Ii

Fig. 5.16. The effect of anomalous skewness for two synthetic profiles using the block model shown covering the time interval 0-3 Ma across a spreading ridge, (a) An ideal anomaly sequence with no anomalous skewness. (b) The same anomaly sequence assuming 20° of anomalous skewness, represented here by phase-shifting the anomalies left of the axis of symmetry by -20° and those to the right of the axis by +20°. The amplitude of a subhorizontal portion of an individual anomalously skewed anomaly tends to decrease more rapidly (or increase less rapidly) than do the anomalies with no anomalous skewness. The bold lines above and below some of the wider anomalies show the differences in the slopes of the anomalies. From Petronotis et al. (1992).

Dyment et al. (1994) showed that anomalous skewness becomes negligible at spreading rates above 50 km Myr"1. This dependence on spreading rate has been explained as being due either to systematic variations of the geomagnetic field intensity within each polarity interval (Cande, 1978) or to sea-floor spreading processes causing the magnetic properties of the oceanic lithosphere to depart significantly from the standard uniformly magnetized rectangular prism model (Cande and Kent, 1976; Cande, 1978; Verosub and Moores, 1981; Raymond and LaBrecque, 1987; Arkani-Hamed, 1989). Dyment et al. (1994) point out that obviously no geomagnetic field behavior can predict the observation, for the same time interval, of a significant anomalous skewness at slow-spreading centers and none at fast spreading centers so that this possible cause can be eliminated. Therefore, the spreading rate dependence of anomalous skewness most likely arises from sea-floor spreading processes.

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