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

Abstract

[1] 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cores from all 16 of the Galapagos Islands, Ecuador, collected by Allan Cox in 1964-1965 but nearly all previously unpublished.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 3 He 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.

Weaker axially dipolar time-averaged paleomagnetic field based on multidomain-corrected paleointensities from Galapagos lavas

“…2D) Islands that formed over the past ∼3 My (13), during which the Nazca plate moved east-southeast relative to the presumed Galapagos hot spot. The dispersed lava sites on different islands ensure a random temporal sampling over this time period (14,15). Previous alternating field (AF) (16) and thermal demagnetization (TD) (13) studies showed shallow average paleomagnetic directions (AF mean I = 1.9°± 3.0°; TD mean I = 2.3°± 3.0°) from 51 lava sites, which agree with the GAD hypothesis.…”

Latitude Dependence of Geomagnetic Paleosecular Variation and its Relation to the Frequency of Magnetic Reversals: Observations From the Cretaceous and Jurassic

“…The equatorial intercept ( a ) of the model curve for the Johnson et al () data (Figure a) is essentially controlled by the two low‐latitude PSV estimates from Costa Rica ( S b = 14 ° , Cromwell et al, ; Johnson et al, ) and Ecuador ( S b = 17.2 ° , Opdyke et al, ). Data subsequently collected from other localities within 10° of the equator (Gromme et al, ; Kent et al, ; Opdyke et al, , ) showed somewhat lower values of VGP dispersion ( S b < 13 ° , Figure b), which led Opdyke et al () to suggest that the results from Costa Rica and Ecuador may have been affected by unrecognized tectonic disturbances (tilting) in these actively deforming regions, resulting in artificially higher dispersion than expected on the basis of the results from tectonically quiescent low‐latitude sites. Because Opdyke et al () and Cromwell et al () included the Johnson et al () data in their compilations, and because their results suggest very similar patterns for the latitude behavior of PSV, we consider that the work of Opdyke et al () and Cromwell et al () supersedes the results of Johnson et al ().…”

“…The equatorial intercept (a) of the model curve for the Johnson et al (2008) data (Figure 14a) is essentially controlled by the two low-latitude PSV estimates from Costa Rica (S b = 14°, Cromwell et al, 2013;Johnson et al, 2008) and Ecuador (S b = 17.2°, Opdyke et al, 2006). Data subsequently collected from other localities within 10°of the equator (Gromme et al, 2010;Kent et al, 2010;Opdyke et al, 2010Opdyke et al, , 2015 showed somewhat lower values of VGP dispersion (S b < 13°, Figure 14b), which led Opdyke et al (2015) to suggest that the results from Costa Rica and Ecuador may have been affected by unrecognized tectonic disturbances (tilting) in these actively deforming regions, resulting in artificially higher dispersion than expected on the basis…”

Illustrated catalogue of phytoliths from modern plants of the Galápagos Islands: Economic species of San Cristóbal Island

“…The Galápagos archipelago comprises approximately 128 named islands and islets formed about three million years ago. The age of the islands increases moving from west to east because of the drift of the Nazca tectonic plate away from the East Pacific Ridge to the southeast over a hotspot [6][7][8]. This study was located in the southeastern highlands of San Cristóbal, the easternmost and one of the oldest islands in the Galápagos.…”