The crustal structure of western Bangladesh from gravity data

341

Tectonophysics, 219 (1993) 341-353 Elsevier Science Publishers B.V., Amsterdam

The crustal structure of western Bangladesh from gravity data A.A. Khan a and B.N.P. Agarwal b

a Department of Geology and Mining, Rajshahi University Rajshahi, Bangladesh b Department of Applied Geophysics, Indian School of Mines, Dhanbad 826 004, India (Received October 21,199l; revised version accepted August 21,1992)

ABSTRACT Khan, A.A. and Agarwal, B.N.P., 1993. The crustal structure of western Bangladesh from gravity data. Tectonophysics, 219: 341-353. The observed Bouguer gravity anomalies along two profiles across the platform flank and foredeep (Faridpur trough) situated in the western part of the Bengal basin have been analysed in terms of their corresponding plausible crustal models. The energy density spectra of these anomaly profiles provide the average depths of the anomalous sources lying along different horizons, which are used to design suitable matched filter responses for deconvolving various individual gravity effects. Under the assumption of the entire anomaly being produced by a fluctuating interface, the nature of interfaces corresponding to various gravity effects have been computed from the (sin X)/X method of Tsuboi (1979). Information obtained from independent sources, such as drill holes, seismic and aeromagnetic data interpretation togetherwith the results of the present investigation, has been used to create plausible crustal models up to the Moho depth to explain the known tectonic history of the region. The analyses reveal that the platform part of the Bengal basin, underlain by continental crust, is characterized by the presence of antithetic and synthetic faults, and that the Faridpur trough is underlain by oceanic crust.

Introduction Bangladesh, a tropical to subtropical country, is located in Southeast Asia between latitudes of 20”N and 26”4O’N and longitudes of WE and 92”41’E. Geographically, western Bangladesh occupies a continental shelf (platfom hank) in the north and a foredeep (Faridpur trough) in the south, as tectonic elements of the Bengal basin. The present study covers only the western part of Bangladesh, where the gravity profiles A-B and C-D (Fig. 1) are taken across the platform flank and Faridpur trough, respectively. During the 1950s and 196Os, the Oil and Gas Development Corporation, and a few major oil companies such as Shell Oil and Stanvac from the

Correspondence to: B.N.P. Aganval, Department of Applied Geophysics, Indian School of Mines, Dhanbad 826 004, India. 0040-1951/93/$06.00

USA and Pakistan Petroleum Limited, conducted reconnaissance gravity and magnetic surveys. The Bangladesh Oil, Gas and Mineral Corporation (BOGMC) continued gravity surveying at a scale of 1: 50000. Around 15000 observations were used to. compile a Bouguer anomaly map on a scale of 1: lOOCjOO0, with a contour interval of 2 mGa1 (Ah and Raghava, 1985). An aeromagnetic survey was conducted in 1981 by Hunting Geology and Geophysics Ltd, U.K. under the sponsorship of the Geological Survey of Bangladesh and BOGMC. An interpreted map (anonymous, 19811, showing the basement depths and faults from aeromagnetic data, has been used as supplementary information to study the tectonics of the area. BOGMC have also acquired 2100 line kilometre on-shore multifold seismic data, covering only a small portion of the present study area. The deepest reflector, R7, corresponds to Upper Cretaceous to Palaeocene sediments (Lietz and Kabir, 1982). No deep seismic sounding work has so far

8 1993 - Elsevier Science Publishers B.V. All rights reserved

A.A. KHAN AND B.N.P. AGARWAL

342

been carried out in the Bengal basin. Many boreholes drilled in the northern portion of the study area have encountered Arehaean gneissic basement at a shallow depth (Zaher and Rahman, 1980>, Thus, in view of the existing database, the only available tool to study the crustat structure is possibly through the analysis of the gravity data of the Bengal basin. The widespread use of spectral analysis in the interpretation of gravity and magnetic field measurements has been recognized in scores of publications (such as Dean, 1958; Odegard and Berg, X965; Spector and Grant, 1970; Syberg, 1972; Bhattacharyya and Leu, 1977; Granser, 1987).

21

MAJOR TECTONIC . =nklr LVRC SEISMIC L’NC . ..- .Db.( l- PKl- EXT,HOR-21 T FAULT

'i

-f-

\ -

-i’KOFILE l

WFLL

8%

Chakraborty and Agarwal (1990) have demonstrated the application of wavelength filtering of the gravity field in det~~n~g the crustal eonfiguration. The basic problem with inte~retation of any observed Bouguer anomaly lies in the separation of the regional component - which is associated with tong-wavelength anomalies originating at a considerable depth - and the residual component - due to shallow inhomogeneities, producing short-wavelength anomalies. However, several efforts have been made to evolve adequate procedures by incorporating a prior condition either on the nature of the regional trend (Paul, 1967) or

Y$ Oewkm

x:\ u

DIRECTION

LOCATION, e9

90

Ll

L 911

1 921

Fig. LA geographical map of Bangladesh, showing locations d the gravity profiles A-B and C-D; the seismic lines PKl/PKl-EXT, HOR-21 and HOR-21 EXT, the major tectonic zones; and basement faults, and the epicentre of an earthquake along with its focat

plane solutiou. &STand Sf are the locations of borehofes at Mufadi and Singra, respectively.

343

THE CRUSTAL STRUCTURE OF WESTERN BANGLADESH

by considering the purely statistical nature of the random distribution of sources (Spector and Grant, 1970). Zurfleuh (1967) has suggested the use of appropriate low-pass filters as a tool to separate out effects due to deep and shallow features. Therefore, a decision on the correct choice of the unknown desired cut-off frequen~/wavelength has to be made carefully in order to achieve a meaningful separation of anomalies. Furthermore, the effects of the shallow features are likely to be passed on to the effects due to sources at large depths. Spector and Grant (1970) have described a method of dete~ining the average ensemble depths to various horizons containing a random distribution of causative sources from the plot of the logarithm of the energy of the magnetic anomalies versus radial frequency. Syberg (1972) has extended the method to gra~tation~ fields and proposed a procedure for designing a matched filter to separate out the gravity effects associated with different horizons. Under the assumption that the gravity effect is produced entirely due to the changes in an interface between two different formations, Tomoda and Aki’s (195% method of (sin X)/X provides a computation of the nature of fluctuations in the interface. The technique of selective wavelength filtering (Syberg, 1972) has been used to deduce crustal structures through the analyses of two gravity profiles, across the platform flank (profile A-B), and across the foredeep part (Faridpur trough, profile C-D) of the Bengal basin (Fig. 1). The results obtained from the analyses of the gravity data, boreholes and seismic info~ation and basement faults have been incorporated to evolve plausible crustal models and to decipher the tectonic history of the region. Forward modelling of these proposed geological cross-sections has been undertaken to compare the observed and the computed fields. Theory With the aim of clarity and completeness, we present here brief theories for the design of a matched filter to deconvolve the gravity effect originating at a certain depth level, and the

(sin X)/X method of Tomoda and Aki (1955) to compute fluctuations in the interface between two different fo~ations. Matched filter

By considering the gravity field, Ag(x) produced by a continuous distribution of point masses throughout the infinite half-space, Syberg (1972) has derived a relation for the energy spectrum, I G(u) I 2 as: IG(u)12=AT

exp[-2h,IuI] +A$ exp[-2k,lul]

+iV@

(11

where the first and the second terms correspond to the regional and residual components of the observed gravity field, respectively, and u?v corresponds to the white noise spectrum. The constants A, and A, depend upon the magnitudes of the anomalous masses situated at ensemble average depths hi and h,, respectively, and u is the angular frequency. In view of the zero phase shift requirement of the majority of geophysical filters used in processing potential field data, Syberg (1972) and Spector (1975) have approximated the amplitude spectrum, G(u), by: IC(u)l

=A, exp[-h&4]

+A, exp[-&lull

+EV

(2)

which can be rearranged

as:

IG(u)l =A,exp[-h,lu where W(u) = 1 +A,

exp((h,

-

+ WN exp( h, I u

It is therefore evident from the above equation that a suitable matched filter response, W-l (W x W- ’ = 1) in the frequency domain can be computed from the values of A,, A,, h, and h,all being computed from the plot of the logarithm of energy versus angular frequency - to pass only one of the two components present in the observed Bouguer anomaly. It is quite likely that an observed anomaly may consist of more than two components (Syberg,

AA. KHAN AND B.N.P. AGARWAL

344

1972). In such a case, eq. (2) can be generalized to incorporate N distinct levels, hi, of the concentration of point masses, as: IG(u)l = zAi

exp(-h,lul)+m

i=l

(5)

from which the gravity effect corresponding to the jth level can be obtained from the application of a matched filter response, ~-“CU>, given by:

+

f%VexP( I U I)/Aj hj

(6)

large as compared to the average depth, h, over which the differential masses are supposed to be distributed. Let the values of the gravity anomalies at a series of equispaced points along the plane of observation be denoted by: . . . . b-3, b_,, b_,, b,, b,, b,, b,,... then the variations in ampiitudes of f(n) Iying at x = nu and depth z = h, required to produce this gravity dist~bution, are given by: f(n)

= f

44Ws,-,

(7)

-M

with Determination of the undulations of the interface

Tomoda and Aki (1955) have proposed a very elegant technique for determining the undulations in an interface, f(x), which are not too

1 4(n) = 2~

h h2 + (nn)’

[(- 1) ’ eh - l]

(8)

and h = D/a; where CTis the density contrast along the interface, y is the gravitational con-

Fig. 2. A map showing major tectonic elements of the Bengal basin (modified from Sengupta, 1966; Guha, 1978; Khan and Rahman, 1992): 1 = northern slope of platform; 2 = stable platform; 3 = intracratonic high; 4 = southern part of platform; 5 = Faridpur trough; 6 = Sylhet trough; 7 t= Barisal-Chandpur gravity high; 8 = Hatia trough; and 9 = eastern folded Tertiary belt.

TKE

CRUSTAL

STRWCNRE

OF

WESTEBN

345

BANGLAUESH

GEOLOGICAL

INTERPRETATION HOR.21

OF

SEISMIC

LINES

HOR.21EXT

LEGEND

Fig. 3, The geofogicat i~te~~~t~o

of the seismic Iine HOR-Zl/HOR-21

stant, a is the data spacing, D is the actual depth of the plane on which the ~uctuations f(n) are superimposed, and M is one-sided filter length.

The Bengal basin, located at the head of the Bay of Bengal, is one of the thickest and largest ~d~enta~ basins in the world. The entire basin area, covering the whole of B~gladesh and neighouring West Bengal, India is, except at its fringes, completely covered by alluvium from the Ganges and Brahmaputra river systems. The Bengal basin is bordered on the western side by the Indian peninsular shield (Fig. 2). The Rajmaha1 h&s outline the northwestern timit of the basin, and in the north it is limited by the Shillong foreland shield. Between the exposed peninsular shield and the Shillong shield lies the socalled Garo-Rajmahaf gap Desikachar, 1974). From the western extreme of the Bengal basin PK-1 IPK-1

EXT (after S&t et al., 1986).

the different structural units are the shield area, the marginal zone, the shelf area, the slope or hinge zone, the basin foredeep and the eastern folded Tertiary belt (Fig. 2). Furthermore, the northwestern part of Bangladesh within the Bengat basin has recently been classified by Khan and Rahman (1992) as the northern platform slope, the stable platform, the NawabganjGaiba~dha intracratonic high, and the southern part of the platform (Fig. 2). The boundaries of these tectonic elements follow a general NE-SW trend. The western part of the Bengal basin is characterized by a virtual absence of folded structures (Salt et al., 1986). However, the presence of some broad low-relief features (grabens) is observed. These are possibly formed by tectonic movements associated with several deep-seated faults. From the western margin of the basin, the basement surface dips gently to the SE under an increasing cover of sediments, to form the broad western foreiand shelf of the basin. This shelf

EXT.

SI,tD:4tOOm

Fii. 4. A two-way travel-time section along the seismic line PK*l/PK-1 EXT (after JLietz and Kabir, 1982).

A.A. KHAN AND B.N.P. AGARWAL

346

et al., 1986): (1) the transgression and onlap of the passive continental margins; and (2) a broad regressive cycle comprising successive periods of delta submarine fan progradation. The passive margin is marked by a series of migrating shelf breaks, the most prominent being the Eocene shelf edge, which is conspicuous on seismic records because of a strong carbonate reflector. This shelf break marks the top of the foresetting Eocene limestone bed; The open-marine platform limestone covers most of the shelf area, except in the northwest where Plio-Pleistocene sediments directly overly the Precambrian basement (Fig. 3). The basic structural configuration of the western part of the Bengal basin is characterized by block-faulted unfolded extensional continental crust, with its extensional margin hav-

area is characterized by a NE-SW to NNE-SSW trend down to the basinal normal faults. The practically undisturbed sedimentary section gradually thickens basinward and becomes more marine, with the development of new marine wedges further down dip. The precontinental collision geology of western Bangladesh is known from the seismic sections HOR-21, HOR-21 EXT and PKl/PK-1 EXT (Figs. 3 and 4). It consists of shelf sediments of Cretaceous to Eocene age overlying upper Palaeozoic-Mesozoic sediments which were deposited in the grabens of western Bangladesh formed during the Gondwana period, as interpreted from the seismic sections. The depositional history of the western Bengal basin from the time of crustal separation in the early Cretaceous involves two distinct phases (Salt -

10r

..’

DISTANCE 1OOKM

-

GRAVITY EFFECT DUE TO -.GRANITIC SURFACE ~~~;Cll\AD

L

OBSERVED BOUGUER ANOMALY

/

8-

GRANITIC BASEMENT A? = 0.25gkmJ

LIP: 0.23 g/cm3 22 .

36 -

A?: 0.2glcmJ

Fig. 5. (a) The observed Bouguer anomaly along profile A-B and its deconvolved gravity effects due to the granitic basement and the Conrad and Moho discontinuities. (b) Undulations in the discontinuity interfaces derived with the indicated density contrasts.

341

THE CRUSTAL STRUCTURE OF WESTERN BANGLADESH

ing a broad, gentle shelf flanking the Precambrian outcrops. Gravity proffle across the platform flank This NNW-SSE gravity profile, 270 km long, is taken across the sub-Himalayan foredeep, the platform flank and the shelf region (Fig. 1) Spectral analysis of the observed Bouguer anomaly along profile A-B (Fig. 5a), digitized at an interval of 3 km, indicates three horizons (Fig. 6) at average depths of 29.3 km, 13.9 km and 3.5 km. The linear segment corresponding to a depth of 29.3 km was based on the upper three spectral points, which deviate considerably from the linear segment for 13.9 km. Furthermore, three spectral points near the intersection of these two linear segments (29.3 km and 13.9 km) have been considered with equal weighting. Thus, the segment for 29.3 km is marked on the basis of six points to yield a reasonable accuracy in esimating the depth to the deepest horizon. It may be mentioned here that in order to have a sufficient number of spectral points in drawing a linear segment, either the profile length should be large or the digitization interval should be small - or a judicious ~mbination of these parameters should be used. (Note that this also applies to the curve-fitting in Fig. 9.) The deconvolved gravity effects (Fig. 5a) of various horizons at different depths have been computed from the application of eqn (6) in the frequecy domain. Following Tomoda and Aki (19551, these deconvolved gravity effects associated with different horizons have been con-

DATA

PROFILE SPACING

AB : 3 KM

40 ANGULAR

FREQUENCY

120 AK / Ii%

K 160

Fig. 6. A plot of the logarithm of the energy of the observed gravity field along profile A-B versus the angular frequency, to determine average depths to various discontinuity interfaces.

vetted into fluctuations of their corresponding interfaces (Fig. 5b) by assigning density contrasts of 0.4 g/cm3 for the bottom interface, 0.23 g/cm3 for the intermediate one and 0.25 g/cm3 for the top interface. An attempt has been made to evolve a probable crustal model (Fig. 7b) by incorporating various pieces of independently known info~ation, such as drill-hole records and the locations of basement faults from the aeromagnetic map of Bangladesh (anonymous, 19811, along with the results obtained from spectral analysis. The computed undulating interfaces (Fig. 5b) can be converted into probable geological structures by visualizing the positions of the strong horizontal gradients as inferred fault positions, The relative motions of various blocks, thus obtained, have been deciphered on the basis of their relative disposition in two-dimensional space. Therefore, Figure 7b resembles Figure 5b, except that the latter has no apparent geological meaning. It is obvious from Figure 7a that the computed gravity anomaly of the proposed model (Talwani et al., 1959) exhibits a reasonably good match with the observed Bouguer anomaly. Thus, the matched crustal model (Fig. 7b) is characterized by: the Moho surface associated with an average depth of 29.3 km and a mantle density of 3.3 g/cm3; the Conrad surface associated with an average depth of 13.9 km and a density of the basaltic layer of 2.9 g/cm3; and the basement surface associated with an average depth of the granitic layer of 3.5 km, having a density of 2.67 g/cm3. Evans and Crompton (1946) have measured the density of the sediments lying within the top 1.5 km, which varies from 2.35 g/cm3 to 2.45 g/cm3 and below 1.5 km (2.45-2.55 g/cm3). Taking this into account, a density of 2.42 g/cm3 has been assigned to the sediments that occur only in the platform flanks where, in general, the thickness of the sediments is small. It is known from the records of a well drilled in this region (Zaher and Rahman, 1980) that the Archaean basement complex occurs at varying depths, ranging from 326 m to more than 4000 m, and is directly overlain by Gondwana sediments deposited in intracratonic basins. Cretaceous and Palaeogene sediments have also been recorded in

A.A.

348

some wells. In the proposed model (Fig. 7b), the Tetulia well (5%) has encountered Archean basement at a depth of 3OCXl m, and the wells ED&10 and Singra ($1) have encountered Gondwana sediments at their respective total depths of 1157 m and 4100 m. As these three wells have encountered mostly Neogene sediments, an average density of 2.42 g/cm3 has been assigned to the overlying sediments, as have 2.67 g/cm3 to the Archaean rocks, 2.9 g/cm3 to the basaltic layer,

KHAN

AND

B.N.P.

AGARWAL

and 3.3 g/cm3 to the mantle in the present modelling work. Several faults inferred on the basis of the cu~at~es in the respective interfaces derived from the present analysis exhibit a reasonably good correlation with the basement faults obtained from the interpreted aeromagnetic map (anonymous, 1981). The crustal model (Fig. 7b) clearly indicates an extensive mantle upwa~ing, which resulted in fracturing and movements within the crust. The tensional forces thus gener-

B

300 t

I)-

f =2.67g/cm3

RASAtTtC LAYER P = 2.9 g/cm* 24

C :Locofion $$I :Infcrrtd

of foulis faults

from orromognrtic wfth

mop.

fr!oVWYW~tS

Fig. 7. (a) A comparison of the observed gravity field with the computed gravity effect due to the proposed geological crustal model (b).

349

THE CRUSTAL STRUCTURE OF WFiSTERN BANGLADESH

DISTANCE

-ii---- *-**-

--•--

TOP OF TOP OF MOHO

IN KM

LEGEh E??ti&‘fESTONE BASALTIC

LAYER

0 4-

NEOGENE P * 2.47

SEDIMENTS g/cm3 /

/ 6-

12 ”

CRETACEOUS PALAEOGENE p = 2.67 g/cm-’

SEDIMENTS

16-

20 21-

BASALTIC Pa 2,9gfcm3

LAYER

MANTLE P l 3.3 9/cm3

Fig. 8. (a) Observed and computed gravity anomalies along profile C-D. fb> The proposed geological crustal Earth model.

produced normal faults and a series of horsts and grabens in which the Gondwana sediments were deposited. This inference is also supported by the wells drilled in this region. The downwarping of the Moho, indicated at the end A of this profile which is toward the Himalayan collision zone, is characterized by the subduction of the Indian plate. The average thickness of the crust is found to be 26 km along this profile. Furthermore, the proposed model can explain the probable tectonics by considering that the platform flank has resulted from mantle upwarping to form horst- and graben-like structures in the crust. Up to a distance of 100 km from point A, synthetic faulting is prominent, enhancing the crustal flexuring. Beyond 100 km, antithetic faulting opposing the crustal flexuring is prominent. These faults ated

can occur in conjugate pairs, and are associated with the crnstal upwarping (Dennis and Kelly, 1980).

PROFILE DATA 2. ‘2 k

SPACING

CD :5

KM

31KM

ANGlll.

AR

FREQUENCY

7fKt64

Fig. 9. A plot of the logarithm of the energy of the observed gravity field along profile C-D versus the angular frequency, to determine average depths to various discontinuity interfaces.

A.A. KHAN AND B.N.P. AGARWAL

350

Gravity profile across the foredeep region

aeromagnetic data (anonymous, 1981). The proposed model has been tested through forward modelling, which indicates a close match with the Spectral analysis of the observed Bouguer observed Bouguer anomaly except for a small anomaly along profile C-D (Fig. 8a>, digitized at mismatch near point C of the profile (Fig. lOa). an interval of 5 km, indicates three horizons (Fig. The assigned densities of the different layers are 9) at average depths of: 31 km, corresponding to as follows: mantle, 3.3 g/cm3; basaltic layer, 2.9 the Moho (Ay = 3.3-2.9 = 0.4 g/cm3); 14 km, as g/cm3; Cretaceous-Palaeogene sediments, 2.67 the top of the basaltic layer (A-y = 2.9-2.67 = 0.23 g/cm3. In view of the large thickness of the g/cm3); and 6.6 km, as the top of the Eocene Neogene sediments in the foredeep region, a limestone (Ay = 2.67-2.47 = 0.2 g/cm3). The density of 2.47 g/cm3 has been assigned (Evans gravity effects of different horizons and the fluctuations of their interfaces are shown in Figs. 8a and Crompton, 1946). All of the reflectors-Rl, R3, R4, R5 and R7 and 8b. The proposed crustal model (Fig. lob) for of seismic line PKl/PKl-EXT (Fig. 4; Lietz and the foredeep part has been constructed on the basis of information derived from: spectral analyKabir, 1982)-correspond to different sedimensis; seismic profiles HOR-21, HOR-21 EXT and tary formations, with ages ranging from Cretaceous to Plio-Pleistocene. The Muladi well (MU) PKl/PKl-EXT (Figs. 3 and 4); the Muladi well (MU, Fig. 1) record (Lietz and Kabir, 1982); and with a total depth of 4732 m, drilled by BOGMC, basement fault data adapted from interpreted has penetrated only Miocene (Bhuban) sediments. The MU well is about 80 km east of the gravity profile C-D, but the seismic line C DISTANCE IN KM PKl/PKl-EXT, intersects the gravity profile < 150 100 50 F C-D at a point close to end C. z 01 The most prominent reflector, R5, corresponds to Eocene limestone. This coincides with a horizon at a mean depth of 6.6 km, derived from spectral analysis. Reflector R7 is the interface between the Upper Cretaceous and Palaeocene or Eocene sediments lying above the @RI NEOGENE SEOIMENTS RI P= .?,47 g/cm3 Gondwana sediments, and the Jurassic volcanic \ _ and volcanoclastics lying below (Lietz and Kabir, lb q 1982). Hence, it is reasonable to assume a sediETACEOUS-PALEOGENE SEDIMENTS mentary thickness of more than 12 km in this x 4+ foredeep region. With such a huge thickness of L ;* I\! 16sediments and a tremendous overburden presBASALTIC LAYER VI _ sure, the sediments in the deeper part should P=2.9glcm3 ,” _ attain a high density value. Evans and Crompton u II 24(1946), on the basis of the measurements of a 0 large number of samples, gave a density value for 2 _ the Eocene sediments of 2.6 g/cm3 below 1.5 km MANTLE depth in this region. Hence, it seems reasonable P * 3.3 g/cmJ L EGENO to assign a density value of 2.67 g/cm3 to the F :Locotion of faults from oeromognetlc mop //$ : Inferred faults with movements Cretaceous-Palaeogene sediments below 10 km Rl-R7:Scismlc Reflecting Horizons depth. Fig. 10. (a) The observed Eiouguer anomaly along profile With a mean depth of 31 km for the Moho and C-D and its deconvolved gravity effects due to the top of the 14 km for the top of the proposed basaltic layer, Eocene limestone, and the Conrad and Moho discontinuities. an average thickness of 17 km for the basaltic (b) Undulations in the various discontinuity interfaces derived with the indicated density contrasts. crust is obtained beneath the foredeep region, in

P

THE

CRUSTAL

STRUCTURE

OF WESTERN

BANGLADESH

contrast to 26 km beneath the platform flank. The study reveals a relatively thin, flat and tectonically less disturbed crust beneath the foredeep region. It is therefore observed that an average sedimentary thickness of 3.5 km, with a m~mum of 4.1 km (well .%‘I),has been found in the platform flank, while the average sedimentary thickness in the fore deep region is 14 km. This shows a differential displacement of about 10 km between the platform flank and the foredeep. However, the geologica and geophysical evidence does not support such a great vertical to subvertical displacement within the same crustal block in this region, except along the Dauki fault zone where a vertical to sub-vertical displacement of about 11 km between the Shillong massif and the Sylhet trough has been obtained (Khan, 1991). This evidence, in conjuction with the evidence from seismic and borehole data, therefore indicates that the foredeep part is underlain by oceanic crust. The more or less flat and tectonically less disturbed nature of the Moho and the basaltic interface with overlying sediments in the foredeep region (Fig. 10) indicate the existence of a marginal sea basin, underlain by oceanic crust, with the deposition of sediments mainly derived from continental and arc areas (Condie, 1982). Tectonic history The tectonic history of the study area can be visualized from the proposed crustal models coupled with plate tectonics. The proposed crustal models indicate two separate geotectonic domains: the platform flank, which is essentially underlain by continental crust; and the foredeep region which is underlain by oceanic crust, as inferred from the present study. The separation of India from Australia (Hamilton, 19791, must have taken place along a palaeocontinental shelf by mantle-activated rifting (Condie, 1982). Sastri et al. (1973) have postulated the existence of such a palaeoshelf between eastern India and western Australia. The crust beneath the platform flank, as determined from the present analysis, is characterized by tensional tectonics. Tensional forces might have created a complex graben system in which the down-

351

dropped and/or tilted blocks were intersected by many normal faults. A prominent mantle upwarp, as observed in the central portion of profile A-B, is responsible for the development of the tensional forces within the crust which generated the m~tle-activated rift system. Fu~he~ore, it is observed that the crustal block in profile A-B (up to a distance of about 100 km from point A) shows synthetic faulting (Dennis and Kelly, 1980), enhancing crustal flexuring and thickening of the crust toward the Himalayan region. Beyond a distance of 100 km from point A, the dip of the postulated antithetic faults opposes that of the crustal flexuring. These faults have been envisaged as occurring in conjugate pairs, and are associated with the crustal upwarping. A downwarp in the Moho near point A has been interpreted as the subduction of crustal blocks toward the Himalayan collision zone. It is also envisaged in the proposed crustal model (Fig. 7b) that a few segments of the don-bending crnstal blocks near the collision zone might be pushed back over the trailing segments, due to pressure from the north exerted by the abducting crustal blocks, to produce features associated with thrust faulting. This view has been put forward on the basis of a thrust fault near point A of profile A-B. In addition, an earthquake (magnitude 5.2, July 25, 1970; Fig. 11, with a focal mechanism solution of thrust faulting with a minor strike-slip component, corroborates the above inference (Khan, 1991). The analysis in this paper has clearly established that the nature of the crust existing beneath the foredeep (profile C-D) is remarkably different from that of the platform flank. The postulated oceanic crust beneath the foredeep region would have evolved by seafloor spreading subsequent to initiation of India’s northeasterly flight from the combined Australia-Antarctica. Movement of the Indian plate was maintained along the two “driving lines” of the Owen fracture zone and Ninetyeast ridge transform. Hence spreading from the Carlsberg ridge, and the oceanic crust generated on both sides of the Ninetyeast ridge transform, began to move northeast. The oceanic crust postulated beneath the foredeep region is relatively old crust which has remained mostly stations, due to the resistance

A.A. KHAN AND B.N.P. AGAUWAL

352

offered by the Shillong massif that now upli~ed along the Da&i fault, since the collision of the Indian plate with the Eurasian plate. A comparatively flat Moho and a less disturbed overlying (postulated) basaltic layer (Fig. 10) suggests that the southwestern part of the foredeep suffered less tectonism, whereas the crust beneath the platform flank (Fig. 7) underwent severe tectonism. However, the regional tectonic pressure that was generated due to the northeast motion of the Indian plate and the resistance offered by the Shillong massif resulted in the development of differential movements within the crustal blocks beneath the foredeep region, as is envisaged in the crustal model for the foredeep region (Fig. 10). Furthermore, it is inferred that the great uplift of the Shillong massif was balanced by the tremendous subsidence of the foredeep, especially of the Sylhet trough in the northeast (Fig. 1) along the Dauki fault. This subsidence was further enhanced by overburden pressure imparted by the large influx of sediment mainly from the continental arc. Conclusions The spectral analyses of two gravity profiles over the platform flank and the Faridpur trough indicate almost the same depth for the Moho interface. Fu~he~ore, it is inferred that the sediments in the Faridpur trough lie directly over the basaltic layer (oceanic crust). Synthetic and antithetic faults in the upper crustal part of the platform flank have been postulated. The analysis in this paper clearly proves that an energy density spectrum coupled with matched filtering, can lead to new information on crust?1 structure that was hitherto unobtainable from existing data analysis.

We are grateful to the Geological Survey of Bangladesh and Bangladesh Oil, Gas and Mineral Corporation for co-operation and the provision of data. We are also grateful to Professor R.K.S. Chouhan, Dr. R.K. Shaw, K. Chakraborty,

Ch. Sivaji and the journal comments.

reviewers

for their

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