Secular variation and excursions of the Earth magnetic field during the Plio-Quaternary: New paleomagnetic data from radiometrically dated lava flows of the Colima volcanic complex (western Mexico)
Detailed rockmagnetic and paleomagnetic investigations were performed on selected lava flows from the Colima volcanic complex (CVC) located in the western sector of the Trans-Mexican Volcanic Belt. Reliable paleomagnetic directions were obtained from 21 Ar-Ar dated lava flows (205 standard paleomagnetic cores) within the age interval from 372 to 34 kyr. Ti-poor titanomagnetite is the main magnetic carrier in most of the samples, while some of them exhibit Curie temperatures ranging from 300°C to 340°C. Nineteen sites yielded normal polarity magnetization as expected from Brunhes Chron lavas, while two remaining lava flows show clearly defined transitional paleomagnetic directions. The mean direction, excluding the transitional data, is Dm=1.2°, Im=38.2° (k=39.5, α95=5.5°). The corresponding paleomagnetic pole is located at a longitude φ=270.1° and a latitude λ=87.6° (K=35.9, A95=5.3°). This paleodirection is practically indistinguishable from the one expected for Pleistocene, as derived from the references poles for the stable North America. This suggests that no major tectonic rotation occurred in the studied area. The paleosecular variation is analysed through the scatter of virtual geomagnetic poles (VGP). The dispersion estimated through the parameter SF=13.0°, with an upper confidence limit SU=15.2° and a lower confidence limit SL=12.7° is consistent with the recent geomagnetic field models for the last 5 Myr. The present study yields the evidence of two "transitional" lava flows, whose age likely correspond to the Mono Lake or Laschamp excursion and to the Calabrian Ridge I or Portuguese Margin excursion, respectively.
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