GRAVITY-ANOMALIES AND CRUSTAL THICKNESS VARIATIONS ALONG THE MID-ATLANTIC-RIDGE BETWEEN 33-DEGREES-N AND 40-DEGREES-N

Multibeam bathymetry and gravity data have been obtained along an simi lar to 800-km-long section of the Mid-Atlantic Ridge from just south o f the Hayes fracture zone at 33 degrees N to the northern edge of the Azores Platform near 40 degrees N. A three-dimensional analysis pf the se gravity and topography data, combined with results from earlier sei smic refraction studies in this area, reveal two different scales of c rustal heterogeneity. A systematic, along-axis, segment-scale (lambda< 20-100 km) variation in crustal thickness is present with the thickest crust (>8-9 km) near the middle of spreading segments and the thinnes t crust (<3-4 km) near segment offsets. The magnitude of this along-ax is variation in crustal thickness is proportional to the length of the spreading segment and the size of the adjacent ridge offset. There al so is a distinct asymmetry in crustal structure across the rift valley near large segment offsets with gravity highs, inferred to be thinner crust, beneath the ''inside corner'' highs adjacent to these offsets. This segment-scale crustal heterogeneity is similar to that reported from the Kane-to-Atlantis section of the Mid-Atlantic Ridge and from p arts of the intermediate-spreading southern Mid-Atlantic Ridge. It is superimposed on a second, longer wavelength variation in gravity and c rustal thickness associated with the Azores hot spot. The most pronoun ced effect of the Azores hot spot on the Mid-Atlantic Ridge occurs bet ween 38 degrees N and 40 degrees N where the ridge axis rapidly shoals by more than 1000 m, the crust thickens by over 2 km, and the rift va lley largely disappears. The absence of a deep axial rift valley on th e Azores Platform and near the midpoints of some ridge segments along the Mid-Atlantic Ridge to the south, agrees well with the predictions of recent ridge crest thermal and theological models that suggest a de pendence of axial ?orphology on both crustal thickness and spreading r ate. The transition from a rift valley to an axial high morphology at these spreading rates (similar to 11 mm/yr half rate) occurs at a crus tal thickness of about 9 +/- 1 km.