Structure and stability of non-transform discontinuities on the Mid-Atlantic Ridge between 24 degrees N and 30 degrees N
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Observations of the median valley within the 24-30 degrees N area of the Mid-Atlantic Ridge (MAR), using the IOSDL high resolution side-scan sonar instrument TOBI, image four separate areas of the median valley, containing part or all of nine spreading segments, and five non-transform discontinuities between spreading segments (NTDs). These high resolution side scan images were inter interpreted in parallel with multibeam bathymetry (Purdy et al., 1990), giving a greater degree of structural precision than is possible with the multibeam data alone. Three distinct types of NTD were identified, corresponding in part to types previously identified from the multibeam bathymetric survey of the area. Type 1 NTDs are termed septal offsets. and are marked by a topographic ridge separating the two spreading segments. The offset between the spreading segments ranges from 9 to 14 km. These can be further subdivided into Type IA in which the septa run parallel to the overall trend of the MAR and Type 1B in which the septa lie at a high angle to the bulk ridge trend. Type 1A NTDs are characterised by overlap of the neovolcanic zones of the segments on each side, and strong offaxis traces, while Type 1B NTDs show no overlap of neovolcanic zones, and weak offaxis traces. Type 2 NTDs are brittle/ductile extensional shear zones, marked by oblique extensional fractures, and associated with rotation of tectonic and volcanic structures away from the overall trend of the MAR. Type 3 NTDs are associated with offsets of less than 5 km, and show no sign of any accommodating structure. In this type of NTD, the offset zone is covered with undeformed volcanics. The type of NTD developed at any locality along the ridge axis appears to depend on the amount of segment offset and segment overlap, the overall trend of the mid-ocean ridge, the width of the zone of discontinuity, the median valley offset and the longevity of the offset. These factors influence the mechanical properties of the lithosphere across the discontinuity, and ultimately the tectonic style of the NTD that can be supported. Thus brittle/ductile extensional shear zones are long-lived structures favoured by large segment offsets, and small or negative segment overlaps. Septa can be short or long lived, and are associated with large segment offsets. Segment overlaps vary from negative (an along axis gap) to zero, for Type 1B septal offsets, or positive to zero for Type 1A septal offsets. Non-tectonised NTDs are generally short lived structures, characterised by small segment offsets and zero or positive overlaps.