Confidential manuscript submitted to Tectonics

1 Surface Interaction in north-west Basin

1,2 2 1 3 2 Sabber Ahamed , Delwar Hossain , Eunseo Choi , and Jahangir Alam

1 3 Center for Earthquake Reserarch and Information, The University of Memphis, Memphis, TN, USA.

2 4 Department of Geological Sciences, Jahangirnagar University, Savar, Dhaka,

3 5 Geological Survey of Bangladesh, Segunbagicha, Dhaka, Bangladesh

Corresponding author: Sabber Ahamed, [email protected]

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6 Abstract

7 The northwest Bengal Basin is one of the least explored areas where the basement surface

8 interaction is still controversial. We analyze satellite images, bouguer anomaly data and con-

9 struct a long-term tectonic model. Satellite images reveal significant spatial changes in the

10 uplifted Barind tract and its surrounding low-lying subsided floodplains. The regional and

11 residual gravity anomalies exhibit an association with the surface geologic structures. For ex-

12 ample, the uplifted Barind tract areas are located on top of the gravity highs represent crustal

13 horst. On the other hand, low-lying flood plains and faults are located on the gravity lows

14 represent graben. To explore the relationship between the surface and basement structures,

15 we construct a geodynamic model. We find that the model produces conjugate thrust faults

16 beneath the horsts. With time the faults reach the surface and push the horst block upward.

17 We do not see such noticeable upliftment in the graben structures. Finally, we conclude that

18 the uplifted surface structures and the surrounding low-lying flood plains may be produced

19 by the regional compression and have a relationship with existing basement structures.

20 1 Introduction

21 The northwestern part of the Bengal Basin (Figure. 1 and Figure. 2) has many unsolved

22 and complex geological history. One of them is the mystery of tectonic evolution of the Pale-

23 oproterozoic basement. Ameen et al. [2007] and Hossain et al. [2007] study the basement

24 extensively and gather petrographic information. They both publish almost the same age

25 1722±6 Ma and 1730±11 Ma respectively. However, their conclusion on the evolution of the

26 basement differs from each other. Ameen et al. [2007] point out that the basement is a sepa-

27 rate and discrete micro-continental fragment that was trapped by the northward migration of

28 during dispersal, contrarily Hossain et al. [2007] report that the basement

29 rock is just a continuation of Central Indian Tectonic Zone and up to Shillong Plateau.

37 Another important geologic unsolved problem is the evolution of the elevated Pleis-

38 tocene terrace locally known as Barind tract (Figure. 2). The tract is underlain by a horst

39 block of the paleoprotoreozoic basement. The surface of the tract is composed of loose sed-

40 iments [Rashid et al., 2015, 2006]. The eastern side of the tarct is bordered by the Brahma-

41 putra river (Figure. 2) which is believed to be linked with the lithospheric flexure of the un-

42 derlying basement [Rajasekhar and Mishra, 2008].

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28 88 E 90 92 94 E

Assam Himalayan Main Boundary Thrust Basin

Mikir Hills 26 N

Dauki Fault C

Shilong Plateau h

i

n

d

R w

i

a n

Padma River Surma B

j a s

Brahmaputra R Brahmaputra

Basin i

m n

T I a n

r d

h o

a a B

p u r

l M

Chittagong–Tripura folded belt a

n Trough

ur R

nge Zone a

idp n

Hi ges

Far

C

e

n

Hatiya t r

22 Barisal Chadpur Grvity high a

l

Trough

B

u r

Bay of Bengal B ma

a s

i

n S

w

a

t

c

h

O

f

20

N

o India

G

r

o

u n d Bangladesh N 18N 1000 Km 100 Km

30 Figure 1. Tectonic map of Bengal Basin and its surrounding area. The map has been prepared modifying

31 from Reimann and Hiller [1993]; Alam [1972]; Johnson and Alam [1991]. The blue square box is the study

32 area shown in Figure. 2. This figure and associated running/plotting scripts available under Ahamed et al.

33 [2017].

43 There has been a long debate among the geoscientists on the tectonic evolution of the

44 Barind tract. For example, Alam [1995] and Rashid et al. [2015] point out that the tract

45 may have been evolved during the Quaternary period due to the north-south and east-west

46 regional compressional stress. Similarly, Morgan and McIntire [1959] show that the region

47 may have been associated with the Quarternary tectonic activity based on the aerial pho-

48 tographic interpretation. Hussain and Abdullah [2001] also point out that the Tract is the

49 product of vertical movements of Pleistocene period. On the Other hand, Monsur [1995]

50 suggests the tract has no connection with the regional compression rather it is just an ero-

51 sional geomorphic feature.

52 Although, the region has the shallow basement depth (∼ 128m)[Khan, 1991] and

53 in the entire Bengal Basin a limited information about the subsurface geologic structures is

54 available. Some geophysical studies [Rahman, 1990a,b; Rabbani Md. Golam, 2000] have

55 done so far. Because of the complex geologic history and lack of information, a couple of

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88°0'0"E 89°0'0"E 90°0'0"E

40 Km N 26°0'0"N

Shallowest Basement Brahmaputra River 25°0'0"N

Barind Tract 24°0'0"N

34 Figure 2. Major tectonic elements of northwestern Bengal Basin. Black square box is the location where

35 the shallowest paleoproterozoic basement (∼ 128)m is reported [Khan, 1991]. This figure and associated

36 running/plotting scripts available under Ahamed et al. [2017].

56 studies published the tectonic maps of the region differently. For example, Alam [1972]

57 first considered the Bengal Basin as an exsogeosyncline and proposed a simple tectonic map.

58 Khan and Rahman [1992] proposed another tectonic map of the region based on the trends,

59 shape and magnitude of gravity anomalies. Later Reimann and Hiller [1993] reported a very

60 different tectonic classification of the region.

61 Our aim in this paper is to explore the relationship between basement and surface

62 structures using time series satellite images, gravity data and constructing a geodynamic

63 model. We start with the investigation of time series remote sensing images to understand

64 the geomorphological changes in time. We then analyze the gravity data to infer subsurface

65 basement structure and to correlate with geomorphological observations. Finally, we con-

66 struct a geodynamic model to explore the role of the existing regional compressional stress

67 and basement structure in the development of the current geomorphological structures.

68 2 Regional and local tectonics

69 The was a part of the ancient super continent of Gondwana. It started

70 breaking up from Gondwana about 176 million years ago and later became a major plate

71 [Chatterjee et al., 2013]. The major tectonic elements of Indian plate started developing with

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72 the northward drift of the Indian plate since and its collision with the Eurasian

73 plate by early to middle Sikder and Alam [2003].

74 The Bengal Basin (Figure. 1) is one of the largest sedimentary basins, located in the

75 north-eastern part of the Indian plate. The Indian shield makes its eastern boundary, whereas

76 the compressional Indo-Burman folded belt make the western boundary. The pop-up Shil-

77 long plateau marks one of the major structural features of the basin's northern portion. Our

78 study area locally known as stable-platform is located in the northwest part of the basin.

79 The area was part of the stable Gondwana continent in the Precambrian [Alam et al., 2003].

80 Many studies [Reimann and Hiller, 1993; Khan and Chouhan, 1996; Hossain et al., 2007]

81 show that this stable-platform is the continuation of the exposed Indian shield. Numerous

82 graben, half graben and horst like structures have formed [Alam et al., 2003] during the rift-

83 ing process of Indian plate from Gondwana in the Early Cretaceous. The area represents the

84 shallowest paleo-protozoic basement in the entire Bengal basin. The sedimentary succession

85 unconformably overlies the basement rocks.

86 The study area (Figure. 2) has a complex geological structure and history. Unique tec-

87 tonic settings and ongoing tectonic activities made a complex relationship between the var-

88 ious geomorphic processes and morphotectonic activities of the region. The dominant sed-

89 iments that cover the region vary in age from Pleistocene to Holocene Reimann and Hiller

90 [1993]. Geomorphologically, the study area consists of the plain land of fluvial-deltaic sedi-

91 ments deposited by the -Brahmaputra and the Meghna river systems. The Pleistocene

92 Uplands known as Barind Tract which is located on the west side of the Brahmaputra river

93 (Figure.2).

94 3 Discussions

95 3.1 Satellite Images analysis

99 We use time series (1972, 1989, 2003 and 2010) Landsat TM satellite images (Fig-

100 ure. 3) to investigate both spatial and temporal geomorphological chnages in the study area.

101 Our visual interpretation of the images relies on grey level(tone) and the relationship to lo-

102 cal geologic features. We apply the same projection system of the gravity data to the images.

103 Satellite images with time indicate that the region has active geomorphic processes. The spa-

104 tial changes detected in the region are mostly limited to Barind tract and its surroundings.

105 The tract is visible in all satellite images (Figure. 3). The elevated Barind tract has persis-

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88°0'0"E 89°0'0"E 90°0'0"E 88°0'0"E 89°0'0"E 90°0'0"E a 1972 b 1989

Teesta Floodplain 26°0'0"N 26°0'0"N

Barind tract Barind tract Brahmaputra River Brahmaputra River 25°0'0"N 25°0'0"N

Lower Atrai Floodplain N N Karatoya-Bangali Floodplain Karatoya-Bangali 36 Km 36 Km 24°0'0"N 24°0'0"N

88°0'0"E 89°0'0"E 90°0'0"E 88°0'0"E 89°0'0"E 90°0'0"E c 2003 d 2010 26°0'0"N 26°0'0"N

Barind tract Barind tract Brahmaputra River Brahmaputra River 25°0'0"N 25°0'0"N

N N

36 Km 36 Km 36 Km 24°0'0"N 24°0'0"N

96 Figure 3. Time series Landsat satellite images of the study area. Floodplains are named after Brammer

97 [1996]. Red arrows show the Barind tract location. This figure and associated running/plotting scripts

98 available under Ahamed et al. [2017].

106 tently white tone compare to it's surrounding low-level flood plains which have a compara-

107 tively darker tone. Alam [1995] associates this tonal variation with moisture content. Sed-

108 iments of Barind tract and surrounding floodplains contain low and high moisture contents

109 respectively. The interpretation is consistent with the local topography and hydrogeology.

110 For example, because of the high elevation of the Barind tract area, the area has groundwater

111 crisis. On the other hand, the surrounding floodplains contain abundance surface and subsur-

112 face water.

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113 The tonal variation of the images makes it easier to identify local faults and lineament.

114 The southern and eastern side of the tract have a sharp tonal contrast with it surrounding

115 floodplains. The variation of grey level in these areas are very linear and consistent with the

116 regional and residual gravity anomalies (Figure. 4 and 5). Since the tract is elevated and

117 makes a sharp linear border with its surrounding low-lying floodplains, it may be extended

118 to deep subsurface basement. Khandoker [1987] show that the tract is a horst block along

119 crustal weakness with compensatory subsidence of the bordering regions.

120 3.2 Gravity data analysis

121 3.2.1 Regional gravity data analysis

124 We use polynomial surface fitting [Telford et al., 1990] from 2nd to 5th order to sep-

125 arate deep crustal structures from shallow ones. Figure. 4 shows the regional gravity of dif-

126 ferent polynomial surfaces. All the four orders show a strong, negative northeast trending

127 regional gravity anomalies with a non-uniform gradient in the northwestern region of the

128 study area. The anomalies have high intensity and long-wavelength. Rahman [1990b] re-

129 ports that these high magnitude anomalies may be due to the combined effects of a thick

130 low-density sedimentary rocks and a north-dipping, denser substrate in the Himalayan colli-

131 sion zone. Like as the north, no such regional gravity trend is present in the south. However,

132 in the middle of the area, some scattered moderate magnitude regional gravity anomalies

133 are present. The magnitude of these anomalies decreases with high order polynomial sur-

134 face fitting. Three distinct gravity highs (a, b, c) are identified in regional gravity anomaly

135 map (Figure. 4). The highs(a − c) are also distinguishable on the residual gravity anomaly

136 map(Figure. 5) indicating that they extend from shallow to likely deeper depth. The shal-

137 lowest basement of the entire Bengal basin has been found in the gravity high b where the

138 reported depth is ∼ 128 m. Interestingly the Barind tract is also located on the top of the

139 gravity high-a and b.

140 3.2.2 Residual gravity data analysis

143 Due to the presence of shallow basement, we emphasize on residual anomalies calcu-

144 lated using Second Vertical Derivative(SVD) [Telford et al., 1990]. The SVD is a measure

145 of curvature, and large curvatures enhance the near-surface effects at the expense of deeper

146 anomalies. Figure. 5 shows the SVD of Bouguer anomaly of numerous gravity lows bounded

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a b

c c b b

a a mGal mGal -23.71 -18.12

-92.87 -112.13

c d N

b b

a mGal -25.42 mGal a -26.14

-123.32 60 Km -125.16

122 Figure 4. Regional Gravity anomaly map of polynomial surface of a) second b) third c) fourth and d) fifth

123 order. This figure and associated running/plotting scripts available under Ahamed et al. [2017].

147 by gravity highs. For interpretation purpose, we group them into two clusters of SVDs (Fig-

148 ure. 5). SVD-1 is in the northern most areas with high gravity anomalies. Interestingly, no

149 notable variation of gravity anomalies in between SVD-1 and SVD-2. In the middle of the

150 study area, several scattered gravity highs and lows are present. They are grouped as SVD-

151 2. We identify a gravity high marked by a dashed arrow, where the shallowest crustal block

152 in Bengal Basin has been reported [Khan, 1991]. Barind tract is also located on the top of

153 SVD-2. A sudden truncation of SVD low anomalies is present on the eastern side of the

154 tract. Along with this margin, Alam [1995] identified a surface lineament from the satellite

155 image analysis. Interestingly, this lineament overlaps the truncation of SVD anomalies. The

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88°0'0"E 89°0'0"E 90°0'0"E

N

1/ms 2 3408.25 26°0'0"N Shallowest Basement

-2979.44 25°0'0"N Possible subsurface Possible of Lineamnet Alam(1995)

40 Km 24°0'0"N

141 Figure 5. Second Vertical Derivative (SVD) of Bouguer anomaly showing shallow crustal features. This

142 figure and associated running/plotting scripts available under Ahamed et al. [2017].

156 path of the Brahmaputra river has a strong correlation with SVD anomalies. The river flows

157 through the boundaries of SVD highs and lows. We interpret that the river is the possible

158 surface expression of the abrupt SVD highs and lows.

159 3.3 Geodynamic model

160 The satellite images and gravity data analysis show that surface features like the Barind

161 tract, Brahmaputra fault and subsurface structures horst and graben spatially correlated.

162 However, it is still not obvious if the basement structures can produce the surficial features

163 in long-term tectonic movement. To test the hypothesis we create a geodynamic model from

164 the existing east-west crustal cross-section from Alam et al. [2003]. We only use our study

165 area of the entire coss-section. The model is a Mohr-Coulomb elastoplastic layer which is

◦ 166 initially 100 km long and 10 km thick (Figure. 6). Since the profile is at 96 angle with av-

167 erage Indian plate velocity(v = 3.6cm/yr)[Mahesh et al., 2012; Socquet et al., 2006], we

◦ 168 use the profile component (|v cos 96 )|) of the velocity(v) to push the left boundary. The

169 right boundary is kept as free slip. The bottom boundary is supported by the Winkler foun-

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170 dation [Watts, 2001, pp.95] and the surface is free surface. To induce strain localization, we

171 decrease cohesion to 4 MPa linearly as plastic strain increases to 1. We impose topographic

−7 2 172 smoothing of the diffusion type with a transport coefficient of 10 m /s [Turcotte and Schu-

173 bert, 2014, pp. 225]. Parameters used in this model are listed in Table 1:

174 Table 1. Parameters for the geodynamic model

Parameter Symbol Sedimentary Layer Basement

Bulk Modulus K 7.24 GPaa 17.89 GPab Shear Modulus G 1.4 GPaa 12.5 GPab Initial Cohesion C 25 MPa 40 MPa

Friction Angle φ 30◦ 30◦ Dilation Angle Ψ 0◦ 0◦ Density ρ 2300 Kg/m3 c 2750 Kg/m3 c Volumetric expansion coefficient α 3.5 K−1 3.5 K−1

a Bulk and Shear modulus are calculated based on density and lower range of P

wave(Vp ) and (Vs ) of porous and saturated sandstone [Bourbié et al., 1987]. b Bulk and Shear modulus have been calculated based on Young’s modulus(37583.70)MPa [Khan, 2006] and Poisson’s ratio(0.3).

c Bourbié et al. [1987].

Horst Sedimentary deposits Graben Basement 10 km

0.38 cm/year 0.38 100 km

175 Figure 6. Tectonic model setup.This figure and associated running/plotting scripts available under Ahamed

176 et al. [2017].

179 The plastic strain distribution of the model of different shortenings (Figure. 7a) is no-

180 ticeable. From the beginning, the plastic strain accumulation is concentrated along the base-

181 ment and sedimentary deposits interface. Conjugate thrust faults with broad and thick plastic

182 strain concentrations start to form only inside the elevated blocks(horsts) of basement. Com-

183 paring to this wide plastic strain concentrated fault, a thin and low amount of plastic strain

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184 faults are present subsided regions (grabens) of the basement (Figure. 7a). At 1.40Km short-

185 ening, conjugate thrust faults start to extend deeper. These faults clearly take the advantage of the existing horst of the basement.

Plastic strain a 0 0.05

1.40 km shortening

PlasticPl ti ststrain in b 0 0.5

2.20 km shortening

2.60 km shortening

3.00 km shortening

177 Figure 7. Plastic strain distribution of four different shortenings. Platic strain scale a) 0-0.05 and b) 0-0.5.

178 This figure and associated running/plotting scripts available under Ahamed et al. [2017].

186

187 The conjugate thrust faults are responsible for the surface topography of this region.

188 Faults first form inside the basement and with time they reach the surface. Since the region

189 is undergone compression, the faults become more active and push the horst blocks upward

190 (Figure. 7b). Since the deformation is accommodated mostly by the conjugate faults in horst

191 area, graben areas are least affected and subsided. That’s why we do not see any noticeable

192 such uplifting beneath the graben structures (Figure. 7b). Uplifting of the hort and subsi-

193 dence of grabens are consistent with our gravity and geomorphological observations.

194 4 Conclusion

195 We analyze time-series satellite images, bouguer gravity anomaly data and construct a

196 long-term tectonic model. Satellite images reveal significant spatial changes in the uplifted

197 Barind tract and its surrounding low-lying subsidence floodplains. The gravity anomalies

198 show that the basement structure may have a relationship with the surface geomorphology.

199 We find that the uplifted Barind tract is located on top of the gravity highs whereas low-lying

200 flood plains and faults are on the lows. We construct a tectonic model to further explore the

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201 relation between the surface and basement structures. The model produces conjugate thrust

202 faults beneath the gravity highs. The faults reach the surface and push the gravity highs

203 block upward with time. However, no prominent upliftment is seen beneath the grabens.

204 Our conclusion is that the elevated surface tract its surrounding low-lying floodplains are

205 produced by the regional compression where existing basement have a significant role.

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