Isostatic response of the Laccadive Ridge from admittance analysis of gravity and bathymetry data

The Laccadive Ridge (L-R), trending roughly parallel to the west coast of India, is an intriguing segment of the northernmost Chagos-Laccadive Ridge (C-L-R) system. Although crustal nature and isostatic response of the southern C-L-R is well known, there are no similar studies on the L-R. In the present study, the isostatic response of the lithosphere beneath the L-R is estimated so as to characterize its crustal nature, total crustal as well as effective elastic plate thickness and mode of compensation. Twelve gravity and bathymetry profiles across the ridge were analyzed using linear transfer function and forward model techniques. The observed admittance function within the diagnostic waveband of 250 < λ > 80 km (0.025 < k > 0.080 km⁻¹) fits well with (i) the Airy model whose average crustal thickness (T_c) and density are 17 ± 2 km and 2.7 × 10³ kg m⁻³, respectively, and (ii) the thin plate flexure model of isostasy with an effective elastic plate thickness (T_e) of 2–3 km. The estimated average crustal thickness and density are in good agreement with published seismic refraction results over the ridge. The results of the present study support an Airy model of isostasy for the L-R. The low T_e value, in view of other published results in the study area, suggests stretched and loaded continental lithosphere of the L-R during the evolution of the western continental margin of India.     

Fractal analysis of bathymetry and gravity profiles across the Chagos-Laccadive Ridge and the Carlsberg Ridge in the Indian Ocean

Gravity and bathymetry data have been extensively used to infer the thermo-mechanical evolution of different segments of the oceanic lithosphere. It is now understood that magmatic fluid processes involved in the accretion of oceanic crust are spatially complex and episodic. The nature of these processes which are in general nonlinear, can be described using fractal analysis of marine geophysical data. Fractal analysis has been carried out for gravity and bathymetry profiles over the aseismic Chagos-Laccadive Ridge and the spreading Carlsberg Ridge. The Iterated Function Systems (IFS) have been used to generate synthetic profiles of known dimension (D) and these are compared with the observed profiles. The D for the data sets are in the range of 1–1.5. The D for gravity profiles is less than those of bathymetry and the D for gravity and bathymetry over spreading ridge is higher than the aseismic ridge. The low fractal dimension indicates that the processes generating them are of low dimensional dynamical systems.     

Tomographic approach for gas hydrate investigation in Kerala-Konkan region,India

Seismic tomography is an effective means of estimating velocity and structure from multichannel seismic (MCS) reflection data. In this study we have followed a 2D approach to arrive at the probable velocity field configuration from multichannel seismic data and infer the presence of gas hydrates/free-gas in the offshore Kerala-Konkan region, along the eastern part of a seismic line on which a bottom simulating reflector (BSR) has previously been identified. Tomographic modeling consists of the identification of reflection phases and picking of respective travel times for various source-receiver positions. These picks were then utilized to arrive at a 2D velocity field following a forward and inversion approach using a ray tracing technique. The modeling for the first time brought out the finer scale velocity structure under the region of investigation. Modeling through the 2D approach shows lateral variation in velocity field along the studied segment of the seismic line. The results indicate a thin (∼50–60 m) sedimentary cover with velocity ranging from 1,770 to 1,850 m/s. A sedimentary layer with high P-wave velocity 1,980–2,100 m/s below the sea floor was interpreted as the hydrate layer. The thickness of this layer varies between 110 and 140 m. The hydrate layer is underlain by a low-velocity layer having velocities in the range 1,660–1,720 m/s. This low velocity may represent a free gas layer, whose thickness varies between 50 and 100 m located below the hydrated layer. The investigation suggests the occurrence of gas hydrate underlain by free gas in some parts of the Kerala-Konkan offshore region.     

Origin and compensation of Chagos-Laccadive ridge, Indian ocean, from admittance analysis of gravity and bathymetry data

The Chagos-Laccadive ridge (CLR) is a prominent aseismic, volcanic ridge in the northern Indian ocean. The ridge, together with the Southern Mascarene plateau (SMP), to which it is genetically related, is considered as a volcanic trace of the Reunion hotspot. We have examined the isostatic compensation of the CLR through transfer function analysis of gravity and bathymetry data along seven profiles. The analysis suggests that the CLR is compensated locally, with an Airy crustal thickness (T_c) of 20 km. The rather low elastic plate thickness (T_e) of about 4 km implies that the volcanism of the ridge took place very near a spreading centre. The proximity of the Chagos fracture zone indicates that the emplacement was probably near a spreading centre-transform junction.     

Fluid flow related features as an indicator of potential gas hydrate zone: western continental margin of India

Multichannel seismic reflection data from the continental margin of western India suggest the potential presence of fluid expulsion features, which may or may not be associated with gas hydrates. No typical bottom simulating reflector was observed on the reflection seismic section. As a result we look for other evidence in seismic sections in a small corridor of the western continental margin of India in order to establish the presence of gas hydrates. We study features including venting through the seafloor, pockmarks, sea floor collapse, faults acting as migration paths for fluid flow, transparent gas-charged sediment, reduction in amplitude strength, diapirism and mud-volcano. Presence of all these gas-escape features on a seismic section implies the probable presence of methane within the zone of hydrate stability field.