Geology & Geophysics Feature

The Bay of Bengal and the Curious Case of the Missing Rift

In a classic detective story, clues from data new and old helped researchers reveal the puzzling chain of tectonic events that followed the Early Cretaceous split between India and Antarctica.

By and Maria Desa

This is a tale of forensic geology—of trying to sort out the ancient history of the crust beneath Bangladesh and the Bay of Bengal, a tectonically complex region that has confounded scientists for decades, and how it’s connected to an area thousands of kilometers south. As with many good detective stories, this one involves conflicting interpretations of incomplete evidence, chance clues that revealed new insights, and, ultimately, we suggest, a compelling resolution that also sheds light on other mysteries.

India and Antarctica Part Ways

The story starts when India and Antarctica—both formerly part of the supercontinent Gondwana—split from each other in the Early Cretaceous, about 130 million years ago. This split occurred along a rift, or spreading center, in the crust between what is now eastern India and a portion of East Antarctica. As the landmasses separated, with India speeding away northward at about 3 centimeters per year, lava erupted and cooled on both sides of the rift, giving birth to the Enderby Basin (off the coast of Queen Maud Land, Antarctica) on one side and the Bay of Bengal on the other. Early on, crust on either side of the rift beneath the newly formed ocean basins essentially mirrored that on the other side.

The area of Gondwana including what would become the Indian subcontinent and its adjacent ocean basin became the Indian Plate. This region was divided by the spreading center from the area including Antarctica and its adjacent ocean basin, which became the Antarctic Plate.

A Puzzling Disconnect

Gravity map of the eastern Indian Ocean, showing the Bay of Bengal and the Enderby Basin, 8,000 kilometers to the south
Fig. 1. Gravity map of the eastern part of the Indian Ocean, showing the Bay of Bengal and its conjugate, the Enderby Basin, 8,000 kilometers to the south. Click image for larger version.

As lava solidifies into new ocean crust, certain types of minerals preserve signatures of Earth’s magnetic field. These signatures can be recorded by ship-towed magnetometers and then used to determine the ages of different portions of crust. Scientists make this determination by inferring the pattern of magnetism seen in the crustal rocks and tying it to the known pattern of periodic reversals of Earth’s magnetic field.

Because seafloor spreading and the formation of new crust are symmetrical on either side of a spreading center, the magnetic patterns on each side are also symmetrical and form mirror images of each other. Thus, it was expected that the magnetic pattern in the Bay of Bengal observed today would mirror the pattern in the Enderby Basin, roughly 8,000 kilometers away (Figure 1).

Scientists at the National Institute of Oceanography (NIO) in Goa, India, used the magnetic measurement method to obtain ages of the crust underlying the Bay of Bengal. In 1992, they launched a massive program, towing a magnetometer behind the NIO research ship Sagar Kanya (“Ocean Daughter”) along six tracks totaling some 8,200 kilometers (Figure 2).

From this effort, Ramana et al. [1994] reported that the oldest crust beneath the bay was about 130 million years old (in agreement with the start of the split between India and Antarctica), whereas the youngest crust was about 120 million years old. These researchers also observed that the opening and the formation of new crust proceeded continuously without any interruptions.

A validation of Ramana et al.’s [1994] results could be made by finding mirror image magnetic variations in the Enderby Basin. However, when Gaina et al. [2003] published magnetic measurements from Enderby Basin, they proved not to be a mirror image of the results from the Bay of Bengal, nor did they show uninterrupted spreading. Gaina’s team’s results were based on magnetic readings collected on three relatively short ship tracks (totaling about 1,100 kilometers) south of Elan Bank, a western salient of the Kerguelen Plateau (Figure 2). These results showed what appeared to be a spreading center not at the boundary of the Enderby Basin, but within it. This pattern was quite different from that in the Bay of Bengal and could have been created only by a rapid shift, or jump, of the spreading center.

Gravity maps of the Bay of Bengal (left) and Enderby Basin (right) with distinctive features labeled
Fig. 2. Gravity map of the Bay of Bengal (left). The Ninety East buried ridge displays a gravity high and is ascribed to motion over a hot spot (plume). The 85°E buried ridge displays a gravity low, which is unusual for an oceanic ridge. Gravity map of the Enderby Basin, Kerguelen Plateau, and its western salient, Elan Bank (right). These features are covered with plume-derived rocks, but their bases are thought to consist of continental rock. Magnetic measurements were made along the tracks shown for the Bay of Bengal and for Enderby Basin. Modified from Talwani et al. [2016].

Reconciling Discordant Magnetic Measurements

The difference in the interpretation of the magnetic patterns in the two basins was puzzling. Which was correct? Interpreting magnetic patterns usually involves making assumptions. To derive an age interpretation of the entire crust of the Bay of Bengal, Ramana et al. [1994] had assumed spreading rates, the most appropriate portion of the magnetic reversal timescale to apply, and the presence of fracture zones (which displace the magnetic pattern). But incorrect assumptions can negate entire interpretations.

Gaina et al. [2003], on the other hand, in tackling only a portion of the Enderby Basin, made no such assumptions. The fact that they saw exact symmetry in the magnetic pattern around a rift was enough to justify the presence of a jump in the spreading center and confirm their interpretation of the magnetic pattern.

The assumptions made by Ramana et al. [1994] did not stand up to closer examination. They had invoked spreading rates that appeared to be much too high. They had assumed the presence of fracture zones that, it turned out, did not exist. And they had ignored the presence of the 85°E Ridge, an important feature of the Bay of Bengal. For these reasons, their interpretation of the magnetic pattern and crustal ages in the Bay of Bengal had to be rejected.

However, we were still left with the uncomfortable conclusion that the magnetic patterns in the two basins were different.

The most plausible course of events that we came up with to explain this geologic mystery proceeds as follows. For about 10 million years after the rift between India and Antarctica opened, the crust on either side—in the Bay of Bengal and in the Enderby Basin—did, indeed, form symmetrically as expected, as indicated by the magnetic lineations in the picture on the left in Figure 3.

But in an unexpected twist, the eastern part of the original spreading center appears to have jumped northward relative to the western part of the spreading center. This newly relocated rift must have been farther from Antarctica and closer to India. As the Indian and Antarctic Plates continued moving apart, spreading along the prejump eastern portion of the original rift stopped. This change left behind a relic symmetrical magnetic pattern south of Elan Bank (as shown in the picture on the right in Figure 3), which is what Gaina and colleagues detected.

Illustration showing positions of the Indian and Antarctic plates and the line of opening between them before and after about 120 million years ago
Fig. 3. This illustration displays the jump of the eastern portion of the line of opening from the Enderby Basin to the Rajmahal-Sylhet line by showing the situations 120 million years ago, just before and just after the jump. The Indian Plate is shown in pink; the Antarctic Plate is shown in blue. Before the jump, magnetic anomalies M12 (130 million years ago) and M2 (124 million years ago) are mirror images in the Indian and Antarctic Plates. After the jump, both limbs of M2 are in the Antarctic Plate on either side of what has become the relic rift. The transform fault that, after the jump, connects the two segments of the line of opening is the negative gravity anomaly strip on land (Figure 5) and the 85°E Ridge at sea (Figure 2). Notice how the ancestral Kerguelen Plateau (yellow) that was in the Indian Plate became a part of the Antarctic Plate. Modified from Talwani et al. [2016].
This sequence of events accounted for the different magnetic results from the Bay of Bengal and the Enderby Basin, but two big questions remained: Where did the eastern part of the spreading center end up, and why did the jump occur?

Big Clues in Decades-Old Data

A clue that could help solve these questions came from an unexpected source—the energy giant Unocal. Unocal had seismic reflection records collected by the German geophysical contractor Prakla in the 1960s. These records showed features called seaward dipping reflectors (SDRs), which represent interfaces between interspersed layers of volcanic and sedimentary material and are characteristic of volcanic passive continental margins. (They are observed, for example, off the U.S. East Coast.) But why would SDRs occur on land beneath Bangladesh rather than close to a continent-ocean boundary?

An answer to this question emerged when Bert Bally, a colleague at Rice University, pointed out a paper about the tectonics of Bangladesh by Lohmann [1995]. The paper, published in the Bulletin of the Swiss Association of Petroleum Geologists and Engineers, had escaped our earlier notice but now gave us a major clue about where the eastern part of the spreading center had wound up after the jump. We looked to previously unassociated volcanic traps.

A fragment of one of the Unocal SDR records and partial illustrations of two of the reflection lines with SDRs are shown in Figure 4, and the portions of the reflection lines where SDRs occurred are indicated in Figure 5. Figure 5 also shows the locations of the Rajmahal and Sylhet Traps, large provinces of volcanic rock that formed when lavas flooded onto Earth’s surface. The rocks constituting the two traps have identical chemical properties and the same age of 117.5 million years. Yet without a compelling geologic explanation to connect the two provinces, which are separated by hundreds of kilometers, most investigators had believed that the Rajmahal and Sylhet Traps came from separate eruptions. But the positions of the SDRs with respect to the traps suggested to us the possibility of a different interpretation: that the traps lay along a continuous line of past volcanic activity representing the unknown location to which the eastern end of the original spreading center had jumped.

Diagram showing seaward dipping reflectors (SDR) in Bangladesh (right) and tracings of two seismic lines showing SDRs
Fig. 4. A fragment from a seaward dipping reflector (SDR) in Bangladesh (right). Tracing of two seismic lines showing SDRs (left, modified from Talwani et al. [2016]). These lines are indicated in Figure 5. Click image for larger version.
Corroboration of this idea came from another geophysical measurement. At passive volcanic margins, SDRs are associated with rocks bearing large amounts of magnetic minerals, which give rise to large magnetic anomalies. Magnetic measurement maps of Bangladesh showed that such an anomaly indeed lies between the Rajmahal and Sylhet Traps. (An international boundary is responsible for the apparent discontinuity at either end of the magnetic anomaly seen in Figure 5, although Mita Rajaram of the Indian Institute of Geomagnetism assured us that the anomaly continues to the traps on either side.) Thus, the continuity of the magnetic anomaly supported our discovery of the relocated line of opening connecting the two traps (and probably extending eastward), which we reported in 2016 [Talwani et al., 2016].

Map of the Bengal Basin, north of the Bay of Bengal, showing important geologic features, including a large magnetic anomaly
Fig. 5. Map of the Bengal Basin (situated north of the Bay of Bengal) showing important geologic features. A large magnetic anomaly between the Rajmahal and Sylhet Traps defines the continuity of the Rajmahal-Sylhet line, which, after the jump, was the new line of opening. The apparent small gaps in the continuity of the magnetic anomaly on either side are artifacts caused by the international boundary. We have been assured that Indian data confirm the continuity on both sides to the traps. Modified from Talwani et al. [2016].

Why the Jump?

We now had a deeper understanding of the crust beneath the Bay of Bengal. After the new rift formed roughly 120 million years ago, the Indian Plate continued marching north while a new ocean opened to the south of the spreading center. About 65 million years later, the Indian Plate collided with Eurasia, uplifting a new mountain chain, the Himalayas. As the mountains rose, they shed enormous amounts of eroded sediments that were carried by two giant rivers, the Ganges and the Brahmaputra, to the newly formed ocean. Gradually, these sediments filled in a portion of ocean—today, this filled-in area is known as the Bengal Basin, which includes Bangladesh and part of the eastern Indian state of Bengal. Bangladesh, thus, lies on a bed of oceanic crust, not continental crust as was once assumed. South of the Bengal Basin is today’s Bay of Bengal, which contains one of the thickest accumulations of sediment in the world and is still in the process of filling up.

The mystery still had one lingering question: Why did the jump occur? The most likely explanation invokes a role for rock rising from deep in the mantle. India, after its Early Cretaceous split from Antarctica and during its march north, passed over the Kerguelen plume. Plumes contain warm rock that rises from the core-mantle boundary to the crust. This buoyant material occasionally erupts as lava at the surface. Several petrologists have argued that the material constituting the Rajmahal and Sylhet Traps did not come directly from the Kerguelen plume, however, but rather, heat conveyed by the plume was responsible for opening a rift that then supplied the magma in the traps. We suggest that this new rift represents the jump of the eastern portion of the original rift.

Contemporaneously with the jump, lava was deposited over a part of the Indian Plate (shown in yellow in Figure 3) that subsequently detached from it. This detached area comprises the Kerguelen Plateau and Elan Bank, now part of the Enderby Basin (Figure 2). Following the jump and the initiation of the new spreading center, rifting along the eastern part of the original spreading center (now in the eastern Enderby Basin) ceased. It was this extinct spreading center that Gaina’s team discovered. The western part of the original spreading center, meanwhile, did not jump. Thus, the original spreading center was split into two segments connected by a fracture zone (transform fault), as seen in Figure 3. The 85°E Ridge in Figure 2 and the negative anomaly strip in Figure 5 represent this fracture zone.

A Second Line of Evidence

Magnetic measurements are not the only method used to determine the nature of the crust. Seismic refraction, in which the velocity of seismic waves traveling through the crust is measured and related to the composition of the crustal rocks, is a viable method as well. This method can also be used to determine crustal thickness.

An excellent refraction experiment was carried out by Sibuet et al. [2016], who collected seismic refraction data along three tracks offshore of Bangladesh (Figure 6, left). Results from such experiments are often shown in graphs in which the velocity of seismic waves in the crust is plotted against depth. An example of this type of plot is seen in Figure 6 (right), which includes results from one of the seismic stations used in Sibuet et al.’s research. Also shown in Figure 6 are velocity-depth data determined in previous research for seismic waves traveling through different areas of oceanic or continental crust.

Map of seismic refraction stations shot by researchers off the coast of Bangladesh (left), and diagram showing the crust velocity depth curve obtained from one of these station compared with an average velocity depth curve for new ocean crust (right)
Fig. 6. Location of seismic refraction stations shot by Sibuet et al. [2016] (left). The velocity depth curve obtained at station 11 is almost identical to the average velocity depth curve compiled by Eldholm and Grue [1994] for “new” ocean crust just seaward of volcanic passive margins (right). Modified from Talwani et al. [2017].
Oceanic crust is typically thinner than continental crust, but it is also denser, so seismic velocities are higher. Sibuet and his colleagues concluded that because their results indicated a thick crust where they’d collected data off Bangladesh, the crust there must be continental in origin, even though the seismic velocity was much higher than expected for continental crust.

But Eldholm and Grue [1994] had shown with data from other passive volcanic margins that newly formed ocean crust could be much thicker than normal. In fact, the average velocity-depth curve that they determined for new ocean crust along volcanic margins was completely coincident with Sibuet et al.’s [2016] curve (Figure 6). Thus, the data that Sibuet et al. took to indicate continental crust off Bangladesh actually provide strong support for the idea that the crust beneath the Bengal Basin is oceanic [Talwani et al., 2017]. (It is important to note, however, that Sibuet et al. [2017] do not agree with our interpretation. They suggest instead that their velocity-depth curves differ from velocity-depth curves at passive volcanic margins, and they attribute higher crustal seismic velocities to volcanic sills intruding into continental crust.)

A Tectonic Tale Revealed

Looking in total at the evidence accumulated, the likely series of events that led to the formation of the crust beneath the Bay of Bengal is as follows:

1. Following the split between India and Antarctica in the Early Cretaceous, the Bay of Bengal and the Enderby Basin evolved symmetrically.

2. About 120 million years ago, a northward jump in the eastern portion of the spreading center led to the creation of a new ocean. This new ocean opened south of the relocated spreading center, which lay along a line defined by the Rajmahal and Sylhet Traps, and the new ocean crust is what underlies the Bengal Basin, including Bangladesh, as well as the eastern basin of the Bay of Bengal.

3. The jump in the spreading center tore the ancestral Kerguelen Plateau from the Indian Plate and relocated it to the Antarctic Plate. This plateau was subsequently the site of extensive lava deposition, but it retained its base of Indian continental crust.

4. The jump also created a transform fault that connected the two segments of the rift. This transform fault is delineated by a negative gravity anomaly on land in modern-day eastern India and at sea at the 85°E Ridge.

With this sequence elaborated, many questions that have perplexed scientists investigating this area have been answered. However, as with many good detective stories, there are details left unresolved—the negative gravity anomaly observed in the 85°E fracture zone has yet to be successfully explained, for example. Perhaps future investigations will continue revealing new insights into this region’s fascinating and complex geology.

References

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Author Information

Manik Talwani ([email protected]), Rice University, Houston, Texas; and Maria Desa, National Institute of Oceanography, Goa, India

Citation: Talwani, M., and M. Desa (2020), The Bay of Bengal and the curious case of the missing rift, Eos, 101, https://doi.org/10.1029/2020EO149707. Published on 02 October 2020.
Text © 2020. The authors. CC BY-NC-ND 3.0
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