VẬT lý địa CHẤN paleomagnetism

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

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)

•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

•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

Apparent Polar Wander

•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

•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

•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

Modeling Seafloor Spreading

Modeling Seafloor Spreading

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.