Paleomagnetism of Miocene volcanics on Sao Tome: Paleosecular variation at the Equator and a comparison to its latitudinal dependence over the last 5 Myr

A collection was made in January 2009 of 10 oriented samples from each of 54 sites in lavas on Sao Tome Island (nominal location 0.3ºN, 6.5ºE). Some sites were affected by lightning leaving a total of 42 sites for analysis of paleosecular variation. Overall magnetic properties were excellent (highly stable magnetizations carried by pseudosingle domain magnetite). After principal component analysis of progressive alternating field demagnetization data for the samples, 22 sites had normal polarity magnetizations (D = 0. 6º, I = -8.3º, α₉₅ = 4.3º, κ = 53.1) and 20 had reverse magnetizations (D = 176.0º, I = 4.2º, α₉₅ = 7.3º, κ = 20.8); the directions are within 5° of antiparallel, yielding a positive reversal test. The combined data set of 42 site mean virtual geomagnetic poles converted to common (normal) polarity yields a pole position at 86.0°N, 211.5ºE, A₉₅=3.1º. Ar/Ar and K/Ar dating reveals that these rocks are Miocene in age (∼5-11 Ma), old enough to allow northerly plate motion to help explain the slightly far-sided pole position. The between-site dispersion in virtual geomagnetic poles was estimated as the angular standard deviation, S_b, and equaled 11.4° with 95% confidence interval between 9.9° and 13.4°.