No abstract

No abstract

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.

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.