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
L. Geli et al., 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, J GEO R-SOL, 99(B2), 1994, pp. 2995-3013
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 th
at its rift valley floor is characterized by en echelon volcanic ridge
s, oriented obliquely relatively to the average strike of the ridge ax
is. These ridges are regularly spaced along the axis, about every 40 k
m, and are separated by nontransform discontinuities. Sharp positive m
agnetic anomalies, centered over the topographic highs, suggest that t
hey are eruptive centers, considered as the surficial expression of ac
tive 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 correspond
s 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 seri
es of spreading cells, the center of one cell being where the Bouguer
anomaly is widest. Short wavelength (less than 10 to 20 km) along-stri
ke variations, such as Bouguer anomaly lows centered on the topographi
c highs, reflect local effects associated with the presence of the eru
ptive centers. Seismic tomography data from a 20 x 10 kin active obliq
ue volcanic ridge near 72-degrees 22'N tend to indicate that the links
between the main, low-velocity body at depth, and the magma injection
s centers which lie within the rift valley inner floor are probably co
mplex.