Characteristic Remanent Magnetization Directions Research Articles

Paleomagnetic constraints on the link between the Comei-Bunbury large igneous province and the Kerguelen mantle plume

The causal link between the Comei-Bunbury large igneous province (CBLIP) and the Kerguelen mantle plume (KMP) remains a topic of debate. Here, a new paleomagnetic study was conducted on the Zhela Formation basalts and sandstones (ca. 147–140 Ma) in the Taga area of the eastern Tethyan Himalaya (TH). The sandstone specimens had erratic demagnetization patterns, and reliable characteristic remanent magnetization (ChRM) directions could not be isolated. Positive fold and reversal tests indicate that the ChRM directions of the Zhela Formation basalts should be the primary magnetization. The tilt-corrected site-mean direction from 18 sites is D/I = 87.8°/70.0° (α95 = 6.8°), yielded a paleopole of 21.6°N/130.2°E (A95 = 12.0°) and a paleolatitude of 54.7° ± 12.0°S for the study area (28.4°N, 91.8°E). Comparison with robust latest Jurassic and Early Cretaceous paleomagnetic, geochemical, and geochronological data from the eastern TH, northeastern Indian craton, and southwestern Australia reveal that (1) the eruption periods of the Comei igneous rocks are similar to the Bunbury basalt (and the Naturaliste Plateau volcanic rocks); (2) the original erupted positions of the Comei-Bunbury (CB) igneous rocks were in agreement with the reconstructed KMP; and (3) the CBLIP originated from the KMP.

On the early development of Huastecs (Gulf of Mexico) revealed by the Earth's magnetic field recorded in domestic hearths

AbstractA detailed magnetic mineralogy and archaeomagnetic study was carried out on recently discovered domestic hearths and burned floors at the Chak Pet archaeological settlement (Tamaulipas, Mexico). The study aimed to obtain reliable absolute chronological constraints on the early development of Huastecs during the Formative period. Oriented hand samples corresponded to four domestic hearths and one burned floor. Continuous thermomagnetic curves revealed mostly irreversible behavior, while titanomagnetites, titanomaghemites, and goethites are assumed to carry the remanent magnetization. In total, 87 specimens were subjected to stepwise demagnetization of natural remanent magnetization using an alternating field procedure. Characteristic remanent magnetization directions were obtained for 29 samples of two hearths and one burned floor. No single, technically acceptable paleointensity determination was obtained. The new archaeomagnetic age intervals for Chak Pet allow locating the origin of this settlement at the Gulf of Mexico within the Middle Formative (900–600 BCE) continuing until the Late Formative period (350–100 BCE). New archaeomagnetic ages are in accordance with the diagnostic pottery analysis. Dated archaeological elements are associated with both ceramic types and different sets of burials, providing a reliable tool to calibrate their chronological and stratigraphic positions.

New paleomagnetic and geochronologic results from late Paleozoic rocks in the Turfan-Hami block (NW China) and implications for the geodynamic evolution of the western Altaids

How and when the ocean-continent transition started in the western Altaids remain controversial. The paleomagnetic signals recorded by late Paleozoic rocks in the Turpan-Hami block can provide critical constraints on this issue. We conducted a new combined paleomagnetic and geochronologic study on the late Paleozoic rocks from the Turpan-Hami block. Laser ablation−inductively coupled plasma−mass spectrometry zircon U-Pb dating of volcanic beds from the Upper Carboniferous Qijiaojing and Julideneng Formations yielded ages of 313.1 ± 4.3 Ma and 309.6 ± 1.9 Ma to 308.1 ± 3.6 Ma, respectively. Meanwhile, the U-Pb age of the granite intruding the Julideneng Formation is 300.3 ± 2.4 Ma. Passing a series of fold tests, the characteristic remanent magnetization (ChRM) directions of the Qijiaojing Formation are likely primary and consistent with the Kiaman reversed superchron (ca. 319−267 Ma). However, the ChRM values of the Dananhu (Middle Devonian) and Julideneng Formations all represent reverse polarity with negative fold tests, which indicate remagnetizations related to the magmatic thermal events during the late Carboniferous. Thus, two high-quality paleomagnetic poles were obtained for the periods ca. 313−308 Ma at 44.4°N, 177.3°E (K = 22.1, A95 = 8.0°) and ca. 300 Ma at 47.8°N, 173.9°E (K = 116.0, A95 = 4.8°), respectively. Comparison with published coeval paleomagnetic poles of the blocks on both sides of the Tianshan sutures suggests that the central oceanic basin of the western Paleo-Asian Ocean (between the Siberian and Tarim blocks) had been closed since the late Carboniferous (ca. 310 Ma), apart from remnant seas. In addition, a sizeable clockwise rotation (∼58°) of the Turpan-Hami block had taken place during the early Permian, with a scissor-style shrinking of the remnant Bogda marine basin in the meantime. This study provides a new perspective for understanding the tectonic evolution of the western Altaids.

New Late Cretaceous paleomagnetic and geochronologic results from the southern Qiangtang Terrane: Contributions to the reliability of redbed paleomagnetic datasets from the Tibetan Plateau and shape of the Qiangtang Terrane

To better clarify the reliability of redbed paleomagnetic datasets from the Tibetan Plateau and shape of the Qiangtang Terrane (QT) prior to the India-Asia collision, a combined paleomagnetic and detrital zircon U-Pb geochronologic study is conducted on the Upper Cretaceous Abushan Formation (Fm) redbeds, dated to be within 89.1−83.6 Ma, in the Anduo area. The tilt-corrected grand mean direction for 37 sites is Ds = 0.9°, Is = +36.4°, k = 43.6, α95 = 3.6°, which provides a paleopole at 78.2°N, 266.8°E (A95 = 3.6°), corresponding to a paleolatitude of 20.5° ± 3.6°N for the study area (32.3°N, 91.4°E). Reliable demagnetization curves, positive fold, and conglomerate tests support the interpretation that characteristic remanent magnetization directions recorded primary magnetizations carried by detrital hematite and did not suffer from the influence of distortional strains. Our paleomagnetic results indicate that the mean inclination observed from the southwest dipping limb (Is = +44.2°) is clearly steeper than that from the north dipping limb (Is = +34.2°). The results of syntectonic-sedimentation-correction and the fluvial gravel deposits present in the Abushan Fm redbeds sampled support the contention that the apparent inclination discrepancy is attributed to the syntectonic growth strata and that the paleomagnetic datasets observed from two limbs of folds are still reliable. Furthermore, reliable Late Cretaceous paleomagnetic datasets show that the shape of the QT west of the Eastern Himalaya Syntaxis (EHS) during the Late Cretaceous was similar to its present-day relatively E-W alignment, and that around the EHS, its shape changed from the Late Cretaceous NE-SW alignment to the present-day approximately NW-SE alignment.

Bending of the Western Mongolian Blocks Initiated the Late Triassic Closure of the Mongol‐Okhotsk Ocean and Formation of the Tuva‐Mongol Orocline

AbstractBending of ribbon continents or arcs is an important tectonic process for reconstructing central Asia. Formation of the striking Tuva‐Mongol Orocline by closure of the Mongol‐Okhotsk Ocean (MOO) has long been proposed, but coupling of the two geological events has not been well illustrated, hindering our understanding of tectonic evolution of central Asia. In order to constrain age of initial closure of the MOO in its western segment and formation of the Tuva‐Mongol Orocline, we performed paleomagnetic studies on the Late Triassic clastic rocks in the Amuria Block (AMB; 17 sites) and the Tarvagatay Block (TVB; 10 sites) of the western Mongolian blocks (WMB) on both sides of the Mongol‐Okhotsk Suture. Rock magnetic investigations show hematite and magnetite as main magnetic carriers of the characteristic remanent magnetization (ChRM). The ChRM directions from both regions pass fold and/or reversal tests and can be considered as primary remanent magnetization. Combining the overlapped new paleomagnetic poles (AMB: λ/φ = 70.4°N/233.8°E, A95 = 4.6°; TVB: λ/φ = 70.5°N/248.2°E, A95 = 9.0°) with geological evidence, we propose that the bending of the WMB following its collision with the Siberia Craton resulted in the Late Triassic closure of the MOO in its western segment, and initiated the formation of the Tuva‐Mongol Orocline.

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

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

Paleomagnetic study of the 1112 Ma Huanchaca mafic sills (SW Amazonian Craton, Brazil) and the paleogeographic implications for Rodinia supercontinent

During the Stenian Period (1200–1000 Ma) Rodinia was being formed, and the Amazonian Craton, presently in northern South America, was an important piece in this scenario. Recently a megacontinent named Umkondia composed by Amazonia/West Africa, Congo/São Francisco, Kalahari and India was proposed to have existed at 1100 Ma. We present a paleomagnetic study on the well-dated (U-Pb, baddeleyite) 1112 Ma mafic sills from the Huanchaca Intrusive Suite, western Mato Grosso State (Amazonian Craton) in order to test the proposed configuration of Umkondia. Alternating field (AF) and thermal stepwise demagnetization revealed northwestern characteristic remanent magnetization (ChRM) directions with low positive inclinations (mean: Dm = 303.2°, Im = 12.2°, N = 10, α95 = 14.5°, K = 12.0). A paleomagnetic pole was calculated for the Huanchaca sills (HU − 225.7°E, 30.1°N, A95 = 9.9°) which is classified with reliability index R = 5. Magnetic mineralogy experiments show that ChRM is carried by Pseudo single domain (PSD) magnetite. The Huanchaca paleopole favors the existence of the Umkondia megacontinent at 1100 Ma, which latter collided with Laurentia forming the Rodinia supercontinent around 1000–900 Ma.

Sinistral displacement along the Tan–Lu Fault during the Cretaceous induced by Paleo-Pacific subduction: Constraints from new paleomagnetic and U–Pb geochronological data

The Jiaodong Peninsula in eastern China is separated from the main mass of the North China Craton by the Tancheng–Lujiang (Tan–Lu) Fault, the largest fault system in eastern Asia. Although accumulating lines of evidence have suggested that the Tan–Lu Fault underwent strike-slip movement in the Mesozoic, there is still debate about the timing and magnitude of the displacement. To decipher this problem, we conducted paleomagnetic and U–Pb geochronological studies of the Lower Cretaceous volcanic rocks of the Qingshan Group on the Jiaodong Peninsula, to the east of the Tan–Lu Fault on the southeastern edge of the North China Craton. LA–ICP–MS U–Pb dating of zircons from five samples yielded consistent ages of ca. 120 Ma. Stepwise thermal demagnetization conducted on 308 samples (40 sites) from two sections revealed stable characteristic remanent magnetization directions. After paleomagnetic tests and rejecting inappropriate data, the mean direction from 288 samples (31 independent units) is 200.3°/–51.4° (α95 = 4.1°), with a corresponding pole at 215.1°/73.0° (A95 = 4.8°). The paleolatitude calculated for the Jiaodong Peninsula is lower than that of the main block of the North China Craton, which indicates that latitudinal offset occurred between these two parts, and this offset can be ascribed to strike-slip movement along the Tan–Lu Fault. Our data provide new constraints on regional tectonic movement and Paleo-Pacific subduction processes during the late Mesozoic.

Late Holocene Paleomagnetic Secular Variation in the Chukchi Sea, Arctic Ocean

AbstractThe geomagnetic field behavior in polar regions remains poorly understood and documented. Although a number of Late Holocene paleomagnetic secular variation (PSV) records exist from marginal settings of the Amerasian Basin in the Arctic Ocean, their age control often relies on a handful of radiocarbon dates to constrain ages over the past 4,200 years. Here we present well‐dated Late Holocene PSV records from two sediment cores recovered from the Chukchi Sea, Arctic Ocean. The records are dated using 26 14C measurements, with local marine reservoir corrections calibrated using tephra layers from the 3.6 cal ka BP Aniakchak eruption in Northern Alaska. These 14C‐based chronologies are extended into the post‐bomb era using caesium‐137 dating, and mercury isochrons. Paleomagnetic measurements and rock magnetic analyses reveal stable characteristic remanent magnetization directions, and a magnetic mineralogy dominated by low‐coercivity minerals. The PSV records conform well to global spherical harmonic field model outputs. Centennial to millennial scale directional features are synchronous between the cores and other Western Arctic records from the area. Due to the robust chronology, these new high‐resolution PSV records provide a valuable contribution to the characterization of geomagnetic field behavior in the Arctic over the past few thousand years, and can aid in developing age models for suitable sediments found in this region.

Remagnetization of Carboniferous Limestone in the Zaduo Area, Eastern Qiangtang Terrane, and Its Tectonic Implications

Robust paleomagnetic results through geological time are one of the keys to understand the drift history of the eastern Qiangtang terrane (EQT). Here, we presented comprehensive petrographic observations and rock magnetic and paleomagnetic analyses of the early Carboniferous Upper Zaduo (ZD) limestone Formation (C1z2) from the Sulucun (SLC) section in the Zaduo area, EQT, to investigate its magnetic originality and geological significance. A total of 12 sites (131 samples) were collected. Photomicrograph observations indicate that the limestone samples were characterized by widespread carbonate veinlets. Electron microprobe and energy dispersive spectrometry analyses confirm that authigenic magnetite formed after pyrite. Rock magnetic analyses reveal the dominant magnetic minerals of pyrite and magnetite, with ‘wasp-waisted’ hysteresis loops and close to the “remagnetization trend” hysteresis parameters. Based on both thermal and alternating field demagnetizations, the characteristic remanent magnetization directions for most samples were isolated: Dg = 6.3°, Ig = 50.1°, kg = 54.9, α95 = 6.2° in-situ, and Ds = 330.2°, Is = 58.9°, ks = 5.9, and α95 = 20.5° after 2-step tilt correction. The κ (α95) value decreases (increases) after tilt-correction, and the ChRM directions failed both the McFadden (1990), Watson and Enkin (1993) fold tests, indicating post-folding magnetizations. The 11 site-mean directions yield a mean in-situ paleopole of 84.4°N, 200.3°E, and A95 = 6.8°, which is coincident with the post ∼53 Myr (especially around 40 Ma) paleopoles of the region. We therefore interpreted that these early Carboniferous limestone samples contain remagnetized magnetizations and that they were obtained after 53 Ma, most likely around 40 Ma, due to the far-field effect of the India–Eurasia collision.

The Early Cretaceous Zaduo Granite, Eastern Qiangtang Terrane (China)—An Attempt to Constrain its Paleolatitude and Tectonic Implications

The Eastern Qiangtang Terrane is an orogenic-like belt around the Eastern Himalayan syntaxis (EHS). The deformation history of this terrane must be known to understand how the EHS region responded to the Lhasa-Qiangtang collision and the closure of the Bangong-Nujiang Ocean (BNO). Here, we present a new paleomagnetic investigation on an Early Cretaceous granite (∼126 Ma) in the Zaduo area, Eastern Qiangtang Terrane. Petrographic observations reflect crystallization from primary melts with only limited subsequent alteration (chloritization of biotite). Magnetite appears to be the dominant carrier of the characteristic remanent magnetization (ChRM) based on stepwise demagnetization of the natural remanent magnetization, supplemented by detailed rock magnetic measurements, including magnetization versus temperature, and acquisition curves of the isothermal and anhysteretic remanent magnetization. End-member modeling of those acquisition curves helped to constrain the paleomagnetic analysis. The inconsistent demagnetization behavior between alternating field (AF) demagnetization at high levels and thermal demagnetization was attributed to the development of gyroremanent magnetization in the AF demagnetization generated by fine-grained single domain magnetite. The ChRM directions from 92 granite samples in geographic coordinates yield an average of declination (Dg) of 2.6° and inclination (Ig) of 38.6° (precision parameter k = 51.4, and 95% confidence cone α95 = 2.1°). The amount of tilting of the granite is poorly constrained which makes proper correction rather tedious. We compared the expected bedding attitudes (Strikeexp = 43.1°, Dipexp = 46.1°) derived from published data (Huang et al., 1992;Tong et al., 2015) with the average observed bedding attitudes (Strikeobs = 54°, Dipobs = 32°) of the Middle-Upper Jurassic sandstones of the Yanshiping Group that was intruded by the Early Cretaceous granite. The discrepancy between the expected and measured bedding attitudes implies that the strata of the Yanshiping Group in the Zaduo area were already tilted prior to the intrusion of the ∼126 Ma Zaduo granite, which was attributed to the Lhasa-Qiangtang collision and the closure of the BNO. The collision led to a series of geological events, such as the tilting of the strata, the ophiolite emplacement, the development of a peripheral foreland basin, and the magmatic activity gap. The tilting/folding of the strata was generally delayed by the layer parallel shortening processes during the early stages of the deformation, thus suggesting an older Lhasa-Qiangtang collision (i.e., >126 Ma).

Paleomagnetic and geochronological results of the Risong Formation in the western Lhasa Terrane: Insights into the Lhasa-Qiangtang collision and stratal age

To constrain the Lhasa-Qiangtang collisional age, a combined paleomagnetic and geochronologic investigation has been performed on the Risong Formation redbeds and volcanic rocks in the Wuma area of the western Lhasa terrane (LT). A U-Pb dating result from the volcanic interlayer at the bottom of the Risong Formation redbeds yields the youngest weighted mean 206 Pb/ 238 U age of 110.6 ± 1.3 Ma, and the two detrital zircon U-Pb chronological results from the Risong Formation redbeds provide the youngest weighted mean 206 Pb/ 238 U age of 106.9 ± 3.3 Ma (the top strata of section A) and 120.5 ± 3.0 Ma (the top strata of section D), respectively. Our new geochronological results suggest that the studied Risong Formation redbeds formed during the Early Cretaceous, rather than during the Late Jurassic as assigned by 1:250,000 scale Wuma regional geological survey report. Characteristic remanent magnetization (ChRM) directions from two different limbs of folds concentrate in two different groups, and both site-mean directions pass positive fold tests. One group including 32 sites yields a tilt-corrected site-mean direction that is D = 356.1°, I = 33.1° with α 95 = 2.3°, corresponding to a paleopole at 75.2°N, 278.1°E with dp/dm = 1.5°/2.6° and a paleolatitude of 18.1° ± 1.5°N for the study area (32.41°N, 83.39°E). The other group including 24 sites provides a tilt-corrected site-mean direction that is D = 60.5°, I = 48.5° with α 9 5 = 2.0°, corresponding to a paleopole at 38.7°N, 159.6°E with dp/dm = 1.7°/2.6° and a paleolatitude of 29.5° ± 1.7°N for the study area (32.41°N, 83.39°E). After syntectonic-sedimentation-correction, the Fisherian site-mean inclination of 41.7° ± 1.6° and corresponding paleolatitude of 24.0° ± 1.2°N obtained from 56 sites of two different limbs are consistent with the inclination-only mean of 39.6 ± 3.1° and corresponding paleolatitude of 22.5° ± 2.2°N deduced from the ChRM directions of all the 56 paleomagnetic sites, suggesting that the observed inclination discrepancy of the Risong Formation redbeds may result from the syntectonic sedimentation. Comparing the Early Cretaceous paleolatitudes observed from the western LT with those from the western Qiangtang terrane shows insignificant paleolatitude difference, supporting that the Lhasa-Qiangtang collision in the western part occurred at or before the Early Cretaceous. • New dating shows the sampled Risong Formation redbeds formed during the Early Cretaceous rather than the Late Jurassic. • The Wuma sampling area was located at ~22.5° ± 2.2°N during ~120.5–106.9 Ma. • The Lhasa-Qiangtang collision occurred before ~120 Ma in the western part.

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

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.

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.

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.

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).

An integrated magnetic survey on lava flows associated to the Paricutin volcano (Western Mexico)

We report a detailed aeromagnetic, field magnetometry and paleomagnetic survey from the most recent volcano on the American continent that emerged in February of 1943. Paleomagnetic measurements were performed to determine the characteristic remanent magnetization directions, considering that the Geomagnetic Field observed in nearby existing observatories at the time of extrusions is well known for the time of eruption of this volcano. The paleomagnetic records of the Paricutin flows showed lower values compared to the Teoloyucan Geomagnetic Observatory and those estimated with the International Geomagnetic Reference Field model (IGRF). Also, an aeromagnetic data modeling provided by the Mexican Geological Service together with field magnetometry along two profiles in perpendicular directions were carried out. The main objective was to model two levels of the internal structure, one that has to do with deep sources (not exclusively associated with the volcano itself), and another one of a more superficial area directly related to recent eruptions and the immediate geological basement.