M M M M

Fig. 5.20. Decreasing amplitude of magnetic anomalies in the Pacific observed from anomalies M19 to M26 approaching the Jurassic Quiet Zone. Ages are indicated according to the time scale of Kent and Gradstein (1986). The reduced amplitude from anomalies older than about 159 Ma corresponds with the minimum in the Mesozoic dipole low determined from paleointensities on land in the time interval 160-170 Ma. After Cande et al. (1978), with the permission of Elsevier Science.

Fig. 5.20. Decreasing amplitude of magnetic anomalies in the Pacific observed from anomalies M19 to M26 approaching the Jurassic Quiet Zone. Ages are indicated according to the time scale of Kent and Gradstein (1986). The reduced amplitude from anomalies older than about 159 Ma corresponds with the minimum in the Mesozoic dipole low determined from paleointensities on land in the time interval 160-170 Ma. After Cande et al. (1978), with the permission of Elsevier Science.

The Jurassic Quiet Zone (JQZ) is observed off the east coast of North America, the northwest coast of Africa, and in the western Pacific. It corresponds to ocean floor of age older than M29 and is characterized by low-amplitude anomalies whose interpretation has been controversial. The known geomagnetic polarity time scale observed from sequences on land (Steiner and Ogg, 1988; Gradstein et al., 1994; Opdyke and Channell, 1996) shows that, contrary to previous perception, there is no long period of a single polarity that could be correlated with this quiet zone. Several explanations have been offered, by Mascle and Phillips (1972), Poehls et al. (1973), Hayes and Rabinowitz (1975), Larson and Hilde (1975), Roots (1976), and Cande et al. (1978). The problem is that many of the quiet zones occur adjacent to continental margins and one suggestion is that they formed during the initial rifting of continents. Spreading rates are at first slow, producing narrow magnetic bodies. When these narrower bodies subside to continental margin depth, their anomalous magnetic field is then attenuated at the ocean surface.

However, one of the most important results not known until recently is that the intensity of the geomagnetic field was much reduced between 130 and 170 Ma, with a minimum of about one-third of its Cenozoic value between 160 and 170 Ma (Prévôt et al., 1990; Perrin et al., 1991; Prévôt and Perrin, 1992; Kono and Tanaka, 1995; Perrin and Shcherbakov, 1997; Kosterov et ai, 1997), as was illustrated in Fig. 1.15. An envelope of decreasing anomaly amplitudes with increasing age is observed from around anomaly M20 (-150 Ma) in the Pacific (Larson and Hilde, 1975). Typical amplitudes of 250-500 nT are observed for anomalies formed after M20, whereas those older than M25 have typical amplitudes of about 50-70 nT (Fig. 5.20). Hayes and Rabinowitz (1975), Larson and Hilde (1975) and Cande et al. (1978) suggested that this could be due to a systematic decrease in field intensity going backwards in time. There is also a remarkable linearity in the low amplitude magnetic anomalies that are seen in the JQZ, especially in the faster spreading Pacific regions (Handschumacher et al., 1988; Sager et al., 1998).

The frequency of reversals during the time of the JQZ is not observed to be remarkably high on land (Gradstein et al., 1994; Opdyke and Channell, 1996), with rates of ~3 per Myr. However, Jurassic ocean floor is deep and anomalies measured at the surface are attenuated. Sager et al. (1998) investigated these anomalies using deep-tow profiles over the western Pacific JQZ. The anomalies show both short- and long-wavelength components. The short-wavelength components are preferentially attenuated when upward continued to the ocean surface. Sager et al. inferred that many of the short-wavelength anomalies represented geomagnetic intensity fluctuations and were therefore cryptochrons according to Cande and Kent (1992b). The polarity reversal model deduced from the long-wavelength anomalies then becomes reasonably consistent with that observed on land. This interpretation, coupled with the known decrease in global field intensity at that time, appears to be a reasonable explanation of the JQZ anomalies.

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