VẬT lý địa CHẤN paleomagnetism

Paleomagnetism
The measurement of remnant magnetism can provide information important
information about where a rock may have come from.
Measuring a paleomagnetic direction:
•An individual lava flow may not record an “average” pole (secular variation), so
samples from a series of flows may be taken
•Oriented (azimuth and dip) rock cores separated by up to a few meters are
drilled (using non-magnetic equipment).
•If the rock has been tilted since its formation, this has to be measured.
•The magnetization direction is measured (by measuring all three axis of the
core) using a very sensitive magnetometer.
•The direction, which is relative to the cylinder is calculated with respect to
north and the vertical.
•The magnetization direction is plotted on a stereonet.


•

Magnetic inclination varies
from vertical in the center to
horizontal at the circumference.

•

Declination is the angle around
the circle clockwise from north.

•

Downward magnetizations
(positive inclination) are plotted

as open circle. Negative
magnetizations are plotted as
solid circles.

•

Plot mean direction and 95%
confidence interval (95%
probability of containing the
true direction).

From Mussett and Khan, 2000

Paleomagnetism


Paleomagnetism

•If the apparent pole is not at the north
geographic pole, the rock must have moved
(assuming secular variations have been
averaged out).
•If the declination is not due north, the rock has
rotated about its vertical axis.
•We cannot tell if the rock has changed its
longitude.
•Uses??
•Climate change

From Taylor et al., 2000

•Now that we now the inclination of the magnetic
field in the rock sample we can calculate its
paleolatitude, and from that the relative location of
the pole at that time.


Apparent Polar Wander

From Mussett and Khan, 2000

•If a landmass has moved north or south over geological time, the paleopoles of
rocks of successive ages will change and trace out an apparent polar wonder
(APW) path. The poles have not moved – but the continent has.

•The APW for North America and Europe are different, as the two have moved
relative to each other.


Magnetostratigraphy

•At even smaller scales we can examine secular
variation within a series of lava flow (assuming a high
resolution series of flows).
•If these flows are historic, we could probably
date them.
•If they are very old, we could use the pattern of
secular variation to correlate between outcrops.
•Archeological applications – dating ancient
fireplaces.

•The resultant magnetic timescale can be used to
date sediments and the seafloor by the recognition of
distinctive reversal patterns.

From Mussett and Khan, 2000

•By measuring the polarity of magnetization of a rock
of know age (radiometric data, sediment on ocean
floor above basement) we can build up a magnetic
polarity timescale.


From Robb et al., 2004

Modeling Seafloor Spreading


From Robb et al., 2004

Modeling Seafloor Spreading


From Robb et al., 2004

Modeling Seafloor Spreading


References Used
1. Mussett, A.E. and M.A. Khan, Looking into the Earth: An introduction to
geological geophysics, 2000.

2. Robb, M., B. Taylor, and A.M. Goodliffe, Re0examination of the magnetic
lineations of the Gascoyne and Cuiver abyssal plains, NW Australia,
submitted to GJI, 2004.
3. Taylor, G.K., J. Gascoyne, and H. Colley, Rapid rotation of Fiji:
Paleomagnetic evidence and tectonic implications, Journal of Geophysical
Research, 105 (B3), 5771-5781, 2000.