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Post-breakup deformations in the Bay of Bengal: Response of crustal strata to the sediment load

Abstract

Passive continental margins are tectonically inactive, but a few of them including the East India Passive Margin (EIPM) show evidences for post-breakup deformations. This unusual process prompted us to investigate the post-rift deformations on EIPM and adjacent deep-water region for understanding the possible mechanisms. Seismic reflection images of the Bay of Bengal reveal a post-rift deformation with a manifestation of extensional faults in Krishna–Godavari (K–G) basin and on flanks of the 85°E Ridge. In K–G basin, one of the rift-related faults reactivated during the Early Miocene time (~16 Ma), while on flanks of the 85°E Ridge new normal faults originated at about 6.8 Ma. From detailed analysis of fault throws, it is observed that the fault in K–G basin recorded a cumulative throw of about 900 m between the basement and Early Miocene horizon (~16 Ma), later the fault was reactivated at 6.8 Ma and continued the activity progressively until 0.3 Ma before cessation. The fault system on the margin spatially extends for about 300 km between offshore extensions of the Pranahita–Godavari graben and Nagavali–Vamshadhara shear zone. The faults on 85°E Ridge, initiated at 6.8 Ma and continued until 0.8 Ma, have cumulative throws of about 60 and 110 m on western and eastern flanks of the ridge, respectively. Back-stripping analysis of the fault from the K–G basin discloses two distinct phases of subsidence history: (i) during the first phase (120–23 Ma) the basement subsided at a rate of 46–18 m/Myr due to thermal cooling of the lithosphere, (ii) during the second phase (23 Ma–Present) rapid subsidence rate (69.56 m/Myr) of basement is noticed as a consequence of deposition of copious amounts of Bengal Fan sediments. The thick sedimentary strata exerted vertical load on underlying heterogeneous crust that led to build excessive internal stress and release through weak zones (lying at intersecting planes of heterogeneous crustal blocks). The stress, thus released through fault planes has caused the deformation of crust as well as overlying sedimentary strata.

Introduction

Passive margins can be categorized based on two simplified models magma-poor (Whitmarsh et al. 2001; Reston 2009) and magma-rich rifted continental margins (Eldholm et al. 1989). The East India Passive Margin (EIPM), evolved by rifting between India and East Antarctica during the Early Cretaceous period (Curray et al. 1982; Gopala Rao et al. 1997; Gaina et al. 2007; Krishna et al. 2009a; Veevers 2009; Talwani et al. 2016), is categorized largely as a model of magma-poor rifted margin. The evolved rifted margin consists of attenuated continental crust with normal faults and half graben structures, and continent–ocean transition involving a broad zone of serpentinized mantle (Nemčok et al. 2012; Ismaiel et al. 2019). The structural architecture of the EIPM revealed varied styles of rifting from sheared to hyper-extended to hypo-extended from south to north (Sinha et al. 2015; Haupert et al. 2016; Ismaiel et al. 2019; Harkin et al. 2019). The normal faults of the margin became passive after completion of the rift process, and then they were completely buried under post-rift sediments.

The lithosphere in the Bay of Bengal is overlain by copious amount of sediments exceeding 16 km in the vicinity of mouth of Ganges and Brahmaputra river systems (Brune et al. 1992; Curray 1994; Radhakrishna et al. 2010; Krishna et al. 2016). The nature of rocks across the Bay of Bengal varies from continental to partially serpentinized mantle (Nemčok et al. 2012; Ismaiel et al. 2019) to oceanic and to volcanic. The thick overlying sediment package put forth a huge mechanical loading on lithosphere, and shown its impact specifically in the regions, where varied crustal rocks juxtapose. This is an interesting topic to study how a passive rifted fault reactivates in margin region and new faults develop on flanks of the 85°E Ridge due to impact of sediment load.

In order to address a cause–effect relationship between the sediment load and response of underneath heterogeneous rocks, we analysed seismic reflection profile data acquired from shelf to deep-water region of the Bay of Bengal (figure 1), thereby the interpreted seismic sequences are correlated to drill wells ‘A’ and ‘B’ results. Earlier seismic studies in the Bay of Bengal brought out architecture of rifted margin and sediment depositional environment (Brune et al. 1992; Curray et al. 2002; Radhakrishna et al. 2012; Nemčok et al. 2012; Krishna et al. 2016; Ismaiel et al. 2019), but not focussed on studies of post-rift deformation in the region. The present study allowed us to identify the occurrence of faults in Bay of Bengal, and then a possible process is envisaged how these faults generated selectively at certain locations in the region.

Figure 1
figure1

(a) The map shows the satellite derived bathymetry image of the north-eastern Indian Ocean and trace of the 85°E Ridge from Mahanadi basin in the north Bay of Bengal to Afanasy Nikitin Seamount (ANS) in the equatorial Indian Ocean. Light yellow coloured zone covering the distal Bengal Fan, Central Indian basin and Ninetyeast Ridge shows location of the northwestern part of the diffuse plate boundary separating Capricorn, Indian, and Australian plates (Royer and Gordon 1997; Krishna et al. 2001). Circles with yellow colour indicate the locations of recorded earthquakes in Bay of Bengal and adjacent central Indian basin and Ninetyeast Ridge (International Seismological Centre, On-line Bulletin, 2016). (b) Satellite free-air gravity field of the central part of the East India Passive Margin (EIPM) continental, north part of the 85°E Ridge and intervening western basin region (Sandwell et al. 2014). Black dotted lines show the network of ION/GXT multi-channel seismic profiles investigated in the present work. Transparent white zone shows the location of northern part of the 85°E Ridge. The solid red circles with white star within show the locations of industry (ONGC) drill wells ‘A’ and ‘B’. Thick black lines show the locations of seismic sections illustrated in figures 24.

Tectonic setting of the Bay of Bengal

The tectonic evolution of Bay of Bengal reveals that the lithosphere conjugate to the EIPM is located at the Western Enderby Basin, East Antarctica and were co-evolved as conjugate passive margins. The spreading reorganisation occurred at about 120 Ma led to separation of two continental fragments – Elan Bank and parts of the Kerguelen Plateau – from the north EIPM (Powell et al. 1988; Gaina et al. 2003, 2007; Borissova et al. 2003; Krishna et al. 2009a; Talwani et al. 2016; Ismaiel et al. 2019). Subsequently, the northward movement of the Indian plate witnessed formation of two linear (in N–S direction) volcanic ridges (85°E and Ninetyeast ridges) in the Bay of Bengal from eruptions of mantle sources (Curray and Munasinghe 1991; Krishna 2003; Sreejith et al. 2011; Michael and Krishna 2011; Krishna et al. 1999, 2012). The 85°E Ridge in middle of the Bay of Bengal extends in near N–S direction and buried under the thick Bengal Fan sediments (figures 1 and 2). Earlier studies (Sreejith et al. 2011; Srinivas et al. 2017) inferred that the ridge structure was exposed above the seafloor in the past, which is presently buried below the fan sediments deposited since the Late Oligocene (Gopala Rao et al. 1997; Michael and Krishna 2011; Krishna et al. 2016; Ismaiel et al. 2017). Considering the process of thermal subsidence of ocean floor and effects of volcanic and sediment loads, Sreejith et al. (2011) and Ismaiel et al. (2017) have proposed that the 85°E Ridge was emplaced on ocean floor at around 80 Ma with a relief of about 3 km, and then deepened and buried under the fan sediments.

Figure 2
figure2

(a) The seismic depth section of a profile E1290, running from shelf to deep-water region depicting the deformation of basement and overlying sediments in margin region and on flanks of the 85°E Ridge. Location of seismic profile is shown in figure 1. (b) The seismic profile E1290 (time section in TWT sec) is shown with chrono-stratigraphy after correlating to drill well ‘A’. Seismic results of important geological features, margin fault and 85°E Ridge within the boxes, are shown in figures 3(d) and 4(b) for better resolution and detailed information. Free-air and Bouguer gravity anomaly data are superimposed on top of the seismic sections.

With the presence of two volcanic ridges, basement topography across the bay shows the presence of two large oceanic basins (western and central) bordered by margin and narrow linear ridges (Gopala Rao et al. 1997; Michael and Krishna 2011). During the Early Tertiary period, the collision between Indian and Eurasian plates led to closure of the Tethys Ocean (Molnar and Tapponnier 1977; Najman et al. 2010; Bouilhol et al. 2013). The collision process advanced further and headed to faster growth of the Himalayan orogeny, as a result, the mountain building has undergone significant erosion that eventually lead to transport terrigenious material into the Bay of Bengal (Curray and Moore 1971; Molnar et al. 1993; Curray et al. 2002; Krishna et al. 2016). Thus, the ocean floor of the Bay of Bengal was covered with copious amounts of sediment reaching up to 22 km in Bengal Offshore basin (Brune et al. 1992; Curray 1994; Radhakrishna et al. 2010; Krishna et al. 2016), from there the sediment thickness has decreased with some uniformity to 7°40′S (Krishna et al. 1998, 2001). During the Miocene Epoch, the crust and overlying sediment section in distal Bengal Fan together got deformed, resulted in formation of long-wavelength folds and high-angle reverse faults (Weissel et al. 1980; Cochran 1990; Bull and Scrutton 1992; Krishna et al. 2001, 2009b). Thus, the Bay of Bengal has become a unique site among the global ocean basins for accumulation of excessive thickness of sediments, thereby showing sediment impact in the form of vertical loading stresses on varied nature of crustal rocks.

Seismic data analyses – fault throws and subsidence history

Under the India Span programme in 2007, ION/GX technology carried out multi-channel seismic reflection, gravity and magnetic surveys along coast-orthogonal and coast-parallel transects over the EIPM and adjacent deep-water region (figure 1). The reflection data were acquired using 10 km long streamer consisting of 400 channels with a group interval of 25 m. The configuration with a short interval of 12.5 m has yielded 400-fold coverage of depth points lying on various seismic horizons within the sediment column, basement and Moho boundary at some places. The data, thus acquired were sampled with 2 msec interval and recorded for 18 sec in two-way travel time (TWT). In the present work, the data were analysed with an objective of mapping fault fabric, particularly for understanding the effect of post-rift tectonic processes on these faults (figure 2a). Following the reflection geometry of unconformities and correlative conformities, eight seismic sequences are identified within the sedimentary column starting from the Early Cretaceous to Present (figure 2b). Further, these sequences were correlated to drill wells A and B for assigning ages to all eight sequence boundaries from basement to seafloor (figures 2 and 3).

Figure 3
figure3

Manifestation of deformed basement and overlying sediment strata in the form of faults and folds on five seismic profiles running in an area between offshore extensions of the Pranahita–Godavari and Nagavali–Vamsadhara grabens. Location of seismic sections are shown in figure 1.

We mapped a major fault in K–G basin region (here onwards we call this as a margin fault) and two small-scale faults on flanks of the 85°E Ridge (figures 24). In order to determine the onset timing of faults and their activity through geologic time, we measured depths to genetically related horizons on both sides of each fault for estimating cumulative fault and absolute fault throws (tables 13). The measurements of depths to horizons on either side of the faults were carried out away from the fault plane so as to avoid local drag effects of reflectors. On margin fault, we measured depths at 10 horizons starting from basement to ~0.3 Ma sediment layer (figure 3d) in terms of two-way travel time, and then in kilometres; thus the determined cumulative and absolute fault throws are presented in table 1. In the same manner, depths are measured along the faults that lie on flanks of the 85°E Ridge from Oligocene (age of the oldest sequence) to late Pleistocene horizons, and estimated cumulative and absolute fault throws are presented in tables 2 and 3. The chrono-stratigraphic information from drill well ‘A’ and time sections are utilized for converting two-way travel time into depths in meters (figure 3d). The vertical displacements (throws), measured on faults lying in K–G basin and on flanks of the 85°E Ridge, were then back-stripped to determine stratigraphic position of horizons that had experienced offsets. This exercise did provide an opportunity to understand behaviour of faults, particularly timing of reactivation/occurrence, and then how they grew in later periods.

Figure 4
figure4

The seismic sections of the 85°E Ridge from three profiles E1240, E1290 and E1400 show the occurrence of faults right above the flanks of the ridge during the Miocene period. Locations of seismic sections are shown in figure 1.

Table 1 Details of fault throw in Krishna–Godavari basin.
Table 2 Details of fault throw on western flank of the 85°E Ridge.
Table 3 Details of fault throw on eastern flank of the 85°E Ridge.

Subsidence history of any geological province is expected to bring out some important insights on how basement of the region went-down through geologic time, particularly with fast and slow post-rift thermal subsidence in respect of varied nature of underneath lithosphere and effect of overlying sediment load. In the present work, back-stripping technique is executed to different age sediment layers with consideration of compaction of stratigraphic units (sediment loading effect) (Steckler and Watts 1978; Roberts et al. 1998). Back-stripping technique is applied to restore the depth to the basement in geological past by removing the effect of deposited sedimentary strata. The procedure followed in 1-D back-stripping method is as follows (Allen and Allen 2013). Sediment layers at various depths are decompacted by removing the effect of weight of overlying sediments. If a sedimentary layer is present at a depth (y2y1), and decompacted to the formation depth (\( y_{2}^{{\prime }} - y_{1}^{{\prime }} \)), its thickness expands due to change in the volume of pore space. So, the new depths can be determined by a mathematical formula proposed by Sclater and Christie (1980):

$$ y_{2}^{{\prime }} - y_{1}^{{\prime }} = y_{2} - y_{1} - \frac{{\varphi_{0} }}{c}\left[ {e^{{ - cy_{1} }} - e^{{ - cy_{2} }} } \right] + \frac{{\varphi_{0} }}{c}\left[ {e^{{ - cy_{1}^{{\prime }} }} - e^{{ - cy_{2}^{{\prime }} }} } \right], $$

where \( \varphi_{0} \) is the surface porosity and c is the compact constant. In general, top of sedimentary layer \( y_{1}^{{\prime }}\) is considered to be 0 and depth to its base (\( y_{2}^{{\prime }}\)) is solved by iteration method. Each sedimentary layer in succession is decompacted to their depositional thickness and then progressively compacted as each layer is added above it, until they reach their present day thickness. The above procedure provides number of restored stratigraphic column, which is used to prepare the basement/total subsidence history of the sedimentary column.

In the present case, we used exponential behaviour of porosity with depth (i.e., \( \varphi = \varphi_{0} e^{ - cy} \)) with values of \( \varphi_{0} = 0.55, c = 4.5 \times 10^{- 4}\quad{\text{m}}^{ - 1} \) and ρsg = 2.65 × 103 kg m−3 (Sawyer 1985) to estimate total subsidence histories of both hanging and foot wall blocks lying on either side of the margin fault (figure 3c). The contributions of paleo-bathymetry and eustatic sea-level changes are not included in calculation of sediment-loaded basement (total) subsidence as we believe that their inputs to subsidence process are minimal. For each stratigraphic unit in hanging and foot wall blocks, subsidence values are measured, and then divided by time interval to estimate dip–slip rate (Wagreich and Schmid 2002). Here, we assumed two pseudo-wells on either side of the fault plane with a spatial distance of about 11 km, and prepared total subsidence curve and apparent dip–slip/dip–slip rate to recognize distinct phases of faulting events in K–G basin. The possible error source in fault throw estimation is time to depth conversion that basically depends on p-wave velocity of sedimentary lithologies and sediment compaction parameters used for the back-striping calculations. Any deviation in the porosity-depth curve of the Bay of Bengal and global oceans would only affect the subsidence reconstruction of deep sedimentary sections due to overpressure loading. We have adopted average porosity-depth curve (Sawyer 1985) established for the global ocean than the low sedimentation rate curve. However, older sedimentary units are deposited before the reactivation of the fault on the margin, and any possible error caused by the compaction parameters adds the same amount of throw values on both sides of the fault plane. Therefore, the error estimates within the sediment strata older than 16 Ma are almost negligible.

Faults development in the Bay of Bengal

The Bay of Bengal region except the locale around the Andman–Sunda subduction zone is generally considered as passive zone from the point of seismicity and also from fault activity as the region belongs to central part of the Indian tectonic plate. Contrary to this, an occurrence of few earthquakes was noticed in the region (Biswas and Majumdar 1997) including the recent one with an epicentre located at ~275 km southeast of Paradip port occurred on 21st May 2014 (Rai et al. 2015). This led to suspect the prevalence of paleo-seismic activity and associated crustal deformation in the Bay of Bengal. In order to retrieve information on past tectonic deformations, we analysed seismic reflection data of the EIPM for identifying the signatures left by process of faults activities, thereby to reconstruct their tectonic history from their onset to cessation. In western Bay of Bengal region, we could identify at least two locations: one on margin of K–G basin (figure 3) and other one over the 85°E Ridge (figure 4) demonstrating once most active fault histories in the Bay of Bengal.

Reactivation of rift-related fault on margin of the Krishna–Godavari basin

The seismic reflection profiles across and along the EIPM reveal details of subsurface structures not only related to rift process and continental breakup, but also related to post-breakup tectonic processes (figures 24). The deformed crustal blocks bounded by extensional faults and half-graben structures are observed in continental margin region (figure 2), which have been evolved during the syn-rift process between the Greater India and East Antarctica. Earlier, Nemčok et al. (2012) and Ismaiel et al. (2019) investigated subsurface structure of the East India margin and inferred that the rift geometry and its spatial extent greatly vary segment-wise due to variable mode of rift evolution. The seismic section shown in figure 2 clearly depicts presence of the Moho boundary with a seaward rise separating, thinned continental crust from the exhumed mantle rocks (Ismaiel et al. 2019).

The fault plane that deformed the basement and sediment strata coincides merely with the dipping Moho at depth beneath the basement. In contrast, the profile E1000 does show the similar dipping Moho reflector, but no fault associated deformation is seen at basement level and in overlying sediment strata (Nemčok et al. 2012; Ismaiel et al. 2019). Thus, it is clear that the dipping Moho is not related to the fault feature that has been observed at basement and in overlying sediments (figure 2). Further, we did not notice any overlapping of basement surfaces and sediment horizons in the vicinity of fault plane, suggestive of reverse fault. The high-gradient segment of the Bouguer anomaly reflects the signature of seaward rising Moho boundary after compensating the contribution of seaward deepening of the basement (figure 2).

Analysis of seismic profile data over the EIPM (figure 1) reveals that all the fault features except the one on margin of the K–G basin became passive after completion of rift evolution between the Greater India and East Antarctica. In offshore K–G basin, particularly in regions of continental slope and rise, the seismic records (E1200, E1240, E1290, E1300 and E1400) exhibit a spectacular post-breakup deformation in the form of faulting at basement level and in lower sedimentary sequences, whereas upper sedimentary sequences deformed in the form of folds (figures 2 and 3). The fault feature most prominently emerged on profile E1290 (figure 3d) is carefully examined to retrieve detailed information on timing of fault’s reoccurrence and its activity through geologic time. The results of industry drill well ‘A’ located on continental rise in K–G basin are utilized for correlations between significant seismic boundaries and core lithologies and also for assigning ages to the boundaries (figure 3d).

The fault throws are measured at levels of basement and 10 prominent sedimentary horizons (figure 3d) for estimating cumulative and absolute fault throws (table 1). The details of fault throws reveal that the fault in K–G basin has a constant cumulative throw of ~900 m from basement to top Early Miocene (~16 Ma) horizon, from there the throws are variable with ~800 m at Late Miocene (6.8 Ma) horizon, ~450 m at Late Pleistocene (0.8 Ma) horizon and ~230 m at 0.3 Ma age horizon (figure 3d, table 1). An absolute throws of about 100, 340, 220 and 230 m are noticed at horizons Early Miocene (~16 Ma), Late Miocene (6.8 Ma), Late Pleistocene (0.8 Ma) and 0.3 Ma age, respectively (figure 3d, table 1).

The seismic profiles that lie towards south and north sides of profile E1290 are further examined for delineating the fault’s spatial extent on the margin. The fault can be traced along profiles E1200, E1240, E1300 and E1400 with different manifestations (figures 1 and 3). Thus, the reactivated fault feature extends along the margin for about 300 km between latitudes 15.6° and 17.7°N (figure 5). Along profiles E1200 and E1240, the basement surface is found to be faulted in near vertical sense, whereas overlying sediment strata is folded in multiples with different wavelengths, but without any expression on seafloor and in shallow sediment layers (figure 3a and b). The fault on profile E1300 is manifested as similar to the one identified on profile E1290 (figure 3c and d) as both the profiles are in close vicinity (figure 1). Whilst on profile E1400, the fault is emerged as a different structure with a faulted plane lacking observable throw on one side and a fold on other side (figure 3e).

Figure 5
figure5

Spatial extents of margin and 85°E Ridge faults are superimposed on depth to the basement map. The faults are discontinuous over the 85°E Ridge, where the ridge feature is absent. P–G graben: Pranahita–Godavari graben, N–V shear zone: Nagavali–Vamshadhara shear zone, COT: Continental–Ocean Transition.

Occurrence of extensional faults on flanks of the 85°E Ridge

The top of 85°E Ridge, its internal structure and overlying sedimentary sequences are well imaged along coast-perpendicular profiles E1240, E1290 and E1400 (figure 4), and coast-parallel profiles E4000 and E6000. On careful examination of sediment strata over the ridge structure, it is found that sedimentary layers above the flanks of ridge are faulted-down towards the oceanic basins (figure 4). The cumulative and absolute fault throws are estimated at horizons of Oligocene-top (~23 Ma), Early Miocene (~16 Ma), Late Miocene (~6.8 Ma) and Late Pleistocene (~0.8 Ma) in sediment strata (figure 4b, tables 2 and 3). Along the profile E1290, the fault offsets the sediment strata up to upper Miocene with a total cumulative throw of about 60 and 110 m on western and eastern edges of the ridge-top, respectively (figure 4b). While shallower sedimentary layers from Upper Miocene to Upper Pleistocene, both the faults offset the sediments with a cumulative throws of about 20 and 40 m on western and eastern sides of the ridge, correspondingly. The fault throws over the ridge are further examined for determining absolute throws at Late Miocene and Late Pleistocene ages. The fault on western edge recorded an absolute throws of about 37 and 23 m at horizons of Upper Miocene and Upper Pleistocene, respectively, whereas on eastern side of the ridge, a large absolute throws of about 65 m at Upper Miocene horizon and 44 m at Upper Pleistocene horizon are calculated (figure 4b).

Development of faults – impact of sediment load on underneath crustal strata

The spatial extents of fault systems observed on margin of the K–G basin and on flanks of the 85°E Ridge are superimposed on depth to the basement map (figure 5). The basement topography reveals that the margin, particularly in shelf-slope region, is dissected by nearly coast perpendicular major grabens/shear zones G2–G5 (figure 5). These features were earlier interpreted as continuity of onshore grabens and suture zone into the offshore region (Krishna et al. 2016; Ismaiel et al. 2019). The fault on the margin spatially extends for about 300 km between the latitudes 15.6° and 17.7° N, and interestingly the feature’s extent is limited by the offshore extensions of Pranahita–Godavari graben in south and Nagavali–Vamshadhara shear zone in north. This implies that the coast-perpendicular graben and shear zone features, G3 and G4 and rheology of intervening cratonic block have not only controlled the nature of continental break-up between the Greater India and East Antarctica (Ismaiel et al. 2019), but also the post-rift tectonic processes.

Both cumulative and absolute throws measured on fault systems (on the margin as well as on flanks of the 85°E Ridge) are plotted against the ages ranging from Early Cretaceous to Present (figure 6). The deformation, which is located at the margin region had initiated at ~16 Ma with a lesser throw of ~100 m, and reactivated again at 6.8 Ma with enhanced activity with a maximum throw of ~340 m (table 1). At this stage (~6.8 Ma), interestingly the faults on both flanks of the 85°E Ridge have not only initiated, but also recorded the maximum slip of about 35 and 65 m throw on western and eastern flanks of the ridge, respectively (figure 6). Although, faults have initiated at different times, maximum activities (in terms of throws) are noticed at about 6.8 Ma. In a later stage at about 0.8 Ma, the intensity of faults activity reduced by about 35% comparing to the activity at 6.8 Ma (tables 13, figure 6). The margin fault activity continued up to 0.3 Ma with nearly same intensity, and then ceased, whereas the faults on flanks of the ridge ceased in an earlier phase at about 0.8 Ma. Overall, along the hypo-extended margin segment, the rift-related extensional fault reactivated first at ~16 Ma, secondly at ~6.8 Ma with an enhanced throw of roughly three-fold, and since then the fault activity continually progressed until 0.3 Ma and become passive, whilst on flanks of the 85°E Ridge, the faults originated at ~6.8 Ma and continued their activity until 0.8 Ma (tables 13, figure 6).

Figure 6
figure6

The faults cumulative and absolute throws are plotted against age to demonstrate the nature and magnitude of fault throws.

Further, we estimated the total subsidence of footwall and hanging wall blocks of the margin fault in K–G basin, and also determined dip–slip rates of the fault to identify activities of intensive phases. The sediments on either side of the fault were back-stripped to retrieve details of basement subsidence with the age (figure 7a) caused by thermal subsidence and sediment load effects (figure 7b). Three distinct subsidence trends have primarily been noticed.

Figure 7
figure7

(a) Seismic image of the margin fault. Two vertical lines (red and blue colours) are the locations on crustal blocks, where subsidence history of the basement and sediment strata are calculated, (b) tectonic subsidence curves of basement beneath both crustal blocks in K–G basin are shown, and (c) fault dip–slip rates are plotted against the time.

During the initial phase 132–65 Ma, the basement subsided to ~3100 m at a rate of ~46 m/Myr due to rapid thermal cooling; in a later phase between 65 and 23 Ma, the basement was further deepened by 700 m at a slower rate of ~18 m/Myr due to reduced level of thermal cooling. The thermal subsidence rates calculated in the present study are comparable with the subsidence curves drawn for other globally known passive continental margins (Xie and Heller 2009; Allen and Allen 2013). During the third phase (23 Ma–Present), the basement further subsided by ~1600 m to a depth of about 5400 m at a higher rate (~69.56 m/Myr) exclusively due to the effect of thick sedimentary overburden. Grall et al. (2018) estimated high subsidence rate ranging from 0.2 to 5 mm/yr in Ganges–Brahmaputra Delta region, particularly during the Holocene period.

On close scrutiny of subsidence curves for post-23 Ma age (figure 7b), it is observed that the basement on seaward-side of the fault is subsided to ~5600 m with an enhanced rate of ~76 m/Myr compared to the depths of ~5000 m to basement and subsidence rate of ~50 m/Myr on landward-side of the fault (figure 7b). During the post-23 Ma period, the rate of subsidence on the seaward side is nearly 50% more than that of landward side of the fault. The reason for this enhancement in the rate of subsidence may possibly be linked to the excess sediment supply by Ganges–Brahmaputra river systems. Earlier, Krishna et al. (2016) inferred a change in the direction of sediment flow from W–E to N–S and high sediment accumulation rate, more than 100 m/Myr in K–G basin for the period, 23–6.8 Ma. Later, the fault initiation at 16 Ma led to form depocentre sort geometry on hanging wall side for receiving more sediment from the Ganges–Brahmaputra river systems, resulting in enhanced subsidence rate on seaward-side (figure 7b). On the whole, the sediments for the post-Oligocene period are thicker on seaward-side of the margin fault, leading to excessive subsidence of hanging wall block in the basin.

Back-stripping dip–slip fault history is reconstructed for the margin fault to determine vertical component of fault-slip and sense of fault movement through time (figure 7c). Dip–slip data along the fault during the period 83–16 Ma indicate that apparent dip–slips are very minimal, suggesting no observable vertical motion during this period. Noticeable increase in slip rate is observed at 16 Ma with 8.4 m/Myr and at 6.8 Ma with 14.66 m/Myr with extensional sense of movement. The most significant increase in fault activities happened during the periods from 6.8 to 0.8 Ma and from 0.8 to 0.3 Ma with slip rates of 406.22 and 327.81 m/Myr, respectively.

The crustal rocks across the Bay of Bengal from EIPM to Sunda trench are variable in nature ranging from rifted continental rocks beneath the margin, mixed continental and oceanic rocks with partial serpentinization beneath the transition zone, oceanic rocks beneath the western, central and eastern basins to volcanic rocks beneath both 85°E and Ninetyeast ridges (Nemčok et al. 2012; Ismaiel et al. 2019). Thus, the crustal strata beneath the Bay of Bengal are highly heterogeneous and at places different lithological rocks are juxtaposed, for example, in the margin region thinned continental rocks adjoin the serpentinized rocks (figure 8a) and in middle and east of the Bay of Bengal volcanic rocks adjoin the oceanic rocks. Since the continental breakup of the Eastern Gondwana land and during the growth of the Bay of Bengal, the ocean floor was being continuously filled with the sediments discharged by peninsular and Himalayan river systems. Thus, the ocean floor in the Bay of Bengal is completely buried under enormous thick pre- and post-continental collision sediments reaching more than 16 km in the northern Bay of Bengal (Brune et al. 1992; Curray 1994; Radhakrishna et al. 2010; Krishna et al. 2016; Ismaiel et al. 2019). In the vicinity of margin fault (between latitudes 15.6° and 17.7°N), there is a difference of about 2 km in sediment thickness between the shelf-slope margin and transition zone region (figure 2a and b). Similar sediment thickness differences are also found on both flanks of the 85°E Ridge (figure 4b) (Misra 2018). The difference in thickness of overlying sediments in both regions may have exerted differential vertical loads on underneath crustal rocks (figure 8a). Since the crustal rocks in the margin region between rifted thinned continental rocks and transitional (mixed oceanic and continental) rocks and in deep-water region between oceanic rocks and volcanic rocks of the 85°E Ridge are different in nature, it is likely that these heterogeneous rocks might have responded differently to the stresses applied by the overlying sediment loads. The excessive stress fields, thus released have caused the deformation of crust as well as overlying sedimentary strata (figure 8b).

Figure 8
figure8

Sketch diagram shows how the location of juxtaposition of different crustal blocks (continental vs. serpentinized rocks) responded to the overlying variable thick sediment strata.

Earlier, Müller et al. (2014) found the presence of crustal deformation, explicitly folds in margin region of the K–G basin and attributed it to anomalously high intraplate stress-field that stretches from Indian subcontinent to the Wharton basin. Using the seismic results of central margin of eastern India (Bastia et al. 2010) and timings of fault activities in the central Indian basin (Krishna et al. 2009b; Bull et al. 2010), Müller et al. (2014) opined that maximum horizontal stress is propagated onto this continental margin region around the latest Miocene (8.0–7.5 Ma). But, our seismic results suggest that operative stresses on margin segment of the K–G basin and on flanks of the 85°E Ridge are extensional in nature and differ from the episodic compressional forces active in the central Indian basin (Krishna et al. 1998, 2001). Although the timings and nature of fault activities in both the regions are to some extent similar, their mechanisms completely differ.

The margin fault’s activity in K–G basin was variable in nature, to begin with episodic starting at about 16 Ma and resumed at 6.8 Ma, from there the activity was continuous until 0.3 Ma. Excessive fault throws (106 and 342 m) occurred over a distance of 300 km at ages of 16 and 6.8 Ma may have prompted generation of intensive tsunami events. Presently, the fault is inactive and may possibly become active whenever the differential stresses on either sides of the fault reach the threshold level and may turn out to be a geohazard event in future.

Conclusions

Analyses of multi-channel seismic reflection data of the EIPM, estimation of cumulative and absolute fault throws, basement subsidence history on either side of the margin fault and dip–slip rates along the fault provided new understanding on occurrence of faults in the Bay of Bengal as a response to excessive sediment depositions. Important observations are as follows.

  1. 1.

    The fault-line on margin segment of the K–G basin was initially formed during the continental rifting process, and later reactivated in Early Miocene time (~16 Ma). The fault’s activity varied in nature began as episodic starting at about 16 Ma and resumed at 6.8 Ma, since then, the activity continued until 0.3 Ma. The fault throw data suggest that the cumulative throw of about 900 m is observed on margin fault from basement to top Early Miocene horizon, subsequently the fault throw was continuously reduced to 800 m at Late Miocene horizon (6.8 Ma), 450 m at Late Pleistocene horizon and 230 m at 0.3 Ma. Absolute throws are also determined at different horizons and found to be about 100, 340, 220 and 230 m at tops of Early Miocene (~16 Ma), Late Miocene (6.8 Ma), Late Pleistocene (0.8 Ma) and 0.3 Ma age horizons, respectively. The fault plane spatial continuity in K–G basin is found to be around 300 km (paralleling the coast between latitudes 15.6° and 17.7°N) and lay between Pranahita–Godavari graben and Nagavalli–Vamshadhara shear zone. This implies that the coast perpendicular structural features and rheology of intervening cratonic block have not only controlled the mode of rift process initially, but also the post-rift tectonic processes.

  2. 2.

    On both flanks of the 85°E Ridge new normal faults are identified. The faults are found to be initiated at 6.8 Ma and continued their activity until 0.8 Ma. They have cumulative throws of about 60 and 110 m on western and eastern flanks of the ridge, respectively.

  3. 3.

    Subsidence history of crustal blocks on either side of the margin fault revealed that the basement was deepened to about 3100 m at a rate of ~46 m/Myr due to thermal cooling in initial phase 132–65 Ma, later the basement was further deepened to about 3800 m at a slower rate of ~18 m/Myr due to reduced level of thermal cooling during the phase of 65–23 Ma. During the last phase (23 Ma–Present), the basement has been further fall-down to a depth of about 5400 m at a higher subsidence rate (~69.56 m/Myr) exclusively due to the effect of copious amount of sediment load. About 50% increase in subsidence rate is observed for the seaward block of the margin fault, particularly for the last 23 Ma period because of excessive deposition of sediments derived from the Ganges–Brahmaputra river systems.

  4. 4.

    The thick sedimentary strata exerted vertical load on underlying heterogeneous crust of the margin and 85°E Ridge that led to build excessive internal stresses and release through weak zones (lying at intersecting planes of heterogeneous crustal blocks). The stress fields, thus released through fault planes have caused the deformation of crust as well as overlying sedimentary strata.

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Acknowledgements

Authors are greatly indebted to D K Pande and S V Rao, former Directors (Exploration), ONGC and Ajay K Dwivedi Director (Exploration), ONGC for their keen interest and immense support for R&D collaborative project between CSIR-NIO and ONGC-KDMIPE. The present work is carried out when the authors, KSK and MI are serving at Centre for Earth, Ocean and Atmospheric Sciences, University of Hyderabad. MI is thankful to DST, New Delhi, for awarding the INSPIRE Faculty position.

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Krishna, K.S., Ismaiel, M., Srinivas, K. et al. Post-breakup deformations in the Bay of Bengal: Response of crustal strata to the sediment load. J Earth Syst Sci 129, 159 (2020). https://doi.org/10.1007/s12040-020-01436-7

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Keywords

  • Reactivation of rifted fault
  • effect of sediment load
  • Bengal Fan sediments, 85°E Ridge, Bay of Bengal