Characteristic Remanent Magnetization Research Articles

1.86 Ga key paleomagnetic pole from the Murmansk craton intrusions – Eastern Murman Sill Province, NE Fennoscandia: Multidisciplinary approach and paleotectonic applications

We present the first 1.86 Ga paleomagnetic key pole of Fennoscandia obtained for the dolerite sills of the Murmansk craton – Eastern Murman Sill Province, that outcrop in the northern part of the Kola Peninsula along the Barents Sea coast for a distance of 200 km (Slat = 68.5°; Slong = 37.9°; N = 16 sites; Plat = 54.7°; Plong = 234.7°; dp/dm = 4.3°/6.3°, Qv = 5). The age of the sills and their characteristic remanent magnetization (ChRM) was determined by four independent geochronometers: U-Pb – 1860 ± 4 and 1863 ± 7 Ma (ID-TIMS, baddeleyite), Sm-Nd – 1889 ± 57 Ma, Rb-Sr – 1850 Ma, Ar/Ar – 1865 ± 8 and 1857 ± 20 Ma (biotite). The primary nature of the ChRM is confirmed by the results of petrographic, geochemical, paleo- and rock magnetic studies, as well as by thermochronological data. The similarity of the petrographic and geochemical characteristics of sills from different localities indicates that these dolerite sills were formed during a single magmatic event and their cooling down to 580 °C occurred at depths of about 10 ± 2 km and lasted ∼2800 years or even faster. Paleogeographic reconstruction of Fennoscandia on the basis of the obtained paleomagnetic pole is in general agreement with the previously suggested configuration of core of the Nuna/Columbia supercontinent (Evans and Mitchell, 2011; Meert and Santosh, 2017). A new reliable Thellier-Coe paleointensity determination for this time reveals a rather low mean VDM = 1.8 (±0.1) × 1022 Am2 that supports the Proterozoic dipole low hypothesis (Biggin et al., 2009).

Palaeomagnetism of Bahariya iron ores and their genetic implications, North Western Desert, Egypt

ABSTRACTIntegrated rock magnetic, palaeomagnetic and magnetic fabric studies have been carried out, in order to rely a better understanding of the age and origin of the Bahariya iron ores. A total of 80 oriented block samples were collected from 10 sites covering the three iron mining areas at Bahariya Oasis (El Gedida, Ghorabi and El Harra) within an area located between latitudes (28° 24` to 28° 29`N) and longitudes (29° 02` to 29° 11`E). Rock magnetic measurements reveal that, the main magnetic carrier is the haematite. Goethite and pyrrhotite, that are also present as subordinate constituents in the studied samples. Careful thermal demagnetization successfully enabled the isolation of the characteristic remanent magnetization of primary origin, which yields a magnetic position at Lat. 75.3º N and Long. 137.9º E revealing that, both the ore and host rocks are of the same age. Magnetic fabric studies on these ores indicated that, they are still holding their primary Fabrics of sedimentary origin.

Magnetization age from paleomagnetism of the Copper Harbor red beds, Northern Michigan, USA, and its Keweenawan geologic consequences

The Copper Harbor Formation on Lake Superior’s Keweenaw Peninsula records the transition from volcanic to sedimentary infilling of North America’s 1.1 Ga Keweenawan rift. Radiometric dating shows that the formation’s primary mafic sediments and interbedded “Lake Shore” flows were deposited between ∼1092 and ∼1082 Ma. Our regional paleomagnetic results for the Copper Harbor’s red beds yield a dominantly prefolding normal-polarity secondary chemical characteristic remanent magnetization in hematite at 18 of 21 sites with a mean direction of declination = 274.9°, inclination = +10.9° (k = 69.5, α95= 4.2°), and a paleopole at 7.4°N, 181.7°E (A95= 3.3°). Using paleopoles from Keweenawan volcanic rocks with U–Pb zircon age dates, an apparent polar wander path is constructed from 1106 ± 2 to 1087 ± 2 Ma. Extrapolation of this path dates oxidation of the Copper Harbor’s primary gray beds to red beds at 1060 ± 5 Ma. The path implies an apparent polar wander rate of ∼18 cm per year from ∼1108 to 1096 Ma and of 6.8 cm per year from 1096 to 1087 Ma, along with a consistent clockwise rotation of 0.30 ± 0.05°per million years for the Laurentian Shield from ∼1108 to ∼1160 Ma. Further, most Keweenawan volcanic rocks around the Lake Superior region carry an endemic ∼1060 Ma normal-polarity hematite remanence overprint, acquired during the initial stages of Grenvillian tectonic uplift, that has caused asymmetry in a unit’s normal and reverse paleopoles. Also, the Copper Harbor paleopole dates emplacement of the White Pine stratiform sedimentary copper mineralization more precisely at 1060 ± 5 Ma.

Paleomagnetic and Geochronological Results From the Zhela and Weimei Formations Lava Flows of the Eastern Tethyan Himalaya: New Insights Into the Breakup of Eastern Gondwana

AbstractThe breakup of eastern Gondwana is among the hottest topics in the Earth sciences because of its effect on global climate during the Jurassic‐Cretaceous, its influence on the evolution of life, and its importance to paleogeographic reconstruction. To better constrain the Jurassic and Cretaceous paleogeographic position of the Tethyan Himalaya and the breakup of eastern Gondwana, a combined paleomagnetic and geochronological study was performed on the Zhela and Weimei Formations lava flows, dated at ~138–135 Ma, in the Luozha area of the eastern Tethyan Himalaya. Both positive fold and reversal tests together with a maximum grouping at 100% unfolding indicate that the characteristic remanent magnetization directions are primary magnetizations acquired before folding. The tilt‐corrected directions yielded a paleopole at 0.9°N, 293.4°E with A95 = 7.0° and a corresponding paleolatitude of 53.5°S ± 7.0°S for the Luozha sampling area (28.9°N, 91.3°E), validating that the original erupted position of the Zhela and Weimei Formations lava flows was located in the center of the Kerguelen mantle plume. Our new results, together with the published paleomagnetic, geochronological, and geochemical results, demonstrate that the Comei‐Bunbury large igneous province originated from the Kerguelen mantle plume. The temporal and spatial relationships between the Comei‐Bunbury large igneous province and the Kerguelen mantle plume indicate that eastern Gondwana initially rifted at ~147 Ma and that the Indian Plate fully separated from the Australian‐Antarctic Plate before ~124 Ma.

First report of coupled Early Permian paleomagnetic and geochronologic data from the Dunhuang block (NW China), and implications for the tectonic evolution of the Paleo-Asian ocean

In order to better understand the tectonic relationship between the Dunhuang block (DHB) and its adjacent blocks, and to constrain the timing of the closure of the Paleo-Asian ocean, a combined geochronologic and paleomagnetic investigation has been undertaken on Early Permian tuff, basalt flows and sandstones from the Shuangbaotang Formation in the northwestern part of the DHB. U-Pb zircon dating indicates that the age of the strata is between 280.6 ± 2.9 Ma and 291.4 ± 2.6 Ma. Thermal demagnetization of a three-component isothermal remanent magnetization (IRM), and Curie point experiment suggest that magnetite dominates in the rock samples analyzed. In addition, there is a minor amount of hematite in some sandstones. Stepwise thermal demagnetization successfully isolated stable characteristic remanent magnetization (ChRM) from 11 tuff layers, two lava flows and nine sandstone beds. Two components were isolated from all samples: a lower temperature component (LTC) and a higher temperature component (HTC). The LTC is near the direction of the present-day geomagnetic field and produced a negative fold test, indicating it is a viscous remanent magnetization in the present-day geomagnetic field. Most of the HTC are reverse polarity (Normal = 4, reverse = 158), which is in accordance with the Kiaman Reversed Superchron that spans the Late Carboniferous-Permian interval. In addition, the HTC of all studied sites passed the fold and reverse tests, suggesting that they likely represent primary remanent magnetization. The tilt-corrected mean direction from all sites (tuff, basalts and sandstones) is Ds = 1.7°, Is = 43.1°, ks = 403, α95 = 1.5°, N = 22. The mean paleopole of the site-mean direction-corresponded VGPs lies at 74.5°N, 268.5°E with A95 = 1.6°. Considering the consistent inclination values recorded among the studied tuff, basalts and sandstones, and the low degrees of anisotropy within all samples, we suggest that there is no significant inclination shallowing caused by depositional compaction in the sedimentary layers of the studied section. Taking into account the results from this study as well as previous Late Paleozoic paleomagentic studies from adjacent tectonic blocks, we conclude that the DHB formed part of the amalgamated Dunhuang-North China-Alxa-Qaidam mega-block during the Early Permian, but was separated from the Tarim block by a small ocean (here named the Qiemo-Xingxingxia paleo-ocean) at this time. A comparison of the Early Permian paleolatitudes of these and other adjacent blocks suggest that the Paleo-Asian ocean (sensu lato) was still open at this time. Combined with other geological evidence, a paleogeographic reconstruction of the Paleo-Asian ocean has been reconstructed for the Early Permian.

Revisiting the paleomagnetism of the Neoarchean Uauá mafic dyke swarm, Brazil: Implications for Archean supercratons

The original connections of Archean cratons are becoming traceable due to an increasing amount of paleomagnetic data and refined magmatic barcodes. The Uauá block of the northern São Francisco craton may represent a fragment of a major Archean craton. Here, we report new paleomagnetic data from the 2.62 Ga Uauá tholeiitic mafic dyke swarm of the Uauá block in the northern São Francisco craton, Eastern Brazil. Our paleomagnetic results confirm the earlier results for these units, but our interpretation differs. We suggest that the obtained characteristic remanent magnetization for the 2.62 Ga swarm is of primary origin, supported by a provisionally-positive baked contact test. The corresponding paleomagnetic pole (25.2°N, 330.5°E, A95 = 8.1°, N = 20) takes the present northern part of the São Francisco craton to moderate latitudes. Based on the comparison of the paleolatitudes of cratons with high-quality paleomagnetic data and magmatic barcodes, we suggest that the northern part of the São Francisco craton could have been part of the proposed Supervaalbara supercraton during the Archean. Supervaalbara is proposed as including (but not limited to) the part of the São Francisco craton as well as the Superior, Wyoming, Kola + Karelia, Zimbabwe, Kaapvaal, Tanzania, Yilgarn, and Pilbara cratons.

Paleomagnetic, rock magnetic and geochemical study of the 1755 tsunami deposit at Boca do Rio (Algarve, Portugal)

A multiproxy study combining paleomagnetic, rock magnetic (magnetic susceptibility; anhysteretic remanent magnetization, ARM; isothermal remanent magnetization, IRM, anisotropy of magnetic susceptibility, AMS) and geochemical (XRF) data was carried out on trench sediments collected in the area of Boca do Rio (Algarve, South Portugal) known to have been stricken by a tsunami wave related to the 1755 Lisbon earthquake. The magnetic mineral assemblage is dominated by multidomain ferrimagnetic grains with a significant contribution (5–9%) of ultra-fine magnetic particles related to the presence of pedogenetic magnetite. The variable amount of this post-depositional magnetite plays a role on the characteristic remanent magnetization (ChRM) of the sediment. The primary magnetization recorded at T1, located farthest from the coast, is representative of the paleosecular variation of the Earth's magnetic field and a comparison of the results with the regional model for the geomagnetic field in Europe (SCHA.DIF.3K) was used to constrain the erosional signature of the tsunami layer. The result indicates that less than eighty years could be missing in the sedimentary column. The eroded sediment (thickness 27–40 cm) was incorporated in the tsunami layer that is characterized by a low magnetic content and a large increase in the Ca content representing allochthonous material.Sedimentological, geochemical, magnetic and AMS data show that the signature of the tsunami layer reflects at least two waves with an erosive capacity that is controlled by the distance from the shore.

Regionally consistent Western North America paleomagnetic directions from 15 to 35 ka: Assessing chronology and uncertainty with paleosecular variation (PSV) stratigraphy

New Paleomagnetic Secular Variation (PSV) data from Fish Lake, Utah, USA, along with previously published regional records, allow us to build an independently dated Western North America PSV stack (WNAM17) from about 35 to 15 ka that quantifies dating and paleomagnetic uncertainties. In February 2014, we recovered a composite 11-m-long sediment record from Fish Lake, Utah that retrieved glacially influenced and post-glacial sediments. Computed tomography (CT) scans and magnetic susceptibility were used to build a precise depth scale, while radiocarbon dating of terrestrial macrofossils and tephrostratigraphy provide tight age-control. Glacially influenced sediments have well-defined characteristic remanent magnetizations with all maximum angular deviation values < 5°, resolving a record of Late Pleistocene PSV. Comparison to the PSV records at Bear Lake, Utah/Idaho, USA and Bessette Creak, British Columbia, Canada, which also contain terrestrial material radiocarbon age-control points, allow us to develop the WNAM17 PSV stack which considers both dating and magnetic acquisition uncertainties. We utilize a regional PSV chronostratigraphic modeling approach to assign chronology and uncertainty to the PSV signal and to evaluate the fidelity of age-control points at the site level that are difficult to objectively evaluate otherwise. The greatest strength of our age-depth model is that by approaching chronostratigraphy from a regional perspective, assuming a common geomagnetic signal, we can address sources of geologic uncertainty that would be difficult to quantify using only one site. We illustrate the potential stratigraphic opportunities using the WNAM17 PSV template through assessing the timing of paleoenvironmental and paleomagnetic events in Western North America, including the timing of glaciations in the Fish Lake and Bear Lake catchments and the stratigraphic context for the Late Pleistocene geomagnetic excursion recorded at Mono Lake.

Insights on the geomagnetic secular variation in the Eastern Mediterranean: First directional data from Cyprus

The first directional archaeomagnetic results from Cyprus are presented, based on the study of seven baked clay structures excavated at the copper smelting site of Agia Varvara-Almyras, situated some 20 km south of Nicosia. This ancient copper working site represents a unique example of complete primary production of the copper metal in Cyprus. In total five furnaces, one ore roasting pit and an oven were sampled in situ for an archaeomagnetic investigation. All the studied structures are well dated, based on pottery finds and radiocarbon dates, with ages ranging from 600 BCE to 50 BCE. Several rock magnetic experiments were performed to determine the main magnetic carrier and to investigate the thermal stability of the studied material. The direction of the Characteristic Remanent Magnetization (ChRM) for each structure has been determined by demagnetization of 7–15 specimens in alternating magnetic fields up to 100–120 mT. Mean directions were calculated assuming a Fisherian distribution and are well defined for five out of the seven structures studied. The new directions are compared with the directional data and the secular variation curves available from the surrounding area, and with the predictions of global geomagnetic field models. They are an important contribution to improving our knowledge of the geomagnetic field variations in the Eastern Mediterranean and offer new insights about fast geomagnetic directional changes during the first millennium BCE that were not previously identified by the available geomagnetic field models’ predictions in this area.

Palaeo- and rock magnetic investigations of Late Quaternary sediments from the Upper Congo deep-sea fan: on the difficulty in obtaining palaeomagnetic secular variation records from low latitudes

We report here on results of palaeo- and rock magnetic investigations of two sediment cores from the Upper Congo deep-sea fan. The sediments have a high organic content and contain a heterogeneous Fe-mineral assemblage with biogenic magnetite and detrital (Ti-)magnetite as the main magnetic carrier minerals. Pyrite, hematite, and Fe-oxyhydroxides were identified by comparing high-temperature magnetic susceptibility curves with those from Fe-minerals of known composition. According to AMS 14C dates, the 6.8 m-long profile spans the last 37 kyr. Sediments older than 20 ka are affected by reductive diagenesis that has led to a loss of the fine-grained magnetic mineral fraction. Sediments younger than 20 ka have stable magnetizations. Characteristic remanent magnetization records of inclination and declination were obtained for each core. There is a little agreement between these records, modelled curves, and other sediment records from Equatorial Africa, so no composite record could be established. The cores are not ideal relative palaeointensity recorders and estimates using different normalizers did not yield consistent signals from both cores. Normalization methods used for relative palaeointensity estimation were not developed for sediments that contain large amounts of ultra-fine-grained biogenic magnetite; therefore, the relative palaeointensity estimates should be considered with caution. However, in view of the incoherent picture given by the scarce available palaeointensity information from the region off South-West Africa, the GeoB6517-2 record may provide a tentative relative palaeointensity record for comparison, at least for the past 10 kyr.

Paleomagnetic Rotation Constraints on the Deformation of the Northern Qaidam Marginal Thrust Belt and Implications for Strike‐Slip Faulting Along the Altyn Tagh Fault

AbstractDiscordant Cenozoic paleomagnetic rotation results obtained throughout the northeastern Tibetan Plateau have yielded different tectonic models for the evolution of the Altyn Tagh Fault and the northeastern Tibetan Plateau. Here based on detailed existing magnetostratigraphic age constraints, we carried out an in‐depth paleomagnetic study along the Hongliugou section in the middle northern Qaidam marginal thrust belt to address the Cenozoic rotations of this region. A total of 1,342 Jurassic‐Neogene siltstone‐sandstone core samples were collected from 128 sites across 18 time intervals. Rock magnetic analysis suggested that magnetite and hematite are the dominant magnetic carriers. Detailed thermal demagnetization analyses were performed, and the characteristic remanent magnetizations of 90 sites (743 samples) were obtained, which yielded positive fold and reversals tests, indicating that they likely represent primary magnetizations. Detailed paleomagnetic declination versus age analyses revealed three periods of rotations: ~12.5 ± 15.7° statistically insignificant counterclockwise rotations during ~45–33 Ma, successive clockwise rotations of ~32.2 ± 15.4° during ~33–14 Ma, and subsequent statistically insignificant counterclockwise rotations of ~8.7 ± 9.4° after ~14 Ma. These three periods of rotations are similar to those calculated based on previously available magnetostratigraphic data from the nearby Dahonggou section. Based on the integration of these data with other lines of geological evidence, we attributed the latter two periods of rotations to fast strike‐slip faulting along the Altyn Tagh Fault.

Paleomagnetism in Lake Pannon: Problems, Pitfalls, and Progress in Using Iron Sulfides for Magnetostratigraphy

AbstractDating of upper Miocene sediments of the Pannonian Basin (Hungary) has proven difficult due to the endemic nature of biota, scarcity of reliable radio isotopic data, and generally inconsistent magnetostratigraphic results. The natural remanent magnetization (NRM) is mostly residing in greigite (Fe3S4), which complicates NRM interpretation. We reinvestigate the viability of these sediments for magnetostratigraphy using samples from recently drilled well cores (PAET‐30 and PAET‐34) from the Paks region. Significant intervals of the cores contain composite NRM behavior. Thermal demagnetization results include multipolarity (M‐type) samples consisting of a low‐temperature (LT, above ~120 °C), a medium‐temperature (MT), and a high‐temperature (HT) component, within distinct temperature ranges and all exhibiting dual polarities. The LT and HT components have the same polarity and are antiparallel to the MT component. Rock magnetic and scanning electron microscopy results indicate that all magnetic components reside in authigenic greigite. The LT and HT components represent the characteristic remanent magnetization and are of early diagenetic origin. The MT component records a late diagenetic overprint. Alternating field demagnetization cannot resolve the individual components: it yields polarities corresponding to the dominant component resulting in erratic polarity patterns. Interpretation of LT and HT components allows a reasonably robust magnetostratigraphic correlation to the geomagnetic polarity time scale with the base of PAET‐30 at ~8.4 Ma and its top at ~6.8 Ma (average sedimentation rate of ~30 cm/kyr). The base of PAET‐34 is correlated to ~9 Ma and its top to ~6.8 Ma (average sedimentation rate of 27 cm/kyr).

Paleomagnetism and Geochemistry of ~1144‐Ma Lamprophyre Dikes, Northwestern Ontario: Implications for the North American Polar Wander and Plate Velocities

AbstractWe present new paleomagnetic and geochemical data from a suite of the ~1144‐Ma ultramafic lamprophyre dikes that outcrop in the Canadian Shield northeast of Lake Superior (Ontario, Canada). Nineteen of 22 sampled dikes yielded consistent characteristic remanent magnetization directions of normal (n = 5) and reversed (n = 14) polarity. The primary origin of characteristic remanent magnetization is bolstered by positive baked contact tests and a reversal test. The group mean direction (D = 306.4°, I = 72.1°, α95 = 5.5°, N = 19) obtained from the lamprophyre dikes is statistically indistinguishable from the group mean direction (D = 297.4°, I = 65.5°, α95 = 8.3°, N = 8) previously reported for the nearly coeval ~1142‐Ma Abitibi dikes. The geochemistry of the lamprophyre dikes suggests strong affinity with magmas derived from ocean island basalt‐type mantle sources, consistent with the mantle plume hypothesis for the formation of the ~1.1‐Ga North American Midcontinent Rift. The similarity in age, trend, paleomagnetism, and geochemistry indicates that the lamprophyre and Abitibi dike suites represent the earliest magmatic event associated with the commencement of rifting. The combined mean direction (D = 303.1°, I = 70.2°, α95 = 4.5°, N = 27) corresponds to a paleomagnetic pole at Plat = 55.8°N, Plong = 220.0°E (A95 = 7.3°). The new pole merits the highest classification on the Q‐scale of paleomagnetic reliability and represents a key pole defining the North American apparent polar wander path during the late Mesoproterozoic. Combined with high‐quality data from the ~1108‐Ma Coldwell Complex, our data indicate an equatorward motion of Laurentia at 3.8 ± 1.4 cm/year, comparable with the present‐day velocities of continental plates, before switching to extremely rapid motion between ~1108 and ~1099 Ma.

Paleomagnetic evidence for the emplacement mechanism of an enigmatic boulder accumulation on a coastal cliff top in New South Wales: implications for the Australian Megatsunami Hypothesis

ABSTRACTPaleomagnetic sampling and measurement of a boulder accumulation on Little Beecroft Head on the Illawarra coastline of New South Wales was undertaken to evaluate potential emplacement mechanisms. This deposit is of central importance in the Australian Megatsunami Hypothesis (AMH) debate, but to date, there has been no unequivocal determination of its provenance. The most likely emplacement mechanisms are by slow collapse during denudation of overlying strata, storm wave overwash or a combination of these. Characteristic Remanent Magnetisation (ChRM) directions were obtained from 15 individual boulders and the in situ bedrock platform on which they currently rest. The in situ Permian bedrock has a normal polarity mean ChRM direction of D/I = 1.6°/–66.7° (α95 = 5.2°; k = 33.9) that is statistically indistinguishable from the Present Earth Field direction at the site. The magnetisation is most likely due to Cenozoic/recent weathering, which is common in surficial rocks throughout the Sydney Basin. ChRM directions for the boulders are stable but scattered, although not random, and the mean boulder direction is indistinguishable in geographic (i.e. current in situ) coordinates, at the 5% significance level, from the mean direction of the in situ bedrock. Further statistical tests confirm that the scatter in the mean directions of the boulders and the in situ bedrock is different, at the 5% significance level, with the boulder mean being more scattered. At an individual boulder level, some blocks have mean ChRM directions that are statistically indistinguishable from the mean in situ rock ChRM direction, whereas others are distinguishable at the 5% significance level.These results indicate that the boulders were magnetised prior to emplacement but were not moved far from their original positions during emplacement. The emplacement age is constrained to the last ca 780 000 years. These observations strongly support the hypothesis that the Little Beecroft Head boulder deposit was emplaced by a non-catastrophic mechanism, namely slow collapse during denudation of pre-existing cliff material or overtopping from severe storms, which occur regularly on the east coast of New South Wales. Even if a catastrophic wave were responsible, the results constrain the age of that event to be older than 780 000 years. Therefore, the results presented here are not supportive of the AMH as it currently stands. Further paleomagnetic work, on similar deposits along the Illawarra coastline and from elsewhere in Australia, is needed to evaluate the interpretations presented here.

Remagnetization of the Lower Ordovician Hongshiya Formation of the southwestern Yangtze Block

Large-scale block migration has been proposed based on Early and early Late Ordovician paleomagnetic data for the South China Block (SCB). However, this is anomalous in terms of the previously reconstructed affinity between the SCB and East Gondwana. A paleomagnetic and petrographic reassessment of the Lower Ordovician sedimentary rocks is therefore necessary to assess the reliability of the Early Ordovician paleopole of the SCB. Consequently, we obtained paleomagnetic data from 47 sites at five localities of the SCB. For most specimens, detailed thermal demagnetization experiments yielded viscous components at ~120 °C, while the stable characteristic remanent magnetization (ChRM) was isolated up to 680 °C. The ChRM passed the fold test and hematite is identified as the remanence carrier. The samples give the site-mean ChRM direction Ds/Is = 313.0°/61.9° (ks = 47.1, α95 = 3.2°) after tilt correction (five localities, 47 sites). Although the ChRM direction passed the fold test that was likely acquired before the Tertiary, petrographic studies reveal the occurrence of widespread secondary hematite in the specimens, indicating that the rocks were remagnetized after deposition. The average paleomagnetic direction overlaps with the Jurassic paleomagnetic direction from sampling areas of the southwestern SCB, implying that the remagnetization event occurred during the Jurassic. The Early Ordovician paleopole of the SCB is therefore urgently needed for plate reconstruction of the SCB within Gondwana.

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.

Constrained Magnetostratigraphic Dating of a Continental Middle Miocene Section in the Arid Central Asia

The Neogene succession of the Aktau Mountains in the Ili Basin, southeast Kazakhstan, is a terrestrial archive well suited for researching the role of Central Asia in Miocene climate evolution. We present an integrated approach for dating the well-exposed Bastau Formation, based on magnetostratigraphy and constraints from cyclostratigraphy and biostratigraphy. Stepwise demagnetization yielded characteristic remanence directions that are consistent with those expected for the Miocene in Central Asia. The reddish-colored alluvial floodplain deposits and grey lacustrine deposits show partly complex magnetic behavior with magnetite and hematite as the main magnetic carriers, with variable demagnetization behavior and non-dipolar normal and reverse polarity directions. The observed magnetic properties are best explained by depositional variability and magneto-mineralogical alteration effects of both dissolution and neo-formation of magnetite, including significant secondary magnetization. The mean of reverse polarity directions is flatter than the expected Middle Miocene Earth magnetic field, which is an indicator for the existence of inclination shallowing that supports a primary origin. Detailed rock magnetic analyses are used to analyze the nature of the characteristic remanent magnetization and to discriminate primary and secondary remanence directions in order to obtain a reliable magnetostratigraphic result. The proposed age of 15.3 Ma to 13.9 Ma for the Bastau Formation corresponds to the known biostratigraphic setting, matches with typical sedimentation rates of foreland basins in Central Asia, and coincides with spectral analysis of geochemical proxies of that section. The resulting age model serves as a robust framework for paleoclimate reconstruction of Neogene climate dynamics in Central Asia.

53–43 Ma Deformation of Eastern Tibet Revealed by Three Stages of Tectonic Rotation in the Gongjue Basin

AbstractThe Gongjue basin from the eastern Qiangtang terrane is located in the transition region where the regional structural lineation curves from east‐west‐oriented in Tibet to north‐south‐oriented in Yunnan. In this study, we sampled the red beds in the basin from the lower Gongjue to upper Ranmugou formations for the first time covering the entire stratigraphic profile. The stratigraphic ages are bracketed within 53–43 Ma by new detrital zircon U‐Pb ages constraining the maximum deposition age to 52.5 ± 1.5 Ma. Rock magnetic and petrographic studies indicate that detrital magnetite and hematite are the magnetic carriers. Positive reversals and fold tests demonstrate that the characteristic remanent magnetization has a primary origin. The Gongjue and Ranmugou formations yield mean characteristic remanent magnetization directions of Ds/Is = 31.0°/21.3° and Ds/Is = 15.9°/22.0°, respectively. The magnetic inclination of these characteristic remanent magnetizations is significantly shallowed compared to the expected inclination for the locality. However, the elongation/inclination correction method does not provide a meaningful correction, likely because of syn‐depositional rotation. Rotations relative to the Eurasian apparent polar wander path occurred in three stages: Stage I, 33.3 ± 3.4° clockwise rotation during the deposition of the Gongjue and lower Ranmugou formations; Stage II, 26.9 ± 3.7° counterclockwise rotation during deposition of the lower and middle Ranmugou formation; and Stage III, 17.7 ± 3.3° clockwise rotation after 43 Ma. The complex rotation history recorded in the basin is possibly linked to sinistral shear along the Qiangtang block during India indentation into Asia and the early stage of the extrusion of the northwestern Indochina blocks away from eastern Tibet.

Paleomagnetic Secular Variation Constraints on the Rapid Eruption of the Emeishan Continental Flood Basalts in Southwestern China and Northern Vietnam

AbstractEstimating the duration of magma eruptions using isotopic dating methods is difficult because of the intrinsic errors of the technique regarding the dated materials (such as zircon). However, the long‐term variation of the geomagnetic field recorded by lava flows can be used to estimate the net duration of an eruption sequence. The Emeishan basalts at Dongchuan, with a thickness of 630 m, yielded a reliable characteristic remanent magnetization of normal polarity and which passed the fold test (Tauxe &amp; Watson, 1994, https://doi.org/10.1016/0012-821X(94)90006-X). Stratigraphic and magnetostratigraphic correlations of the Emeishan basalts in the Dongchuan section with other sections indicate that the eruption of the Emeishan basalts at Dongchuan spans the entire normal polarity zone in the early stage of the Emeishan large igneous province. A flow‐by‐flow analysis of geomagnetic directions of the Emeishan basalts at Dongchuan indicates that four directional groups and fifteen individual lava directions were recorded, with a net duration (excluding quiescent intervals) of no more than 3100 years. The averaged site directions from the Emeishan basalts with normal polarity conforming to a geocentric axial dipole direction indicate that this interval is longer than 104–105 years. In addition, a magnetostratigraphic study indicates that the normal polarity interval recorded by the Emeishan basalts was shorter than 2–20 × 104 years. Thus, the total duration of the normal polarity stage of the Emeishan large igneous province was roughly 105 years. There is a possible relationship between the rapid eruption and the Late Capitanian mass extinction (259.8 ± 0.4 Ma, Henderson et al., 2012).

Magneto- and biostratigraphy across the Jurassic-Cretaceous boundary in the Kurovice section, Western Carpathians, Czech Republic

An integrated magneto- and biostratigraphic study of the Jurassic-Cretaceous (J-K) boundary section at Kurovice, Czech Republic was carried out. The acquired paleomagnetic data showed dual polarity characteristic remanent magnetization (ChRM) with counter-clockwise rotation direction. Rock magnetic data indicated magnetite as the principal carrier of remanent magnetization as well as presence of e.g. hematite and goethite. Magnetozones from M21n up to M17r were identified and dated as early Tithonian (dinoflagellate Semiradiata and nannofossil NJT15b zones) to late early Berriasian (Calpionella elliptica and nannofossil NK-1 (sub)zones).