Characteristic Remanent Magnetization Research Articles

Paleomagnetism as a geochronological gauge for deformational events: Insights from the Najd-related Nugrus Shear Zone, Northern Nubian Shield

Abstract The Nugrus Shear Zone (NSZ) of southern central Egypt is a major ductile shear structure with controversial origins. It lies east of the Hafafit Gneissic Complex (HGC) and separates the Central and South Eastern Desert terranes of Egypt. The NSZ has been correlated with the Najd Fault system in Saudi Arabia, and shares orientation and kinematic characteristics with the Najd fault. Previous microstructural studies of the NSZ and HGC have concluded a polydeformation history for the Nugrus area, involving early NW-ward thrusting, followed by NW-SE transcurrent shearing. This contribution reports a combined rock magnetic and palaeomagnetic study of the shear zone and wallrocks that gives a conclusive evidence for the previously mentioned tectonic scenario. Furthermore, the palaeopoles obtained correlate well with the available geochronologically dated poles from Northern Africa, leading to inferred ages for the deformation and thermal events of the area. These ages confirm the previous proposed tectonic models. Three components of Natural Remanant Magnetization (NRM) direction (CA, CB and CD) are identified. Component CA represents the Characteristic Remanent Magnetization (ChRM). Components CB and CD are considered to be secondary magnetization. CA is consistent with arc-related deformation and magmatism at ∼ 700–680 Ma. CB and CD represent the protracted Najd-related shear deformation and transpression at ∼ 650–550 Ma.

Combined Magnetostratigraphy From Three Localities of the Rainstorm Member of the Johnnie Formation in California and Nevada, United States Calibrated by Cyclostratigraphy: A 13 R/Ma Reversal Frequency for the Ediacaran

A combined magnetostratigraphy for the Rainstorm Member of the Ediacaran Johnnie Formation was constructed using the sediment accumulation rates determined by rock magnetic cyclostratigraphy for three localities of the Rainstorm Member to provide a high resolution, time-calibrated record of geomagnetic field reversal frequency at a critical time period in Earth history. Two previously reported magnetostratigraphy records from Death Valley, California, the Nopah Range and Winters Pass Hills (Minguez et al., 2015), were combined with new paleomagnetic and cyclostratigraphic results from the Desert Range locality of the Rainstorm Member in south central Nevada, United States . The Johnnie oolite marker bed is at the base of each of the three sections and allows their regional correlation. The Nopah Range and Desert Range localities have similar sediment accumulation rates of ∼5 cm/ka, so their stratigraphic sections can be combined directly. The Winters Pass Hills locality has a higher sediment accumulation rate of 8.4 cm/ka, therefore its stratigraphic positions are multiplied by 0.6 to combine with the Desert Range and Nopah Range magnetostratigraphy. The thermal demagnetization results from the Desert Range locality isolates characteristic remanent magnetizations that indicate two nearly antipodal east-west and shallow directions and a mean paleopole (11.7˚N, 348.4˚E) that is consistent with “shallow” Ediacaran directions. The Desert Range also yields a magnetic susceptibility rock magnetic cyclostratigraphy that records short eccentricity, obliquity, and precession astronomically-forced climate cycles in the Ediacaran. The high-resolution combined magnetostratigraphy with nearly meter-scale stratigraphic spacing (nominally 23 ka, based on the Desert Range sediment accumulation rate), indicates 11 polarity intervals in a cyclostratigraphy-calibrated duration of 849 ka, indicating a reversal frequency of 13 R/Ma. The Rainstorm Member records the Shuram carbon isotope excursion, hence its age is ∼574 Ma. Given the recent cyclostratigraphy-calibrated reversal frequency of 20 R/Ma from the Zigan Formation (Levashova et al., 2021) at 547 Ma, our results show that reversal frequency was high but fluctuated during the Ediacaran.

Rapid eruption of the Emeishan continental flood basalts: New paleomagnetic and geochronologic constraints

The duration of the eruption of the Emeishan large igneous province is hotly debated. We conducted a magnetostratigraphic and geochronological study of the core area of the large igneous province in the Binchuan area of Yunnan Province, southwestern China, in order to constrain the duration of the eruption. The results of detailed thermal demagnetization experiments revealed two remanent magnetic components from the volcanic rocks of 11 composite sections. A low-temperature component separated below 300 °C is interpreted as a recent viscous remanence. Additionally, reliable characteristic remanent magnetizations were revealed above 400 °C, with unblocking temperatures up to 580−680 °C, which passed the fold test and record three magnetozones. Zircons from the felsic ignimbrites exposed in the final stage of the mafic volcanism are dated to 258.2 ± 0.7 Ma (n = 15; mean square of weighted deviates = 1.3) by sensitive high-resolution ion microprobe. Stratigraphic and magnetostratigraphic correlations of the Emeishan basalts in the Binchuan sections indicate that the eruption of the mafic rocks of the Emeishan large igneous province can be clearly divided into early (reverse polarity subzone), middle (normal polarity subzone), and late (reverse polarity subzone) stages, with a total duration of less than 1.7 m.y. (260.8−259.1 Ma). However, by combining this chronology with previously reported conodont biostratigraphic results from locations around the Emeishan large igneous province, and comparing the dominant normal-reverse polarity sequence in the Emeishan large igneous province with the geomagnetic polarity time scale, we obtain a much shorter duration of the main eruptive stage of <0.8 m.y. (260.4−259.6 Ma). About three quarters of the basalts of the Emeishan large igneous province record have a normal polarity and erupted within 0.4 m.y., while the other quarter, mainly distributed in the central zone, shows a reverse polarity and much shorter duration. Given the short duration of the eruption, gas volatiles would have been released into the atmosphere at high rates, which might provide a causal link between the rapid eruption and the end-Guadalupian mass extinction. Before the mantle plume eruption, localized eruptions probably occurred. After eruption of the mafic Emeishan flood basalts, an acid volcanic eruption occurred in the early Wuchiapingian, which was sporadically distributed in the Emeishan large igneous province.

Remagnetization of the Jurassic limestones in the Zaduo area, Eastern Qiangtang Terrane (Tibetan Plateau, China): implications for the India–Eurasia collision

SUMMARYA series of terranes were accreted to Eurasia in the region of what is now the Tibetan Plateau, including the Qaidam-Qilian, the Songpan-Ganzi, the Qiangtang, the Lhasa and the Tethyan Himalaya terranes. The drift history of the Qiangtang Terrane and the timing of the Lhasa–Qiangtang collision are controversial. To contribute to this topic, here, we palaeomagnetically investigate the Middle-Upper Jurassic limestones of the Yanshiping group in the Zaduo area (32.5°N, 95.2°E), in the Estern Qiangtang Terrane. Twelve sites (133 samples) were processed. A major challenge in palaeomagnetism is the possibility of remagnetization that interferes with palaeogeographic reconstructions. Both thermal and alternating field demagnetizations were carried out to isolate the characteristic remanent magnetization (ChRM). Despite the positive reversals test, rock magnetic information points to a remagnetized ChRM. The ChRM is residing in stable single-domain (SSD) magnetite grains with cogenetic superparamagnetic (SP) particles. The co-occurreance of SSD and SP magnetites generates distinct rock-magnetic properties often refer to as the ‘remagnetized fingerprint’ in limestones. This remagnetization process is also manifested by the widespread occurrence of gypsum veinlets in the limestones. The site-mean direction of the 12 sites after tilt-correction is Ds = 30.6°, Is = 35.6°, κs = 182.9, α95 = 3.2°, corresponding to a palaeolatitude of ∼19.7°± 2.8°N for the study area. The corresponding palaeopole (59.8°N, 202.7°E with A95 = 2.8°) points to an NRM acquired after the India–Eurasia collision. The original sediments were likely anoxic because of the high organic carbon fluxes that prevailed during their deposition. After the India–Eurasia collision, it is envisaged that conditions became more oxic, giving rise to oxidation of iron sulphides to authigenic magnetite and the CRM acquisition. The Zaduo area in the Eastern Qiangtang Terrane has experienced ∼15.7° ± 3.2° (∼1740 ± 350 km) of latitudinal crustal shortening since the Eocene. In addition, the clockwise rotation responding to the India–Eurasia collision is also detected in the Zaduo area.

Intermediate field directions recorded in Pliocene basalts in Styria (Austria): evidence for cryptochron C2r.2r-1

Pliocene volcanic rocks from south-east Austria were paleomagnetically investigated. Samples were taken from 28 sites located on eight different volcanoes. Rock magnetic investigations revealed that magnetic carriers are Ti-rich or Ti-poor titanomagnetites with mainly pseudo-single-domain characteristics. Characteristic remanent magnetization directions were obtained from alternating field as well as from thermal demagnetization. Four localities give reversed directions agreeing with the expected direction from secular variation. Another four localities of the Klöch–Königsberg volcanic complex (3) and the Neuhaus volcano (1) have reversed directions with shallow inclinations and declinations of about 240° while the locality Steinberg yields a positive inclination of about 30° and 200° declination. These aberrant directions cannot be explained by local or regional tectonic movements. All virtual geomagnetic pole positions are located on the southern hemisphere. Four virtual geomagnetic poles lie close to the geographic pole, while all others are concentrated in a narrow longitude sector offshore South America (310°–355°) with low virtual geomagnetic pole latitudes ranging from − 15° to − 70°. The hypothesis that a transitional geomagnetic field configuration was recorded during the short volcanic activity of these five localities is supported by 9 paleointensity results and 39Ar/40Ar dating. Virtual geomagnetic dipole moments range from 1.1 to 2.9·1022 Am2 for sites with low VGP latitudes below about 60° and from 3.0 to 9.3·1022 Am2 for sites with higher virtual geomagnetic pole latitudes. The new 39Ar/40Ar ages of 2.51 ± 0.27 Ma for Klöch and 2.39 ± 0.03 Ma for Steinberg allow for the correlation of the Styrian transitional directions with cryptochron C2r.2r-1 of the geomagnetic polarity time scale.Graphic abstract

Micro‐Blocks in NE Asia Amalgamated Into the Unified Amuria Block by ∼300 Ma: First Paleomagnetic Evidence From the Songliao Block, NE China

AbstractAmuria formed in the late Paleozoic through amalgamation of the Mongolia and Songliao‐Jiamusi blocks in the eastern Central Asian Orogenic Belt (CAOB). The exact timing of the assembly is critical towards unraveling the Phanerozoic evolution of the Paleo‐Asian Ocean (PAO) and assembly of the Asian blocks in Pangea. Here, we report the first Permo‐Carboniferous paleomagnetic data from the Songliao‐Jiamusi block, aiming to constrain its relative paleoposition with respect to Mongolia. A stable characteristic remanent magnetization (ChRM) was isolated from 313–292 Ma volcanic‐sedimentary sequences of the Dashizhai Formation. The ChRM yields a primary record of the geomagnetic field with positive fold, reversal and paleosecular variation tests. Along with published paleomagnetic data and geological constraints, we proposed a new reconstruction for the CAOB at 300 Ma. Our model posits that the Songliao‐Jiamusi and Mongolia blocks were welded into the unified Amuria block situated along the northern side of the PAO (∼34°N). On the southern side of the PAO, the Xilinhot and North China blocks were situated at equatorial paleolatitudes and were not amalgamated with Amuria until 250 Ma. This finding provides new insights into Asian paleogeography and the reconstruction of the Pangea supercontinent.

Magnetostratigraphy and magnetic properties of the Jurassic to Lower Cretaceous Girón Group (northern Andes, Colombia)

AbstractWe report paleomagnetic results from the Jurassic to Lower Cretaceous continental sedimentary succession exposed in the eastern limb of the Los Yariguíes anticlinorium, Eastern Cordillera, Colombia. About 820 m of a stratigraphic section of the upper part of the Girón Group (Angostura del Río Lebrija and Los Santos Formations) was sampled to construct a magnetic polarity stratigraphy. A total of 199 independent samples that yield interpretable and acceptable data have a characteristic remanent magnetization component (ChRM) isolated between 400 °C and 680 °C in progressive thermal demagnetization. Demagnetization behavior and rock magnetic properties are interpreted to indicate that hematite is the principal magnetization carrier with a possible contribution by magnetite in some parts of the section. After tilt correction, 123 samples are of normal polarity (declination [D] = 44.9°, inclination [I] = +9.7°, R = 110.64, k = 9.87, and α95 = 4.3°), and the other 76 accepted samples are of reverse polarity (D = 216.4°, I = −6.1°, R = 68.29, k = 9.72, and α95 = 5.5°). The statistical reversal test conducted on virtual geomagnetic poles is positive (class B). Based on paleontologic age estimates for the Cumbre and Rosablanca Formations, we assume a Berriasian age for the Los Santos Formation. The magnetostratigraphic data from the Girón Group strata are interpreted to suggest an age for the sampled part of the section between early Kimmeridgian and early Valanginian (ca. 157–139 Ma). The age of the Angostura del Río Lebrija Formation is estimated as between early Kimmeridgian and early Tithonian (ca. 157–146.5 Ma). The age of the Los Santos Formation is estimated between early Tithonian and early Valanginian (146.5–139.3 Ma). With our proposed, but nonunique, correlation with the Geomagnetic Polarity Time Scale, the Jurassic-Cretaceous boundary is interpreted to be located within the Los Santos Formation. The Girón Group is characterized by two periods of high (>8 cm/k.y.) and two periods of low (< 2 cm/k.y.) sedimentation rates. An inferred clockwise rotation of ∼44°, based on paleomagnetic declination data from the Girón Group, is similar to rotation estimates reported in some previous studies in the general area, and this facet of deformation could be related to local and regional response to displacement along regional-scale strike-slip faults.

Paleomagnetism of the Guanyang Devonian sedimentary successions in Guangxi province, South China

The Devonian position of the South China Block (SCB) remains debated because apparently contradictory Devonian paleopoles have been documented. Here we report two characteristic remanent magnetization components from the mildly deformed Guanyang Devonian successions (Lochkovian-Famennian) within the cratonic interior of the SCB. Both components pass reversal tests and occur in different samples across the successions. Component A and associated paleopole DA (228.9°E, 37.7°N, A95 = 3.7°) are comparable with published Devonian data and were likely acquired before Permian-Triassic folding and likely reflect syn-depositional magnetization. Component B and its associated paleopole DB (204.6°E, 4.5°N, A95 = 5.5°) are distinct from published results and were probably acquired during minor tilting associated with the Late Devonian Liujiang orogeny. If paleopole DA was acquired prior to the Late Devonian Liujiang orogeny, the paleolatitude of the Guanyang successions varied little throughout the Devonian, but the azimuthal orientation of recorded declinations exhibits a 40° difference. The distinct positions of the two paleopoles are broadly consistent with existing enigmatic Devonian field records. Alternatively, given the uncertainty in the acquisition ages of both components in the Guanyang successions, a case can be made for component A being a secondary magnetization acquired during the Permian-Triassic Indosinian orogeny. Nonetheless, the similar distribution of Devonian paleopoles from Gondwana to those of cratonic interior of the SCB suggests a Devonian connection between the two continents, and adds further support for a non-uniformitarian geomagnetic field during this enigmatic time interval.

Bidirectional growth of the Altyn Tagh Fault since the Early Oligocene

The Cenozoic evolution of the Altyn Tagh Fault (ATF) is still under debate. Here, we present new paleomagnetic rotation evidence on curvatures south of the ATF in the northeastern Tibetan Plateau to shed new light on this issue. A total of 498 core samples representing 53 sites and 6 time intervals (a-f) were collected from the post-Oligocene sequence at the Heishiqiu locality that in the junction of the northwestern and middle of the reverse S-type northern Qaidam marginal thrust belt. Rock magnetic analysis indicated that the main magnetic carriers were hematite and magnetite. After detailed thermal demagnetization analyses, the characteristic remanent magnetizations of 48 sites (387 samples), which exhibited dual polarities and passed a regional fold test, are most likely the primary magnetizations. Two periods of remarkable rotations are identified at Heishiqiu: a clockwise rotation of ~16.0 ± 10.3° during ~27–17 Ma and a subsequent counterclockwise rotation of ~24.3 ± 14.5° during ~17–10 Ma. The rotations were attributed to two periods of strike-slip faulting along the ATF during the Early Oligocene to Late Miocene, yielding at least ~290–380 km of offset at Heishiqiu. The integrated lines of paleomagnetic and geological evidence suggest that the deformation along the ATF has propagated both southwestward and northeastward from Heishiqiu in the middle ATF since the Early Oligocene. We propose a bidirectional growth model for the ATF that might be applicable to the evolution of the Tibetan Plateau.

Evidence of the Matuyama-Brunhes transition in cave sediment in Central Europe

In this study, we offer significant improvement over previous results that identified the Matuyama-Brunhes magnetic reversal in cave sediments from the Czech Republic in Central Europe. We collected discrete samples from the sedimentary profile in Za Hajovnou cave located in the eastern part of the Czech Republic. The rock magnetism measurements indicated that the magnetic carrier of most of the samples is maghemite. Characteristic remanent magnetization (ChRM) directions and related virtual geomagnetic pole (VGP) paths indicated that the Matuyama-Brunhes transition boundary was within 5.7 cm of the sediment, located in the upper part of the sampled sedimentary section. This result showed a new, more detailed behavior of the polarity transition from that of the central European location. The migration of the paleopole between eastern Africa and western North America was established as a significant marker for the central European paleomagnetic record in terms of global magnetic data. The transition duration was 8.1 ± 0.2 kyr, and the precursor of the reversal occurred 4 ± 0.2 kyr before the transition. In addition, we estimated the sedimentation rate of the studied section (~35 cm) in the cave as 0.7 ± 0.2 cm/kyr.

Jurassic fast polar shift rejected by a new high-quality paleomagnetic pole from southwest Greenland

A selective compilation of paleomagnetic data from North America indicates that a vast amount of rapid polar motion occurred in Late Jurassic time. The over 30° polar shift that accumulated during a relatively short time interval (~160–145 Ma) suggests an episode of fast true polar wander (TPW) and was referred to as the Jurassic “monster polar shift” by some workers. However, this rapid TPW event is not supported by paleomagnetic data on a global scale. Here, we scrutinize the Jurassic apparent polar wander path (APWP) by virtue of a new paleomagnetic and 40Ar/39Ar geochronology study of Mesozoic coast-parallel dykes exposed in southwest Greenland. Combined with existing geochronological data, our results show that the dykes were emplaced during a prolonged period centered at 147.6 ± 3.4 Ma (2σ). A primary nature of the characteristic remanent magnetization is supported by multiple positive baked contact tests and a reversal test. The paleomagnetic pole calculated from 40 site-mean paleomagnetic directions is located at Plat = 69.3°S, Plong = 5.0°E (A95 = 4.6°), or at Plat = 73.9°S and Plong = 0.4°E when reconstructed to North America. Our new high-quality paleomagnetic pole and an updated global APWP do not support the fast Jurassic polar shift but instead indicate steady polar motion with moderate rates of about 0.7°/Myr. The new pole effectively reduces the mismatch between the APWPs for Laurentia and Europe. Our critical reassessment of the monster polar shift indicates that it may be an artifact of paleomagnetic and geochronological data that were previously used to argue for its existence.

Extending and refining the paleomagnetic secular variation database for south-eastern Africa (Madagascar) to 2500 cal BP

Today's knowledge about paleomagnetic secular variations in (southern) Africa is extremely limited. Here, we derive paleomagnetic records from independently radiocarbon dated sediment records from three maars in Madagascar: Andraikiba and Amparihibe are terrestrial maars located on the main island of Madagascar and on a small island in the Northwest, whereas Crater Lake is a maar which has an open connection to the Indian Ocean. Studied through alternating field demagnetization of u-channel samples, characteristic remanent magnetization (ChRM) directions document similar inclination and declination patterns in two of the archives for the past 2,500 years (except for the last 400 cal BP). These new data are the first allowing to test the reliability of previous records which often suffer from low resolution and other obstacles. This will further allow to start to distinguish between robustly confirmed paleomagnetic secular variation data for this region from potentially problematic data.Considering a much lower resolution and a shorter covered time interval of archaeomagnetic data from La Réunion, South Africa, Botswana and Zimbabwe the temporal successions of maxima and minima reveal a coherent picture. Although slightly shifted in time, similarities also exist to the global geomagnetic field reconstruction model SHAWQ2k (Campuzano et al., 2019) which includes the above mentioned data. Surprisingly also similarities, though only in declination, are observed to the CALS3K.4 model (Korte and Constable, 2011) which is used as Northern Hemisphere biased model example. In contrast to this, no declination similarities but remarkable inclination similarities are observed to a lake record from Lake Malawi. An analog inclination pattern is also observed in a record from the Makran Accretionary Wedge which is ∼5,000 km to the north of the investigated sites. Interestingly the spatial distribution of archives showing these inclination similarities resembles the spatial distribution of inclination anomalies detected in model predictions. PSV similarities over such a large area are suggestive of a large-scale core dynamic origin independent of westward drift of non-dipole field components often associated with PSV records. This study emphasizes the potential of maar lakes in Madagascar for paleo reconstructions but also suggests that shallow marine and marine-brackish systems should be avoided if possible when trying to expand the (South) African paleomagnetic database.

Paleolatitude and tectonic rotations of the Early Carboniferous Fountain Lake Group, Cobequid Highlands, Nova Scotia, Canada

The ca. 355 Ma Fountain Lake Group in the Cobequid Highlands of Nova Scotia, is part of the transtensional basin fill that formed during dextral strike-slip motion between Avalonia and the Meguma terranes following the Acadian Orogeny. Paleomagnetic analysis of the Fountain Lake Group offers a paleolatitude estimate for the Laurentian accretionary margin in the Early Carboniferous as well as locality-specific paleomagnetic directions, which indicate clockwise-sense block rotations during dextral strike-slip motion along the Cobequid Fault zone. Stepwise demagnetization of 142 specimens from 20 sites in three Fountain Lake Group localities across the Cobequid Highlands (Squally Point, West Moose River, and Wentworth exposures) reveals remanence consisting of an easily removed component of probable recent origin, and more persistent components carried by magnetite and hematite, which in petrographic and electron beam analysis appear to be of primary igneous and volcanic oxidation origins, respectively. Sites from all three localities carry stable characteristic remanent magnetization (ChRM) directions that assume similar moderate downward inclinations when tilt-corrected. A Block Rotation Fisher analysis inclination-only fold test demonstrated best agreement at 90% unfolding, showing that remanence acquisition pre-dates Alleghenian deformation in the Late Carboniferous and is most likely of primary 355 Ma age. Paleomagnetic results for the Squally Point, West Moose River, and Wentworth localities show relative rotations between the blocks that are variously clockwise-rotated compared with a Laurentia cratonic reference frame. Inclinations at all three localities imply a subtropics paleolatitude for the margin (at Squally Point, 27.2° ± 9.4°; N = 7 sites), directly supporting the depicted location of Laurentia and its Appalachian accretionary margin in most Devonian to Early Carboniferous reconstructions.

Reply to Comment by Zhao et al. on “Paleomagnetism of the Late Cretaceous Red Beds From the Far Western Lhasa Terrane: Inclination Discrepancy and Tectonic Implications”

AbstractIn this manuscript, we checked all the pieces of evidence separately and conclude that the evidence Zhao et al. (2021, https://doi.org/10.1029/2020TC006520) provided cannot be regarded as arguments supporting their points. Our observations show that (i) sampling units and depositional environment support that the Jingzhushan Formation red beds could be affected by the syntectonic sedimentation, (ii) characteristic remanent magnetization directions observed from the Jingzhushan Formation red beds represent primary magnetization acquired during the syntectonic sedimentation, not secondary remagnetization due to strain reorientation of remanence, and (iii) inclination discrepancy of the Jingzhushan Formation red beds could be attributed to syntectonic sedimentation. Therefore, the paleomagnetic results obtained from the Jingzhushan Formation red beds are reliable for paleogeographic reconstructions.

Paleomagnetism of Permian-Triassic volcanic sequences from Son La province, northwest Vietnam

Nineteen sites with 198 oriented-core samples have been collected from the Upper Permian-Lower Triassic volcanic rocks of Vien Nam Formation at Quynh Nhai locality, Son La Province, northwestern Vietnam. The characteristic remanent magnetization components carried by magnetite and hematite were successfully isolated from secondary components reveal a mean paleomagnetic direction Ds = 48.3°, Is = -10.0°, a95 = 8.0°, corresponding to a virtual geomagnetic pole located at l = 35.7°N, f = 217.4°E and a paleo-latitude of study area situated at 5.1°S during the Permian time. Compared with the Late Permian-Early Triassic pole of the South China Block (SCB), the data show that crustal elements of NW Vietnam have been close to, but not unequivocally a coherent part of the SCB, since the Late Permian. Development of the parallel NW-SE striking Song Ma and Song Chay orogenic belts did not involve the closure of wide (> 500 km) ocean basins.

Paleomagnetism of Permian-Triassic volcanic sequences from Son La province, northwest Vietnam

Nineteen sites with 198 oriented-core samples have been collected from the Upper Permian-Lower Triassic volcanic rocks of Vien Nam Formation at Quynh Nhai locality, Son La Province, northwestern Vietnam. The characteristic remanent magnetization components carried by magnetite and hematite were successfully isolated from secondary components reveal a mean paleomagnetic direction Ds = 48.3°, Is = -10.0°, a95 = 8.0°, corresponding to a virtual geomagnetic pole located at l = 35.7°N, f = 217.4°E and a paleo-latitude of study area situated at 5.1°S during the Permian time. Compared with the Late Permian-Early Triassic pole of the South China Block (SCB), the data show that crustal elements of NW Vietnam have been close to, but not unequivocally a coherent part of the SCB, since the Late Permian. Development of the parallel NW-SE striking Song Ma and Song Chay orogenic belts did not involve the closure of wide (> 500 km) ocean basins.

West Africa in Rodinia: High quality paleomagnetic pole from the ~ 860 Ma Manso dyke swarm (Ghana)

The paleogeography of the Meso-Neoproterozoic Rodinia supercontinent remains debated partly because many stable cratons still lack reliable paleomagnetic data for this period. A new geochronological and paleomagnetic study was conducted on the NNW-trending Manso dyke swarm of southern West Africa (Ghana) to clarify the position of this unconstrained continent in Rodinia. Two UPb apatite ages of 857.2 ± 8.5 Ma and 855 ± 16 Ma agree with one previous baddeleyite age, indicating a ~ 860 Ma emplacement age for the Manso dykes. A characteristic remanent magnetization (ChRM) was isolated in stable single to pseudo-single domain (SD-PSD) magnetite. Well constrained site mean directions obtained for 13 dykes lead to a mean direction for the Manso dyke swarm of Dm = 181.9°, Im = −77.2° (N = 13, α95 = 7.6°, k = 30.6), yielding a paleomagnetic pole at 177.6°E, 28.3°S, (A95 = 12.7 °K = 11.6). Two directional clusters of opposite inclination pass a reversal test (C-class) and the primary origin is supported by a positive baked contact test, satisfying all the seven R-criteria to provide the first West African Tonian key paleomagnetic pole. This key pole indicates a high latitude for the West Africa Craton during the emplacement of the ~860 Manso dykes. A compilation of reliable paleomagnetic poles for West Africa, Baltica, Amazonia and Congo-São Francisco cratons suggests that these cratons were together between ~1200 and 800 Ma in a long-lived WABAMGO configuration. We suggest that the collision of this block with Laurentia along the Grenvillian-Sunsás orogens closed the external Nuna Ocean and formed Rodinia by extroversion.

High-resolution palaeomagnetic results of Ethiopian trap series from Lima Limo section: implications for the Oligocene geomagnetic field behaviour and timing of volcanism

SUMMARY We present high-resolution palaeomagnetic and rock magnetic results from 92 successive volcanic units on a 2 km tall Lima Limo (LL) section of the Oligocene Ethiopian flood basalts (OEFB). A total of 85 well-defined flow-mean characteristic remanent magnetization directions representing seven stable magnetic polarity zones with six transitional/excursional directions were determined. These results indicate the identification of four more polarity reversals than previous palaeomagnetic studies; the resulting magnetostratigraphy has, therefore, more polarity zonation. Comparison of this result with the most recently revised geomagnetic polarity time scale (GPTS, hereafter GPTS2016), by using available radiometric age data, could not find a unique correlation. Instead, it strongly suggests the recording of at least one very brief polarity chron or putative geomagnetic excursion. Our magnetostratigraphic interpretation infers that the relative timing between the OEFB eruptions and the Oi-2 global cooling estimated two possible scenarios: one where the lowermost reverse-polarity (R1) interval for the LL section is correlated to Chron C11r of GPTS2016, same as the interpretations of earlier studies, would make a causal linkage between the OEFB eruptions and the Oi-2 cooling less possible; and, the eruption timing estimated in another scenario correlating the R1 to Chron C12r might preserve the probability of its causal influence in the Oi-2 cooling. The resulting overall mean pole for the LL section of this study, 78.2° N in latitude and 213.0° E in longitude (A95 = 3.4°, N = 79), is closer to other co-eval palaeomagnetic poles from different regions and the reference pole of Africa. Our analysis of geomagnetic palaeosecular variation (PSV) shows that, compared to the Plio-Pleistocene, the Early Oligocene has higher degrees of PSV but its slightly weaker latitude dependence.

Remagnetization of Permian Emeishan basalts: Constraints on the timing of native copper mineralization in northeast Yunnan Province, China

New paleomagnetic results from the Permian Emeishan basalts in the Zhaotong area, NE Yunnan province, China show four natural remanent magnetization components. Detailed stepwise thermal demagnetization of basaltic samples from 16 flows from the Dadi section, which represent basalt units III and IV, isolated two groups of characteristic remanent magnetizations. Samples in unit IV (five flows) record a southwest declination and a moderate downward inclination that is considered to be a partial remagnetized remanence. The bottom flows from unit III (11 flows) record a normal polarity direction, interpreted as a remagnetization, which yields a tilt-corrected mean direction of Ds/Is= 8.8°/31.6° (N = 9, ks= 39.7,α95= 8.3°), with a corresponding paleomagnetic pole at 77.1°N, 240.0°E (K = 49.2, A95= 7.4°). The secondary directions have steeper inclinations than primary ones that have been successfully recovered from other studies in this area of the Emeishan basalts. By comparison with the Phanerozoic paleomagnetic poles of the South China Block, the preferred timing of remagnetization is the Lower-Middle Jurassic. Field relationships suggest that the remagnetization of the Emeishan basalts is coeval with the spatially related, but localized, copper mineralization. Thus the timing of the main copper mineralization hosted in the Emeishan basalts is hypothesized to occur in the Early-Middle Jurassic.

Magnetostratigraphy of the Hominin Sites and Paleolakes Drilling Project (HSPDP) Baringo-Tugen Hills-Barsemoi core (Kenya)

The principal objective of the Hominin Sites and Paleolakes Drilling project (HSPDP) is to study the relationship between climate and environmental change and the implications on human evolution in eastern Africa. For this purpose, HSPDP has recovered a 228 m core in the Chemeron Formation of the Baringo Basin (Kenya). The Chemeron Formation spans approximately 3.7 Myr, from around 1.6 to 5.3 Ma, and has yielded many vertebrate fossils, including fossil hominins. The magnetostratigraphy of the Baringo core contributes to the chronological framework. A total of 567 individual paleomagnetic samples were collected from 543 levels at regular intervals throughout the core and 264 were processed using thermal and alternative field stepwise demagnetizations. In most samples, distinct Low-Temperature (LT; 20–150 °C) and High-Temperature (HT; 150–550 °C) Characteristic Remanent Magnetization (ChRM) could be determined. Typical demagnetization behaviors and some rock magnetic experiments suggest titanomagnetite acts as the main carrier of the HT ChRM with pervasive secondary overprints in normal polarity expressed by the LT component. Normal and reversed polarities were identified based on the secondary overprints LT ChRM directions, either parallel or antiparallel to the HT ChRM directions respectively. Our study identified four paleomagnetic reversals interpreted as the Matuyama-Gauss, Gauss-Kaena, Kaena-Gauss and the Gauss-Mammoth transitions. These boundaries provide chronostratigraphic tie-points that can be combined with those derived from 40Ar/39Ar dating of tuffs (Deino et al., 2020) and together indicate that the HSPDP Baringo core has an age range of ~3.3 Ma to ~2.6 Ma. The consistent paleomagnetic and radioisotopic age constraints are incorporated into a Bayesian age model of the core (Deino et al., 2020).