Basement Surface Interaction in North-West Bengal Basin
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Confidential manuscript submitted to Tectonics 1 Basement Surface Interaction in north-west Bengal 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, Bangladesh 3 5 Geological Survey of Bangladesh, Segunbagicha, Dhaka, Bangladesh Corresponding author: Sabber Ahamed, [email protected] –1– Confidential manuscript submitted to Tectonics 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 India during Gondwana 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]. –2– Confidential manuscript submitted to Tectonics 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 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 West Bengal Far C e n Hatiya t r 22 Barisal Chadpur Grvity high a l Trough B u r Bay of Bengal ma B a s i n S w a t c h O f 20 N o India G r o u n d Bay of Bengal 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 –3– Confidential manuscript submitted to Tectonics 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 Indian Plate 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 –4– Confidential manuscript submitted to Tectonics 72 the northward drift of the Indian plate since Cretaceous and its collision with the Eurasian 73 plate by early to middle Eocene 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 Ganges-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.