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■13.302 ^O 13.510 O 13.703 14.076 14.178 14.612 14.800 14.888 15.034 15.155 16.014 16.293 16.327 16.488 16.556 16.726 O 17.277 17.615

Fig. 4.7. Detail of the Cande and Kent (1995) time scale for the past 18 Myr. This time scale includes only those subchrons that are recorded both in marine magnetic anomaly records and in magnetostratigraphic sections.

40Ar/39Ar dating. Kent and Gradstein (1986) presented a geomagnetic and geologic time scale for the past 160 Myr based on the integration of marine magnetic anomaly data with biostratigraphic, magnetostratigraphic, and radiometric data.

Despite having a rather slow rate of spreading, the South Atlantic has not suffered major plate reorganization and both flanks of the ridge crest are well preserved. Thus, in the first major analysis of magnetic anomaly profiles from the world's ocean basins since that of Heirtzler et al. (1968), Cande and Kent (1992a) concluded that the South Atlantic magnetic anomalies remained the most appropriate as the basis for a GPTS. However, their revised GPTS for the Late Cretaceous and Cenozoic had significant changes from the earlier time scales. This time scale was later refined by Cande and Kent (1995), who still used the South Atlantic as a reference ocean but assumed that its spreading rate had smooth variations, which they approximated by splining between calibration points. Additionally, the Cande and Kent (1995) time scale (Table 4.2) adopted astrochronological estimates for the ages of the Plio-Pleistocene reversals. Only those subchrons that are recorded both in marine magnetic anomaly records (see Chapter 5) and in magnetostratigraphic section (see §4.3) are included in the Cande and Kent (1995) time scale. Because of the relatively rapid reversal rate, Fig. 4.7 shows details of this time scale back to 18 Ma.

In general, the GPTS for the past 10 or 20 Myr has stabilized significantly, with minor improvements continuing to occur. For example, the time scales of Baksi (1993) for the past 18 Myr and of Cande and Kent (1995) differ only in minor detail. However, there are two issues of possible contention; calibration points and assumptions about the South Atlantic spreading rate (§5.3.1). Wei (1995) disputed five of the nine calibration points used by Cande and Kent (1992a, 1995) and used other calibrations instead. This has led to a time scale that differs significantly from the Cande and Kent time scales, with the greatest differences being in the Miocene, as may be seen in Table 4.2.

In Fig. 4.8 the Late Jurassic and Early Cretaceous part of the of Kent and Gradstein (1986) reversal chronology has been combined with the Cande and Kent (1995) chronology to give a complete reversal time scale for the past 160 Myr. The presence of two suggested zones of mixed polarity between 100 and 107 Ma (Gradstein et al., 1994) has been omitted, since these are not seen in marine magnetic anomalies, and their existence in magnetostratigraphic data can be disputed (Opdyke and Channell, 1996). Extending the time scale to ages older than 160 Ma where the marine magnetic anomaly record ends relies on the integration of magnetostratigraphic sections sampled around the world. This is discussed further in §4.3.

a e2

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