OCEAN CRUST FORMATION PROCESSES AT VERY SLOW-SPREADING CENTERS - A MODEL FOR THE MOHNS RIDGE, NEAR 72-DEGREES-N, BASED ON MAGNETIC, GRAVITY, AND SEISMIC DATA

The Mohns Ridge, in the Norwegian Greenland Sea, is one of the slowest spreading centers of the mid-ocean ridge system (8 mm/yr half rate). Sea Beam data acquired with R/V Jean Charcot near 72-degrees-N show that its rift valley floor is characterized by en echelon volcanic ridges, oriented obliquely relatively to the average strike of the ridge axis. These ridges are regularly spaced along the axis, about every 40 km, and are separated by nontransform discontinuities. Sharp positive magnetic anomalies, centered over the topographic highs, suggest that they are eruptive centers, considered as the surficial expression of active spreading cells. Over the rift valley, Bouguer anomalies obtained by subtracting the predicted effects due to seafloor topography from the measured free-air gravity field are consistent with a low density body within the lower crust having its upper surface lying at about 2 km below the sea surface. This body, if it exists, probably corresponds to the zone of low viscosity that can be inferred from the model of Chen and Morgan (1990b), which predicts the existence of a decoupling region, between the upper crust and the asthenophere below. Its width varies rapidly along-strike, from less than about 5 km to more than 15 km. In plan view, it has a pinch and swell form, which defines a series of spreading cells, the center of one cell being where the Bouguer anomaly is widest. Short wavelength (less than 10 to 20 km) along-strike variations, such as Bouguer anomaly lows centered on the topographic highs, reflect local effects associated with the presence of the eruptive centers. Seismic tomography data from a 20 x 10 kin active oblique volcanic ridge near 72-degrees 22'N tend to indicate that the links between the main, low-velocity body at depth, and the magma injections centers which lie within the rift valley inner floor are probably complex.