Gravity anomalies, seismicity, subducting slab folding and surface deformations in the orogenic belts—an example from the Andaman-Nicobar region

JOURNAL OF GEODYNAMICS12, 39-63 (1990)

39

GRAVITY A N O M A L I E S , SEISMICITY, S U B D U C T I N G SLAB F O L D I N G AND S U R F A C E D E F O R M A T I O N S IN THE O R O G E N I C BELTS---AN E X A M P L E F R O M THE A N D A M A N - N I C O B A R R E G I O N

SURENDAR K U M A R

Wadia Institute 0[" Himalayan Geology, Dehra Dun 248001, hzdia (Received February 9, 1990; accepted March 30. 1990)

ABSTRACT Kumar, S., 1990. Gravity anomalies, seismicity, subducting slab folding and surface deformations in the orogenic bclts An example from the Andaman-Nicobar region. Journal o['Geodynamics, 12:39 63. Geophysical data in relation to the geometry associated with the Andaman-Nicobar arc are less strongly constrained. The lithosphere, after buckling, becomes convex downwards in a series of plunging ridges (anticlines) and depressions (synclines). The variation of tectonic activity is greater in front of the subducting folded oceanic lithosphere whereas volcanoes are confined to the shear zones present over the common limbs of plunging ridges and depressions. The representation of the subducting slab geometry over the surface leads to tectonic variations in the orogenic belts.

INTRODUCTION

The Andaman-Nicobar (Fig. 1) region is characterized by highly seismic zones and less seismic zones with shallow- to intermediate-focus earthquakes. This region falls in the tectonically continuous Alpine-Himalayan seismic belt between Burma and Sumatra. Recently, many efforts have been made to understand the involvement of geodynamics in the AndamanNicobar region through various approaches, such as geology (Moore and Karig, 1982; Roy, 1983) tectonic stresses and seismicity (Fitch, 1970a, b; Sclater and Fisher, 1974; Kumar, 1981) and seismotectonics (Mukhopadhyay, 1984; M u k h o p a d h y a y and Dasgupta, 1988). While working on the geodynamics of this region, the present author came across a few observations which show a different state of lithospheric geometry and a prevalence of stresses at depth related to the volume of lithosphere subducted in a smaller available space. Wortel (1982) has given a rheologic model on the basis of seismic data and calculated temperatures, while Vlaar and Wortel (1976) have shown that the depth of an earthquake 0264-3707/90/$3.00

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is directly proportional to the age of the downgoing oceanic lithosphere. The down-going lithosphere is further associated with the differentiation of material and phase t'~'ansformation (Ringwood, 1969). Meissner and Strehlan (1982) have determined the limits of stresses in continental crusts and their relation to the depth-frequency distribution. The presence of large gravity anomalies and intermediate-depth earthquakes in the trench region supports the argument for a subduction zone. The relative gravity "highs" and "lows", clustering of earthquakes and high heat flow corresponding to the surface geology indicate the involvement of some different type of lithospheric deformation in this region. It is in consideration of all these aspects that the present study has been carried out.

TECTONIC F R A M E W O R K O F THE ANDAMAN-NICOBAR REGION

The tectonic activity of the Andaman-Nicobar region is at present intense along two broad belts: the inner and outer volcanic arcs. Late-Cretaceous ophiolites occur in limestone, sandstone and shale radiolarites. There are basic and ultrabasic submarine flows in the flysch as sills and dykes. Acid submarine tufts occur interbedded in the fossiliferous Lower Miocene or post-Pliocene rocks. Also plugs of basic and intermediate lava and agglomerate rocks occur in the fossiliferous limestone. The most active of these two belts is the axis of the outer island arc along which the lithospheric convergence has taken place. The dominant structural features of the Andaman-Nicobar region have been related to the Indonesian island arc (Katili, 1975). The Andaman-Nicobar island arc is concave towards the overriding Malayan peninsula plate. The Andaman Basin, between the Andaman island arc and the BurmaMalayan Orogen, is dominated by youthful structures (Fig. 1) which are either tensional in origin or have resulted due to combined processes of tensional and strike-slip movements. Most of the tensional and tension-transcurrent structures belong to two sets of trends, i.e. N15~E and N 5 ° W (Rodolfo, 1969). The Nicobar rift valley is the deepest feature of the Andaman Basin and this has been explained as due to tensional strain (Kumar, 1981). Another basin which is very prominent is the depression south of the Nicobar island, on the Andaman-Nicobar arc. The seismic profiles have identified various geotectonic elements of the

Fig. 1. Map of the Burma and Andaman-Nicobar region, showing different prominent geologic and geotectonic elements. AA', BB', CC' & DD' are the lines along which the cross-sections have been taken for geologic and seismic zones (Modified after Mukhopadhyay and Dasgupta, 1988). A, B, C, J and R are the lines along which the seismic profiles have been taken (modified after Roy, 1983).

42

KUMAR

Andaman-Nicobar region (Fig. 1). The region consists of series of folds at an angle to the island-arc system. The associated wrench faulting has occurred as a result of transcurrent movements along the rift zone. The islands extending between latitude 1 4 ° N and 7 ° N constitute the outer high which consists of series of narrow parallel folds, faults and thrusts. A high pore pressure is indicated in some of the fold covers which are associated with diapiric movements (Fig. 12).

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The analysis of shallow earthquakes recorded at Port-Blair (Andaman Island) yielded a Pg velocity of 5.3 km/sec and a Pn velocity of 7.8 km/sec., with a Pn time intercept of 374 sec. The crustal thickness obtained in this region is of the order of 20 km. The high temperatures are attributed to normal faulting to the east of Andaman, parallel to the inner and outer volcanic arc axes that are the main zones of shear and intense folding (Kumar, 1981). JOG 121-4

44

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T H E 90 E R I D G E

The totally submerged 9 0 ° E ridge is a meridional feature in the eastcentral Indian Ocean (Fig. 1 ). It extends North-South as a topographic high for more than 4500 km (31 ° S to 10 ° N). The ridge is en e n c h e h m in form south of 7 ° S. All along and parallel to the eastern margin of the 90" E ridge, a fracture zone is present. This fracture zone divides the central Indian Ocean plate and the Wharton Basin plate. The ridge consists mostly of gabbro and serpentinized peridotites. The northern portion (3 N to 10 ~ N) of the ridge is an active seismic zone where both vertical and strikeslip motions have occurred (Stein and Okel, 1978). The strike-slip motion is left lateral, which is consistent with the Indian (West) side encountering (east) side is subducting smoothly at the Sumatra-Java trench. B u r m e s e arc

Bearing in mind the heterogeneity of the Burmese tectonostratigraphic province (Kumar, 1981), the region is divided into four sectors (Mukhopadhyay and Dasgupta, 1988). From the middle of the sectors four

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G R A V I T Y A N O M A L I E S , SEISMICITY, S U B D U C T I N G SLAB F O L D I N G

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E-W cross-sections were drawn (Fig. 1) and the distributions of foci in the inclined seismic zone (Fig. 2) configuration were used and the depth variations of the Chindwin Basin from Sector I to Sector IV were quite prominent in the cross sections AA', BB', CC' and DD' (Fig. 2) which show a correlation with the varying dips of the subduction angle. The volcanoes in Burma are confined to the deepest part of the Benioff zone. The Bouguer anomalies over Burma and India (Evans and Crompton, 1946; Gulatee, 1956) show two prominent and distinct trends, i.e., E-W over the Shillong Plateau and a N-S over the Bengal Basin and Burmese arc. Many gravity "lows" and "highs" are marked by relative amplitude upto 40 mgal or more (Fig. 3). These clearly correspond to tectonic ridges and depressions underlying the basin. These relative gravity "lows" and "highs" also indicate a thinner and thicker sedimentary basin present in the central region over the Benioff zone (Fig. 2). The variation in subduction angle indicates lateral folding of the subducted lithosphere after buckling.

FREE-AIR GRAVITY A N O M A L I E S

The hypothesis of free-air gravity anomalies is a useful standard of comparison because it represents an extreme case such that the internal anomalies of density are just sufficient to cancel the extra mass of the visible inequalities and are as near the surface as they could possibly be. For a very local irregularity of the topography, their effect on the gravity is nearly the same whether they are compensated or not. The study of free-air gravity anomalies (Fig. 4) over the AndamanNicobar island arc shows an important belt of positive anomalies seaward of the deep-sea trench. This positive gravity high can be explained by a stress system associated with the convergence of the lithosphere at the island arc due to the horizontal compressive stress. The maximum belt of positive anomalies occurs in the vicinity of the Andaman-Nicobar arc and it decreases gradually landward but again marked by a belt of negative anomalies between the land and the arc, indicating the deposition of lighter material of great thickness. The belt of positive anomalies over the Andaman-Nicobar is split up by negative anomalies extending transverse to the main trend. Free-air gravity anomalies reach a maximum of - 100 mgal in the north of the Andaman Basin and deepens southward towards the Fig. 5. Synoptic s u m m a r y in the north-south cross-section along line RR (Fig. 1) of the available geological and geophysical data and the lateral folding (contortion) of the ocean floor in the subduction zone. The geological data are based on available seismic profiles (after O N G C published reports and Roy, 1983).

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GRAVITY ANOMALIES, SEISMICITY, SUBDUCTING SLAB FOLDING

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Sunda arc. The minimum of the negative anomalies correlates with the axis of the deep-sea trench. The relative anomaly difference corresponds to the thickness and extent of the sedimentary basin. The transverse negative anomalies are mostly confined to the regions of inter-arc basins at about 9'5°N and 5 ° N and the equator. However, the landward walls of the trench are steep. The island arc in the transverse negative-anomal zones is also steeply cut and shows a narrow steep-walled depression. A correlation between free-air anomalies and the oceanic crust of the trench is shown in Fig. 5.

SEISMICITY

The relationship between depth and intensities (Figs. 5 & 6; data from the NOAA, IMD and local laboratory files for 1960-1988: routine bulletin depths based on travel times have been used) show distinct gaps with regions of lower seismicity and a considerable reduction in the seismic activity northward. The variation in depth of earthquake foci from 200 km near Sumatra to 45 km near Nicobar and again to 180 km near Andaman, with five distinct depth zones of 33 km or 60 km, indicates a wavey nature of the stress zones in the subduction area. When the depth contours based on earthquake foci of the subducted slab are taken into consideration, the geometry of the subduction zone becomes quite clear (Fig. 5). It is likely that the maximum deformation occurred over the common limb of a depression and the adjoining ridge, which are dominated by strike-slip and thrust faults, depending upon the behaviour of the subducting oceanic crust. This type of activity also disturbs the asthenosphere below the crust and relates to a different type of dynamics in the mantle heat flow. The concentration of a funnel-shaped zone of earthquake activity may be an expression of horizontal movements of the heterogeneous materials present in the variably thick lithospheric layers which are associated with the movements. This indicates variable density and isostatic imbalance in the zone. This is further supported by the fault-plane solutions (Fig. 7), as strike-slip faults are associated with shallow depth and normal faults with deep-seated earthquakes (Table I). Events 9 and 11 in Fig. 7 are near to the north Andaman fault, which is of normal type and extends over 800km in NE-SW direction (Kumar, 1981 ). The focal depth for these events ranges from 19 km to 36 km, which is of transform nature. The sudden decrease in seismic activity at shallow depth (25-33 km) between 12 ° N and 15 ° N (Fig. 5) may be a consequence of the gradual conversion of subduction into a transform fault. The depth and focal mechanism suggest an extensional stress in this region. Events

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confined to 10.5 ° N to 13.5 ~ N and to between 6.5 ° N and 8 ° N are mostly of shallow to intermediate type (Fig. 5). The strike-slip normal faulting is confined to the back-arc region, while the strike-slip thrust faulting in the east of the outer volcanic arc (Fig. 7) indicates a tensional stress in the back-arc region and in the frontal part of the accretionary prism. The events confined to the region between 8 ° N and 10'5~'N are mostly strike-slip thrust faults of shallow depth. The stress rates estimated from the source mechanism of interplate earthquakes in the back-arc region (Mb 5'5-5'6; Fitch, 1972; Equchi et al., 1979) created an extensional basin in the back-arc region-like the Andaman Sea. Adequate data on the slab geometry of the subduction zone (length, dip and maximum depth of the seismic zone (Fig. 8) relevant plate kineamatrics and age of the downgoing slab) were taken from Fitch and Molnar, 1970 and Hamilton, 1979 (Table II). The direction of subduction of different-aged segments creates areas of instability and maximum seismic activity which give rise to the variation in subduction geometry. This change in the subducted lithosphere causes a reduction of the rate and thickening of the lithosphere, where accumulation of cumulative stresses and their release produce earthquakes along the slip planes. The earthquakes occurring at depths of 100 km or more are accepted as being in the subducted lithosphere. The depth of earthquakes, the melting temperature and the rheology of the subducted lithosphere can be understood in terms of thermal assimilation or rheological deactivation. When the subducted lithosphere cannot sustain the stress necessary to generate seismic events, this shows the critical temperature, Tcr(Z), which governs the rheology of the subducted lithosphere (Wortel, 1982). Hence, the seismic activity is confined to those parts of the subducted segments which have temperatures below the critical value Tcr(Z) (Fig. 8). High heat flow in the form of volcanism is reported in regions where the temperature is greater than the critical value Tcr(Z).

NATURE OF VOLCANISM AND SEDIMENTARY STRUCTURES

In the Andaman Sea, according to Uyeda and Kanamori (1979), the back-arc spreading was longitudinal (Karig, 1972), so the arc became thin and was pushed away from the continent, forming the Andaman Sea Basin. Nakamura (1977) correlated the tectonic stresses with the structure of

Fig. 7. Fault-plane solutions of some of the major events based on available data (Table 1), indicating thrust and normal fault components associated with strike-slip in different regions.

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TABLE 1

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07 03 54.0

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54

KUMAR

15' N

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INDIAN ~

PLATE

UNDER T H R U S T

PLATE

BOUNDARY

EARTHQUAKE E P I C E N T E R S ZONE OF UNDER THRUSTING CONTOUR (X I00 K M ) Fig. 8.

Expected underthrusting slab geometry at the Andaman-Nicobar margin.

GRAVITY A N O M A L I E S , SEISMICITY, S U B D U C T I N G SLAB F O L D I N G

55

TABLE 1I

Zone

Age {my)

v~'''

Length (km)

Dip (degres)

Depth (km)

70 60 140-115

5'8 6'9

400 860

35 75

200 600

1. Sumatra 2. Java

v' ''~ (Cm Yr ~): C o m p o n e n t of convergence rate normal to plate contact.

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Fig. 9. Seismic section 'B' (Fig. 1) across the Inner slope showing folded Neogene sediments. H o r Scale: 25, S P = 1 km (after Roy, 1983).

56

KUMAR

volcanoes, i.e., large strato-volcanic edifices grow in Chilean-type environments, whereas many small monogenic volcanoes are found in the Marinatype areas. Accordingly, in the Andaman-Nicobar region, the volcanics indicate the effects of both the above examples because the difference in magma eruption is also caused by the general conditions under a tensional regime. A conspicuous break in the pattern of shallow-earthquake activity occurs between 5 and 6.5°N and 11 ~ and 13' N. From 6.5 ~ to 8' N, the volcanicity is high, back-arc deformation is limited and developed locally, and cross-arc faulting seems to be well developed (Fig. 5). Part of the volcanicity and deformation may be closely related to the effects of subduction. In general, all of the Quaternary volcanism in the island arc is situated in the area of the epicenters of shocks with focal depths of more than 120 km (Fig. 5). These volcanoes are arranged linearly at an angle to the subduction plane. The inner slopes of the accretionary basins are commonly structured by compressional folds and listric thrust faults and are filled with marine sediments derived from an uplifted subduction complex. Seismic reflection profiles (Figs. 9 to 12) taken along (Fig. 1, B, C, D and

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ANOMALIES,

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Seismic section 'D" (Fig. 1 ) across the inner slope showing folded sediments overlain by a published

y o u n g e r slope basin with less deformed sediments. Hor. Scale 25, S P = 1 k m [ a f t e r O N G C

report R o y , 19831.

J) and across the trench in E-W direction show folded Neogene sediments in the variable younger slope basins, with less deformed sediments indicating the formation of these basisn in the depressed regions of the folded subducting lithosphere. A seismic reflection profile across R - R in Fig. 1 (Fig. 5) in N-S direction shows the sedimentary sequences present in different depression regions over the oceanic crust in the trench region (Curray et al., 1977; Karig et al., 1979; Morre and Karig, 1980) and sedimentary studies (Roy, 1983} indicate that the Andaman-Nicobar region is probably a geologically recent feature (Table lI). The age decreases from Java (140-115my) to Sumatra (70-60 my) to Andaman-Nicobar and further north (Larson, 1972; Curray el al., 1979).

GRAVITY ANOMALIES, SEISMICITY, SUBDUCTING SLAB FOLDING

59

DEPRESSION

DRAGGING OF SEDIMENTS

TENSION

URAL HIGH COMPRESSION

/ ;~

~

LiNE OF VOLCANICS

TION

,%/ FORE-ARC FOLDS FLEXURE FLEXURE SLIP SHEAR Fig. 13. Surface deformations over plunging depressions and ridges formed over the subducting slab due to horizontal compression because of availability of trench space.

The nature of the volcanism suggests a constant subduction dip variation from the known magnetic anomalies from 120 to 80 my and a low-angle subduction after 80 my, showing a direct relationship with the decrease in dip of the subducting slab. This flattening and steepening of the subduction controls the position of epicenters and depth of earthquakes. The shearing J()(; 121-5

60

KUMAR

and thinning produced at the contact zone of a relative dip difference between two segments give the direction and position of volcanism on the surface, which is always at an angle to the subduction zone (Fig. 13).

DISCUSSION

Geophysical and geological evidence indicates an inclined zone produced by the downward movement of the lithospheric plate relative to the island arc. The varying epicentral depth zones suggest that buckling played a considerably role in the development of the subduction angle in different zones (Fig. 8). The long-wavelength, positive gravity anomalies indicate the parts of the subduction zone that are relatively hot and cold, and dense. The convergence rate at subduction boundaries in the Andaman Sea is 5.5 cm/year (Fitch, 1972; Minister et al., 1974). The flexural stresses that are created due to loading produce a vertical displacement which shows 100 regal over the bottom of the slope and its extension towards landward side of the load. Seismological (Press, 1970) and heat-flow (Mckenzie, 1967) studies give the thickness of the oceanic lithosphere, which is about one half or three quarters that of a continent. The lowering leading edge in the asthenosphere undergoes intense local pressures at right angles to the subduction zone. The elastically compressed portions may produce some depressions (synclines) due to compression and across due to tension in the edge of the slab. The accumulation of sediments between the island regions is comparatively greater and is localized. The observed free-air anomaly shows a low of - 2 5 to - 7 5 m g a l superimposed over the gravity highs of the islands, while a negative anomaly exists over the foot of the slope of about - 2 5 to - 5 0 m g a l (Fig. 4). The positions and amplitudes of the gravity "lows" between the islands show good relative bending stresses for flexural models (Walcott, 1972). The deep structures of the continental margin between Sumatra and Burma (Karig et al., 1979), and the correlation of gravity (Fig. 5) with the known geometry from surface geology and seismic studies (Figs. 9 to 12), have provided valuable structural information about the folded and faulted nature of the subducting lithosphere in this region. These observations are important in defining the elastic failure due to flexure and bending stress and to changes in the subduction angle from low to high due to sediment load in the oceanic lithosphere. The other process which may also have helped in down-dip fault and oblique-slip faults in the accumulated sediments is that of buckling at the subduction zones of the oceanic lithosphere with the apex towards the oceanic lithosphere or towards the trench (Fig. 13).

GRAVITY ANOMALIES, SEISMICITY, SUBDUCTING SLAB FOLDING

61

The forces which prevail on an oceanic plate on approaching a deep-sea trench are important in studies of gravity anomalies seaward of trenches. If the down-going slab is de-coupled from the approaching slab by fracturing (Lliboutry, 1969; Kanamori, 1971; Abe, 1972) caused by gravitational instability (Molner and Gray, 1979), perhaps, due to rebuoyancy, the bottom, denser part of the crust is pulled down, resulting in the formation of faults on the surface in this region (Fig. 13). The compression increases when the segments continue to thrust towards each other and when it reaches a critical value of stress in the plate, buckling may occur in the intermediate depths, hence producing areas of instability and maximum seismic activity (Fig. 5). When the descending segment buckles, stress is extensional but due to the confined space, the free subducted end folds and produces ridges and depressions (Figs. 5 and 13). This indicates the stress distribution in the Andaman-Nicobar region and the zone of deformation. The earthquakes at 120 km depth indicate the region where the subducted lithosphere cannot sustain the stresses necessary to generate seismic events. There, the critical temperature governs the melting and rheology of the subducted lithosphere. The change in the geometry of the bending lithosphere in relation to the surface configuration gives an explanation for repeated earthquake events at particular points. Such points are the regions of different deformation which are associated with the different magmatic suites of the volcanic arc, indicating the high heat flow, deep seismicity and shearing in the fault zones in the subducted lithosphere. The segments of the lithosphere along the Andaman-Nicobar region with intermediate focal mechanisms are probably under greater tension that the adjacent segments in which only down-dip compressional events and more gentle bending are found. The depth and focal mechanisms suggest a pattern of extensional stresses down to 25-30 km and compressional stresses in the 40-50 km range. In the Andaman-Nicobar subduction zone, the high stress regime, of a maximum stress level of about 100 MPa, subducts at a shallow dip and is strongly coupled with the lesser stress level of a low value of 10-30 MPa with steep dip of the subducted slab. The geometric characteristics of the subducting lithosphere control the geographic position of sedimentary basins, the positions of volcanoes and the relative positions of epicenters.

ACKNOWLEDGMENTS

The author thanks Dr. V. C. Thakur (Director, WIHG), Dr. K. Sudo and Dr. S. Hattori, IISEE Japan and Prof. I. Yokoyama, Dept. of Geophysics,

62

KUMAR

Hokkaido University, Japan for encouragement and Prof. K. Oike, Disaster Prevention Research Institute, Kyoto University, Japan for critically reading the first draft of the manuscript. The work was supported by the W.I.H.G. Northeast Himalaya Project and a Department of Science & Technology grant for the Seismicity and Seismotectonic Project of the Himalaya.

REFERENCES Abe, K., 1972. Focal processes of the south Sandwich island earthquake of May 26, 1964. Phys. Earth. Planet. Inter., 5: 110-122. Chandra, U., 1975. Seismicity, earthquake mechanisms and tectonics of Burma, 20 N-28 N. Geophy. J. R. Astron. Soc., 40: 267-281. Cahuhan, R. S. K. and Srivastava, V. K., 1975. Focal mechanisms in northeastern India and their tectonic implications. Pure Appl. Geophys., 113: 467~,82. Curray, J. R., Shor, G. G., Raitt, R. W. and Henrt, M., 1977. Seismic refraction and reflection studies of crustal structure of the eastern and western Banda arcs. J. Geophys. Res., 82:2479 2489. Curry, J. R., Moore, D. G., Lawvev, L. A., Emmel, F. J., Raitt, R. W., Henry, M. and Kieckhefer, R., 1979. Tectonics of the Andaman Sea and Burma. Am. Assoc. Pet. Geol. Mem., 29: 189-198. Eguchi, T., Uyeda, S. and Maki, T., 1979. Seismotectonics and tectonic history of the Andaman Sea. In: S. Uyeda (Editor), Processes at subduction zones. Tectonophysics, 57:35 51. Evans, P. and Crompton, W., 1946. Geological factors in gravity interpretation by evidence from India and Burma. Q.J. Geol. Soc. London, 102:211 249. Fitch, T. J., 1970a. Earthquake mechanism and island arc tectonics in the Indonesian-Phillippine region. Geol. Soc. Am. Bull., 60: 565-591. Fitch, T. J., 1970b. Earthquake mechanism in the Himalayan, Burmese and Andaman regions and continental tectonics in central Asia. J. Geophys. Res., 75: 2699-2709. Fitch, T. J. and Molnar, P., 1980. Focal mechanisms along inclined earthquake zones in the IndonesianPhillippine region. J. Geophys. Res., 75: 1431-1444. Fitch, T. J., 1972. Plate convergence, transcurrent faults and the internal deformation adjacent to south east Asia and the western Pacific. J. Geophys. Res., 77: 4432~4460. Gulatee, B. L., 1956. Gravity data in India. Sur. India. Tech. Pap., 10: 195. Hamilton, W. B., 1979. Tectonics of the Indonesian region. U.S. Geol. Surv. Prof. Pap., 1078:345 pp. Kanamori, H., 1971. Great earthquake at island arc and the lithosphere. Tectonophysics, 12:187 198. Karig, D. E., 1972. Remnant arcs. Geol. Soc. Am. Bull., 83:1057 1068. Karig, F. E., Suparka, S., Moore, G. F. and Hehanussa, P., 1979. Structure and Cenozoic evolution in the Sunda arc in the central Sumatra region. Mem. Am. Assoc. Pet. Geol., 29:223 237. Katili, J. A., 1975. Volcanism and plate tectonics in the Indonesian arc. Tectonophysics, 26:165 188. Kumar, S., 1981. Geodynamics of Burma and Andaman-Nicobar region on the basis of tectonic stresses and regional seismicity. Tectonophysics, 79: 75-95. Larson, R. L., 1972. Bathymetry, magnetic anomalies and plate tectonic history of the mouth of the Gulf of California. Geol. Soc. Am. Bull., 83: 3345-3360. Lliboutry, L., 1969. Sea floor spreading, continental drift and lithosphere sinking at melting point. J. Geophys. Res., 74: 6525-6540. Meissner, R. and Strehlan, J., 1982. Limits of stresses in continental crusts and their relation to the depth frequency distribution of shallow earthquakes. Tectonics, 1: 73 89. Mckenzie, D. P., 1967. The viscosity of the mantle. Geophys. J.R. astron. Soc., 14:297 305.

GRAVITY ANOMALIES, SEISMICITY, S U B D U C T I N G SLAB F O L D I N G

63

Minster, J. B., Jordan, T. H., Molnar, P. and Haines, E., 1974. Numerical modeling of instantaneous plate tectonics. Geophys. J.R. astron. Soc., 36:541 576. Molnar, P. and Gray, D., 1979. Subduction of continental lithosphere some constraints and uncertainities. Geology, 7:58 62. Moore, G. F. and Karig, D. E., 1980. Structural geology of Nias island, Indonesia. implication for subduction zone tectonics. Am. J. Sci., 280: 193-223. Mukhopadhyay, M., 1984. Seismotectonics of subduction and block arc rifting under the Andaman sea. Tectonophysics, 108:229 239. Mukhopadhyay, M. and Dasgupta, S., 1988. Deep structure and tectonics of the Burmese arc: constraints from earthquake and gravity data. Tectonophysics, 149: 299-322. Nakamura, R, 1977. Volcanics as possible indicators of tectonic stress orientations--Aleutains & Alaska. Pure Appl. Geophys., 115:87 112. Press, F., 1970. Earth models consistent with geophysical data. Phys. Earth Planet. Inter., 3:3 22. Qureshy, M. N., Kumar, S. and Gupta, G. D., 1989. The Himalaya megalineament -Its geophysical characteristics. Mem. Geol. Soc. of India, 12:207 222. Rastogi, B. K., Singh, J. and Verma, R. K., 1973. Earthquake mechanisms and tectonics in the AssamByrma region. Tectonophysics, 18:355 366. Ringwood. A. E., 1969. Composition and evolution of the upper mantle. In P.J. Hart (Editor). The earth's crust and upper mantle. Geophys. monogr,, B.AM. Geophys. Union Washington, D.C.. pp. 1 -18. Ritsema, A. R. and Valdcamp, J., 1960. Fault plane mechanism of south east Asian earthquakes. Meded. Verh. Kon. Ned. Meteorol. Inst., 76:63 85. Rodolfo, K. S.. 1969. Bathymetry and marine geology of the Andaman basin and tectonic implications for south east Asia. Geol. Soc. AM. Bull., 80:1203 1230. Roy, T. K., 1983. Geology and hydrocarbon prospects of Andaman-Nicobar basin. Petrol. Asia. Jour.. 6:37-50. Sclater, J. G. and Fisher, R. L., 1974. Evolution of east-central Indian ocean with emphasis on the tectonic setting of the ninety-east ridge. Geol. Soc. AM. Bull., 85: 683-702. Stein, S. and Okal, A., 1978. Seismicity and tectonics of the ninety east ridge area, evidence for internal deformation of the Indian plate. J. Geophys., 83:2283 2246. Uyeda, S. and Kanamori, H., 1979. Back arc opening and the mode of the subduction. J. Geophys. Res., 84: 1094-1061. Vlaar, N. J. and Wortel, M. J. R., 1976. Lithospheric instability and subduction. Tectonophysics. 32:331 351. Walcott, R. I., 1972. Late Quaternary vertical movements in eastern north America. Quantitalivc evidence of glaceoisostatic rebound. Rev. Geophys. Space Phys., 10:849 889. Wortel. R., 1982. Seismicity and rheology of subducted slabs. Nature, 296:553 555.