M.V. Ramana

Mesozoic anomalies in the Bay of Bengal

Abstract The analysis of 8200 line km of total magnetic intensity data in the Bay of Bengal, northeastern Indian Ocean, revealed the presence of approximately N30°E-trending seafloor spreading type magnetic anomalies. These anomalies resemble the Mesozoic anomaly series (M11–M0) reported elsewhere. The oldest anomaly (M11, 132.5 Ma) identified close to the east coast of India is followed by the younger series of Mesozoic anomalies towards the offshore. Some of the anomalies are offset by 60–80 km. The configuration of the offsets of the isochrons allowed us to propose approximately N120°E-trending oceanic fracture zones. The Mesozoic crust of 132.5–118 Ma is estimated to evolve with an average half-spreading rate of 3.5 cm/yr, except for the ocean floor between the M9 and M4 magnetic isochrons. This part of the crust appears to be affected by the buried subsurface 85°E Ridge.

Seafloor spreading magnetic anomalies in the Enderby Basin, East Antarctica

Abstract The timing of the early separation of India from the contiguous Antarctica–Australia is still an unresolved problem although it is well established that Antarctica and India formed a single Indo-Antarctic platform prior to the fragmentation of eastern Gondwanaland. Inadequate age information either in the form of magnetic anomaly isochrons or dating of oceanic rocks from the conjugate margins of Antarctica and India perhaps led several authors to propose incomplete plate reconstruction models particularly for the early separation of India from Antarctica. Analysis of magnetic and satellite-derived gravity data in the Enderby Basin, East Antarctica, reveals the presence of seafloor spreading type linear magnetic anomalies and eight new fracture zones. The observed magnetic anomalies can be interpreted as the younger sequence of Mesozoic anomalies M11–M0. Half-spreading rates range from 6.5 to 2.8 cm/yr and are comparable with those measured in the Bay of Bengal. These similarities in the Mesozoic magnetic anomaly sequence and in the spreading rates provide evidence that these two basins are conjugate and contemporary. A consistent plate reconstruction model can be derived from the identified conjugate patterns of Mesozoic magnetic anomalies and fracture zones. The occurrence of the oldest magnetic anomaly M11 close to the coasts in these two offshore basins unequivocally suggests that the break-up of India from Antarctica occurred before ∼134 Ma.

Tectonics of the Bay of Bengal: new insights from satellite-gravity and ship-borne geophysical data

Abstract Recently released satellite-derived free air gravity anomalies and the existing ship-board geophysical data provide new insights into the tectonics of the Bay of Bengal with respect to the structure and regional extension of the buried 85°E ridge and the tectonics of the Eastern Continental Margin of India (ECMI). The 85°E ridge can be visualized extending inland via the Mahanadi basin volcanics to the Rajmahal Traps. A large volcanic province in eastern India encompassing the Rajmahal and Sylhet Traps and volcanics in the Bengal and Mahanadi basins, almost on the scale of the Deccan volcanic province along the west coast, can be envisaged taking into account the occurrences of intrusive and extrusive rocks around the age of 117 Ma. The 85°E ridge represents the deep-ocean volcanic trace of this magmatic activity. Towards the south, the ridge continues in an arcuate manner to the Afanasy–Nikitin seamount at equatorial latitudes in the central Indian Ocean. Gravity models of the ridge are indicative of hotspot-related crustal underplating processes beneath the ridge. The ECMI can be divided into a southern transform and northern rifted segments on the basis of gravity and bathymetry data, which bear similarities with the conjugate East Antarctica margin.

Tectonic model for the evolution of oceanic crust in the northeastern Indian Ocean from the Late Cretaceous to the Early Tertiary

Bathymetry and magnetic studies (part of the Trans Indian Ocean Geotraverse investigations) in the northeastern Indian Ocean revealed seafloor topographic features, magnetic lineations (19 through 32B) and abandoned spreading centers. The seafloor topography of the Ninetyeast Ridge is relatively wider and shallower south of 15°S. The magnetic anomalies indicate nine fracture zones. Two of them are newly identified. Some of the fracture zones are reflected in the bathymetry. Abandoned spreading centers between 86°E Fracture Zone (FZ) and 92°E FZ are interpreted as the western extensions of the Wharton Ridge. They ceased spreading along with other spreading centers in the Wharton Basin soon after the formation of magnetic anomaly 19 (around 42 Ma) and merged the Indian and Australian plates as single Indo-Australian plate. The pattern of magnetic lineations between 86°E FZ and 90°E FZ indicate a series of southerly ridge jumps at anomalies 30, 26 (Royer et al., 1991 and other workers) and 19. These ridge jumps transferred portions of the Antarctic plate to the Indian plate. The captured portions and offset along 86°E FZ between India-Antartica Ridge and Wharton Ridge resulted in an anomalous extra oceanic crust between 86°E FZ and Ninetyeast Ridge spanning 11° in latitude.

Structure and origin of the 85°E ridge

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 (<−60 mGal) anomalies with variable widths of 100–180 km. Magnetic model studies revealed that the rocks of the 85°E Ridge are magnetized with reversed polarity. The well-defined geophysical anomalies and lack of magnetic polarity reversals together with the deep burial nature of the ridge may not favor a hotspot origin. Two alternative processes for the ridge emplacement have been suggested. Ridge emplacement may be (1) due to shearing of the lithosphere caused by stretching and compressional forces associated at the time of major plate reorganization immediately after the evolution of the early Cretaceous crust in the Bay of Bengal, more precisely at M0 isochron or during the middle Albian reversals within the Cretaceous long normal polarity (K-T superchron) epoch when the Earths magnetic polarity changed from normal to reversed polarity, and/or (2) due to sagging followed by deformation produced by the buckling instability of the oceanic plate caused by horizontal compressional forces on the passive continental margin. However, more marine geophysical data are required to support the postulated coincidence of the ridge with a reversed polarity magnetic anomaly and their associated model. Further, the Rajmahal traps (normally polarized) and the 85°E Ridge (reversely polarized) appear to be associated with two different episodes of eruption that might have been triggered by the Kerguelen mantle plume. The 85°E Ridge seems to extend into the onshore West Bengal Basin as a subsurface ridge and merges with the reported NNE-SSW trending zone of strong geophysical anomalies east of Rajmahal traps up to 25°N latitude.

Periodic deformation of oceanic crust in the central Indian Ocean

New seismic reflection profiles of ≈5370 km, running through the Ocean Drilling Program Leg 116 sites and Deep Sea Drilling Project Sites 215 and 218, were obtained to investigate the spatial extent, timing, and nature of the Tertiary deformation of the equatorial central Indian Ocean. Analyses of the data revealed basement and sedimentary structures and structural unconformities that resulted from the release of compressional forces. Long-wavelength (150–300 km) anticlinal basement structures with 1–2 km relief and tight folding and high-angle faulting (5–20 km long) of oceanic basement and overlying sediments exist between the Afanasy Nikitin seamount and the Ninety east Ridge. The deformation zone extends from 10°S to the north up to ≈7°N latitude. The changes in the regional basement trend along 87°E longitude between latitudes 11°S and 15°N coincide with the significant tectonic events of the first major plate reorganization occurred in the eastern Indian Ocean at 90±5 Ma and changes in the Indian plate motion at 65±5 Ma. The basement deformation at selected places, a widespread unconformity of the upper Miocene, and subsequent younger unconformities (lower Pliocene and upper Pleistocene) indicate that the deformation activity might have begun earlier than the generally believed age of 7.5 Ma and appears to be periodic. The upper Miocene and upper Pleistocene deformational unconformities are, in general, observed south of 1°S, while the basement deformation and lower Pliocene deformational unconformity are mostly present in the area north of 1°S. It is surmised that the compressional stresses built up since the hard collision of India with Eurasia may have released for a short period prior to the early Miocene time and deformed the oceanic basement in the northeastern Indian Ocean. Thereafter the stress regime seems to have transferred to the north of the Indian shield and caused deformation of the continental lithosphere. Later, the activity reoccurred during the late Miocene (≈7.5 Ma), early Pliocene (≈4 Ma) and late Pleistocene (≈0.8 Ma) with a cyclicity of ≈3.5 m.y. and deformed the oceanic crust and sedimentary strata in the central Indian Ocean.

Analyses of multichannel seismic reflection, gravity and magnetic data along a regional profile across the central-western continental margin of India

Abstract Analyses of multichannel seismic reflection, gravity, magnetic and bathymetry data along a regional profile across the central-western continental margin of India have revealed the depositional pattern of sediments, crustal structure and tectonics. The four most distinct and varied crustal regions of the margin are palaeo-shelf edges, shelf margin basin, Prathap and Laccadive Ridges and the Arabian Basin. The shelf margin basin is carpeted by ∼4.5 km maximum thick aggraded and prograded Paleocene to Holocene sediments. Six major seismic sequences of the sediments of the margin are identified and their ages are assigned on correlation with drill-well results. Development of the sequence boundaries is attributed to the events of rifting of western India, eustatic sea-level changes, Indian and Eurasian plate collision and Himalayan orogeny. Tilted fault blocks (half-grabens) located almost equi-distance from the igneous construct of the ‘Prathap Ridge’ in the shelf margin basin suggest a failed rift associated with stretched continental crust of the basin. 2-D model studies of gravity and magnetic anomalies, constrained by the seismic results, have revealed 6 to 27 km thick crust across the margin. The Laccadive Ridge crust limited by two volcanic intrusives and a steep scarp at its western end is ∼16 km thick. It gradually thins towards offshore and juxtaposed with early Tertiary normal oceanic crust ∼6 km thick of the Arabian Basin. The crustal thickness and velocity and density structure of the ridge are comparable to that of the Laxmi Ridge, a continental sliver. The inferences and abrupt change in magnetic and gravity anomaly signatures across the western end of the Laccadive Ridge mark the zone of transition from continental to oceanic crust.

Analysis of multi-channel seismic reflection and magnetic data along 13° N latitude across the Bay of Bengal

Analysis of the multi-channel seismic reflection, magnetic and bathymetric data collected along a transect, 1110 km long parallel to 13° N latitude across the Bay of Bengal was made. The transect is from the continental shelf off Madras to the continental slope off Andaman Island in water depths of 525 m to 3350 m and across the Western Basin (bounded by foot of the continental slope of Madras and 85° E Ridge), the 85° E Ridge, the Central Basin (between the 85° E Ridge and the Ninetyeast Ridge), the Ninetyeast Ridge and the Sunda Arc. The study revealed eight seismic sequences, H1 to H8 of parallel continuous to discontinuous reflectors. Considering especially depth to the horizons, nature of reflection and on comparison with the published seismic reflection results of Currayet al. (1982), the early Eocene (P) and Miocene (M) unconformities and the base of the Quaternary sediments (Q) are identified on the seismic section. Marked changes in velocities also occur at their boundaries.

Occurrence of gas hydrates along the continental margins of India, particularly the Krishna‐Godavari offshore basin

The presence of gas hydrates along the Indian continental margins has been inferred mainly from the bottom simulating reflection/reflector (BSR) and the gas hydrate stability zone thickness map of India. Multidisciplinary investigations have been carried out in the Krishna‐Godavari offshore area along the eastern continental margin of India which is known for its hydrocarbon potential. Processed multibeam data provided a high resolution seafloor mosaic with a fine scale geomorphology. Deep tow digital side scan sonar, multifrequency chirp sonar and 3.5 KHz sub‐bottom profiler records depict various kinds of gas escape features over the regions where BSRs are prominent. Geochemical analyses of the 5 m‐long cores show a general decrease trend in the porewater sulphate concentration, while the gas chemistry reveals an increase trend of methane concentration with core depth. Total Organic Carbon varies from 0.6 to >2.0% and CaCO3 from 5.0 to >29%. Observed geophysical, geochemical and microbial proxies suggest the likely presence of gas hydrates in the Krishna‐Godavari offshore area. Recent drilling work carried out onboard JOIDES Resolution confirmed the presence of massive (>80 m thick) accumulation of gas hydrates, and fully developed gas hydrate system in the Mahanadi offshore area and the Andaman Sea.

Gravity anomalies and crustal structure of the Bay of Bengal

Abstract The Bengal Fan is covered afresh by systematic geological and geophysical investigations by National Institute of Oceanography (NIO), India and a detailed free-air gravity map of the fan is prepared. The map shows two strong gravity lows – one corresponding to the continental shelf and the other to the 85°E Ridge. The Ninetyeast Ridge is brought out as a gravity high. The anomalies are inverted to determine the anomaly-producing interfaces, which suggest that the 85°E Ridge anomaly could not be explained by an isolated geophysical model invoking a negative density contrast for the ridge material. The 85°E Ridge anomaly and several other isolated gravity lows are attributed mostly to the depression-like structures in the Moho. Each depression of the Moho is associated with a basement high. The depression beneath the 85°E Ridge is about 6 km deep from the regional Moho boundary, which is at variance to the earlier results. It is suggested that the depressions may possibly have developed due to the surface volcanic loads emplaced on already evolved oceanic crust of the Bay of Bengal.