Alternating Field Demagnetization Research Articles

Composition law of oblique anhysteretic remanent magnetization and its relation to the magnetostatic interaction

AbstractThe basic properties of oblique anhysteretic remanent magnetization (OARM) acquired in a weak and steady magnetic field with an arbitrary angle to the alternating field direction were studied. OARM and rock‐magnetic experiments were conducted on samples of basalt, granite, and sediment containing noninteracting single domain (SD), interacting SD, pseudo‐single‐domain, and multidomain low‐Ti titanomagnetites. The intensity of OARM (MOARM) systematically increased or decreased with increasing angle between alternating and steady field directions (θSF), while the angle between alternating field and OARM directions (θOARM) increased with increasing θSF for all samples. During stepwise alternating field demagnetization, the OARM vector shows a single component parallel to the steady field direction for θSF = 0° (ARMǁ) and 90° (ARM⊥). The median destructive field of ARM⊥ is larger than that of ARMǁ. For intermediate angles (θSF = 30°, 45°, and 60°), the OARM vector did not parallel the applied steady field but gradually increased θOARM with higher coercivity. These experiments indicate that the OARM vector is approximately given by the sum of two orthogonal magnetizations coinciding with ARMǁ and ARM⊥, respectively. Thus, the OARM vector can be determined by acquisition efficiencies of ARMǁ and ARM⊥ in an individual sample. Based on these experiments and associated rock‐magnetic measurements, noninteracting SD samples show lower ARM⊥/ARMǁ ratios, compared to other samples. This result suggests that OARM can be used as a useful tool to detect noninteracting SD particles in the paleomagnetic samples.

Paleomagnetic results from Jurassic-Cretaceous strata in the Chuangde area of southern Tibet constrain the nature and timing of the India-Asia collision system

The Tethyan Himalaya sits between the India craton and the Lhasa block, the contact being marked by the Indus-Tsangpo suture zone (ITSZ). The amalgamation of the Tethyan Himalaya with the Lhasa block has been traditionally regarded as representing the India-Asia collision. Recently, however, a dual-collision model has been proposed, in which Tethyan Himalayais was considered as a separate block that rifted from Greater India’s northern margin in Late Cretaceous. The Tethyan Himalaya subsequently underwent two episodes of collision, i.e., the first with Lhasa block to the north ~50 Ma and the second collision with India to the south ~23 Ma. The dual-collision model hinges on the speculated rifting of Tethyan Himalaya from Greater India in Late Cretaceous, which, however, has not been paleomagnetically constrained because the Late Cretaceous paleomagnetic data from Tethyan Himalaya are lacking. Clearly, elucidating the Mesozoic, particularly the Late Cretaceous, tectonic evolution of Tethyan Himalaya is crucial to understanding the India-Asia collisional processes. To better constrain the paleogeographic position and drift history of Tethyan Himalaya during the Mesozoic, particularly in Late Cretaceous, a paleomagnetic investigation was conducted of the Xiare and Weimei (Jurassic) and Chuangde (Cretaceous) formations in the Chuangde areas of southern Tibet. Paleomagnetic samples were collected from 39 sites, among which 5 sites were from grey andesite of the Xiare Formation, 25 sites were from siltstone and sandstones of the Weimei Formation, and 9 sites were from limestones of the Chuangde Formation. A total of 118 paleomagnetic specimens were subjected to stepwise thermal or alternating field demagnetization. The demagnetization results show that samples from the Xiare Formation and the Weimei Formation did not yield characteristic remanent magnetization (ChRM). Specimens from the Chuangde Formation yield stable ChRMs upon demagnetization that display both normal and reversed polarities, and passed a reversal test. The anisotropy of magnetic susceptibility (AMS) data of samples from the Chuangde Formation show that the minimum axes are sub-vertical and the intermediate and maximum axes are subparallel to the bedding, indicative of a depositional-type fabric. Temperature-dependence magnetic susceptibility ( k - T curves) and stepwise thermal demagnetization of the composite isothermal remanent magnetization (IRM) indicate that the remanence is carried principally by hematite and magnetite. Taken together, the sedimentological, rock magnetic, and paleomagnetic results suggest that the Chuangde ChRMs likely represent primary remanence that was acquired during 86.3–74.0 Ma, the age being constrained based on the foraminera data. The mean of the ChRMs of specimens from the Chuangde Formation is D s=152.0°, I s= - 52.9°, κ =18.0, α 95=5.9° after tilt correction. The paleomagnetic results are interpreted to represent remanence of a normal polarity based on geological constraints and thus indicate that Tethyan Himalaya was located at 33.1°±5.6°S in Late Cretaceous (80.1±6.2 Ma). The paleolatitude of Tethyan Himalaya is similar to the coeval paleolatitude of India (reference site: 28.0°N, 88.5°E), suggesting that Tethyan Himalaya and India were not separated by oceans, but likely formed a single crustal block in the Campanian. It is possible that rifting took place afterwards. However, provenance studies of sediments in the middle Eocene (~45 Ma) Bhainskati Formation in Lower Himalaya have components sourced from the Tethyan Himalaya, suggesting that the Tethyan Himalya and India was then in immediate proximity. If the Tethyan Himalaya rifted from India after 74 Ma, it must have collided with India by ~45 Ma. This would require the development of and elimination of a ~1700-km-wide ocean within ~30 Myr, which is tectonically highly unlikely. Therefore, the paleomagnetic data obtained from the Chuangde Formation do not support the dual-collision model, but instead support the traditional model, that is the collision between Tethyan Himalaya and Lhasa block represents the India-Asia collision.

Further evidence for early lunar magnetism from troctolite 76535

AbstractThe earliest history of the lunar dynamo is largely unknown and has important implications for the thermal state of the Moon and the physics of dynamo generation. The lunar sample with the oldest known paleomagnetic record is the 4.25 billion year old (Ga) troctolite 76535. Previous studies of unoriented subsamples of 76535 found evidence for a dynamo field with a paleointensity of several tens of microteslas. However, the lack of mutual subsample orientation prevented a demonstration that the magnetization was unidirectional, a key property of thermoremanent magnetization. Here we report further alternating field demagnetization on three mutually oriented subsamples of 76535, as well as new pressure remanent magnetization experiments to help rule out shock magnetization. We also describe new 40Ar/39Ar thermochronometry and cosmogenic neon measurements that better constrain the rock's thermal history. Although the rock is unbrecciated, unshocked, and slowly cooled, its demagnetization behavior is not ideal due to spurious remanence acquisition. Despite this limitation, all three subsamples record a high coercivity magnetization oriented in nearly the same direction, implying that they were magnetized by a unidirectional field on the Moon. We find no evidence for shock remanence, and our thermochronometry calculations show no significant reheating events since 4249 ± 12 million years ago (Ma). We infer a field paleointensity of approximately 20–40 μT, supporting the previous conclusion that a lunar dynamo existed at 4.25 Ga. The timing of this field supports an early dynamo powered by thermal or thermochemical core convection and/or a mechanical dynamo but marginally excludes a dynamo delayed by thermal blanketing from radiogenic element‐rich magma ocean cumulates.

Palaeomagnetic results and new dates of sedimentary deposits from Klasies River Cave 1, South Africa


 
 
 Palaeomagnetic data from Klasies River main site Cave 1 (Eastern Cape Province, South Africa) are reported. Natural remanent magnetisation directions obtained from 77 oriented samples were determined by progressive alternating field demagnetisation methodology. Three palaeomagnetic samplings from the Witness Baulk from the Middle Stone Age (MSA) Late Pleistocene White Sand member and the Holocene Later Stone Age (LSA) middens in Cave 1 were dated and analysed to obtain the palaeomagnetic directions recorded in the sediments. Here we provide new optically stimulated luminescence (OSL) dates for the White Sand Member, and new accelerator mass spectrometry (AMS) radiocarbon dates for the LSA midden of areas not previously dated. The palaeomagnetic analysis took into account rock magnetism and directional analysis. The former reveals that the main magnetic carrier was magnetite; the latter shows that characteristic remanent magnetisation of normal and anomalous directions was observed in the lower portion of the White Sand Member and LSA midden. Normal directions correspond to the palaeosecular variation record for South Africa during the Late Pleistocene. On the other hand, the anomalous directions recorded in the LSA midden might represent the likely Sterno-Etrussia geomagnetic field excursion which occurred during the Late Holocene and is observed in other places on the planet. Finally, the directional data obtained are a potential tool for discussing the age of deposits corresponding to those periods.
 
 
 
 
 Significance: 
 
 
 
 New dates confirm and extend previous age determinations for the LSA and White Sand Member from Klasies River
 
 
 

Distinguishing and correlating deposits from large ignimbrite eruptions using paleomagnetism: The Cougar Point Tuffs (mid‐Miocene), southern Snake River Plain, Idaho, USA

AbstractIn this paper, we present paleomagnetic, geochemical, mineralogical, and geochronologic evidence for correlation of the mid‐Miocene Cougar Point Tuff (CPT) in southwest Snake River Plain (SRP) of Idaho. The new stratigraphy presented here significantly reduces the frequency and increases the scale of known SRP ignimbrite eruptions. The CPT section exposed at the Black Rock Escarpment along the Bruneau River has been correlated eastward to the Brown's Bench escarpment (six common eruption units) and Cassia Mountains (three common eruption units) regions of southern Idaho. The CPT records an unusual pattern of geomagnetic field directions that provides the basis for robust stratigraphic correlations. Paleomagnetic characterization of eruption units based on geomagnetic field variation has a resolution on the order of a few centuries, providing a strong test of whether two deposits could have been emplaced from the same eruption or from temporally separate events. To obtain reliable paleomagnetic directions, the anisotropy of anhysteretic remanence was measured to correct for magnetic anisotropy, and an efficient new method was used to remove gyroremanence acquired during alternating field demagnetization.

Automated paleomagnetic and rock magnetic data acquisition with an in-line horizontal “2G” system

Today's paleomagnetic and magnetic proxy studies involve processing of large sample collections while simultaneously demanding high quality data and high reproducibility. Here we describe a fully automated interface based on a commercial horizontal pass-through “2G” DC-SQUID magnetometer. This system is operational at the universities of Bremen (Germany) and Utrecht (Netherlands) since 1998 and 2006, respectively, while a system is currently being built at NGU Trondheim (Norway). The magnetometers are equipped with “in-line” alternating field (AF) demagnetization, a direct-current bias field coil along the coaxial AF demagnetization coil for the acquisition of anhysteretic remanent magnetization (ARM) and a long pulse-field coil for the acquisition of isothermal remanent magnetization (IRM). Samples are contained in dedicated low magnetization perspex holders that are manipulated by a pneumatic pick-and-place-unit. Upon desire samples can be measured in several positions considerably enhancing data quality in particular for magnetically weak samples. In the Bremen system, the peak of the IRM pulse fields is actively measured which reduces the discrepancy between the set field and the field that is actually applied. Techniques for quantifying and removing gyroremanent overprints and for measuring the viscosity of IRM further extend the range of applications of the system. Typically c. 300 paleomagnetic samples can be AF demagnetized per week (15 levels) in the three-position protocol. The versatility of the system is illustrated by several examples of paleomagnetic and rock magnetic data processing.

A whole rock absolute paleointensity determination of dacites from the Duffer Formation (ca. 3.467 Ga) of the Pilbara Craton, Australia: An impossible task?

We have conducted a whole-rock type magnetic and absolute paleointensity determination of the red dacite of the Duffer Formation from the Pilbara Craton, Australia. The age of the dated rock unit is 3467±5Ma (95% confidence). Vector analyses results of the step-wise alternating field demagnetization (NRM up to 100mT) and thermal demagnetization (from NRM up to 650°C) yield three components of magnetization. Curie point determinations indicate three characteristic temperatures, one at 150–200°C, a second one at ∼450°C and a third one at ∼580°C. Magnetic grain-size experiments were performed on small specimens with a variable field translation balance (VFTB). The coercivity of remanence (Hcr) suggests that the NRM is carried by low-coercivity grains that are associated with a magnetite fraction as is shown by the high-temperature component with blocking temperatures above 450°C and up to at least 580°C. The ratios of the hysteresis parameters plotted as a modified Day diagram show that most grain sizes are scattered within the Single Domain (SD) and the Superparamagnetic and Single Domain SP-SD domain ranges. In addition to the rock magnetic experiments we have performed absolute paleointensity experiments on the samples using the modified Thellier-Coe double heating method to determine the paleointensities. Partial-TRM (p-TRM) checks were performed systematically to document magnetomineralogical changes during heating. The temperature was incremented by steps of 50°C between room temperature and 590°C. The paleointensity determinations were obtained from the slope of Arai diagrams. Our paleointensity results indicate that the paleofield obtained was ∼6.4±0.68 (N=11) micro-Teslas with a Virtual Dipole Moment (VDM) of 1.51±0.81×1022Am2, from a medium-to high-temperature component ranging from 300 to 590°C that has been interpreted to be the oldest magnetization yet recorded in paleomagnetic studies of the Duffer Formation. The absolute paleointensity is relatively low and we interpret this low-paleofield bias a result of a thermochemical remanent magnetization (TCRM) process that indicates a possible underestimate of the paleofield by a factor of four for the red dacite of the Duffer Fm.

Preliminary magnetostratigraphic results from the late Miocene Maragheh Formation, NW Iran

Maragheh in northwestern of Iran is a world famous Miocene fossil-bearing area. The area has yielded classical late Miocene Turolian age fauna that has been collected and studied sporadically over the last 150 years. However, the precise correlation of these sediments to the Global Time Scale (GTS) has remained ambiguous. To address this, 115 levels along an approximately 27-m-thick interval were collected from the middle Maragheh Formation at Dareh Gorg (Gort Daresi) section. Characteristic remanent magnetization directions obtained by alternating field demagnetization produce a polarity pattern that is supported by thermal demagnetization on a set of sister specimens. Three polarity intervals were recognised, the middle part of the section at around 15–21 m showing reversed polarity, bounded by normal polarities above and below. Based on the palaeontological constraints and recent K-Ar age determinations from the Maragheh Fm, three correlations to the geomagnetic polarity time scale appear likely. According to these correlations, recently discovered hominoid locality is correlated to C3Br.1n, C4n.1n, or to C4n.2n. For a unique correlation, however, additional palaeomagnetic data is required from the upper and lower parts of the section.

Magnetostratigraphy of the middle-upper Jurassic sedimentary sequences at Yanshiping, Qiangtang Basin, China

A series of important geological events occurred in the Tibetan Plateau area during the Jurassic, such as the collision of the Lhasa and Qiangtang Terranes, the closure of the Meso-Tethyan Ocean, the opening of the Neo-Tethyan Ocean and the cessation of the mega-monsoon. The similar to 3000 m thick Jurassic sedimentary sequence in the Qiangtang Basin on the central Tibetan Plateau, which is called the Yanshiping (YSP) Group, recorded these geological events. However, the chronology of the sequence is surprisingly poorly constrained. Here, we perform a detailed palaeomagnetic analysis on the similar to 1060 m thick middle and upper portions of the YSP Group (the Xiali and Suowa Formations) in the YSP section of the eastern Qiangtang Basin. Three bivalve zones at stratigraphic intervals of similar to 40-140, 640-800 and 940-1040 m are identified, which yield a Bathonian-Callovian age for the Lower Xiali Fm., a Callovian-Oxfordian age for the Lower Suowa Fm. and an Oxfordian-Kimmeridgian age for the Upper Suowa Fm. A total of 544 oriented palaeomagnetic samples were collected from the section. By combining thermal and alternating field demagnetizations, clear characteristic remanent magnetization (ChRM) directions are isolated for most of the samples. The robust ChRM directions pass fold and reversals tests, which support the primary nature of the ChRMs and yield a palaeopole at 76.8A degrees N/297.2A degrees E (dp = 2.2A degrees, dm = 3.7A degrees). A total of 27 normal and 26 reversed polarity zones were successfully recorded in the section. Combined with fossil age constraints, results suggest that the section is plausibly composed of a Callovian-Early Kimmeridgian age sedimentary sequence.

A new protocol for three-axis static alternating field demagnetization of rocks

Static three-axis alternating field (AF) demagnetization is the most common method regularly implemented for removing magnetic components of rock samples. This method is so widely used that one of its main limitations, the acquisition of gyroremanence (GRM), is often not accounted for or even discussed. The presence of GRM likely interferes more than is recognized in accurate determination of the most stable remanence. The accepted method proposed by Dankers and Zijderveld (1981) for excluding GRM affected measurements requires nearly triple the amount of lab work, and by consequence, is almost never regularly implemented on large batches of samples. Here, we present a laboratory procedure and subsequent analysis (SI method) that removes the effects of GRM in static AF demagnetization without requiring extra laboratory work. This paper, therefore, describes a new standard protocol for efficient static AF demagnetization of rocks.

Development of a 0.5 T magnetic‐core alternating‐field demagnetizer

AbstractWe have constructed an alternating‐field (AF) demagnetizer with a magnetic core in a passively air‐cooled coil that can routinely operate at fields up to 0.5 T, almost 3 times higher than we could attain before in our commercial instrument. The field is powered by a commercial 1 kW power amplifier and is transverse to the bore, uniform to ±2% over a 25 mm paleomagnetic sample, and compatible with our existing sample handler for automated demagnetization and measurement. Even harmonics are ≤1 ppm of the fundamental and so generate negligible anhysteretic remanence. The much higher peak alternating field, 2 and 5 times that commonly available in air‐core solenoidal and Helmhotz coil configurations, respectively, enables successful AF demagnetization of many samples that could not be completely demagnetized with commercially available equipment. This capability is especially useful for high‐coercivity sedimentary and igneous rocks and extraterrestrial materials that contain magnetic minerals that alter during thermal demagnetization. In addition to the benefits, this instrument brings to our own research, a much broader potential impact is that it could replace the transverse coils of most automated AF demagnetization systems in use today, whether for discrete or continuous U‐channel measurements, which are commonly limited to peak fields of ∼100 mT. Manual and tumbling demagnetizers would benefit as well by the ∼2 times increase in maximum field over those that can be attained by commercial solenoidal coils. Furthermore, we expect that it and similarly designed magnetic‐core instruments will be capable of attaining even higher fields, of order 1 T.

Paleomagnetic and geochronological study of Carboniferous forearc basin rocks in the Southern New England Orogen (Eastern Australia)

We present results of a paleomagnetic study from Carboniferous forearc basin rocks that occur at both limbs of the Texas Orocline (New England Orogen, eastern Australia). Using thermal and alternating field demagnetizations, two remanence components have been isolated from rocks sampled from the Emu Creek terrane, in the eastern limb of the orocline. A middle-temperature Component M is post-folding and was likely acquired during low-temperature oxidation at 65–35Ma. A high-temperature Component H is pre-folding, but its comparison with the paleomagnetic data from coeval rocks in the northern Tamworth terrane on the other limb of Texas Orocline does not indicate rotations around a vertical axis, as expected from geological data. A likely explanation for this apparent discrepancy is that Component H postdates the oroclinal bending, but predates folding in late stages of the 265–230Ma Hunter Bowen Orogeny. The post-Kiaman age of Component H is supported by the presence of an alternating paleomagnetic polarity in the studied rocks. A paleomagnetic study of volcanic and volcaniclastic rocks in the Boomi Creek area (northern Tamworth terrane) revealed a stable high-temperature pre-folding characteristic remanence, which is dated to c. 318Ma using U–Pb zircon geochronology. The new paleopole (37.8°S, 182.7°E, A95=16.2°) is consistent with previously published poles from coeval rocks from the northern Tamworth terrane. The combination of our new paleomagnetic and geochronological data with previously published results allows us to develop a revised kinematic model of the New England Orogen from 340Ma to 270Ma, which compared to the previous model, incorporates a different orientation of the northern Tamworth terrane at 340Ma.

Reassessment of Aguapeí (Salto do Céu) paleomagnetic pole, Amazonian Craton and implications for Proterozoic supercontinents

The Aguapeí paleomagnetic pole obtained for mafic sills and dykes from Salto do Céu region at the western margin of the Amazonian Craton constrains its links with Baltica and Laurentia in Rodinian reconstructions. A new U–Pb age on baddeleyite at 1439±4Ma for the intrusives, contrasts strongly with a previous Ar–Ar age at 981±2Ma, with important consequences for paleogeographic reconstructions. We report new paleomagnetic and magnetic anisotropy results for sills from the Salto do Céu region and reassess the paleomagnetic data in view of the new geochronological age. A total of 155 samples were collected from thirteen new paleomagnetic sampling sites, five of them corresponding to sedimentary rocks located at the borders of the sills in an attempt to perform baked contact tests. After thermal and alternating field demagnetization, the sills provided a characteristic magnetic component at Dm=208.2°, Im=68.5° (N=8, α95=6.4°), with a corresponding paleomagnetic pole at 46.4°S; 277.0°E (A95=10.2°). Directions obtained in this study are similar to those reported previously for other mafic sills and dykes in the same region. A pole integrating the new results for the sills with those of the previous Aguapeí pole is situated at 56.0°S; 278.5°E (A95=7.9°). This new combined Salto do Céu pole supersedes the previous Aguapeí pole. Magnetic mineralogy studies, optical and electronic microscopy indicates PSD magnetite as the main magnetic carrier in these rocks. The baked contact tests were inconclusive, but the similarity between Salto do Céu pole and other high-quality poles with ages around 1420–1430Ma suggests they carry a primary thermoremanence of that age. The Salto do Céu and other coeval poles are compatible with a connection between Amazonia and Baltica at the Mesoproterozoic in a paleogeographic configuration slightly different from SAMBA (South America-Baltica). At the same time, the new geochronological and paleomagnetic data imply that all paleogeographic interpretations for the position of Amazonia in Rodinia based on the previously published Aguapeí pole (and now renamed as Salto do Céu pole) must be revised.

Paleomagnetism of Miocene volcanics on Sao Tome: Paleosecular variation at the Equator and a comparison to its latitudinal dependence over the last 5 Myr

AbstractA collection was made in January 2009 of 10 oriented samples from each of 54 sites in lavas on Sao Tome Island (nominal location 0.3ºN, 6.5ºE). Some sites were affected by lightning leaving a total of 42 sites for analysis of paleosecular variation. Overall magnetic properties were excellent (highly stable magnetizations carried by pseudosingle domain magnetite). After principal component analysis of progressive alternating field demagnetization data for the samples, 22 sites had normal polarity magnetizations (D = 0. 6º, I = −8.3º, α95 = 4.3º, κ = 53.1) and 20 had reverse magnetizations (D = 176.0º, I = 4.2º, α95 = 7.3º, κ = 20.8); the directions are within 5° of antiparallel, yielding a positive reversal test. The combined data set of 42 site mean virtual geomagnetic poles converted to common (normal) polarity yields a pole position at 86.0°N, 211.5ºE, A95=3.1º. Ar/Ar and K/Ar dating reveals that these rocks are Miocene in age (∼5–11 Ma), old enough to allow northerly plate motion to help explain the slightly far‐sided pole position. The between‐site dispersion in virtual geomagnetic poles was estimated as the angular standard deviation, Sb, and equaled 11.4° with 95% confidence interval between 9.9° and 13.4°.

Preservation and detectability of shock‐induced magnetization

AbstractAn understanding of the effects of hypervelocity impacts on the magnetization of natural samples is required for interpreting paleomagnetic records of meteorites, lunar rocks, and cratered planetary surfaces. Rocks containing ferromagnetic minerals have been shown to acquire shock remanent magnetization (SRM) due to the passage of a shock wave in the presence of an ambient magnetic field. In this study, we conducted pressure remanent magnetization (PRM) acquisition experiments on a variety of natural samples as an analog for SRM acquisition at pressures ranging up to 1.8 GPa. Comparison of the alternating field (AF) and thermal demagnetization behavior of PRM confirms that AF demagnetization is a more efficient method for removing SRM overprints than thermal demagnetization because SRM may persist to unblocking temperatures approaching the Curie temperatures of magnetic minerals. The blocking of SRM to high temperatures suggests that SRM could persist without being eradicated by viscous relaxation over geologic timescales. However, SRM has been rarely observed in natural samples likely because of two factors: (1) other forms of impact‐related remanence (e.g., thermal remanent magnetization from impact‐related heating or chemical remanent magnetization from postimpact hydrothermal activity) are often acquired by target rocks that overprint SRM, and (2) low SRM acquisition efficiencies may prevent SRM from being distinguished from the underlying primary remanence or other overprints due to its low magnetization intensity.

Paleomagnetic results from the Early Cretaceous Lakang Formation lavas: Constraints on the paleolatitude of the Tethyan Himalaya and the India–Asia collision

To better constrain the Early Cretaceous paleogeographic position of the Tethyan Himalaya and the India–Asia collision process, a paleomagnetic study was performed on the Lakang Formation lava flows in the Cuona area in the southeastern Tethyan Himalaya. Stepwise thermal and alternating field demagnetizations successfully isolated reliable characteristic remanent magnetization (ChRM) directions that include antipodal dual polarities and pass positive fold tests at the 99% confidence level and reversal tests at 95% confidence level, indicating prefolding primary magnetizations. The distribution patterns of ChRM directions from the Lakang Formation lava flows are consistent with young lava flows at similar latitudes, suggesting that secular variation has likely been averaged out. The tilt-corrected site-mean direction for 31 sites is D=261.6°, I=−68.5° with α95=3.6°, which provides a paleopole at 26.8°S, 315.2°E (A95=5.7°), corresponding to a paleolatitude of 52.2°±5.7°S for the study area. Comparison of the paleolatitude observed from the Lakang Formation lava flows with that expected from the apparent polar wander paths of India at 130 Ma show a paleolatitude difference of ∼2.1° (∼230 km), indicating that neither a great north-south continental crustal shortening occurred between the Indian craton and the Tethyan Himalaya after 130 Ma, nor that a wide ocean separated them at that time. Comparison with reliable Cretaceous–Paleocene paleomagnetic results observed from the Tethyan Himalaya and the Lhasa terrane indicates that the latitudinal width of the Neo-Tethyan Ocean could have been up to ∼7000 km at 134–130 Ma and an extension should have existed between the Indian craton and the Tethyan Himalaya during the Late Cretaceous and Paleocene. Furthermore, reliable paleomagnetic results suggest that the India–Asia collision was a dual-collision process, consisting of a first collision of the Tethyan Himalaya with the Lhasa terrane (Asia) at 54.9±2.3 Ma and a final continent–continent collision of the Indian craton with the Tethyan Himalaya at 40.0±3.3 Ma.

Late Triassic paleolatitude of the Qiangtang block: Implications for the closure of the Paleo-Tethys Ocean

To better constrain the Late Triassic paleolatitude of the Qiangtang block and the closure of the Paleo-Tethys Ocean, a combined paleomagnetic and zircon U/Pb geochronological study has been conducted on the Upper Triassic Jiapila Formation volcanic rocks on the northern edge of the Qiangtang block of Central Tibet (34.1°N, 92.4°E). These rocks are dated to 204–213 Ma. Progressive thermal or alternating field demagnetization successfully isolated stable characteristic remanent magnetizations (ChRM) that pass both the fold and reversal tests, consistent with a primary magnetization. These are the first volcanic-based paleomagnetic results from pre-Cretaceous rocks of the Qiangtang block that appear to average secular variation well enough to yield a reliable paleolatitude estimate. Based on our new paleomagnetic data from Upper Triassic lavas, we conclude that the Late Triassic pole of the Qiangtang block was located at 64.0°N, 174.7°E, with A95=6.6° (N=29). We compile published paleomagnetic data from the Qiangtang block to calculate a Late Triassic latitude for the Qiangtang block at 31.7 ± 3.0°N. The central Paleo-Tethys Ocean basin was located between the North China (NCB) and Tarim blocks to the north and the Qiangtang block to the south during Late Paleozoic–Early Mesozoic. A comparison of published Early Triassic paleopole from the Qiangtang block with the coeval paleopoles from the NCB and Tarim indicates that the Paleo-Tethys Ocean could not have closed during the Early Triassic and that its width was approximately ∼32–38° latitude (∼3500–4200 km). However, the comparison of our new combined Late Triassic paleomagnetic result with the Late Triassic poles of the NCB and Tarim, as well as numerous geological observations, indicates that the closure of the Paleo-Tethys Ocean at the longitude of the Qiangtang block most likely occurred during the Late Triassic.

Archeointensity study on baked clay samples taken from the reconstructed ancient kiln: implication for validity of the Tsunakawa-Shaw paleointensity method

In 1972, a reconstruction experiment of a kiln had been done to reproduce an excavated kiln of the seventh century in Japan. Baked clay samples were taken from the floor surface and −20 cm level, and they have been stored after determinations of the paleomagnetic directions by partial alternating field demagnetizations. We recently applied the Tsunakawa-Shaw method to the samples to assess how reliable archeointensity results are obtained from the samples. A suite of the rock magnetic experiments and the scanning electron microscope observations elucidate that dominant magnetic carriers of the floor surface samples are Ti-poor titanomagnetite grains in approximately 10 nm size with single-domain and/or super-paramagnetic states, whereas contributions of multi-domain grains seem to be relatively large for the −20-cm level samples. From the floor surface samples, six out of the eight successful results were obtained and they give an average of 47.3 μT with a standard deviation of 2.2 μT. This is fairly consistent with the in situ geomagnetic field of 46.4 μT at the time of the reconstruction. They are obtained with a built-in anisotropy correction using anhysteretic remanent magnetization and without any cooling rate corrections. In contrast, only one out of four was successful from the −20-cm level samples. It yields an archeointensity of 31.6 μT, which is inconsistent with the in situ geomagnetic field. Considering from the in situ temperature record during the firing of the kiln and the unblocking temperature spectra of the samples, the floor surface samples acquired full thermoremanent magnetizations (TRMs) as their natural remanent magnetizations whereas the −20-cm level samples only acquired partial TRMs, and these differences probably cause the difference in the archeointensity results between the two sample groups. For archeointensity researches, baked clay samples from a kiln floor are considered to be ideal materials.

Paleomagnetism of the Late Cretaceous ignimbrite from the Okhotsk-Chukotka Volcanic Belt, Kolyma-Omolon Composite Terrane: Tectonic implications

New Late Cretaceous paleomagnetic results from the Okhotsk-Chukotka Volcanic Belt in the Kolyma-Omolon Composite Terrane yield stable and consistent remanent directions. The Late Cretaceous (86–81Ma) ignimbrites from the Kholchan and Ola suites were sampled at 19 sites in the Magadan area (60.4°N, 151.0°E). We isolated the characteristic paleomagnetic directions from 16 sampled sites using an alternating field demagnetization procedure. The primary nature of these directions is ascertained by dual polarities and positive fold tests. A tilt-corrected mean direction (D=42.8°, I=84.7°, k=46.0, α95=10.0°) yields a paleomagnetic pole of 66.7°N, 168.5°E (A95=18.8°) which appears almost identical to the 90–67Ma pole reported from the Lake El’gygytgyn area of the Okhotsk-Chukotka Volcanic Belt (Chukotka Terrane). This consistency suggests that the Kolyma-Omolon Composite Terrane and Chukotka Terrane has acted as a single tectonic unit since 80Ma without any significant internal deformation. Accordingly, we calculate a combined 80Ma characteristic paleomagnetic pole (Long.=164.7°E, Lat.=68.0°, A95=10.9°, N=12) for the Kolyma-Omolon-Chukotka Block which falls 16.5–17.5° south of the same age poles from Europe and East Asia. We ascribe this discrepancy in pole positions to tectonic activity in the area and infer a southward displacement of 1640±1380km for the Kolyma-Omolon-Chukotka Block with respect to the North American and Eurasian blocks since 80Ma; more than 260km of it is attributed to tectonic displacement in the Arctic Ocean due to the opening of the Canadian Basin.

The performance of various palaeointensity techniques as a function of rock magnetic behaviour – A case study for La Palma

Three different palaeointensity methods were applied to six historical and three carbon-dated flows from the island of La Palma (Spain); in total fifteen sites were processed. The two 20th-century flows were sampled at multiple locations as their obtained directions and intensities can be compared directly to those from the International Geomagnetic Reference Field (IGRF). After determination of the declinations and inclinations of the natural remanent magnetisation (NRM) by thermal and alternating-field demagnetisation, the samples were subjected to standard rock magnetic analyses to determine their Curie and alteration temperatures. Based on these characteristics, the sites were allocated to one of four rock magnetic groups labelled L∗, L, C, and H, a division primarily based on the temperature-dependent behaviour of the low-field susceptibility that has been used in studies of other volcanic edifices. Scanning electron microscope (SEM) observations revealed little oxidation and exsolution (oxidation classes I to III). Palaeointensities were determined using the classic Thellier–Thellier method (Aitken and IZZI protocols), the microwave method and the domain-state-corrected multispecimen method. Thellier–Thellier and microwave results were analysed using the ThellierTool A and B sets of selection criteria as modified by Paterson et al. (2014). Their combined success rate was around 40%. Of the eight IGRF sites, two yielded average intensities within 10% of the IGRF value. For the microwave method, three sites reproduced the IGRF intensity within 10%. In the domain-state-corrected multispecimen protocol, just one site (site 9, 1971) passed the ‘ARM-test’ (applied in retrospect) and showed less than 3% progressive alteration. Its multispecimen result reproduced the palaeofield within error. The other IGRF sites over- or underestimated the palaeofield by up to 50%. The seven older sites produced plausible palaeointensities, generally within a few μT of model data, and if multiple methods were successful, the results were within error of each other. For all three PI methods, it seems that sites with low Curie temperatures (<150°C; group L∗), are more likely to pass all selection criteria while substantially over- or underestimating the palaeofield. It is hypothesised that time-dependent processes after cooling of the lava would be a prime reason for this discrepancy: PI experiments with a laboratory thermoremanent magnetisation (TRM), imparted at a temperature above the site’s dominant Curie temperature but below its alteration temperature, yielded the correct intensity of the laboratory-imparted TRM. When two or three methods agree to within a few μT, the obtained palaeointensity is close to the palaeofield. Multi-method consistency provides an additional palaeointensity reliability check.