Marine Geosciences

Frauke Klingelhoefer, Louis Géli , Luis Matias

A series of 8 previously unpublished high resolution seismic refraction profiles from the ultra-slow spreading Mohns Ridge in the Norwegian-Greenland Sea have been treated with modern inversion software. The joint refraction and gravity interpretation of the deep crustal structure sheds new light on the relative thickness of the various layers of oceanic crust at ultra-slow spreading centers. The profiles where shot parallel to the ridge at an off axis distance of 0-135 km corresponding to crustal ages of 0-20 Ma. The structural evolution of oceanic crust generated at a very slow spreading centers can be traced back through time in these profiles. The crustal thickness in all profiles is below the global average for typical oceanic crust, and shows a mean thickness of 4 km. Thickness varies only slightly with respect to distance from the axis. On all profiles the crust is locally thicker beneath basement highs and thinner beneath basement lows, in agreement with gravity data.

Three layers with different velocity gradients could be identified. The top of the basement (Layer 2a) consists of a zone with low p-wave velocities, ranging from 2.5-3.0 km/s at the ridge and increasing to 3.0-3.2 km/s in the profile furthest from the axis. This layer exhibits variable thickness along each profile, generally thinning with increasing distance from the ridge. Beneath lies a layer with velocities of 3.5-4.5 km/s at the MOR (Layer 2b). The velocity of this layer increases away from the axis to a velocity gradient between 4.5 and 5.0 km/s. The thickness shows less variability along a given profile and an overall increase with age. The main increase of the velocity for the upper two layers occurs between 0-3 Ma. Together the thickness of the two layers remains nearly constant. The third layer displays a nearly constant velocity and thickness for all profiles, although for the two profiles directly on the ridge a model without third layer, incorporating only a constant gradient up to upper mantle velocities fits the data equally well. Velocities range from 5.5-6.5 km/s.

Generally, upper mantle arrivals with velocities > 8 km/s are scarce. The deepest arrivals most likely originate from the steep gradient layer directly above the Moho and may not fully penetrate into the mantle. The outermost 4 profiles, reveal a crust overlain by a 0.5 -1.0 km thick sediment cover and show distinct S-wave arrivals, which have been modelled in order to constrain the Poisson's ratio.

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