Asymmetric crustal structure of the slow-spreading Mohns Ridge

We investigate the relationship among hotspot influence, local magma supply, and asymmetric topography and crustal structure of the conjugate flanks of the slow-spreading Mohns Ridge. Residual mantle Bouguer anomaly (RMBA) was calculated by subtracting from free-air gravity the predicted attractions of water-sediment, sediment-crust, and crust-mantle interfaces as well as the effects of lithospheric cooling. Similarly, residual topography (RT) was calculated by removing from the observed topography the predicted effects of sediment thickness and thermal subsidence associated with lithospheric cooling. The calculated RT and RMBA reveal distinctive asymmetric crustal structure on conjugate flanks of the Mohns Ridge during the last 35 Ma. During Period I (35 Ma to 10 Ma), significant asymmetric crustal structure was observed: Large regions of the western flank are associated with relatively negative RMBA and higher RT in comparison to the conjugate eastern flank, indicating possible off-axis crustal accretion 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 (10 Ma to 5 Ma), the RT and RMBA are roughly symmetric on both flanks. During Period III (5 Ma to present), however, significant asymmetric crustal structure was observed again, but with different characteristics than that of Period I. During Period III, the western flank has less magma supply but higher RT relative to the eastern conjugate; the overall magma supply for this period 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 minimum (~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 10 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 5 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 observed higher RB and relative positive RMBA 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 interactions of hotspot influence, local magma supply, and tectonic deformation in controlling the asymmetric crustal structure of slow-spreading ridges.