The central anomaly magnetic high (CAMH) is a long, lineated, short-wavelength (<10 km wide at the sea surface) magnetic anomaly overlying many of the world's midocean spreading centers. This anomaly is particularly well developed over the northern Juan de Fuca Ridge, including the Northern Symmetrical and Endeavour segments. The ubiquitous presence of this zone of high magnetic contrast directly in the zone of active oceanic plate formation is a key to our understanding of near-axis crustal plate evolution. Recently obtained deep-tow magnetometer data from segments of the Juan de Fuca spreading center have caused us to revise the present models of the source of the CAMH, and to review the implications the new model would have on our understanding of the evolution of the magnetic structure of oceanic plates. In this model, we attribute the near-axis magnetization contrast to tectonically controlled low-temperature alteration of the upper vertical section of extrusive basement. Extensive and systematic fissuring of the upper extrusive rocks allows penetration of seawater and rapid alteration of the magnetic minerals in fissured areas approximately 3 km off axis. The correlation between fissured crustal blocks and near-bottom magnetic lows, and unfissured crustal blocks with magnetic highs which is observed at the axis is also seen in a single off-axis tow over an outer flanking ridge. ¿ American Geophysical Union 1987