Reliability of Relative Paleointensity Recorded in Chinese Loess–Paleosol Sediments

A detailed paleomagnetic and rock-magnetic investigation spanning loess L7 to paleosol S8 has been carried out at the Baoji and Xifeng sections. Results of anisotropy of magnetic susceptibility confirm that the studied loess–paleosol sediments retain primary sedimentary fabrics. Stepwise thermal demagnetization shows that two well-defined magnetization components can be isolated from both loess and paleosol specimens. A low-temperature component, isolated between 100°C and 200°C, is consistent with the present geomagnetic field direction, and a high-temperature component, isolated between 200–300°C and 620–680°C, includes clearly normal and reversed polarities. Isothermal remanent magnetization and thermomagnetic analyses indicate that characteristic remanent magnetization is mainly carried by magnetite and hematite. The Day plot, together with the stratigraphic variations of rock-magnetic parameters, shows that the uniformity of magnetic mineralogy and grain size fulfills the criteria for relative paleointensity (RPI) studies. RPI records have been constructed using natural remanent magnetization (NRM) intensity after thermal demagnetization at 300°C normalized by low-frequency magnetic susceptibility (NRM300/χ). The results show that the RPI record from the Baoji section, where pedogenesis is quite weak, is compatible with the stacked PISO-1500 paleointensity record, suggesting that it might reflect the paleointensity variation of the geomagnetic field. The RPI record from the Xifeng section, where pedogenesis is rather strong, indicates a clear dissimilarity with the stacked PISO-1500 paleointensity record, implying that it does not reflect the paleointensity variation of the geomagnetic field. Our new results show that the NRM300/χ from the strongly pedogenetic paleosols does not completely eliminate the pedogenetic (climatic) influence, so it might be unsuitable for a reliable paleointensity study.     

Late Cenozoic paleomagnetism of West Siberian Plate

On the basis of combined (paleomagnetic, lithological, and paleontological) data, a scale of Neogene geomagnetic polarity is proposed for the West Siberian Plate (WSP). It comprises 17 large orthozones of normal and reversed polarity. The scale was compiled by comparing and correlating the Neogene key sections of the Kulunda and Baraba plains, Irtysh regions between Omsk and Pavlodar and near Tara, and Ob’ region near Tomsk. The reliability of paleomagnetic data is confirmed by component analysis of natural remanent magnetization and by a possibility of determining its primary component. In the studied Neogene rocks this is a high-temperature component related to magnetite, hematite, and maghemite, which decays at 420–675 °C. In the period from Early Miocene to Late Pliocene, the Late Cenozoic geomagnetic field reconstructed from NRM vectors for WSP rocks of Neogene age experienced 17 reversals (at the level of orthozone boundaries), with eight regimes of normal polarity and nine regimes of reversed polarity. Comparison of the WSP Neogene scale with Berggren’s scale provided absolute age boundaries of the Late Cenozoic series on the WSP regional stratigraphic scale. The 23.8 Ma boundary between the Oligocene and Miocene is recorded in the regional scale at the sole of the Lyamin beds of the Abrosimovka Formation, at the bottom of Orthochron R₁N₁aq, and the Miocene-Pliocene 5.2 Ma boundary (Chron C3r), in the Novaya Stanitsa Formation (top of the Cherlak beds).     

An example of multicomponent magnetization in welded tuffs: a case study of Upper Cretaceous welded tuffs of eastern Russia

Four distinct components of natural remanent magnetization were isolated from a single site in welded tuffs in the Upper Cretaceous Kisin Group of the Sikhote Alin mountain range, Russia. In order to contribute toward a basis for an interpretation of multicomponent magnetization, rock magnetic experiments were performed on the welded tuffs. All four magnetization components essentially reside in magnetite. The lowest-temperature component up to 300 °C (component A: D=349.3°, I=60.9°, α₉₅=7.3°, N=7) is a present day viscous magnetization. The third-removed component (component C: D=41.4°, I=51.8°, α₉₅=3.5°, N=8), isolated over the temperature range of 450–560 °C, is a primary remanence. The second- and fourth-demagnetized components (component B: D=174.7°, I=−53.1°, α₉₅=21.2°, N=3 and component D: D=188.1°, I=−64.5°, α₉₅=4.0°, N=8, respectively) are secondary magnetizations related to a thermal event in Sikhote Alin between 66 and 51 Ma. Components B and D were acquired through different remagnetization processes. Component B is ascribed to a thermoviscous remanent magnetization carried by single-domain magnetite, and component D is a chemical remanent magnetization.     

Middle Devonian Paleomagnetism of Geological Complexes of Central Tuva

New paleomagnetic data are obtained for Middle Devonian rocks of Central Tuva. The rocks contain one-, two-, or three-component magnetization. The low-temperature (LT) components of magnetization are close to the directions of the present-day or Cenozoic magnetic field in Tuva. Based on the directions of the high-temperature (HT) components of magnetization, which were distinguished in the magnetite spectrum of blocking temperatures of up to 580оС, we revealed a prefolding magnetization of different polarity. The time when Middle Devonian rocks acquired the prefolding HT component of magnetization almost does not differ from the time of rock formation. Middle Devonian sequences were formed at low latitudes (19°–25° N). We calculated the Middle Devonian paleomagnetic pole (Φ =–13°, Λ = 106°, A₉₅ = 7), which can be used to describe the movement of the Caledonian block in Central Asia, and probably Siberia, if these blocks had been tectonically coupled by the Devonian.     

Reversals of the geomagnetic field,magnetostratigraphy, and relative magnitude of paleosecular variation in the phanerozoic

The percentage of normal and reversed magnetization in land-based paleomagnetic studies of Phanerozoic rocks (0 to ? 570 m.y.) have been compiled in order to determine the long-term variation in polarity bias of the geomagnetic field. Where possible the results are compared with the record from marine magnetic anomalies. Only rarely is there an even balance between normal and reversed polarity. During the past 350 m.y. two quiet intervals can be recognized when few reversals occurred, the Cretaceous (KN about ? 81 to ? 110 m.y.) and Permo-Carboniferous (PCR about ? 227 to ? 313 m.y.). Less firmly established are two other quiet intervals, one in the Jurassic (JN about ? 145 to ? 165 m.y.), and one in the Triassic (TRN about ? 205 to ? 220 m.y.). Between these quiet intervals there are disturbed intervals when reversals were comparatively frequent. From ? 680 to ? 350 m.y. the paleomagnetic record is inadequate to delineate a succession of quiet and disturbed intervals although one is probably present. Maximum entropy spectral analysis reveals three periodicities, a dominant one at about 300 m.y. and others, less well-defined, at 113 and 57 m.y. The variations in polarity bias are compared with the paleosecular variation, and it is shown that the magnitude of the paleosecular variation is greater in disturbed than in quiet intervals. This indicates that the magnitude of paleosecular variation and polarity bias are governed by variations in the balance between non-dipole and dipole components of the field, and that these variations probably had their origin in processes near the core—mantle interface. The correspondence between the dominant periods of 300 m.y. and plate tectonics is noted and a causal relationship suggested.     

三门峡地区黄土L9的重磁化现象及原因探析

对黄河三门峡地区曹村黄土剖面L₁至L₁₃(年代持续约1.1Ma)的高分辨率古地磁研究发现:布容/松山界线位于第8层古土壤(S₈)的顶部,贾拉米洛正极性亚时的顶、底界分别位于S₁₀和L₁₃的顶部,上述古地磁极性转换界线与洛川标准剖面一致。“上粉砂层”(L₉)的绝大多数样品特征剩磁方向与现代地磁场方向一致,而其中的弱发育古土壤层(L₉SS₁)则呈预期的反极性特征。综合L₉的厚度、岩性和岩石磁学特征,认为这一巨厚的正极性带可能反映了完全由岩性所控制的重磁化特征。对于L₉重磁化的原因,似乎很难用后生的化学剩磁和粘滞剩磁来解释。文章尝试性地提出,可能在L₉沉积后的间冰期(对应S₈)和布容正极性时内气候较湿润的时期,由于雨水的向下渗透而诱导了其中强磁性矿物(磁铁矿)在沉积后随地磁场方向的变化而进行了重新排列。     

Magnetostratigraphy across the Triassic-Jurassic boundary in the main Karoo Basin

The end-Triassic mass extinction and the transition and explosive diversification of fauna over the Triassic-Jurassic boundary is poorly understood and poorly represented in the rock record of the Southern Hemisphere. This is despite the rich diversity in both body and trace fossils of Triassic-Jurassic age in southern Africa, which is not found in coeval Northern Hemisphere localities. We report here the first palaeomagnetic polarity zonation of the Upper Triassic-Lower Jurassic continental red bed succession (Elliot Formation; Stormberg Group) in southern Africa. The results from 10 partially overlapping sections, with a composite thickness of ~ 280 m, provide a magnetic polarity chronology of the main Karoo Basin in South Africa and Lesotho. Palaeomagnetic analyses reveal that heating samples to between 150 °C and ~ 300 °C removes the secondary, moderately inclined (~ 48°) normal-polarity component of remanent magnetization. This component overlaps with the present-day field and is comparable to the overprint direction expected from Lower Jurassic Karoo dolerite intrusions. In contrast, a likely primary, high unblocking temperature component, of dual polarity, consistently is of steeper inclination (~ 63°). This characteristic remanence passes the reversals test, except where means are based on small sample populations. There are only two resulting polarity zones for the ~ 200 m thick lower Elliot Formation (LEF) with potential for a thin 3rd magnetozone in the uppermost part. The upper Elliot Formation (UEF), in contrast, which was sampled over a thickness of ~ 80 m, has five polarity zones. The failure of the reversal test for the UEF and combined Elliot Formation (LEF + UEF) indicates that the normal polarity samples may be biased by a younger overprint of either the Jurassic normal polarity of the Karoo Large Igneous Province or present day field. The separate poles calculated for the four sites in the LEF and ten sites in the UEF overlap with the Late Triassic and Early to Middle Jurassic Gondwana poles, respectively. The combined Elliot Formation and UEF pole positions are better constrained than the LEF and therefore considered more reliable. Overall the LEF shows considerable overlap with the Late Triassic Apparent Polar Wander Paths (APWP) poles.     

Magnetostratigraphic Study of Meishan Permian-Triassic Section, Changxing, Zhejiang Province, China

INTRODUCTIONBecause many P/ T boundary sections around the worldare stratigraphically unconformed,som e possible exceptionsdeveloped in Greenland,Iran,Russia and South China are ofcourse of international importance.Especially successive sedi-ments from the L ate Paleozoic to Early Mesozoic widely ap-peared in South China,for example,the Meishan Section inChangxing County,Zhejiang Province,and som e analogies inGuangyuan,Wulong and Shangsi counties,Sichuan Province.Some geologists…     

塔里木地块上石炭统的古地磁研究

本文对塔里木地块西北缘柯坪地区的3条上石炭统康克林组剖面进行了古地磁研究。95个定向岩心样品的逐步热退磁和交变退磁揭示出3个磁性组分,第一个是具有低解阻温度谱或低矫顽力谱在现代地磁场作用下形成的粘滞剩磁A(D=10°;I=56°,α95=4°);第二个是红色石英砂岩的具有反极性的高温组分B(D=217°,I=-43°,λ=56°,Φ=184°E;A25=6°详细研究表明,该分量受附近晚二叠世脉岩侵入的重磁化;第三个是灰岩高矫顽力谱的反极性组分C（D=241°、I=-51°,λ=41°N,Φ=160°E,A（?）=4°）。这一分量通讨倾斜检验,在95%概率水平上明显不同于塔里木地块目前已有的石炭纪以后的古地磁方向,它代表了原生剩磁。研究结果表明:（1）晚石炭世塔里木地块已和哈萨克斯坦地块、苏联西天山地块以及西伯利亚地块发生碰撞:（2）苏联西天山地块相对塔里木地块曾逆时针旋转了70°（欧拉极位置λ=8°N,Φ=69°E）,而塔里木地块则相对西伯利亚地块逆时计旋转了21°（欧拉极位置λ=20°N,Φ=34°E）。从该区已有的晚古生代至早三叠世占地磁数据看,这些地块之间的相对旋转运动发生于早三叠世之后。     

Palaeomagnetic Study on the Precambrian-Cambrian Boundary Candidate Stratotype Section at Meishucun, Yunnan

The Meishucun secton has been recommended as an international candidate stratotype secton of thePrecambrian-Cambrian boundary. The paper deals with the palaeomagnetic study on the section. A total of159 palaeomagnetic samples were successively collected from the platform-facies sequence of carbonates andphosphates at the section. Thermal demagnetization results indicate a great majority of the rocks at the sectionhave been strongly overprinted by recent magnetic field, but 57 samples have preserved remanentmagnetization with antipodal directions (mean D/I=4.2°/ 7.1°, K=9, α_(95) = 6.6°). Baaed on calculation,the location of the palaeomagnetic pole was at 68.8°N and 270.7°E, which is different from any palaeopolesobtained from younger Phanerozoic rocks in South China. The results reveal a polarity zonation which in-cludes at least 9 reversal events. A comparison of China's magnetostratigraphic records with those fromSiberia, Australia and the western U.S.A. shows that all the sections are characterized by frequent polarity re-versals.     

塔里木盆地腹地玛扎塔格山隆升时限探讨及其环境意义

位于塔里木盆地腹地(N38°40.911', E80°18.484') 的玛扎塔格褶断带东西向延伸约300km,南缘发育出露连续、完全的早更新世地层,岩性主要以灰黄色泥岩、粉砂质泥岩为主。本文以其中出露完全的一段河湖相泥岩、粉砂岩为研究对象,共采集古地磁样品130块16个采点。通过岩石磁学研究,说明主要载磁矿物是磁铁矿,107块样品的磁化率各向异性结果显示地层校正后其最小轴近直立,并且磁面理较磁线理发育,具有沉积组构特征。地层校正后磁化率各向异性最大轴的方向指示了早更新世时研究区域NWW-SEE的古流向。系统热退磁结果揭示出了正反极性,高分辨率的磁性地层学研究限定研究剖面时代约为1.3~0.9Ma。从而推测玛扎塔格剖面中第四纪地层记录的最近一次构造活动时间约为0.8Ma,即早更新世末中更新世初。河湖相泥岩、粉砂质泥岩的岩性特征代表了当时温湿的气候。研究区近东西向的古流向与玛扎塔格山脊近于平行,附合快速隆升构造地貌特征,暗示该套地层沉积时代(1.3~0.9Ma)对应了玛扎塔格山的快速隆升过程。     

DEFORMATION OF THE EASTERN HIMALAYAN SYNTAXIS: EVIDENCES FROM STRUCTURES AND KINEMATICS OF ITS WESTERN BOUNDARY

DEFORMATION OF THE EASTERN HIMALAYAN SYNTAXIS: EVIDENCES FROM STRUCTURES AND KINEMATICS OF ITS WESTERN BOUNDARYtheNationalNaturalScienceFoundationofChina (grants 49732 10 0and 4980 2 0 2 0 )     

Paleomagnetic and rock magnetic results from lower crustal rocks of IODP Site U1309: Implication for thermal and accretion history of the Atlantis Massif

Integrated Ocean Drilling Program (IODP) Expeditions 304/305 recovered a total of 1.4 km sequence of lower crustal gabbroic and minor ultramafic rocks from the Atlantis Massif oceanic core complex on the western flank of the Mid Atlantic Ridge (MAR) at 30°N. We conducted an integrated paleomagnetic and rock magnetic study on this sequence to help address the interplay between magmatism and detachment faulting. Detailed thermal and alternating-field demagnetization results demonstrate that stable components of magnetization of mainly reversed polarity with unblocking temperatures below the Curie temperature of magnetite are retained in gabbroic rocks at IODP Site U1309. Several samples also contain multicomponent remanences of both normal and reversed polarities that were acquired over sharply defined blocking temperature intervals, providing evidence for localized reheating of some intervals during both normal and reversed polarity periods. Results from a series of rock magnetic measurements corroborate the demagnetization behavior and show that titanomagnetites are the main magnetic carrier rocks recovered at Site U1309D. The overall magnetic inclination of Hole U1309D is -35°, implying significant (up to ~ 50° counterclockwise, viewed to the north) rotation of the footwall around a horizontal axis parallel to the rift axis (010°) may have occurred. The tectonic rotations inferred by the paleomagnetic data suggest that the original fault orientation dipped relative steeply toward the spreading axis and subsequently rotated to a shallower angle. Coupled with the newly published U–Pb zircon ages for Hole U1309D rocks [Grimes, C.B., John, B.E., Wooden, J.L., 2008. Protracted construction of gabbroic crust at a slow-spreading ridge: Constraints from ²⁰⁶Pb/²³⁸U zircon ages from Atlantis Massif and IODP Hole 1309D, (30°N, MAR). Geochem. Geophys. Geosyst. 9, Q08012. doi:1029/2008GC002063], the new paleomagnetic data provide temporal and thermal constraints on the accretion history of the Atlantis Massif.     

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

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

Paleomagnetism and magnetostratigraphy of Upper Cretaceous and Cretaceous-Paleogene boundary intervals,southern Kulunda basin (West Siberia)

The study presents new paleomagnetic data on the Upper Cretaceous and Cretaceous-Paleogene boundary intervals of the southern Kulunda basin (Alei area), which were obtained from core samples collected from a 305-m-thick section penetrated in two wells. The paleomagnetic sections of each well were compiled and correlated based on the characteristic remanent magnetization (ChRM). Paleomagnetic, geological, stratigraphic, and paleontological data were used to compile the Upper Cretaceous and Cretaceous-Paleogene magnetostratigraphic section of the southern Kulunda basin. The magnetostratigraphic section consists of five magnetozones, one normal polarity zone, and four reversed polarity zones spanning the Upper Cretaceous and Lower Paleogene. The lower part of the Gan’kino Horizon, showing normal polarity, forms a single normal polarity magnetozone N. The upper part of the Gan’kino Horizon comprises two reversed polarity magnetozones (R₁km and R₂mt). The Talitsa and Lyulinvor Formations of Lower Paleogene age correspond to two reversed polarity magnetozones (R₁zl and R₂i). The compiled Upper Cretaceous and Lower Paleogene magnetostratigraphic section was correlated with the geomagnetic polarity time scale. Two options were considered for correlating the lower normal polarity part of the section with geomagnetic polarity time scale of Gradstein.     

Late Paleozoic geomagnetic-field estimates from studies of Permian lavas in northeastern Kazakhstan

Paleomagnetic studies of thick lava series are one of the most reliable sources of data on the ancient geomagnetic field. However, most of such data are younger than 5 Ma, with much fewer results on the rest of the Cenozoic and the Mesozoic. Two wholesome results are available for the Precambrian but none for the Paleozoic. Late Permian basalts and rhyolites from northeastern Kazakhstan were studied to obtain first estimates of the geomagnetic-field characteristics during that period. We present preliminary results on part of the collection (66 flows (sites)) from a section ~ 1600 m thick. The characteristic component of reversed polarity was isolated by stepwise demagnetization at all the sites with a slight error. This component is of prefolding age and, most likely, primary. No abnormal magnetization direction is observed in the data, and the average directions of the characteristic component at the sites are tightly clustered (D = 243.3°; I = − 57.0°; k = 79.1; α₉₅ = 2.0°; 65 sites). As compared with the published data on Cenozoic and Mesozoic thick lava series, secular variation was much weaker in the Late Permian than in the Mesozoic or Cenozoic, and the geomagnetic field was less disturbed. Secular-variation models based on the Late Cenozoic data show even more dramatic differences.     

Paleomagnetism of the Late Cretaceous intrusions from the Minusa trough (southern Siberia)

The paper summarizes paleomagnetic and rock-magnetic data on the Late Cretaceous diatremes and associated dikes from the Minusa trough located within the southwestern Siberian Platform. It is shown that the stable characteristic component of magnetization is superimposed magnetization (in physical sense). It is linked to Fe-rich titanomagnetite produced by the decay and oxidation of Ti-rich titanomagnetite derived from a primary magma. This process, however, coincides in time with the intrusion cooling, which is supported by paleomagnetic tests. Correlation of magnetic polarity with ³⁹Ar/⁴⁰Ar ages suggests that the acquired stable characteristic component of magnetization corresponds to magnetic Chrons C33-C32 and characterizes the Middle Campanian magnetic field (74–82 Ma). The mean paleomagnetic pole for this span is located at 82.8° N, 188.5° E, with α₉₅ = 6.1 and, within confidence intervals, coincides with the reference data from the European part of the Eurasian plate. The excellent agreement between virtual paleomagnetic poles testifies that the intraplate motions in the Mesozoic resulting in the crust deformation of Central Asia ceased in the late Cretaceous or were so small that elude detection by the paleomagnetic method.     

The recognition of multiple magmatic events and pre-existing deformation zones in metamorphic rocks as illustrated by CL signatures and numerical modelling: examples from the Ballachulish contact aureole, Scotland

The combination of cathodoluminescence (CL) analysis, temperature and temperature–time calculations, and microstructural numerical modelling offers the possibility to derive the time-resolved evolution of a metamorphic rock. This combination of techniques is applied to a natural laboratory, namely the Ballachulish contact aureole, Scotland. Analysis of the Appin Quartzite reveals that the aureole was produced by two distinct magmatic events and infiltrated by associated fluids. Developing microstructures allow us to divide the aureole into three distinct regions. Region A (0–400?m, 663°C?<?T _max?<?714°C) exhibits a three-stage grain boundary migration (GBM) evolution associated with heating, fluid I and fluid II. GBM in region B (400–700?m, 630°C?<?T _max?<?663°C) is associated with fluid II only. Region C (>700?m of contact, T _max?<?630°C) is characterised by healed intragranular cracks. The combination of CL signature analysis and numerical modelling enables us to recognise whether grain size increase occurred mainly by surface energy-driven grain growth (GG) or strain-induced grain boundary migration (SIGBM). GG and SIGBM result in either straight bands strongly associated with present-day boundaries or highly curved irregular bands that often fill entire grains, respectively. At a temperature of ~620°C, evidence for GBM is observed in the initially dry, largely undeformed quartzite samples. At this temperature, evidence for GG is sparse, whereas at ~663°C, CL signatures typical for GG are commonplace. The grain boundary network approached energy equilibrium in samples that were at least 5?ka above 620°C.     

Neogene rotations of the Chenoua massif,northern Algeria,from remagnetizations

The tectonics of the Chenoua massif suggests block rotations of Neogene nappes associated with the African–European plate convergence. To estimate the extent of these rotations, a Paleomagnetic study on rhyolites and andesites of Langhian–Serravallian age and sandstones of Burdigalian age was carried out on 23 sites (200 specimens). The sites are distributed in the northwestern, southeastern and southern Chenoua massif. One or two components of magnetization, mainly carried by magnetite, pyrrhotite and/or hematite, were isolated in sandstones and volcanics. The sandstone sites reveal magnetizations in sandstones from the Cap Blanc syncline that are post-folding. However, both polarities are found, which is consistent with data from Africa during the Upper Miocene. Clockwise and counterclockwise rotations were recorded, dating back to the Neogene times in volcanics and sediments. From the faulted Cap Blanc syncline counterclockwise rotations of 1?±?4° to 18?±?28° around a vertical axis occurred in sediments since the Miocene with respect to Africa. In fact, remagnetizations occurred at several periods of time and in different sites, providing information on the evolution of post-tectonic rotations. Some volcanics record counterclockwise rotations of about 30° since the Miocene, whereas others do not show any significant rotation. This can be explained by the direction of the principal compressive stress axis σ ₁ and by lateral extrusions related to an indentation model, in which we expect both clockwise and counterclockwise rotations.     

Low-latitude and multiple geomagnetic reversals in the Neoproterozoic Puga cap carbonate, Amazon craton

Palaeomagnetic study of the carbonates that ubiquitously cap glacial deposits may constrain the latitudinal extent of Neoproterozoic glaciations and the duration of the greenhouse recovery. We present the first palaeomagnetic data on the Neoproterozoic cap carbonates covering the Amazon craton, which are folded along the Paraguay Belt. Samples collected at deformed beds along the Paraguay Belt present a single‐polarity secondary magnetization acquired by the end of the Brasiliano orogeny (540–520 Ma). In the cratonic area, a dual‐polarity component was isolated in dolostones at the base of the sequence. The presence of a stratabound reversal stratigraphy along with high unblocking temperatures strongly suggest that this magnetization is primary. This result implies a low palaeolatitude (22+6/?5°) for the Amazon block just after deposition of Puga diamictites. In addition, the presence of multiple reversals across the first 20 m of the cap carbonate sequence suggests that their sedimentation must have spanned hundreds of thousands of years at least.