MORPHOLOGY AND CRUSTAL STRUCTURE OF A SMALL TRANSFORM-FAULT ALONG THEMID-ATLANTIC RIDGE - THE ATLANTIS FRACTURE-ZONE

The Atlantis Fracture Zone (30 degrees N) is one of the smallest trans form faults along the Mid-Atlantic Ridge with a spatial offset of 70 k m and an age offset of similar to 6 Ma. The morphology of the Atlantis Fracture Zone is typical of that of slow-slipping transforms. The tra nsform valley is 15-20 km wide and 2-4 km deep. The locus of strike-sl ip deformation is confined to a narrow band a few kilometers wide. Ter rain created at the outside corners of the transform is characterized by ridges which curve toward the ridge-transform intersections. and de pressions which resemble nodal basins. Hooked ridges are not observed on the transform side of the ridge-transform intersections. Results of the three-dimensional inversion of the surface magnetic field over ou r survey area suggest that accretionary processes are sufficiently org anized within 3-4 km of the transform fault to produce lineated magnet ic anomalies. The magnetization solution further documents a 15-km, we stward relocation of the axis of accretion immediately south of the tr ansform about 0.25 Ma ago. The Atlantis Transform is associated with a band of high mantle Bouguer anomalies, suggesting the presence of hig h densities in the crust and/or mantle along the transform, or anomalo usly thin crust beneath the transform. Assuming that all the mantle Bo uguer anomalies are due to crustal thickness variations,we calculate t hat the crust may be 2-3 km thinner than a reference 6-km thickness be neath the transform valley, and 2-3 km thicker beneath the mid-points of the spreading segments which bound the transform. Our results indic ate that crustal thinning is not uniform along the strike of the fract ure zone. Based on studies of the state of compensation of the transfo rm, we conclude that the depth anomaly associated with the fracture zo ne valley is not compensated everywhere by thin crust. Instead, the re gional relationship between bathymetry and gravity is best explained b y compensation with an elastic plate with an effective thickness of si milar to 4 km or greater. However, the remaining isostatic anomalies i ndicate that there are large variations away from this simple model wh ich are likely due to variations in crustal thickness and density near the transform.