New absolute paleointensity results from the Parana Magmatic Province (Uruguay) and the Early Cretaceous geomagnetic paleofield

Abstract

Thellier paleointensity experiments are carried out on the Early Cretaceous Arapey Formation, which belongs to the Parana Magmatic Province (PMP) in northern Uruguay. Twenty‐seven samples out of 225 analyzed yield reliable absolute intensity determinations. For these samples the NRM fraction f used for determination ranges between 0.33 to 0.83 and the quality factor q ranges from 4.5 to 14.1, being generally greater than 5. The flow‐mean paleointensity values obtained in this study range from 16.3 ± 2.3 to 41.5 ± 3.8 μT, with corresponding virtual dipole moments (VDMs) from 3.4 ± 0.6 to 8.2 ± 0.8 (1022 Am2). This corresponds to mean VDM value of 6.4 ± 2.3 × 1022 Am2. 40Ar/39Ar dating on the PMP sequence by Feraud et al. (1999) gave a mean age of 132.2 ± 0.5 Ma. Our results in conjunction with previous studies (Kosterov et al., 1998; Goguitchaichvili et al., 2002) suggest that the paleostrength during the early Cretaceous might be as variable and strong as Brunhes data and not anomalously low as previously suggested. The scatter observed in our study is similar to recent findings by Granot et al. (2007) for the Troodos Ophiolite.

Paleosecular variation and absolute geomagnetic paleointensity records retrieved from the Early Cretaceous Posadas Formation (Misiones, Argentina)

“…Globally, early Cretaceous paleointensities appear similar to the Brunhes data. This reinforces some previous suggestions (Goguitchaichvili et al, 2002(Goguitchaichvili et al, , 2008 about the invalidity of the Mezozoic Dipole Low hypothesis (Prévot et al, 1990). High geomagnetic intensities have been reported (Tarduno et al, 2001(Tarduno et al, , 2002 for Early Cretaceous, which are consistent with some inferences from computer simulations (Glatzmaier et al, 1999).…”

“…The Posadas Formation mean paleointensity agrees reasonably well with values from the coeval Arapey and Serra Geral Formations (Goguitchaichvili et al, 2008) while the slightly older Ponta Grossa dikes yielded relatively lower geomagnetic field strength. This suggests high geomagnetic intensities for the Early Cretaceous reinforcing some previous suggestions (Goguitchaichvili et al, 2002(Goguitchaichvili et al, , 2008 about the invalidity of the Mezozoic Dipole Low hypothesis (Prévot et al, 1990).…”

“…This corresponds to a mean value of 7.7  2.1  10 22 Am 2 , almost 96% of the present day geomagnetic field strength. The mean paleointensities value obtained from the Posadas Formation agree reasonably well, within the uncertainties, with those retrieved from the coeval Arapey (Uruguay) and Serra Geral (Southern Brazil) Formations (Goguitchaichvili et al, 2008). The slightly older Ponta Grossa dikes however, yielded relatively low geomagnetic field strength.…”

Paleomagnetism of early cretaceous arapey formation (Northern Uruguay)

“…In this section we will only recall some main results of our rock-magnetic experiments since these samples were already partly investigated by Goguitchaichvili et al (2008). The susceptibility vs. temperature experiments indicates, in most cases, the presence of Ti-poor titanomagnetites.…”

“…In this study, we present new detailed rock-magnetic and paleomagnetic directional data from 35 lavas (302 standard paleomagnetic cores) sampled in the Central-Northern region of Uruguay in order to contribute to both paleosecular variation of Earth's magnetic field during early Cretaceous and paleomagnetic pole positions for the stable south America. This work comes to complete the reconnaissance paper (first sample collection campaign in Uruguay in 2005) of Goguitchaichvili et al (2008). These authors reported five absolute paleointensity results and no directional data.…”