Abstract

The structure and seismo-tectonics of the Vema Fracture Zone and the adjacent regions of its transverse ridge to the south and the Mid-Atlantic Ridge spreading centre to the north is discussed, based on an analysis of seismic refraction profiles, micro-earthquake activity and gravity anomalies. Crustal structures were determined by analysis of wide angle seismic data from explosive and airgun sources recorded on arrays of ocean bottom seismometers. Micro-earthquakes were recorded by six instruments for a period of 14 days in an array to the north of the ridge-transform intersection.

Immature crust, similar to that reported by Fowler (1976) from the FAMOUS area, lies beneath the spreading centre. The lower crust thins and normal mantle appears as the fracture zone is approached. The thinnest crust lies beneath the northern end of the topographic deep at the fracture zone intersection. This is also the locus of current microearthquake activity at sub-bottom depths of 3–4 km, which a composite fault-plane solution suggests may originate on normal faults that parallel the trend of the local topography on the eastern wall of the median valley.

Within the central region of the fracture zone valley, the crust thickens but still retains an anomalous velocity structure. Seismic layer 3 is absent and crustal velocities are low. This is probably the consequence of attenuated crustal accretion, of faulting and cracking, and of hydrothermal alteration. The pattern of residual gravity anomalies, after correction for topography, for sediment thickness and for mantle temperature variations, is consistent with the crustal structure defined by the seismic experiments. The gravity measurements indicate that there is a broad thinning of the crust over 20–40 km in the vicinity of the fracture zone, on which are superimposed localized regions of low and high densities.

To the south of the fracture zone, the crust thickens and assumes a normal velocity structure including a mature seismic layer 3. The seismic Moho is warped upwards under the transverse ridge, suggesting a tectonic origin from buckling rather than uplift by serpentinization.

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