Paleosecular variation record of geomagnetic full vector during late Miocene, from the Nayarit area, Mexico

Details Paleosecular variation record of geomagnetic full vector during late Miocene, from the Nayarit area, Mexico Paleosecular variation record of geomagnetic full vector during late Miocene, from the Nayarit area, Mexico

Nov 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002pepi..134...71g&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 134, Issue 1-2, p. 71-88.

Physics

3

Scientific paper

We have sampled a sequence of 45 late Miocene consecutive lava flows in the Tepic area (Nayarit State, Mexico). The age of the volcanic units lies between 8 and 9 million years (Ma) according to available radiometric data. All samples were stepwise demagnetized, partly with alternating field (AF), partly thermally with very similar results. Most of the rocks exhibited well-defined one component remanent magnetisation with high unblocking temperatures (mostly above 525°C) and high median destructive fields (MDF) (40-50mT). Rock-magnetic experiments combined with microscopy show that, in most cases, the main magnetic mineral is Ti-poor titanomagnetite associated with exsoluted ilmenite. Continuous susceptibility measurements with temperature and hysteresis experiments yield in most cases nearly reversible curves with Curie points close to that of magnetite and pseudo-single-domain characteristics. Characteristic remanent magnetisations (ChRM) isolated after the first steps of demagnetisation are all normal polarity. According to the dispersion of virtual geomagnetic pole (VGP) directions, paleosecular variation was abnormally lower than the one observed in general during Miocene. Considering our paleomagnetic results together with available radiometric data, it seems that the volcanic units have been emplaced during a very short time span of about 0.08 million years. The mean paleomagnetic directions obtained from this study do not differ significantly from that expected for the middle Miocene. Thirty-one samples from eight individual flows yielded acceptable paleointensity estimates. The site mean paleointensities range from /27.8+/-0.9 to /42.0+/-7.9μT. The virtual dipole moments (VDM) range from 5.9 to 9.5×1022Am2. This corresponds to a mean value of 7.6+/-1.4×1022Am2, which is higher than the average VDM value for late Miocene. Altogether our data suggest the existence of relatively high geomagnetic field strength undergoing low fluctuations. These results support the theoretical suggestion about an inverse relationship between secular variation and local field strength as result of electromagnetic coupling between the solid inner core and liquid outer core, with the inner core tending to stabilise core convection, and hence the field, when intensity is high. Some fluctuation of absolute intensity was detected within the same directional group (DG) indicating that the intensity of the geomagnetic field varies faster than its direction.

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