Pre-Cretaceous tectonic evolution of the Pacific plate and extension of the geomagnetic polarity reversal time scale with implications for the origin of the Jurassic ``Quiet Zone''
Linear magnetic anomalies resulting from seafloor spreading were mapped in the vicinity of the magnetic bight in the western Pacific Ocean. New aeromagnetic data allowed the magnetic bight to be more accurately mapped from M21 to M28 and enabled the identification of low-amplitude magnetic lineations in the Jurassic "Quiet Zone". These lineations were formed by magnetic field reversals prior to M29. A revised Jurassic geomagnetic polarity reversal time scale was constructed with nineteen reversals older than M29, numbered M30-M38. These reversals extend the record of geomagnetic polarity back in time by approximately 8 Ma and are important constraints on the origin of the quiet zone. In particular, they imply that the Jurassic was not a period of constant normal polarity, an explanation offered by some authors. Further, they cast doubt on a model of systematically decreasing geomagnetic field strength with increasing age during this period. The early history of the northern Pacific plate and Pacific-Farallon-Izanagi (P-F-I) triple junction was traced by mapped magnetic isochrons. The Pacific plate seems to have evolved from a small plate that formed near the Phoenix-Farallon-Izanagi triple junction about 180-188 m.y. ago at approximately 17°N, 160°E in present coordinates. Until M21 time the evolution of the northern Pacific plate was relatively simple and the P-F-I triple junction migrated north-northwest with respect to the Pacific.