Permo-Carboniferous Reversed Superchron Research Articles

Intensity of the Earth's magnetic field: Evidence for a Mid-Paleozoic dipole low

The Mesozoic Dipole Low (MDL) is a period, covering at least ∼80 My, of low dipole moment that ended at the start of the Cretaceous Normal Superchron. Recent studies of Devonian age Siberian localities identified similarly low field values a few tens of million years prior to the Permo-Carboniferous Reverse Superchron (PCRS). To constrain the length and timing of this potential dipole low, this study presents paleointensity estimates from Strathmore (∼411 to 416 Ma) and Kinghorn (∼332 Ma) lava flows, United Kingdom. Both localities have been studied for paleomagnetic poles (Q values of 6 to 7), and the sites were assessed for their suitability for paleointensity from paleodirections, rock magnetic analysis, and microscopy. Thermal and microwave experiments were used to determine site mean paleointensity estimates of ∼3 to 51 μT (6 to 98 ZAm2) and 4 to 11 μT (9 to 27 ZAm2) from the Strathmore and Kinghorn localities, respectively. These, and all the sites from 200 to 500 Ma from the (updated) Paleointensity database (PINT15), were assessed using the Qualitative Paleointensity criteria (QPI). The procurement of reliable (QPI ≥ 5) weak paleointensity estimates from this and other studies indicates a period of low dipole moment (median field strength of 17 ZAm2) from 332 to 416 Ma. This "Mid-Paleozoic Dipole Low (MPDL)" bears a number of similarities to the MDL, including the substantial increase in field strength near the onset of the PCRS. The MPDL also adds support to the inverse relationship between reversal frequency and field strength and a possible ∼200-My cycle in paleomagnetic behavior relating to mantle convection.

A new Siberian record of the ∼1.0 Gyr-old Maya superchron

We report new magnetostratigraphic data from the Late Mesoproterozoic Linok Formation consisting of clayed carbonates, together with paleomagnetic data from the Neoproterozoic carbonate Sukhaya Tunguska, Burovaya and Turukhansk formations, located in the Turukhansk region (northwestern Siberian platform). We also re-analyzed demagnetization data previously obtained from the Derevnya Formation. Paleomagnetic analyses revealed up to four magnetization components. First, a steep-inclination component, likely of recent origin, is isolated in all sections in the low temperature range. Second, a magnetization is recovered in most samples between ∼250 °C and ∼350–400 °C. It shows a single magnetic polarity and is interpreted as a remagnetization acquired during the Kiaman Permo-Carboniferous reverse superchron. Third, a magnetization component is isolated between ∼400 °C and ∼500 °C in samples from the Burovaya and Turukhansk formations. This component is interpreted as a remagnetization that occurred before the end of the Neoproterozoic. A fourth high-temperature (HT) magnetization component is observed up to ∼575 °C in samples from the Linok and Burovaya formations and up to ∼680 °C for those of the Derevnya and Turukhansk formations. A series of magnetic polarity reversals is observed in the lower and middle parts of the Linok Formation, whereas a single polarity interval is found in its upper part. The syn-depositional origin of this magnetization is attested by positive reversal, conglomerate and fold tests. The HT magnetization component isolated in the Derevnya and Burovaya formations shows a single magnetic polarity. Its primary origin is supported by the fact that the Derevnya and Burovaya HT mean directions are positioned in the continuity of the directional evolution observed through the Linok Formation. It is further constrained by very consistent Late Mesoproterozoic to Early Neoproterozoic apparent polar wander paths obtained from different regions of the Siberian platform. A single magnetic polarity reversal is observed in a ∼40 m-thick section of the Turukhansk Formation. The HT directions found in the upper part of the Linok Formations, over a stratigraphic thickness of ∼200 m, and in the Derevnya and Burovaya formations strengthen the existence of the normal polarity Maya superchron around 1.0 Ga, at the transition between the Mesoproterozoic and the Neoproterozoic.

New palaeomagnetic results from the Oslo Graben, a Permian Superchron lava province

We have performed an extended palaeomagnetic study of the Oslo Graben volcanics, compared to the study of half a century ago by van Everdingen, using modern techniques and a four times larger amount of sites, plus additional rock magnetic experiments. We conclude that the average direction (D = 204.0, I = −37.9, k = 46.9, α95 = 2.0) and associated palaeomagnetic pole (λ = 48.3, ϕ = 155.5, K = 52.2, A95 = 1.9) of the Krokskogen and Vestfold volcanics together are statistically identical to those of the earlier study. This gives confidence in the fact that older palaeomagnetic studies can be reliable and robust, even though methods have improved. Our larger number of samples, and better age constraints, enable us to separate the data into two major intervals: the younger, on average, Krokskogen area and the older Vestfold area. The results show firstly that palaeolatitudes are slightly higher than predicted by the latest apparent polar wander path (APWP) for Eurasia by Torsvik et al. These data support an early Permian Pangaea A configuration and do not necessitate a Pangaea B configuration. The larger data set also allows us to assess the distribution of the characteristic remanent magnetization directions of the Oslo Graben in terms of geomagnetic field behaviour, which were acquired during a long period of dominantly single polarity the Permo-Carboniferous Reversed Superchron (PCRS). The distributions show a significantly lower virtual geomagnetic pole (VGP) scatter at the observed (low) latitudes than expected from a compilation from lavas of the last 5 Myr. The data do however show excellent agreement with the scatter observed both during the Cretaceous Normal Superchron and the PCRS. A comparison of the directional distributions in terms of elongation is less discriminating, since the large errors in all cases allow a fit to the predicted elongation/inclination behaviour of the TK03.GAD model.

Magnetostratigraphy and mid-palaeolatitude VGP dispersion during the Permo-Carboniferous Superchron: results from Paraná Basin (Southern Brazil) rhythmites

SUMMARY A detailed magnetostratigraphic and rock-magnetism study of two Late Palaeozoic rhythmite exposures (Itu and Rio do Sul) from the Itarare Group (Parana Basin, Brazil) is presented in this paper. After stepwise alterning-field procedures and thermal cleaning were performed, samples from both collections show reversed characteristic magnetization components, which is expected for Late Palaeozoic rocks. However, the Itu rocks presented an odd, flat inclination pattern that could not be corrected with mathematical methods based on the virtual geomagnetic pole (VGP) distributions. Correlation tests between the maximum anisotropy of the magnetic susceptibility axis (K1) and the magnetic declination indicated a possible mechanical influence on the remanence acquisition. The Rio do Sul sequence displayed medium to high inclinations and provided a high-quality palaeomagnetic pole (after shallowing corrections of f = 0.8) of 347.5°E 63.2°S (N = 119; A95 = 3.3; K = 31), which is in accordance with the Palaeozoic apparent wander pole path of South America. The angular dispersion (Sb) for the distribution of the VGPs calculated on the basis of both the 45° cut-off angle and Vandamme method was compared to the best-fit Model G for mid-latitudes. Both of the Sb results are in reasonable agreement with the predicted (palaeo) latitudinal S-λ relationship during the Cretaceous Normal Superchron (CNS), although the Sb value after the Vandamme cut-off has been applied is a little lower than expected. This result, in addition to those for low palaeolatitudes during the Permo-Carboniferous Reversed Superchron (PCRS) previously reported, indicates that the low secular variation regime for the geodynamo that has already been discovered in the CNS might have also been predominant during the PCRS.

Magnetostratigraphy of the Lodève Basin, France: Implications for the Permo-Carboniferous reversed superchron and the geocentric axial dipole

New results from the La Lieude Formation now complete the magneto-stratigraphic coverage of the Permian redbeds preserved in the Lodeve Basin of southern France. The majority of the samples yield reversed polarity, with a mean of declination D = 187.4°, inclination I = −0.5° (Fisherian statistics k = 44.2, α95 = 5.6°, n = 16). Together with previously published results, these data indicate that the entire basin lies within the Permo-Carboniferous Reversed Superchron (PCRS). But the stratigraphically highest sample exhibits normal polarity, suggesting that the transition marking the end of the PCRS may be close. The unusual behaviour of the geodynamo that generates superchrons prompts one to ask if there are concomitant influences on the morphology of the field. The intersecting palaeomeridian method offers a means of pursuing this question. An updated analysis suggests that there are currently no compelling reasons for adding significant higher-order terms to the geocentric axial dipole (GAD) model for the Late Permian.

Palaeomagnetism and magnetostratigraphy of the Permian and Triassic of Spitsbergen: a review of progress and challenges

Permian and Triassic sediments from Svalbard provide a testing ground for evaluating concepts about the polarity and configuration of the geomagnetic field during the Palaeozoic–Mesozoic transition. This review examines existing palaeomagnetic and magnetostratigraphic data, and also re-examines the issue of partial remagnetization of these sediments. Permian and Triassic palaeomagnetic poles from Spitsbergen demonstrate a close similarity to the stable Europe apparent polar wander path. Magnetostratigraphy from the Gipsdalen and Tempelfjorden groups demonstrates the dominance of reverse polarity (the Permo-Carboniferous Reverse Superchron), with evidence for three normal magnetozones of short duration, one of which may provide a useful marker near the base of the Permian. The other two normal magnetozones in the middle and lower parts of the Kapp Starostin Formation are most likely to represent equivalent magnetozones in the Wordian and Capitanian (mid- Permian). Magnetostratigraphy of the Lower Triassic up to the lowest parts of the Upper Triassic in central and western Spitsbergen provides greater chronostratigraphic detail than has been available hitherto from biostratigraphy alone. This demonstrates the scale of a number of suspected hiatuses, and also hints at improved chronostratigraphic correlations between east and west Spitsbergen. Upper Triassic sediments provide the best palaeomagnetic properties, indicating that future magnetostratigraphic studies in this interval may improve chronostratigraphic understanding of this poorly dated interval. A persistent partial remagnetization affecting both Permian and Triassic sediments is not of a single origin. It may be both a Brunhes-age viscous remanence and a Palaeogene heating/burial-induced magnetization, or a fluid flow related remagnetization locally in western Spitsbergen. The remagnetization is often composite (Cenozoic plus older) in nature.

Palaeomagnetism and magnetostratigraphy of the Permian and Triassic of Spitsbergen: a review of progress and challenges

Permian and Triassic sediments from Svalbard provide a testing ground for evaluating concepts about the polarity and configuration of the geomagnetic field during the Palaeozoic–Mesozoic transition. This review examines existing palaeomagnetic and magnetostratigraphic data, and also re-examines the issue of partial remagnetization of these sediments. Permian and Triassic palaeomagnetic poles from Spitsbergen demonstrate a close similarity to the stable Europe apparent polar wander path. Magnetostratigraphy from the Gipsdalen and Tempelfjorden groups demonstrates the dominance of reverse polarity (the Permo-Carboniferous Reverse Superchron), with evidence for three normal magnetozones of short duration, one of which may provide a useful marker near the base of the Permian. The other two normal magnetozones in the middle and lower parts of the Kapp Starostin Formation are most likely to represent equivalent magnetozones in the Wordian and Capitanian (mid- Permian). Magnetostratigraphy of the Lower Triassic up to the lowest parts of the Upper Triassic in central and western Spitsbergen provides greater chronostratigraphic detail than has been available hitherto from biostratigraphy alone. This demonstrates the scale of a number of suspected hiatuses, and also hints at improved chronostratigraphic correlations between east and west Spitsbergen. Upper Triassic sediments provide the best palaeomagnetic properties, indicating that future magnetostratigraphic studies in this interval may improve chronostratigraphic understanding of this poorly dated interval. A persistent partial remagnetization affecting both Permian and Triassic sediments is not of a single origin. It may be both a Brunhes-age viscous remanence and a Palaeogene heating/burial-induced magnetization, or a fluid flow related remagnetization locally in western Spitsbergen. The remagnetization is often composite (Cenozoic plus older) in nature.

Paleomagnetism of the Newcastle Range, northern Queensland: Eastern Gondwana in the Late Paleozoic

The Newcastle Range is an extensive (2500 km2) and well‐exposed caldera system erupted on the trailing edge of Eastern Gondwana between 325 and 295 Ma. Paleomagnetic samples were collected from ignimbrites and associated microgranitoid intrusions from the central, northern and southern calderas from which three components of magnetization are recognized. Component 1 is considered to be a viscous magnetization acquired during the Brunhes Chron. A presumed Permian component, C2, is found in seven paleomagnetic sites with a mean pole at 30.9°S, 139.7°E (K = 13.9, A95 = 16.8°, ASD = 21.7°), agreeing with previously reported Permian data from Australia. Carboniferous units have a well‐defined characteristic component, C3, distinguished by dual polarity (predominantly reversed) and moderate to steep inclination directions. Paleomagnetic polarities in the Newcastle Range Volcanics are formation dependent and new constraints on the timing of Carboniferous volcanism (∼325–317 Ma) are consistent with recent reanalysis of the base of the Permo‐Carboniferous Reversed Superchron (PCRS). A mean paleomagnetic pole, calculated from 15 VGPs, lies at 63.4°S, 125°E (K = 26.22, A95 = 7.6°, ASD = 15.8°), suggesting that Australia remained at midlatitudes into the Middle Carboniferous. This paleomagnetic pole is consistent with similarly aged poles from Western Gondwana, the conformity of which indicates contributions from nondipole components of the Earth's paleofield were probably not significant in the time immediately preceding the PCRS.

Late Paleozoic remagnetization of Precambrian crystalline rocks along the Precambrian/Carboniferous nonconformity, Rocky Mountains: a relationship among deformation, remagnetization, and fluid migration

Paleomagnetic and rock magnetic data and petrographic observations demonstrate that Archean to late middle Proterozoic rocks (three localities in metamorphic rocks, two in granitic plutons) in parts of the Rocky Mountains, along and below a regional nonconformity with Carboniferous strata, have acquired a secondary magnetization. This magnetization is exclusively of reverse polarity and, based on its unique direction, was presumably acquired during the Permo-Carboniferous Reverse Superchron. At most sites, the remanence is carried exclusively by hematite, although it also resides in magnetite at some localities. Based on comparison between paleomagnetic poles derived from locality mean secondary magnetizations (e.g. Decl.=137.2°, Incl.=−18.2° α95=4.1°, k=221.4, N=7 of seven samples, Archean gneiss, northern Laramie Range; Decl.=148.7°, Incl.=−15.0°, α95=4.6°, k=77.0, N=14 of 23 samples, Sherman Granite, southern Laramie Range; Decl.=174.3°, Incl.=−18.9°, α95=6.6°, k=48.6, N=11 of 13 samples, metavolcanic rocks, Sandia Mountains) and late Paleozoic reference poles for North America, this secondary magnetization does not appear to have been acquired as a direct consequence of subareal erosion prior to deposition of younger sedimentary strata. At two localities, the age of the remanence is demonstrably younger than the oldest overlying strata. Locally, the nonconformity between basement rocks and late Paleozoic strata, as well as steep shear zones within the basement rocks, may have been efficient channelways for brines flushed out of basins created during Ancestral Rocky Mountains deformation. On a continental scale, migration of fluids and attending remagnetization is consistent with epeirogenic uplift of the Pangean supercontinent and a relative lowering of ground water levels. The fact that these basement rocks do not show evidence of subsequent remagnetization during latest Cretaceous to early Tertiary (Laramide) contraction in this area suggests that the characteristics of the late Paleozoic remagnetization phenomenon were unique in time.

The implications of non-suppressed geomagnetic secular variation during the Permo-Carboniferous Reversed Superchron

We have investigated variations in the natural remanent magnetisation (NRM) in red beds which were deposited during the Permo-Carboniferous Reversed Superchron (PCRS) near the paleo-equator. These red beds are reliable recorders of the geomagnetic field, because the NRM is carried by detrital hematite. We analysed the angular standard deviation (ASD) and spectral content of the directional records to quantify the amount of paleosecular variation (PSV) during the time of deposition. The ASD of a high-resolution directional record of the Dôme de Barrot which spans approximately 27 kyr is 10°. This is comparable to recent (0–5 Ma) low-latitude lavas from the PSVRL database ( McElhinny and McFadden, 1997) and to recent lake sediments from the SECVR database ( McElhinny and Lock, 1996). The frequency spectra of declination and inclination are also similar to those from the recent lake sediments: spectral power significant at the 95% level is concentrated in periods of ∼1–5 kyr, higher frequencies are not significant. From the ASD and spectral analyses, it appears that the amount of PSV during—at least part of—the PCRS was comparable to that of recent times. Several heuristic and numerical geodynamo models are discussed in the context of Superchrons and PSV. There is a discrepancy between the amount of SV shown by our data and that suggested by numerical models which use lateral heat flux variations at the core–mantle boundary (CMB). However, computational limitations still force some model parameters to have values that are unrealistic for the Earth. For the phenomenological geodynamos, ‘low-energy state’ models which suggest that PSV is suppressed during a Superchron are not supported by the Dôme de Barrot data. Rather, a ‘high-energy state’ of the geodynamo, in which outer-core convection is vigorous, is favoured as a prerequisite for a Superchron to occur.

Secular variation in Permian red beds from Dôme de Barrot, SE France

The Dôme de Barrot red beds (SE France) were deposited during the Permo–Carboniferous Reversed Superchron (PCRS), near the palaeo-equator. The natural remanent magnetisation (NRM) resides in detrital hematite, which is very resistant against thermal demagnetisation. Microprobe analyses reveal the presence of titanium in hematite grains, indicating the volcanic and thus detrital origin. Therefore, the NRM is a detrital remanent magnetisation and NRM variations reflect changes in the geomagnetic field. Tuning of the proxy parameter magnetic susceptibility to periods of the Earth orbit during Permian times provides relative time control. The inferred average sedimentation rate is 11.8 cm 1000 yr −1. The variation in NRM directions is similar to present-day secular variation (SV). This suggests that SV was not suppressed during (at least part of) the PCRS.

Magnetostratigraphic investigations on an Emeishan basalt section in western Guizhou province, China

A magnetostratigraphic investigation has been conducted on a late Permian Emeishan basalt section at Duge in western Guizhou province, China. The characteristic remanent magnetization (ChRM) directions resolved at high temperatures from the basalts pass the fold test with dual polarity and define a normal magnetozone of five sixths of the total thickness in the lower part and a reversed magnetozone of one sixth of the thickness in the top part of the section. The ChRM directions from the basalt clasts of pyroclastic rocks fail the conglomerate test, but the direction and polarity of the ChRM of the clasts from the upper reversed zone are consistent with those of the basaltic lavas in the reversed zone and the normal directions also dominate the ChRM of the clasts taken from the lower normal zone. This suggests that either the original remanence of the clasts was reset by cementing basaltic lavas or the ChRM of the basalts was acquired late but not beyond a field reversal. The magnetostratigraphic pattern (N–R) observed at Duge is as simple as that observed from some major continental flood basalt provinces in the world, where very few magnetic polarity zones were observed from volcanic sequences of great thickness; the emplacement of the Emeishan basalts in western Guizhou is estimated to be less than two million years. The normal polarity magnetozone observed indicates that the extrusion of the Emeishan basalts in western Guizhou postdates the Permo-Carboniferous reversed superchron (PCRS). The apparent shortness of the reversed magnetozone explains why far fewer reverse directions than normal ones have been so far reported from the Emeishan basalts.

A palaeomagnetic and palaeointensity study of the Mauchline lavas, south-west Scotland

The Mauchline lavas of SW Scotland erupted close to the Carboniferous-Permian boundary and near the middle of the Permo-Carboniferous Reversed Superchron (PCRS). Rock magnetic studies have identified mixed magnetite-hematite assemblages; both minerals record reversed components of magnetisation. A fold test is not diagnostic but the mean remanence recovered in 23 stable samples at five sites ( D I = 192.5 −26.1 °, α 95 = 14.3°) accords with the regional field at the time of eruption and the pole position (346.8°E, 47.2°S) is coincident with late Carboniferous-Early Permian poles. A palaeointensity study of 15 samples employing the modified Thellier-Thellier method yields nine acceptable estimates of ancient field intensity with a mean value of 13.6 ± 4.5 μT from four lava flows. The corresponding virtual dipole moment of 3.3 × 10 22 Am 2 is approximately 40% of the present day value. This result supports recent studies from western Europe which suggest that the geomagnetic field strength was low during the PCRS but is in conflict with Russian data which suggest that the field strength was close to, or slightly higher than the present day value.

Palaeointensity results from the Permo-Carboniferous (Kiaman) reversed superchron: the Great Whin and Midland Valley sills of the northern United Kingdom

SUMMARY Fifty Thellier palaeointensity determinations were made from the Great Whin Sill (GWS) of northern England and the Midland Valley Sill (MVS) of central Scotland, which form the major part of a late Carboniferous quartz-dolerite complex. Palaeodirectional analysis confirms that the reversed polarity characteristic remanence carried by samples from both sills is consistent with the Permo-Carboniferous reversed superchron. Palaeomagnetic results suggest that the two sills may not be exactly contemporaneous. The MVS was intruded rapidly some time during the emplacement of the GWS, which was itself emplaced over a considerable time period. The MVS may well represent the northernmost expression of the instrusive activity. A mean palaeointensity of 22.9 ± 2.6 μT was calculated for the Great Whin Sill, with a corresponding VDM value of 5.9 ± 1022 A m2. The palaeointensity value calculated from the Midland Valley Sill was much lower, 13.0 ± 0.5 μT, with a corresponding VDM value of 3.3 ± 1022 A m2. Detailed rock magnetic analyses and stepwise thermal demagnetization studies reveal that the remanence carrier in both sills is magnetite of mixed domain state. Multidomain grains are less important in the Midland Valley Sill and thus greater confidence is attached to palaeointensity results from this sill. VDM values from this and other studies of the Permo-Carboniferous reversed superchron (P-CRS) indicate that the dipole strength showed similar variations to that witnessed for the past 5 Ma. There is no conclusive support in favour of either the weak or strong field models that have been suggested for superchrons. More palaeointensity results are needed from the termination of the P-CRS and the late Permian/early Triassic in order to understand how the dipole field evolved during the P-CRS and its relationship to the so-called Mesozoic dipole low.

A paleomagnetic investigation of the Minturn Formation, Colorado: A study in establishing the timing of remanence acquisition

One hundred sixty-five oriented hand samples were collected from 55 sites in the Minturn Formation located between Wellsville and Howard, Colorado, in an attempt to locate the base of the Permo-Carboniferous Reversed Superchron (PCRS) in western North America. After the samples were demagnetized using thermal and alternating field cleaning techniques, the normal polarity magnetizations (which marks the base of the Permo-Carboniferous Reversed Superchron (PCRS)) were not found in the Minturn Formation. Reversed polarities were consistent throughout the section studied. A conglomerate test was conducted on syndepositional rip-up clasts taken from the formation. The test indicates that a post-depositional, secondary component of magnetization has been acquired by the clasts and remains stable up to temperatures of 610–640°C. However, the high temperature components for three of the four test sites show random directions indicating that a primary remanent magnetization was acquired shortly after deposition. The conglomerate test demonstrates that a secondary Permian direction can be successfully isolated from a primary Pennsylvanian one revealed at blocking temperatures above 650°C. In this study, the two components were isolated using step-wise thermal demagnetization. The results of the study indicate that the base of the PCR superchron lies stratigraphically below the Minturn Formation, probably in Early Atokan or Morrowan age sediments.

Magnetic Polarity Stratigraphy of the Permian and Carboniferous: The Extension of the Magnetic Reversal Record into the Paleozoic

Magnetic polarity stratigraphy has revolutionized stratigraphic studies in Jurassic to Pleistocene sediments. These studies have been greatly facilitated by the reversal record that is recorded in rocks of the ocean floor. For times prior to the Jurassic, the reversal history of the magnetic field must be determined and eventually related through the type section concept. The magnetic reversal history of the late Paleozoic is dominated by the Permo-Carboniferous reversed superchron (PCRS), which extends from the late Permian to the Carboniferous (Westphalian). Recent studies by the author and his students in Middle Carboniferous sediments of eastern Canada, Pennsylvania, Arizona, and Nevada reveal that the magnetic field has reversed frequently in late Mississippian and early Pennsylvanian times (Meramecian through late Morrowan). The polarity of the magnetic field over this interval is approximately 50% normal and 50% reversed. The frequency of reversal appears to be about one reversal per m.y. The possibility, therefore, exists that this pattern may be used for continental and intercontinental correlation. Attempts are currently underway to correlate this magnetic stratigraphy to fossiliferous marine sections. The base of the PCRS is probably of Atokan age.