Imaging a Subduction Margin with an Extreme Sediment Blanket

Motivation

The Indo-Burman Margin is one of the most unusual subduction systems on Earth. Here, the Indian plate descends beneath Southeast Asia whilst covered by an enormous pile of sediments. Delivered from the Himalayas by the Ganges-Brahmaputra River system, these sediments form the largest delta in the world and strongly influence deformation, earthquake behavior, and the transfer of material from the Earth's surface into the mantle across the subduction zone.

Despite its importance, the subsurface structure of the incoming plate remains poorly understood. Thick sediment cover obscures the crust and upper mantle, making it difficult to determine the composition, structure, and fluid distribution of material entering the subduction zone. How do sediments evolve as they are buried and deformed? What is the structure of the underlying crust and mantle? By answering these questions, we can better understand the locked nature of this plate boundary and the seismic hazards facing one of the most densely populated regions on the planet.

To investigate these questions, we analyzed seismic data from the Bangladesh-India-Myanmar Array (BIMA), a broadband seismic experiment spanning the subduction margin in Bangladesh. We focused on two complementary seismic observations: (1) Rayleigh waves (a type of surface waves) generated by earthquakes and ambient seismic noise, and (2) body waves scattered across strong subsurface boundaries. Surface waves provide robust constraints on seismic velocity throughout the crust and upper mantle, while scattered waves are particularly sensitive to abrupt changes in structure. By combining these datasets, we can image both gradual changes in Earth properties and sharp geological boundaries that would be difficult to resolve using either method in isolation.

Before exploring the results, it is helpful to understand why surface waves are such powerful tools for imaging Earth's interior.

Evolving Sediments in the Accretionary Wedge

Our results reveal strong lateral variations within the sediment layer across the Indo-Burman Margin. Preferred models show extremely slow velocities within an upper sediment layer that thickens from a region near the plate boundary in the southwest (A) to the northeast in the direction of subduction (B). Velocities increase in the layer as it thickens, reflecting sediment compaction as pore space closes and any liquid water is squeezed out. Beneath this layer, reduced seismic velocities within the metamorphosing sediments (i.e., metasediments) suggest that fluids may remain trapped at depth. These low velocities could reflect either elevated pore-fluid pressures or the formation of water-bearing minerals as sediments are progressively buried and altered.

A Hidden Crustal Boundary

One of the fundamental unanswered questions along the Indo-Burman Margin is where the Indian continent ends and oceanic crust begins. Determining the location of this transition is surprisingly difficult. The thick sediments and metasediments of the Ganges-Brahmaputra Delta obscure the underlying crust, and many of these materials have similar seismic velocities. As a result, the crustal boundary has remained elusive despite decades of geophysical investigation.

At greater depths, our observed seismic variations support two possible interpretations. One scenario involves a relatively thin crust underlain by an unusually slow uppermost mantle, perhaps reflecting inherited material from the formation of the eastern Indian Ocean. The other involves a thick layer of magmatic material added to the base of the crust during ancient tectonic events, potentially associated with the passage of the Kerguelen Mantle Plume.

Regardless of which interpretation proves correct, our results demonstrate that the structure beneath the Indo-Burman Margin is far more heterogeneous than previously recognized. They also suggest that structures formed tens of millions of years ago continue to influence modern tectonic processes.

Outstanding Questions

Our study provides one of the most detailed views yet of the Indo-Burman Margin, but important questions remain. Where is the transition between continental and oceanic crust within the downgoing plate? How much of the observed structure reflects ancient magmatic processes versus ongoing modification? How do these structures influence deformation, fluid transport, and earthquake hazard along the plate boundary? Future work combining seismic imaging, geodynamic modeling, and geological observations will help us to better understand Earth's most sediment-rich convergent margin.

Updated August 2026.