Geology Reference
In-Depth Information
Therefore, in the calculation of the field com-
ponents through (
5.49
)and(
5.50
), it is not neces-
sary to know the corresponding sequence of pa-
leopoles (
p
0
1
,
p
0
2
, :::,
p
0
n
) for the conjugate plate.
A conjugate pair of magnetized prisms on the
two sides of a spreading ridge has a unique
inclination
,
I
,
and a unique paleostrike
'.
The expected magnetic anomaly profile
is built calculating, for a series of equally
spaced locations
r
along the projection line, the
contribution,
F
k
(
r
), to the total crustal field
from each magnetized prism included in the
model. The individual contribution of a block
is determined through expressions (
5.49
)and
(
5.50
). Then, a vector summation of these terms
will give the total anomalous field associated
with the magnetization model. Finally, the local
anomaly is computed by projecting the total field
F
(
r
) onto the reference field axis (Eq.
5.3
). This
procedure is repeated for each point
r
along the
projection line. The resulting magnetic anomaly
profile can be compared with the observed data
to evaluate if the assumed velocity is appropriate.
In general, the forward modelling of marine
magnetic anomalies requires successive adjust-
ments of the spreading velocity function
v
D
v
(
T
), and eventually of the magnetization inten-
sity
M
D
M
(
T
), until an acceptable visual match
between model and observed anomalies is ob-
tained. In this trial and error procedure, the inves-
tigator first identifies the characteristic wiggles
associated with the major anomalies (e.g., 2, 2A,
3, 4, 5, :::) on the observed profile, then he/she
tries to change the spreading rate
v
of groups
of chrons to improve the match. According to
(
5.60
), a change of the spreading velocity
v
k
during the
k
-th chron determines a variation of the
horizontal width
w
k
of the corresponding block,
because the time interval
T
k
is fixed by the
geomagnetic polarity time scale. A good rule of
thumb is to match a well-known anomaly close
to the profile end through an average constant
velocity
v
. Then, we select an “easy” anomaly
within the sequence and try to match the corre-
sponding wiggle changing the average velocity
of the lower half to some value
v
0
. This oper-
ation will require an adjustment of the veloc-
ity of the upper half sequence to a new value
v
00
. Such “divide-et-impera” algorithm can be
repeated iteratively until we obtain a satisfactory
fit of the major anomalies. In general, it is not
recommended to use different velocities for the
sub-chrons of a major chron, because the char-
acteristic shape of the corresponding anomaly
depends precisely upon the relative duration of
the various sub-chrons. Therefore, changing arbi-
trarily the width of the blocks in the model could
lead to a misinterpretation of the anomalies. It is
important to note that the magnetization models
resulting from procedures of forward or inverse
modelling are never unique, because there are
infinitely many block models that generate the
same magnetic signal. When interpreting marine
magnetic anomaly profiles, it is necessary to
take into account that the shape of the major
anomalies mainly depends from the following
factors:
1. The bathymetric profile, which determines the
geometry of magnetized prisms;
2. The presence of sea mounts and other volcanic
features;
3. The present day latitude of the prisms,
4. The paleostrike, ', of the magnetized blocks;
5. The paleolatitude of the blocks, which deter-
mines their inclination;
6. The present day strike, “, of the magnetized
prisms;
7. The profile obliquity §;
8. The presence of ridge jumps
For example, Fig.
5.12
illustrates the shape of
the magnetic anomalies 1-12 along a N-S profile
at various latitudes (hence, for different values
of the reference field inclination
I
0
). We note
that the profiles that would be observed in the
southern hemisphere are specular with respect
to those observed in the northern hemisphere.
The effect of another important factor influencing
the shape of the magnetic anomalies, which is
the paleo-strike of the spreading ridge, is illus-
trated in Fig.
5.13
. While the inclination of the
reference field essentially modifies the
ampli-
tude
of the anomalies (within the same hemi-
sphere), the paleo-strike, ', of the magnetized
blocks has a strong effect on the
shape
. Ridge
jumps and strong spreading asymmetry are other
sources of complexity in the analysis of marine
