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 Mussett, A.E and M.A Khan, Looking into the Earth: An introduction to geological geophysics, 2000 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 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  ... 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... Paleomagnetic evidence and tectonic implications, Journal of Geophysical Research, 105 (B3), 577 1-5 781, 2000