Behavior of the Paleosecular Variation During the Permian‐Carboniferous Reversed Superchron and Comparisons to the Low Reversal Frequency Intervals Since Precambrian Times

Abstract

We investigated the (paleo)latitudinal dependence of angular dispersion sets of virtual geomagnetic poles (VGPs) for the Permian‐Carboniferous Reversed Superchron (PCRS: 262–318 Ma). In order to analyze the paleosecular variation during this period, we prepared different paleomagnetic data sets from scientific databases and recent literature, based on selection criteria which provided high degree of refinement. Model G of McFadden et al. (1988) was fitted to the VGP dispersion data, providing the shape parameters a and b, which were further compared with similar results for the Cretaceous Normal Superchron (CNS) and other periods of different geomagnetic reversal rates throughout the last 3 Ga. Our results indicate high similarity between the angular VGP dispersion from the PCRS and CNS (a low VGP dispersion at low paleolatitudes, and strong paleolatitudinal dependence), in contrasting difference for periods of higher reversal rates (the last 5 Ma). Despite the geodynamic differences related to both Phanerozoic superchrons, such evidence could point out similar heat flux conditions in the CMB, which may favor compatible stability conditions throughout these magnetozones. Notably, two additional observations which arose from the shape parameters relations for both Phanerozoic superchrons and Precambrian data sets demand further investigation: (i) a pattern of increase for the b/a ratio throughout the 0.5–1.6 Ga interval was observed, which could be partially explained by the Maya Superchron, recently proposed by Gallet et al. (2012) and (ii) the b × a relationship for both Phanerozoic superchrons and the 2.45–2.82 Ga interval pointed to the presence of one or more long‐term magnetozones throughout the Paleoproterozoic.

Latitude Dependence of Geomagnetic Paleosecular Variation and its Relation to the Frequency of Magnetic Reversals: Observations From the Cretaceous and Jurassic

“…Variations in Model G parameters estimated in this study and previously published studies of paleosecular variation for Carboniferous through 0–5 Ma (Biggin, van Hinsbergen, et al, ; Cromwell et al, ; de Oliveira et al, ; Johnson et al, ; McFadden et al, ; Opdyke et al, ). Triangles show the parameters obtained by fitting paleosecular variation estimates derived from virtual geomagnetic pole data sets truncated by the variable cutoff angle (Vandamme, ); circles show the corresponding estimates based on data sets trimmed by a fixed (40° or 45°) cutoff angle.…”

“…These values are similar to those obtained by Biggin, van Hinsbergen, et al () and are compatible with our estimates for the CNS. We note that de Oliveira et al () did not correct the S values for the within‐site scatter. After correcting their values for S w (available for eight of nine data sets), we obtained a = 9.3° and b = 0.21, which are close to our estimates for the CNS.…”

“…In Figure , we compare the estimates of Model G parameters ( a , b , and their ratio b/a ) available from the literature for different intervals from Carboniferous to recent time (Biggin, van Hinsbergen, et al, ; Cromwell et al, ; de Oliveira et al, ; Johnson et al, ; McFadden et al, ; Opdyke et al, ) with the results obtained here. Among these studies, only the recent work of de Oliveira et al () has not been already discussed. This study provided estimates of VGP dispersion for the long interval of dominantly reverse geomagnetic polarity spanning Late Carboniferous through Late Permian time, the PCRS (~262–318 Ma).…”

“…This study provided estimates of VGP dispersion for the long interval of dominantly reverse geomagnetic polarity spanning Late Carboniferous through Late Permian time, the PCRS (~262–318 Ma). The results suggested that secular variation during the PCRS was characterized by a strong increase of VGP scatter with latitude, which can be described by a Model G curve with the parameters a = 9.4° + 1.5°/−1.9° and b = 0.27 + 0.02/−0.05 (based on nine data sets from igneous rocks that de Oliveira et al, , considered the most reliable). These values are similar to those obtained by Biggin, van Hinsbergen, et al () and are compatible with our estimates for the CNS.…”

“…No data from rocks suspected to be remagnetized were included. Data from igneous formations that were tectonically folded or tilted after their emplacement must be structurally corrected (restored to paleohorizontal). Data from areas that may have been affected by uncertain post‐emplacement tilting and those showing evidence for local block rotations between the sampling sites were not used. Following the earlier studies of PSV for the Mesozoic, Paleozoic, and Precambrian (Biggin, van Hinsbergen, et al, ; Biggin, Strik, et al, ; de Oliveira et al, ; Smirnov et al, ; Veikkolainen & Pesonen, ), we required each data set to comprise paleomagnetic directions from at least nine sites ( N ≥ 9 independent spot readings of the magnetic field), and each site‐mean direction to be based on ChRM directions from at least three independent samples ( n ≥ 3). We will discuss the influence of imposing stricter criteria (higher minimum values of N and n ) on the estimates of latitude dependence of PSV in section . The ChRM components must have been isolated using stepwise demagnetization techniques.…”