STRUCTURE AND STABILITY OF NON-TRANSFORM DISCONTINUITIES ON THE MID-ATLANTIC RIDGE BETWEEN 24-DEGREES-N AND 30-DEGREES-N

Observations of the median valley within the 24-30 degrees N area of t he 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-tran sform discontinuities between spreading segments (NTDs). These high re solution side scan images were inter interpreted in parallel with mult ibeam bathymetry (Purdy et al., 1990), giving a greater degree of stru ctural 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 ar ea. Type 1 NTDs are termed septal offsets. and are marked by a topogra phic ridge separating the two spreading segments. The offset between t he spreading segments ranges from 9 to 14 km. These can be further sub divided into Type IA in which the septa run parallel to the overall tr end of the MAR and Type 1B in which the septa lie at a high angle to t he bulk ridge trend. Type 1A NTDs are characterised by overlap of the neovolcanic zones of the segments on each side, and strong offaxis tra ces, while Type 1B NTDs show no overlap of neovolcanic zones, and weak offaxis traces. Type 2 NTDs are brittle/ductile extensional shear zon es, marked by oblique extensional fractures, and associated with rotat ion 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, a nd show no sign of any accommodating structure. In this type of NTD, t he offset zone is covered with undeformed volcanics. The type of NTD d eveloped at any locality along the ridge axis appears to depend on the amount of segment offset and segment overlap, the overall trend of th e 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 over laps. Septa can be short or long lived, and are associated with large segment offsets. Segment overlaps vary from negative (an along axis ga p) to zero, for Type 1B septal offsets, or positive to zero for Type 1 A septal offsets. Non-tectonised NTDs are generally short lived struct ures, characterised by small segment offsets and zero or positive over laps.