Geology Reference
In-Depth Information
100
I
0
= +90
°
0
−
100
0
50
100
150
200
250
300
350
400
450
100
I
0
= +45°
0
0
50
100
150
200
250
300
350
400
450
−
100
100
I
0
=
−
45
°
0
0
50
100
150
200
250
300
350
400
450
−
100
100
I
0
=
−
90
°
0
−
100
0
50
100
150
200
250
300
350
400
450
1
2
2A
3
3A
4
4A
5
5A
5B
5C
5D 5E
6 6A
6B
6C
7
8
9
10
11
12
Fig. 5.12
Theoretical magnetic anomaly profiles, gener-
ated at a ridge with constant full spreading rate
v
D 30 mm
year
1
, originally located at 40
ı
N with a paleo-strike
'
orientation with respect to the geographic North). The
magnetic signal changes according to the present day
latitude of the prisms, as evidenced by the variation with
the inclination
I
0
of the ambient geomagnetic field
D 180
ı
. It is assumed that the present day strike of
the prisms is “
0
ı
(so that the ridge has not changed
D
100
0
0
−
100
50
100
150
200
250
300
350
400
450
1
2
2A
3
3A
4
4A
5
5A
5B
5C
5D 5E
6 6A
6B
6C
7
8
9
10
11
12
the prisms is “ D 35
ı
(so that the ridge has not changed
orientation with respect to the geographic North). The
ambient geomagnetic field inclination is
I
0
D 45
ı
Fig. 5.13
Theoretical magnetic anomaly profile, gener-
ated at a ridge with constant full spreading rate
v
D 30 mm
year
1
, originally located at 40
ı
N with a paleo-strike
'
135
ı
. It is assumed that the present day strike of
D
magnetic anomalies. The effect of ridge jumps
is to introduce gaps and duplications in the se-
quence of chrons. Figure
5.14
shows an example
of magnetization model distorted by the presence
of a ridge jump, along with the corresponding dis-
continuities in the age progression plot as a func-
tion of the distance from the ridge. The detection
of minor ridge jumps in a real magnetic profile
is not a simple task, especially when the profile
does not encompass both flanks of the spreading
axis.
Therefore, it is good practice to be cautious
when introducing a ridge jump in the block
model: very often a reinterpretation of the
anomalies allows to obtain anyway an acceptable
fit of the data.
