Evidence for widespread Remagnetizations in South America, case study of the Itararé Group rocks of Brazil

Abstract

Paleomagnetism of South American Jurassic/Cretaceous rocks has been troubled by elongated distributions of poles which has led to contrasting interpretations. Moreover, many discordant paleomagnetic poles from the Carboniferous to the Triassic have also been recognized and systematically explained by a variety of processes, but this portion of the South American apparent polar wonder path (APWP) still remains problematic. We have conducted a paleomagnetic study of the sedimentary Permo-Carboniferous Itararé Group rocks and three intruding mafic sills of likely Cretaceous age within the state of São Paulo, Brazil. The site-mean VGP distributions obtained from the sedimentary rocks define elongations that include the VGPs of the mafic intrusions. We interpret these distributions as remagnetization paths toward the directions characteristic of the sills. Careful analysis of the paleomagnetic data of the Itararé sedimentary rocks enables isolation of a primary VGP distribution that is consistent with the reference Carboniferous pole position. The paleomagnetic directions of the sills are partially overprinted by the present time averaged and current Earth’s magnetic field. Combined rock- and paleomagnetic data suggest that interacting SD grains carry a very recent magnetic overprint that is visco-chemical in origin and cannot be fully erased. The dominant distribution of PSD-MD grains carries the high-temperature component, which is either a primary magnetization coincidentally close to the time averaged dipole field direction, or a secondary thermo-viscous magnetization. Extending our study to other Carboniferous to Triassic South American paleomagnetic records reveals that the majority of these data are elongated, similarly to the Itararé Group rocks. Regardless of the age of the rocks, the elongations systematically intersect at the location of the Late Cretaceous reference pole, and at a long- recognized problematic location (“X”) observed in certain Jurassic and Cretaceous rock formations. We interpret the elongated VGP distributions to reflect remagnetizations from the primary VGP positions toward Jurassic-Cretaceous and “X” pole locations, which occurred as a result of the widespread magmatic events associated with the opening of the South Atlantic. The extent of the remagnetizations is formation-specific and other rock-formations should be carefully re-evaluated.