Unblocking Temperature Spectra Research Articles

New palaeomagnetic results of Permian tectonic blocks in the western segment of the Yarlung Zangbo Suture Zone and their tectonic implications

The composition and structure of the Yarlung Zangbo Suture Zone (YZSZ) provide important constraints on long‐standing disputes about the evolution of the Neo‐Tethys Ocean and continental collision. The Permian tectonic blocks (mainly composed of limestones and volcanic rocks) are widely distributed in the YZSZ. There are still different understandings about the origin of these blocks and their relationship with the evolution of the Neo‐Tethys Ocean. Here, we report the first reliable palaeomagnetic results of the Permian tectonic blocks in the Gyanyima and Mayum areas, the western segment of the YZSZ. Fossil records and our new zircon U–Pb results provide 254–252 Ma and 249 ± 2 Ma age constraints for the strata in the Gyanyima and Mayum sections, respectively. A total of 109 specimens were subjected to stepwise thermal demagnetizations. Among them, 83 specimens yielded interpretable results. After removing a recent geomagnetic field direction at low to intermediate temperature ranges, characteristic remanent magnetization (ChRM) directions can be isolated from the upper Permian–lower Triassic Gyanyima Formation (Fm). Unblocking temperature spectra of the ChRM indicate magnetite and haematite as the main carriers in the basalt and limestone/andesitic‐basalt samples, respectively. The ChRM passed the fold tests and a reversal test at 95% confidence level, indicating that it is most likely a primary remanence. The tilt‐corrected mean direction for 14 sites is D = 18.0°, I = −14.4°, k = 20.5, and α95 = 9.0°, which corresponds to a palaeolatitude of 7.3°S and a palaeopole at 48.3°N, 233.3°E with A95 = 8.1°. Combining the rock assemblage, palaeomagnetic, palaeontological, and geochronologic data, we conclude that these blocks were most likely seamounts formed in the low‐latitude area of the Neo‐Tethys Ocean. The palaeolatitude and origin of these blocks indicate that the evolution of the Neo‐Tethys Ocean was a complex process interspersed with the development of a series of seamounts and microblocks.

Investigating the utility of a high-temperature Thellier-style paleointensity experimental protocol

High-quality data are vital to the research field of paleointensity, which has long suffered from poor-quality and/or sparse data. Previous paleointensity work has established that repeatedly heating specimens increases the opportunity for thermochemical alteration to decrease the reliability of paleointensity data. In addition, recent work has shown that repeatedly heating specimens in paleointensity experiments can also exaggerate the effects of non-ideal, non-single domain grains. Arai plots resulting from paleointensity experiments containing such grains are often curvilinear (two-slope) across most of the specimen’s unblocking temperature spectrum, except in the temperature range nearest to the grains’ Curie temperature. This study tests the following strategy to mitigate these effects: that of performing a Thellier paleointensity experiment using fewer temperature steps that are also concentrated at higher temperatures. For this purpose, we use natural specimens with well-constrained rock magnetic data from the Hawaiian Scientific Observation Hole 1 (SOH1) drill core in paleointensity experiments with starting temperatures ranging from 200 °C to 500+ °C. Those experiments that focused in on the portion of the unblocking temperature spectrum nearest the Curie temperature of the specimen (HiTeCT) had an exceptionally low success rate, whereas those with initial temperatures at comparatively moderate temperatures (200–400 °C) had high success rates (~ 70%). Thermochemical alteration was minimized with a start temperature of 400 °C, but the curvature of the Arai plots had no clear dependance on start temperature. We conclude herein that increasing the start temperature can help avoid the effects of low-temperature alterations. Additionally, we found that the approach of focusing in on the highest temperature range is not a feasible one to apply in paleointensity experiments, in general, and consider this likely to be a result of a lack of intermediate-temperature checks for alteration and insufficient independence of temperature steps.Graphical abstract

Understanding Nonideal Paleointensity Recording in Igneous Rocks: Insights From Aging Experiments on Lava Samples and the Causes and Consequences of “Fragile” Curvature in Arai Plots

AbstractThe theory for recording of thermally blocked remanences predicts a quasilinear relationship between low fields like the Earth's in which rocks cool and acquire a magnetization. This serves as the foundation for estimating ancient magnetic field strengths. Addressing long‐standing questions concerning Earth's magnetic field requires a global paleointensity data set, but recovering the ancient field strength is complicated because the theory only pertains to uniformly magnetized particles. A key requirement of a paleointensity experiment is that a magnetization blocked at a given temperature should be unblocked by zero‐field reheating to the same temperature. However, failure of this requirement occurs frequently and the causes and consequences of failure are understood incompletely. Recent experiments demonstrate that the remanence in many samples typical of those used in paleointensity experiments is unstable, exhibiting an “aging” effect in which the (un)blocking temperature spectra can change over only a few years resulting in nonideal experimental behavior. While a fresh remanence may conform to the requirement of equality of blocking and unblocking temperatures, aged remanences may not. Blocking temperature spectra can be unstable (fragile), which precludes reproduction of the conditions under which the original magnetization was acquired. This limits our ability to acquire accurate and precise ancient magnetic field strength estimates because differences between known and estimated fields can be significant for individual specimens, with a low field bias. Fragility of unblocking temperature spectra may be related to grain sizes with lower energy barriers and may be detected by features observed in first‐order reversal curves.

Remagnetization of Red Beds on the Tibetan Plateau: Mechanism and Diagnosis

AbstractRed beds are important targets for paleomagnetic studies, yet discriminating secondary chemical remanent magnetization (CRM) from primary depositional remanent magnetization (DRM) in them remains challenging. Here we reanalyze the thermal demagnetization data of and conduct comprehensive rock magnetic, Mössbauer spectroscopic and petrographic studies on red beds from four Cenozoic basins on the eastern Tibetan Plateau (China): the Gongjue, Nangqian, Shanglaxiu, and Jinggu basins. The red beds in the latter two basins are remagnetized, as are most Nangqian red beds. The Gongjue red beds and some Nangqian red beds retain a DRM. Our results reveal that detrital (titano)magnetite and hematite are well preserved in red beds retaining the DRM, whereas remagnetized red beds contain large amounts of authigenic hematite and goethite with detrital Fe‐bearing minerals strongly altered. Postdepositional diagenetic alteration induced by heating and/or fluid circulation related to magmatism and/or crustal deformation is probably the main reason for the remagnetization. Hematite carrying the CRM in remagnetized red beds has wide distribution of grain size and unblocking temperature spectra, while hematite carrying the DRM is usually coarse and has confined unblocking temperature spectrum. This can be used as a criterion for diagnosing remagnetization. Nanoscale goethite appears to occur only in remagnetized red beds: another sensitive criterion for discriminating CRM from DRM. These property‐based criteria constrain the origin of the NRM in red beds better than the classic paleomagnetic field tests. Our research emphasizes that integrated rock magnetic, Mössbauer spectroscopic and petrographic studies provide robust tools to diagnose remagnetization in red beds.

Effects of titanomagnetite reordering processes on thermal demagnetization and paleointensity experiments

AbstractTitanomagnetite (Fe3‐xTixO4, 0 ≤ x ≤ 1) is a common, naturally occurring magnetic mineral critical to many paleomagnetic studies. Underlying most interpretations is the assumption that, lacking chemical alteration, Curie temperature (Tc) remains constant. However, recent work has demonstrated that Tc of many natural titanomagnetites varies strongly as a function of thermal history, independent of chemical alteration. This is inferred to arise from reordering of cations and/or vacancies in the crystal structure, and changes occur at temperatures and times relevant to standard paleomagnetic thermal treatments. Because changes take place at T < Tc, they have the potential to dramatically affect thermal remanence acquisition or demagnetization, impacting interpretation of paleomagnetic results. Here we have modeled the effects of reordering on standard thermal demagnetization and paleointensity experiments. Results suggest that Tc changes during laboratory heating make it impossible to accurately measure the unblocking temperature spectrum without modifying it. Samples with a starting Tc0 less than the closure temperature (Tclose) for the reordering process will develop a high‐temperature “tail” that did not exist prior to heating. Samples with a starting Tc0 > Tclose will have their original Tb spectrum truncated at T ≈ Tclose. Predicted behavior during Thellier‐type paleointensity experiments results in only modest deviations in NRM‐lost or pTRM*‐gained from the nonreordering case. Much larger deviations are predicted for pTRM checks. Compared to paleointensity results from titanomagnetite‐bearing pyroclastic deposits, modeled nonideal behavior occurs in the same temperature intervals, but is much more systematic. Reordering is likely one contributing factor to failure of paleointensity experiments.

Validity of archaeomagnetic field recording: an experimental pottery kiln at Coppengrave, Germany.

Palaeomagnetic data obtained from archaeological materials are used for reconstructions of the Earth's magnetic field of the past millennia. While many studies tested the reliability of this recorder for palaeointensity only a few studies did this for direction. The study presents an archaeomagnetic and rock magnetic investigation applied to an experimental pottery kiln, which was operated in 2003 to produce stone ware. This kind of high-quality pottery needs a temperature of at least 1160 °C. Shortly before heating of the kiln direct absolute measurements of the absolute geomagnetic field vector have been carried out close to it. After cooling of the kiln 24 oriented palaeomagnetic samples have been taken. Although Curie temperatures are about 580 °C, that is the typical temperature for magnetite, thermal as well as alternating field demagnetisations reveal also a considerable amount of hematite as magnetic carrier. This mixture of magnetite and hematite is dominated by pseudo-single domain grains. Demagnetisation removed in some cases weak secondary components, but in most cases the specimens carried a single component thermoremanent magnetisation. The mean characteristic remanent magnetisation direction agrees on 95per cent confidence level with the directly measured field direction. Archaeointensity was obtained from five specimens with the Thellier-Coe method and with the multiple-specimen palaeointensity domain-state corrected method. Six of these specimens also provided a result of the Dekkers-Böhnel method, which overestimated the archaeointensity by about 9per cent compared to the direct value, while after correction for fraction the value agrees very well. For the multiple-specimen palaeointensity domain-state corrected method only fractions between 25 and 75per cent have been used and specimens showing alteration have been excluded. Above 450 °C many specimens showed alteration of the magnetic grains. Because median destructive temperatures were often above this value in most cases the fraction was less than 50per cent. Nevertheless the obtained intensity (48.48± 0.24 μ) is on 95per cent confidence level in agreement with the direct observation. Behaviour of the specimens during the Thellier-experiments was not ideal because of narrow unblocking temperature spectra and alteration. Nevertheless, the obtained mean archaeointensity is also in agreement with the direct field observation. Here the relative palaeointensity error is about 6per cent and very high compared the multiple-specimen palaeointensity domain-state corrected method. The investigation demonstrates that a pottery kiln can provide a very precise estimate of the ancient geomagnetic field vector.

Paleomagnetism of the Upper Triassic rocks from south of the Ailaoshan Suture and the timing of the amalgamation between the South China and the Indochina Blocks

A high temperature remanence with thermally-discrete unblocking temperature spectra has been isolated from the redbed samples of the Upper Triassic Yiwanshui Formation collected from south of the Ailaoshan Suture in southern Yunnan Province of China. Two groups of directions were observed: the tilt-corrected mean is D/I=141.4°/31.2° with α95=13.0° based on 12 sites for the first group, and D/I=207.5°/4.9° with α95=16.0° based on 5 sites for the second group. The second group of directions was derived from massively bedded rocks with large uncertainties in determination of the paleo-horizontal and is thus deemed inaccurate. The first group of directions passes the foldtest with dual polarity, and is consistent with results previously reported from the same type section, and probably represents primary magnetization of the rock formation. Comparison with the Triassic paleomagnetic poles for the Baoshan Terrane and the South China Block indicates that the Simao and the Baoshan terranes were probably already amalgamated by late Triassic times but the coalescence between them and the South China Block may have not been completed until near the end of the Triassic.

Acquisition of chemical remanent magnetization during experimental ferrihydrite–hematite conversion in Earth-like magnetic field—implications for paleomagnetic studies of red beds

Hematite-bearing red beds are renowned for their chemical remanent magnetization (CRM). If the CRM was acquired substantially later than the sediment was formed, this severely compromises paleomagnetic records. To improve our interpretation of the natural remanent magnetization, the intricacies of the CRM acquisition process must be understood. Here, we contribute to this issue by synthesizing hematite under controlled ‘Earth-like’ field conditions (≲100 μT). CRM was imparted in 90 oriented samples with varying inclinations. The final synthesis product appeared to be dominated by hematite with traces of ferrimagnetic iron oxides. When the magnetic field intensity is ≳40 μT, the CRM records the field direction faithfully. However, for field intensities ≲40 μT, the CRM direction may deviate considerably from that of the applied field during synthesis. The CRM intensity normalized by the isothermal remanent magnetization (CRM/IRM@2.5 T) increases linearly with the intensity of growth field, implying that CRM could potentially be useful for relative paleointensity studies if hematite particles of chemical origins have consistent properties. CRM in hematite has a distributed unblocking temperature spectrum from ∼200 to ∼650 °C, while hematite with a depositional remanent magnetization (DRM) has a more confined spectrum from ∼600to680°C because it is usually coarser-grained and more stoichiometric. Therefore, the thermal decay curves of CRM with their concave shape are notably different from their DRM counterparts which are convex. These differences together are suggested to be a potential discriminator of CRM from DRM carried by hematite in natural red beds, and of significance for the interpretation of paleomagnetic studies on red beds.

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 impact breccias from the Chicxulub crater – Implications for ejecta emplacement and hydrothermal processes

Results of a paleomagnetic study of 89 samples from impact breccias from the Chicxulub crater are presented and used to investigate on ejecta emplacement, hydrothermal and alteration processes. The impactite sequence sampled in cores from Yaxcopoil-1 borehole is ∼100 m thick and formed by six subunits characterized by petrographic, scan images, geochemical and textural analyses, which point to different emplacement modes. Impact occurred within reverse polarity chron C29r; therefore, magnetization acquired at crater formation and shortly thereafter will present reverse polarity, which is supported by paleomagnetic measurements on melt samples from Yucatan-6 borehole. Results after step-wise thermal and alternating field demagnetization document a pattern of 23 upward and 29 downward inclinations at Yaxcopoil-1 around −33° and 38°, respectively. Measurements of magnetic hysteresis and variation of low-field susceptibility with temperature suggest low-Ti titanomagnetites and magnetite as main magnetic carriers. Petrographic observations indicate the presence of fine-grained magnetite, hematite and Fe-oxyhydroxides, related to hydrothermal alteration processes. Curie points are in the range of 520–580 °C. The wide range of unblocking temperature spectra points to variable grain sizes of a magnetic phase that unblocks around 580 °C. Hysteresis ratio plots indicate most samples fall in the pseudo-single domain field. In some samples, wasp-waist constrained hysteresis loops suggest magnetite and hematite. Analyses of vector plots and coercivity and unblocking temperature spectra do not show apparent differences within and between subunits. Subunits are characterized by distinct textural and compositional differences in size, type and relative abundance of clasts and melt-rich or carbonate-rich matrix types. Bulk properties vary with composition and clast contents, particularly within the Middle Suevite and Brecciated Melt Rock subunits that show higher values, while low values characterize the Lower Suevite carbonate-rich with rare basement clasts subunit. Magnetization acquisition mechanisms are discussed, with reference to impactite characteristics, and may relate to remagnetization from hydrothermal and post-impact alteration processes.

Testing determinations of absolute paleointensity from the 1955 and 1960 Hawaiian flows

A new exhaustive study of the flows erupted in 1955 and 1960 from the Kilauea volcano (Big Island of Hawaii) has been conducted in order to constrain the dispersion inherent to the data of absolute paleointensity reported so far and ultimately to determine what are the most critical parameters to obtain suitable determinations. The mean directions derived from both flows agree with the expected field within less than 2°. Paleointensity experiments have been performed using a double heating Thellier procedure completed by systematic pTRM checks. Almost all samples display a linear NRM–TRM straight line but only 20% of the samples have provided suitable results less than 10% away from the expected field. In the other cases, the deviation from the expected paleointensity is related to the quality of the pTRM checks, which thus provides a threshold value for acceptance of pTRM checks. All successful determinations less than 10% away from the expected value have a maximum DRAT ( Selkin and Tauxe, 2000) that never exceeds 5% while the mean value of all deviations is lower than 3.5%. Another interesting feature is the existence of a link between the unblocking temperature spectra and the accuracy of the paleofield, which points out the origin of difficulties in the presence of a wide distribution of grain sizes and/or the existence of several magnetic phases. The results also indicate that magnetite with a narrow range of high unblocking temperatures and little variability in grain size is a favorable situation to obtain suitable determinations with a high success rate. These observations may well explain the dispersion inherent to the previous studies of the Hawaiian flows.

Properties of partial thermoremanence in magnetite: Testing the blocking versus unblocking temperature spectrum using the phenomenological model

To better understand the fundamentals of Thellier‐type absolute geomagnetic field intensity determinations, a map of χ was constructed from the experimental blocking versus unblocking relation. Once constructed, the map of χ can predict the outcome of a Thellier experiment. Thellier predictions were compared with an independent Thellier experiment. The contribution from low‐ and high‐temperature partial thermoremanent magnetization (pTRM) tails increased as the grain size of magnetite increased, indicating a progressive violation of pTRM reciprocity in the multidomain (MD) range. Such a violation of reciprocity with increasing grain size generated enhancement of down‐sagging features in Arai plots, a primary reason for the failure of Thellier experiments. As anticipated, MD magnetites displayed broad unblocking spectra, while single‐domain grains showed a narrow inflection. Overall, the mapping of χ was useful in predicting key features in the Thellier experiment. However, as the grain size increased, there was a growing discrepancy between prediction and experiments, possibly caused by the initial state dependence of pTRM intensity.

Palaeomagnetic and rock magnetic study of charnockites from Tamil Nadu, India, and the ‘Ur’ protocontinent in Early Palaeoproterozoic times

Palaeomagnetic and magnetomineralogical results are reported from charnockites in basement terrane at the eastern sector of the WSW–ENE granulite belt of South India. Magnetite is the dominant ferromagnet identified by rock magnetic and optical study; it is present in several phases including large homogeneous titanomagnetites and disseminated magnetite in microfractures linked to growth stages ranging from primary charnockite formation to uplift decompression and exhumation within the interval ∼2500–2100 Ma. Several components of magnetization are resolved by thermal demagnetization and summarized by four pole positions; in the northern (Pallavaram) sector these are P1 (33°N, 99°E, d p/d m = 8/9°) and P2 (79°N, 170°E, d p/d m = 3/6°), and in the southern (Vandallur) sector they are V1 (23°N, 116°E, d p/d m = 8/9°) and V2 (26°S, 136°E, d p/d m = 5/10°). These magnetizations are linked to uplift cooling of the basement and unblocking temperature spectra suggest acquisition sequences P1 → P2 and V1 → V2 in each case implying movement of the shield from higher to lower palaeolatitudes sometime between 2500 and 2100 Ma. Palaeomagnetic poles from the cratonic nuclei of Africa, Australia and India all identify motion from higher to lower palaeolatitudes in Early Palaeoproterozoic times, and this is dated ∼2400 and ∼2200 Ma in the former two shields. The corresponding apparent polar wander (APW) segments match the magnetization record within the charnockite basement terranes of southern India to yield a preliminary reconstruction of the ‘Ur’ protocontinent, the oldest surviving continental protolith with origins prior to 3000 Ma. Although subject to later relative movements these nuclei seem to have remained in proximity until the Mesozoic break-up of Gondwana.

Low-latitude paleosecular variation and the time-averaged field during the late Pliocene and Quaternary—Paleomagnetic study of the Michoacan-Guanajuato volcanic field, Central Mexico

AbstractWe report paleomagnetic, rock magnetic, and paleointensity studies for 24 volcanic lava flows from the Michoacan-Guanajuato volcanic field (MGVF), erupted between 2.27 Ma to present according to available radiometric ages and historic records. The MGVF located in the central-western sector of the Plio-Quaternary Trans-Mexican volcanic belt is mainly composed by cinder cones and shield volcanoes. Rock magnetic experiments show remanence is carried in most cases by Ti-poor titanomagnetites, resulting from oxy-exsolution of original titanomagnetites during flow cooling. Unblocking temperature spectra and high coercivities point to “small” pseudo-single domain grains for the titanomagnetites. Single component, linear vector plots are obtained after alternating field and thermal demagnetization. Seven flows yield reverse polarity magnetization while sixteen flows are normally magnetized. The overall mean paleodirection obtained (with 14 flows normal and 6 flows reverse) is I=28.4°, D=357.9°, k=21, α95=7.3°, with a paleomagnetic pole position ofPlat=85.7°,Plong= 104.5°, K=27, A95=6.4°. The paleodirection is undistinguishable from expected Plio-Quaternary paleodirections derived from reference poles for the North American polar wander curve, and previously reported paleodirections for central Mexico. Paleointensity experiments give high quality results for only twelve samples from two flows with mean values of 7.3 and 8.1 × 1022Am2, which are close to present geomagnetic field intensity. The combination of new MGVF directional results with currently available paleomagnetic data from central Mexico yield angular dispersion estimates ofSF= 15.4 withSU= 19.6 andSL= 12.7, which are in agreement with the latitude-dependent PSV model of McFaddenet al.(1988, 1991) for the last 5 Ma, and show no significant inclination anomaly.

Paleomagnetic Study of Lavas from the Popocatepetl Volcanic Region, Central Mexico

Results are presented for a paleodirectional and paleointensity study of 16 Popocatepetl lava flows. Popocatepetl is a tall, active, dacitic-andesitic stratovolcano; it forms the southern end of a N-S-trending Quaternary volcanic range at the eastern edge of the Basin of Mexico. Studied units are mainly andesites and one trachyandestite, and all possess similar trace- and rare-earth-element compositions. Rock-magnetic experiments show that remanence is carried in most cases by Ti-poor titanomagnetite, resulting from oxy-exsolution of original titanomagnetite during flow cooling. Unblocking temperature spectra and high coercivities point to "small" pseudo-single domain magnetic grains for these (titano)magnetites. Single-component, linear demagnetization plots were observed in most cases. A strong, lightning-produced magnetization overprint was detected for one site. Combining the new paleomagnetic data provides a mean paleodirection of I = 35.4°, D = 345.7°, k = 21, a95 = 8.5°, N = 15 for the Popocatepetl volcano region. All studied flows yield normal polarity magnetization, which supports a maximum age within the Brunhes chron. Twenty-five samples were selected for Thellier palaeointensity experiments based on magnetic properties, stable single-component remanent magnetizations, and reasonably reversible continuous thermomagnetic curves. Fourteen samples from four different flows yield reliable paleointensity estimates with the flow-mean virtual dipole moments (VDM) ranging from 5.9 to 9.2 x 1022 Am2. The NRM fractions used for paleointensity determination range from 35 to 96%, and the quality factors vary between 3.4 and 46.9, being normally greater than 6. Mean VDM obtained in this study is 7.2 ± 1.4 x 1022 Am2, slightly lower than the present-day dipolar value.

A pilot rock magnetic and ore microscopy study of xenolith-bearing young basaltic rocks from the Camargo cinder cone field, Chihuahua, Northern Mexico

Initial results of a rock magnetic, paleomagnetic, microscopic study of the Camargo volcanic field from Chihuahua (northern Mexico) are reported. The field is formed by more than 300 cinder cones and extends over an area of 200,000 km 2 Mantle and lower crust xenoliths in the alkaline basaltic rocks of La Olivina, in the northern sector of the field, suggest rapid magma ascent rates. This study concentrates on the rock magnetic properties of the cinder cone field. Remanences are carried in most cases by Ti-poor titanomagnetites and magnetites as a result of the oxy-exsolution of original titanomagnetite during initial flow cooling. For a few units, remanence seems to be carried by (titano)hematites, as shown by unblocking temperature spectra and secondary titano(maghemite). Judging from the ratios of hysteresis parameters, all samples fall in the pseudo-single domain state region. Most units (13) show reverse polarity magnetizations, and only 2 show normal polarity. Radiometric dating gives the field's age range as between 2 and 1.7 Ma, which, together with the magnetostratigraphic results, suggests a relatively short emplacement time for the volcanic field. The mean preliminary paleomagnetic direction obtained for the Camargo volcanic field is I=30.6°, D=341.3°, k=19, and α 95=9.8°, which deviates counterclockwise from the expected direction estimated from the North American apparent polar wander path. In turn, this suggests a vertical axis tectonic counterclockwise rotation of approximately 20° relative to stable North America.

Laboratory depositional and compaction-caused inclination errors carried by haematite and their implications in identifying inclination error of natural remanence in red beds

SUMMARY Undetected depositional and/or compaction-caused inclination errors may result in an overestimation of tectonically caused latitudinal offset. Hodych & Buchan used a single-component isothermal remanent magnetization (IRM) acquired in a DC field at a 45 ◦ angle to the bedding to test for inclination errors in Silurian red beds. This approach was criticized by Stamatakos et al. We produced synthetic depositional and compaction-caused inclination errors to test Hodych and Buchan’s approach. Red bed samples collected from the Eocene Suweiyi Formation, Tarim Basin (northwest China) were disaggregated using an ultrasonic cleaner and the sediments were mixed with distilled water to make a sediment slurry. The sediment slurry spontaneously separated into silt-dominated and clay-sized parts, so deposition and compaction experiments were conducted with three categories of slurries: coarse grained, the fine grained and a 1:1 volume ratio mixture of the coarse- and fine-grained slurries. During compaction clay-sized sediments experienced 17 ◦ ‐19 ◦ inclination shallowing at 58 ◦ magnetic field inclination, while coarser sediments showed little laboratory compaction-caused inclination error. Acquisition of IRM, coercivity spectra and unblocking temperature spectra reveal that the magnetic carrier for the fine-grained sample is dominated by pigmentary haematite. A deposition experiment was also conducted with the coarse-grained sediments, which showed a range of depositional inclination error, ∼0 ◦ ‐30 ◦ , carried by larger high unblocking temperature/coercivity particles. The intermediate unblocking temperature (or coercivity) component carried presumably by pigmentary haematite is an accurate record of the ambient magnetic field direction. Our data indicate that the single-component IRM method of Hodych & Buchan can be used to identify and correct for the compaction-caused inclination error of the fine-grained samples, while it failed to detect any depositional inclination error in the coarse-grained samples. Therefore, to better quantify the inclination error in red beds, it is suggested that multiple IRMs be measured in various directions.

Magnetic Mineralogy, Paleomagnetism, and Magnetostratigraphy of Nayarit Volcanic Formations, Western Mexico: A Pilot Study

The first rock-magnetic, paleomagnetic, and magnetostratigraphic study was carried out on Late Miocene volcanic formations from the Nayarit area (western Mexico) in order to provide constraints for the tectonic evolution of the western Trans-Mexican volcanic belt (TMVB) and southern Gulf of California extensional province (GEP), and to establish a reference magnetostratigraphy for the volcanic sequences. Rock-magnetic experiments show that remanence is carried in most cases by Tipoor titanomagnetite, resulting from oxy-exsolution of original titanomagnetite during the initial flow cooling. Unblocking temperature spectra and relatively high coercivities point to "small" pseudo-single domain magnetic grains for these (titano)magnetites. Single-component, linear demagnetization plots were observed in most cases. The mean paleodirection obtained in this study is I = 33.7°, D = 358.4°, k = 140, a95 = 3.0°, N = 17. This mean direction is in agreement with the expected paleodirections for the Late Miocene, as derived from reference poles for the North American plate. These results suggest that no major tectonic deformation has occurred in the Nayarit area since ~11 Ma, in contrast to the left-lateral transtension and extensional faulting proposed for the TMVB and GEP during this period. Magnetic polarities obtained for the studied units are consistent with their stratigraphic position and with radiometric dating for individual units. By combining the available geochronologic data with the polarity of paleomagnetic samples, better constraints on the age of emplacement are achieved.

Late Carboniferous remagnetisation of Palaeozoic rocks in the NE Rhenish Massif, Germany

During stepwise thermal and alternating field demagnetisation experiments on Devonian and Lower Carboniferous carbonate and clastic rocks from the north-eastern part of the Rhenish Massif, Germany, three components of magnetisation (A, B, C) are identified. Component A is a recent viscous overprint that parallels the local present day geomagnetic field. Component B is mainly observed from 260 up to 550 °C during thermal demagnetisation and is carried by magnetite. In two localities, where red siltstones and red carbonate rocks were sampled, component B is stable up to 670 °C, indicating the presence of hematite. Three clusters of in situ B directions can be identified, which are controlled by the tectonic position of the sampling areas. These are from NW to SE: the Remscheid anticline (RA), the Lüdenscheid syncline (LS) and the Attendorn and Wittgenstein synclines (AS/WS). Standard and inclination-only fold tests, using parametric resampling, yield optimal statistical parameters at increasing amounts of untilting ranging from 0% in the South up to 57% in the North of the NE Rhenish Massif. Despite the variations in optimal untilting, the resulting site mean directions of component B do not differ significantly in inclination. These results are interpreted to reflect the acquisition of magnetisation during progressive northward migration of the deformation front in Late Carboniferous times. The resulting palaeolatitudes (RA: 1°S +2° −3°; LS: 2°S +3° −2°; AS/WS: 1°S +3° −4°) are in good agreement with the predicted position of the sampling area in the Late Carboniferous, as derived from a published Apparent Polar Wander Path for Baltica and Laurentia. The unblocking temperature spectra and the synfolding nature of B yield strong evidence that chemical processes, possibly driven by fluid migration during orogenesis, caused this remagnetisation. A third component C was observed in zones of tight folding with steeply dipping to overturned bedding planes and is dominantly carried by hematite. The resulting palaeolatitude (27°N +10° −8°) suggest a Late Triassic to Early Jurassic age of component C, which is interpreted to be caused by either hematite-bearing post-Variscan mineralisation or oxidising fluids percolating from the weathering surface and penetrating zones of enhanced permeability in the Mesozoic.

Towards pyrrhotite/magnetite geothermometry in low-grade metamorphic carbonates of the Tethyan Himalayas (Shiar Khola, Central Nepal)

In Mesozoic metacarbonates of the Tethyan Himalayas (Shiar Khola area, Central Nepal) two characteristic remanent magnetisations (ChRM1 and ChRM2) were identified by their unblocking temperature spectra. The ChRM1 is carried by pyrrhotite (unblocking temperature: 270–360°C) and the ChRM2 by magnetite (unblocking temperature spectra: 430–580°C). The temperature-related formation of pyrrhotite at the expense of primary magnetite during low-grade metamorphism in marly carbonates allows the determination of thermal gradients by the pyrrhotite/magnetite ratio. This new method can be used as a geothermometer for T≤300°C in low-grade metamorphic carbonates, where other methods are not available. This method is applied for the first time in the Tethyan Himalayas of Central Nepal.In the Shiar Khola valley, systematic variations in the ferrimagnetic content of the metacarbonates along an E–W profile were detected by the ratio of remanence intensity of pyrrhotite to magnetite, derived from natural remanent magnetisation (RPYR/MAG) and saturation magnetisation (SPYR/MAG). Over a stretch of 10km the RPYR/MAG and SPYR/MAG increase from W to E from ~0.42 to ~0.91 and ~0.48 to ~1.0, respectively. Based on temperature estimates, the eastern part experienced upper anchizone–epizone (~250–300°C) conditions, while the western part underwent only diagenesis (~200°C). The temperature gradient and the temperature ranges suggested are consistent with the findings of the calcite twin lamellae geothermometry which is a non-magnetic method.