Structure and early evolution of the Arabian Sea and East Somali Basin

The Laxmi Ridge is a large-scale basement high buried beneath the sediments of the Indus Fan. The location of the ocean–continent transition (OCT) on this margin has previously been proposed at either the southern edge of the Laxmi Ridge or beyond it towards the India–Pakistan shelf. The former explains the margin-parallel Laxmi Basin as thinned continental crust, the latter as a failed rift of earlier seafloor spreading. To examine the structure of this margin, a reassessment of marine magnetic data has detailed seafloor-spreading magnetic anomalies prior to anomaly 24 in both the Arabian and East Somali basins. The previously identified anomaly 28 is not interpreted as a seafloor-spreading anomaly but as a magnetized basement feature adjacent to, and merging with, the ridge—the Laxmi Spur. New gravity models across the Laxmi Ridge and adjacent margin using ship and satellite data corroborate the existence of underplated crust beneath the Laxmi Ridge and Basin and the location of the OCT at the southern edge of the Ridge. The results are not compatible with the existence of a pre-anomaly 28 phase of seafloor spreading, although large-scale intrusions may be the origin of some of the basement features in the Laxmi Basin. The models also identify the Laxmi Spur as a low-density feature with a natural remanent magnetization (NRM) compatible with serpentinization. The Laxmi Ridge is mapped to the southeast, where it appears to terminate at a point coinciding with the appearance of E–W magnetic lineations and gravity anomalies at 15.5°N. Thereafter it becomes indistinct. This is interpreted as necessary in the reconstruction to the Mascarene Plateau to avoid continental overlap.     

Contribution of serpentinized ultramafics to marine magnetic anomalies at slow and intermediate spreading centres: insights from the shape of the anomalies

Detailed characteristics of marine magnetic anomalies 33r and 20r suggest that the magnetization of the deeper magnetic layers, including the lower crust and possibly the uppermost mantle, is horizontally displaced with respect to that of the upper crust. We examine the possibility that serpentinization of ultramafics in the lower crust and possibly the uppermost mantle delays the acquisition of magnetization and introduces a shift between the upper- and lower-crustal magnetization patterns. Thermal evolution models and the resulting magnetization patterns of the oceanic lithosphere are calculated for a wide range of physical parameters such as the Nusselt number and the depth of hydrothermal circulation in the crust, and the temperature range of serpentinization. The models with moderate hydrothermal cooling of the whole crust and serpentinization temperatures ranging between 200 and 300 C successfully explain the anomalous skewness and the 'hook shape' of observed sea-level magnetic anomalies created at slow and intermediate spreading rates.