NEAR-BOTTOM MAGNETIC SURVEY OF THE MID-ATLANTIC RIDGE AXIS, 24-DEGREES-24-DEGREES-40'N - IMPLICATIONS FOR CRUSTAL ACCRETION AT SLOW-SPREADING RIDGES

Near-bottom magnetic field measurements provide increased spatial reso lution over sea surface magnetic data and allow a detailed analysis of the accretionary and tectonic processes at work in a slow spreading e nvironment. Through the use of magnetic inversion methods and forward modeling, we investigate the fine-scale magnetic structure of young oc eanic crust along four near-bottom profiles that cross the two bathyme tric segments immediately north of the Kane transform on the Mid-Atlan tic Ridge. Our results show the presence of a narrow, peaked central a nomaly magnetic high (CAMH) located over the zone of most recent volca nism. We hypothesize that the pattern of high magnetization at segment ends and relatively low values at their centers is largely the result of increased iron and titanium content away from segment centers. Mag netization lows, which are not observed in the sea surface data, flank the CAMH and are associated with the axial valley walls. These lows m ay locate areas of intense magnetic source layer disruption. Blocks of relatively high magnetization found outside the valley walls suggest a cyclic faulting process, whereby discrete blocks of relatively unalt ered crust are transported out of the rift valley by adjacent zones of faulting. These observations of fine-scale magnetic structure show th at while the emplacement of crust at slow spreading ridges is highly f ocused, subsequent tectonic disruption and alteration at the rift vall ey walls degrades the crustal magnetization signal and may be the prim ary reason for Atlantic magnetic anomaly variability.