Geology Reference
In-Depth Information
Fig. 4.17
Magnetic field intensity anomaly,
F
, caused by the crustal magnetic field at a satellite altitude of 400 km
above the Earth's surface, as given by the MF3 model (Maus et al.
2006
)
on the basis of POGO (Polar Orbiting Geophysi-
cal Observatory) and MAGSAT (Magnetic Field
Satellite) low-orbiting satellite measurements of
magnetic intensity since the 1960s (Maus et al.
2006
and references therein). The first data were
scalar total field magnitudes, whereas starting
from MAGSAT the satellites were also equipped
with vector magnetometers. One of the most
recent data sets comes from the CHAMP (CHAl-
lenging Minisatellite Payload) mission, a German
satellite mission for geologic and atmospheric
research that started in July 2000. An example
of crustal field model based on these data is
showninFig.
4.17
. This model provides long
wavelength features of the crustal component of
the internal field, which can be useful in studies
of global scale tectonics.
Maps of crustal magnetic field on continents
can be used in conjunction with gravity and
geological maps to identify tectonic provinces,
dikes, faults, and any other geologic feature hav-
ing a magnetization contrast with the surround-
ing. In recent years, the World Digital Magnetic
Anomaly.
Map (WDMAM) has been compiled under the
auspices of the International Association for Ge-
omagnetism and Aeronomy (IAGA). This map,
which is shown in Fig.
4.18
combines ground,
airborne, and marine magnetic data and includes
all the wavelengths that could be useful for the
geological and tectonic mapping of the Earth's
crust. We have already stressed the fact that
ocean floor magnetization represents a primary
source of data in plate tectonics. In Sect.
1.3
we
have shown that the oceanic crust has a layered
uppermost of the igneous layers. These subma-
rine basalts cool quickly from high temperatures
in the seawater environment. Consequently, their
titanomagnetite content is fine-grained and car-
ries an intense TRM. This mineral has an average
composition Fe
2.4
Ti
0.6
O
4
and Curie point in the
range 150-200
ı
C (McElhinny and McFadden
2000
). We know that the geothermal gradient
close to mid-ocean ridges is
100
ı
C/km, while
for a 120 Ma old oceanic crust it is reduced
to the Curie isotherm is very shallow near the
spreading centers and increases with the distance
from the ridge. Regarding the lower layers (2B,
2C, and 3), they cool slowly and have Curie
points generally higher than 500
ı
C, but the
magnetization is at least one order of magnitude
less than the overlying pillow lavas. It is also
