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Time-averaged paleomagnetic field at the equator: Complete data and results from the Galapagos Islands, Ecuador

Sherman Gromme·Edward A. Mankinen·Michel Prevot
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Abstract

We present here the complete paleomagnetic laboratory results from a collection of approximately 1500 oriented cores from all 16 of the Galapagos Islands, Ecuador, collected by Allan Cox in 1964–1965 but nearly all previously unpublished. The islands are located in the eastern Pacific Ocean within 1.4° of latitude from the equator and range in age from historically erupted to 3 Ma, mostly determined by published K-Ar and 3He isotopic dating. The number of sites collected on each island ranges from 1 to 28, for a total of 186. After combining duplicate site mean directions, 149 are used for an overall mean direction and 8 represent excursions and one reversal path. Divided by geomagnetic polarity chron, 110 site means are Brunhes or Jaramillo (normal polarity), 27 are Matuyama (reversed polarity), and 12 are Gauss (both polarities). We have completed the magnetic cleaning that was commenced in the late 1960s. Secondary (mostly viscous) magnetizations were nearly all removed by alternating field demagnetization at 10 mT. We have used the so-called blanket cleaning method, generally at 10 mT. All sites were in basalt flows and gave good paleomagnetic results; none was rejected in toto, and only a few core specimens were magnetically unsatisfactory. Nearly all sites had eight independently oriented cores, and within-site angular standard deviations of directions range from 1° to 8°. We used both Fisher and Bingham statistics to analyze the data and found that many of the direction populations are strongly elongate along the paleomagnetic meridian, while the corresponding virtual pole (VGP) populations are essentially circularly distributed. The paleomagnetic poles, calculated as the means of VGPs, are as follows: Brunhes and Jaramillo, north latitude = 86.9°, east longitude = 245.1°, and 95% confidence radius A 95 = 1.9°; Matuyama, latitude = 87.2°, longitude = 158.2°, and A 95 = 3.8°; Gauss, latitude = 83.0°, longitude = 204.7°, and A 95 = 7.0°. These paleomagnetic poles do not differ significantly from one another, but the Brunhes and Jaramillo combined pole is significantly near-sided with respect to the Galapagos, as is the overall mean pole, which is at latitude = 87.1° and longitude = 227.6°, with A 95 = 1.7°. Omitting the excursion and reversal path data, the overall angular standard deviation of VGPs is 11.7° with lower and upper 95% confidence limits of 10.8° and 12.7°, respectively, in good agreement with previously published values for near-equatorial sampling latitudes.

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Sherman Gromme, Edward A. Mankinen, Michel Prevot. 2010-11-18. Time-averaged paleomagnetic field at the equator: Complete data and results from the Galapagos Islands, Ecuador. https://doi.org/10.1029/2010gc003090

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