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We review previous models for the Paleogene tectonic evolution of the Arabian and Eastern Somali basins and present a model based on a new compilation of magnetic and gravity data. Using plate reconstructions, we derive a self-consistent set of isochrons for Chron 27 to Chron 21 (61–46 Ma). The new isochrons account for the development of successive ridge propagation events along the Carlsberg Ridge, leading to an important spreading asymmetry between the conjugate basins. Our model predicts the growth of the outer and inner pseudofaults associated with the ridge propagation events. The location of outer pseudo-faults appears to remain very stable despite a drastic change in the direction of ridge propagation before Chron 24 (c. 54 Ma). The motion of the Indian plate relative to the Somalian plate is stable in direction through Paleogene time; spreading velocities decrease from 6 to 3 cm a−1. Our reconstructions also confirm that the Arabia-India plate boundary was located west of the Owen Ridge along the Oman margin during Paleogene time; some compression is predicted at about Chron 21 (47 Ma) between the Indian and Arabian plates.

We report evidence of paleo–cold seep associated activities, preserved in methane‐derived carbonates in association with chemosynthetic clams (Calyptogena sp.) from a sediment core in the Krishna‐Godavari basin, Bay of Bengal. Visual observations and calculations based on high‐resolution wet bulk density profile of a core collected on board R/V Marion Dufresne (May 2007) show zones of sharp increase in carbonate content (10–55 vol %) within 16–20 meters below seafloor (mbsf). The presence of Calyptogena clam shells, chimneys, shell breccias with high Mg calcite cement, and pyrite within this zone suggest seepage of methane and sulfide‐bearing fluid to the seafloor in the past. Highly depleted carbon isotopic values (δ13C ranges from −41 to −52‰ VPDB) from these carbonates indicate carbon derived via anaerobic oxidation of methane. Extrapolated mean calendar age (∼58.7 ka B.P.) of the clastic sediments at a depth of 16 mbsf is close to the upper limit of the U‐Th based depositional age (46.2 ± 3.7 and 53.0 ± 1.6 ka) of authigenic carbonates sampled from this level, thereby constraining the younger age limit of the carbonate deposition/methane expulsion events. The observed carbonate deposition might have resulted from the flow of methane‐enriched fluids through the fracture network formed because of shale diapirism.

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Results obtained from compilation and reinterpretation of about 21,200 line km of bathymetry, magnetic and satellite gravity data between 10°S to 10°N latitudes and 75 to 90°E longitudes south off Sri Lanka are presented here. Magnetic data and the synthetic seafloor spreading model reveal the presence of Mesozoic anomaly sequence M11 through M0 south of Sri Lanka. The oldest magnetic anomaly M11 (134 Ma) occurs between 110 and 140 km away from the Sri Lankan coast. The seafloor created during the Early Cretaceous is estimated to have evolved with variable half-spreading rates ranging from 5.5 to 1.53 cm/yr. The trends of the fracture zones inferred from the offsets in the magnetic anomalies have been constrained using the satellite gravity mosaic. The Cretaceous Magnetic Quiet Zone (CMQZ, 121-84 Ma) crust between the isochrons M0 and A34 has an unequal width and widens from about 170 km in the west (80°E longitude) to about 500 km towards east (85°E longitude). Plate reconstruction models for 160 (Fit), 134 (M11), 121 (M0) and 84 Ma (A34) are generated under the constraints of newly identified magnetic anomaly isochrons and fracture zones. The mismatch in the A34 reconstruction is attributed to the emplacement of the Ob, Lena and Marion Dufresne seamounts at the spreading ridge axis and/or the frequent ridge jumps in the Middle Cretaceous during the major plate reorganization.

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