Characteristic Remanent Magnetization Research Articles

Paleocurrent analysis of Pleistocene turbidite sediments in the forearc basin inferred from anisotropy of magnetic susceptibility and paleomagnetic data at the gas hydrate production test site in the eastern Nankai Trough

To understand the paleocurrent directions and depositional processes in the forearc basin along the active convergent margin of the eastern Nankai Trough, analysis of the anisotropy of magnetic susceptibility (AMS) has been applied to Pleistocene turbidite sediments in a borehole core obtained in the vicinity of the gas hydrate production test site. Sixty-one core specimens mainly sampled from silty and sandy sediments were measured, and the AMS results show generally oblate magnetic fabrics characterized by girdle-distributed weakly-clustered maximum and intermediate susceptibility axes. Diagnostic magnetic fabric parameters show that the sediments generally preserve primary sedimentary structures without significant sediment disturbance due to events such as coring and bioturbation. The characteristic remanent magnetization was also measured to determine the reorientation of each core. Paleocurrent directions estimated from the offset of the minimum susceptibility axes in the stratigraphic coordinates show two predominant directions: one toward the southwest (SW) and the other toward the northwest (NW). On the basis of the interpretations of 3D seismic data, the principal SW direction is generally parallel to the trough-shaped basin axis; this suggests that the sediments were carried by axial turbidity currents within the channel. The subordinate NW direction corresponds to the downslope direction of the Daini–Atsumi Knoll; this may be due to a sediment supply from the southeast associated with a submarine slope failure and landslide evolution or it may be due to flow reflections from the slope toward the basin center. The results of the paleocurrent analysis suggest that the Pleistocene depositional system around the Daini–Atsumi Knoll is affected by the basin configuration. These results are in good agreement with previous studies of the coeval depositional system for the adjacent forearc minibasin around the eastern Nankai Trough.

Independently dated paleomagnetic secular variation records from the Tibetan Plateau

Magnetostratigraphy has been serving as a valuable tool for dating and confirming chronologies of lacustrine sediments in many parts of the world. Suitable paleomagnetic records on the Tibetan Plateau (TP) and adjacent areas are, however, extremely scarce. Here, we derive paleomagnetic records from independently radiocarbon-dated sediments from two lakes separated by 250 km on the southern central TP, Tangra Yumco and Taro Co. Studied through alternating field demagnetization of u-channel samples, characteristic remanent magnetization (ChRM) directions document similar inclination patterns in multiple sediment cores for the past 4000 years. Comparisons to an existing record from Nam Co, a lake 350 km east of Tangra Yumco, a varve-dated record from the Makran Accretionary Wedge, records from Lakes Issyk-Kul and Baikal, and a stack record from East Asia reveal many similarities in inclination. This regional similarity demonstrates the high potential of inclination to compare records over the Tibetan Plateau and eventually date other Tibetan records stratigraphically. PSV similarities over such a large area (>3000 km) suggest a large-scale core dynamic origin rather than small scale processes like drift of the non-dipole field often associated with PSV records.

Early Cretaceous counterclockwise rotation of Northeast Africa within the equatorial zone: Paleomagnetic study on Mansouri ring complex, Southeastern Desert, Egypt

The Mansouri ring complex (132Ma) is, paleomagnetically, studied to shed light on the paleo-tectonic position of Northeast Africa during the Early Cretaceous. Progressive thermal demagnetization of all samples verified a general bi-vectorial decay of the natural remanence. After the removal of the present-day field overprint, the decaying anchored component was either:1.A dual-polarity, shallow NW–SE directed component residing in magnetite (400–585°C) of shiny fresh samples, or,2.A normal-polarity, medium-inclination, north-oriented component stored in haematite of few reddish ferruginous sites. This component was considered as chemical remagnetization carried by secondary haematite.Due to its steady stability, overwhelming existence in most sites, positive reversal test and its residence in fresh-samples’ magnetite, the first dual-polarity, shallow NW–SE component, was considered as the characteristic remanent magnetization [ChRM] representing the paleomagnetic field during cooling of the Mansouri ring complex. The mean paleomagnetic pole of the isolated ChRM was at 47°N/259°E, Dp/Dm=3.4°/6.6°.This Hauterivian pole from Egypt shows reasonable consistency with its coeval poles rotated from the main tectonic units to Northeast Africa. It reveals that in Early Cretaceous:1.Northeast Africa was equatorial, lying just south the Equator. Cairo, which is now at 30°N, was at −1.5° paleo-latitude.2.The Azimuth of the African Plate was NE–SW, about 30° clockwise with respect to the present-dayN–S trend.Comparing this Hauterivian pole to that of the Wadi Natash basalts [107±4Ma], which was at [55°N/250°E] during the Albian, the African Plate seems to have rotated counter-clockwise about 10° with Northeast Africa moving northwards [Cairo was moving from 1.5°S to 1.5°N] within the equatorial zone, during the Early Cretaceous.

Holocene archeointensities from mid European ceramics, slags, burned sediments and cherts

The Earth’s geomagnetic field intensity in the past can be determined from archeological artifacts. These archeointensity data are important inputs for geomagnetic field models and local reference curves of Earth’s magnetic field. Although archeointensities have been measured on materials for more than half a century ago, data are still scarce before 1000 BC and for the Alpine area in general. This investigation presents new absolute archeointensity data from a time period of 5000–700 BC from Italy and Switzerland. The archeological materials that were studied are ceramics, copper slag, and burned sediments from fireplaces. In addition, we investigated archeointensities from burned cherts, in order to uncover if they are a suitable material for paleomagnetism. Rock magnetic properties of all samples indicate magnetite, and small amounts of maghemite and hematite in the pseudosingle domain range as the ferromagnetic carriers. The IZZI protocol was used for 96 specimens to obtain absolute intensities; 23 ceramics, slags and burned cherts passed the threshold criteria, which we applied. The choice of the threshold values allowed us to obtain the linear part in the Arai diagram, which corresponds to the characteristic remanent magnetization. Burned sediments did not pass the threshold criteria, most probably because they acquired a thermochemical remanent magnetization during their formation. We demonstrate that magnetic susceptibility and saturation isothermal remanent magnetization can be used to select cherts that are suitable for paleointensity determinations. After applying anisotropy and cooling rate corrections, the new archeointensity values are lower for some samples, but fit well with available models and other archeomagnetic data.

New results for Palaeozoic volcanic phases in the Prague Basin – magnetic and geochemical studies of Lištice, Czech Republic

Palaeo-, rock magnetic and geochemical studies were conducted on volcanic samples from the Listice area to improve the knowledge of Palaeozoic volcanic evolution in the Prague Basin. The magnetic data display no significant differences between two studied localities, indicating one magnetizing event for both localities. Geochemical data suggest that Listice basalt could have originated from deep melting of the garnet peridotite mantle source during the attenuation and rifting of the continental lithosphere connected with asthenospheric mantle upwelling. The dataset furthermore supports the evidence of syn- or post- intrusive fluid interactions and low-temperature stages of alteration. The Ti-magnetite within amygdales of the samples was found to be carrying the characteristic remanent magnetization and reflects probably the Permo-Carboniferous remagnetization of volcanic phases.

Magnetic polarity of Upper Triassic sediments of the Germanic Basin in Poland

Palaeomagnetic results are presented for 205 samples of cores from the Ksiąz Wielkopolski IG-2, Woźniki K1 and Patoka 1 wells, drilled in the Polish part of Germanic Basin. The magnetic polarity stratigraphy is based on the inclination of the characteristic remanent magnetization, isolated in 60% of the total samples and found to be in general agreement with the expected Late Triassic inclination at the sampling sites. A total of 22 magnetozones from the integration of the three records correspond to about 25% of the published polarity zones for the Upper Triassic sediments that were combined in the worldwide composite polarity-time scale. The magne- tic polarity pattern, defined for the Schilfsanstein, fits very well with the one defined in the Tethys area for the upper part of the Julian sub-stage. According to the magnetostratigraphic data, the uppermost part of the Upper Gypsum Beds (equivalent to the Ozimek Member of the redefined Grabowa Formation) and the lowermost part of the Patoka Member, containing the Krasiejow bone-breccia horizon, can be correlated with the latest Tuvalian (~228.5 Ma) or with the middle part of Lacian (~225 Ma). However, if the “Long-Tuvalian” option for the Late Triassic Time Scale is taken into consideration, the parts of these substages mentioned above should be correlated with ~221.5 Ma and ~218.5 Ma, respectively.

Can a primary remanence be retrieved from partially remagnetized Eocence volcanic rocks in the Nanmulin Basin (southern Tibet) to date the India‐Asia collision?

AbstractPaleomagnetic dating of the India‐Asia collision hinges on determining the Paleogene latitude of the Lhasa terrane (southern Tibet). Reported latitudes range from 5°N to 30°N, however, leading to contrasting paleogeographic interpretations. Here we report new data from the Eocene Linzizong volcanic rocks in the Nanmulin Basin, which previously yielded data suggesting a low paleolatitude (~10°N). New zircon U‐Pb dates indicate an age of ~52 Ma. Negative fold tests, however, demonstrate that the isolated characteristic remanent magnetizations, with notably varying inclinations, are not primary. Rock magnetic analyses, end‐member modeling of isothermal remanent magnetization acquisition curves, and petrographic observations are consistent with variable degrees of posttilting remagnetization due to low‐temperature alteration of primary magmatic titanomagnetite and the formation of secondary pigmentary hematite that unblock simultaneously. Previously reported paleomagnetic data from the Nanmulin Basin implying low paleolatitude should thus not be used to estimate the time and latitude of the India‐Asia collision. We show that the paleomagnetic inclinations vary linearly with the contribution of secondary hematite to saturation isothermal remanent magnetization. We tentatively propose a new method to recover a primary remanence with inclination of 38.1° (35.7°, 40.5°) (95% significance) and a secondary remanence with inclination of 42.9° (41.5°,44.4°) (95% significance). The paleolatitude defined by the modeled primary remanence—21°N (19.8°N, 23.1°N)—is consistent with the regional compilation of published results from pristine volcanic rocks and sedimentary rocks of the upper Linzizong Group corrected for inclination shallowing. The start of the Tibetan Himalaya‐Asia collision was situated at ~20°N and took place by ~50 Ma.

Posterior archaeomagnetic dating: An example from the Early Medieval site Thunau am Kamp, Austria

The Early Medieval valley settlement of Thunau am Kamp in Lower Austria has been under archaeological excavation for 10years. The site was occupied during the 9th and 10th centuries AD according to potsherds, which seem to indicate two phases of activity: in the older phase ovens were placed in the corners of houses while during the younger phase they are found in the middle of the wall. The present study has been conducted in order to increase the archaeomagnetic database and fill the temporal gap around 900 AD. For this purpose 14 ovens have been sampled for their paleaomagnetic signals. Laboratory treatment generally confirmed that the baked clay has preserved stable directions. Apart from one exception, all the mean characteristic remanent magnetisation directions are concentrated on the Early Medieval part of the directional archaeomagnetic reference curve of Austria at about 900 AD. Using this curve archaeomagnetic dating provides ages between 800 and 1100 AD, which are in agreement with the archaeological dating. Together with the archaeological age estimates and stratigraphic information the new data have been included into the database of the Austrian curve and it has been recalculated using a new version of RenCurve. The new data confine the curve and its error band considerably in the time interval 800 to 1100 AD. This calibration process also provides probability density distributions for each included structure, which allows for posterior dating and refines temporal errors considerably. Because such dating includes archaeological information it is not an independent age estimate but is a combination of all available dating methods.

The curved Magallanes fold and thrust belt: Tectonic insights from a paleomagnetic and anisotropy of magnetic susceptibility study

The Magallanes fold and thrust belt (FTB) presents a large-scale curvature from N-S oriented structures north of 52°S to nearly E-W in Tierra del Fuego Island. We present a paleomagnetic and anisotropy of magnetic susceptibility (AMS) study from 85 sites sampled in Cretaceous to Miocene marine sediments. Magnetic susceptibility is lower than 0.0005 SI for 76 sites and mainly controlled by paramagnetic minerals. AMS results indicate that the sedimentary fabric is preserved in the undeformed areas of Tierra del Fuego and the more external thrust sheets units, where an incipient lineation due to layer parallel shortening is recorded. Prolate AMS ellipsoids, indicating a significant tectonic imprint in the AMS fabric, are observed in the internal units of the belt. AMS results show a good correlation between the orientation of the magnetic lineation and the fold axes. However, in Península Brunswick, the AMS lineations are at ~20° counterclockwise to the strike of the fold axes. Pretectonic stable characteristic remanent magnetizations (ChRM) were determined in seven sites. A counterclockwise rotation (21.2° ± 9.2°) is documented by ChRM data from four sites near the hinge of the belt in Península Brunswick and near Canal Whiteside while there is no evidence of rotation near the nearly E-W oriented Vicuña thrust within Tierra del Fuego. The curved shape of the Cenozoic Magallanes FTB is not related to vertical axis rotation, and thus, the Magallanes FTB can be considered as a primary arc.

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.

Further paleomagnetic results for lower Permian basalts of the Baoshan Terrane, southwestern China, and paleogeographic implications

The Baoshan Terrane of southwestern China is considered to have been part of the Cimmerian block during the late Paleozoic; consequently, knowledge of its paleoposition and geological evolution can provide constraints on the Permian breakup of northern East Gondwana. Therefore, we conducted paleomagnetic and rockmagnetic studies on lower Permian basalts from four localities in the Baoshan Terrane. The basalts hold a stable characteristic remanent magnetization (ChRM) at high temperatures (300–680°C) that is carried by magnetite, maghemite, and hematite with both pseudo-single and multiple domains. To test the reliability of data from these volcanic rocks, we analyzed the geomagnetic secular variation (GSV) and reliability of both the present data and previous paleomagnetic data. The results from 23 sites yield a single reversed polarity directed downwards to the southwest, giving a site-mean direction of Dg/Ig=156.7°/56.6° (kg=8.0, α95=11.4°) before tilt correction, and Ds/Is=218.3°/60.1° (ks=14.1, α95=8.4°) after tilt correction. The result passed the fold test, but the GSV was able to be averaged out in only two sections. All available data were examined section-by-section using the angular dispersion (SB) of virtual geomagnetic poles (VGPs) to ensure that the GSV was completely averaged out. Because the dispersion in declinations is likely to have been affectedby subsequent tectonic deformation, the paleosecular variation (PSV) could not be evaluated from all the data amassed from different sections, and the PSV was able to be removed from only four (combined) sections. A small-circle fit of these VGPs gives an averaged paleocolatitude of 51.9°±3.7° (N=31 sites) centered on 24°N, 99°E. The result indicates that the sampled area of the Baoshan Terrane was located at a latitude of 38°S±3.7° during the late early Permian. A comparison of this result with early Permian data from Gondwanan blocks suggests that the Baoshan Terrane was situated near the junction of northern India and northwestern Australia, and that it broke away from western Australia after the early Permian.

Dating of ancient kilns: A combined archaeomagnetic and thermoluminescence analysis applied to a brick workshop at Kato Achaia, Greece

Abstract We present here the results of a detailed archaeomagnetic and thermoluminescence investigation performed on bricks from two ancient kilns excavated at Kato Achaia, Greece. Magnetic mineralogy measurements have been carried out to determine the main magnetic carrier of the samples. The directions of the characteristic remanent magnetization of each structure have been obtained from standard thermal demagnetisation procedures and the absolute archaeointensity has been determined with the Thellier modified by Coe method, accompanied by regular partial thermoremanent magnetization (pTRM) checks. The full geomagnetic field vector was used for the archaeomagnetic dating of the two kilns, after comparison with the reference secular variation curves calculated directly at the site of Kato Achaia. Independent dating has also been obtained from thermoluminescence (TL) analysis on four brick samples from each kiln. The dating results obtained from the two methods have been compared and the last firing of each kiln has been estimated from the combination of the two techniques. Using the independent date offered by TL dating, the new archaeomagnetic data have been compared with other data from the same time period and they can further be used as reference points to enrich our knowledge about the past secular variation of the Earth's magnetic field in Greece.

Magnetostratigraphic dating of the Shanshenmiaozui mammalian fauna in the Nihewan Basin, North China

Timing of the mammalian faunas in the Nihewan Basin, North China has provided insights into our understanding of Quaternary biochronology and biostratigraphy in East Asia. Here we contribute to this topic with detailed magnetostratigraphic investigation, coupled with mineral magnetic measurements on a fluvio-lacustrine sequence in this basin, which contains the Shanshenmiaozui mammalian fauna. Magnetite and hematite were identified as the main carriers for the characteristic remanent magnetizations. Magnetostratigraphic results show that the Shanshenmiaozui sedimentary sequence recorded the Brunhes chron, the Jaramillo subchron, and the late Matuyama chron. Stratigraphic correlation in terms of lithology, magnetic susceptibility and magnetic polarity sequences between the Shanshenmiaozui, Xiaochangliang and Dachangliang sections indicates that the Shanshenmiaozui mammalian fossil layer is younger than the Xiaochangliang and Dachangliang artefact layers, which have been previously estimated to be about 1.36 Ma. The age of the Shanshenmiaozui mammalian fossil layer at the bottom of the section is estimated to be about 1.2 Ma. The new magnetostratigraphy of the Shanshenmiaozui section provides useful constraints on lithostratigraphic and biostratigraphic correlations in the Nihewan Basin.

Challenging the sensitivity limits of Paleomagnetism: Magnetostratigraphy of weakly magnetized Guadalupian–Lopingian (Permian) Limestone from Kyushu, Japan

Despite their utility for bio- and chemostratigraphy, many carbonate platform sequences have been difficult to analyze using paleomagnetic techniques due to their extraordinarily weak natural remanent magnetizations (NRMs). However, the physical processes of magnetization imply that stable NRMs can be preserved that are many orders of magnitude below our present measurement abilities. Recent advances in reducing the noise level of superconducting magnetometer systems, particularly the introduction of DC-SQUID sensors and development of a low-noise sample handling system using thin-walled quartz-glass vacuum tubes, have solved many of these instrumentation problems, increasing the effective sensitivity by a factor of nearly 50 over the previous techniques of SQUID moment magnetometry.Here we report the successful isolation of a two-polarity characteristic remanent magnetization from Middle–Late Permian limestone formed in the atoll of a mid-oceanic paleo-seamount, now preserved in the Jurassic accretionary complex in Japan, which had proved difficult to analyze in past studies. Paleothermometric indicators including Conodont Alteration Indices, carbonate petrology, and clumped isotope paleothermometry are consistent with peak burial temperatures close to 130°C, consistent with rock magnetic indicators suggesting fine-grained magnetite and hematite holds the NRM. The magnetic polarity pattern is in broad agreement with previous global magnetostratigraphic summaries from the interval of the Early–Middle Permian Kiaman Reversed Superchron and the Permian–Triassic mixed interval, and ties the Tethyan–Panthalassan fusuline zones to it. Elevated levels of hematite associated with the positive δ13Ccarb of the Kamura event argue for a brief spike in environmental oxygen. The results also place the paleo-seamount at a paleolatitude of ~12° S, in the middle of the Panthalassan Ocean, and imply a N/NW transport toward the Asian margin of Pangea during Triassic and Jurassic times, in accordance with the predicted trajectory from its tectono-sedimentary background. These developments should expand the applicability of magnetostratigraphic techniques to many additional portions of the Geological time scale.

Archaeological, archaeomagnetic and thermoluminescence investigation of a baked clay kiln excavated at Chieri, northern Italy: contribution to the rescue of our cultural heritage

<p>Combining different dating techniques is fundamental to constrain the ages of archaeological findings, mainly when direct evidences for their chronological context are not available. This paper presents the results of a combined archaeological, archaeomagnetic and thermoluminescence study of a kiln discovered during a rescue excavation at Chieri, northern Italy. The archaeological site is quite complex mainly due to the interposition of different stratigraphic levels that span from Roman times to present day. The studied kiln belongs to the post-medieval stratigraphic level but the lack of datable diagnostic objects strongly limits the possibility of its accurate dating only by means of archaeological evidences. Archaeomagnetic study was performed on 26 baked clay samples, isolating stable characteristic remanent magnetizations. The statistic comparison of the averaged direction with reference secular variation curves suggests two possible dating intervals. Independent dating from thermoluminescence study of 2 samples is consistent with the second time interval proposed by archaeomagnetism, suggesting that the kiln was abandoned at the beginning of the 17th century. The successful combination of the two independent laboratory techniques provides accurate dating of archaeological baked clays and can be used as routine for future archaeological investigations of rescue excavations.</p>

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.

Paleomagnetism and U‐Pb zircon geochronology of Lower Cretaceous lava flows from the western Lhasa terrane: New constraints on the India‐Asia collision process and intracontinental deformation within Asia

AbstractTo better understand the India‐Asia collisional process and intracontinental deformation within Asia, a paleomagnetic study has been conducted on the Qushenla Formation lava flows dated at ~132–120 Ma from the Yanhu area in the western Lhasa terrane. Stepwise thermal demagnetization isolates stable characteristic remanent magnetizations, which include dual polarity and pass fold tests at a 99% confidence level, indicating primary magnetizations. The tilt‐corrected site‐mean direction for 51 sites is D = 28.2°, I = 34.5°, and k = 74.3° with α95 = 2.3°, corresponding to a paleopole at 61.4°N, 192.9°E (A95 = 2.1°). Our new paleomagnetic data, combined with previous Cretaceous volcanic paleomagnetic data from the Lhasa terrane, show that the precollisional southern margin of Asia was at ~16.8°N with a relatively E‐W alignment and that the Lhasa terrane did not experience significantly discrepant north‐south movement although local vertical axis rotations did take place after the Cretaceous. Comparison with the apparent polar wander paths (APWPs) of India and the Cretaceous‐Paleocene paleopoles of the Tethyan Himalaya shows that the India‐Asia collision was at ~54.3 Ma and the Greater India during the Paleocene had a large northern extent of ~2000 km (~18.1°) beyond its present northern margin. Comparison with the Cretaceous Eurasian APWP indicates that a latitudinal convergence of ~1000 km has taken place between the Lhasa terrane and Eurasia since the India‐Asia collision. The amount of north‐south shortening deduced from Cretaceous paleomagnetic data is consistent with that accommodated by the Cenozoic fold and thrust belts between the Lhasa terrane and the Hexi corridor.

Paleomagnetic and 40Ar/39Ar geochronological results from the Linzizong Group, Linzhou Basin, Lhasa Terrane, Tibet: Implications to Paleogene paleolatitude and onset of the India–Asia collision

The Linzizong Group (64–44Ma) of the Lhasa Terrane in Tibet is critically positioned for establishing the paleoposition of the southern leading edge of the Asian continent during Paleogene times and constraining onset of the India–Asia collision. Here we report paleomagnetic results from a collection comprising 384 drill-core samples from 34 sites embracing all three formations of this group. Comprehensive demagnetization and field tests isolate characteristic remanent magnetizations (ChRM) summarized by overall tilt-corrected formation-mean directions of D=183.6°, I=−12.4° (α95=8.1°) for the Dianzhong (64–60Ma), D=1.0°, I=18.1° (α95=8.1°) for the Nianbo (60–50Ma), and D=12.4°, I=23.2° (α95=7.3°) for the Pana (50–44Ma). Fold tests are positive in each formation suggesting a pre-folding origin and we interpret the magnetizations as quasi-primary and acquired at, or slightly later than, formation of the Linzizong Group. Revised Paleogene paleopoles with Ar–Ar age constraints for the Lhasa Terrane indicate that onset of the India–Asia collision occurred no later than ∼60.5±1.5Ma at a low paleolatitude of ∼10°N. Analysis of 60 site-mean observations from a range of studies of the Pana Formation in the higher part of the succession highlight a large dispersion of ChRM directions; a number of possible causes are suggested but further study of this formation over a wider area is required to resolve this issue.

New insights into regional tectonics of the Indochina Peninsula inferred from Lower-Middle Jurassic paleomagnetic data of the Sibumasu Terrane

The post-Jurassic occurrence of differential tectonic rotation between the Indochina and South Sundaland blocks remains an issue to be properly investigated. New paleomagnetic study is used here to find the role of Sibumasu Terrane in this rotation, which is located between a clockwise rotated Indochina Block and a counter-clockwise rotated South Sundaland Block. For this purpose, lower to middle Jurassic red sandstones of the Umphang Group in the Sibumasu Terrane were sampled at 21 sites in the Ratchaburi area (13.6°E, 99.6°E), Thailand. Stepwise thermal demagnetization by 680°C unblocked a pre-folding characteristic remanent magnetization. A mean direction of this component at 100% unfolding is Ds=348.5°, Is=24.7°, α95=10.5°, k=10.7, N=20, corresponding to an Early-Middle Jurassic pole of λ=78.6°N, ϕ=10.6°E (A95=9.3). Comparison of this direction with those reported from other localities of the Umphang Group (Kalaw, Mae Sot and North Trang Syncline localities) reveal variable declinations (between 348.5° and 44.7°) for the Sibumasu Terrane. We ascribe this variation to differential tectonic deformation in the Sibumasu Terrane, as reflected from sinusoidal shaped structural features in the study area. The presence of such features in the granitic rocks indicates the occurrences of deformational activities after their intrusion, which took place in the period between 130Ma and 51Ma. The Sibumasu Terrane behaved as an independent fragment at a time when Indochina was undergoing a clockwise rotation and southward displacement, as a result of extrusion tectonics after the gigantic India–Asia collision. Taking into consideration a westerly deflected declination (D=342.8°) from the West Trang area in Peninsular Thailand, a counterclockwise rotation of 15° is estimated for the Sibumasu Terrane, as a result of continuous northward indentation of the Australian Plate into South Sundaland Block.

Quaternary remagnetization of the Neoproterozoic limestone of Negash Synclinorium (Arabian–Nubian Shield, northern Ethiopia): With implications of no paleomagnetic testing for the proposed Snowball Earth events

Eighty-one paleomagnetic cores were collected from 10 locations across a black limestone unit within the core of Negash Synclinorium, northern Ethiopia in order to test the proposed Snowball Earth events for the diamictite unit of the Tambien Group. Cores were cut into two standard paleomagnetic specimens and were subjected to stepwise demagnetizations using both Thermal (TH) and alternating field (AF) techniques. Rock magnetic analyses on representative specimens were done and results revealed goethite, pyrrhotite, titano-magnetite, and titano-hematite to be the major magnetic materials carrying the magnetizations with PSD (pseudo single domain) grain size range. In most cases paleomagnetic directions are defined by a single component of magnetization, where a viscous component is present it is usually removed by heating to a temperature of ∼200°C or an AF of ∼10mT. The high stability component isolated above temperature of 200°C or AF of 15mT, defined straight line trajectories directed towards the origin and considered as the Characteristic Remanent Magnetization Direction (ChRM). The direction of magnetization of the ChRM is determined for samples with stable straight line segments by the best-fit line using the least square technique of Kirschvink (1980). In the cases of overlapping spectra and unblocking temperatures, direction of magnetization is determined by remagnetization circles of Halls (1976, 1978). When site mean ChRM directions are plotted on stereogram, their distribution is relatively clustered in geographic coordinates and the overall mean direction is Decg=358.5°, Incg=16.6° (α95=3.8°, K=162.8, N=10). After a structural restoration to the horizontal is made the directions disperse and fail the fold test of both McElhinny’s and McFadden’s tests and the mean direction is Decs=353.5°, Incs=8.8° (α95=18.9°, K=7.5, N=10). This is interpreted to result from a later remagnetization of the black limestone. All directions are normal in polarity and have mean unrestored paleomagnetic directions comparable to the Quaternary paleomagnetic directions. Virtual Geomagnetic poles (VGP) in the unrestored position is used to calculate overall mean VGP position resulting long=235.7°E, latg=84.5°N (A95=3.0°, N=10). Comparison of the obtained pole with the apparent polar wander path (APWP) curve for Africa of Besse and Courtillot (1991, 2003) and with the 2Ma reference pole of stable Africa (Kidane et al., 2003) is found to be consistent with remagnetizations during the Quaternary period. Hence supporting evidence for the proposed Snowball Earth event of the Sturtian glaciation in the Negash rocks could not, unfortunately, be obtained from paleomagnetism.