A discussion concerning the floor of the northwest Indian Ocean - An area on the crest of the Carlsberg Ridge: petrology and magnetic survey

Abstract

Rocks collected in the vicinity of a transcurrent fault cutting the crest of the Ridge have been affected by brecciation and, in some cases, metamorphism and hydrothermal action. These processes have led to the formation of spilites from crystalline basalts, and ultramafic rocks from basalt glasses Further hydrothermal action has taken the form of replacement of some ultramafic rocks by quartz, ending in a nearly pure quartzite. The mineralogy is characteristic of greenschist facies meta-morphism. Fresh basalts were collected from a nearby hill, which seems to be a recent volcano post-dating the faulting and metamorphism. The magnetic survey reveals a marked parallelism between the anomalies and the trend of the ridge, regardless of bathymetry. Computations confirm that uniform magnetization of the material represented by the bathymetry can in no way simulate the observed anomalies. Application of the vector fitting technique suggests that the remanent magnetization of this material is often reversed and from this a very crude and simple model is developed to account for the observed anomalies. The model is consistent with an ocean floor spreading hypothesis and periodic reversals in the earth's magnetic field. If substantiated it would have important implications in deducing the history of the ocean basins. Above all it provides a plausible explanation to account for the magnetic gradients and amplitudes observed over ridges without implying improbable magnetic contrasts, structures, or changes in petrology.