Search

International audience; The Central Indian Ocean, namely the Central Indian, Crozet, and Madagascar basins, formed by rifting and subsequent drifting of India (now Capricorn), Antarctica, and Africa (now Somalia). We gathered a comprehensive set of sea surface magnetic anomaly profiles over these basins and revisited location and identification of magnetic isochrons between C34ny (83.0 Ma) and C20ny (42.536 Ma) using the objective analytic signal technique. We present high-resolution magnetic isochrons for 29 periods based oñ 1,400 magnetic anomaly picks. From the conjugate sets of picks, we derive two-plate finite rotation parameters for both the Capricorn-Antarctica and Capricorn-Somalia motions. These finite rotations are compared to three-plate reconstructions of the plate boundaries between the Capricorn, Antarctica, and Somalia plates, constrained by the closure of the Indian Ocean Triple junction. In general, the three-plate reconstructions slightly overrotate the reconstructed isochrons with respect to the Capricorn-Antarctica and Capricorn-Somalia two-plate reconstructions. Conversely, the two-plate reconstructions for Somalia-Antarctica slightly underrotate the isochrons compared to the Capricorn-Antarctica-Somalia three-plate reconstructions. We suspect that the discrepancies between the two-plate and three-plate methods result from the recent seafloor deformation in the Capricorn-India diffuse plate boundary and/or from the contrasted nature and geometry of magnetic isochrons at different spreading rates (i.e., magnetic structure of the three spreading centers). Three-plate reconstructions better constrain the closure of the triple junction but spread any misfit among all three plate boundaries. When enough quality data are available, two-plate reconstructions may lead to more realistic plate motion estimates from which additional geological problems can be identified and solved.

[1] For the first time, we report the amplitude variation with angle (AVA) pattern of bottom-simulating reflectors (BSRs) beneath fracture-filled gas hydrate deposits when the effective medium is anisotropic. The common depth point (CDP) gathers of two mutually perpendicular multichannel seismic profiles, located in the vicinity of Site NGHP-01-10, are appropriately processed such that they are fit for AVA analysis. AVA analysis of the BSR shows normal-incidence reflection coefficients of −0.04 to −0.11 with positive gradients of 0.04 to 0.31 indicating class IV pattern. The acoustic properties from isotropic rock physics model predict class III AVA pattern which cannot explain the observed class IV AVA pattern in Krishna-Godavari basin due to the anisotropic nature of fracture-filled gas hydrate deposits. We modeled the observed class IV AVA of the BSR by assuming that the gas hydrate bearing sediment can be represented by horizontally transversely isotropic (HTI) medium after accounting for anisotropic wave propagation effects on BSR amplitudes. The effective medium properties are estimated using Backus averaging technique, and the AVA pattern of BSRs is modeled using the properties of overlying HTI and underlying isotropy/HTI media with or without free gas. Anisotropic AVA analysis of the BSR from the inline seismic profile shows 5–30% gas hydrate concentration (equivalent to fracture density) and the azimuth of fracture system (fracture orientation) with respect to the seismic profile is close to 45°. Free gas below the base of gas hydrate stability zone is interpreted in the vicinity of fault system (F1).

The submerged 85°E Ridge in the Bay of Bengal trends approximately N-S between 19°N and 6°N latitudes. Off the southeast coast of Sri Lanka it takes an arcuate shape and seems to terminate with the northward extension of the Afanasy Nikitin seamounts situated around 2°S latitude. The ridge is characterized by positive magnetic (100–400 nT) and negative free-air gravity (

In fine-grained, faulted sediments, both stratigraphic and fault-induced structural variations can simultaneously determine the gas hydrate distribution. Insights into hydrate distribution can be obtained from P-wave velocity (VP) and attenuation (QP -1 ) character of the gas hydrate stability zone (GHSZ). In this paper, we apply frequency-domain full-waveform inversion (FWI) to surface-towed 2D multichannel seismic data from the Krishna-Godavari (KG) Basin, India, to image the fine-scale (100 X 30 m) VP and QP -1 variations within the GHSZ. We validate the inverted VP model by reconciling it with a sonic log from a nearby (~250 m) well. The VP model shows a patchy distribution of hydrate. Away from the faults-dominated parts of the profile, hydrates demonstrate stratigraphic control which appears to be permeability driven. The QP -1 model suggests that attenuation is relatively suppressed in hydratesbearing sediments. Elevated attenuation in non-hydrate-bearing sediments could be driven by the apparent pore-fluid immiscibility at seismic wavelengths. The VP and the QP -1 models also suggest that fault zones within the GHSZ can be hydrate- or free-gas-rich depending on the relative supply of free gas and water from below the GHSZ.

[1] A combination of diffusion and advection in fine grained sediments can create a patchy Bottom Simulating Reflector (BSR) which has little to no apparent correlation with the overlying hydrate distribution. Using 2D seismic data from faulted, clay-rich sediments in the Krishna-Godavari (KG) basin, we show both the hydrate distribution and the BSR structure are fault controlled. Our demonstration hinges upon a kinematically accurate P wave velocity (VP) model which is estimated using a composite traveltime-inversion, depth-migration method in an iterative manner. The flexibility of the method allows simultaneous usage of traveltimes from multiple, discontinuous reflectors. The application begins with a simple VP model from time processing which is reflective of a diffusive, continuous, hydrate– and free gas–bearing system. The application converges in three iterations yielding a final VPmodel which is suggestive of a patchy distribution of hydrates and free gas possibly developing through a combined diffusive-advective system. The depth image corresponding to the final VP model can be interpreted for faults that suggest ongoing tectonism. The BSR appears to be truncated at active faults zones. Both the final VP model and the corresponding depth image can be reconciled with the hydrate distribution and BSR depth at logging/coring sites located ∼250 m away from the line by projecting the sites along the strike direction of the regional faults.