Pacific Plate Apparent Polar Wander from the Skewnes of Marine Magnetic Anomalies
Channel: Paul Wessel
Date: 2020-02-05

Dr. Richard Gordon (Rice University) gave a guest lecture via Skype in Spring 2012 for a graduate class on Advanced Plate Tectonics in SOEST, University of Hawaii at Manoa.  The students recorded the lecture and produced video podcasts of the event.  This is one of 8 such lectures from the course which was taught by Prof. Paul Wessel.

ABSTRACT:

"The asymmetry or skewness of marine magnetic anomalies due to seafloor spreading contains information about the location of the paleomagnetic pole. The amount of asymmetry or skewness of the magnetic anomalies is quantified by its phase, θ, which is the inverse tangent of the ratio of the antisymmetric part of the anomaly to the symmetric part of the anomaly. Knowing the structure of the source of marine magnetic anomalies and the value of θ constrains the paleomagnetic pole to lie along a great semi-circle if the strike of the magnetic lineations is exactly known. For total intensity magnetic anomaly profiles it is also necessary to know the inclination and declination of the main magnetic field along the anomaly profile. Obtaining estimates of θ at different locations of crossings of the same anomaly can be used to determine a unique paleomagnetic pole.
Paleomagnetic skewness analysis has presented several challenges: (1) Developing statistically appropriate methods for determining the best estimate of paleomagnetic poles and their uncertainties.
(2) Determining accurate poles when diurnal variation and, especially, the electrojet are significant sources of noise. (3) How best to account for so-called “anomalous” skewness and its spreading-rate dependence.
At the same time such analysis has presented some excellent opportunities: (1) Paleomagnetic poles determined from the analysis of a single anomaly or short sequence of anomalies have well-determined ages and can have compact confidence limits. (2) The geometry of Pacific-Farallon spreading is fortuitously favorable for obtaining excellent constraints on the location of the paleomagnetic pole.
(3) Low paleolatitude Pacific-Farallon anomaly crossings are especially information rich. (4) Vector aeromagnetic data are available and have superior signal to noise characteristics in low latitudes relative to shipboard data. (4) Poles from skewness data are in excellent agreement with paleomagnetic data from azimuthally unoriented cores of igneous rock recovered from deep sea drilling.
Skewness data have been providing useful paleomagnetic information for the Pacific plate for more than three decades. Poles from skewness data have shown that the Hawaiian hotspot has shifted southward relative to the spin axis since early Tertiary and Late Cretaceous time. Together with other paleomagnetic data, they show that the global plate motion circuit through Antarctica fails the paleomagnetic test, most likely indicating unmodeled motion between East and West Antarctica. Poles from skewness data indicate consistency of the apparent polar wander path of Pacific hotspots with that of the Indo-Atlantic hotspots for the past 65 Ma or longer. Both paths move in apparent unison presumably recording true polar wander."