Asymmetric crustal structure of the slow-spreading Mohns Ridge

The calculated residual topography (RT) and residual mantle Bouguer anomaly (RMBA) reveal distinctive asymmetric crustal structure on conjugate flanks of the Mohns Ridge during the last 35 Ma. During Period I (35 Ma to 17 Ma), large regions of the western flank are associated with relatively negative RMBA and higher RT (i.e., shallower seafloor) in comparison to the conjugate eastern flank. The average differences in RMBA and RT between both flanks are about 35 mGal and 400 m, respectively, indicating possible off-axis crustal accretion on the western flank associated with the Iceland hotspot. Furthermore, the RMBA on both flanks are associated with relative negative values, suggesting overall enhanced magma supply at the axis of the Mohns Ridge during this period. During Period II (17 Ma to 6 Ma), the average asymmetry of RT and RMBA between the two flanks gradually reduced to zero. During Period III (6 Ma to present), the western flank are associated with higher RT and more positive RMBA values relative to the eastern conjugate; the overall magma supply is the least robust since 35 Ma. We infer the following interpretations from the above analyses: (1) The gradual decrease in magma supply of the Mohns Ridge from 35 Ma to present could be caused by the Mohns Ridge moving farther away from the Iceland hotspot. The separation distance between the Mohns Ridge and Iceland hotspot reached a minimum value (~300 km) at about 35 Ma, but gradually increases to the present-day value of ~630 km. (2) The observed significant asymmetric crustal structure in crustal ages of 35 Ma to 17 Ma (Period I) might be related to the intensive off-axis volcanism of the Iceland hotspot on the western flank. (3) The observed asymmetric crustal structure in crustal ages of 6 Ma to present (Period III), however, might be best explained by a model of relatively low overall ridge-axis magma supply and greater degree of tectonic deformation. Furthermore, the higher RT and relative positive RMBA values on the western flank might be associated with thinner crust and uplifted footwalls of large normal faults on the western flank. Together the systematic variations along the Mohns Ridge reveal the important role of interactions among hotspot influence, local magma supply, and tectonic deformation, in controlling the asymmetric crustal structure of slow-spreading ridges.