Rock Magnetic Results Research Articles

Rock magnetic and paleomagnetic investigations of Sylhet traps, Shillong Plateau, NE India

Magnetic signals in basalts from the Sylhet traps (ST), Shillong Plateau, India is investigated using rock magnetism as a proxy to explain the potentiality of the ST basalts as an indicator of magnetic mineralogy. A paleomagnetic study was carried out in order to unravel the Paleopole. In detail, rock magnetic and Anisotropy of Magnetic susceptibility (AMS) results have been reported for the first time on the ST basalts. A total of 38 oriented block samples were collected from 8 sites along Cherrapunji-Shella bazaar road section, Meghalaya, India. Samples were subjected to rock magnetic measurements such as magnetic susceptibility (K), Isothermal Remanent Magnetization (IRM), hysteresis loops, backfield (coercivity remanence) and k-T (Susceptibility vs. Temperature) analysis to identify the magnetic properties of basalts. Advanced Variable Field Translation Balance (AVFTB) was used for some rock magnetic studies. A few samples were also subjected to Anisotropy of Magnetic susceptibility (AMS) study to understand the petrofabric orientations and shape anisotropy in the ST basalts. The Remanence ratio (Mrs/Ms) and the Coercivity ratio (Bcr/Bc) ranging from 0.105 to 0.333 and from 1.25 to 1.87 respectively indicate sizes of magnetic grains are mainly in the range of Pseudo Single Domain (PSD) limits with minority of them in the ranges of Single Domain (SD) limits. The results of k-T study in combination with other rock magnetic results shows that magnetite (Fe3O4) [Curie Temperature = 565-590 °C] as the dominant magnetic carrier with a subordinate content of titanomagnetite Fe2+(Fe3+Ti)2O4 [Curie Temperature = 380–450 °C]. From the AMS study, parameters like lineation, foliation are calculated, and results define all the three shape anisotropies present in the ST basalts. The ST basalts display paleomagnetic directions with mean declination = 255°, mean inclination = -57° (α95 = 3.44°; k = 16.74). The pole position of the ST at 117 Ma is -26.79 °N, 330.75 °E (dp = 3.6; dm = 5) with a paleolatitude 37.6 °S. This infers that the ST is a later volcanic event than Rajmahal traps (RT) which is considered contemporary to the ST.

Paleomagnetism of the Hart Dolerite (Kimberley, Western Australia) – A two-stage assembly of the supercontinent Nuna?

We present new paleo- and rock magnetic results from the ca. 1792 Ma Hart Dolerite sills that intrude the strata of the Kimberley craton, Western Australia. From 24 sites sampled, 23 are directionally clustered and the site mean directions were used to calculate a grand mean direction. Ten of the 23 sites have 95% confidence intervals (α95) less than 16°. A positive reversal test, dissimilarity of the corresponding paleopole from known younger Australian poles, and a previously reported positive baked contact test, indicate that our paleomagnetic results are of primary origin, and the results are consistent with the geologic interpretation that the Hart Dolerite sills represent a multiphase magmatic emplacement over the duration of several million years. The new ca. 1792 Ma paleopole for the Kimberley craton is applicable to the entire North Australian Craton. A comparison of Paleoproterozoic poles from the North and West Australian cratons supports the occurrence of a previously proposed 40° intracontinental rotation between the two cratons during latest Neoproterozoic time. Furthermore, comparing these Australian poles with similar aged Laurentian poles shows that the apparent polar wander paths of the two continents have a similar history between ca. 1800 and 1730 Ma and between ca. 1650 and 1400 Ma. To achieve a fit within uncertainty limits, an additional relative latitudinal motion is necessary during the transition interval (1730–1650 Ma). We suggest that a two-stage evolution occurred between Laurentia and Australia during the assembly of the supercontinent Nuna. Australia and Laurentia traveled together between ca. 1800 Ma until ca. 1730 Ma in a semi-stable configuration. During 1730–1650 Ma, they underwent a net relative latitudinal motion, leading to the final assembly of Nuna after ca. 1650 Ma.

Magnetostratigraphic dating of the Shixia red sediments and implications for formation of Nihewan paleo-lake, North China

The sedimentary sequence in Nihewan Basin, northern China, is rich in mammalian fossils and Paleolithic sites. Extensive magnetostratigraphic investigations have been carried out in the Nihewan sediments, but precise age control on the earliest Nihewan red sediments, which have a fluvio-lacustrine origin in the upper part and an eolian origin in the lower part, has remained unavailable. The formation process of Nihewan paleo-lake, therefore, remains unclear. Here we contribute to understanding the age of the early Nihewan reworked and eolian red clays by presenting detailed magnetostratigraphic and rock magnetic results from the Shixia section coupled with geochemical and petrographic analyses. Magnetostratigraphic correlation to the geomagnetic polarity timescale indicates that the Shixia sedimentary sequence recorded part of the Gilbert Chron with the onset of deposition of the Nihewan Formation occurring prior to the Gilbert-Gauss geomagnetic reversal at 3.6 Ma. With tectonic development of a graben basin, early red clay was deposited first in Nihewan Basin during the Early Pliocene before giving way to lacustrine environments in Nihewan paleo-lake. Geochemical and petrographic results indicate a wind-blown origin for the lower red clay deposits, which have been reworked and mixed with conglomerates produced by local tectonic activity to produce the basal Nihewan sediments. Later formation of carbonate nodules has given rise to the sediments that are now exposed in Nihewan Basin.

Preliminary Rock Magnetic and Paleomagnetic Results of the Holešice and Libkovice Member Transition of the Most Basin (Burdigalian, Czech Republic)

Preliminary Rock Magnetic and Paleomagnetic Results of the Holešice and Libkovice Member Transition of the Most Basin (Burdigalian, Czech Republic)

Preliminary paleomagnetic and rock magnetic results from 17 to 22\xa0ka sediment of Jeju Island, Korea: Geomagnetic excursional behavior or rock magnetic anomalies?

Paleomagnetic and rock magnetic investigations were performed on a 64-cm-thick section of nonmarine unconsolidated muddy sediment from the Gosan Formation on Jeju Island, Korea. This sediment was recently dated to have been deposited between 22 and 17 kyr BP calibrated, with a sedimentation rate of 13–25 cm/kyr, based on many radiocarbon ages. Interestingly, stepwise alternating field (AF) demagnetization revealed characteristic natural remanent magnetizations with anomalous directions, manifested by marked deviations from the direction of today’s axial dipole field, for some separate depth levels. On the other hand, stepwise thermal (TH) demagnetization showed more complex behavior, resulting in the identification of multiple remanence components. For all TH-treated specimens, consistently two different components are predominant: a low-temperature component unblocked below 240–320 °C entirely having normal-polarity apparently within the secular variation range of the Brunhes Chron, and a high-temperature component with unblocking temperatures (Tubs) between 240–320 and 520–580 °C that have anomalous directions, concentrated in the ~ 13–34-cm-depth interval (~ 17–19 ka in inferred age) and possibly below ~ 53 cm depth (before ~ 20 ka). Rock magnetic results also infer the dominance of low-coercivity magnetic particles having ~ 300 and ~ 580 °C Curie temperature as remanence carriers, suggestive of (titano)maghemite and/or Ti-rich titanomagnetite and magnetite (or Ti-poor titanomagnetite), respectively. A noteworthy finding is that AF demagnetizations in this study often lead to incomplete separation of the two remanence components possibly due to their strongly overlapping AF spectra. The unusual directions do not appear to result from self-reversal remanences. Then, one interpretation is that the low-temperature components are attributable to post-depositional chemical remanences, associated possibly with the later formation of the mineral phase having Tub ~ 300 °C, whereas the high-temperature components are of primary detrital origin that survived later chemical influence. Accordingly, the unusual directions might record geomagnetic instability within the ~ 17–22 ka period manifested by multiple excursional swings, partly associated with the Tianchi/Hilina Pali excursion. However, further work is needed to verify this interpretation and distinguish it from alternative explanations that invoke rock magnetic complexities as the cause of the unusual directions.

Rock magnetism and geomagnetic field strength of the rare Iron Age (300–500 BC) artifacts from Tamilnadu: The first Virtual Axial Dipole Moment determination from India

Archaeological artifacts are abundant in India to undertake archaeointensity (AI) research. High quality AI data from this region are essential to improve global geomagnetic field (GGF) model secular variation curve for the past few millennia for its applicability to the Indian region. Rock magnetic and AI investigations are carried out on 15 rare Megalithic/Iron Age (300–500 BC) pottery samples from the Sengalur site, Tamilnadu, India. Rock magnetic results indicate that either SD/PSD type of ferrimagnetic mineral (magnetite/titanomagnetite) is responsible for magnetic remanence. Temperature versus susceptibility experiments for most of the samples yield reversible heating and cooling curves with Curie temperatures of 565–585 °C. AI values are determined by the Thellier–Thellier method as modified by Coe 1967 (Zero-field/In-field method). The AI data of the present study meets the reliability and quality criteria adopted for the AI determinations worldwide. The mean AI of 47.48 ± 1.72 µT and a mean Virtual Axial Dipole Moment of 11.7 ± 0.4 × 1022 Am2 are estimated. This new AI data are in good agreement with the predictions of ARCH3K.1 GGF model for the period of 300–500 BC for India derived from the GEOMAGIA. V3 updated database. Other models incorporating sediment data are not consistent with the actual values of direct determination.

Paleomagnetism of the Upper Paleozoic of the Novaya Zemlya Archipelago

The rock magnetic and paleomagnetic results from the Upper Paleozoic sedimentary sequences composing the isles of the Novaya Zemlya Archipelago are presented. The recorded temperature dependences of the magnetic susceptibility, the magnetic hysteresis parameters, and the results of the first-order reversal curve (FORC) measurements suggest the presence of single-domain or pseudo-single-domain magnetite and hematite grains in the rocks. The Upper Paleozoic deposits overall are promising for unraveling the tectonic evolution of the Barents–Kara region. Together with the rock magnetic data, the positive fold and reversal tests testify to the primary origin of the indentified magnetization components. However, the interpretation of the paleomagnetic data should take into account the probable inclination shallowing. New substantiation is offered for the paleomagnetic poles for Early Devonian and Late Permian. For the first time, paleomagnetic constraints are obtained for the Late Carboniferous boundary. It is shown that the Early Cimmerian deformation stage within the Paikhoi–Novaya Zemlya region is associated with the sinistral strike slip displacement along the Baidaratskii suture during which the internal structure of the Southern Novaya Zemlya segment could undergo shear in addition to the nappe-thrust transformations. The Northern Novaya Zemlya segment, which is shifted northwest with respect to the Southern segment, was deformed in the thrusting mode with an overall clockwise rotation of this segment relative to the East European Craton.

Comment on “Remagnetization of the Paleogene Tibetan Himalayan carbonate rocks in the Gamba area: Implications for reconstructing the lower plate in the India‐Asia collision” by Huang et al.

AbstractThe recent publication of “Remagnetization of the Paleogene Tibetan Himalayan carbonate rocks in the Gamba area: Implications for reconstructing the lower plate in the India‐Asia collision” by Huang et al. [J. Geophys. Res. Solid Earth, 122, doi:10.1002/2016JB013662] argued for a complete chemical remagnetization on the characteristic remanence of carbonate rocks from the Zongpu Formation (Fm) at Gamba, southern Tibet, and discussed its implications for the reconstruction of India‐Asia collision. To support their conclusion, the authors performed nonparametric fold tests on the paleomagnetic data obtained from the Zongpuxi section, we previously published in Yi et al. [Earth Planet. Sci. Lett., 2011, 309 (1–2):153–165] and argued for a synfolding or postfolding origin. In this comment, we demonstrate that their reinterpretation of the fold tests is invalid, and a prefolding origin of the ChRMs from the Zongpu Fm is still supported by the data. We agree that the new rock magnetic and SEM results are robust evidence for a secondary origin of the magnetite fraction in the Zongpu Fm; however, the associated secondary remanence was most likely acquired shortly after deposition of the carbonates, or even in an early diagenetic stage. We conclude that the paleomagnetic results from Gamba area can still be used for reconstructing the precollisional extent of Greater India.

Generation of billow-like wavy folds by fluidization at high temperature in Nojima fault gouge: microscopic and rock magnetic perspectives

Microscopic billow-like wavy folds have been observed along slip planes of the Nojima active fault, southwest Japan. The folds are similar in form to Kelvin–Helmholtz (KH) instabilities occurring in fluids, which implies that the slip zone underwent “lubrication” such as frictional melting or fluidization of fault gouge materials. If the temperature range for generation of the billow-like wavy folds can be determined, we can constrain the physical properties of fault gouge materials during seismic slip. Here, we report on rock magnetic studies that identify seismic slip zones associated with the folds, and their temperature rises during ancient seismic slips of the Nojima active fault. Using a scanning magneto-impedance magnetic microscope and a scanning superconducting quantum interference device microscope, we observed surface stray magnetic field distributions over the folds, indicating that the folds and slip zones are strongly magnetized. This is due to the production of magnetite through thermal decomposition of antiferromagnetic or paramagnetic minerals in the gouge at temperatures over 350 °C. The presence of micrometer-sized finely comminuted materials in the billow-like wavy folds, along with our rock magnetic results, suggests that frictional heating-induced fluidization was the driving mechanism of faulting. We found that the existence of the magnetized KH-type billow-like wavy folds supports that the low-viscosity fluid induced by fluidization after frictional heating decreased the frictional strength of the fault slip zone.Graphical abstract.

Rock magnetic investigation and its geological significance for vein‐type uranium deposits in southern China

AbstractTo characterize the metallogenic environment of a typical vein‐type uranium deposit, samples from diabase dykes, alteration zones including metamorphic diabase and uranium ore, and granites were systematically investigated for six boreholes from southeastern China. Rock magnetic results indicate that coarse‐grained magnetites (pseudosingle domain, PSD, and multidomain, MD) are dominant magnetic carriers in diabase. In contrast, the uranium ore is dominated by fine‐grained magnetites (superparamagnetic, SP, and single‐domain, SD). The concentration of magnetic particles in fresh granites is low. Magnetic properties of metamorphic diabases exhibit much greater variability of magnetic properties and higher degrees of sulfuration than unaltered diabase and granite, due to contact metasomatism and reduction effects close to the vein. Compared with diabase, magnetic remanence of the uranium ore is much lower, but displays much higher stability. The Koenigsberger ratio Q peaks in the uranium ore with a value of ∼1.00. Using the systematic rock magnetic results to constrain the interpretation, the contribution of the intersection zone of diabase dyke and silicified fault to magnetic anomalies was further modeled, and the effects of the ore body are significant for magnetic exploration. Overall, rock magnetic investigations of vein‐type uranium deposit provide a better understanding of the interactions between different rock types, and further facilitate regional magnetic surveys on the ground.

Magnetic biomonitoring of roadside pollution in the restricted Midagahara area of Mt. Tateyama, Toyama, Japan.

Magnetic biomonitoring techniques and in situ topsoil magnetic susceptibility measurements have been shown to be rapid, cost-effective, and useful methods for investigating roadside pollution. However, combustible vegetation in samples makes it very difficult to use them in high-temperature magnetic experiments although the thermal alteration of spontaneous magnetization is a fundamental magnetic property and can be used to identify reliably the magnetic minerals. Here, we report the first magnetic biomonitoring results of dust deposited on plant leaves along the Tateyama-Kurobe Alpine route at the highly protected Midagahara wetland areas of Mt. Tateyama in Toyama, Japan. In-field magnetic susceptibility from 15 sites (161 points) shows higher susceptibilities near the roadside. Dust deposited on the leaves of Sasa kurilensis, or dwarf bamboo, has been wiped off at 12 sites (64 samples) with a commercial ethanol wipe sheet or silica wool damped in ethanol and subjected to rock magnetic analyses. Thermomagnetic remanence curves and low-temperature behaviors for samples collected near the roadside using silica wool show clearly that the main magnetic mineral in the dust is partially oxidized magnetite. Further, detailed rock magnetic analyses and elemental analyses of leaves in the study area indicate that (a) the magnetic mineralogy on leaves' surface is consistent throughout the study area and (b) higher saturation isothermal remanent magnetization intensities as well as higher concentrations of Pb, Fe, Cr, and Y are observed near the roadside, i.e., the closer to the roadside, the more anthropogenic materials, including partially oxidized magnetite, are present. Also, microscopic observations show the lack of spherical grains, indicating that dust on the roadside leaves is derived from passing vehicle rather than industrial process. Both rock magnetic and geochemical results show that S. kurilensis would be an excellent candidate for investigating air pollution. Also, the proposed wiping-off method of collecting dust from plant surfaces is an effective non-destructive method that can be applied even in highly restricted collection areas, because detailed magnetic mineralogy, including superparamagnetic minerals, could be determined with the use of silica wool for use in both high- and low-temperature experiments.

New archaeomagnetic direction results from China and their constraints on palaeosecular variation of the geomagnetic field in Eastern Asia

We carried out an archaeomagnetic directional study on 38 oriented samples (bricks and baked clays) collected from four archaeological locations at three provinces in China. The ages of our samples, spanning from ∼3000 BCE to ∼1300 CE, were constrained using a combination of archaeological context, radiocarbon dating and stratigraphic information. Rock magnetic results demonstrate that the main magnetic minerals of the studied samples are magnetite and/or hematite in single domain and superparamagnetic states. A total of 20 new reliable archaeodirectional data from 12 independent sites are obtained after thermal demagnetization experiments. These are the first set of archaeodirectional data in China produced since the 1990s. The published data are largely from the past 2 kyr and data from older time periods are rare. Our new data, especially those from period older than 3 ka, fill many gaps of the presently published dataset and will provide strong constraints on palaeosecular variation of the geomagnetic field in Eastern Asia and on the improvement of global models. Quite a few inflection points in the direction of the geomagnetic field are recorded in Eastern Asia over the past 10 kyr and some of them synchronize with the maximums or minimums of the palaeointensity. The palaeosecular variation rates are very low (based on present data distribution) before 2000 BCE and then start to increase and fluctuate afterward, which is generally consistent with the pattern of palaeointensity variations in this area.

Paleomagnetism of the Central Iberian curve's putative hinge: Too many oroclines in the Iberian Variscides

The Variscan mountain belt in Iberia defines a large “S” shape with the Cantabrian Orocline in the north and the Central Iberian curve, an alleged orocline belt of opposite curvature, to the south. The Cantabrian Orocline is kinematically well constrained, but the geometry and kinematics of the Central Iberian curve are still controversial. Here, we investigate the kinematics of the Central Iberian curve, which plays an important role in the amalgamation of Pangea since it may have accommodated much of the post-collisional deformation. We have performed a paleomagnetic study on Carboniferous granitoids and Cambrian limestones within the hinge of the curve. Our paleomagnetic and rock magnetic results show a primary magnetization in the granitoids and a widespread Carboniferous remagnetization of the limestones. Syn-kinematic granitoids show ca. 70° counter-clockwise rotations consistent with the southern limb of the Cantabrian Orocline. Post-kinematic granitoids and Cambrian limestones show consistent inclinations but very scattered declinations suggesting that they were magnetized coevally to and after the ~70° rotation. Our results show no differential rotations between northern, southern limb and the hinge zone. Therefore, we discard a late Carboniferous oroclinal origin for the Central Iberian curve.

A Greigite-Based Magnetostratigraphic Time Frame for the Late Miocene to Recent DSDP Leg 42B Cores from the Black Sea

Throughout the Late Neogene, the Black Sea experienced large paleoenvironmental changes, switching between (anoxic) marine conditions when connected to the Mediterranean Sea and (oxic) freshwater conditions at times of isolation. We create a magnetostratigraphic time frame for three sites drilled during Deep Sea Drilling Project (DSDP) Leg 42B to the Black Sea (drilled in 1975). At the time, magnetostratigraphic dating was impossible because of the presence of the little understood iron sulfide mineral greigite (in sediments a precursor to pyrite) as magnetic carrier. Our rock-magnetic results indicate that only anoxic conditions result in poor magnetic signal, likely as a result of pyrite formation in the water column rather than in the sediment. The magnetostratigraphic results indicate that Hole 379A, drilled in the basin center, has a continuous sedimentary record dating back to 1.3 Ma. Hole 380/380A is subdivided into three consistent intervals, 0-700 mbsf, 700-860 mbsf and 860-1075 mbsf. The top unit covers the Pleistocene but the magnetostratigraphy is likely compromised by the presence of mass transport deposits. The middle unit spans between 4.3 and 6.1 Ma and records continuous deposition at ~10 cm/kyr. The lower unit lacks the independent age constraints to correlate the obtained magnetostratigraphy. Hole 381 is drilled on the Bosporus slope and as a result, hiatuses are common. A correlation to the nearby Hole 380/380A is proposed, but indicates deposits cannot straightforwardly be traced across the slope. Our improved age model does not support the original interpretation based on these cores of a desiccation of the Black Sea during the Messinian salinity crisis.

Paleosecular variations of the geomagnetic field during the Holocene from Eastern Asia

High-resolution paleomagnetic secular variation (PSV) records bear great information of dynamics processes of the Earth’s geomagnetic field, and can be further used for inter-profile correlation and for dating sediments. However, effects of changes in the depositional environment on PSV records have not been fully determined. This study constructed Holocene PSV records for the gravity piston core (ZSQD34) obtained from the northern South China Sea. Rock magnetic and Transmission Electron Microscope (TEM) results indicate that single (SD) and PSD domain magnetites are the main carrier of the natural remanent magnetization. Comparable to the records derived from the freshwater lakes and the modeling results, we observed that direction curves from these two environments of contrasting salinity content are rather consistent. The direction curves are independent of the constructed salinity. However, the gradual increasing trend of relative intensity since about 5kyr might be related to the decreasing sea surface salinity. Furthermore, on the centennial and millennial time scale, the relative intensity and salinity show some positive relation, suggesting a potential contribution of salinity to the paleomagnetic relative intensity recording processes.

Intensified tectonic deformation and uplift of the Altyn Tagh range recorded by rock magnetism and growth strata studies of the western Qaidam Basin, NE Tibetan Plateau

As the tectonical and geographical northern edge of the Tibetan Plateau, the evolution of the Altyn Tagh range has attracted wide attention. Precise dating of its activities is believed essential for understanding the possible mechanisms of the Tibetan Plateau uplift and its effects on climate changes. Under the framework of basin–mountain coupling, both magnetic susceptibility and rock magnetic researches were carried out in this study on the Late Cenozoic sediments of the Honggouzi (HGZ) section (ca. 17–5Ma) in the western Qaidam Basin to explore the tectonic and climatic evolution as well as their interactions of the Altyn Tagh range.The obtained magnetic susceptibility record in the HGZ section displayed a two-step variation, which kept relatively low and stable values for sediments from the stratigraphic levels of 120–596m (ca. 17–10Ma) (stage I), but increased rapidly from 596 to 1014m (ca. 10–5Ma) (stage II). The rock magnetic results revealed that paramagnetic minerals or clay minerals, maghemite and hematite are dominant in stage I, which were replaced by magnetite and maghemite in stage II. A detailed comparison of magnetic susceptibility record in the HGZ section with regional tectonic and climate records was carried out. Combined with sedimentary facies, lithology and angular unconformity in the sequence, as well as seismostratigraphy data, paleocurrent and provenance analyses, the possible mechanisms for the magnetic susceptibility variation were explored. The results indicated a direct link between magnetic susceptibility change and the uplift of the Altyn Tagh range at ca. 10Ma.

Paleomagnetic constraints on the tectonic evolution of the Costa Rican subduction zone: New results from sedimentary successions of IODP drill sites from the Cocos Ridge

AbstractThe near‐flat subduction of the Cocos Ridge (CR) along the Middle American Trench (MAT) plays a pivotal role in governing the geodynamic evolution of the central American convergent margin. Elucidating the onset of its subduction is essential to understand the tectonic evolution and seismogenesis of the Costa Rican convergent margin, a typical erosive convergent margin and modern example of a flat‐slab subduction. Initial subduction of the CR has been previously investigated by examining upper plate deformation that was inferred to have resulted from the initial CR subduction. However, little attention has been paid to the extensive sedimentary archives on the CR that could hold important clues to the initial CR subduction. Drilling on the CR during IODP Expedition 344 discovered a pronounced sedimentary hiatus at Site U1381. Here we present paleomagnetic and rock magnetic results of the Cenozoic sedimentary sequences at this site that bracket the hiatus between ca. 9.61 and 1.52 Ma. We also examine the areal extent, timing, and geologic significance of the hiatus by analyzing sedimentary records from five other ODP/IODP sites on CR and Cocos plate. The analyses show that the hiatus appears to be regional and the presence/absence of the sedimentary hiatus at different locations on CR implies a link to the onset of CR shallow subduction, as a result of either bottom current erosion or CR buckling upon its initial collision at the MAT. Records directly from CR thus provide a new window to unraveling the geodynamic evolution of the central American margin.

A sedimentary paleomagnetic record of the upper Jaramillo transition from the Lantian Basin in China

The termination of the Jaramillo (normal to reverse) subchron is a key chronostratigraphic marker for dating global Pleistocene sedimentary sequences. However, the stratigraphic position of the geomagnetic polarity reversal varies greatly across the Chinese Loess Plateau (CLP), from near the bottom of paleosol unit S9 to the middle-upper part of S10. Here, we present paleomagnetic and rock magnetic results from high-resolution sampling of the Yushan loess section of the Lantian Basin located within the southern CLP. Our combined analyses determine that the polarity reversal is located in the middle-lower part of the paleosol unit S10. This stratigraphic position is lower than most of other studies conducted throughout the CLP. We attribute the difference in the location of the reversal to a deeper lock-in depth of remanence acquisition, which may have occurred from postdepositional processes under favorable hydrothermal conditions along the southern margin of CLP. It is important to note that age determinations through magnetic stratigraphy on sedimentary sections, particularly in discontinuous and/or imperfect sequences, should be treated with caution; there are significant differences with respect to the location of the polarity reversal throughout the CLP.

Oligocene‐Miocene magnetostratigraphy and magnetic anisotropy of the Baxbulak section from the Pamir‐Tian Shan convergence zone

AbstractAs the northernmost part of the Indo‐Eurasian collision belt, the Pamir‐Tian Shan convergence zone (PTCZ) is a strategic location for understanding intracontinental deformation. Here we present a magnetostratigraphic investigation of a continuous section from the Baxbulak region, to better constrain regional tectonic history. Rock magnetic analyses indicate that hematite and magnetite are the main carriers of characteristic remanent magnetization. The resulting polarity sequence allows a distinct correlation to the geomagnetic polarity time scale, showing that the section spans the interval of 29.1–20.7 Ma. Rock magnetic results further suggest that paramagnetic and antiferromagnetic minerals dominantly contribute to anisotropy of magnetic susceptibility (AMS) of the sequence. Thus, the AMS would indicate the preferred orientations of the mineral grains that are sensitive to tectonic strain. At around 26 Ma, the grouped principal minimum perpendicular to the bedding diverts to a girdle distribution in a N‐S direction, demonstrating the overprint of tectonic fabric to previous weakly deformed sedimentary fabric. This would be interpreted as a marked increase in tectonic strain, consistent with various evidence from the Pamir and the neighboring basin that show the Pamir began to migrate northward. Moreover, the coincident changes in distribution of AMS principal axes, in both direction and magnitude, are comparable to the regional counterclockwise rotations observed from paleomagnetic data, likely related to orogenesis.

Paleomagnetism of the Miocene Jantetelco Granodiorites and Tepexco Volcanic Group and inferences for crustal block rotations in central Mexico — Reevaluation

Miocene igneous rocks from a location in central Mexico provide paleomagnetic data that allow re-evaluation of previous data by Urrutia-Fucugauchi (1981) that were interpreted to suggest a counter clockwise rotation of a crustal block by about 50°. We sampled 31 sites from the Miocene trondhjemitic Chalcatzingo domes, the Tepexco volcanic group, and the Xalostoc diorite, covering the same area of approximately 50km2 about 90km SE of Mexico City. Magnetic analysis shows that these rocks contain magnetic phases of variable composition, with Curie temperatures characteristic for magnetite, but often accompanied by lower Curie temperature components. Magnetic hysteresis measurements point to the presence of PSD particles, and in combination these properties suggest that these rocks are also suitable for paleomagnetic study. Scanning electron microscope analysis supports the rock magnetic results, indicating the presence of magnetite and high-intermediate titanium titanomagnetite in many samples. Demagnetization experiments showed in most cases characteristic remanence directions of reasonable to good quality, and for 26 sites mean directions could be determined. Of these 14 (12) are of normal (reverse) polarity. Overall mean directions of normal and reverse sites are antipodal and pass a reversal test at the 95% probability level. The paleodirection from 26 sites is D=348.1°E, I=35.7°, α95=7.4°, and the paleopole is located at Lat=78.3°N, Long=184.2°E, A95=7.0°, which is indistinguishable from the 20Ma reference pole for the stable North America plate. These data do not support any tectonic deformation of the sampling area since the Miocene. Based on the number of sites studied, their rock magnetic characteristics, and the quality of the magnetic remanence that results in a positive reversal test, we consider our data to be reliable and we therefore suggest that this result supersedes that of Urrutia-Fucugauchi (1981). The reason for this previous and much different result remains unknown.