Crustal structure and tectonics of Bangladesh: New constraints from inversion of receiver functions

Tectonophysics 680 (2016) 99–112

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Crustal structure and tectonics of Bangladesh: New constraints from inversion of receiver functions Arun Singh a,⁎, Kirti Bhushan a, Chandrani Singh a, Michael S. Steckler b, S. Humayun Akhter c, Leonardo Seeber b, Won-Young Kim b, Ashwani K. Tiwari a, Rahul Biswas a a b c

Department of Geology and Geophysics, Indian Institute of Technology Kharagpur, India Lamont-Doherty Earth Observatory, Columbia University, USA Department of Geology, University of Dhaka, Dhaka, Bangladesh

a r t i c l e

i n f o

Article history: Received 24 November 2015 Received in revised form 20 April 2016 Accepted 28 April 2016 Available online 11 May 2016 Keywords: Moho Sedimentary basins Receiver functions Seismogenic faults

a b s t r a c t An understanding of the sedimentary and crustal structure of the Bengal Basin and of the tectonics deforming it remains elusive due to lack of seismic data from Bangladesh. Taking advantage of recently available data from 11 seismic stations deployed over Bangladesh, we determine the crustal structure beneath each station using 2768 high quality receiver functions (RFs). Inversion of the RFs reveals a highly variable thickness of the overlying sediments beneath the Bengal Basin. The thickness of the sediments increases dramatically across the Hinge Zone of the Early Cretaceous passive margin from 3 to 17 km. The thick sediments partly represent progradation of the continental margin due to the influx of clastic sediments from the Himalayas. The Moho shallows across the region. This reflects thinning of the crystalline crust from 38 km in the Indian Craton to 34 km at the Hinge Zone to b 16 km in the Bengal Basin. The thickness of the sediments increases dramatically from 3 to 17 km south of Madhupur tract which reflects the regions of highest influx of clastic sediments from the Himalayan collision zone. RFs display strong dipping reflectors (strike 67°) for a station over Hinge Zone and seem to be associated with the transition from continental to oceanic crust. The thinning of significant crustal thickness (16–19 km) beneath 15–17 km of sediment and associated velocities (N4.0 km/s) at lower crustal levels supports an influence of the Kerguelen plume igneous activity during rifting. We invert data for a station near the Dauki Fault, which marks the southern boundary of the uplifted Shillong plateau, for dip and anisotropic effects. Our results show the Dauki as a north-dipping thrust fault at Jaflong with a dip angle of 32° and strike (110°) close to its surficial expression. A strong anisotropy (~15%) and the sense of shear (plunge: 24°, trend: 79°) compliment the dipping geometry and deformation seems to be related to the initiation of the Dauki Fault. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The Bengal Basin is a region with extreme sediment thickness (N12 km, Murphy, 1988; Johnson and Nur Alam, 1991) situated between the Indian Craton (Singhbhum) in the west, the highly deformed Indo-Burmese subduction zone towards its east (Fig. 1) and the elevated (~1.2 km) Shillong plateau and Mikir Hills to the north. The Bengal Basin was formed by the rifting of Antarctica from India in the Early Cretaceous (Storey et al., 1992; Coffin et al., 2002). The NNE–SSW striking Hinge Zone, the trace of the Eocene shelf edge, also marks the start of the transition from the thick continental crust to thinned, extended crust of the continental margin. The location of the transition from attenuated continental crust to oceanic crust is unknown, but often associated with the Barisal–Chandpur gravity high (Fig. 1; Alam et al., 2003). Mitra et al. (2008) identified a high velocity lower crust at Agartala (AGT) that they associated with oceanic crust. ⁎ Corresponding author. E-mail address: [email protected] (A. Singh).

http://dx.doi.org/10.1016/j.tecto.2016.04.046 0040-1951/© 2016 Elsevier B.V. All rights reserved.

Since the Eocene, sediments from the Himalaya carried primarily by the Ganges and Brahmaputra Rivers have prograded the shelf edge by 3–400 km from the Hinge Zone to its present location at 20–20.2°N filling the Bengal Basin. Previous geophysical studies from the Bengal Basin do not provide strong constraints on its total sediment thickness and or the nature of the crust beneath the thick sediments. Lindsay et al. (1991) image basement increasing from 3–5 km landward of the Hinge Zone to N 10 km east of it. Offshore refraction data suggest 16– 22 km (Curray, 1991) and reflection data indicate N15.5 km (Maurin and Rangin, 2009). Gravity modelling across the Bengal Basin suggests 13–14 km (Khan and Agarwal, 1993), (Rajasekhar and Mishra, 2008). Stratigraphic reconstructions involving well and seismic data suggest 13 to 16 km of sediments (Johnson and Nur Alam, 1991). Receiver functions (RFs) along the northern and eastern edges of the Bengal Basin indicate 21–24 km for the sediment thicknesses. Thus the total thickness of strata is large, but poorly constrained in the center of the Bengal Basin. The Indo-Burmese subduction zone obliquely overthrusts the Bengal Basin from the east (Fig. 1). About 20 mm/y of the shear motion is

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A. Singh et al. / Tectonophysics 680 (2016) 99–112

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Fig. 1. Tectonic setting and location of broadband seismic stations (inverted triangle) from the Bengal Basin and surrounding region. Faults and other tectonic features, such as the Hinge Zone and Barisal–Chandpur gravity high, are from published sources (Steckler et al., 2008; Alam et al., 2003). The possible thrust front of Comilla tract and its possible extension towards the NE are taken from Steckler et al. (2008). MFT: Main Frontal Thrust; MBT: Main Boundary Thrust; MCT: Main Central Thrust; RT: Rajmahal Traps.

accommodated by the Sagaing Fault (Maurin and Rangin, 2009). Some of the rest is absorbed by faults within the foldbelt, such as the Churachandpur–Mao Fault (Gahalaut et al., 2013). Recent GPS results indicate a convergence rate of 13–17 mm/y in addition to the partitioned shear motion (Steckler et al., submitted for publication). The subduction of the thick strata of the Bengal Basin has built a 250 km wide accretionary prism that extends almost half way across the basin (Steckler et al., 2008). Its front is blind, buried by the Ganges–Brahmaputra Delta sediments. The Shillong plateau overthrusts the Bengal Basin from the north, depressing the Sylhet Basin (Johnson and Nur Alam, 1991; Najman et al., 2012). The plateau is a large anticlinorium floored by Archean and Proterozoic basement rocks with remnants of the passive margin strata on its steep southern face. The evolution of the Shillong plateau as “pop up tectonics” has been much discussed (Bilham and England, 2001; Kayal et al., 2012), but lacks geophysical observations (apart from seismicity) which may define the structure and mechanism at greater depths (N10 km). The southern side of plateau is demarcated by Dauki thrust fault (Fig. 1), responsible for accommodating part of the shortening between the Himalaya and India (Bilham and England, 2001; Vernant et al., 2014). However the nature of deformation, which includes dextral convergent motion between Shillong and the IndoBurman foldbelt requires more knowledge and understanding. The great Assam earthquake, which occurred on 12 June 1897 (Mw = 8.1, Bilham and England, 2001), affected this region. However, the source of Assam Earthquake has been attributed to multiple faults around the Shillong plateau including the Dauki, Dapsi and Oldham Faults (Fig. 1, Oldham, 1899; Kayal and De, 1991; Bilham and England, 2001).

Most geophysical investigations from the northeast India collision zone are concentrated on either north or further northeast of the Shillong plateau (Singh et al., 2015). The southern side of the Shillong plateau and Bengal Basin are poorly resolved regions due to lack of seismic stations from Bangladesh. To understand the nature and character of faults within the continents with ability to create great earthquakes is of prime concern. The pop up model requires a mechanism where steeply dipping Dauki and Oldham faults bound the popped up structure from south and north respectively. The faults are expected to cross at crustal levels beneath the Shillong plateau with possible slips on Oldham fault extending from 9 to 40 km and dipping SSW at 57° (Bilham and England, 2001). With further constraints on geometry of Oldham fault (dip and depth) based on the extent of exposed basement and sedimentary rocks having a lack of sedimentary deposits surrounding the plateau, it is viewed as a backthrust to a master north-dipping fault based at depth (Clark and Bilham, 2008). Earlier considerations of a 5 to 10° dip on the Dauki Fault require few tens of kilometers of horizontal tectonic transport to attain the present elevation of the Shillong plateau (Johnson and Nur Alam, 1991). These observations lack the information about geometry of faults at depths and are supported with observations with meagre datasets from Bengal Basin. Previous geophysical studies from the Bengal Basin do not contribute much to the estimation of its sedimentary thickness and nature of the crust beneath Bengal Basin. The gravity modelling across various profiles accounted for a thinner crust (~30 km) with thicker sedimentary deposits (N7 km) (Khan and Agarwal, 1993; Rajasekhar and Mishra, 2008). Sylhet, a complex sub-basin in Bangladesh has been traced through various seismic lines due to hydrocarbon prospects in the region. Stratigraphic reconstructions involving seismic data have

A. Singh et al. / Tectonophysics 680 (2016) 99–112

provided a detailed picture of the Sylhet Basin to be filled with thick sedimentary strata (12 to 16 km) of late Mesozoic to Cenozoic (Johnson and Nur Alam, 1991). Modeling of receiver functions for various broadband seismic stations is limited to north of Dauki Fault (Fig. 1). The Moho depth estimates (33–35 km) beneath the Shillong plateau reveal a crust with Poisson's ratio estimates similar to that of the Indian shield (Kumar et al., 2004). Contrasts between the depth of Moho beneath the Shillong plateau (~ 38 km) and Bengal Basin (~ 44 km) have been determined for a station near the edge when events are modelled for northern and southern azimuths separately (Mitra et al., 2005). Most geophysical investigations from the northeast India collision zone are concentrated on either north or further northeast of the Shillong plateau (Singh et al., 2015) under various projects. Recent global efforts to investigate the Ganges–Brahmaputra Delta have generated some new data sets of hitherto less studied Bengal Basin. The present study makes use of these new public data, first to capture the lateral variations in the crustal structure, and second is to demonstrate the geometry (strike, dip) and deformation (anisotropy) of the region using available seismic datasets. To accomplish our objectives, we use receiver function modeling for dipping and anisotropic structures (Frederiksen et al., 2003) and the recently introduced technique of Schulte-Pelkum and Mahan (2014b) in order to quantify strike and depth of the dipping boundaries. In the present study, we are able to better constrain the crustal structure beneath the Bengal Basin and demonstrate the geometry of Dauki Fault, which has the potential to create major earthquakes. 2. Data and methodology 2.1. Teleseismic data: BanglaQuake We have used data from 10 seismic stations which were operated under joint research program between Dhaka University, Bangladesh

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and Lamont Doherty Earth Observatory (LDEO) of Columbia University at New York (BanglaQuake, XI network, IRIS, doi: 10.7914/SN/XI_2007). Additional data came from two permanent seismic stations DHAK (BI network, IRIS 2003–till present) and SHL (2007–till present) operated by the Dhaka University, Bangladesh and Indian Meteorological Department, India, respectively. The group of seismic stations under BanglaQuake Network were operated during 2007–2010 with a sampling rate of 100 samples per second. The units were a combination of short period Mark Products L4C (1-Hz) and broadband seismometers (Guralp CMG40T) with Reftek 130 Dataloggers. In the present study, we have used teleseismic earthquakes of magnitude ≥5.5 recorded in the distance range of 30° to 100°. As a first step, we have removed the instrument response from each seismogram to isolate the receiver effects of different instrument types (broadband, short period). Further visual inspection is performed over seismic waveforms which are of good signal to noise ratio (≥2.5). Only 30% of waveforms could pass our selection criteria and we are left with 2768 good quality waveforms from 1041 earthquakes (Fig. 2) which are used to compute the receiver functions. 2.2. Receiver functions The technique of receiver functions is an effective way to isolate the source signatures from the response of Earth's structure beneath a seismic station (Langston, 1979; Ammon et al., 1990). It is usually composed of P-to-s converted energy from the various discontinuities in the crust and upper mantle. We have adopted the approach of Vinnik (1977) which requires the rotation of Z, N, and E components into a ray coordinate system using optimized backazimuth and incidence angle. The coordinate system transformation decomposes the wavefield into P, SV and SH components. To isolate the effects of P-to-s conversions from the P-coda, we further deconvolved the SV and SH components by the P component. Deconvolution operation is performed in the frequency domain by spectral division using a water-level stabilization (Clayton

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Fig. 2. Epicentral locations of earthquakes in the distance range of 30–100° used in this study. The region of study is indicated by a filled square.

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Fig. 3. This section depicts SV component of P-RFs at various seismic stations, stacked in narrow slowness bins. Moveout corrected stacked traces for conversions (lower trace) and firstmultiples (upper trace) are shown at the top of each section. The seismic stations for which inversions are performed (Fig. 6) are not shown here to avoid repeatability. The conversions from Moho (P MS) and expected multiples (Pp MS and Ps MS) are shown for stations JURI, FENC and JAFL, with average crustal velocities searched in the range of 6.0–6.8 km/s for Vp and 3.5 to 3.8 km/s for Vs.

and Wiggins, 1976). A water level parameter of 0.0001 and a Gaussian filter of 2 Hz are used throughout the analysis for computing RFs. The final receiver functions produced are of relatively lower frequency and this limits the possibility to resolve much finer scale features. A moveout correction is applied before stacking to improve coherence of the converted phases; thereafter they are binned at regular intervals of 0.15 s/° in the slowness range of 4.44 to 8.44 s/°. Moveout correction is done for the conversions and first-multiples separately which helps to differentiate between actual reflectors and their multiples, respectively (Fig. 3). In the approach, to perform the moveout correction the times of receiver functions are adjusted with respect to arrival time of Ps conversions at a reference epicentral distance (67°). Common conversion point (CCP) stacks can illuminate the lateral variations in the crustal thickness and other intracrustal features (e.g. Gilbert and Sheehan, 2004). To obtain the migrated image, the receiver

function amplitudes are back projected to their respective depths using a standard Earth's velocity model. The velocity models obtained through modeling (RFs in this case) or high resolution crustal models (Laske et al., 2013) can be used to replace the standard velocity models. Replacements in the velocity model can reduce the effects of lateral variations in the velocity over depth along a profile. However, the highly complex and attenuated crust (Figs. 3 and 4) and a low resolution of CRUST1.0 for the region prompted us to initially use the standard IASP91 velocity model (Kennett and Engdahl, 1991). However, due to the velocity contrast between the Shillong basement and the Bengal Basin, we migrated each section of the profile with velocities from the RF results for each section. Finally, amplitudes are averaged at specific depths for various event–station pairs and are projected along a profile to get the 2D-migrated image (Fig. 5).

Fig. 4. SV and SH components of RFs arranged in the narrow backazimuthal bins. Two separate panels are shown for the group of seismic stations located in the east and west of Bangladesh. The piercing points of individual rays at a depth of 40 km are shown by a cross. The backazimuthal patterns obtained at seismic stations MPUR, JAFL, SKPR and CAL are clear indications of dipping boundaries. Profiles A1–A2 represent the positions along which the migrated image has been produced, shown in Fig. 5.

A. Singh et al. / Tectonophysics 680 (2016) 99–112

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2.3. Inversion: dip and anisotropic effects Dipping and anisotropic behavior of various layers beneath a seismic station may produce a significant amount of energy on both the radial and transverse components of RFs (Frederiksen and Bostock, 2000; Ozacar and Zandt, 2004). The energy produced out of source receiver plane of RFs follows a specific pattern as a function of back azimuth. Back azimuthal variation of the RFs generally displays a 180° symmetry for azimuthal anisotropy, while for dipping layers it is 360° (Cassidy, 1992; Levin and Park, 1997; Savage, 1998). The combination of dipping and anisotropic effects can produce complex patterns of amplitude on both the radial and transverse components of RFs and in some cases it may be difficult to resolve (Frederiksen and Bostock, 2000). In the present study we have used the technique of Frederiksen et al. (2003) to model the dipping and anisotropic effects, while for improved constraints of strike of dipping reflectors we have used that of SchultePelkum and Mahan (2014b). The Frederiksen et al. (2003) technique is applied to invert the data of seismic station (JAFL) located near the Dauki Fault, by incorporating the dipping and anisotropic effects. Clear back azimuthal patterns and a good azimuthal distribution of waveforms were the key reasons for the selection of JAFL station for inversion. We have performed simple 1D-modelling at seismic stations DHAK, SUST and SHL, where no distinct pattern in the backazimuthal distribution is observed (Fig. 6). The simple 1D-modelling is also performed for seismic station MPUR, where dipping/anisotropic reflectors are seen as the attempts to model for dip and anisotropic effects resulted in large uncertainties. The inversion scheme of Frederiksen et al. (2003) relies on the nearest neighborhood algorithm (Sambridge, 1999) to search for the model parameters which will provide the lowest data misfit. In a given model space random points are generated and the whole region is divided into neighborhoods (Sambridge, 1999) by creating Voronoicells, which are regions of space closer to one sampled point than to the other (Frederiksen et al., 2003). Misfits are obtained for each sampled point and those few with the lowest misfits are retained for the next iteration, the process then continues until we reach an optimal solution. In certain cases, we have also used a recently developed technique of Schulte-Pelkum and Mahan (2014b) in order to quantify the strike and depth of dipping layers. The technique of Schulte-Pelkum and Mahan (2014b) has an advantage for seismic stations located over the Indian sub-continent, as it fills the crucial gaps in azimuthal distribution (western azimuths) by shifting transverse components by 90° to fill in gaps in the radial component. We prepared the data that was used in the inversion by binning the RFs into narrow backazimuthal bins (10°) and each backazimuthal bin was further subdivided into smaller slowness bins. For 1D-modelling the RFs are stacked into the slowness bins of 0.15 s/° in the range of 4.44 to 8.44 s/°. We have used 2 to 4 layer models in the inversion (Fig. 6), with a maximum number of 12 variable model parameters.

The choice of number of layers (SHL, 2; SUST, 4; MPUR, 2; DHAK, 4) with proper smoothing (here 2) of the RFs is made to simplify the inversions. At most of the seismic stations from the study region, the Moho (P MS) conversions and its multiples (Pp MS and Ps MS) are not clear (Figs. 3 and 6) and limit our scope to provide velocity models for each seismic station. For 1D-modelling we have used seismic stations DHAK, MPUR, SUST and SHL arranged in an order of increasing slowness (Fig. 6). There is a bit of ambiguity involved for certain stations (SUST, multiples at 10 s and 15 s), where various multiple arrivals are seen. While defining the model space to perform inversion for a specific station, we have also taken into account the information from available seismic data of nearby stations. For example, stations DHAK and MANK are very similar in their appearance and complements each other (Fig. 4) and lateral variations in the crustal structure for seismic stations SKPR, MANK and DHAK help to identify the continuity of (P MS) reflections (after 5 s) from Moho (Figs. 3 and 4). The initial inversions are performed with various seed values to generate the random numbers. This is done to check the stability of inversion due to highly nonlinear nature of the parameterization involved in NA-algorithm. The results are quite consistent and the final results are plotted for the random seed value of 150,000. The inversions are initiated by generating 25 models in the model space. Based on the convergence rate and efficiency of the search, we have performed 100 to 300 iterations for each seismic station using a sample size of 25 models per cycle and retaining 8 models per cycle with lowest misfit values. The recovery of model parameters is quite good for seismic stations, an example for station SHL is shown in Fig. 7 to show the robustness of inversions. At station JAFL, we modeled the data for both dip and anisotropic effects using technique of Frederiksen et al. (2003), however the strike of dipping layer is constrained using the technique of Schulte-Pelkum and Mahan (2014b), as it helps to reduce the uncertainty in the strike with better azimuthal coverage. To account for the dips of stations JAFL located near the Dauki Fault and MPUR located over Madhupur Tract, the SV and SH components of RFs are arranged on the basis of the method of Schulte-Pelkum and Mahan (2014b). The first panel in Fig. 8 shows SV component of receiver functions by subtracting azimuthally averaged amplitude for each individual receiver functions, the middle panel represents SH component of receiver functions with 90° added to back azimuth, the last panel is a stacked section of both. The strike (green line, 110° for JAFL and 67° for MPUR) is obtained by fitting a degree-1 function over back azimuth. 3. Results We present here the first results of crustal thickness estimates (using RFs) from the seismically active region of Bangladesh. Sediments are thick and variations (3–17 km) in the thickness are consistent with the local geology. RF inversion revealed very thick sediments beneath the Bengal Basin (SKPR, 12.2 km; DHAK, 15.9 km; and SUST, 17.4 km)

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Fig. 6. The real and synthetic RFs generated by the best model obtained through 1D inversion. Specifics of line color are, blue: Vp/Vs ratio; red: Vs (km/s); green: density(gm/cm3). The respective crustal parameters from CRUST1.0 are also plotted at each station (dotted brown lines) for comparison. SYNT: Synthetics (SV); OBSD: Observed (SV). We were able to perform a meaningful inversion at only a few stations where data is sufficient and the phases are clear. RFs are binned in narrow slowness bins with sufficient smoothing to perform the inversion. A significant interference at deeper levels from the multiples of sedimentary layers can be seen at stations DHAK, SKPR and SUST. (For interpretation of the references to color in this figure legend, the reader is referred to the online version of this chapter.)

and thinner sediments near the Hinge Zone (MPUR, 2.8 km and CAL, 3.1 km). The uncertainties in the estimates of crustal parameters are shown in Table 1. Cultural noise and reverberations in the thick sediments are issues for the data. Thus, the complexity and quality of the data restrict us to invert only a few seismic stations for crustal thickness estimates (MPUR, SUST, SHL and DHAKA; Fig. 6). For seismic stations CAL and SKPR, we are able to model only for sedimentary thickness estimates as uncertainties involving crustal multiples are too high for deeper layers. Nevertheless, the final estimates obtained through NA-inversion provided a fair estimate of crustal configuration of the various segments within the Bengal Basin. We obtained a shallower Moho beneath the Bengal Basin and

Shillong plateau (DHAK, 31.6 km; MPUR, 36.9 km; SUST, 36.6 km; SHL, 34.5 km), which is also evident in the migrated image (Fig. 5). Intriguing are a comparison of seismic stations DHAK and MPUR in the central portion of Bangladesh. Inversions accounted for a deeper Moho (36.9 km) with relatively lower crustal velocities (3.75 km/s) beneath MPUR closer to the Hinge zone while we observed thicker sediments and a shallower Moho (31.6 km) with higher seismic velocities (N4.0 km/s) for station DHAK. The thinner crust beneath DHAK (Dhaka) exhibits higher velocities consistent with oceanic crust. However the thickness is still quite large, MPUR, in contrast exhibits velocities similar to what is reported for the seismic stations located over Indian shield to its northwest (see Singh et al., 2015).

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Fig. 7. Tradeoff curves for station SHL observed through NA-inversion of receiver functions. The final values for the misfit 0.46 are shown by cross ‘x’, for the two layer model. Average crustal velocity obtained through inversion (x) are shown with results (diamond) obtained through (Zhu and Kanamori, 2000) technique for the same station (taken from (Kumar et al., 2004)). Misfit values with various models tested are shown in the bottom right corner.

Beneath station JAFL (Jaflong), the RFs revealed a very complex structure and have specific variations with back azimuth (Figs. 8 and 9). In the initial trials, we have considered the effect of dip (0–50°) for the layer at ~1.5 s (SH-component, Fig. 9) ignoring the effects of anisotropy. The inversion resulted in unreasonably large data misfits (0.6). The larger misfits and tilt in the pattern necessitate incorporating the anisotropic effects along with the dip. Inclusion of anisotropic effects is not so farfetched approach as highly deformed faults and shear zones exhibit a high degree of anisotropy (Ozacar and Zandt, 2004; Sherrington et al., 2004; Ozacar and Zandt, 2009). As a first step we fixed the strike of the dipping layer (110°) using the technique of Schulte-Pelkum and Mahan (2014b) (Fig. 8). This helps us to minimize the number of model parameters used in inversion technique of Frederiksen et al. (2003). Inverting for strike, in the inversion results in large uncertainties in the strike of the dipping layer due to lack of events from western azimuths (Fig. 9). The data is further inverted for a wide range of parameters involving dip (dip and strike) and anisotropic (trend and plunge, Vp and Vs% anisotropy) effects. We have achieved lower misfits (0.22) when the data is inverted for both the dip and anisotropic effects. The synthetics obtained using the model parameters for the lowest misfit value (0.22) are in close agreement of the observed data (Fig. 10). We finally conclude, based on the recovered parameters that the Dauki Fault is a north-dipping thrust fault at Jaflong (Frederiksen et al., 2003). Strong anisotropy (~15%) and the sense of shear (plunge: 24°, trend: 79°) actually compliment the dipping effect (Fig. 10). 4. Discussion 4.1. Sedimentary thickness Sedimentary thickness variations are quite high in the basin and are similar to earlier estimates of very thick sediments (12–20 km) in the Bengal Basin (Curray, 1991; Johnson and Nur Alam, 1991; Brune et al., 1992; Rajasekhar and Mishra, 2008). Complications in the subsurface

structure beneath the Bengal Basin required a two to three layer sedimentary model, except for the thinner sediments at stations MPUR and CAL (single layer) reflecting compaction of the sediments with increasing depth. Our assumption of a multi layered model to account for the sediments is appropriate given the dominance of arrivals of multiples from the shallow layers at around 10 s (Fig. 6). Considering the arrivals at around 3 to 3.5 s as a first multiple of the reflector from 1–1.5 s may not satisfy the later arrivals (multiples at ~10 s) and we achieved a relatively higher velocity through inversions (N 2.2 km/s, Vs) than we may expect for the soft overpressured sediments in the Bengal Basin (Zahid and Uddin, 2005). To satisfy the multiples at delay times of ~ 10 s we require a conversion above the Moho at around 3–4 s, achieved in the present study by introducing a two layer sedimentary model. The distinct positive peak in the 1–1.5 s delay time range for the seismic stations (DHAK, SKPR and SUST) reflects the upper boundary layer with velocities 1.3 to 1.5 km/s (Vs) and the lower layer (3.5 s) exhibits velocities close to 2.6 km/s (Vs). This may reflect the contrast between the younger syn-Himalayan clastics and the Eocene Sylhet limestone and older strata. We also note that with the large sediment thicknesses observed, the base of the sediments may be in greenschist facies with velocities close to the crust. The conversion could be off the top of metamorphism instead of the possible limited velocity contrast at the metamorphosed sediment–crust boundary. The LASE refraction experiment off the U.S. east coast obtained strong refractions from within the sediment column and the upper/lower crust, but the top of the crust was not determined in the deepest parts of the basin (LASE study group, 1986). If this is the case here, the sediment thickness could be a few kilometers greater than the values we obtained and the crystalline crust correspondingly thinner. Seismic station MPUR is located over the Madhupur Tract, a slightly elevated region of Pleistocene outcrop in the Bengal Basin. It is an interfluve between the current Jamuna channel of the Brahmaputra River and the Old Brahmaputra channel occupied before ~ 1800. Arguments as to whether it is tectonically uplifted and whether its western margin

A. Singh et al. / Tectonophysics 680 (2016) 99–112

107

JAFL SH(Φ−900)

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Fig. 8. The receiver functions for station JAFL and MPUR located over Dauki and Madhupur Fault. The SV and SH components are arranged based on the method of Schulte-Pelkum and Mahan (2014b). The first panel shows SV component of receiver functions by substracting azimuthally averaged amplitude for each individual receiver functions, the middle panel represents SH component of receiver functions with 90° added to back azimuth, the last panel is stacked section of both. The strike (green lines) is obtained by fitting a degree-1 function over back azimuth. (For interpretation of the references to color in this figure legend, the reader is referred to the online version of this chapter.)

is faulted have lasted over 50 years (e.g. Morgan and McIntire, 1959; Steckler et al., 2008; Maitra and Akhter, 2011; Hosain, 2014). The 1885 Bengal earthquake has been associated with the proposed Madhupur Fault (Akhter, 2010). MPUR required only one layer model for the sediments based on the clear arrivals of the conversion and its multiples at around 1 s. A sedimentary thickness of 2.8 km and velocities in the range of 2.2 km/s (Vs) obtained through inversion clearly reflect the local surface geology (Pleistocene).

A significant increase in the sedimentary thickness just south of the MPUR, past the Hinge Zone is evident from the observed RFs (Fig. 4). Inversions indicate a very thick sedimentary strata for seismic stations SKPR and DHAK. The increase in thickness from MPUR (2.8 km) to SKPR (12.2 km) is similar to seismic reflection profiles (Lindsay et al., 1991; Frielingsdorf et al., 2008) that show the sediment thickness increasing from ~2 km to 5 km at the Hinge Zone and then increasing to N10 km as the end of the lines. The total sedimentary thickness

Table 1 Station locations crustal parameters obtained in the present study using inversion of RFs (Frederiksen et al., 2003) and technique of Schulte-Pelkum and Mahan (2014b) to define strike of dipping reflectors. The results of inversion (for dip and anisotropic effects) using the technique of Frederiksen et al. (2003) for JAFL seismic station is shown in Fig. 10. Station

Latitude (°)

Longitude (°)

Sedimentary thickness (km)

Crustal thickness (km)

Strike (°)

Method

CAL MPUR

22.54 24.60

88.30 90.03

3.1 ± 0.6 2.8 ± 0.3

– 36.9 ± 2.0

SKPR DHAK SUST SHL JAFL

24.32 23.73 24.92 25.57 25.18

90.17 90.40 91.83 91.88 92.02

12.2 ± 0.4 15.9 ± 1.3 17.4 ± 1.6 –

– 31.6 ± 1.7 36.6 ± 1.6 34.5 ± 2.0

– – 67 – – – – 110

Frederiksen et al. (2003) Frederiksen et al. (2003) Schulte-Pelkum and Mahan (2014b) Frederiksen et al. (2003) Frederiksen et al. (2003) Frederiksen et al. (2003) Frederiksen et al. (2003) Schulte-Pelkum and Mahan (2014b)

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JAFL

360 SYNT−SV

SYNT−SH

270 BAZ [Deg]

270 BAZ [Deg]

arrivals from the Moho are masked due to conversions from the sediments and the arrival at 3.5 s is from the bottom of sediments (Fig. 6) and that conversions from the Moho (~5.5 s, 36.6 km) are significantly attenuated due to the multiples from above. The NA-inversion results (Fig. 6) which accounted for various multiples for the seismic station SUST provided a crust having a similar thickness as observed for nearby stations. The arrivals between 8 and 15 s are better matched with multiples from intracrustal layers and the expected arrivals from the Moho does match with amplitudes at 16.5 and 22.5 s (Fig. 6, SUST). We incorporate the effects of thicker sediments beneath Sylhet Basin and the inversion is performed over a broader model space. This results in a sedimentary thickness of 17.4 km, akin to the earlier findings (Johnson and Nur Alam, 1991). However, we refrain from making further interpretations given the complexity of the data for this site.

360

180

180

90

90

0

0

360

360

OBSD−SV

OBSD−SH 4.2. Image of the crust

270

90

0

0 0

5 Delay Time [s]

10

0

5 Delay Time [s]

10

Fig. 9. Synthetic and observed receiver functions (radial and transverse components) for station JAFL located over the Dauki Fault. The strike is set to 110° as shown in Fig. 7.

increases to 15.9 km at DHAK. Not surprisingly, inversions for the thicker sediments show multiple layers. The upper layer has a thickness of 3 and 4 km for the stations SKPR and DHAK, and velocities reflective of soft sediments (Vs, 1.3 to 1.5 km/s). The lower layers correspond to more compacted sediments with higher velocities close to 2.6 km/s for the lower layer. We have explored various possibilities for the seismic station SUST located in the Sylhet Basin. If we consider the arrival around 3.5 s as Moho, it implies resulting a thinner crust beneath Sylhet Basin with much thinner sedimentary strata than previously reported (12– 16 km) (Johnson and Nur Alam, 1991). Furthermore, a shallower Moho beneath Sylhet Basin (25 km) may not account for the multiple conversions as visible in the data in the time range of 10–15 s (Fig. 6, SUST). It is expected that sediment thicknesses in Sylhet are larger due to flexural loading from Shillong (Johnson and Nur Alam, 1991; Najman et al., 2012). We therefore consider it much more likely that

180

trend(deg)

120 90

0.3

0.4

30

50

0.4

120 90

0.3

60

0.4

60

0.4

150

0.4

0.4

trend(deg)

150

60

30

0.5

0 5

10

15

%P−anis

20

25

0.4

0

5

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15

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1.0

40

0.8

30

0.6 0.4

20 0.5

0.2 10

0.6

0 0

dip(deg)

180

20

misfit

90

180

4

180

The Hinge Zone is considered to be the eastern boundary of Indian Craton. Beyond it is a transition to thinned continental crust and finally to oceanic crust somewhere towards the southeast in the deeper parts of the basin. Our results support this interpretation. Interestingly, the two stations MPUR and SKPR closer to Hinge zone have shown compelling evidence of a dipping Moho, as seen on their SH components (Fig. 4). Station CAL, from the western side of Bangladesh reflects conversions which seem to be a reflection of fossilized fabric as data lacks polarity reversals at zero time, which is indicative of a dipping boundary. We interpret that, if the Hinge zone is supposed to be the Paleocontinental slope and a boundary which separates the Bengal Basin from the Precambrian Indian platform, then these two stations (MPUR, SKPR) may lie over the transitions from the continental to oceanic Moho. The close similarity in the strike of the dipping Moho and Hinge zone orientation (67°, MPUR, Fig. 8) further supports the idea of existence of a transitional boundary along the Hinge zone. While not precisely matching the Hinge Zone trend, the observation where the Hinge Zone begins to bend eastward to align with the southern edge of Shillong. The sediment thickness (3 km) and Moho (37 km) under MPUR indicate a crystalline crustal thickness of 34 km. This is only slightly thinned relative to stations farther west at the edge of the craton (38 km, Fig. 11) consistent with the position of MPUR near the Hinge Zone. In contrast, the thicker sediments (16 km) and shallower Moho (32 km) indicate a 16 km thick crust at DHAK. Considering the thick sediments (12–17 km) with low velocity (b 2.6 km/s) and high Poisson's ratio(~2.0) as observed in the present study for the seismic stations we have modeled, we expect a much thinner crust beneath the Bengal Basin. While the higher crustal velocity (N4.0 km/s) suggests a possible oceanic nature, the thickness is more than twice that of normal oceanic crust. The rifting that created this continental margin is associated with

0.

BAZ [Deg]

BAZ [Deg]

270

0.0

0 25

0

10

20

30

40

50

60

plunge(deg)

Fig. 10. Tradeoff curves for station JAFL observed through NA-inversion of receiver functions. The final values for the misfit 0.22 are shown by a cross ‘x’. The specific results are characteristics of a north dipping fault at Jaflong (Dauki).

A. Singh et al. / Tectonophysics 680 (2016) 99–112

109

27˚ 47.0 DJLG 44.0 ILAM

42.0 TEZ

57.0 BKD 50.0 BKD

35.0 TEZ

46.0 BIRA 35.0 JPA 41.0 JPA

42.0 BAI

26˚

40.0 GAU

33.0 HMN

35.0 BPN

SHL

35.0 NGB

38.0,44.0 CHP 35 km

JAFL 17 km SUST 37 km

25˚ 3 km MPUR 37 km 12 km SKPR

38.0 PAKR

39.0 KMG

24˚

35.0 AGT

16 km DHAK 32 km 38.0 DGPR

23˚ 3 km

CAL 38.0 KGP

3

6

9 12 15 18

sed thickness (km) 22˚ 88˚

89˚

90˚

91˚

92˚

93˚

Fig. 11. Summary of results. The sedimentary thicknesses obtained through inversion of RFs are shown by a colored inverted triangle. Yellow bars indicate the strike of dipping structures revealed at seismic stations JAFL and MPUR using the method of Schulte-Pelkum and Mahan (2014a) (JAFL: 110°; MPUR: 67°). The RF crustal thickness estimates from the region derived using RF inversions are shown by gray filled square (Singh et al., 2015), while stations with crustal parameters from the present study are shown by yellow filled square. Sedimentary thickness obtained through inversion of RFs for each station is shown in italics, while Moho depth is shown in bold letters. Faults and other tectonic features are the same as shown in Fig. 1. (For interpretation of the references to color in this figure legend, the reader is referred to the online version of this chapter.)

the Kerguleun plume (Storey et al., 1992; Coffin et al., 2002; Ray et al., 2005). The Rajmahal and Sylhet Traps outcrop to the NW and N of the Bengal Basin and several wells in West Bengal bottomed in basalts up to 332 m thick (Baksi et al., 1987; Frielingsdorf et al., 2008) interprets seaward dipping-reflectors as volcanics on a seismic line across the Hinge Zone just south of Shillong. Kent (1991) interprets much of the Bengal Basin to contain seaward dipping reflectors. Thus the thicker crust may indicate a volcanically thickened crust, reflecting the addition of either volcanics, intrusions or underplating. The slightly thicker crust interpreted at JAFL (19 km) despite the thicker sediments (17 km) is consistent with ~ 4 km of subsidence from flexural loading from Shillong, similar to the flexure seen in seismic and well data (Johnson and Nur Alam, 1991; Najman et al., 2012). The flexural loading of the Bengal Basin by Shillong across the Dauki Fault can be seen in a migrated receiver function image (Fig. 5). The image has been produced along a N–S profile (24.0°,91.5°; 26.5°,91.5°). Although we have only two seismic stations from the north of Dauki Fault, it helps us to map the continuation of the Moho towards north. To account for the velocity difference between the outcropping basement on Shillong and the thick sediments of the Bengal Basin, we migrated using the inverted velocity profiles from SHL and JAFL for north and south of the Dauki Fault, respectively. The low velocity sediments of the Sylhet Basin can delay the RFs arrivals to much greater depths (Hetényi et al., 2006) and even attenuate the Moho conversions as

seems to be the case here (Fig. 4). Despite the crustal Ps image being affected by low-velocity sedimentary structures the clear first arrivals (P MS) from seismic stations JURI, FENC, JAFL and SHL are able to illuminate the Moho boundary (Fig. 3). Lateral variations are small and the migrated image (Fig. 5) clearly revealed a simple crust and shallower Moho beneath the Shillong plateau and Bengal Basin (~32–37 km) along the profile. The downward flexure from seismic stations SUST and KNGT northwards to JAFL is clearly seen in the migrated image (Fig. 5). Earlier studies also account for a shallower Moho beneath the Shillong plateau (~33 km), (Kumar et al., 2004) using the approach of Zhu and Kanamori (2000). Mitra et al. (2005) obtained a crustal thickness of 35–38 km beneath Shillong plateau through isotropic 1D-inversion performed over a single stack of RFs. Moho deepens to 42 km beneath the Assam Valley. We similarly observe evidence of upwarp in the Moho (Fig. 4) for SHL seismic station as suggested by Mitra et al. (2005). The Shillong plateau region is an amalgamation of Proterozoic and Archean basement rocks (Clark and Bilham, 2008). The upwarp of Moho over Shillong is consistent with the 1–2 km elevation of the massif and 3–6 km of erosion (Biswas et al., 2007; Clark and Bilham, 2008). 4.3. Implication for faulting Sedimentary thickness variations are quite high in the basin and are similar to earlier estimates of very thick sediments (12–16 km) in the

110

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Bengal Basin (Curray, 1991; Johnson and Nur Alam, 1991; Brune et al., 1992), (Rajasekhar and Mishra, 2008). Complications in the subsurface structure beneath the Bengal Basin required a two to three layer sedimentary model, except for the station MPUR (single layer). Our assumption of a multi layered model to account for the sediments is appropriate given the dominance of arrivals of multiples from the shallow layers at around 10 s (Fig. 6). Considering the arrivals at around 3 to 3.5 s as a first multiple of the reflector from 1–1.5 s may not satisfy the later arrivals (multiples at ~ 10 s) and we achieved a relatively higher velocity through inversions (N 2.2 km/s, Vs) than we may expect for the soft sediments in the Bengal Basin. To satisfy the multiples at delay times of ~ 10 s we require a conversion above the Moho at around 3–4 s, achieved in the present study by introducing a two layer sedimentary model. The distinct positive peak in the 1–1.5 s delay time range for the seismic stations (DHAK, SKPR and SUST) reflects the upper boundary layer with velocities 1.3 to 1.5 km/s (Vs) and the lower layer (3.5 s) exhibits velocities close to 2.6 km/s (Vs). The role of Dauki (and Oldham) Faults in the formation of Shillong plateau and their ability to create large magnitude earthquakes have been extensively studied (Bilham and England, 2001; Kayal et al., 2012). However the dip of the Dauki Fault and its continuation at crustal depths has been an issue (Bilham and England, 2001; Biswas et al., 2007; Clark and Bilham, 2008; Vernant et al., 2014; Najman et al., 2012). Shallow dips of a 5 to 10° on Dauki Fault require a few tens of kilometers of horizontal tectonic transport to create the present elevation of Shillong plateau (Johnson and Nur Alam, 1991). Our results (dip: 32° and strike: 110°), consistent with optimal thrust fault dips, are a step forward in resolving the ambiguities revolving around the geometry of the Dauki Fault. Johnson and Nur Alam (1991) have pointed out that if the Dauki Fault is dipping at shallower angles (5 to 10°), then a few tens of kilometers of horizontal tectonic transport are need to uplift the Shillong plateau (~2 km). Our findings do not support a low angle dipping Dauki Fault. The moderate angle of 32° also does not require intersection of the Dauki and Oldham faults at mid crustal levels. The fault geometry also affects the calculations to interpret the movement of Shillong plateau based on GPS velocity vectors (Clark and Bilham, 2008; Vernant et al., 2014). Vernant et al. (2014) argue for a substantial increase in the clockwise rotation rate of the Shillong plateau at 3–4 Ma as being needed to match geological uplift rates and the convergence rate across the Dauki Fault. However, this estimate was based on a Dauki Fault with a steeper dip of 45°. The possible Madhupur Fault (Morgan and McIntire, 1959; Hosain, 2014) is perpendicular to the observed strike directions (67°, MPUR) near the subsurface (1–1.5 s, Figs. 8, 11). We conclude that the dip is related to the structural boundary appertaining to the transition from continent to oceanic crust. The RF results do not address the existence of a possible Madhupur Fault. The RF inversion also revealed a highly deformed zone (anisotropy ~ 15%) along the Dauki thrust fault, the sense of shear (Fig. 10) is in close agreement with the geometry of a NNE-dipping fault. The anisotropy (79°) and strike of the velocity discontinuity (110°) lie on either side of the approximately E–W trend of Shillong. The inversion scheme adopted here (Frederiksen et al., 2003) has been implemented successfully to map the anisotropic characteristics of other parts of the Himalayan and Tibet collision zone (Schulte-Pelkum et al., 2005; Sherrington et al., 2004; Singh et al., 2010). Significantly high anisotropy (~ 20%) observed beneath the Nepal Himalaya at mid-crustal levels has been attributed to the shear at the base of the decollement surface (SchultePelkum et al., 2005), which gently dips towards the north. A completely different mechanism, possibly because of midcrustal transcurrent deformation, has been invoked to explain the convergence parallel alignment of the fast-axis and strong anisotropy (~ 17%) beneath the Sikkim Himalaya (Singh et al., 2010). There are certain instances, where the orientation of the anisotropic rock fabric and present day sense of shear due to the recent tectonic activity are largely inconsistent (Ozacar and Zandt, 2009), but in most

of the cases they are related to the present day deformation patterns in the crust (Ozacar and Zandt, 2004; Schulte-Pelkum et al., 2005; Sherrington et al., 2004). The extent of these deformed zones should be sufficiently wide (N3 km), or else RFs may undergo a destructive interference with having a signal pulse width in the order of 0.5 to 1.0 s (Schulte-Pelkum and Mahan, 2014a). Based on the modeling and amplitude arrival patterns (Fig. 9), the deformed zone seems to be relatively wider (N 5 km). We conclude that the anisotropic fabric in the north-dipping Dauki Fault is related to the recent deformation. The possible mechanism to create such large scale deformation could be the initiation of the faulting along southern side of the Shillong plateau, estimated as Pliocene by Najman et al. (2016). The IndoBurman foldbelt overthrusts eastern Shillong, further contributing to the fabric of the region (Ferguson et al., in preparation). Lateral motion between the foldbelt and Shillong may also be a factor in the anisotropy.

5. Summary The results are summarized in Fig. 11. In summary, 1) The crust is thinner beneath the basin and no significant lateral variations are apparent across a N–S profile covering Shilling plateau and Bengal Basin. The sedimentary thickness in the Bengal Basin increases from ~3 km to ~17 km across the Hinge Zone. In conjunction, the crust thins from 38 km at the Indian Craton to 34 km at the Hinge Zone to b16 km in the deep Bengal Basin. This represents the transition from the Indian Craton across the Cretaceous passive margin of eastern India. 2) The 16–19 km thick, higher-velocity crust in the deep Bengal Basin may be thickened by igneous activity related to the Kerguelen plume, which was active near the margin at the time of rifting. 3) Dipping layers seen on the transverse component of receiver functions for the stations close to the Hinge Zone appear to be a the part of continent to oceanic transition. 4) The Moho is at similar depths under Shillong and the Bengal Basin, despite the contrast of craton vs thick sediments. It indicates uplift of the overthrusting Shillong plateau. Downward flexure of the Bengal Basin by 4–5 km can be seen towards Shillong on a migrated RF profile and is consistent with the sediment and crustal thicknesses determined in the basin. 5) The Dauki Fault is a north dipping fault and dips along a highly deformed zone with a moderate (32°) dip angle. The highly deformed zone and strong anisotropy created in the rock fabric are resultant of the fault initiation. Thus, we are able to provide valuable new constraints in terms of the nature and thickness of the crust in the Bengal Basin previously subject to conjecture due to lack of data.

Acknowledgments The waveform data used in the study are from IRIS data management center. We thank the students involved in servicing and maintaining the BanglaQuake array and the people at many sites that hosted the seismic stations. The manuscript was improved by thoughtful and constructive reviews by the editor and two anonymous reviewers. We thank M. Ravi Kumar for his constant support. The Indian contribution to this work was performed under the project IAI, IITKGP funded by the Ministry of Earth Sciences Govt. of India. This is Lamont-Doherty Earth Observatory contribution number 8002. The U.S. and Bangladeshi contribution project was supported by grants NSF INT 9900487, NSF EAR-06 36037, and NSF 09-68354. The figures were made using the Generic Mapping Tools software (Wessel et al., 2013).

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