Geomagnetic reversals and crustal spreading rates during the Miocene

Statistical analysis of the geomagnetic time scale suggests that the rate of reversals was anomalously low during the Miocene. To determine whether undetected reversals actually occurred in the Miocene, 14 magnetic profiles from a survey of the northeast Pacific by NOAA are analyzed by signal-averaging techniques. The data suggest that during the time period 7.3 to 22.7 m.y., eight short-wavelength anomalies are present that are not predicted by previously published time scales. Anomaly 5 in particular, originally considered a major polarity epoch, is actually composed of numerous shorter events. Additional data, both from the east Pacific rise and the Indian-Antarctic ridge, corroborate these observations. A statistical study of the spreading rates of all the data indicates apparent accelerations of the order of 30 km/m.y.2 for the Pacific, Indian, and antarctic tectonic plates. These abrupt changes in spreading rate are considered artifacts due to errors in the published time scale, and a new Miocene time scale is derived on the assumption of constant spreading rates for the three plates. The new time scale contains 18 additional reversals and 14 short events ranging from 10,000 yr to 0.1 m.y. in length. The widths of the transition zones from normal to reversely magnetized crust are determined to be 6 km for the northeast Pacific data, which is consistent with a recently proposed two-layer crustal model.