Invited article The use and misuse of magnetic methods to monitor environmental pollution in urban areas Uso y abuso de métodos magnéticos en el monitoreo de contaminación ambiental en áreas urbanas
We report detailed rock-magnetic and paleomagnetic directional data from 35 lava flows (302 standard paleomagnetic cores) sampled in the Central-Northern region of Uruguay in order to contribute to the study of the paleosecular variation of the Earth's magnetic field during early Cretaceous and to obtain precise Cretaceous paleomagnetic pole positions for stable South America. The average unit direction is rather precisely determined from 29 out of 35 sites. All A 95 confidence angles are less than 8°, which points to small within-site dispersion and high directional stability. Normal polarity magnetizations are revealed for 19 sites and 10 are reversely magnetized. Two other sites yield well defined intermediate polarities. The mean direction, supported by a positive reversal test is in reasonably good agreement with the expected paleodirection for Early Cretaceous stable South America and in disagreement with a 10° clockwise rotation found in the previous studies. On the other hand, paleomagnetic poles are significantly different from the pole position suggested by hotspot reconstructions, which may be due to true polar wander or the hotspot motion. Our data suggest a different style of secular variation during (and just before) the Cretaceous Normal Superchron and the last 5 Ma, supporting a link between paleosecular variation and reversal frequency. K e y wo r d s : paleosecular variation, Cretaceous paleomagnetic poles, Paraná Magmatic Province, Uruguay M.C. Solano et al. 534 Stud. Geophys. Geod., 54 (2010)
The Early Cretaceous may be considered a key period for understanding the evolution of the Earth's magnetic field. Some still unsolved problems are related to the mode of paleosecular variation (PSV) of the Earth's magnetic field before and during the Cretaceous Normal Superchron. We report here a detailed rock-magnetic, paleomagnetic and paleointensity investigation from 28 lava flows (331 standard paleomagnetic cores) collected in the Argentinean part of the Parana Flood Basalts (Formation Posadas) in order to contribute to the study of PSV during the early Cretaceous and to obtain precise Cretaceous paleomagnetic pole positions for stable South America. The average paleofield direction is precisely determined from 26 sites, which show small within-site dispersion and high directional stability. Five sites show evidences for the self-reversal of thermoremanent magnetization. 23 sites yielded normal polarity magnetization and only 3 are reversely magnetized. Moving windows averages were used to analyze the sequential variation of virtual geomagnetic pole's (VGP) axial positions. Interestingly, the axial average VGP path traces an almost complete cycle around the geographical pole and passes near the location of all previously published Paraná Magmatic Province poles. Both paleomagnetic poles and average VGP paths are significantly different from the pole position suggested by fixed hotspot reconstructions, which may be due to true polar wander or the hotspot motion itself. Only 15 samples from 5 individual basaltic lava flows, yielded acceptable paleointensity estimates. The site mean paleointensities range from 25.2 2.2 to 44.0 2.2 T. The virtual dipole moments (VDMs) range from 4.8 to 9.9 10 22 Am 2 . This correspond to a mean value of 7.7 2.1 10 22 Am 2 which is 96% of the present day geomagnetic field strength. These intensities agree with the relatively high values already reported for Early Cretaceous, which are consistent with some inferences from computer simulations previously published. Continuation of LATINMAG Special Issue #4, Stud. Geophys. Geod., 54 (2010) M. Mena et al. 280 Stud. Geophys. Geod., 55 (2011) K e y w o r d s :
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