B

Ridge axis

Ridge axis (b)

Ridge axis (C)

Fig. 5.1. Latitude variation of the shape of the magnetic anomaly due to magnetized crustal blocks adjacent to the axis of a spreading ridge. Normal polarity block is shaded. The external magnetic field B due to an axial geocentric dipole is shown for three cases, (a) At the north pole the intensity of the field is increased over the ridge axis, (b) At the equator a north-south oriented ridge spreading east-west produces no magnetic anomaly (crosses and dots indicate north-south horizontal fields perpendicular to the page, in and out respectively, along the ridge), (c) At the equator, for an east-west oriented ridge spreading north-south, the intensity of the field is decreased over the ridge axis.

shown. The central block will be magnetized in the same direction (normal polarity) as the present geomagnetic field and the adjoining blocks on either side will be magnetized in the opposite direction (reverse polarity). During normal polarity time the magnetic anomaly is positive at the pole (thus reinforcing the field intensity as in Fig. 5.10a). At the equator the magnetic anomaly depends on the ridge orientation and for a north-south ridge spreading east-west there is no magnetic anomaly (Fig. 5.10b). However, for an east-west ridge spreading north-south the magnetic anomaly is negative (thus reducing the field intensity as in Fig. 5.10c).

It should be noted that certain distributions of magnetization (e.g., that illustrated in Fig. 5.10b) will not produce an external field. Such distributions are known as magnetic annihilators (Parker, 1977, 1994) and clearly demonstrate the non-uniqueness of magnetic inversion. Of particular interest here is that an infinite slab with uniform magnetization is such an annihilator. Thus any anomaly observed over a large slab with constant thickness must arise from changes or discontinuities in the magnetization. In the ocean floor there are discontinuities at each change in polarity and it is these discontinuities that give rise to the anomalies as shown in Fig. 5.10. When the magnetized blocks are sufficiently narrow, as is often the case in the ocean floor, then these anomalies can become superimposed.

The shape and intensity of the observed magnetic anomalies will depend on several factors, including latitude as described above, the direction of the profile in relation to the orientation of the magnetized blocks, and the spreading rate. The effect of each of these factors is illustrated in Fig. 5.11. Figure 5.11a shows a more detailed pattern of the latitude effect described in Fig. 5.10 for north-south profiles at various latitudes. Note that at intermediate latitudes the symmetry of the anomaly about the ridge crest becomes obscured as individual blocks are no longer associated with a single positive or negative anomaly. Figure 5.11b shows the variation of the magnetic anomaly pattern with the direction of the profile for a location where the magnetic inclination is 45°. The variation arises because only the component of the magnetization vector lying in the vertical plane through the magnetic profile affects the anomaly. This component is a maximum for east-west ridges with north-south profiles and a minimum for north-south ridges. The spreading rate affects the detail shown in the observed magnetic anomalies and the ability to detect short subchrons as illustrated in Fig. 5.1 lc for slow- and fast-spreading ridges.

Because of the variation in the observed magnetic anomalies at different places, they obviously cannot simply be stacked to improve the signal to noise ratio. Blakely and Cox (1972) overcame this problem by applying the geocentric axial dipole assumption (§1.2.3) and the anomalies were transformed to what they would be if the observation site were at the pole. This process is referred to as reduction to the pole. After reduction to the pole and adjustment for different

Fig. 5.11. The effects of latitude, profile orientation and spreading rate on the magnetic anomaly patterns. After Kearey and Vine (1996) and DeMets el al. (1994).

(a) Variation with geomagnetic latitude for north-south profiles. Angles refer to magnetic inclination.

(b) Variation with profile orientation at a fixed latitude, where the magnetic inclination is 45°.

(c) Variation with spreading rate. More detail can be obtained from the fast-spreading ridge.

»■ ■!■ iii i—i in mm hi ii ■« in i —i hi w ii

Fig. 5.11. The effects of latitude, profile orientation and spreading rate on the magnetic anomaly patterns. After Kearey and Vine (1996) and DeMets el al. (1994).

(a) Variation with geomagnetic latitude for north-south profiles. Angles refer to magnetic inclination.

(b) Variation with profile orientation at a fixed latitude, where the magnetic inclination is 45°.

(c) Variation with spreading rate. More detail can be obtained from the fast-spreading ridge.

spreading rates, magnetic anomalies may then be stacked so as to reduce the noise and enhance the signal.

The first applications of the Vine-Matthews crustal model to the interpretation of marine magnetic anomalies depended critically on the geomagnetic time scale available at the time. Improvements in the geomagnetic polarity time scale, especially the discovery of the Jaramillo event, led to improved analyses of the ridge anomalies. Four of these are shown in Fig. 5.12 from the analysis of Vine (1966) for four widely separated areas on the mid-ocean ridge system. In the first two profiles from the Juan de Fuca ridge and the East Pacific Rise, the profiles have been reversed for comparison. The remarkable symmetry displayed is noteworthy. Note that the three profiles from mid-latitudes have central positive anomalies, whereas the equatorial profile from the northwest Indian Ocean on the Carlsberg ridge has a central negative anomaly (c.f. Fig. 5.10). Spreading rates deduced varied from 15 to 44 km Myr"1 over the time interval 0-4 Ma, as far back as the land-based geomagnetic polarity time scale extended at that time.

Juan de Fuca Ridge Profile Reversed

46"N

East Pacific Rise Profile Reversed

Juan de Fuca Ridge Profile Reversed

500 nT

Model 29 km Myr'

500 nT

Model 29 km Myr'

East Pacific Rise Profile Reversed

100 km

Northwest Indian Ocean 5°N Observed Profile C

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