GEOMAGNETIC POLARITY REVERSAL MODEL OF DEEP-TOW PROFILES FROM THE PACIFIC JURASSIC QUIET ZONE

The Jurassic magnetic ''Quiet Zone'' (JQZ) contains magnetic lineation s, but their low amplitudes make correlation and interpretation diffic ult. Part of the problem is the separation of source and sensor for ol d, deep ocean crust. We increased anomaly amplitudes by collecting mag netic data along two deep-tow profiles over western Pacific JQZ lineat ions. A magnetic polarity reversal timescale was constructed by matchi ng deep-tow anomalies with a simple, rectangular block magnetization m odel for oceanic crust. The polarity sequence covers similar to 11 m.y . (156-167.5 Ma) and contains 88 pre-M29 polarity chrons extending to Chron M41. A limitation of this model is its poor representation of th e oldest anomalies upward continued to sea level. On deep-tow profiles these anomalies have both long-and short-wavelength components, but o nly the latter are easily modeled on a datum close to the source. An a lternative polarity model was constructed to match the anomalies upwar d continued to sea level. This model retains only 44% of the deep-tow model polarity chrons because of short-wavelength attenuation by upwar d continuation. Because of the inferred periods and magnetization cont rasts, we think many of the short-wavelength anomalies represent paleo field intensity fluctuations. In contrast, polarity reversals have bee n documented by prior magnetostratigraphic work for the younger part o f the timescale covered by our model. Thus our data may show a transit ion from a geomagnetic field behavior dominated by intensity fluctuati ons to one dominated by reversals.