Geology Reference
In-Depth Information
180
°
140
°
W
100
°
W
60
°
N
North America
n
40
°
N
40
°
N
.
Pacific Ocean
0
2000 km
Fig. 7.30
Pattern of linear magnetic
anomalies and major fracture zones
in the northeast Pacific Ocean.
20
°
N
20
°
N
180
°
140
°
W
(Kearey &Vine 1996) and on views of the formation of
oceanic lithosphere. Early magnetic surveying at sea
showed that the oceanic crust is characterized by a pat-
tern of linear magnetic anomalies (Fig. 7.30) attribut-
able to strips of oceanic crust alternately magnetized in a
normal and reverse direction (Mason & Raff 1961).The
bilateral symmetry of these linear magnetic anomalies
about oceanic ridges and rises (Vine & Matthews 1963)
led directly to the theory of sea floor spreading and
the establishment of a time scale for polarity transitions
of the geomagnetic field (Heirtzler
et al
. 1968).
Consequently, oceanic crust can be dated on the basis of
the pattern of magnetic polarity transitions preserved
in it.
Transform faults disrupt the pattern of linear mag-
netic anomalies (see Fig. 7.30) and their distribution can
therefore be mapped magnetically. Since these faults lie
along arcs of small circles to the prevailing pole of rota-
tion at the time of transform fault movement, individual
regimes of spreading during the evolution of an ocean
basin can be identified by detailed magnetic surveying.
Such studies have been carried out in all the major
oceans and show the evolution of an ocean basin to be a
complex process involving several discrete phases of
spreading, each with a distinct pole of rotation.
Magnetic surveying is a very useful aid to geological
mapping. Over extensive regions with a thick sedi-
mentary cover, structural features may be revealed if
magnetic horizons such as ferruginous sandstones and
shales, tuffs and lava flows are present within the sedi-
mentary sequence. In the absence of magnetic sedi-
ments, magnetic survey data can provide information on
the nature and form of the crystalline basement. Both
cases are applicable to petroleum exploration in the
location of structural traps within sediments or features
of basement topography which might influence the
overlying sedimentary sequence.The magnetic method
may also be used to assist a programme of reconnaissance
geological mapping based on widely-spaced grid sam-
ples, since aeromagnetic anomalies can be employed to
delineate geological boundaries between sampling
points.
Problems
1.
Discuss the advantages and disadvantages of
aeromagnetic surveying.
2.
How and why do the methods of reduction of
gravity and magnetic data differ?
3.
Compare and contrast the techniques of inter-
pretation of gravity and magnetic anomalies.
4.
Assuming the magnetic moment of the Earth
is 8
¥
10
22
Am
2
, its radius 6370 km and that its
Continued
