Primary Remanence Research Articles

Late Devonian carbonate magnetostratigraphy from the Oscar and Horse Spring Ranges, Lennard Shelf, Canning Basin, Western Australia

The Late Devonian was a time of major evolutionary change encompassing the fifth largest mass extinction, the Frasnian–Famennian event. In order to establish a chronological framework for global correlation before, during, and following the Frasnian–Famennian mass extinction, we carried out a coupled magnetostratigraphic and biostratigraphic study of two stratigraphic sections in the Upper Devonian carbonate reef complexes of the Lennard Shelf, in the Canning Basin, Western Australia. Magnetostratigraphy from these rocks provides the first high-resolution definition of the Late Devonian magnetic polarity timescale. A 581-m-reference section and an 82-m overlapping section through the marginal slope facies (Napier Formation) of the Oscar Range as well as a 117-m section at Horse Spring (Virgin Hills Formation) were sampled at decimeter to meter scale for magnetostratigraphy. Conodont biostratigraphy was used to correlate both sections, and link magnetostratigraphic polarity zones to a globally established biostratigraphy. A stable, Characteristic Remanent Magnetization (ChRM) with dual polarities (NE, shallowly upward and SW, shallowly downward) is recovered from ∼60% of all samples, with magnetite inferred to be the chief magnetic carrier from thermal demagnetization characteristics. These directions define a geomagnetic pole at 49.5°S/285.8°E and α95=2.4 (n=501), placing the Canning Basin at 9.9°S during the Late Devonian, consistent with carbonate reef development at this time. A conservative interpretation of the magnetostratigraphy shows the recovery of multiple reversals from both sections, not including possible cryptochrons and short duration magnetozones. Field tests for primary remanence include positive reversal tests and matching magnetozones from an overlapping section in the Oscar Range. A strong correlation was found between magnetic polarity stratigraphies of the Oscar Range and Horse Spring sections, and we correlate 12 magnetostratigraphic packages. The relative stratigraphic thicknesses of the isochronous sediments from these two sections indicate that carbonate accumulation was ∼4.5× faster in the middle slope deposits at Oscar Range than in the more distal, lower slope Horse Spring deposits for the middle Frasnian through Famennian. The magnetic field during the Late Devonian underwent a relatively high reversal frequency with good potential for regional and global correlation, and should prove useful in deciphering a high-resolution chronostratigraphy across the Lennard Shelf to enable higher confidence examination of reef development across a major biotic crisis.

New paleomagnetic results from the Ediacaran Doushantuo Formation in South China and their paleogeographic implications

A new paleomagnetic pole is obtained from the top of Member 3 of the Doushantuo Formation in the Jiulongwan section of the Yangtze Gorges area in South China. A total of 191 samples from 23 sites were collected and subjected to stepwise alternating field (AF) and thermal demagnetization. After the removal of a soft component (SC) of viscous magnetic remanence acquired in present geomagnetic field, a high temperature component (HC), likely carried by magnetite, was isolated. The HC includes vectors from 147 samples using principal component analysis and arc constraints from 44 samples using remagnetization great-circle analysis. Both the vectors only and the combined vectors and arc constraints of the HC passed a reversal test. This is the first Ediacaran paleomagnetic remanence from the South China Block (SCB) that passes a reversal test, and it is interpreted as a primary remanence. The HC vectors were thus used to calculate the paleomagnetic pole, and in combination with arc constraints, were used for polarity interpretation. The vectors give a mean direction of (D=75.2°, I=41.0°, α95=2.5°, N=147) after bedding correction, and a corresponding paleomagnetic pole at 23.9°N, 187.0°E (dm/dp=3.0/1.8). This pole differs significantly from previously published results of the Doushantuo Formation, which may have been subjected to Paleozoic remagnetization. Our results provide a 23.5±1.8°N paleolatitude for the Yangtze Gorges area of the SCB and suggest several alternative paleogeographic options for the Ediacaran–Cambrian amalgamation of East Gondwana, depending on chronostratigraphic correlations.

Haematite pigmentation events and palaeomagnetic recording: implications from the Pilbara Print Stone, Western Australia

SUMMARY Haematite pigment is a common constituent of sedimentary rocks, but its contribution to the natural remanent magnetization of rocks is poorly understood. Here, we describe magnetic properties of two distinct pigment types that produce a characteristic decorative ‘print stone’ foundinthe ∼2.5GaMount McRae ShaleFormation,Hamersley Province,WesternAustralia. Distinct magnetic remanence directions observed in the Print Stone can be correlated to each pigment type. By comparison with the Australian apparent polar wander path, the remanence carried by uniformly distributed pigment can be dated to ∼15–25 Ma, while two age options, the Mesoproterozoic (∼1.5 Ga) or the middle Carboniferous (∼320–310 Ma), are permissible for the remanence carried by the pigment responsible for the distinctive ‘newsprint’ pattern. Magnetic properties and demagnetization characteristics of the different pigment types overlap significantly, and thus are not predictive of the dominant remanence carrier. Magnetic characteristics of the uniformly distributed pigment vary significantly on short spatial scales. Strong local control on pigment formation raises the possibility that a primary remanent magnetization may survive locally in pockets within sedimentary red bed formations.

Hydrostatic pressure effect on magnetic hysteresis parameters of multidomain magnetite: Implication for crustal magnetization

Hydrostatic pressure effects on magnetic parameters for crustal rock have been poorly investigated yet, while it is important for an understanding of source of long-wavelength magnetic anomaly, which is considered to reside in deep crust. In this study we have conducted the in situ magnetic hysteresis measurements on multidomain (MD) magnetite under high pressure up to 1GPa. With special attention to hydrostatic condition and sample preparation, pressure dependences of its magnetic hysteresis parameters (saturation magnetization, Ms; saturation remanence, Mrs; coercivity, Bc; coercivity of remanence, Bcr) are revealed as follows: (1) Bc monotonically increases with pressure at a rate of +91%/GPa; (2) Ms is constant under high pressure up to 1GPa; (3) Mrs increases with pressure up to 0.5GPa by ∼30% and reaches to saturation above the pressure; (4) Bcr is nearly constant at low pressure, and it increases above ∼0.6GPa; and (5) the changes in ratios Mrs/Ms and Bcr/Bc correlate with each other, resulting in systematic movement on the Day plot. These findings allow us to estimate change in a relaxation time of magnetic remanence carried by MD magnetite as a function of depth in the continental crust. In the model calculation, we consider no effect of plastic deformation on magnetic properties of magnetite, and the relaxation time is calculated using the theoretical thermal gradient. In consequence, the relaxation time monotonously decreases with depth, and primary remanence is considered to be replaced by a viscous remanent magnetization (VRM) over the Brunhes chron. Therefore, it is suggested that MD magnetite in deep crustal rocks can contribute to the source of the anomaly over the continental crust by VRM and induced magnetization.

Paleomagnetism and U–Pb age of the 2.4 Ga Erayinia mafic dykes in the south-western Yilgarn, Western Australia: Paleogeographic and geodynamic implications

We present results from a paleomagnetic study of the previously undated Erayinia dykes intruding the south-western Yilgarn Craton. The U–Pb TIMs baddeleyite age of these dykes is now 2401±1Ma, which is about 10m.y. younger than the 2418–2410Ma Widgiemooltha dyke swarm. The paleomagnetic study isolated a stable primary remanence with steep downward direction, and the paleomagnetic pole (22.7°S, 150.5°E, A95=11.4°) is similar, but not identical to that of the previously studied Widgiemooltha dykes. We interpret this difference as the result of the movement of the Yilgarn Craton toward the pole at ∼1°/m.y. angular speed, which is comparable with tectonic plates’ velocities during the Phanerozoic. Paleomagnetic polarities of Widgiemooltha and Erayinia dykes suggest that at least one geomagnetic reversal occurred between these two magmatic events. The estimated amplitude of geomagnetic secular variations at c. 2400Ma is slightly higher than predicted by the existing models for the last 5m.y. at the c. 64° latitude. The paleomagnetic data and patterns of c. 2.6–2.1Ga mafic dyke swarms permit the recently suggested reconstruction of the Paleoproterozoic supercontinent.

Paleointensity of the geomagnetic field in the Late Cretaceous and earliest Paleogene obtained from drill cores of the Louisville seamount trail

AbstractInformation on the strength of the geomagnetic field is important for understanding the behavior of the geodynamo. Polarity reversal frequency increases toward the late Cenozoic since the end of the Cretaceous Normal Superchron. Accumulating reliable paleointensity data in this time interval is still desired for elucidating the possibility of a link between reversal frequency and paleointensity. We conducted a paleointensity study on 288 samples from four seamounts from ∼74 to ∼50 Ma in age that constitutes the Louisville seamount trail. Based on thermomagnetic curves and scanning electron microscopy, we first excluded 134 samples that showed evidence for low‐temperature oxidation. We then applied the Tsunakawa‐Shaw method with low‐temperature demagnetization and double heating after rejecting some samples that do not preserve stable primary remanence, and 52 successful results were obtained. An average virtual axial dipole moment (VADM) of 3.75 ± 1.52 (1022 Am2) was obtained at Site U1372 on Canopus seamount (∼74 Ma) based on 16 independent measurements. Sites U1373 and U1374 on Rigil seamount (∼70 Ma) yielded a mean of 3.79 ± 1.40 (n = 16), and Site U1376 on Burton seamount (∼65 Ma) yielded a mean of 3.70 ± 1.37 (n = 8). These VADMs are similar to the long‐term average of the last 200 Myr by Tauxe et al. (2013) and the mean of the last 5 Myr by Yamamoto and Tsunakawa (2005). The volcanic sequence drilled at Site U1372 probably belongs to Chron C33n of 5.59 Myr long, whereas others belong to shorter chrons. Our results support no discernible relationship between polarity length and paleointensity.

Paleomagnetic secular variation study of Ar–Ar dated lavas flows from Tacambaro area (Central Mexico): Possible evidence of Intra-Jaramillo geomagnetic excursion in volcanic rocks

More than 350 oriented paleomagnetic cores were obtained for rock-magnetic and paleomagnetic analysis from radiometrically dated (40Ar–39Ar) magmatic rocks occurring in the southern segment (Jorullo and Tacámbaro areas) of the Michoacán-Guanajuato Volcanic Field in the Trans-Mexican Volcanic Belt. Most of the lavas (37) stem from monogenetic volcanoes dated at less than 4Ma. Two additional sites were sampled from the plutonic basement dated at 33–30Ma. Primary remanences carried by low-Ti titanomagnetites allowed to determining 34 reliable site-mean directions of mostly normal (27) but also reversed (7) polarities. The mean directions of these two populations are antipodal, and suggest neither major vertical-axis rotations with respect to the North America craton nor tilting in the region for the last 4Ma (rotation and flattening of the inclination parameters being less than −5.9±3.8 and 0.1±3.9, respectively). The corresponding paleomagnetic pole obtained for Pliocene–Pleistocene times is PLAT=83.4°, PLON=2.4° (N=32, A95=2.7°). Virtual geomagnetic poles also contribute to the time averaged field global database and to the paleosecular variation (PSV) investigations at low latitudes from lavas for the last 5Ma, showing a geomagnetic dispersion value that is in agreement with available PSV models. When comparing the magnetic polarities and corresponding radiometric ages of the studied sites with the Cenozoic geomagnetic polarity time scale (GPTS), a good correlation is observable. This finding underscores the suitability of data obtained on lavas in Central Mexico for contributing to the GPTS. Furthermore, the detection of short-lived geomagnetic features seems possible, since the possible evidence of Intra-Jaramillo geomagnetic excursion could be documented for the first time in these volcanic rocks.

A palaeomagnetic and 40Ar/39Ar study of mafic dykes in southern Sweden: A new Early Neoproterozoic key-pole for the Baltic Shield and implications for Sveconorwegian and Grenville loops

We present the results of palaeomagnetic and 40Ar/39Ar studies of the Proterozoic mafic dykes in the Norrköping and Falun areas of the southern Sweden. The primary remanence of two 939±3Ma dykes is supported by the rigorous baked contact test. The remanence direction of two other dykes, one of which was previously U–Pb dated at 946±1Ma is close to the reverse direction of 939Ma dykes. Using these results together with previously published 935±5Ma palaeomagnetic data from the Göteborg-Slussen mafic dykes and some dykes from the Falun area we calculated the mean 946–935Ma palaeopole for Baltica (0.9°S, 240.7°E, A95=6.7), which can be qualified as the key pole. Using this pole together with other date we conclude that the Grenville and Sveconorwegian loops of Laurentian and Baltican Apparent Polar Wander Paths are temporary displaced by 100–150m.y. We propose new palaeogeographic reconstructions of Baltica and Laurentia at ca. 940Ma and ca. 850Ma. We also present two new Mesoproterozoic non-key poles from 1410Ma and 1595Ma dykes.

Paleoproterozoic mafic dyke swarms from the Dharwar craton; paleomagnetic poles for India from 2.37 to 1.88 Ga and rethinking the Columbia supercontinent

Here we report new paleomagnetic and geochronologic results from the Dharwar craton (south India) from 2.37 to 1.88Ga. The presence of a ∼85,000km2 radiating dyke swarm with a fanning angle of 65° is confirmed within Peninsular India at 1.88Ga. North of the Cuddapah basin the dykes are oriented NW-SE and progress to an E-W orientation further south, converging at a focal point southeast of the basin. The Grand Mean dual polarity paleomagnetic pole falls at 36.5°N and 333.5°E (D=129.1°, I=4.2°, α95=4.5°, λ=2.1°) for 29 sites from the present study combined with previously published sites. Our continental reconstruction for India at ∼1.9Ga conflicts with the archetypal Columbia model, suggesting that the exact configuration needs modification. We also report two separate paleomagnetic directions from NW-SE (D=3.2°, I=56.4°, α95=17.9°, λ=37°) and N-S (D=240.1°, I=−65.5°, α95=10.9°, λ=47.7°) trending ∼2.2Ga dykes. We attribute this difference in directions to the separate magmatic pulses at 2.18 and 2.21Ga identified by French and Heaman (2010). Our results place India at intermediate latitudes from 2.21 to 2.18Ga and are supported by a positive baked contact test. New paleomagnetic results from E-W and NW-SE trending 2.37Ga dykes, combined with previous work in the Dharwar craton, yields a Grand Mean dual polarity paleomagnetic pole at 15.1°N and 62.2°E (A95=4.0°), placing India at polar latitudes (D=88.7°, I=−81.7°, α95=4.8°, λ=73.7°). Here we also report a shallow NE direction (D=52.2°, I=−1.5°, α95=6.3°) previously classified as a secondary magnetization from three dykes near the Cuddapah basin. A baked contact test and petrophysical analysis of two cross-cutting dykes supports a primary remanence. Finally we present a Paleoproterozoic Apparent Polar Wander Path (APWP) for the Dharwar craton, and examine paleogeographic relationships between India and other cratonic blocks for the 2.37–1.88Ga time interval

Was there SAMBA in Columbia? Paleomagnetic evidence from 1790 Ma Avanavero mafic sills (northern Amazonian Craton)

The Amazonian Craton represents an important piece in the Paleoproterozoic paleogeography of the Earth. This study presents paleomagnetic data obtained on well-dated (U–Pb) 1790Ma mafic sills from the Avanavero magmatism in northern Amazonian Craton (Guiana Shield). AF and thermal treatments revealed southeastern, low downward/upward inclination, remanent magnetization directions that are carried by moderate to high-Hc and high-TB Ti-poor titanomagnetite. The site mean directions cluster around the mean Dm=138.2°, Im=−3.4° (N=13, α95=13.0°), which yields a robust paleomagnetic pole (AV pole) at 27.9°E, 48.4°S (A95=9.6°) with a Q-value of 5. The characteristic component disclosed for the Avanavero sill matches that obtained for sediments collected along the baked contact and are distinct from those away from the sill. The Avanavero directions are also significantly different from those obtained for younger units, with ages spanning from 1420 to 520Ma, suggesting the sills carry a primary remanence. The Avanavero pole helps in constraining the paleogeography of the central pieces of Columbia. It is compatible with the Baltica and Amazonian Cratons SAMBA link in the Columbia Supercontinent at about 1780Ma, but other configurations are also possible. When compared to other Paleo- to Mesoproterozoic paleomagnetic poles from the southern Amazonian Craton (Central Brazil Shield), our new paleomagnetic pole suggests intracratonic motions within the southern area of the Craton, probably after 1420Ma ago. We tentatively suggest that these movements are related to the collision of the Paraguá Block with the proto-Amazonian Craton at about 1350–1320Ma ago.

Paleomagnetism of Mesoproterozoic Satakunta sandstone, Western Finland

A paleomagnetic study of the Mesoproterozoic Satakunta sandstone formation, Western Finland is presented here. Two components of natural remanent magnetization (NRM) were isolated in the sandstone with alternating field (AF) and thermal demagnetization treatments. The first is a high coercivity/unblocking temperature component which, after tilt and inclination shallowing corrections, yields a remanent magnetization of D=25.2°, I=3.9° with α95=9.1° (10 sites), corresponding to a paleomagnetic pole of Plat=27.8°N, Plon=173.2°E, with A95=6.5°. Positive reversal and tilt tests, an inverse baked contact test, as well as a stratigraphic order of the site mean poles, indicates a primary remanence. The relative age of the sandstone is estimated at ca. 1600 based on the proximity to other well defined poles of 1540–1770Ma from Baltica. However, because the pole also overlaps with poles with the ages of 1452–1630Ma, the age of the sandstone cannot be accurately defined. The second component is a lower coercivity/unblocking temperature component with a remanent magnetization direction D=34.0°, I=−40.7° with α95=8.1° (13 sites) similar to that of the Postjotnian Satakunta diabase intrusions, which are widespread in the area. Comparing APWPs of Baltica and Laurentia during 1.77–1.27Ga gives strong support to the Mesoproterozoic NENA configuration at 1.6Ga.

The palaeomagnetism of glauconitic sediments

The palaeoenvironmental significance of glaucony has long been appreciated, but accurate palaeomagnetic dating of events recorded by glauconitic horizons requires an understanding of how glauconitic sediments acquire a remanent magnetization. Pure glauconitic minerals are paramagnetic, but glauconite grains are large and slow-forming (over periods that can exceed 100kyr), with complex and variable morphologies. It is, thus, possible that small magnetic grains within glaucony particles may carry a significant fraction of the remanence in weakly magnetized sediments. Any remanence carried by glauconitic grains may therefore represent the geomagnetic field at a time significantly later than the time of deposition, or a time-averaged signal over some or all of the formation period. We investigated this problem using weakly magnetic Palaeocene glauconitic siltstones from southern New Zealand. We disaggregated the rock and separated it magnetically into glauconitic and non-glauconitic fractions. Results from stepwise isothermal remanent magnetization (IRM) acquisition, alternating-field demagnetization, temperature dependence of magnetic susceptibility, and stepwise thermal demagnetization of a triaxial IRM were used to demonstrate that the remanent magnetization is carried by single-domain or pseudo-single-domain magnetite in the non-glauconitic sediment fraction, and that the glauconite grains themselves make no contribution to the remanent magnetization. However, accurate measurement of the primary remanence is complicated by a strong viscous overprint and mineral alteration during thermal demagnetization studies. Identification of magnetite as the remanence carrier in sediments within a reducing diagenetic environment gives confidence that the remanence has a depositional origin. Glauconite does not carry a remanence; therefore, its effect is to dilute and weaken the overall magnetization. Furthermore, the use of rock magnetic parameters may be problematic when glauconite concentrations are (as in the studied sediments) orders of magnitude greater than remanence carrier concentrations, because in such cases the glauconite susceptibility can dominate that of the remanence carriers.

Analyses of primary remanence vector data from a large collection of lava flows: towards improved methodology in paleo-geomagnetism

Research into the properties of past geomagnetic fields (paleo-geomagnetism) has been carried out worldwide for over half a century. This research utilizes rocks which are to a varying degree inadequate for that purpose, and evaluation of the results is therefore not always on firm ground. One resource which can be utilized to constrain several aspects of the interpretation of other paleo-geomagnetic data, consists of stable primary remanence vectors in large homogeneous collections of dated fresh lava flows. We outline how the available remanence data from several thousand lavas in Iceland may be analysed in a semi-quantitative way to test some of current results, methodology and concepts of paleo-geomagnetism. Among topics where such analysis provides new insights, are the geomagnetic polarity time scale, the frequency distribution of virtual geomagnetic poles in latitude and longitude, and the relative intensity of the magnetic field as a function of virtual pole latitude. Comparison of the scatter in remanence intensities and in absolute paleointensity determinations on Icelandic lavas indicates, along with other evidence, that quality criteria for the latter are in need of revising. It is also confirmed here that long-term changes have occurred in the amplitude of the geomagnetic secular variation; they should be taken into account in studies on other properties of the paleo-geomagnetic field.

Phanerozoic Paleomagnetism Characteristics of the Qomolangma Area in Tibet

Abstract:This paper conducts systematic test research on the 2920 paleomagnetic directional samples taken from Ordovician‐Paleogene sedimentary formation in the north slope of Qomolangma in south of Tibet and obtains the primary remanent magnetization component and counts the new data of paleomagnetism the times. Based on the characteristic remanent magnetization component, it calculates the geomagnetic pole position and latitude value of Himalaya block in Ordovician‐Paleogene. According to the new data of paleomagnetism, it draws the palaeomagnetic polar wander curve and palaeolatitude change curve of the north slope of Qomolangma in Ordovician‐Paleogene. It also makes a preliminary discussion to the structure evolution history and relative movement of Himalaya bloc. The research results show that many clockwise rotation movements had occurred to the Himalaya block in northern slope of Qomolangmain the process of northward drifting in the phanerozoic eon. In Ordovician‐late Cretaceous, there the movement of about 20.0° clockwise rotation occurred in the process of northward drifting. However, 0.4° counterclockwise rotation occurred from the end of late Devonian epoch to the beginning of early carboniferous epoch; 6.0° and 8.0° counterclockwise rotation occurred in carboniferous period and early Triassic epoch respectively, which might be related with the tension crack of continental rift valley from late Devonian period to the beginning of early carboniferous epoch, carboniferous period and early Triassic epoch. From the Eocene epoch to Pliocene epoch, the Himalaya block generated about 28.0° clockwise while drifting northward with a relatively rapid speed. This was the result that since the Eocene epoch, due to the continuous expansion of mid‐ocean ridge of the India Ocean, the neo‐Tethys with the Yarlung Zangbo River as the main ocean basin closed to form orogenic movement and the strong continent‐continent collision orogenic movement of the east and west Himalayas generated clockwise movement in the mid‐ Himalaya area. According to the calculation of palaeolatitude data, the Himalaya continent‐continent collusion orogenic movement since the Eocene epoch caused the crustal structure in Indian Plate‐ Himalaya folded structural belt‐ Lhasa block to shorten by at least 1000 km. The systematic research on the paleomagnetism of Qomolangma area in the phanerozoic eon provides a scientific basis to further research the evolution of Gondwanaland, formation and extinction history of paleo‐Tethys Ocean and uplift mechanism of the Qinghai‐Tibet Plateau.

Further evidence of widespread Permian remagnetization in the North Patagonian massif, Argentina

A paleomagnetic study on accurately dated (472–476Ma) Early Ordovician undeformed granitoids exposed in the northeastern margin of the North Patagonian massif (41.5°S, 65.0°W) was carried out, with the main goal of comparing the Early Ordovician paleogeographic position of Patagonia relative to Gondwana. About one hundred specimens were processed, from ten sites on the granitoids plus two sites on the quartzites and sandstones of the unconformably overlying Early Devonian Sierra Grande Formation. Seven out of the twelve sites, all in the granitoids, provided consistent directions of a characteristic remanent magnetization. Structural correction of paleomagnetic data from plutons could only be applied at a few sites, from bedding attitudes of overlying Devonian or Tertiary sedimentary rocks. The paleomagnetic study failed to recover the primary remanence. Thermal demagnetization isolated a reversed magnetic component with high unblocking temperatures. Magnetic properties and petrographic analyses suggest that remagnetization is carried by secondary magnetic minerals that grew from circulating high temperature fluids inferred from the pervasive to localized hydrothermal alteration. This led to the formation of magnetite and/or hematite after biotite. A paleomagnetic pole (PP) was computed from the mean site directions. The position of this PP (011°E, 65.0°S; A95=12°, K=24.5) on the apparent polar wander path of South America suggests that this magnetization was probably acquired during the Early Permian. Syntectonic remagnetization during Permian times has been reported for sandstones of the lower member of the Devonian Sierra Grande Formation at nearby outcrops. Our data suggest that the remagnetization was pervasive and affected the Ordovician granitoids as well. Many South American Permian paleomagnetic poles have been derived from syn- to postectonic magnetizations along the southwestern Gondwana margin, from the Argentine Precordillera to the Sierras Australes belt and related to the ca. 290–270Ma San Rafael tectonic phase. Slightly younger remagnetizations have been found affecting sedimentary rocks exposed in the foreland both in Argentina and Uruguay, suggesting a migration of the remagnetization front towards the North-Northeast. The similarity of the position of our new remagnetized pole from the North Patagonian massif to those from units in southern Gondwana confirms that Patagonia was already part of Gondwana by the Early Permian. Whether the remagnetization of the Ordovician granitoids is associated to the widespread and well dated in 290–280Ma magmatism that affected the northern Patagonia and a major part of southwestern Gondwana during the Permian or can be attributed to tectonic processes associated to the collision of Patagonia, or post-collision adjustment of minor plates, is discussed.

Pre-Rodinia supercontinent Nuna shaping up: A global synthesis with new paleomagnetic results from North China

The existence of a pre-Rodinia Precambrian supercontinent, variously called Nuna or Columbia, has been widely speculated in the past decade, but the precise timing of its existence and its configuration have been uncertain due to the lack of unequivocal paleomagnetic and geological constraints. Here we report high-quality paleomagnetic results from the well dated ∼1780Ma Xiong'er Group in southern North China Block (NCB). A total of 110 paleomagnetic samples from 14 sites were collected and subjected to stepwise thermal demagnetization. After removing a low temperature component (CL) of viscous magnetic remanence acquired in recent geomagnetic field, a high temperature component (CH), carried by hematite and magnetite in redbeds and volcanic samples, has been isolated. It gives a mean direction of (D=18.4°, I=−3.7°, α95=7.6°, N=14) after bedding correction, and a corresponding paleomagnetic pole at 50.2°N, 263.0°E (A95=4.5°). The CH passed a reversal test and was interpreted as a primary remanence. This new pole plus three other high-quality poles from the NCB that have been more precisely dated at 1769±3Ma, 1560–1440Ma and 1437±21Ma define a 1780–1440Ma apparent polar wander path (APWP) for the NCB. This, together with an update of global high quality paleomagnetic dataset, allows us to demonstrate that the pre-Rodinia supercontinent Nuna likely existed at least between ∼1780Ma and ∼1400Ma. Our paleomagnetism-based global reconstruction, for the first time, quantitatively assembles all major cratons together; it encompasses previously proposed regional links including the SAMBA connection between Baltica, Amazonia and Western Africa (Johansson, 2009), connections between Laurentia, Baltica and Siberia at the core of Nuna (Evans and Mitchell, 2011), the proto-SWEAT connection between Laurentia, East Antarctica and Australian blocks (Payne et al., 2009), and the NCB–India connection (Zhao et al., 2011).

Upper-crustal, basement-involved folding in the East Range of the Sudbury Basin, Ontario, inferred from paleomagnetic data and spatial analysis of mafic dykes

Tilting of crystalline basement rocks associated with folding strain at uppermost crustal levels is difficult to recognize if basement rocks are devoid of traceable marker planes. Here we use the spatial variation in strike of Paleoproterozoic mafic dyke segments complemented by compiled paleomagnetic data to identify tilting in Archean basement rocks associated with kilometre-scale folds of the eastern Sudbury Basin, Ontario. Spatial analysis of the strike of dyke segments is consistent with generation of the NE lobe and a newly identified anticline, referred to as the West Bay Anticline, in the layered Sudbury Igneous Complex (SIC). This anticline accounts better for the structural characteristics of the eastern Sudbury Basin than a previously proposed anticline with west-plunging hinge line. The West Bay Anticline is characterized by abrupt plan-view thickness variations in the lower SIC and curved faults displaying significant strike separations of SIC contacts. These structural characteristics are consistent with folding strain imparted to the SIC and adjacent Archean rocks during formation of the West Bay Anticline. Sublayer embayments and associated quartz diorite dykes likely served as zones of mechanical weaknesses, at which the higher-order folds localized. Unfolding magnitudes of the NE lobe based on primary paleomagnetic remanence directions are significantly smaller than inferred magnitudes that are based on the assumption that the basal SIC contact was initially planar. Thus, the basal SIC contact in the NE lobe likely had a trough-like geometry at the time of remanence acquisition. We advocate a scenario for the formation of the NE lobe, in which the trough geometry of the SIC is primary rather than a consequence of tilting prior to solidification of, and remanence acquisition in, the SIC. Finally, we caution the interpretation of photo lineaments in eroded basement terranes purely as a consequence of faulting.

Magnetostratigraphic dating of the prime-time sedimentary record of Himalayan tectonics and climate: new age constraints (13-10 Ma) from the Siwaliks of the Tinau Khola north section, Nepal

SUMMARY A molasse sequence comprising 1.5-km-thick sediments of the Lower and Middle Siwaliks Group in the sub-Himalayan North Belt along the Tinau Khola River in west-central Nepal was studied for the low-field magnetic susceptibility and anisotropy using single core specimens of siltstone/sandstone from 127 stratigraphic levels. The magnetic fabric comprises primary sedimentary-compactional and secondary tectonic components. It is characterized by (i) predominantly oblate magnetic susceptibility anisotropy ellipsoids, (ii) low anisotropy (P′ mostly <1.1) and low magnetic susceptibility (<ca. 10−7 m3 kg−1) contributed mainly by paramagnetic and diamagnetic minerals and (iii) WNW–ESE magnetic lineations subparallel to the fold axes/bedding strikes/thrust front, and hence normal to the direction of palaeotectonic compression. Thermal demagnetization of single core specimens from 103 levels across 1120 m of the lower part revealed a characteristic remanence of the high unblocking temperature (> 600 °C) in hematite. Remanence ratios derived from demagnetization data allowed the first-order estimation of remanence contributions from magnetic minerals (goethite, maghemite, magnetite and hematite), and discrimination of rockmagnetic zones correlatable with distinct lithofacies, which will facilitate objective mapping. We correlated a magnetic polarity sequence, constructed from normal and reverse polarity directions from 77 levels that passed the reversal test and represented primary remanences, with the standard geomagnetic polarity timescale to constrain the depositional age between ca. 13.2 Ma (base of Chron C5AAN, 13.015–13.183 Ma) and the middle of Chron C5n.2n (9.987–11.040 Ma). We calculated the sediment accumulation rate for polarity zones from the chronologically better constrained part below Chron C5n.2n (i.e. below 11.040 Ma) to be 25–61 cm kyr−1 (average, 39 cm kyr−1), which is consistent with the value of 32–50 cm kyr−1 reported from Siwaik sections in Nepal. The notable increase in accumulation rate after 12.1 Ma probably reflects the peak of the earlier phase of uplift and/or unroofing of the Himalayan source region followed by rapid accumulation in the foredeep, and a link to monsoon initiation/intensification. Compared to the expected remanence from the latest APWP for the Indian Plate 10–13 Ma, the best-defined mean (351.8°/20.9°) is rotated anticlockwise by 9.2° and records an inclination shallowing of 25°. Constraining the base of the Tinau Khola north section to 13.2 Ma (i.e. older than the Tinau Khola south by 1.7 Myr) should open up new horizons for multidisciplinary and multiproxy research targeting geotectonic/climatic/environmental palaeoreconsructions of Himalaya-wide events.

Oroclinal bending of the Middle and Late Paleozoic volcanic belts in Kazakhstan: Paleomagnetic evidence and geological implications

Paleomagnetic data on Middle- and Late-Paleozoic rocks from the central part of the Ural-Mongolian Belt in Kazakhstan are considered. The primary remanences in the Permian rocks and secondary magnetization components of the same age in pre-Permian rocks of central and northern Kazakhstan are not rotated relative to the East European Platform. In southern Kazakhstan adjoining the Tien Shan almost all data point to large, up to 90°, counterclockwise rotation of blocks. These rotations, related to the regional wrench fault zone, must be subtracted from older paleomagnetic data to ensure their correct interpretation. The paleomagnetic declinations in Upper Carboniferous rocks coincide more or less over all of Kazakhstan, whereas the Silurian and Early Devonian declinations in the north and south of Kazakhstan differ approximately by 180°. It can be suggested that the Devonian volcanic belt, having a horseshoe outline, was initially an almost rectilinear NW-trending feature. Its oroclinal bending took place in the Devonian and Early Carboniferous and completed by the Late Carboniferous. We compared the model of the Kazakh Orocline based on paleomagnetic data with the geological events in this territory. It turned out that a slow bending of an initially rectilinear subduction zone is consistent with lateral migration of active volcanism and folding inside a developing loop, whereas extension outside the loop was accompanied by subsidence and rifting. In general, the proposed model connects the main tectonic events in Kazakhstan with the movements established from paleomagnetic data.

Minor counterclockwise rotation of the Tatra Mountains (Central Western Carpathians) as derived from paleomagnetic results achieved in hematite-bearing Lower Triassic sandstones

Paleomagnetic studies accompanied with tectonic fieldworks have been carried out within hematite-bearing Lower Triassic red beds from the Tatra Mountains (Central Western Carpathians) in order to determine tectonic rotations of regional scale. The studied rocks contain primary remanent magnetization recorded by hematite and display both polarities. The obtained inclination is slightly lower than expected for stable Europe, which could imply either minor latitudinal separation or come from the inclination error. The results from six sampling sites representing rocks situated in autochthonous cover of the crystalline basement and inclined only during the Neogene uplift imply 19° counterclockwise rotation. Considering the tectonic position of the studied rocks, this rotation could be attributed to the entire Tatra Block. In turn, results from rocks tilted in two tectonic events: during Late Cretaceous compression and further in Neogene uplift requires double tectonic correction as normal unfolding procedure results in spurious declination. It is suggested that the latter mechanism together with local rotations of allochthonous nappes may explain part of variable rotations reported in previous paleomagnetic studies of Mesozoic rocks from the Central Western Carpathians.