How fast is the Ganges-Brahmaputra-Meghna Delta (GBMD) sinking under the weight of its own Holocene sediments? Krien et al. (2019) used the first 3-D viscoelastic model of the delta to show that lateral contrasts between the strong Indian Craton and the weakened Indo-Burman plate margin amplify this sediment-loading (isostatic) subsidence toward the east, into the most densely populated part of the delta, but the modeled amplitude spans roughly 1.5 to more than 3.5 mm/yr across plausible mantle-viscosity and lithosphere-thickness end-members, and the predicted isostatic signal has never been separated observationally from shallow compaction, groundwater withdrawal, and tectonics. The sediment-loading contribution remains model-derived and unconstrained: the Bengal Basin rheology and eastern-margin structure are unresolved, and the observational decomposition is still lacking.

Background

The Ganges-Brahmaputra-Meghna Delta (GBMD), covering most of Bangladesh and part of the Indian state of West Bengal, is the world's most populous delta: roughly 150 million people live within a few meters of mean sea level. There, land subsidence (sinking of the ground surface) adds to rising seas to produce relative sea-level rise, the quantity that governs coastal flood exposure.

Measured vertical land motion in the GBMD is the sum of several processes acting at different depths, spatial scales, and time scales. Sediment compaction, the dewatering and consolidation of the muddy Holocene (last ~11,000 years) delta deposits, dominates near the surface. Groundwater withdrawal produces locally very fast sinking: InSAR (interferometric synthetic aperture radar, a satellite technique that measures millimeter-scale ground motion from phase differences between repeated radar images) detected rates up to ~18 mm/yr around Dhaka. Tectonic folding and uplift act near the eastern mountain front. The deep component at issue here is sediment isostatic adjustment (SIA): the viscoelastic (slowly flowing) solid Earth continues to sink under the weight of the sediments the rivers have deposited since sea level stabilized ~11,000 years ago, so the delta surface subsides on its own, regardless of human activity. The rate of this sinking is controlled by the viscosity (effective strength) of the mantle and the thickness of the lithosphere (the stiff outer shell comprising the crust and uppermost mantle), both of which vary sharply across the GBMD: the cold, strong Indian Craton (the ancient, thick continental core of India) underlies the northwest, while the east abuts the Indo-Burman subduction zone, where the Indian plate bends beneath Myanmar along a mechanically weakened plate boundary.

The main prior results are: (i) a millennial-scale subsidence map derived from delta-wide sediment stratigraphy by Grall et al. (2018), showing rates rising from under 1 mm/yr near the craton to about 5 mm/yr at the southeastern delta edge; (ii) Karpytchev et al. (2018), who computed the present-day sediment-loading contribution with a laterally uniform (1-D) Earth model and found at most ~1.6 mm/yr; and (iii) Krien et al. (2019), who built the first 3-D viscoelastic model of the GBMD with laterally varying mantle viscosity and lithosphere thickness, including the craton and a weakened Indo-Burman margin. Krien et al. (2019) found that these lateral contrasts shift and amplify the sediment-driven subsidence toward the east, with 'likely' rates of 2-3 mm/yr, comparable to present-day global mean sea-level rise. However, the modeled amplitude ranged from about 1.5 mm/yr (hard mantle, thick lithosphere) to more than 3.5 mm/yr (soft mantle) depending on the assumed rheology (the deformation properties of the mantle and lithosphere); the spatial pattern depended on whether the eastern margin is represented as a narrow, localized weak zone or a broad, diffuse one; and seismological evidence that the mantle beneath the delta plain is weak (Li et al., 2008) constrains the viscosity only loosely. Because measured subsidence is dominated by shallow compaction and pumping, the deep isostatic component has never been separated observationally: GNSS (satellite geodetic positioning with Global Navigation Satellite Systems such as GPS), InSAR, tide- and river-gauge records, and sediment coring each average over different depth ranges and time scales, and no single observation type isolates the isostatic signal by itself.

Problem Statement

Determine, with quantified uncertainty, the present-day contribution of Holocene sediment loading to land subsidence across the Ganges-Brahmaputra-Meghna Delta, both its amplitude and its spatial pattern, including the predicted eastward amplification toward the Indo-Burman margin, where current 3-D viscoelastic models admit end-members from roughly 1.5 mm/yr to more than 3.5 mm/yr (Krien et al., 2019). This single objective requires three jointly supporting constraints: (i) the mantle viscosity and lithosphere thickness beneath the Bengal Basin and their lateral variation between the Indian Craton and the Indo-Burman margin, constrained by observations independent of the land-motion data being explained; (ii) the geometry, width, and degree of weakening of the eastern plate margin, that is, whether it behaves as a localized or a diffuse weak zone; and (iii) a separation of the sediment-isostatic signal from sediment compaction, groundwater withdrawal, and tectonic motion, using spatially dense modern land-motion observations that resolve different depth ranges. The sediment-loading component of present-day GBMD subsidence and its uncertainty are to be quantified, including whether the 2-3 mm/yr estimate and the eastward amplification hold and which share of the measured vertical land motion it accounts for across the delta.

References

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