HR: 13:40h
AN: GP33B-01    [Abstracts]
TI: Paleomagnetic Poles From the Skewness of Marine Magnetic Anomalies
AU: Horner-Johnson, B C
EM: benjh@pdx.edu
AF: Portland State University, Geology, Portland, OR 97207 United States
AU: * Gordon, R G
EM: rgg@rice.edu
AF: Rice University, Earth Science--MS 126 6100 Main St., Houston, TX 77005 United States
AB: The skewness of marine magnetic anomalies due to seafloor spreading depends strongly on the direction of the paleomagnetic field as oceanic lithosphere is created and cooled. Thus, anomaly skewness can be used to estimate paleoomagnetic poles and is attractive, in part, because the ages of the poles can easily be related to the geomagnetic polarity time scale. Here we review methods for determining paleomagnetic poles from anomaly skewness and examine the accuracy of the poles. A concern has been the influence of ``anomalous" skewness, recognized in pioneering studies of anomaly skewness (Cande 1976). Anomalous skewness can be thought of as a systematic difference between observed skewness and that expected from simple models of the marine magnetic source that assume vertical boundaries between reversals. Anomalous skewness is now understood to be the consequence of non-vertical curving reversal boundaries expected from the thermal and magmatic evolution of oceanic lithosphere (Dyment & Arkani-Hamed 1995). Anomalous skewness depends on spreading rate if spreading is slow, but is independent of spreading rate if spreading is fast (Dyment & Arkani-Hamed 1995). The profiles we examine for paleomagnetic analysis dominantly record fast spreading and thus we are able to approximate anomalous skewness as being independent of spreading rate. We can therefore treat anomalous skewness as a third adjustable parameter, along with pole latitude and pole longitude, when determining paleomagnetic poles from skewness estimates. At any one crossing of a magnetic anomaly, local effects could cause the estimate of the paleomagnetic direction to be in error. Aside from anomalous skewness, however, errors in skewness estimates are expected to be uncorrelated from site to site except at exceptionally closely spaced sites. Thus we can treat misfits as independent random errors and reduce the statistical error in the pole position by obtaining many estimates of skewness from widely separated sites. The plate geometry of the Pacific plate throughout much of Late Cretaceous and Tertiary time, in particular the long north-south-striking paleo-East Pacific Rise, provides a nearly ideal geometry for recording critical information on the location of Pacific plate paleomagnetic poles. Moreover, a large skewness gradient with paleolatitude is expected, which helps to limit the location and, to a lesser degree, the strike of the paleoequator and thus strongly constrains the pole. If the accuracy of all skewness estimates is uniform, estimates of skewness near the paleo-equator can be shown to contain much more information than those from higher paleo-latitudes. On total-intensity magnetic profiles the amplitudes of anomalies near the paleo-equator are very low and likely have higher than average uncertainties. On vector aeromagnetic profiles, however, clear anomalies are recorded near the paleo-equator and provide strong constraints on the pole position. The assumptions made in determining paleomagnetic poles from skewness are subject to several critical tests including comparisons of anomalous skewness estimated from the single-plate method with those determined from cross-ridge analysis, comparisons with other skewness poles of similar age, and comparisons with poles determined from other types of paleomagnetic data. We present several such tests and show that the skewness results pass all these tests of consistency. Thus, there are many reasons to believe that poles determined from skewness give unbiased and accurate paleomagnetic poles.
DE: 1517 Magnetic anomalies: modeling and interpretation
DE: 1525 Paleomagnetism applied to tectonics: regional, global
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005