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Detailed survey of the ocean bottom structure in the Central Indian Ocean Intense Deformation Zone: Tectonic implications
Marine Geology, 115 (1993) 165-171
165
Elsevier SciencePublishers B.V., Amsterdam
Letter Section
Detailed survey of the ocean bottom structure in the Central Indian Ocean Intense Deformation Zone: Tectonic implications Oleg V. L e v c h e n k o a, Y u r i D. E v s j u k o v a, C. S u b r a h m a n y a m b, G.S. M i t a l b a n d R . K . D r o l i a b'l aInstitute of Oceanology, USSR Academy of Sciences, 23, Krasikova Street, Moscow 117218, Russian Federation bNational Geophysical Research Institute, Hyderabad 500007, India
(Received May 26, 1993; revision accepted October 7, 1993)
ABSTRACT Levchenko, O.V., Evsjukov, Y.D., Subrahmanyam, C., Mital, G.S. and Drolia, R.K., 1993. Detailed survey of the ocean bottom structure in the Central Indian Ocean Intense Deformation Zone: Tectonicimplications. Mar. Geol., 115: 165-171. New bathymetric and seismic reflection data on the bottom structure of a small area in the Intense Deformation Zone (IDZ) of the Central Indian Ocean show highly deformed crustal blocks. The blocks seem to be controlled by E-W trending faults reflectedin the ocean bottom topography. The data also suggest N-S trending fractures. Structural analysis of the data sets confirm a complex mosaic-block framework of the deformation zone. Tectonic fragmentation occurs on a smaller scale than that suggestedearlier. The presenceof secondarystrongly-offsetlocalisedrises suggests an advanced stage of deformation in this segment of the IDZ.
Introduction The intense tectonic deformation o f sediments and the basement in the equatorial Central Indian Ocean Basin (CIOB) displays clear evidence of intraplate compressive lithospheric deformation on both long-wavelength (100-300km) and shortwavelength (5-20 km) scales as reflected by seismicity (Bergrnan and S o l o m o n , 1985; Petroy and Weins, 1989), geoid and gravity anomalies (Stein et al., 1989) and anomalous heatflow (Stein and Weissel, 1990). Seismic reflection profiles (Eittreim and Ewing, 1972; Weissel et al., 1980; Neprochnov et al., 1988; Bull, 1990; Bull and Scrutton, 1990, 1992; Leger and Louden, 1990) show undulation of the basement in an E - W lineated pattern with basement highs and lows correlating with the geoid anomalies (Geller et al., 1983; Kazmin and Levchenko, 1987). Large faulted basement rises IAuthor to whom all correspondenceshould be addressed 0025-3227/93/$06.00
(50--200 km wide, 1-2 km high) characterize the style of deformation. The overlying asymmetrically folded and thrusted sediments form hummocky swells in the bottom topography rising to 500 m height in an intensely deformed area (0°-5°S, 80°-85°E) of IDZ. Neprochnov et al. (1988) proposed these large-scale deformation features as sets of individual tectonised blocks bounded by preexisting N - S trending fracture zones and E - W trending high-angle faults, alternating with less deformed portions of the sea floor (Kazmin and Levchenko, 1987). These tectonised blocks are superimposed by a second order deformation in the form of faulted blocks (5-20 km in width) bounded by E - W reverse faults with a throw up to 600 m. These reverse faults dip at 35°-40 ° and extend down to the Moho level (Bull and Scrutton, 1990). In sedimentary cover they are steeper (40°-90°). This paper deals with the analysis of detailed bathymetric and seismic reflection profiling data
© 1993 -- Elsevier SciencePublishers B.V. All rights reserved.
SSDIOO25-3227(93)EOI 25-P
166 from Polygon I (70x 100km in area) centred around 2°S, 81.5°E (Fig. 1) collected during cruise 22 of R/V Prof. Shtockman. The seismic reflection profiling data were collected using a small airgun of 0.3 dm 3 (liters). The profiles were 50-100 km long and spaced at 5-10 km. The study area forms a part of the E - W trending Wide Boundary Zone (WBZ) separating distinct India and Australia plates (Gordon and Stein, 1992). The objective of the study was to delineate the bottom topography, to map the boundaries of the tectonised blocks, to examine the style of deformation and its tectonic implications.
Bottom topography The analysis of the bathymetric data reveals that the study area is characterised by very complicated bottom topography with depths varying from 4347 to 4950 m (Fig. 2), which is unusual for deep ocean
o.v. LEVCHENKOETAL. basins. This anomalous character of the deep ocean basin floor is further enhanced by the fact that Polygon I is in the distal part of the Bengal fan where thick sediments obscure the original roughness of the oceanic crustal surface (Eittreim and Ewing, 1972; Moore et al., 1974). The bathymetric map (Fig. 2) clearly shows three distinct geomorphological features: An abyssal plain in the northwest where bottom depths vary from 4770 to 4827 m (4800 m in average), and two hummocky swells having relief of 630 m and 450 m in the northeast and south, respectively. The northeastern swell (swell 1) is characterised by a series of E - W trending, 10-18 km long and 40-150 m high rises. Approximately in the middle portion of the swell, there is a rise 35 km long and 350 m high. All the rises have distinct asymmetric contours with steeper southern slopes (Fig. 2). A comparatively large block of a complex structure is recognised in the southern part of the swell if
Fig. 1. Bathymetry map of Polygon I of cruise 22, R/V Prof. Shtockman. 1 = Polygon boundary, 2 = geophysical profiles, (a) bathymetry, (b) Seismic reflection, 3 = isobaths (m), 4 = swell contours. Inset shows location of the study area.
CENTRAL INDIAN OCEAN INTENSE DEFORMATIONZONE
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Fig. 2. Stacked bathymetry profiles for Polygon I.
which the northward-tilted 6 km wide top is fringed by small rises• In the south, the block is terminated by a steep (15°-17 °) stepped scarp, which is almost 600 m high. The southern swell (swell 2) is defined by a series of E-W trending asymmetric rises• Its northern flank is composed of 6-8 km long and 70-100 m high rises, whereas in the central part, the rises are 30 km long and 250-300 m high. All the rises have steep (7°-9 °) southern slopes and gentle (20-4 °) northern slopes. However, unlike swell 1, there are asymmetric rises with steeper northern slopes• Between these two swells there are small undulations 15-20 m high with gentle slopes within a smooth bottom band of more than 40 km width• According to seismic reflection data these undulations are the crests of large sedimentary folds which are obscured by undeformed flat-lying sediments (Fig. 3). Sediment structure
Examples of seismic reflection records obtained during the survey are shown in Fig. 3, where the
thin bedded sediments typical of the Bengal fan (Cochran et al., 1990) are observed• Both the hummocky swells (swell 1 and swell 2) are formed by the deformed bedded sediments, which are overlain by the undeformed bedded sediments within flat-bottom areas• Narrow depressions between asymmetric rises are also filled by similar flat-lying sediments• We infer from the seismic reflection profiling data that the bottom topography of both swells is of tectonic origin rather than accumulative-erosion origin. Asymmetric rises are, in fact, topographic manifestations of folded sediments. They seem to have originated over the thrusted scarps of the basement rocks similar to those observed in other parts of the CIOB deformation zone (Eittreim and Ewing, 1972; Weissel et al., 1980; Kazmin and Levchenko, 1987; Cochran et al., 1990). In case of major asymmetric folds with steep limbs, the thrusts may have deformed the sediments as well. Within Polygon I, vertical displacements along the faults may reach 450-500 m. From the Ocean Bottom Seismometer data of the present survey, the thickness of the sedimentary
168
o.v. LEVCHENKOET AL.
8..
1
BOKat
Fig. 3. Seismic reflection profiles: (a) southern swell, (b) northeastern swell. Location of the profiles shown in Fig. 4.
cover in Polygon I appears to be about 2 km (V.V. Sedov, pers. commun., 1992), which is similar to that reported by Cochran et al. (1990) for sediments of the distal end of the Bengal fan in the neighbouring area. The acoustic basement could not be imaged because of the small airgun source except in the southeastern part of the Polygon. Fragments of the basement recognised at depths of 1-2 km probably imply the presence of a single tectonically uplifted crustal block. The entire fan section consists of terrigenous turbidites, primarily derived from the Ganges-Brahmaputra delta, interbedded with thin layers of pelagic clays and silts. The main control on the Bengal fan sedimentation seem to be the uplift and erosion of the Himalayas (Brune et al., 1992). The uppermost sequence consists of relatively undeformed sediments which unconformably lap onto a second sequence of intensely deformed sediments. The well-developed unconfirmity 'A', separating the pre-deformation sequence from a post-deformation sequence, marks the onset of the Late Miocene (7.5 Ma) deformation phase of CIOB lithosphere (Cochran et al., 1990). In Polygon I, the topography of the top of the deformed sedimentary sequence (unconfirmity 'A') follows that of the basement. It suggests that the compressive forces have acted on the whole crust. The post-deformational sediment isopach map (Fig. 4) could allow us to construct the tectonic
framework of this region. The data reveal that for most part of the study area, the post-deformational sediments up to 50 m thick, occur as fragments in the form of narrow E-W trending lenses in some interhill depressions. Only in the northwestern part of the area they appear as an entire sedimentary cover whose thickness rapidly reached 200-300 m off the foothills of both the swells. The entire post-deformation sedimentary cover continues to the north and west of the study area. The central seismic reflection profile shows that swell 2 runs continuously southward and postdeformation sediments more than 200 m thick appear only near 3°05'S suggesting that the deformed rise for unconfirmity 'A' is 1l0 km long and 1 km high. We suggest that the western foothills of the swell extends from 81°35' to 81°45'E in N-S direction beyond which the entire postdeformation sedimentary cover occurs. Swell 1 extends outside the survey area. The northern foothill of this swell is approximately at 1°35'S. A deep depression separates it from a local swell composed of three asymmetric rises 100-200 m high with steeper southern slopes (Fig. 3b). In the depression, the top horizon of the exposed deformed sediments is 300 m deep under the seafloor. In the north the depression is terminated by a fault with maximum vertical displacement of 500 m of deformed sediments. Swell 1 and the local swell seem to be parts of a single major
CENTRAL INDIAN OCEAN INTENSE DEFORMATION ZONE 1810
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Fig. 4. Post-deformation sediments (7.5 Ma to present) isopach m a p o f the Polygon I. 1 = Seismic reflection profiles with dotted lines f r o m other cruises, 2 = isopachs (for v = 2 k m s - 1), 3 = interhill depressions, 4 = entire cover.
deformed rise of the basement. The regional upgrade of the deformed sequence starts near 82°55'E with 600 m thick post-deformation sediments. Thus, the rise for unconfirmity 'A' is 1.1 km high and 130 km long. Because of the absence of seismic reflection data from the southern and eastern parts of Polygon I the complete configuration of the intensely deformed tectonic block could not be mapped. Tectonics
Because of sparse data, the contour, inner structure and relationship of the two major tectonic features (northeastern and southern swells) of the area could be limitedly defined and the basement structure was recognised by indirect evidence. The most distinct tectonic features are the E - W trending asymmetric folds and flexures which seem to correspond to thrusts in the basement (Fig. 5). The sharp morphological changes may be the evidence of the presence of N - S trending faults/fracture zones. We have identified three N - S trending faults/fracture zones (Fig. 5). The most
prominent one is along the western foothill of swell 1 along 81°25'E. Various tectonic blocks may abut along these inferred faults. A similar juxtaposition of major tectonic blocks along an old N - S trending transform fault (Indrani Fracture zone) has been inferred from geophysical signatures near 3°-6°S (Kazmin and Levchenko, 1987; Neprochnov et al., 1988) based on differences in the structure of the deeper crustal layers on both sides of the fault as well as a sharp change in the morphology of the basement and sedimentary cover along the fault. The morphological signatures of the tectonic deformation in Polygon I suggest that major deformed tectonic blocks are controlled by sets of small E - W trending and probably large N - S trending faults. It is possible that there are also smaller N - S trending faults which have brought about a smaller scale tectonic fragmentation of the crust (Figs. 3 and 5). The structural analysis of the data set confirms earlier inferences (Neprochnov et al., 1988) of complex mosaic-block framework of the deformation zone as a result of the N - S compressive regime prevailing in the CIOB.
170
o.v. LEVCHENKOETAL.
Fig. 5. Tectonicsof the study area. 1 = Seismic reflection profiles, 2 = swell contours, 3 = E-W trending folds, the teethed patterns show the direction of the steeper limbs of asymmetric the folds, the size of the teeth being proportional to the fold amplitude, 4 = proposed N-S trending faults.
Discussion Seismic reflection profiles gave evidence for the occurrence of gentle folds and high-angle reverse faults in sedimentary cover (Neprochnov et al., 1988; Bull and Scrutton, 1992). The area is characterised by widespread anticlinal folds ( 5 - 1 0 k m wavelength) with or without associated faulting. These faults are either limited to deeper part of the sedimentary cover or extend to both the sedimentary cover and the basement. Anticlines become hanging walls when faulting occurs. On most seismic profiles, the unconfirmities 'A' and/or 'B' and the onlap representing onset of deformation are recognizable upto 3 or more onlap patterns above the unconfirmity (Bull and Scrutton, 1992). All the observed faults are north-dipping highangle reverse faults spaced 8-10 km with mean length of 10 km and throw upto 600 m. The dip changes from 20 ° just above the basement to subvertical higher in the sedimentary succession. These high-angle reverse faults appear to extend down to the Moho, although the Moho is not clearly imaged in seismic sections. In one of the multichannel seismic profiles of Bull and Scrutton (1992,
fig. 3), there is evidence of decollement within the oceanic crust with a dip of 17°-18 °. Figure 3a depicts large amplitude displacement on north-dipping faults with characteristic features such as hanging-wall anticline, fault plane dips of 37 ° in the basement and a large ( > 6 0 0 m) offset of the basement/cover interface (see also Bull and Scrutton, 1992, fig. 4). Conclusions Major deformed tectonic blocks in the study area are controlled by sets of small E - W trending and probably large N - S trending faults. It is possible that there are also smaller N - S trending faults which have brought about a smaller scale tectonic fragmentation of the crust. Structural analyses of the data show a complex mosaic-block framework of the IDZ. The study area falls in an anomalous region (0°-5°S, 80°-85°E) of the E - W trending Wide Boundary Zone (Gordon and Stein, 1992). The occurrence of tight asymmetric folds with steeper north-dipping limbs and high-angle north-dipping thrust faults (Fig. 3), occurrence of decollement
CENTRAL INDIAN OCEAN INTENSE DEFORMATION ZONE
(Bull and Scrutton, 1992), high seismicity, anomalous heatflow, and a N - S compressive regime suggest that the area is in an advanced stage of deformation localization in this segment of the IDZ. The clusters of north-dipping thrust faults may eventually give way to trench formation in the near geological future, which is likely to develop into a site of lithospheric subduction.
Acknowledgements The authors wish to express their appreciation to the scientific staff and crew of the R/V Prof. Shtockman, who contributed to the successful implementation of the bathymetric and seismic reflection survey during cruise 22. The helpful comments of D. Jongsma and S.L. Eittreim improved the text of the manuscript. CS, GSM and R K D thank The Director, National Geophysical Research Institute, for his kind permission to publish this paper.
References Bergman, E.A. and Solomon, S.C., 1985. Earthquake source mechanisms from body wave form inversion and intraplate tectonics in the northern Indian ocean. Phys. Earth Planet. Inter., 40: 1-23. Brune, J.N., Curray, J., Dorman L. and Raitt, R., 1992. A proposed super-thick sedimentary basin, Bay of Bengal. Geophys. Res. Lett., 19: 656-568. Bull, J.M., 1990. Structural style of intra-plate deformation, Central Indian Ocean Basin: Evidence for role of fracture zones. Tectonophysics, 184: 213-228. Bull, J.M. and Scrutton, R.A., 1990. Fault reactivation in the
171 Central Indian Ocean Basin and the rheology of the oceanic lithosphere. Nature, 344: 855-858. Bull, J.M. and Scrutton, R.A., 1992. Seismic reflection images of intraplate deformation, Central Indian Ocean and their tectonic significance. J. Geol. Soc. London, 144: 955-966. Cochran, J.R., Stow, D.A.V. et al., 1990. Proc. ODP, Sci. Results, 116, 445 pp. Eittreim, S.L. and Ewing, J., 1972. Mid-plate tectonics in the Indian Ocean. J. Geophys. Res., 77: 6413-6421. Geller, C.A., Weissel, J.K. and Anderson, R.N., 1983. Heat transfer and intraplate deformation in the Central Indian Ocean. J. Geophys. Res., 88: 1018-1032. Gordon, R.G. and Stein, S., 1992. Global Tectonics and Space Geodesy. Science, 256: 333-342. Kazmin, V.G. and Levchenko, O.V., 1987. Recent deformation of the Indian Ocean Lithosphere. In : Yu.M. Pusharovsky (Editor), Recent Tectonic Activity of Earth and Seismicity. Nauka, Moscow, pp. 159-175 (in Russian). Leger, G.T. and Louden, K.E., 1990. Seismic refraction measurements in the Central Indian Basin: Evidence for crustal thickening related to intraplate deformation. Proc. ODP, Sci. Results, 116: 291-309. Moore, D.G., Curray, J.R., Raitt, R.W. and Emmel, F.J., 1974. Stratigraphic seismic section correlations and implications to Bengal Fan history. Init. Rep. DSDP, 22: 403-412. Neprochnov, Yu.P., Levchenko, O.V., Merklin, L.R. and Sedov, V.V., 1988. The structure and tectonics of the intraplate deformation area in the Indian Ocean. Tectonophysics, 158: 89-106. Petroy, D.E. and Weins, D.A., 1989. Historical seismicity and implications for diffuse plate convergence in the Northeast Indian Ocean. J. Geophys. Res., 94: 12301-12319. Stein, C.A. and Weissel, J.K., 1990. Constraints on the Central Indian Basin thermal structure from heatflow,seismicity and bathymetry. Tectonophysics, 176:315 332. Stein, C.A., Cloetingh, S. and Wortel, R., 1989. SEASAT derived gravity constraints on stress and deformation in the Northeastern Indian Ocean. Geophys. Res. Lett., 16: 823-826. Weissel, J.K., Anderson, R.N. and Gellar, C.A., 1980. Deformation of the Indo-Australian plate. Nature, 287: 284-291.
165
Elsevier SciencePublishers B.V., Amsterdam
Letter Section
Detailed survey of the ocean bottom structure in the Central Indian Ocean Intense Deformation Zone: Tectonic implications Oleg V. L e v c h e n k o a, Y u r i D. E v s j u k o v a, C. S u b r a h m a n y a m b, G.S. M i t a l b a n d R . K . D r o l i a b'l aInstitute of Oceanology, USSR Academy of Sciences, 23, Krasikova Street, Moscow 117218, Russian Federation bNational Geophysical Research Institute, Hyderabad 500007, India
(Received May 26, 1993; revision accepted October 7, 1993)
ABSTRACT Levchenko, O.V., Evsjukov, Y.D., Subrahmanyam, C., Mital, G.S. and Drolia, R.K., 1993. Detailed survey of the ocean bottom structure in the Central Indian Ocean Intense Deformation Zone: Tectonicimplications. Mar. Geol., 115: 165-171. New bathymetric and seismic reflection data on the bottom structure of a small area in the Intense Deformation Zone (IDZ) of the Central Indian Ocean show highly deformed crustal blocks. The blocks seem to be controlled by E-W trending faults reflectedin the ocean bottom topography. The data also suggest N-S trending fractures. Structural analysis of the data sets confirm a complex mosaic-block framework of the deformation zone. Tectonic fragmentation occurs on a smaller scale than that suggestedearlier. The presenceof secondarystrongly-offsetlocalisedrises suggests an advanced stage of deformation in this segment of the IDZ.
Introduction The intense tectonic deformation o f sediments and the basement in the equatorial Central Indian Ocean Basin (CIOB) displays clear evidence of intraplate compressive lithospheric deformation on both long-wavelength (100-300km) and shortwavelength (5-20 km) scales as reflected by seismicity (Bergrnan and S o l o m o n , 1985; Petroy and Weins, 1989), geoid and gravity anomalies (Stein et al., 1989) and anomalous heatflow (Stein and Weissel, 1990). Seismic reflection profiles (Eittreim and Ewing, 1972; Weissel et al., 1980; Neprochnov et al., 1988; Bull, 1990; Bull and Scrutton, 1990, 1992; Leger and Louden, 1990) show undulation of the basement in an E - W lineated pattern with basement highs and lows correlating with the geoid anomalies (Geller et al., 1983; Kazmin and Levchenko, 1987). Large faulted basement rises IAuthor to whom all correspondenceshould be addressed 0025-3227/93/$06.00
(50--200 km wide, 1-2 km high) characterize the style of deformation. The overlying asymmetrically folded and thrusted sediments form hummocky swells in the bottom topography rising to 500 m height in an intensely deformed area (0°-5°S, 80°-85°E) of IDZ. Neprochnov et al. (1988) proposed these large-scale deformation features as sets of individual tectonised blocks bounded by preexisting N - S trending fracture zones and E - W trending high-angle faults, alternating with less deformed portions of the sea floor (Kazmin and Levchenko, 1987). These tectonised blocks are superimposed by a second order deformation in the form of faulted blocks (5-20 km in width) bounded by E - W reverse faults with a throw up to 600 m. These reverse faults dip at 35°-40 ° and extend down to the Moho level (Bull and Scrutton, 1990). In sedimentary cover they are steeper (40°-90°). This paper deals with the analysis of detailed bathymetric and seismic reflection profiling data
© 1993 -- Elsevier SciencePublishers B.V. All rights reserved.
SSDIOO25-3227(93)EOI 25-P
166 from Polygon I (70x 100km in area) centred around 2°S, 81.5°E (Fig. 1) collected during cruise 22 of R/V Prof. Shtockman. The seismic reflection profiling data were collected using a small airgun of 0.3 dm 3 (liters). The profiles were 50-100 km long and spaced at 5-10 km. The study area forms a part of the E - W trending Wide Boundary Zone (WBZ) separating distinct India and Australia plates (Gordon and Stein, 1992). The objective of the study was to delineate the bottom topography, to map the boundaries of the tectonised blocks, to examine the style of deformation and its tectonic implications.
Bottom topography The analysis of the bathymetric data reveals that the study area is characterised by very complicated bottom topography with depths varying from 4347 to 4950 m (Fig. 2), which is unusual for deep ocean
o.v. LEVCHENKOETAL. basins. This anomalous character of the deep ocean basin floor is further enhanced by the fact that Polygon I is in the distal part of the Bengal fan where thick sediments obscure the original roughness of the oceanic crustal surface (Eittreim and Ewing, 1972; Moore et al., 1974). The bathymetric map (Fig. 2) clearly shows three distinct geomorphological features: An abyssal plain in the northwest where bottom depths vary from 4770 to 4827 m (4800 m in average), and two hummocky swells having relief of 630 m and 450 m in the northeast and south, respectively. The northeastern swell (swell 1) is characterised by a series of E - W trending, 10-18 km long and 40-150 m high rises. Approximately in the middle portion of the swell, there is a rise 35 km long and 350 m high. All the rises have distinct asymmetric contours with steeper southern slopes (Fig. 2). A comparatively large block of a complex structure is recognised in the southern part of the swell if
Fig. 1. Bathymetry map of Polygon I of cruise 22, R/V Prof. Shtockman. 1 = Polygon boundary, 2 = geophysical profiles, (a) bathymetry, (b) Seismic reflection, 3 = isobaths (m), 4 = swell contours. Inset shows location of the study area.
CENTRAL INDIAN OCEAN INTENSE DEFORMATIONZONE
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'"
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Fig. 2. Stacked bathymetry profiles for Polygon I.
which the northward-tilted 6 km wide top is fringed by small rises• In the south, the block is terminated by a steep (15°-17 °) stepped scarp, which is almost 600 m high. The southern swell (swell 2) is defined by a series of E-W trending asymmetric rises• Its northern flank is composed of 6-8 km long and 70-100 m high rises, whereas in the central part, the rises are 30 km long and 250-300 m high. All the rises have steep (7°-9 °) southern slopes and gentle (20-4 °) northern slopes. However, unlike swell 1, there are asymmetric rises with steeper northern slopes• Between these two swells there are small undulations 15-20 m high with gentle slopes within a smooth bottom band of more than 40 km width• According to seismic reflection data these undulations are the crests of large sedimentary folds which are obscured by undeformed flat-lying sediments (Fig. 3). Sediment structure
Examples of seismic reflection records obtained during the survey are shown in Fig. 3, where the
thin bedded sediments typical of the Bengal fan (Cochran et al., 1990) are observed• Both the hummocky swells (swell 1 and swell 2) are formed by the deformed bedded sediments, which are overlain by the undeformed bedded sediments within flat-bottom areas• Narrow depressions between asymmetric rises are also filled by similar flat-lying sediments• We infer from the seismic reflection profiling data that the bottom topography of both swells is of tectonic origin rather than accumulative-erosion origin. Asymmetric rises are, in fact, topographic manifestations of folded sediments. They seem to have originated over the thrusted scarps of the basement rocks similar to those observed in other parts of the CIOB deformation zone (Eittreim and Ewing, 1972; Weissel et al., 1980; Kazmin and Levchenko, 1987; Cochran et al., 1990). In case of major asymmetric folds with steep limbs, the thrusts may have deformed the sediments as well. Within Polygon I, vertical displacements along the faults may reach 450-500 m. From the Ocean Bottom Seismometer data of the present survey, the thickness of the sedimentary
168
o.v. LEVCHENKOET AL.
8..
1
BOKat
Fig. 3. Seismic reflection profiles: (a) southern swell, (b) northeastern swell. Location of the profiles shown in Fig. 4.
cover in Polygon I appears to be about 2 km (V.V. Sedov, pers. commun., 1992), which is similar to that reported by Cochran et al. (1990) for sediments of the distal end of the Bengal fan in the neighbouring area. The acoustic basement could not be imaged because of the small airgun source except in the southeastern part of the Polygon. Fragments of the basement recognised at depths of 1-2 km probably imply the presence of a single tectonically uplifted crustal block. The entire fan section consists of terrigenous turbidites, primarily derived from the Ganges-Brahmaputra delta, interbedded with thin layers of pelagic clays and silts. The main control on the Bengal fan sedimentation seem to be the uplift and erosion of the Himalayas (Brune et al., 1992). The uppermost sequence consists of relatively undeformed sediments which unconformably lap onto a second sequence of intensely deformed sediments. The well-developed unconfirmity 'A', separating the pre-deformation sequence from a post-deformation sequence, marks the onset of the Late Miocene (7.5 Ma) deformation phase of CIOB lithosphere (Cochran et al., 1990). In Polygon I, the topography of the top of the deformed sedimentary sequence (unconfirmity 'A') follows that of the basement. It suggests that the compressive forces have acted on the whole crust. The post-deformational sediment isopach map (Fig. 4) could allow us to construct the tectonic
framework of this region. The data reveal that for most part of the study area, the post-deformational sediments up to 50 m thick, occur as fragments in the form of narrow E-W trending lenses in some interhill depressions. Only in the northwestern part of the area they appear as an entire sedimentary cover whose thickness rapidly reached 200-300 m off the foothills of both the swells. The entire post-deformation sedimentary cover continues to the north and west of the study area. The central seismic reflection profile shows that swell 2 runs continuously southward and postdeformation sediments more than 200 m thick appear only near 3°05'S suggesting that the deformed rise for unconfirmity 'A' is 1l0 km long and 1 km high. We suggest that the western foothills of the swell extends from 81°35' to 81°45'E in N-S direction beyond which the entire postdeformation sedimentary cover occurs. Swell 1 extends outside the survey area. The northern foothill of this swell is approximately at 1°35'S. A deep depression separates it from a local swell composed of three asymmetric rises 100-200 m high with steeper southern slopes (Fig. 3b). In the depression, the top horizon of the exposed deformed sediments is 300 m deep under the seafloor. In the north the depression is terminated by a fault with maximum vertical displacement of 500 m of deformed sediments. Swell 1 and the local swell seem to be parts of a single major
CENTRAL INDIAN OCEAN INTENSE DEFORMATION ZONE 1810
i\
l/+0'
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--
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169
120'
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Fig. 4. Post-deformation sediments (7.5 Ma to present) isopach m a p o f the Polygon I. 1 = Seismic reflection profiles with dotted lines f r o m other cruises, 2 = isopachs (for v = 2 k m s - 1), 3 = interhill depressions, 4 = entire cover.
deformed rise of the basement. The regional upgrade of the deformed sequence starts near 82°55'E with 600 m thick post-deformation sediments. Thus, the rise for unconfirmity 'A' is 1.1 km high and 130 km long. Because of the absence of seismic reflection data from the southern and eastern parts of Polygon I the complete configuration of the intensely deformed tectonic block could not be mapped. Tectonics
Because of sparse data, the contour, inner structure and relationship of the two major tectonic features (northeastern and southern swells) of the area could be limitedly defined and the basement structure was recognised by indirect evidence. The most distinct tectonic features are the E - W trending asymmetric folds and flexures which seem to correspond to thrusts in the basement (Fig. 5). The sharp morphological changes may be the evidence of the presence of N - S trending faults/fracture zones. We have identified three N - S trending faults/fracture zones (Fig. 5). The most
prominent one is along the western foothill of swell 1 along 81°25'E. Various tectonic blocks may abut along these inferred faults. A similar juxtaposition of major tectonic blocks along an old N - S trending transform fault (Indrani Fracture zone) has been inferred from geophysical signatures near 3°-6°S (Kazmin and Levchenko, 1987; Neprochnov et al., 1988) based on differences in the structure of the deeper crustal layers on both sides of the fault as well as a sharp change in the morphology of the basement and sedimentary cover along the fault. The morphological signatures of the tectonic deformation in Polygon I suggest that major deformed tectonic blocks are controlled by sets of small E - W trending and probably large N - S trending faults. It is possible that there are also smaller N - S trending faults which have brought about a smaller scale tectonic fragmentation of the crust (Figs. 3 and 5). The structural analysis of the data set confirms earlier inferences (Neprochnov et al., 1988) of complex mosaic-block framework of the deformation zone as a result of the N - S compressive regime prevailing in the CIOB.
170
o.v. LEVCHENKOETAL.
Fig. 5. Tectonicsof the study area. 1 = Seismic reflection profiles, 2 = swell contours, 3 = E-W trending folds, the teethed patterns show the direction of the steeper limbs of asymmetric the folds, the size of the teeth being proportional to the fold amplitude, 4 = proposed N-S trending faults.
Discussion Seismic reflection profiles gave evidence for the occurrence of gentle folds and high-angle reverse faults in sedimentary cover (Neprochnov et al., 1988; Bull and Scrutton, 1992). The area is characterised by widespread anticlinal folds ( 5 - 1 0 k m wavelength) with or without associated faulting. These faults are either limited to deeper part of the sedimentary cover or extend to both the sedimentary cover and the basement. Anticlines become hanging walls when faulting occurs. On most seismic profiles, the unconfirmities 'A' and/or 'B' and the onlap representing onset of deformation are recognizable upto 3 or more onlap patterns above the unconfirmity (Bull and Scrutton, 1992). All the observed faults are north-dipping highangle reverse faults spaced 8-10 km with mean length of 10 km and throw upto 600 m. The dip changes from 20 ° just above the basement to subvertical higher in the sedimentary succession. These high-angle reverse faults appear to extend down to the Moho, although the Moho is not clearly imaged in seismic sections. In one of the multichannel seismic profiles of Bull and Scrutton (1992,
fig. 3), there is evidence of decollement within the oceanic crust with a dip of 17°-18 °. Figure 3a depicts large amplitude displacement on north-dipping faults with characteristic features such as hanging-wall anticline, fault plane dips of 37 ° in the basement and a large ( > 6 0 0 m) offset of the basement/cover interface (see also Bull and Scrutton, 1992, fig. 4). Conclusions Major deformed tectonic blocks in the study area are controlled by sets of small E - W trending and probably large N - S trending faults. It is possible that there are also smaller N - S trending faults which have brought about a smaller scale tectonic fragmentation of the crust. Structural analyses of the data show a complex mosaic-block framework of the IDZ. The study area falls in an anomalous region (0°-5°S, 80°-85°E) of the E - W trending Wide Boundary Zone (Gordon and Stein, 1992). The occurrence of tight asymmetric folds with steeper north-dipping limbs and high-angle north-dipping thrust faults (Fig. 3), occurrence of decollement
CENTRAL INDIAN OCEAN INTENSE DEFORMATION ZONE
(Bull and Scrutton, 1992), high seismicity, anomalous heatflow, and a N - S compressive regime suggest that the area is in an advanced stage of deformation localization in this segment of the IDZ. The clusters of north-dipping thrust faults may eventually give way to trench formation in the near geological future, which is likely to develop into a site of lithospheric subduction.
Acknowledgements The authors wish to express their appreciation to the scientific staff and crew of the R/V Prof. Shtockman, who contributed to the successful implementation of the bathymetric and seismic reflection survey during cruise 22. The helpful comments of D. Jongsma and S.L. Eittreim improved the text of the manuscript. CS, GSM and R K D thank The Director, National Geophysical Research Institute, for his kind permission to publish this paper.
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171 Central Indian Ocean Basin and the rheology of the oceanic lithosphere. Nature, 344: 855-858. Bull, J.M. and Scrutton, R.A., 1992. Seismic reflection images of intraplate deformation, Central Indian Ocean and their tectonic significance. J. Geol. Soc. London, 144: 955-966. Cochran, J.R., Stow, D.A.V. et al., 1990. Proc. ODP, Sci. Results, 116, 445 pp. Eittreim, S.L. and Ewing, J., 1972. Mid-plate tectonics in the Indian Ocean. J. Geophys. Res., 77: 6413-6421. Geller, C.A., Weissel, J.K. and Anderson, R.N., 1983. Heat transfer and intraplate deformation in the Central Indian Ocean. J. Geophys. Res., 88: 1018-1032. Gordon, R.G. and Stein, S., 1992. Global Tectonics and Space Geodesy. Science, 256: 333-342. Kazmin, V.G. and Levchenko, O.V., 1987. Recent deformation of the Indian Ocean Lithosphere. In : Yu.M. Pusharovsky (Editor), Recent Tectonic Activity of Earth and Seismicity. Nauka, Moscow, pp. 159-175 (in Russian). Leger, G.T. and Louden, K.E., 1990. Seismic refraction measurements in the Central Indian Basin: Evidence for crustal thickening related to intraplate deformation. Proc. ODP, Sci. Results, 116: 291-309. Moore, D.G., Curray, J.R., Raitt, R.W. and Emmel, F.J., 1974. Stratigraphic seismic section correlations and implications to Bengal Fan history. Init. Rep. DSDP, 22: 403-412. Neprochnov, Yu.P., Levchenko, O.V., Merklin, L.R. and Sedov, V.V., 1988. The structure and tectonics of the intraplate deformation area in the Indian Ocean. Tectonophysics, 158: 89-106. Petroy, D.E. and Weins, D.A., 1989. Historical seismicity and implications for diffuse plate convergence in the Northeast Indian Ocean. J. Geophys. Res., 94: 12301-12319. Stein, C.A. and Weissel, J.K., 1990. Constraints on the Central Indian Basin thermal structure from heatflow,seismicity and bathymetry. Tectonophysics, 176:315 332. Stein, C.A., Cloetingh, S. and Wortel, R., 1989. SEASAT derived gravity constraints on stress and deformation in the Northeastern Indian Ocean. Geophys. Res. Lett., 16: 823-826. Weissel, J.K., Anderson, R.N. and Gellar, C.A., 1980. Deformation of the Indo-Australian plate. Nature, 287: 284-291.