3-DIMENSIONAL INVERSION OF MARINE MAGNETIC-ANOMALIES - IMPLICATIONS FOR CRUSTAL ACCRETION ALONG THE MID-ATLANTIC RIDGE (28-DEGREES-31-DEGREES-30'N)

We present magnetic field data collected over the Mid-Atlantic Ridge i n the vicinity of the Atlantis Fracture Zone and extending out to 10 M a-old lithosphere. We calculated a magnetization distribution which ac counts for the observed magnetic field by performing a three-dimension al inversion in the presence of bathymetry. Our results show the well- developed pattern of magnetic reversals over our study area. We observ e a sharp decay in magnetization from the axis out to older lithospher e and we attribute this decay to progressive low temperature oxidation of basalt. In crust which is similar to 10 Ma, we observe an abrupt i ncrease in magnetic held intensity which could be due to an increase i n the intensity of magnetization or thickness of the magnetic source l ayer. We demonstrate that because the reversal epoch was of unusually long duration, a two-layer model comprised of a shallow extrusive laye r and a deeper intrusive layer with sloping polarity boundaries can ac count for the increase in the amplitude of anomaly 5. South of the Atl antis Fracture Zone, high magnetization is correlated with bathymetric troughs at segment end points and lower magnetization is associated w ith bathymetric highs at segment midpoints. This pattern can be explai ned by a relative thinning of the magnetic source layer toward the mid point of the segment. Thickening of the source layer at segment endpoi nts due to alteration of lower oceanic crust could also cause this pat tern. Because we do not observe this pattern north of the fracture zon e, we suggest it is a result of the nature of crustal formation proces s where mantle upwelling is focused. South of the fracture zone, rever sals along discontinuity traces only continue to crust similar to 2 Ma old. In crust > similar to 2 Ma, we observe bands of high, positive m agnetization along discontinuity traces. We suggest that within the di scontinuity traces, a high, induced component of magnetization is prod uced by serpentinized lower crust/upper mantle and this masks the cont ribution of basalts to the magnetic anomaly signal.