Dating the ocean floor

The use of the geomagnetic timescale to date the oceanic lithosphere is based on the identification of characteristic patterns of magnetic anomaly lineations and their relation to the dated reversal chronology. Particularly conspicuous markers which are widely used are anomalies 5, 12-13, 21-26, and 31-32. Also of interest is the prolonged period of normal polarity in the Cretaceous. This period corresponds to magnetic quiet zones within the oceans where there are no linear magnetic anomalies. In many instances, however, the recognition of particular anomalies is not possible, and the usual approach is to construct the anomaly pattern expected for relevant parts of the timescale and to compare it with the observed sequence.

Once the reversal chronology has been established, lineations of known age can be identified on magnetic maps and transformed into isochrons so that the sea floor can be subdivided into age provinces (Scotese et al., 1988). Summaries of the isochrons derived from the linear oceanic magnetic anomalies are also provided by Cande et al. (1989) and Müller R.D. et al. (1997). (Plate 4.1 between pp. 244 and 245). Lineations of the same age on either side of a mid-ocean ridge can be fitted together by employing techniques similar to those used for continental margins (Section 3.2.2). In this way reconstructions of plate configurations can be made for different times, and the whole evolution of the present day ocean basins determined (Scotese et al., 1988). Figure 4.14 shows this method applied to the Mesozoic and Cenozoic history of the North Atlantic. Examples of areas with more complex spreading histories, involving extinct ridges and ridge jumps, include the Indian Ocean (Norton & Sclater, 1979) and the Greenland-Iceland-Scotland region (Nunns, 1983).

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