Stepwise Thermal Demagnetization Research Articles

New paleomagnetic data from the sedimentary cover of the Tandilia System: Further geodynamic or geomagnetic complexities in the Late Ediacaran

Anomalous paleomagnetic data have been found worldwide during the Ediacaran period, giving rise to several non-actualistic hypothesis. In order to get more information about this period, paleomagnetic, magnetic fabric and rock magnetic studies were carried out in the Avellaneda Formation (∼570–560 Ma) from two drill cores of the Alicia quarry in the Olavarría area of the Tandilia System, in the Río de la Plata craton (Argentina). Anisotropy of magnetic susceptibility studies indicate a pre-tectonic origin for the magnetic fabric of this Formation. Rock magnetic studies suggest the presence of magnetite and hematite in different proportions as the main ferromagnetic minerals carrying the remanence. After stepwise thermal demagnetization, two different characteristic remanence directions were obtained for the same unit, one corresponding to the marls (“b1”) of the lower section of the Avellaneda Formation, and the other from the claystones (“b2”) of the upper section of this unit. These results were combined with the remanence directions obtained by Franceschinis et al. (2022) for the same unit at La Cabañita quarry, located 10 km away. This procedure allows the calculation of two paleomagnetic poles for the Avellaneda Formation. The AV1 pole is located at: 2.0° S, 311.1° E, A95: 5.0°, N: 58 while the AV2 pole is at: 3.3° N, 348.9° E, A95: 11.7°, N: 7. These results confirms that the Rio de la Plata craton also presents anomalous paleomagnetic data during the Ediacaran, implying extremely fast movements in very short periods of time. This can be interpreted as evidence of two inertial interchange true polar wander events during this time, as was already proposed by other authors. An alternative possibility suggests a non-actualistic behaviour of the Earth Magnetic Field, switching from axial to equatorial positions, with an intermediate stable position between them. The likelihood and implications of these two hypotheses is discussed.

Paleomagnetic and Rock Magnetic Study of the Gulerghat Area Amravati, Maharashtra

Magnetic Polarity and Stratigraphicstudies of the Narnala section from Akot and Amravatidistricts area(Lat. 21000'-21015'N; Long. 770–77015'E) are carried out. Total thousandsof oriented samples from fresh basalts were collected at close intervals. Detailed palaeomagnetic investigation where carried on 250 samples collected from 20 lava flows belonging to 17 selected traverses were subjected to progressive thermal demagnetizer at 15 different steps of temperatures from 50˚C to 600˚C. Traverse attaining a total thickness of 800m in the study area. Some rocks may fail to retain the magnetic record due to the modification subsequence to their formations by a number of natural phenomena like geotectonic disturbance, metamorphism and alteration, which can produce various degrees of secondary magnetisation in the rock. The secondary magnetisation acquired by the rock is unstable as compared to naturally remain magnetism which can be removed by cleaning techniques like stepwise Alternating field demagnetisation (AFD) and Thermal demagnetisation (TD). The palaeomagnetic results indicate that the most of the flows showed irregular natural remnant directions while a few specimen even from other flows for which a meaningful NRM could be deciphered also showed inconsistent directions. Only 35% of the samples showing irregular directions initially improved on alternating field and stepwise thermal demagnetization. High degree of alteration of the flows appears to have affected the stability of the palaeomagnetic directions. The mean palaeomagnetic directions indicate the presence of lower transitional directions at the bottom part of the sequence and upper reversed polarity in remaining part of the sequence with a few samples showing scattered behavior. The polarity transition has take place in the study area. Ancient latitude calculated for the study area is 30o S and that of Nagpur is 31o S indicating that India has drifted by 51o due N which is more than 5000 Kms. since the formation of this lavas.

Magnetic polarity & stratigraphy of the Bundelkund area Amravati, Maharashtra

Magnetic Polarity and Stratigraphic studies of the Bundelkund area Amravati districts (Lat. 21000'-21015'N; Long. 770–77015'E) are carried out. Total thousands of oriented samples from fresh basalts were collected at close intervals. Detailed palaeomagnetic investigation where carried on 250 samples collected from 20 lava flows belonging to 03 selected traverses were subjected to progressive thermal demagnetizer at 15 different steps of temperatures from 50˚C to 600˚C. Traverse attaining a total thickness of 800m in the study area. Some rocks may fail to retain the magnetic record due to the modification subsequence to their formations by a number of natural phenomena like geotectonic disturbance, metamorphism and alteration, which can produce various degrees of secondary magnetisation in the rock. The secondary magnetisation acquired by the rock is unstable as compared to naturally remain magnetism which can be removed by cleaning techniques like stepwise Alternating field demagnetisation (AFD) and Thermal demagnetisation (TD). The palaeomagnetic results indicate that the most of the flows showed irregular natural remnant directions while a few specimen even from other flows for which a meaningful NRM could be deciphered also showed inconsistent directions. Only 35% of the samples showing irregular directions initially improved on alternating field and stepwise thermal demagnetization. High degree of alteration of the flows appears to have affected the stability of the palaeomagnetic directions. The mean palaeomagnetic directions indicate the presence of lower transitional directions at the bottom part of the sequence and upper reversed polarity in remaining part of the sequence with a few samples showing scattered behavior. The polarity transition has take place in the study area. Ancient latitude calculated for the study area is 30o S and that of Nagpur is 31o S indicating that India has drifted by 51o due N which is more than 5000 Kms. since the formation of this lavas.

Middle Jurassic Paleolatitude of the Tethyan Himalaya: New Insights Into the Evolution of the Neo‐Tethys Ocean

AbstractThe paleogeography of the Tethyan Himalaya (TH) during the Mesozoic is vital to constrain the evolutionary history of the Neo‐Tethys Ocean, but reliable paleomagnetic data from Jurassic rocks in this area are scarce. Here, we report the first high‐quality paleomagnetic results from the Middle Jurassic (∼175–173 Ma) Lanongla Formation limestones in the Nyalam area of the TH. For most specimens, stepwise thermal or hybrid demagnetization reveals two well‐defined magnetization components. A low‐temperature component, which is isolated between natural remanent magnetization and 200–300°C, is consistent with the present geomagnetic field direction. A high‐temperature component, which is isolated between 300–350°C/10–25 mT and 500–580°C/60–120 mT, passes fold tests at the 95% and 99% confidence level, indicating a prefolding primary magnetization. The tilt‐corrected site‐mean direction for 28 paleomagnetic sites is Ds = 331.0° and Is = −49.9° with α95 = 2.7°, which provides a Fisherian site‐mean paleopole at 23.7°N, 292.9°E with A95 = 2.8° and a paleolatitude of 31.7 ± 2.8°S for the Nyalam study area (28.6°N, 86.1°E). A comparison between the reliable Middle Jurassic‐Early Cretaceous poles obtained from the TH and those observed from the Lhasa terrane reveals that the Neo‐Tethys Ocean for the reference point (29.1°N, 86.1°E) had a latitudinal width of 3,500 ± 1,000 km at ∼174 Ma, reached its greatest width of 7,000 ± 1,000 km at ∼137 Ma, and had an average latitudinal spreading rate of ∼10.4 cm/year during ∼174–137 Ma.

New Paleomagnetic Constraints on the Amalgamation of the Tuva and Mongolia Blocks From Late Carboniferous Andesites in the Mongolia Block

AbstractThe Tuva‐Mongolia collage system comprises the Tuva Block (TVB) and the Mongolia Block (MOB), which are located in the eastern Central Asian Orogenic Belt (CAOB). The timing of amalgamation of the TVB and MOB is unclear. To address this, we conducted a combined geochronological and paleomagnetic study of the andesitic Member 1 of the Baoligaomiao Formation, in the southeastern MOB. The andesites have a U‐Pb zircon age of 314.2 ± 1.7 Ma. A total of 65 paleomagnetic samples from 10 sites were subjected to stepwise thermal demagnetization and had stable magnetic signals. After removal of the low‐temperature viscous components acquired in the recent geomagnetic field, stable high‐temperature components (HTCs) were isolated from most samples. The HTCs passed the fold, conglomerate, and reversal tests, demonstrating their primary origins. Moreover, these andesites have averaged out paleosecular variations. The paleomagnetic pole for the MOB is located at 30.5°N/14.6°E (A95 = 7.4°) at ca. 314 Ma. Combining our new results with published late Paleozoic paleomagnetic poles for the MOB and TVB suggests that the two blocks were distant from each other during the Middle Devonian and merged into a single TVB‐MOB by ca. 315 Ma. The Devonian‐Carboniferous paleomagnetic, paleobiogeographic, and geological data indicate that the multiple microcontinents in the CAOB were separated by numerous ocean basins during the Middle Devonian. At ca. 315 Ma, the TVB‐MOB, Kazakhstan, and Siberia were amalgamated to form a ribbon‐like continent, which was associated with the evolution of the Mongol‐Okhotsk Ocean.

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.

Tectonic Rotation Pattern at the Northern End of the Red River Fault System in SE Tibet: New Paleomagnetic Evidence From Cretaceous Red Beds

AbstractIn order to study the rotations of crustal material along the boundary faults in the southeastern Tibetan Plateau, and to better understand the tectonic evolution of this region, in this study we conducted a paleomagnetic study of Cretaceous red beds in the Binchuan and Xiangyun sections near the Chenghai‐Binchuan fault, a branch fault at the northern end of the Red River fault in China. Analyses of rock magnetic properties and magnetization components, combined with scanning electron microscopy, show that two types of hematite are present in the Lower Cretaceous rocks: pigmentary hematite and specularite. High‐temperature components (HTCs) were isolated by stepwise thermal demagnetization between 300 and 660°C carried by pigmentary hematite and all of the HTCs are of normal polarity. An additional magnetization component carried by specular hematite cannot be isolated efficiently. Progressive unfolding of both the Binchuan and Xiangyun sections shows that the best grouping of the precision parameter was obtained at 75%–80% untilting, indicating that the remanence of syn‐folding origin is carried by pigmentary hematite. These syn‐folding remanent magnetizations indicate the Binchuan and Xiangyun sections accumulated 20.5 ± 5.3° of clockwise rotation and 9.9 ± 4.6° of counterclockwise rotation, respectively, with respect to East Asia since the early Cretaceous, despite the two sections being only ∼45 km apart. The counterclockwise rotation of the Xiangyun section was likely caused by the left‐lateral shear motion of the Red River Fault; and since the Miocene, the Binchuan section experienced integral clockwise rotations, together with most parts of the Chuan‐Dian Block.

Middle Miocene forearc alkaline magmatism in Amami-Oshima Island, central Ryukyu Arc: implications for paleoreconstruction of Shikoku Basin

During the middle Miocene, forearc alkaline magmatism occurred in the Outer Zone of Southwest Japan and the northern Ryukyu Arc, resulting in the formation of forearc alkaline basaltic rocks with ocean island basalt (OIB)-like chemical signatures. In contrast, subduction-related magmatism was present in the central Ryukyu Arc. However, the southwestern margin of the forearc alkaline magmatism was poorly constrained in the Ryukyu Arc. We examined two basaltic dikes in the Chichibu accretionary complex of Amami-Oshima Island, the central Ryukyu Arc. The dikes cut massive basalt, reddish chert, varicolored shale, and grey chert. The chemical compositions of basaltic dikes are characterized by the enrichment of incompatible trace elements, possibly representing a low degree of partial melting from a deep mantle source. 40Ar/39Ar dating analyses indicate that the basaltic dikes yield ages of 16.37 ± 0.14 Ma and 16.51 ± 0.10 Ma. The paleomagnetic analyses on stepwise thermal demagnetizations allowed extracting the direction of stable magnetizations with unblocking temperatures of 450–575 °C for the two dikes [(Dec, Inc) = (138.1°, − 13.3°), (124.0°, − 24.9°)]. The magnetization could be primary, acquired either as part of a secular variation or a geomagnetic excursion during reversed polarity chron/subchron. The 40Ar/39Ar ages and paleomagnetic directions within the error range imply that they intruded almost simultaneously during C5Cn.2r (16.532–16.434 Ma). The age and trace element patterns of the basaltic dikes are comparable to those of Miocene alkaline basaltic rocks, which resulted from forearc alkaline magmatism during the initial subduction of the young and warm Shikoku Basin. The discovery of alkaline basaltic dikes on Amami-Oshima Island suggests that the distribution of middle Miocene forearc alkaline magmatism may extend to the central Ryukyu Arc. Hence, the northern end of the Kyushu-Palau Ridge (i.e., southern end of Shikoku Basin) could have been located south of Amami-Oshima Island around 16.5–16.4 Ma, then moved eastward to the current location.Graphical

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.

Did the Boreal Realm extend into the equatorial region? New paleomagnetic evidence from the Tuva–Mongol and Amuria blocks

Fossil organisms of the Boreal Realm in the Tuva–Mongol Block (TMB) and Amuria Block (AMB) are commonly thought to have lived at mid to high latitudes during the Carboniferous Period; however, reliable paleomagnetic constraints have not been available previously to test this hypothesis. Here we report newly obtained combined paleomagnetic and geochronological results from the Upper Mississippian Gunbayan Formation in the AMB and the Lower Pennsylvanian Altan-Ovoo Formation in the TMB. Zircon U–Pb dating of tuff beds from the Gunbayan and Altan-Ovoo formations yield ages of 331.0 ± 2.7 and 315.5 ± 2.4 Ma, respectively. A total of 263 paleomagnetic specimens underwent stepwise thermal demagnetization. After removing the viscous remanent magnetizations of the recent geomagnetic field, stable high-temperature components (HTCs), carried by magnetite, were successfully acquired from 240 specimens. The HTCs of the Gunbayan Formation passed a fold test and a reversal test, and those of the Altan-Ovoo Formation passed a fold test, suggesting that they represent primary remanence magnetizations. We used the elongation/inclination method to test and correct for all the HTC directions of the clastic rocks of the studied formations. Their corresponding paleomagnetic poles are 46.0°N/320.0°E (A=952.0°) at ca. 330 Ma for the AMB and 43.5°N/355.9°E (A=952.9°) at ca. 315 Ma for the TMB. The updated paleomagnetic database indicates with certainty that both the TMB and the AMB were located in northern hemisphere low-latitude regions and close to the equator at ca. 330 Ma, suggesting that the Boreal Realm might have extended into the equatorial region at the onset of the large ice age of the Carboniferous. The paleobiogeographic characteristics and distribution of climate-sensitive lithologies indicate that this realm occupied a wide northern temperate belt during 330–315 Ma.

Primary Carboniferous paleomagnetic and geochronologic results from the Aqishan–Yamansu Belt, Eastern Tianshan: Implications for the tectonic evolution of the Paleo-Asian Ocean

To constrain the closure time of the Paleo-Asian Ocean and better understand the movement processes of different terranes in the Northern Xinjiang region, we present a combined geochronologic and paleomagnetic study of Carboniferous andesite and limestones from the Yamansu and Dikaner Formations in the Aqishan–Yamansu Belt (AYB). Zircon UPb dating of the Dikaner Formation andesite yielded an age of 313.2 ± 2.0 Ma, and the zircon εHf(t) values ranged from −15.6 to 16.3. Stepwise thermal demagnetization successfully isolated a lower temperature component (LTC) and a stable higher temperature component (HTC) from 18 limestone beds. The LTC represents a viscous remanent magnetization of the present geomagnetic field. In comparison, the HTC is interpreted to represent Late Carboniferous magnetization, generating two paleopoles at 84.6°N, 38.3°E (A95 = 11.6°) for ~313 Ma and 77.2°N, 301.5°E (A95 = 5.5°) for 321–318 Ma. These results show that the AYB moved northward from 30.5 ± 6.5°N to 44.8 ± 8.3°N during the interval between 321–318 Ma and ~ 313 Ma. Compared to other published coeval paleomagnetic poles from surrounding blocks, it can be concluded that the western Paleo-Asian Ocean (between Tarim and Siberia) had been closed since the Late Carboniferous. In contrast, the eastern Paleo-Asian Ocean (between Siberia and Dunhuang-Alxa-NCB) was still open during this time. Besides, the counterclockwise rotations of AYB with respect to Siberia-Mongolia-Tuva and Tarim are estimated to be 88.5° ± 14.1° and 46.5° ± 13.4° since ~313 Ma respectively. Combined with other geological evidence, we propose that Yili-Junggar-Turpan-Hami-AYB had to move eastward to the still open eastern Paleo-Asian Ocean to accommodate the compressive tectonic stress field during the Late Carboniferous, and a Late Carboniferous tectono-paleogeography of the Paleo-Asian Ocean has been reconstructed.

North China craton: The conjugate margin for northwestern Laurentia in Rodinia

Abstract In the Rodinia supercontinent, Laurentia is placed at the center because it was flanked by late Neoproterozoic rifted margins; however, the conjugate margin for western Laurentia is still enigmatic. In this study, new paleomagnetic results have been obtained from 15 ca. 775 Ma mafic dikes in eastern Hebei Province, North China craton (NCC). Stepwise thermal demagnetization revealed a high-temperature component, directed northeast or southwest with shallow inclinations, with unblocking temperatures of as high as 580 °C. Rock magnetism suggests the component is carried by single-domain and pseudo-single-domain magnetite grains. Its primary origin is supported by a positive reversal test and regional remanence direction correlation test, and the paleomagnetic pole (29.0°S, 64.7°E, A95 = 5.4°) is not similar to any published younger poles of the NCC. Matching the late Mesoproterozoic to early Neoproterozoic (ca. 1110–775 Ma) apparent polar wander paths of the NCC and Laurentia suggests that the NCC could have been the conjugate margin for northwestern Laurentia in Rodinia, rather than sitting off the northeast coast of the main Rodinian landmass. Geological data indicate that breakup of the NCC and Laurentia occurred between ca. 775 and 720 Ma.

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.

Experimental test of the cooling rate effect on blocking temperatures in stepwise thermal demagnetization

SUMMARYUpon cooling, most rocks acquire a thermoremanent magnetization (TRM); the cooling rate at which this happens not only affects palaeointensity estimates, but also their unblocking temperatures in stepwise thermal demagnetization experiments, which is important, for example, to estimate volcanic emplacement temperatures. Traditional single-domain (SD) theory of magnetic remanence relates relaxation times to blocking temperatures— the blocking temperature is the temperature at which the relaxation time becomes shorter than the experimental timescale—and therefore strictly only applies to remanence acquisition mechanisms at constant temperatures (i.e. viscous remanent magnetizations, VRMs). A theoretical framework to relate (constant) blocking temperatures to (time-varying) cooling rates exists, but this theory has very limited experimental verification—partly due to the difficulty of accurately knowing the cooling rates of geological materials. Here we present an experimental test of this ‘cooling rate effect on blocking temperatures’ through a series of demagnetization experiments of laboratory-induced TRMs with controlled cooling rates. The tested cooling rates span about 1 order of magnitude and are made possible through (1) extremely accurate demagnetization experiments using a low-temperature magnetic properties measurement system (MPMS) and (2) the use of a ‘1-step-only’ stepwise thermal demagnetization protocol where the relaxation process is measured over time. In this way the relaxation time corresponding to the blocking temperature is measured, which can be done to much higher accuracy than measuring the blocking temperature directly as done in traditional stepwise thermal demagnetization experiments. Our experiments confirm that the cooling rate relationship holds to high accuracy for ideal magnetic recorders, as shown for a synthetic weakly interacting SD magnetoferritin sample. A SD-dominated low-Ti titanomagnetite Tiva Canyon Tuff sample, however, showed that natural samples are unlikely to be sufficiently ‘ideal’ to meet the theoretical predictions to high accuracy—the experimental data agrees only approximately with the theoretical predictions, which may potentially affect blocking temperature estimates in stepwise thermal demagnetization experiments. Moreover, we find a strongly enhanced cooling rate effect on palaeointensities for even marginally non-ideal samples (up to 43 per cent increase in pTRM for a halving of the cooling rate).

New Middle–Late Permian Paleomagnetic and Geochronological Results From Inner Mongolia and their Paleogeographic Implications

AbstractA combined paleomagnetic and geochronological study was conducted on the red beds of the Zhesi Formation (ZSF) in the Xilinhot–Songliao Block (XSB) and the andesites and sandstones of the Qingfengshan Formation (QFF) in the North China Block (NCB). The ZSF tuffs and the QFF andesites were dated to 265.3 ± 1.8 Ma and 254.8–257.1 Ma, respectively, using the zircon U–Pb laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) method. A total of 246 paleomagnetic samples were subjected to stepwise thermal demagnetization. After removal of the low‐temperature viscous components acquired in the recent geomagnetic field, stable high‐temperature components (HTCs) were isolated from most samples. The HTC of the ZSF passed a fold test, and the HTC of the QFF passed both fold and reversal tests, indicating their primary origins. The corresponding paleomagnetic poles are 57.4°N/342.0°E (A95 = 4.3°) for the XSB at ~265 Ma and 54.3°N/20.2°E (A95 = 3.3°) for the NCB at ~255 Ma. Comparison of high‐quality Permian paleomagnetic poles of the XSB and NCB reveals a latitudinal convergence and relative rotation between them that led to a scissors‐like closure of the Solonker Ocean from west to east between ~265 and ~246 Ma, which controlled the mixing of the Cathaysian and Angara floras in space and time. The XSB and NCB moved rapidly northward between ~265 and ~255 Ma, which probably accelerated the end‐Guadalupian mass extinction in East Asia.

Paleomagnetism and U-Pb Geochronology of Early Cretaceous Volcanic Rocks from the Qiangtang Block, Tibetan Plateau: Implications for the Qiangtang-Lhasa Collision

The collision between the Qiangtang and Lhasa blocks is a critical factor in understanding the geodynamics of the central Tibetan Plateau. However, the process of the Qiangtang-Lhasa collision remains contentious. A direct way to study the history of the Qiangtang-Lhasa collision would be to determine the evolution of the paleolatitude positions of the Qiangtang and Lhasa blocks during the Jurassic-Cretaceous. In this study, we present a combined paleomagnetic and geochronological study of the Early Cretaceous volcanic rocks dated at ~120–115 Ma in the western Qiangtang block. Stepwise thermal demagnetization succeeded in isolating the high-temperature characteristic directions of the samples. The tilt-corrected mean direction of the 16 sampling sites was Ds = 60.9°, Is = 45.9° with ɑ95 = 4.4°, which indicates that the Qiangtang block was situated at 27.6 ± 5.0°N during the Early Cretaceous (~120–115 Ma) (reference point: 32.9°N, 83.5°E). Our new data combined with previous reliable Cretaceous paleomagnetic results for the Lhasa block indicate that the collision between the western parts of the Qiangtang and Lhasa blocks may occur later than 115 Ma. The extent of the western segment of the Bangong-Nujiang Tethys Ocean was ~825 ± 600 km (7.5° ± 5.5° in latitude) at 120–115 Ma.

Downward remagnetization of a ∼74-m-thick zone in lake sediments from palaeo-Lake Idaho (NW United States)—Locating the Gauss/Matuyama geomagnetic boundary within a dual-polarity zone

SUMMARY Remagnetization is an important issue in palaeomagnetism. Here, we discuss an extraordinarily thick (∼74 m) dual-polarity transition zone between the Gauss and Matuyama Chrons. The studied succession is from a drill core through lacustrine sediments of palaeo-Lake Idaho (Snake River Plain, NW United States of America) that are intercalated with basalt units. We identified detrital Ti-rich titanomagnetite and magnetite in lamellar exsolutions as the main carriers of a primary remanence, likely derived from the basalts that erupted in the Snake River Plain. Stepwise thermal demagnetization revealed a single-component remanent magnetization with reversed and normal polarities above and below the transition zone, respectively. Based on rock-magnetic results, microscopic observations, and previously known events in the evolution of palaeo-Lake Idaho, the reversed-polarity component in the transition zone represents a secondary chemical remanent magnetization caused by magnetic mineral alteration or partial neo-formation of magnetite, in association with strong depletion of the primary detrital magnetic minerals that affected a wide depth range below the level where the remagnetization event occurred. This remagnetization event was most likely related to lake-level lowering and partial desiccation of palaeo-Lake Idaho. Understanding the nature and origin of the remagnetization allows to identify the polarity boundary in the unusual case of a secondary magnetization with reversed polarity produced downward in a sequence to an extraordinary large depth. Based on available age information, the observed reversal represents the Gauss/Matuyama boundary, which provides an important age constraint for palaeoclimatic interpretation of the succession.

A combined geochronological and paleomagnetic study on ∼1220 Ma mafic dikes in the North China Craton and the implications for the breakup of Nuna and assembly of Rodinia

A new combined geochronological and paleomagnetic study was carried out on the late Mesoproterozoic mafic dikes in the northeastern and central North China Craton (NCC). Three dikes were dated at 1224.2 ± 5.1 Ma, 1219.9 ± 6.0 Ma and 1215.2 ± 7.7 Ma through use of the secondary ion mass spectrometry (SIMS) zircon dating method and one dike was dated at 1219.1 ± 4.3 Ma by using SIMS baddeleyite dating method, indicating that these dikes emplaced at ∼1220 Ma. A total of 286 paleomagnetic core samples were collected from seven ∼1220 Ma dikes and their host rocks, as well as two ∼775 Ma dikes for field tests. All the samples were subjected to stepwise thermal demagnetization. Rock magnetic experiments of representative samples demonstrate that the dominant magnetic carriers of the ∼1220 Ma dikes are single-domain and pseudo-single domain magnetite grains. Two paleomagnetic components were identified from the ∼1220 Ma dikes. The low temperature component is a viscous magnetic remanence acquired in the recent geomagnetic field. The high temperature component (HTC) was isolated between 500 and 580°C, which directs east and down with moderate inclination. A paleomagnetic pole (11.3°N, 175.8°E, A95 = 7.6°) was determined by averaging seven virtual geomagnetic poles (VGPs), each VGP corresponding to the mean HTC direction of an individual dike. The ∼1220 Ma pole passed a baked-contact test and appeared to adequately average out secular variation. It differs from any younger poles of the NCC, and is thus considered as a reliable pole for the NCC. While our new results place the NCC at intermediate paleolatitudes (∼28.8°N for site located at 36.8°N, 113.4°E) at ∼1220 Ma, the coeval high-quality poles in the global paleomagnetic database, however, indicate that the Australian cratons were positioned near the pole and Laurentia was located around the Equator, depicting a dispersed paleogeographic pattern of the three continents at ∼1220 Ma. The updated paleomagnetic data together with geological evidence may suggest that the NCC separated from northern Australia between ∼1320 and ∼1220 Ma, and later joined northwestern Laurentia between ∼1220 and ∼1110 Ma.

New paleomagnetic results from the ca. 1.68–1.63 Ga mafic dyke swarms in Western Shandong Province, Eastern China: Implications for the reconstruction of the Columbia supercontinent

The existence of Paleo-Mesoproterozoic supercontinent Columbia (aka Nuna) was established decades ago, but the position of North China Craton within Columbia is still highly debated due to the paucity of available high-quality paleomagnetic and reliable geological constraints. Precise geochronological dating of extensive Mesoproterozoic mafic dyke swarms in Western Shandong Province (also named the Luxi area), China reveals two phases of dyke emplacement at ~1.68 Ga and ~1.63 Ga. In this paper, we report new paleomagnetic and rock magnetic results obtained from approximate 160 samples (16 sites) collected from these two phases of mafic dyke swarms in the Luxi area with the aim of pinpointing the location of North China Craton within Columbia supercontinent in this time interval. Rock magnetic experiments confirm that either magnetite or titanomagnetite is the main magnetic carrier in these dykes. Stepwise thermal demagnetization revealed two paleomagnetic poles. For the ~1.63 Ga dykes, normal and reversed high-temperature remanent magnetization directions yield a mean direction (D/I) of 86.1°/53.5° (κ = 43.3, α95 = 7.9°, N = 9). These directions pass a reversal test and are interpreted as primary remanences. The corresponding paleomagnetic pole is calculated at 20.8°N, 182.5°E (κ = 28.3, A95 = 8.3°, N = 9). This pole passes the examination of secular variation of geomagnetic field (Deenen et al., 2011, 2014). It fulfills a Van der Voo (1990) value Q = 6 and is therefore suggested to be a ‘key’ pole demarcating the Precambrian North China Craton. For the ~1.68 Ga dykes, only the normal directions are isolated with a mean direction (D/I) of 89.1°/47.1° (κ = 35.0, α95 = 13.1°, N = 5) with a corresponding paleomagnetic pole of 17.8°N, 184.9°E (κ = 29.6, A95 = 14.3°, N = 5). This pole passes the examination of secular variation of geomagnetic field. Finally we select the ~1.63 Ga high-quantity paleomagnetic pole in order to depict a more detailed apparent pole wander path (APWP) to compare with the other major Precambrian cratons. Combined with other geological evidence, our reconstruction scenario supports the spatio-temporal connection between the Baltica, North Australian Craton and North China Craton.

Experimental Evaluation of Remanence Carriers Using the Microcoercivity‐Unblocking Temperature Diagram

AbstractKnowledge of remanence carriers in recording media is crucial for obtaining reliable paleomagnetic data. In this paper, we report a newly developed technique to determine remanence carriers in natural rock samples that uses anhysteretic remanent magnetization (ARM) spectra in the microcoercivity (hc) and unblocking temperature (TUB) planes. The distribution of ARM carriers was determined via systematic stepwise demagnetization experiments of ARM combining thermal demagnetization and alternating field demagnetization treatments and calculating the change in ARM intensity on the hc‐TUB plane (hc‐TUB diagram). Samples from three basaltic lavas and two welded tuffs were analyzed. Combining the hc‐TUB diagram with the change in ARM coercivity spectra, both decreases and increases in magnetic minerals due to the thermal alteration during laboratory heating could be identified. From experimental samples, ARM carrier components were identified as being original rather than produced as a result of thermal alteration. In the case of the Izu‐Oshima 1986 A lava flow (1986La) sample, it was confirmed from the hc‐TUB diagram that high‐hc components can be separated using stepwise alternating field demagnetization treatment but not stepwise thermal demagnetization treatment. Paleointensity experiments were also conducted on Izu‐Oshima 1986 A lava flow samples. Two of the three paleointensity estimates were greater than the expected field intensity, suggesting a certain rock‐magnetic property that affects the paleointensity estimation. The hc‐TUB diagrams of the Izu‐Oshima 1986 A lava flow samples suggest that the differences in the paleointensity estimates were likely caused by different degrees of the high‐temperature oxidation state.