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Figure 3.16. An infinitely long block of magnetized oceanic crust. The
magnetization of the block M is resolved into three components
( M x , M y and M z ). M y is the horizontal component parallel to the block
and so cannot contribute to the magnetic field outside the block.
M z (the vertical component) and M x (the horizontal component
perpendicular to the block) both contribute to the magnetic field
outside the block and hence to the resultant magnetic anomaly
produced by the block.
M x
x
M
M y
M z
z
y
component M x is complicated, but it usually produces an asymmetry in the mag-
netic anomalies. In the vicinity of the magnetic poles, where the Earth's magnetic
field is almost vertical, the effects of the x component of magnetization are
almost negligible; however, close to the magnetic equator the effect becomes
important.
Calculation of synthetic magnetic anomalies for any general block model
(e.g., Fig. 3.8(b))isperformed in the following way.
1. Calculate remanent magnetization in the x and z directions for each block by assuming
the orientation and latitude at which blocks were formed.
2. Calculate the field produced by these blocks along a line perpendicular to the blocks
and at a constant distance above them (i.e. sea-surface level).
3. Add this field to the Earth's magnetic field (e.g., IGRF 1995) at the block's present-day
latitude. This is the field that would be 'measured'.
4. Calculate the difference in magnitude between this 'measured' field and the Earth's
field. (For more details, see McKenzie and Sclater (1971).)
To visualize the effect of present-day latitude on the magnetic anomalies pro-
duced by a given block structure, consider a model oceanic crust formed by
east-west spreading of a mid-ocean ridge at 40 S. Figure 3.17 shows the mag-
netic anomalies that this magnetized oceanic crust would produce if it later moved
north from 40 S. The present-day latitude is clearly an important factor in the
shapes of the anomalies; note in particular that, if the plate moves across the
equator, positively magnetized blocks give rise to negative anomalies, and vice
versa. (This is a result of B r at today's latitude and M z having opposite signs.)
Magnetic anomalies are dependent on the orientation of the ridge at which the
crust was formed as well as the latitude. The orientation determines the relative
values of M x and M y .Figure 3.18 shows anomalies that would be observed at
15 N. The magnetized blocks were produced by ridges at 40 S, striking in the
three directions shown. The anomalies produced by the ridge striking N45 E and
the ridge striking N45 W are identical. This ambiguity in the strike of the ridge
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