Evidence for Widespread Remagnetizations in South America, Case Study of the Itararé Group Rocks From the State of São Paulo, 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. To this end, 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. Furthermore, we show that the remanence directions of the sills are partially overprinted by the present time-averaged and current Earth's magnetic field. Our interpretations are supported by extensive rock-magnetic data that provides a viable mechanism for the secondary magnetizations. The current study focuses on the paleomagnetism and rock-magnetism of the mafic intrusions and how they affected the paleomagnetic recording of the sedimentary rocks, however, 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. 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 observed in some Jurassic and Cretaceous rock formations. Based on multiple lines of evidence, we interpret the elongations to reflect remagnetizations from the primary VGP positions toward Jurassic-Cretaceous pole locations that occurred as a result of the widespread magmatic events associated with the opening of the South Atlantic. We suggest that the extent of the remagnetizations is formation-specific, and that other rock-formations should be carefully re-evaluated.

Comment on “New Late Pennsylvanian Paleomagnetic Results From Paraná Basin (Southern Brazil): Is the Recent Giant Gaussian Process Model Valid for the Kiaman Superchron?” by Brandt et al.

“…Journal of Geophysical Research: Solid Earth BILARDELLO Ernesto et al (1999) across the state of Sao Paulo (see Bilardello et al, 2018). Moreover, the mean intersection is close to that obtained from a compilation of South American Carboniferous to Triassic formation-derived VGP distributions (Figures 2c and 2d), convincingly suggesting that these late Paleozoic and early Mesozoic rocks were affected by the Jurassic-Cretaceous volcanism, resulting in overprinting by thermochemical magnetization events, regardless of their age and location.…”

“…In total, 11% of the total data set was eliminated, and it cannot be entirely ruled out that the different distribution of the thermal data set is due to the selection process. Bilardello et al (2018) had isolated discordant ChRM site-mean directions from the Itararé Group in the northern Paraná Basin even within the same drill cores. They attributed this behavior to heterogeneities in grain size, which is characteristic of glacial sediments, with different grain sizes recording different components of magnetization.…”

“…These distributions are better evaluated when plotted as (South) virtual geomagnetic poles (VGPs), allowing for a better comparison among results from different localities. In Figure 2a, VGPs and best fit distribution of the Mafra Fm rhythmites are reported, together with those obtained from the Itararé Group rocks from the Rio do Sul and Itu rhythmite exposures (Franco et al, 2012) and the Itararé Group rocks sampled across the state of Sao Paulo (Bilardello et al, 2018). Notably, Franco et al (2012) had concluded that the magnetic mineralogy of the Rio do Sul and Itu rocks consists of primary magnetite and hematite grains which both acquired their magnetization during early stages of diagenesis and further presumed the Itu rocks to contain additional authigenic magnetite formed during the same time interval.…”

“…In particular, the Rio do Sul Figure 1. (a) Mean ChRM and secondary directions isolated from the Mafra Fm rythmites reported by Brandt et al (2019) from both AF and thermal (Th) demagnetization, and the expected mean early Cretaceous (eK) direction for the area, together with their best fit plane plotted on a lower hemisphere stereonet; (b) paleomagnetic specimen directions from the Itararé Group Mafra Fm reported by Brandt et al (2019) and best fit plane (n = 148, black), together with the mostly south and down specimen directions only isolated by Bilardello et al (2018) from the Itararé Group rocks from the state of Sao Paulo and their best fit plane (n = 302, purple), and their site-mean directions and best fit plane (N = 14, cyan and larger symbols), plotted on a lower hemisphere stereonet. Despite the scatter, the Mafra and Sao Paulo distributions have a large overlap and similar orientations regardless of whether the latter are analyzed at the specimen or site-mean level.…”

Reply to Comment by Dario Bilardello on “New late Pennsylvanian Paleomagnetic Results From Paraná Basin (southern Brazil): Is the Recent Giant Gaussian Process Model Valid for the Kiaman Superchron?”

“…The influence of the Paraná magmatism is much more evident in Bilardello et al (2018) data set. About 50% of the remanent directions given by those authors correspond to normal polarity and low inclinations.…”