPECTS OF INTRAPLATE SEISh4iCITY. NE INDIAN OCEAN 133

60’

CENTRAL

INDIAN

BASIN 0 .

.

0 0

70” 80'

WHARTON

,BROKEN RIDGE ’

Fig. 6. Focal mechanisms for intraplate earthquakes in the oceanic part of the Indo-Australian plate. I = isobath of 4 km: 2-5 = are

the same as symbols 1-4 in Fig. 1; 6 = strike-slip faulting; 7 = thrust faulting; 8 = normal faulting.

dian Peninsula show a dominant N-S oriented compressive stress field and left-lateral strike-slip faulting along SW-NE striking planes. Chandra (1977) suggested that these compressive deformations, followed by a low to moderate seismicity in

the Indian peninsula, are affected by the continental collision caused by the northward movement

of India. In the Bay of Bengal, and in the Central Indian Basin, almost all the intraplate earthquakes with known focal mechanisms are characterized by thrust faulting, except for the October 10, 1970 and June 25,1974 events, which are characterized by a strike-slip mechanism combined with a component of thrust faulting. In general, the thrust

134 O.V. LEVCHENKO

faulting occurred on a nearly E-W striking plane, and both left-lateral strike-slip faulting events occurred on a nearly N-S striking plane (Bergman and Solomon, 1985). The direction of the maxi- mum horizontal compressive stress in the Bay of Bengal and in the northern Central Indian Basin is slightly west of north. Bergman and Solomon (1985) and other authors relate these seismogenic deformations in the area to the continental collision between India and Asia. If one cause is responsible both for compressional stress and intraplate deformation within the northern Indo- Australian plate, then the different type of focal mechanism observed in the Indian peninsula (strike-slip faulting) and in the Bay of Bengal and Central Indian Basin (thrust faulting) can be explained by the differences in the thickness and rheology of continental and oceanic lithosphere.

The three earthquakes on the Ninetyeast Ridge with known focal mechanisms (March 21, 1939; May 25, 1964 and April 7,1973) are characterized by a left-lateral strike-slip faulting on approximately N-S planes (Bergman and Solomon, 1985). One-half March 21,1939 event is characterized by thrust faulting on a N-S plane steeply dipping to the west (Stein and Okal, 1978). Bergman and Solomon (1985) determined a thrust faulting mechanism indicating NW-SE compression for the fourth earthquake (December 2, 1981). Nor- mal faulting mechanisms have not yet been found on the ridge. In general, Bergman and Solomon (1985) as well as Stein and Okal (1978) have suggested a left-lateral strike-slip motion along the Ninetyeast Ridge from the northern end of the ridge, where it intersects the Sunda arc, to as far as south as 10”s. These and other authors (Bergman and Solomon, 1980; Sir@, 1988) esti- mated NW-SE compression in the region of the Ninetyeast Ridge.

To the east of the Ninetyeast Ridge normal, thrust and strike-slip faulting occur in the Cocos Basin. Stein and Okal (1978) characterized the March 22, 1955 earthquake as normal faulting along a steep NE-SW plane and suggested it to be associated with graben structures. Bergman and Solomon (1985) doubt this solution and believe that there is no satisfactory explanation for the occurrence of normal faulting east of the Nine-

tyeast Ridge. Sykes and Sbar (1974) determined a mechanism combining thrust faulting with a sig- nificant component of strike-slip motion for the earthquake of June 26, 1971. The strike-slip faulting is a dominant type of faulting in the area, focal mechanisms of the three earthquakes (Oc- tober 31,1965 and November 21,1969 and in part June 26, 1971) are characterized by left-lateral strike-slip motion on fault planes oriented within about 20’ of N-S (Fitch, 1972; Bergman and Solomon, 1985). It was suggested that strain deformation arises there from the NW-SE compression generated by the continental collision along the Himalayas and the NE-SW extension generated by body forces acting on the inclined part of the plate beneath the Sunda arc (Fitch et al., 1973; Stem and Okal, 1978).

Thus, analysis of the earthquake focal mechanisms shows that either thrust faulting on a nearly E-W trending plane, within the Bay of Bengal and Central Indian Basin, or strike-slip faulting on nearly N-S trending plane, along the Nine- tyeast Ridge, dominate the suggested anomalous intraplate seismicity zone and the adjacent areas. Bergman and Solomon (1985) showed that the stress field in this region changes smoothly from the N-S oriented compression in the Indian peninsula to NW-SE directed compression in the northeastern Indian Ocean, and the presence of the Ninetyeast Ridge does not seem to influence the orientation of the stress field. Therefore, a different type faulting in the ridge and to the west-northwest of it can be explained rather by the differences in thickness and structure of the oceanic crust in these areas. Because of its peculiarity, the crust appears to respond to the common compressive stress there in different ways. Different average centroid depth of the earthquakes in the Ninetyeast Ridge (less than 15 km) and in the Bay of Bengal and Central Indian Basin (more than 20 km) may confirm this sugges- tion.

In the Central Indian Basin the focal mechanism solutions agree well with the seismic reflection data. The seismic survey at slow ship speed reveals steeply dipping reverse faults nearly E-W trending, approximately parallel to the magnetic lineations (Eittreim and Ewing, 1972; Weissel et


al., 1980; Geller et al., 1983), which disrupt the oceanic crust and most of the overlying sedimen-

tary section (Figs. 3a and 5). Weissel et al. (1980) showed that the common trend of these intraplate faulted blocks changes from E-W to ENE-WSW near the Ninetyeast Ridge and seismic reflection

profiles over the Ridge reveal high-angle faulting; unfortunately, its type was not specified by these authors. It can be inferred from the bathymetric and seismic reflection data that the northern

Ninetyeast Ridge is broken by wrench-faults oriented NE-SW into a series of en-echelon blocks.

The faults disrupt not only the crust, but also the overlying sediments (Milanovsky et al., 1981a; Milanovsky, 1984). We suggested that the unique area of intraplate deformation of the sediments and the basement is continuous from the northern Central Indian Basin into the northern Ninetyeast Ridge, and the peculiar structure of the northern part of the ridge relates to rejuvenation of older faults in the oceanic crust (Kazmin and Levchenko, 1987). Recent movements on the faults are re-

flected by the distribution of earthquake epi-

centers: cases are noted (l-2 o S and 9 o S) in which epicenters cluster in chains trending northeast and appear to coincide with fault zones (Fig. 2). Thus, for the northern Ninetyeast Ridge earthquakes, as well as the October 10, 1970 event in the Central

Indian Basin, it is necessary to explain an evident discrepancy between the results of focal mechanism solutions (strike-slip faulting on N-S trending planes) and the results of morphological analysis of bathymetric and seismic reflection data (wrench-faults oriented NE-SW).

In the Cocos Basin, the Nicobar Fan sediments are also deformed, but not as intensively as the

Bengal Fan deposits (Bowles et al., 1978). The nature of movement on the faults appears to be reverse, and the horizontal component of displace-

ment cannot be determined (Geller et al., 1983). Regarding the focal mechanism of earthquakes, although known solutions are rather mixed, strike-slip faulting dominates there. The intraplate seismicity has some features common to both that of the island arcs (number of large events) and the mid-ocean ridges (shallow centroid depths).

Thus, it can be assumed from the analysis of the earthquake source mechanisms in the north-

eastern Indian Ocean that the basins on both sides of the Ninetyeast Ridge are characterized by dif-

ferent types of faulting. Following Fitch et al.

(1973) and Stein and Okal (1978), it seems reasonable to suggest that the faulting in the Central Indian Basin was controlled principally by the

collision resistance in the Himalayan region, while that in the Cocos Basin was affected by the normal subduction in the Sunda arc.

Discussion: the subduction jump

Most of the investigators who have studied the geophysics of the central Indian Ocean believe that recent deformation and modem seismicity in northern Indo-Australian plate are the result of India and Asia continental collision (Sykes, 1970; Eitreim and Ewing, 1972; Fitch et al., 1973; Stein and Okal, 1978; Weissel et al., 1980; Levchenko et al., 1985 and others). There are two tectonic ex- planations of this anomalous intraplate seismicity in the northeastern Indian Ocean according to the

epicenters correlation; both of them, as was noted

above, are not devoid of contradictory points. Firstly, it was assumed that the Sri Lanka-

Cocos Island-Australia seismic zone is a nascent island arc (Sykes, 1970). Later, it was suggested that the seismicity is concentrated in a broad zone along the northern segment of the Ninetyeast Ridge, and is related to the strike-slip motion between the Indian and Australian parts of the plate (Stein and Okal, 1978). None of the proposed seismic zones have been verified by the

available seismological data analysis. An extended linear fracture zone along the northern Ninetyeast Ridge north of 7”s is not evident from the bathy-

metric data (Milanovsky et al, 1981b; Milanovsky, 1984). The anomalous seismicity in the Indo- Australian plate seems to be in a diffuse broad

equatorial band, trending nearly E-W (Figs. 1 and 2). It represents the initial stage of intraplate deformation rather than the strike-slip motion

along the N-S trending transform fault-like boundary proposed by Stein and Okal (1978). The intraplate deformation there is diffuse in nature. That is confirmed by other anomalous geophysical phenomena in the northern Central Indian Basin: abnormally high heat flow (Geller et al., 1983),

136 O.V. LEVCHENKO

and intensive folding and faulting of the sediments and the basement (Weissel et al., 1980; Levchenko et al., 1985), which are also irregularly distributed within a widespread area almost coinciding with the suggested equatorial seismic zone. These anomalous geophysical phenomena bear evidence to the presence of tectonic activity in the northern Indo-Australian plate since the Late Miocene (Cochran et al., 1987) up to the present time.

The location and orientation of the suggested seismic zone is in good accord with the area of internal deformation and ~thosphere contraction within the Indo-Australian plate predicted from the global plate motion model (Minster and Jordan, 1978). The indistinct boundaries and com- paratively large size of the anomalous seismicity zone seem to testify to the earliest stage of struct- ural reorganization in the region. The plate’s deformation has begun and is not localized. In the future, it is likely to be completed by the formation of a new tectonic feature in the Indian Ocean. That allows a revival of Sykes’ idea of the nascent island arc, but with regard to the suggested near- latitudinal anomalous zone being almost parallel to the zone of continental collision at the northern margin of the plate (Fig. 7). With India and Asia being drawn together, the compressive stress has

50 100 150

50 100 150

Fig. 7. Position of the intraplate seismicity zone relative to

boundaries of the conventional Indo-Australian plate. I - 3 =

plate boundaries (I = strike-slip zone; 2 = spreading ridge;

3 = convergence zone); 4 = suggested anomalous intrapiate

seismicity zone.

also been increasing and influencing the interior of the Indo-Australian plate. At last, after having been blocked by the Indian subcontinent in the Late Miocene (?), the subduction might have jumped over 3000 km southward towards the sub- ducting oceanic plate. South of the “light” Indian continental block a diffuse area of intraplate deformation has started to form which in due course may turn into a new subduction zone. Such a change in the convergent boundary spatial position is suggested each time when a continental or subcontinental block happens to be involved with subduction processes. The subduction jump models were proposed for the evolution of the North- em Caribbean (Mattson, 1979), Philippine Sea (Juan et al., 1983) and some other regions. The collision of subcontinental plateaus and rises with convergent plate boundaries in the Japan, Kurile and Aleutian trenches may have caused and would probably cause shifting of the subduction zones (Ben-Avraham and Uyeda, 1983). Thus, the suggested seismic zone would have been the logical line for the new subduction. Some difference in orientations of “old” and “new” subduction zones might be influenced by the shape of the India and Sri Lanka continental margins.

The proposed hypothesis, which combines two previous hypothesis by Sykes (1970) and Stein and Okal (1978) and eliminates some of their contradictions, had already been presented at the 1985 Geodynamics Symposium on Intraplate De- formation at Texas A&M University (Levchenko, 1985). It is necessary to note that similar results were obtained by Wiens et al. (1985), who suggested that “the current deformation may repre- sent the pre-subduction phase of an evolving convergent boundary and part of the process of regional plate boundary reorganization”. It is very important, because similar results have been obtained by different investigators at nearly the same time. Althou~ the location of the suggested seismic zone was determined to be almost identi- cal, the tectonic conclusions about the generation of this high intraplate seismicity are different. Wiens et al. (1985) proposed a new diffuse plate boundary between the combined Indo-Arabian and Australian plates and assumed that the motion between India and Arabia along the Owen


Fracture Zone is negligible. They suggested that this diffuse boundary trends E-W from the Central Indian Ridge near Chagos Bank to the Ninetyeast Ridge and north along the Ninetyeast Ridge to the Sunda Trench and that this diffuse boundary is variable in nature: N-S extension near Chagos, N-S compression in the Central Indian Basin and left-lateral strike-slip along the Ninetyeast Ridge. However, as was shown above, the presence of left-lateral shear along the Ninetyeast Ridge is not consistent with the observed structure (Kazmin and Levchenko, 1987). Furthermore, the zone of high seismicity can probably be followed far to the east of the Ninetyeast Ridge into the Cocos Basin, where recent deformation of the sediments and the basement (Bowles et al., 1978; Weissel et al., 1980) and high heat flow (Geller et al., 1983) have also been registered. The “rigid” tectonic scheme, which was proposed by Wiens et al. (1985), cannot be applied to the deformation occurring at present in the Indo-Australian plate.

Let us now suppose that recent deformation of the sediments and the basement, and modem seismicity within the oceanic part of the Indo- Australian plate are probably different phases of the common tectonic process. We have already suggested using seismic reflection data only that the recent intraplate deformation in the oceanic crust in the northern Central Indian Basin might have been generated under the conditions of a large-scale zone of shearing strains (Levchenko et al., 1985; Neprochnov et al., 1985a). Although short fragments of such wrench-faults were actu- ally observed, the available seismic reflection data have not proved their extension throughout the whole of the basin. Therefore, we had to confirm a speculative character of the then suggested model. Analysis of seismological data has led to the con- clusion about a nascent subduction zone which includes the previously identified wrench-fault zone. These two models do not contradict each other as the island arcs evolutional pattern on the background of domineering normal subduction can involve some elements of wrench-fault tectonics, for example the Aleutian Island Arc (Geist et al., 1988). The trouble is that geologists deal with the already formed island arcs, and the subduction initiation mechanics are not clear.

It would be reasonable to suggest that the intraplate deformation in the northeastern Indian Ocean is episodic in behaviour-the prolonged periods of relative stability may have been alter- nating with short pulses of seismic activity when some segments of an old fault set became active due to the increase in compressional stress within the Indo-Australian plate. The whole sediment section up to the Uppermost Miocene was affected by recent faulting and folding. These re- cently deformed sediments are overlain by flat lying Pliocene-Quatemary sediments along a sharp unconformity which indicates that the recent intraplate deformation might have been brought about by a short pulse of tectonic movements near the Miocene-Pliocene boundary, Compressional stress within the Indo-Australian plate, caused by the continental collision between India and Asia, seems to have reached its critical level in the Late Miocene. It was then that the observed deformation of the oceanic crust could have taken place. After a short relaxation the stress began to increase again, while the unde- formed sediments were being deposited. Modem tectonic activity, manifesting itself in high seismicity, can be related to a new pulse of intraplate deformation. In general, the motion of the litho- spheric plates is of interrupted behaviour during their convergence, and the deformation within the colliding zones is pulsating in nature.

Although in my view, the episodic intraplate deformation in the northeastern Indian Ocean region, which is characterized by tectonic activity unusual in a mid-plate setting, may manifest a future site of subduction along the southern margin of the Indian subcontinent, there may be other opinions. For instance, one cannot disregard the fact that the high present-day intraplate seismicity in the northeastern Indian Ocean may partly be connected with the recent involvement of large oceanic crust rises (the Ninetyeast Ridge and Cocos Rise) into the Sunda subduction zone. With this in mind, the author presents his paper in the form of a discussion. The author believes that the seismological observations at the ocean floor by means of the Ocean Bottom Seismometers may bring the most fruitful results which will give rise to new hypotheses explaining the high rate of

138 O.V. LEVCHENKO

intraplate seismicity in the northeastern Indian Ocean.

Acknowledgements

I benefited greatly from discussion with Dr. V.G. Kazmin on the Eastern Indian Ocean tectonics and I am very much obliged to him for numer- ous offered helpful suggestions. I am greatful to Mrs. S.B. Kulnitzkaya for her help in preparing the paper in English.

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Tectonic aspects of intraplate seismicity in the northeastern Indian Ocean