Components Of Remanent Magnetization Research Articles

Low paleomagnetic field in the proterozoic: new 1.72–1.76 ga paleointensity data obtained on the proterozoic volcanics from the Ukrainian shield

A collection of igneous rocks from the Ukrainian Shield sampled from the Korsun-Novomyrhorod pluton (age interval 1760–1735 Ma, Ingul Domain) and from the Korosten pluton (age 1760–1750 Ma, North-Western Domain) is studied. To obtain reliable determinations of paleointensity (Banc), the magnetic and thermomagnetic properties of samples were studied and X-ray diffraction analyses were carried out. It is shown that the carriers of the characteristic component of natural remanent magnetization are single- and small pseudo-single-domain magnetite grains. To determine Banc, two methods were used: the Thellier‒Coe procedure with the pTRM-checks and the Wilson method. Paleointensity determinations are obtained from five sites and are shown to meet quality criteria. For all five sites, the values of the Banc and the virtual dipole moment (VDM) are extremely low, varying within the range of 3.6–9.76 μT and (0.92–2.43)×1022 Am2, respectively. The analysis of the data from the paleointensity world database (WDB) has shown that the operating mode of the geodynamo in the Proterozoic can be characterized by a succession of strong and weak dipole regimes, but the reality of this conclusion entirely depends on the reliability of the data reported in the literature and presented in the WDB.

Anomalous features of the geomagnetic field behavior at the end of the Cretaceous normal superchron based on the results of the study of the Turonian–Santonian in the Southwestern Crimea

In 394 samples characterizing 266 stratigraphic levels in four Turonian–Santonian sections in the Southwestern Crimea, characteristic remanent magnetization components (ChRM) formed at the stage of diagenesis is identified. The data obtained represent the record of the Paleocene geomagnetic variations of high amplitude (rms deviation S = 25.9° with a fixed cut-off angle of 45°, which is about twice as high as the model S for this latitude) in the sediments formed in ~5–6 Ma and are interpreted as anomalous behavior of the geomagnetic field in the Turonian, Coniacian, and Santonian.

Restoration and Transformation: The Response of Shocked and Oxidized Magnetite to Temperature

AbstractLarge impact craters on Earth are associated with prominent magnetic anomalies, residing in magnetite of the shocked target rocks and impactites. Shock experiments on magnetite suggest that up to 90% of magnetic susceptibility is lost at pressures >5 GPa, but can be partially restored by post‐shock thermal annealing. The magnetic property changes are caused by shock induced grain size reduction and fragmentation, as well as domain wall‐pinning at crystal lattice defects. A recent study of granitoids from the peak‐ring of the Chicxulub crater found that annealing may occur naturally, but can also be overprinted by high‐temperature hematite‐to‐magnetite transformation in non‐oxidizing environments. In this study, we isolate the effect of defect annealing and hematite‐to‐magnetite transformation using the evolution of hysteresis, isothermal remanent magnetization components and first order reversal curve (FORC) diagrams at different high‐temperature steps. We used a laboratory‐shocked magnetite‐quartz ore, a non‐shocked naturally oxidized granite, and a naturally shocked and oxidized granite. Our findings suggest that annealing of shock‐induced lattice defects partially restores some pre‐shock magnetic behavior and causes an apparent average bulk‐sample domain state increase. Hematite‐to‐magnetite transformation creates new fine‐grained magnetite that strongly overprints the original signal, and decreases the average bulk‐sample domain state. Where annealing and hematite‐to‐magnetite transformation both occur, the new magnetite masks the annealing‐induced property restoration and apparent domain state modification in the shocked magnetite. As magnetite oxidation is a ubiquitous process in surface rocks, these findings are fundamental to understand hematite‐to‐magnetite transformation as a potential overprint mechanism, and could have broad implications for paleomagnetic interpretations.

First Results of the Intraformational Conglomerate Test of the Late Riphean Katav Formation (Southern Urals)

Detailed petrographic, electron-microscopic, and paleomagnetic studies were conducted on flatbedded synsedimentary carbonate breccias from three blocks of rocks (one block from Minyar, two blocks from Katav-Ivanovsk) to verify the ideas about the time of formation of the high-temperature component (HTC) of natural remanent magnetization (Jn) of limestones from the Upper Riphean Katav Formation, Southern Urals. Petrographic and electron-microscopic observations revealed that the composition of pebbles corresponds to that of the host matrix rocks, thus enabling the use of an intraformational conglomerate test. In all the studied blocks, the distribution of paleomagnetic directions for pebbles is generally chaotic, with the concentration parameter not exceeding 3. These test results meet the Graham criterion. The pebbles’ average Jn have a larger confidence oval and differ significantly from the matrix’s directions with a much smaller confidence oval. The Hodges-Ajne test was applied to the blocks from Katav-Ivanovsk. The Rayleigh test was used for the block from Minyar. The results obtained at this stage indicate that the HTC of magnetization of the Katav limestones may have a primary origin. If this is confirmed, the Katav Formation would be a good paleomagnetic record of the Earth’s Neoproterozoic history and provide valuable insights into the geomagnetic field behavior in the Late Precambrian.

Paleomagnetism and Cyclostratigraphy of the Permian-Triassic Boundary Interval of the Staroe Slukino Section, Vladimir Region

The paper presents the results of paleomagnetic, rock magnetic, and cyclostratigraphic studies of continental red beds that compose the Permian-Triassic boundary interval of the Staroe Slukino section in the Vladimir Region, Russia. Based on the directions of the characteristic components of the natural remanent magnetization of the studied rocks in the sampled stratum, the intervals of normal and reverse polarity related to the regional magnetozones r2RnP, r3RnP, and N3P-T were identified. Within the r3RnP zone, there is an interval of anomalous paleomagnetic directions, which has similar characteristics to those identified earlier in the coeval intervals of the Nedubrovo, Zhukov Ravine, and Okskiy Siyezd sections. Revising the biostratigraphy allows us to assume that the two zones of anomalous paleomagnetic directions in the composite magnetic polarity scale of the upper Permian of the Russian Platform are a reflection of the same epoch of an anomalous geomagnetic field configuration. It is estimated by the cyclostratigraphic method that the studied 16 m interval of the Staroe Slukino section took 900±20 thousand years to accumulate, which limits the duration of the geomagnetic field anomalous state near the Permian-Triassic boundary to ~110 000 years. A new Permian-Triassic (~252 Ma) paleomagnetic pole of the East European Platform is calculated: plat = 36.3°; plong = = 155.0°; dp/dm = 2.8°/4.8°.

Palaeomagnetism of the Vendian traps of Volyn, southwestern margin of the East European Platform, P. 2: magnetostratigraphy

We present new results of palaeomagnetic studies of the Vendian (Ediacaran) rocks of the Volyn Basalt Province revealed by six boreholes on the NW Ukraine. This is a continuation of previous studies of upper part of the Volyn series described by Bakhmutov et al. [2021]. In the recent works [Shcherbakova et al., 2020; Thallner et al., 2022], the results of palaeointensity determinations of the Volyn series basalts showed the ultra-low dipole moment of Earth’s magnetic field which coincident with other palaeomagnetic data for Ediacaran indicating an extremely weak geomagnetic field. Clear stratification and correlation of the basalt and tuff layers by magnetic parameters allow us to determine the magnetic field reversals throughout the stratigraphic succession of the Volyn series of about 400 m thick. In basalts, tuffs and baked contact rocks, a high-temperature characteristic component of remanent magnetization (ChRM) with all signs of primary magnetization have been isolated. For the directions of inclination of ChRM-components at least six magnetic polarity reversals were revealed. The geochronological ages of rocks is in the range of 580—545 Ma, but the errors in the age estimation are too large for separation of individual formations within the pulses of activity or differentiation of individual eruptions. Therefore the formation time of the entire stratum remains uncertain, and the key issue for interpretation of the magnetic polarity reversal frequency is the duration of accumulation of traps. Two possible interpretations of palaeomagnetic results are considered. In the first, which takes into account different stages of magmatic activity, the time interval of accumulation can be about 10 million years. Then the average frequency of geomagnetic inversions is close to mean for the Phanerozoic. We prefer the other interpretation when the formations of the Large Igneous Provinces have been occurred over a short time interval (e.g. 0.5 Myr). Taking into account the results of palaeointensity estimations for the same samples, which shown the extremely weak geomagnetic field, the hypothesis of the «hyperactivity» of the field with a frequency of at least 12 reversals per one million years on the end of the Ediacaran gain the additional confirmation.

New Early Neoproterozoic Paleomagnetic Constraints of the Northwestern China Blocks on the Periphery of Rodinia

AbstractRecent studies highlight the debates about the role of the northwestern China blocks, for example, north Tarim block, south Tarim block (STB), Altyn Tagh block (ATB) and Qaidam‐Qilian block (QQB) during the assembly of Rodinia. We herein present a combined paleomagnetic, geochronological, and geochemical study on 16 mafic dikes in the northern ATB. After removing a recent viscous remanent magnetization, three characteristic remanent magnetization components (CHs) were isolated. CH1 was identified in eight NW‐trending diabase dikes, one of which was newly dated at 895.3 ± 7.7 Ma through use of SHRIMP baddeleyite U‐Pb methodology. It yields a mean direction (D/I) of 123.3°/−42.2° (α95 = 12.1°). The paleopole (40.9°S/184.6°E, A95 = 11.7°) was calculated by averaging eight virtual geomagnetic poles (VGPs) from each dike. A partial baked‐contact test supports the primary origin of CH1. CH2 was isolated from three E‐W trending diabase dikes, pointing northwest with considerably shallower downward inclinations. CH3 was obtained from five lamprophyre dikes, directing southeast with moderate to shallow upward inclinations. However, the VGPs of either CH2 or CH3 are insufficient to average a precise mean pole. Geochemical analyses also reveal petrogenetic discrepancy among the dikes with distinct CHs. Combining the new ∼895 Ma pole of CH1 with the global paleomagnetic database and being aided by geological evidence, we propose that the STB‐ATB‐QQB and the north China craton were located along the northwestern side of Laurentia in Rodinia at ∼895 Ma, and suggest the connection might have been intact until ∼780 Ma.

Palaeomagnetic data from the late Cenozoic Tagay section (Olkhon Island, Baikal region, Eastern Siberia)

One of the most important fossil fauna localities in the Baikal region is the Tagay section. It is located on the shore of Tagay bay at the northwestern coast of Olkhon Island in the Baikal Lake and consists of two different exposures, Tagay-1 and Tagay-2. The Tagay-1 section exposes predominantly lake sediments of Miocene age and contains a faunal complex. The Tagay-2 section is composed of slope wash and subaerial sediments of Early Pleistocene (Calabrian?) age with remains of small mammals. A total of 95 samples for palaeomagnetic studies were taken in both sections. Magnetic susceptibility (MS), its frequency dependence (Fd) and anisotropy were measured from all samples. The stable component of the natural remanent magnetization (NRM) was obtained using stepwise alternating field (AF) demagnetization. The magnetic polarity pattern was constrained for the Tagay-1 section. It includes an extended zone of reversed polarity, complicated by three normal subzones of minor rank. Palaeontological data allow correlation of the Tagay-1 pattern with the magnetic chrons C5Dn to C5Cn.1n (late Burdigalian – early Langhian). Palaeomagnetic directions in the upper part of the Tagay-2 section are randomly distributed due to intensive slope-wash processes resulted in mechanical perturbation of the sediments. The lower part of Tagay-2 section demonstrates only reversed polarity, although the scatter of directions is quite large due to landsliding. This reversed interval may be correlated with the Matuyama chron between Cobb Mountain and Olduvai subchrons. Synchronous increase of MS and Fd values in palaeosols suggests a pedogenic (or “Chinese”) mechanism influencing magnetic properties of the Tagay-2 section under warm palaeoclimatic conditions.

Paleomagnetism of Gedemsa magmatic segment, Main Ethiopian Rift: Implication for clockwise rotation of the segment in the Early Pleistocene

One-hundred-seventy oriented paleomagnetic core samples from 23 sites were collected from volcanic lava flows belonging to the Nazareth Group (1.7–1.9 Ma) and the Wonji group (≤0.3 Ma) along the Gedemsa magmatic segment (MS) in the, Main Ethiopian Rift (MER). Samples were subjected to progressive thermal (Th) and Alternating Field (AF) demagnetization for directional analyses, and representative samples were used for rock-magnetic experiments. The rock-magnetic study indicated that titano-magnetite, titano-hematite and magnetite are the primary carrier of remanent magnetization. Two components of Natural Remanent Magnetization (NRM) were revealed: the first, low stability component is removed by AF of 20-30mT or heating up to 200 °C. The second, high stability component isolated above these steps is directed toward the origin and considered Characteristic Remanent Magnetization (ChRM). All specimens we studied show normal geomagnetic polarity. The overall mean direction for the Wonji group is Decs = 0.60, Incs = 5.10 (α95 = 6.50, K = 35.8, N = 14) while for the Nazareth group is Decs = 18.90, Incs = 13.40 (α95 = 10.30, K = 35.6, N = 7). When these mean directions from the two groups are compared with their respective reference geomagnetic dipole fields (Besse and Courtillot, 1993, 2002), the former shows no significant block rotation and a clockwise rotation R = 170 ± 3.40 were detected in the latter. These results indicate the Gedemsa magmatic segment is characterized by complex deformation due to the opening of DebreZeit-Butajira (DzB) and Yerer – Tullu Wellel Volcano-tectonic Lineament (YTVL segments. Our study indicates that detailed paleomagnetic observations in the Rift are important to identify quantitative deformation history of the region.

Rock Magnetic Signatures of Hydrothermal Mineralization in the Trans‐Atlantic Geotraverse (TAG) Hydrothermal Field

AbstractThe Ocean Drilling Program Leg 158 drill holes from the Trans‐Atlantic Geotraverse hydrothermal field are investigated to understand the rock magnetic signatures of hydrothermal mineralization. A composite columnar section has been constructed through hole correlation to understand the stratigraphic variation of magnetomineralogy within the stockwork. Isothermal remanent magnetization components unmixing, first‐order reversal curve diagrams, low‐temperature magnetic signatures, and electron microscopic analyses disclose magnetic minerals of disparate occurrences related to predominating hydrothermal mineralization reactions in three broad zones: For basaltic basements, serpentinization of olivine phenocrysts during preliminary hydrothermal alteration produces magnetite, in addition to primary titanomagnetite; Chloritized and silicified zone samples contain relict titanomagnetite and exsolved magnetite that survived hydrothermal dissolution; Anhydrite and sulfide zone samples are dominated by magnetite and hematite, likely from oxidation of polymetallic sulfides due to exposure in oxidative seafloor environments during drilling. Our findings suggest that seafloor oxidation potentially modifies the magnetic properties of polymetallic sulfides in hydrothermal deposits, which applies to magnetic tomography of sophisticated subseafloor vent structures and prospecting seafloor massive sulfides (SMS) deposits therein. Meanwhile, we alert future deep‐sea mining that drilling may promote physicochemical alteration of SMS deposits, causing environmental risks. The established magnetic signatures ultimately contribute to understanding the in situ geological preservation of SMS deposits and optimizing exploitation procedures in the future.

A 62 kyr geomagnetic palaeointensity record from the Taymyr Peninsula, Russian Arctic

Abstract. This work presents unprecedented, high-resolution palaeomagnetic data from the sedimentary record of Lake Levinson-Lessing, the deepest lake in northern central Siberia. Palaeomagnetic analyses were carried out on 730 discrete samples from the upper 38 m of the 46 m long core Co1401, which was recovered from the central part of the lake. Alternating field demagnetization experiments were carried out to obtain the characteristic remanent demagnetization. The relative palaeointensity is determined using the magnetic susceptibility, the anhysteretic remanent magnetization, and the isothermal remanent magnetization for normalization of the partial natural remanent magnetization. The chronology of Co1401 derives from correlation of the relative palaeointensity of 642 discrete samples with the GLOPIS-75 reference curve, accelerated mass spectrometer radiocarbon ages, and optically stimulated luminescence dating. This study focuses on the part > 10 ka but also presents preliminary results for the younger part of the core. The record includes the geomagnetic excursions Laschamps and Mono Lake and resolves sufficient geomagnetic features to establish a chronology that continuously covers the last ∼ 62 kyr. The results reveal continuous sedimentation at high rates between 45 and 95 cm kyr−1. The low variability of the magnetic record compared to datasets of reference records with lower sedimentation rates may be due to a smoothing effect associated with the lock-in depths. Because Co1401 was cored without core segment overlap the horizontal component of the characteristic remanent magnetization can only be used with caution. Nevertheless, the magnetic record of Co1401 is exceptional as it is the only high-resolution record of relative palaeointensity and palaeosecular variations from the Arctic tangent cylinder going back to ∼ 62 ka.

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.

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.

Paleolatitude estimation and premises for geomagnetic field instability from the Proterozoic drilling core material of the south-western part of the East European Craton

This study addresses new paleomagnetic, geochemical, mineralogical, and rock-magnetic data from four drill cores of the western part of the East European Craton, representing mainly siliciclastics supplemented by volcaniclastics and a few basaltic rock samples. The cores comprise Mesoproterozoic samples of 1400–1030 Ma and Neoproterozoic (Ediacaran) samples of 588–541 Ma. According to the magnetic data, the dominant magnetic carrier in basalts is magnetite, while two genetic types of hematite (detrital and syngenetic/early diagenetic) carry magnetization in sedimentary rocks. The characteristic components of the natural remanent magnetization (NRM), carried by the fine-grained hematite (intermediate temperature component [CM]) and the coarse-grained hematite (high-temperature component [CH]), were identified during thermal cleaning. Mineralogical and geochemical studies provided evidence for detrital and synsedimentary/early post-sedimentary origin of fine-grained hematite and suggested hot and humid environmental conditions on the western part of Baltica during Meso- and Neoproterozoic time. Because of a low-temperature diagenetic history, which affected the Mesoproterozoic and Ediacaran rocks of Pinsk 26, Ochrymy IG-1, and Šaškai 2 boreholes, no post-Ediacaran paleomagnetic overprint is expected there. On the basis of these findings, the CM component in these profiles was classified as nearly primary (synsedimentary/early post-sedimentary) in origin. The CH component in these profiles and the Ediacaran sandstones of the Kaplonosy IG-1 section is also considered primary and acquired as detrital NRM (DRM). Paleomagnetic results yielded a reliable record with the domination of shallow-to-moderate inclinations, showing both polarities. The calculated paleolatitudes for 550–541 Ma time span, regardless of correction for the inclination error, fall within the subequatorial zone. Steeper inclinations, suggesting subpolar paleolatitudes, were observed in the volcaniclastics and basalts interlayering with sedimentary rocks and, sporadically, in the Ediacaran sedimentary successions. The origin of this steep component, however, was attributed to the anomalous behavior of the geomagnetic field, as a consistently warm environmental facies pattern did not support a fast and far-distance, cross-latitudinal, poleward movement of Baltica during the Ediacaran time and then back to the equator. These results complement previous studies that sought to resolve uncertainty of the paleogeographic position of Baltica in the Proterozoic time.

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.

Loess deposits in southern Tajikistan (Central Asia): Magnetic properties and paleoclimate

Abstract The continental accumulation of dust during the Quaternary led to the formation of widespread loess deposits in southern Tajikistan. In this area, the accumulation of loess is commonly associated with the occurrence of dust storms and the widespread distribution of loess provides evidence of dust storms becoming more frequent in arid Central Asia at least since the Early Pleistocene. Southern Tajikistan represents one of the largest loess deposits in Central Asia. We conducted a thorough study on the magnetostratigraphy, grain size, and magnetic susceptibility of the Chashmanigar section to reconstruct the stratigraphy of loess deposits and paleoclimate of Tajikistan. Based on our new data, the lower boundary of the basal ages of the Olduvai and Reunion subchrons were established for the studied Chashmanigar section. Rock magnetic analyses showed that the predominant ferrimagnetic minerals are large pseudo-single domain grains of magnetite, including limited maghemite. Standard demagnetization techniques yielded a characteristic component of natural remanent magnetization, which was used to obtain a well-defined magnetostratigraphy. In southern Tajikistan, paleosols consistently exhibit finer grain size distribution and higher magnetic susceptibility than loess horizons, suggesting that the environment of the Chashmanigar section was colder, drier, and dustier during glacial periods than during interglacial periods. Through correlation with astronomically tuned oxygen isotope records, sophisticated dating of the loess-paleosol sequence at Chashmanigar could be achieved, and the global significance of the recorded paleoclimatic variations could be revealed. The resulting grain size, magnetic susceptibility, and correlation with astronomically tuned oxygen isotope clearly provide information about the climatic pattern during the Early Pleistocene.

Верхнедевонские отложения (франский ярус) северо-запада Русской платформы: палеомагнитные данные

Paleomagnetic determinations for the Devonian strata of the Main Devonian Field, available in the international paleomagnetic data base, do not make it possible to construct both detailed magnetostratigraphic scales and reliable trajectories of the apparent polar wander path of the paleomagnetic pole. This is primarily due to the insufficient amount of data that determine modern reliability criteria. Obtaining more complete series of reliable paleomagnetic determinations is one of the important tasks of paleomagnetic studies. The paper presents new paleomagnetic determinations from a collection of rocks of the Frasnian stage of the Upper Devonian (Ilmen and Bureg Beds of the Semiluky Formation), sampled on the southern shore of Lake Ilmen, east of the village. Korostyn in 2009. Magnetomineralogical studies were carried out to determine the magnetic minerals of carriers of natural remanent magnetization. Three characteristic components of natural remanent magnetization have been identified. Two components correspond to the Late Paleozoic magnetization reversal (in the Carboniferous and Permian times). The third, bipolar component of the Frasnian age passes the reverse polarity test of the geomagnetic field.. The obtained position of the paleomagnetic pole by the bipolar component of the natural remanent magnetization is consistent with the available data on the section of the Late Upper Devonian of the Main Devonian Field.

Unblocking temperature of secondary magnetic component to outline anomaly thermomagnetic maps of archaeological fireplaces from the southern region of Mexico City, Mexico

We report a magnetic study of two preceramic hearths located at the southern region of Mexico City. Thermal demagnetization was applied between 200 to 540 °C to define the remanent magnetization components. The maximum unblocking temperature of the secondary magnetic component reached due to the last heating was identified in most samples. They were used to develop thermomagnetic anomaly source maps. The direction of this component was used to place the blocks to their last cooling spot and configurate a real and primal magnetic anomaly map produced by the thermoremanent magnetization acquired. A protocol to relocate blocks frame is proposed using geomagnetic secular variation curves. The thermomagnetic mapping revealed the position and temperature (220 to 460 °C) of the last heat sources. The distribution of these sources allowed to model an extensional zone of benefit and probable uses of the hearths.