Page 97 - Global Tectonics

84 CHAPTER 4

4.1.7 Dating the ocean floor

The use of the geomagnetic timescale to date the
oceanic lithosphere is based on the identifi cation 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 pro-
vided 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
Figure 4.12 (a) Location map of drilling sites on Leg 3 can be fitted together by employing techniques similar

of the DSDP in the South Atlantic. (b) Relationship to those used for continental margins (Section 3.2.2).
between greatest sediment age and distance from the
In this way reconstructions of plate confi gurations can
Mid-Atlantic Ridge crest (after Maxwell et al., 1970,
be made for different times, and the whole evolution
Science 168, 1047–59, with permission from the AAAS).
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
The discovery of Larson & Pitman (1972) of older
spreading histories, involving extinct ridges and ridge
magnetic anomalies in three regions of the western
jumps, include the Indian Ocean (Norton & Sclater,
Pacific allowed the Heirtzler geomagnetic timescale to

1979) and the Greenland–Iceland–Scotland region
be extended back to 160 Ma. Lineations of similar pattern
(Nunns, 1983).
were also found in the Atlantic. The timescale was
extended by assuming a constant spreading rate in the

Pacific, calibrated by DSDP sites in the Pacifi c and
Atlantic. The longer periods of reversed polarity in this 4.2 TRANSFORM
sequence are numbered M0 to M28 (M representing
Mesozoic). It appears that spreading in the major ocean
basins has been continuous as all polarity events are FAULTS
present, although the rate of spreading has varied.
The version of the reversal timescale to 160 Ma shown
in Fig. 4.13 combines the timescale of Cande & Kent 4.2.1 Introduction
(1995), for the Late Cretaceous and Cenozoic (anomalies

1–34), with that of Kent & Gradstein (1986) for the Early The theory of sea floor spreading proposes that oceanic
Cretaceous and Late Jurassic (anomalies M0–M28). lithosphere is created at mid-ocean ridges and is