Ocean-floor Asymmetry and Its Relationship with Transform Faults

Based on elevation data along 20 profiles across major divergent plate boundaries, a recent study (Doglioni et al., 2003 Tectonics) found that the eastern sides of the major oceanic and some continental rifts are on average 100-300 m higher than the conjugate western sides. The same study proposed that the asymmetry is caused by a global-scale westward shift of the lithosphere relative to the depleted and lighter asthenosphere. To verify the intriguing observation, we extract elevation data at about 473,000 locations along 2130 profiles of 1100 km in length on each side of the Mid-Atlantic Ridge (MAR), from a database of ocean-floor topography, and obtained the elevation difference between the east and west sides of the MAR. Interestingly, we find that the difference at most of the lithospheric blocks confined by the 22 longest transforms faults (TFs) is closely related to the direction and amount of displacement along the faults. Positive difference (e.g., higher on the eastern than the western sides of MAR) is observed on blocks that drifted westward, and negative difference is observed on blocks that drifted eastward, suggesting that westward drift of lithosphere relative to the asthenosphere is not a global feature. On average, the east-west difference has a mean value of 155+/-7 m along the MAR in the latitude range between 50S and 50N latitudes. In addition, we found that the difference in east-west elevation is positively correlated with the amount of displacement along the TFs. The best-fitting curve suggest that a displacement of 100 km along the TFs leads to an east-west elevation difference of about 200 - 300 m. We propose a model involving movement of the lithosphere along the TFs relative to the location of maximum buoyancy in the asthenosphere to explain the observation.