Search

Curved mountain belts are spectacular natural features that contain crucial 3D information about the tectonic evolution of orogenic systems in the absence of other kinematic markers. The Mesozoic units exposed in the Mexican Fold and Thrust Belt in northeastern Mexico show a striking curvature, whose kinematic history has not been studied. The existing tectonic models of the region simply assumed the shape of the tectonic units as an inherent feature to the orogen. We investigated the kinematic history of this curvature through paleomagnetism and rock magnetism analyses, coupled with an exhaustive review of available published literature. The studied data sets indicate a protracted history of (re)magnetizations that occurred during the Late Jurassic‐Paleocene times at least during the Late Jurassic, Cretaceous and early Eocene. More significantly, they show significant counterclockwise rotations in the northern flank of the curvature and moderate clockwise vertical axis rotations along its southern flank. This data set suggests that the Sierra Madre Oriental was a linear belt that experienced oroclinal bending or buckling during the Cretaceous to early Eocene period (120–50 Ma). Plain Language Summary: The geological history of the American Cordillera is complex due to its extensive geological history. We have investigated a section of this Pacific‐Panthalassa region: the remarkable curved sector of the Mexican Fold and Thrust Belt in northeastern Mexico. This winding area, known as the Sierra Madre Oriental, potentially holds important clues about the evolution of the North American crust. To contribute to a better understanding of this area and to complement the scarce geophysical and geological studies, we used the magnetic properties of rocks and reviewed existing published research to investigate. We've uncovered a complex history of rock magnetizations with at least two main events dating back to the Late Jurassic and Cretaceous periods. Our findings also reveal that these mountain ranges were rotated from their original position on vertical axes, suggesting they were originally linear and were bent or buckled during the period from approximately 120 to 50 million years ago. Key Points: With Paleomagnetism we explore the nature of the striking curvature of the Northeastern section of the Mexican Fold‐and‐Thrust BeltOur study shows evidence of Orocline bending or Buckling during the major horizontal crustal shortening event of the Mesozoic in Mexico [ABSTRACT FROM AUTHOR]

Crustal deformation and hydrothermal percolation related to the India‐Asia collision have caused extensive remagnetization of the Tethyan Himalaya Terrane (THT). The present work identified three phases of regional remagnetization during 62.3–50.0 Ma for the east‐central THT. Consequently, a model of three‐stage India‐Asia collision was proposed. The east‐central THT first collided with the southward migrated southern margin of the Lhasa Terrane (LT) at 5.4 ± 0.9°N during 62.3–60.9 Ma. Subsequently, the THT continuously moved northward and pushed the southern margin of the LT back to its original position prior to the initiation of fore‐arc and back‐arc extension on both sides of the Gangdese magmatic arc. Since the final suturing of the THT with Asia at ∼10°N during 59.8–58.0 Ma, the east‐central THT remained stationary until India collided with it at 10.9 ± 5.1°N at ∼50.0 Ma. Plain Language Summary: The collision of India and Asia caused intense tectonic deformation and hydrothermal alteration throughout the Tethyan Himalaya Terrane (THT), which resulted in the large‐scale remagnetization in the THT. The regional remagnetization of the THT can be used to constrain the India‐Asia collision process, on the premise that the time of remagnetization can be determined. Based on this assumption, we measured two representative Paleocene remagnetized components from Early Jurassic limestones in the Gyangze Basin in the east‐central THT. These remagnetized components, combined with non‐remagnetized components and remagnetization events recorded in the adjacent areas, suggest that the east‐central THT experienced three phases of regional remagnetization during 62.3–50.0 Ma. The first and second phases of remagnetization in the north‐central part of the east‐central THT occurred at the paleolatitude of 5.4 ± 0.9°N at 62.3–60.9 Ma and 10.3 ± 1.0°N–9.5 ± 1.1°N at 59.8–58.0 Ma, respectively. The third phase of remagnetization occurred in the southern part of the east‐central THT, at the paleolatitude of 10.9 ± 5.1°N at ∼50.0 Ma. Consequently, a model of three‐stage India‐Asia collision and southward spreading tectonic deformation of the THT was proposed based on these successive remagnetizations. Key Points: The east‐central Tethyan Himalaya Terrane (THT) experienced three phases of large‐scale remagnetization during 62.3–50.0 MaThe collision of THT with the Lhasa Terrane commenced at 62.3–60.9 Ma and finished at 59.8–58.5 MaIndia finally collided with the THT at the paleolatitude of ∼10.9 ± 5.1°N at ∼50.0 Ma [ABSTRACT FROM AUTHOR]

Paleomagnetic records of middle Neoproterozoic (820 to 780 Ma) rocks display high amplitude directional variations that lead to large discrepancies in paleogeographic reconstructions. Hypotheses to explain these data include rapid true polar wander (TPW), a geomagnetic field geometry that deviates from a predominantly axial dipole field, a hyper‐reversing field (>10 reversals/Ma), and/or undiagnosed remagnetization. To test these hypotheses, we collected 1,057 oriented cores over a 85 m stratigraphic succession in the Laoshanya Formation (Yangjiaping, Hunan, China). High precision U‐Pb dating of two intercalated tuff layers constrain the age of the sediments between 809 and 804 Ma. Thermal demagnetization isolates three magnetization components residing in hematite which are not time‐progressive but conflated throughout the section. All samples possess a north and downward directed component in geographic coordinates at temperatures up to 660°C that is ascribed to a Cretaceous overprint. Two components isolated above 660°C reveal distinct directional clusters: one is interpreted as a depositional remanence, while the other appears to be the result of a mid‐Paleozoic (460 to 420 Ma) remagnetization, which is likely widespread throughout South China. The high‐temperature directions are subtly dependent on lithology; microscopic and rock magnetic analyses identify multiple generations of hematite that vary in concentration and distinguish the magnetization components. A comparison with other middle Neoproterozoic paleomagnetic studies in the region indicates that the sudden changes in paleomagnetic directions, used elsewhere to support the rapid TPW hypothesis (ca. 805 Ma), are better explained by mixtures of primary and remagnetized components, and/or vertical axis rotations. Plain Language Summary: Paleomagnetic directions recorded in 820 to 780 million year old rocks from South China exhibit large amplitude changes that vary rapidly, which have been interpreted to indicate extraordinarily fast motion of Earth's crust and mantle, up to 90° within a 5 million year span, with respect to the spin axis of the core. This hypothetical phenomenon, called rapid true polar wander (TPW), could be responsible for dramatic global environmental change at that time. To test this theory, we collected over 1,000 samples from a well exposed section where the incongruous directions are found. Our measurements suggest that some of the rocks acquired a new magnetic signal during a pervasive remagnetization event in South China around 440 million years ago, long after original deposition of the rocks. New hematite growth has a demagnetization spectrum that partially overlaps or completely obscures the original magnetic signal, which was previously unrecognized. This implies that rapid TPW is likely an artifact of magnetic overprinting in ancient rocks from South China. Our results suggest that South China was in a relatively stable position at high latitudes 809 to 804 million years ago. We find no evidence to support exceptionally fast continental drift or an abnormal geomagnetic field geometry during that time. Key Points: High‐precision ID‐TIMS U‐Pb dating yields deposition ages of the Laoshanya Formation (Yangjiaping, South China) between 809 and 804 MaDiscordant paleomagnetic directions in the red beds are a conflation of three temporally distinct field records residing in hematiteOur results do not support rapid true polar wander or abnormal magnetic field geometry around 805 Ma [ABSTRACT FROM AUTHOR]

Objective This study explores the accuracy of drilling core reorientations by using remanent magnetization. Methods To this end, paleomagnetic analyses were carried out on 43 Silurian sandstone samples collected from five boreholes (TKQ101, SHUN9, TAT19, TZ18, and TS108)in the Tarim Basin. Meanwhile, rock magnetic measurements, scanning electron microscope (SEM) and energy dispersive spectral (EDS) observations were conducted on representative samples to identify the predominant magnetic carriers. Furthermore, the paleocurrent direction inferred from the corrected maximum magnetic susceptibility (Kmax)axis of the anisotropy of magnetic susceptibility (AMS) using remanent magnetization was analyzed. Results AMS results indicate a sedimentary fabric preserved in the studied drilling cores, suggesting their stratigraphy are overall horizontal.Rock magnetic results, SEM and EDS observations reveal that magnetite is the dominant magnetic carrier for the TKQ101 samples, with small amounts of goethite and hematite, while pyrrhotite and magnetite are the dominant magnetic carriers for the other samples. The demagnetization results indicate that the viscous remanent magnetization (VRM) acquired in the present geomagnetic field and the characteristic remanent magnetization(ChRM) of the Silurian formation can be isolated for the TKQ101 samples, where the original azimuth rotations (R, R') estimated by VRM and ChRM are consistent. Furthermore, the paleocurrent direction inferred from the corrected Kmax is supported by the geological evidence, suggesting a counterclockwise rotation of 258.0°-262.0°of the TKQ101 drilling cores. Only one remanence component was isolated for the majority (~90%) of samples from the other four boreholes, which is a superposition component of the VRM acquired in present geomagnetic field and the chemical remagnetization caused by fluid activities, such as oil-gas migration and accumulation, during the Himalayan period. Therefore, it is more reliable to reorient these drill cores by using the VRM component, with confirmation of the paleocurrent direction inferred by the corrected Kmax and geological evidence. Conclusion In summary, to restore the original orientations of these drilling cores, the following rotation angles are required: 258.0°-262.0° counterclockwise rotation for the TKQ101 drill cores; 148.1°, 221.2°, and 318.2° counterclockwise rotation for the 4th, 5th and 6th sections from the borehole SHUN9, respectively; 269.8° and 155.9° counterclockwise rotation for the sections 3 and 5 from the borehole TAT19, respectively; 239.3° and 256.6° counterclockwise rotation for drill cores from the boreholes TS108 and TZ18, respectively.

Abstract—We present the rock-magnetic and palaeomagnetic results of the Sarmatian-Maeotian (Upper Miocene) sediments of the key sections of Cape Popov Kamen and Cape Panagia from Taman Peninsula (Eastern Paratethys) and forming the flanks of a single synclinal fold. A three-component composition of magnetization has been detected. The first is the viscous component A, the second is the component B, which is intermediate between the components A and the third component C, which tends to zero in the Zijderveld diagram. The directions of component B are characterized by northern–northeastern–eastern declinations and positive inclinations in the stratigraphic coordinate system; the fold test for the Popov Kamen and Panagia results is negative, and the magnetization is post-folding. The directions of component C have negative inclinations and southern declinations in the stratigraphic coordinate system. Paleomagnetic results for two sections showed the test fold positive, magnetization preliminary and the magnetization is pre-folding. Components B and C, on the basis of which the polarity of the magnetozones is determined, are of different ages, and the palaeomagnetic record of component B is the result of complete or partial remagnetization, which makes it impossible to use magnetostratigraphy for section dating. Our work affects an important question about the reliability of palaeomagnetic data on the Miocene deposits of the Kerch-Taman region (Eastern Paratethys). [ABSTRACT FROM AUTHOR]

To improve the poor Gondwana paleomagnetic database for Devonian times, detailed paleomagnetic analyses were performed on red chert-like rocks and partly silicified paleosols within the Lower Devonian Ikniouen level (fine-grained sandstones including red ironstone) in conformity within the sub-horizontal Tadrart coarse white formations of the Murzuq basin. Silicification, limited to this level that is only a few meters thick, was probably due to tropical warm climatic conditions during and shortly after the rock deposition. In two sections 40 km away each other, paleomagnetic data point out a high-temperature Characteristic Remanent Magnetization (ChRM) with very well-defined mean direction, positive reversal test and relatively high (5) Q and R scores. Rock magnetic data indicate minerals of the hematite family, but the presence of a minor amount of other mineral phases remains possible. At least part of the ChRMs are Chemical Remanent Magnetizations, likely acquired during or shortly after deposition. The corresponding paleomagnetic results (paleomagnetic pole at 28.6° E and 71.1° S, with K = 1004, A95 = 1.5°) could have major geodynamical implications for the Gondwana supercontinent. In fact, ChRM acquired in this level during or shortly after deposition should imply a much-unexpected fast latitudinal continental drift of the Gondwana during the Lower Devonian or a significant and fast true polar wander. Though much more difficult to match with the ChRM and geological characteristics, the only possible alternative interpretation for the Ikniouen data should be a chemical remagnetization acquired during the Late Cretaceous–Early Paleocene times. [ABSTRACT FROM AUTHOR]

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

Paleomagnetic analysis has been carried out on Late Neoproterozoic dike swarms from two areas along the Red Sea coast: 13 dikes (87 samples) close to the Um Rus gold mine and eight dikes (59 samples) at Sadi Salem. Rock magnetic experiments indicate that the main carrier of magnetization of these dikes is titanomagnetite and/or magnetite. Most samples display a component of magnetization (CA) that is in agreement with previously published Paleo-Late Neoproterozoic poles from other dikes and from one ore complex. But they cluster around the Cenozoic (0–60 Ma) portion of Torsvik's Gondwana apparent polar wander path (APWP) when plotted in southern African coordinates. We suggest two causes of this behavior (remagnetization related to the opening of the Red Sea or later emplacement of the dikes along pre-existing Paleo-Late Neoproterozoic fractures) will have to be tested with new reliable paleomagnetic data on carefully dated rocks. A second component (CB) seen in many of our samples but which is difficult to estimate is in agreement with Late Neoproterozoic granite poles and a dike supposed to be Triassic in age. However, all these poles plot on the Jurassic-Cretaceous portion (80–200 Ma) of Torsvik's path. These results, as well as the Cambro-Carboniferous sediments from Sinai and the Late Neoproterozoic Dokhan volcanic formation that cluster around the Late Paleozoic portion (280–300 Ma) of Torsvik's APWP, are tentatively interpreted as global viscous remagnetizations acquired, respectively, during the Cretaceous Normal and the Kiaman Reverse superchrons. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The purpose of this work is to use paleomagnetic data to determine the tectonic evolution of the inverted High Atlas basin in the area of the Taguelft syncline. This syncline shows two well defined rock types of Jurassic age: marly limestones and red beds, and therefore provides the opportunity of comparing paleomagnetic results from rocks with different magnetic mineralogy. Forty-three sites, in an area of 250 km2 were the subject of a paleomagnetic and rock magnetic study. Both lithologies display a stable interfolding remagnetization, alternatively carried by magnetite in marly-limestones and hematite in red beds. The small circle intersections (SCI) method applied separately to the mean directions of the two rock types indicate a synchronous record of remagnetization, that can be dated as Mid-Cretaceous (100 Ma) by comparing the direction (SCI) with the expected directions obtained from the Global Apparent Wander Path in African coordinates. Small circle techniques were used to reconstruct the syncline geometry at the remagnetization time. Paleodips quantification reveals that this area behaved as an extension-related mini-basin, with strong control by salt migration, before and during the sedimentation of Bathonian red beds. • Application of paleomagnetism to investigate evolution of the Taguelft mini-basin. • NRM of Jurassic marly-limestones and red beds is dominated by a synfolding remagnetization acquired arround 100Ma and pre-dates the atlasic inversion events. • Two stages of folding are revealed from paleomagnetism and structural analysis. • The first stage is extensional and the second one is linked to basin inversion during the Cenozoic. • The paleogeometry of the Taguelft mini-basin is reconstructed using paleomagnetic data. [ABSTRACT FROM AUTHOR]

The western Iberian margin underwent complex evolution during the Mesozoic and Cenozoic. It was subjected to successive extensional and compressional phases, where salt tectonics played a significant role in both. The role of onshore structures during basin formation and inversion is not fully understood, and different chronological relationships between structures have been proposed. To better understand the history of salt body emplacement, we present paleomagnetic and magnetic fabric data from Jurassic carbonates, comprising cover rocks of the diapiric system. Magnetic fabrics show two types of behavior. Type 1 fabrics show magnetic foliation parallel to bedding—the magnetic lineation, after bedding correction, is horizontal and parallel to the diapiric contacts. This fabric is a consequence of overpressure during salt migration, whose effect is limited to the area near the diapirs. Type 2 fabric is characterized by a magnetic lineation parallel to the strike of bedding and—the intermediate and minimum magnetic axes are dispersed on a vertical girdle parallel to the shortening direction. This is interpreted as a compressional fabric that developed during basin inversion. Paleomagnetic results reveal a stable paleomagnetic component carry by magnetite with characteristic unblocking temperatures (175–450 °C) and coercivities (30–100 mT). Twelve of the 14 calculated mean directions show homogeneous behavior in their magnetic properties and directions, with positive inclinations, northward directions, with synfolding to postfolding behavior. We calculated the remagnetization direction (Dec: 347.5°, Inc.: 51.3°) and compare it to the APWP. It was likely acquired during the Cretaceous Normal Superchron, following the extensional deformation phase and prior to basin inversion. The paleodips (i.e., the bedding at remagnetization times) indicate that tilting occurred mainly during the extensional stage, meaning that the diapirs had already been developed. This indicates a strong Mesozoic salt tectonic component in the present-day structuring of this sector of the Lusitanian basin. • Jurassic rocks of the Lusitanian basin record widespread Mesozoic remagnetization. • Paleomagnetic directions are mainly postfolding due to Mesozoic diapiric activity. • Magnetic fabrics of the host rock diapirs are related to salt migration. • Salt migration drove a change in the stress axes due to horizontal overpressure. [ABSTRACT FROM AUTHOR]

Magnetic properties of rocks can be useful for determining paleoenvironmental changes. A dramatic climate change that occurred in the Paleocene-Eocene Thermal Maximum (PETM) modified the environment and, hence, the magnetic properties recorded in the sediments. New paleomagnetic data from marine records of the PETM in the Southern Pyrenean zone displays a variation of the magnetic parameters in four different sections. The magnetic signal reveals a positive excursion of magnetic values starting before the onset of the marly interval of La Faja de las Flores Mb (beginning of the PETM record, Ilerdian) in the Carriata section with a maximum value in the marly interval. A similar magnetic signal is observed in the Bujaruelo section (~10 km south of Carriata at PETM times) that is related directly to the marly interval of Faja de las Flores Mb. However, towards the south, the PETM interval does not appear in the sedimentary record; therefore, in the southern Gallisué section, no magnetic excursion occurs. In the southernmost-studied Entremón section, a positive magnetic excursion occurs in a thin marly interval unrelated to the PETM, and in two lower intervals of the column. All sections were later subjected to deformation during the pyrean orogeny and the three northernmost sections in the regional cleavage front, where pressure solution and remagnetizations have been described. A post-folding remagnetization component is found in the three northern sections.

The secondary (metachronous) magnetization components identified in the Precambrian complexes of the Karelian Craton have been analyzed. The average directions of high-temperature components (deviations from the true direction) have been calculated based on the contribution of secondary magnetization components resulting from tectonomagmatic events of uneven ages. The Precambrian key poles often coincide with the vector sum of the Phanerozoic magnetization components of uneven ages. The conclusion on the primary/secondary origin of the Precambrian paleomagnetic poles should be made on the basis of the integrated petro- and paleomagnetic and isotope data and geological correlations, rather than based on only the paleomagnetic reliability tests. [ABSTRACT FROM AUTHOR]

Abstract Large-scale block migration has been proposed based on Early and early Late Ordovician paleomagnetic data for the South China Block (SCB). However, this is anomalous in terms of the previously reconstructed affinity between the SCB and East Gondwana. A paleomagnetic and petrographic reassessment of the Lower Ordovician sedimentary rocks is therefore necessary to assess the reliability of the Early Ordovician paleopole of the SCB. Consequently, we obtained paleomagnetic data from 47 sites at five localities of the SCB. For most specimens, detailed thermal demagnetization experiments yielded viscous components at ~120 °C, while the stable characteristic remanent magnetization (ChRM) was isolated up to 680 °C. The ChRM passed the fold test and hematite is identified as the remanence carrier. The samples give the site-mean ChRM direction Ds/Is = 313.0°/61.9° (ks = 47.1, α 95 = 3.2°) after tilt correction (five localities, 47 sites). Although the ChRM direction passed the fold test that was likely acquired before the Tertiary, petrographic studies reveal the occurrence of widespread secondary hematite in the specimens, indicating that the rocks were remagnetized after deposition. The average paleomagnetic direction overlaps with the Jurassic paleomagnetic direction from sampling areas of the southwestern SCB, implying that the remagnetization event occurred during the Jurassic. The Early Ordovician paleopole of the SCB is therefore urgently needed for plate reconstruction of the SCB within Gondwana. Highlights • The previously published early Ordovician pole from the Hongshiya Formation of the SCB was remagnetized. • The remagnetization may have occurred in the Jurassic. • The CRM is carried by secondary hematite. [ABSTRACT FROM AUTHOR]

An extremely sharp magnetic reversal observed in lacustrine sediments in central Italy has been interpreted as a record of the Matuyama-Brunhes geomagnetic polarity reversal that may represent less than a decade. Here, we report new results from the same Sulmona Basin outcrops that question this interpretation. In particular, we find evidence of reversed (Matuyama) directions well above the proposed Matuyama-Brunhes Boundary (MBB). Coercivity spectra of anhysteretic remanent magnetization imply a three-component magnetic mineralogy: low-, intermediate- and high-coercivity. The low-coercivity component is found in all but one of the samples and carries a strong modern overprint seen throughout the section. The high-coercivity component is dominated by volcanic material which is prone to remagnetization. Since it is much more magnetic than the surrounding lacustrine sediments, it may influence the remanence signal even when present at very low concentrations. The intermediate-coercivity component is the main carrier of any true primary remanence, but whether or not this can be isolated depends on the blocking temperature and coercivity spectra of individual samples and on the demagnetization method used. The complexity of the magnetization, the reversed zones above the proposed MBB and the normal zones that Sagnotti and colleagues found below it lead to the conclusion that this section does not carry a reliable high-resolution record of the geomagnetic field. Thus, we feel that inferences about the stratigraphic position and duration of the MBB are premature. [ABSTRACT FROM AUTHOR]

Abstract: To reconstruct the paleoposition of the Antarctic Peninsula relative to the South American Plate during the breakup of Gondwana, as well as the opening kinematics of the Drake Passage, we conducted detailed paleomagnetic, rock magnetic, and isotopic chronology studies of Byers Peninsula (Livingston Island) and Fildes Peninsula (King George Island) of the South Shetland Islands. The 40Ar/39Ar ages of the Agate Beach Formation to the Long Hill Formation in Fildes Peninsula range from 56.38 ± 0.2 Ma to 52.42 ± 0.19 Ma. Low natural remanent magnetization/isothermal remanent magnetization ratios, inconsistency with the polarity constrained by the paleomagnetic results and 40Ar/39Ar age constraints, as well as the widespread cation‐deficient titanomagnetite and Ti‐free magnetite of secondary origin, indicate that the volcanic and sedimentary rocks of Fildes Peninsula were remagnetized at about 55 Ma. Combining our results with previous data from the South Shetland Islands and the Antarctic Peninsula, we calculated the paleopoles for 110 Ma and 55 Ma for the South Shetland Islands and the Antarctic Peninsula. The paleomagnetic reconstruction of the relative paleoposition of the Antarctic Peninsula and South America shows that these plates were connected and experienced a southward movement and clockwise rotation from 110 to 55 Ma. Subsequently, southward translation and clockwise rotation of the Antarctic Peninsula between 55 and 27 Ma separated the Antarctic Peninsula and South America, forming the Drake Passage. Northward translation of South America after 27 Ma increased the N‐S divergence and increased the distance between the Antarctic Peninsula and the South American Plate. [ABSTRACT FROM AUTHOR]

Late Eocene-early Oligocene red beds of the Ninglang Formation were sampled from 23 sites in the Yongsheng area, which comprises the northwestern part of the Chuan Dian Fragment (CDF) of the South China Block. The higher temperature components (HTC) were isolated by stepwise thermal demagnetization between ∼300–680 °C and they passed the fold test. However, all the HTCs are of normal polarity, which appears to conflict with the frequent occurrence of reversed polarities during the late Eocene-early Oligocene, and therefore the possibility of remagnetization needs to be considered. Widespread secondary hematite was detected in the red beds that further indicates the remagnetization of samples. From the magmatic–metallogenic events in the sampling area, we propose an early Oligocene (∼35 Ma) remagnetization event that was most likely related to porphyritic intrusions induced by fluid activity. Comparison of the pole calculated from the remagnetized remanent directions with the ∼35 Ma paleomagnetic pole for Eurasia indicated that the degree of clockwise rotation in the Yongsheng area is 17.0 ± 4.1° relative to stable Eurasia. The rotation value is consistent at the 95% confidence level with results obtained from Paleogene and Cretaceous strata in other areas of the CDF. Paleomagnetic data indicate that a consistent clockwise rotation of 20.6 ± 6.3° occurred in different areas in the CDF: at Yongsheng, Zhupeng, Bailu, Dayao, Chuxiong west and Jianchuan. Careful analysis of the paleomagnetic data and the geometrical characteristics of the Ailao Shan–Red River shear zone indicates that the rotation process was separated into two discrete intervals: Approximately 11° of quasi-rigid clockwise rotation occurred between ∼35 and 12.7 Ma compared to stable Eurasia, which may have been accompanied by strike-slip movement of the Ziyun-Luodian fault, caused by west-to-east compression induced by the northeastern indention of the Eastern Himalayan Syntaxis. Subsequently, a clockwise rotation of about 10° of the CDF with respect to stable Eurasia is inferred from the paleomagnetic results since 12.7 Ma, which is consistent with the change in the geometry of the Ailao Shan–Red River shear zone around the western Chuxiong Basin. The later CW rotation was related to shearing movement of the Xianshuihe–Xiaojiang fault. [ABSTRACT FROM AUTHOR]

The Small Circle (SC) methods are founded upon two main starting hypotheses: (i) the analyzed sites were remagnetized contemporarily, acquiring the same paleomagnetic direction. (ii) The deviation of the acquired paleomagnetic signal from its original direction is only due to tilting around the bedding strike and therefore the remagnetization direction must be located on a small circle (SC) whose axis is the strike of bedding and contains the in situ paleomagnetic direction. Therefore, if we analyze several sites (with different bedding strikes) their SCs will intersect in the remagnetization direction. The SC methods have two applications: (1) the Small Circle Intersection (SCI) method is capable of providing adequate approximations to the expected paleomagnetic direction when dealing with synfolding remagnetizations. By comparing the SCI direction with that predicted from an apparent polar wander path, the (re)magnetization can be dated. (2) Once the remagnetization direction is known, the attitude of the beds (at each site) can be restored to the moment of the acquisition of the remagnetization, showing a palinspastic reconstructuion of the structure. Some caveats are necessary under more complex tectonic scenarios, in which SC-based methods can lead to erroneous interpretations. However, the graphical output of the methods tries to avoid ‘black-box’ effects and can minimize misleading interpretations or even help, for example, to identify local or regional vertical axis rotations. In any case, the methods must be used with caution and always considering the knowledge of the tectonic frame. In this paper, some utilities for SCs analysis are automatized by means of a new Python code and a new technique for defining the uncertainty of the solution is proposed. With pySCu the SCs methods can be easily and quickly applied, obtaining firstly a set of text files containing all calculated information and subsequently generating a graphical output on the fly. [ABSTRACT FROM AUTHOR]

Paleomagnetic data may contribute to studying the formation history of orogens; in particular, these data can promote identifying the pattern and scales of deformations at the final stages of orogeny. We have conducted paleomagnetic studies of the Paleozoic and Neoproterozoic sediments in the western part of the Western Ural Megazone in South Urals. The detailed thermal demagnetization revealed the intermediate temperature magnetization component in most samples. This magnetization has a reversed polarity and has been acquired before folding or at the early stages of the deformations. The directions of this component are narrowly grouped in rocks of a different age in all the segments of the studied part of South Urals, and the regional average direction closely agrees with the reference paleomagnetic direction of 270 Ma for the East European Platform. The results of our study suggest the following conclusions: (1) the main magnetization component in the studied sedimentary rocks has a secondary origin; (2) this component has an age of ~270 Ma and has been formed during the Kungur deformations (279-272 Ma ago) of the western part of South Urals; (3) neither a general rotation of the studied part of the Urals relative to the East European Platform nor local rotations of the individual tectonic blocks relative to each other are revealed; (4) the changes in the strike of the structures from NE within the Karatau uplift to the submeridional in the remaining part of the Urals is not an oroclinal bend. [ABSTRACT FROM AUTHOR]

The Caravia-Berbes district is a major fluorite producing area in Europe. The fluorite occurs as mantos replacing breccio-conglomerates of the Middle Permian Caravia Formation and as veins and irregular replacive bodies in the unconformably underlying limestone of the Late Carboniferous Caliza de Montaña Formation. Paleomagnetic analyses were done using alternating field and thermal step demagnetization and saturation remanence methods. Ten sites (82 specimens) in massive and disseminated fluorite ore from the Emilio manto yielded a stable chemical remanent magnetization (CRM) in hematite inclusions with a direction of Decl. 20.1°, Incl. 67.3° (α 95 = 6.1°). After correction for Neogene Pyrenean tilt, the manto's paleoinclination gives a CRM acquisition age of 206 ± 8 Ma that dates a major hydrothermal and ore emplacement event. The age is coeval with the onset of Pangea's break up as the Iberian microplate began to split from the Armorican terrane of Eurasia as part of the ocean-forming CAMP (Central Atlantic Magmatic Province) event. Another 11 sites (109 specimens) of silicified dolostone of the Caliza de Montaña Formation just below the Cueto L'Aspa manto yielded a stable CRM that resides in single and pseudosingle domain magnetite inclusions that has a direction of Decl. 328.7°, Incl. 76.6° (α 95 = 4.9°). After Neogene tilt correction, the altered zone's paleopole gives a CRM acquisition age of 115 ± 3 Ma. This age shows that the western Cantabrian basin also underwent a major hydrothermal alteration and remagnetization event by fluid flow through steep re-activated faults when Iberia rotated 35 ± 2° away from Eurasia during Aptian-Albian time. [ABSTRACT FROM AUTHOR]

The Variscan mountain belt in Iberia defines a large “S” shape with the Cantabrian Orocline in the north and the Central Iberian curve, an alleged orocline belt of opposite curvature, to the south. The Cantabrian Orocline is kinematically well constrained, but the geometry and kinematics of the Central Iberian curve are still controversial. Here, we investigate the kinematics of the Central Iberian curve, which plays an important role in the amalgamation of Pangea since it may have accommodated much of the post-collisional deformation. We have performed a paleomagnetic study on Carboniferous granitoids and Cambrian limestones within the hinge of the curve. Our paleomagnetic and rock magnetic results show a primary magnetization in the granitoids and a widespread Carboniferous remagnetization of the limestones. Syn-kinematic granitoids show ca. 70° counter-clockwise rotations consistent with the southern limb of the Cantabrian Orocline. Post-kinematic granitoids and Cambrian limestones show consistent inclinations but very scattered declinations suggesting that they were magnetized coevally to and after the ~ 70° rotation. Our results show no differential rotations between northern, southern limb and the hinge zone. Therefore, we discard a late Carboniferous oroclinal origin for the Central Iberian curve. [ABSTRACT FROM AUTHOR]

Precambrian paleomagnetic data are crucial for various tectonic and geophysical applications, the reconstructions of fusion and fission of supercontinents in particular. Due to many objective reasons, the variety of stability tests for these data are abridged in comparison to Phanerozoic results and is further hindered by the rather common refusal to consider the resemblance between Precambrian and younger poles. In this paper, we selected Precambrian paleomagnetic data with ages of <1.9 Ga from Fennoscandia, Ukraine and the Uralian margin of Baltica and compared them with Phanerozoic apparent polar wander path for this craton. Our most general finding is that there is a remarkable agreement between Precambrian data and younger segments of the Baltica APWP. We argue that this agreement is more than mere coincidence and may indicate that the remanence in the Precambrian rocks is not primary. [ABSTRACT FROM AUTHOR]

No paleomagnetic data exist for Paleo-Mesoproterozoic times of the West African Craton (WAC). Therefore, paleogeographic reconstructions for such old geological times are difficult to constrain. Gaps on the sedimentary record and intense remagnetizations are the major problems that paleomagnetic studies come across. Recent geochronological results for dyke swarms that intrude several Proterozoic inliers of WAC in the Anti-Atlas Belt (southern Morocco) revealed ages between Paleoproterozoic and early Neoproterozoic, opening for the first time a window of opportunity to conduct paleomagnetic studies and tentatively infer about the paleoposition of WAC during Proterozoic. On this scope we conducted a paleomagnetic study on seven Proterozoic dykes of the Iguerda inlier. The meaning of the obtained paleomagnetic directions was evaluated by rock magnetic and mineral analyses, complemented by petrographic observations. Our samples record the presence of a complex history of remagnetization, mostly assigned to several Phanerozoic thermal/chemical events, in particular to the late stages of Pan African orogeny (s.l.), to the Late Carboniferous Variscan orogeny, and even to more recent events. The recognized remagnetization processes are related to widespread metamorphic events under greenschist facies followed by low-temperature oxidation, both responsible for the formation of new magnetic phases, like magnetite and hematite. These events obliterated the primary (magmatic) thermo-remanent magnetization and promoted multiple remagnetizations of the dykes, thermally and chemically. For only one dyke the presence of primary magnetization is possible to infer, though not to confirm, and would place WAC at an equatorial position around 1750 Ma. [ABSTRACT FROM AUTHOR]

As a resulting of comprehensive studies of Paleoproterozoic igneous complexes a regular remagnetization trend is revealed. It is shown that the remagnetization front propagated from the northwest to southeast. The obtained remagnetization trend probably reflects the direction of the collision in the Paleoproterozoic. Correlation between remagnetization processes and rock compositions was found. The degree of conservation of secondary earlier and later magnetization components is likely associated to both the compositions of protoliths and different transformation conditions of rocks, including their fluid saturation. [ABSTRACT FROM AUTHOR]

The studied area of the Velebit Mt., a part of the Adria microplate, belonged to a NE margin of Gondwana during the Carboniferous and Permian. While the Carboniferous to the Early Permian was characterised by deposition of clastic rocks, younger sedimentation was dominated by a thick sequence of carbonate rocks. The Lower Permian deposits of the core part of the Velebit Mt. at Košna and Crne Grede localities were investigated using palaeomagnetic and rock magnetic measurements. The main remanence carriers were recognized as haematite with an increasing contribution of SP/SD magnetite in younger subsections. The AMS fabric with low anisotropy ratio (1–3%) is strongly oblate at Košna and weakly prolate at Crne Grede, reflecting differences in the contribution of magnetic phases. A significant remagnetization of the Permian rocks, as proved by results of a conglomerate test, probably caused by a combination of elevated temperatures and fluid migration, may be assigned to burial-related processes that affected the rocks before the final uplift of the Dinarides. Characteristic remanent magnetizations recorded in haematite are apparently similar to the Permian direction for Africa (shallow inclination with NNW declination), as expected for Velebit Mt. coordinates. Paradoxically, this orientation is observed in situ within the almost vertically dipping beds. We explain this relationship assuming a syn-folding Cretaceous remagnetization of the rocks at their subhorizontal position (ca. 30°S), in which a mean vector of the secondary remanence overlaps with the Cretaceous direction, expected for Africa at the Velebit Mt. geographical coordinates. Consequently, our results indirectly point to the Cretaceous time of incipient stages of the Dinaric tectonism, and suggest African geotectonic affinity of the Velebit rocks. No important vertical-axis rotation is implied by our results, in contrast to previously published data. The puzzling complete remagnetization carried by haematite at relatively low-to-moderate temperatures is also discussed. [ABSTRACT FROM AUTHOR]

The detailed petro-paleomagnetic study of the Paleoproterozoic eclogite complexes of the White Sea mobile belt in the area of Gridino dike field revealed two stages of rock remagnetization. The Early Paleoproterozoic gabbronorite dikes contain a secondary component of NRM that is pointed NNW with a strong positive slope that is correlated with the post-orogenic collapse of 1.95-1.88 Ga B.P. Formation of the second alternating magnetization phase of rocks is estimated at 1.8-1.75 Ga B.P., which corresponds to the influence of post-orogenic hydrothermal fluids. [ABSTRACT FROM AUTHOR]

The Aït Attab syncline, located in the Central High Atlas, displays a curved geometry in plan view, and is considered as one of the most spectacular fold shapes in the Central High Atlasic belt. We conducted a paleomagnetic study in Jurassic-Cretaceous red beds to investigate the origin of this geometry. The Natural Remanent Magnetization (NRM) is dominated by a secondary magnetization carried by haematite with unvarying normal polarity that has been dated at about 100 Ma. The regional fold test performed in both limbs of the syncline is positive and the paleomagnetic vectors (after tectonic correction) are parallel throughout the curvature, indicating a negative oroclinal bending test. These results are inconsistent with previous works that consider the bent geometry of this syncline to result from subsequent distortion of originally NE–SW trending structures by rotation about a vertical axis. We interpret the NRM data to demonstrate that the changing trend of the Aït Attab syncline is a primary feature, resulting from the influence of pre-existing, NE–SW and E-W-striking extensional faults that developed during a strike-slip regime. Paleomagnetic results also reveal that the tilting observed in the sampled red beds is post Albian, probably linked to the Cenozoic inversion of the High Atlasic belt. [ABSTRACT FROM AUTHOR]

The Early Devonian segment of the Apparent Polar Wander Path (APWP) for Baltica was defined by seven poles almost all from British Islands. Among them only one was obtained for Devonian sediments from Ukraine. The lack of data from other parts of Baltica than Britain makes this segment of the path dubious. To improve this situation we carried out palaeomagnetic study of the Devonian sedimentary sequence in Podolia (Ukraine). The studied rocks represent grey limestone and red beds and were sampled in seven outcrops in the basin of the Dniester River. The standard palaeomagnetic experiments reveal Permian remagnetization corresponding to the pole position: 47°S/351.5°E for red beds and 45°S/340°E for grey limestone. The second component corresponding to the nearly Devonian pole position (2.3°S/338.4°E for red beds and 0°S/329°E for grey limestone) was determined as primary based on the fact that they were recognized in different sediments--limestone and sandstone and carried by different magnetic minerals: multidomain (MD) magnetite in grey limestone, and detritic grains composed of primary hematite with ilmenite intergrowths in red beds. The fabric of anisotropy of magnetic susceptibility of red sandstone and these samples of grey limestone which preserved Devonian magnetization are typical for sedimentary structure. The preservation of primary component of natural remanence was supported by analysis of mechanism of remagnetization and acquisition of secondary components. Hysteresis properties indicate that red beds underwent chemical remagnetization whereas grey limestone were not altered. Variscan orogeny caused the elevated heat and fluid flows along the edge of the Trans European Suture Zone and deep burial of Podolia. We suppose that red beds underwent chemical remagnetization caused by fluid flows, whereas grey limestone could acquire thermoviscous remanence due to elevated temperature caused by burial. Hot fluids could easily penetrate into sandstone and cause chemical alteration and creation of new iron oxides. As detrital hematite is chemically very stable it could preserve original magnetization. As grey limestone cemented at the beginning of digenesis was not penetrable by fluids during burial their remagnetization was caused mainly by increase of temperature and was not accompanied by alteration of rock. MD magnetite present in grey limestone could easily acquire thermoviscous magnetization. The remagnetization pattern is in agreement with the thermal history of Lower Palaeozoic rocks from Podolia. Maximum palaeotemperatures during burial reached 200 °C. The age of maximum palaeotemperature is about 300 Ma in excellent agreement with the position of poles calculated from remagnetization on APWP. [ABSTRACT FROM AUTHOR]

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 and by lateral extrusions related to an indentation model, in which we expect both clockwise and counterclockwise rotations. [ABSTRACT FROM AUTHOR]

Eighty-one paleomagnetic cores were collected from 10 locations across a black limestone unit within the core of Negash Synclinorium, northern Ethiopia in order to test the proposed Snowball Earth events for the diamictite unit of the Tambien Group. Cores were cut into two standard paleomagnetic specimens and were subjected to stepwise demagnetizations using both Thermal (TH) and alternating field (AF) techniques. Rock magnetic analyses on representative specimens were done and results revealed goethite, pyrrhotite, titano-magnetite, and titano-hematite to be the major magnetic materials carrying the magnetizations with PSD (pseudo single domain) grain size range. In most cases paleomagnetic directions are defined by a single component of magnetization, where a viscous component is present it is usually removed by heating to a temperature of ∼200 °C or an AF of ∼10 mT. The high stability component isolated above temperature of 200 °C or AF of 15 mT, defined straight line trajectories directed towards the origin and considered as the Characteristic Remanent Magnetization Direction (ChRM). The direction of magnetization of the ChRM is determined for samples with stable straight line segments by the best-fit line using the least square technique of Kirschvink (1980). In the cases of overlapping spectra and unblocking temperatures, direction of magnetization is determined by remagnetization circles of Halls (1976, 1978). When site mean ChRM directions are plotted on stereogram, their distribution is relatively clustered in geographic coordinates and the overall mean direction is Dec g = 358.5°, Inc g = 16.6° ( α 95 = 3.8°, K = 162.8, N = 10). After a structural restoration to the horizontal is made the directions disperse and fail the fold test of both McElhinny’s and McFadden’s tests and the mean direction is Dec s = 353.5°, Inc s = 8.8° ( α 95 = 18.9°, K = 7.5, N = 10). This is interpreted to result from a later remagnetization of the black limestone. All directions are normal in polarity and have mean unrestored paleomagnetic directions comparable to the Quaternary paleomagnetic directions. Virtual Geomagnetic poles (VGP) in the unrestored position is used to calculate overall mean VGP position resulting lon g = 235.7°E, lat g = 84.5°N ( A 95 = 3.0°, N = 10). Comparison of the obtained pole with the apparent polar wander path (APWP) curve for Africa of Besse and Courtillot (1991, 2003) and with the 2 Ma reference pole of stable Africa (Kidane et al., 2003) is found to be consistent with remagnetizations during the Quaternary period. Hence supporting evidence for the proposed Snowball Earth event of the Sturtian glaciation in the Negash rocks could not, unfortunately, be obtained from paleomagnetism. [ABSTRACT FROM AUTHOR]

Haematite pigment is a common constituent of sedimentary rocks, but its contribution to the natural remanent magnetization of rocks is poorly understood. Here, we describe magnetic properties of two distinct pigment types that produce a characteristic decorative ‘print stone’ found in the ∼2.5 Ga Mount McRae Shale Formation, Hamersley Province, Western Australia. Distinct magnetic remanence directions observed in the Print Stone can be correlated to each pigment type. By comparison with the Australian apparent polar wander path, the remanence carried by uniformly distributed pigment can be dated to ∼15–25 Ma, while two age options, the Mesoproterozoic (∼1.5 Ga) or the middle Carboniferous (∼320–310 Ma), are permissible for the remanence carried by the pigment responsible for the distinctive ‘newsprint’ pattern. Magnetic properties and demagnetization characteristics of the different pigment types overlap significantly, and thus are not predictive of the dominant remanence carrier. Magnetic characteristics of the uniformly distributed pigment vary significantly on short spatial scales. Strong local control on pigment formation raises the possibility that a primary remanent magnetization may survive locally in pockets within sedimentary red bed formations. [ABSTRACT FROM AUTHOR]

We report on a palaeomagnetic study from Mesozoic sedimentary and volcanic rocks from the conjugate areas of the Western Black Sea Basin; that is, the Crimean Peninsula in the north and the Western and Central Pontides in the south, to better constrain their palaeogeographic relationships within the southern margin of Eurasia.From the study of 87 sites in Crimea, we found that Triassic to Lower Jurassic sandstones and siltstones from the Tavric series, and Middle–Upper Jurassic sandstones, siltstones and limestones exhibit remagnetization. Both fold and conglomerate tests confirm a widespread remagnetization in Crimea. Comparison of palaeopoles with the expected reference apparent polar wander path (APWP) of Eurasia and results from conglomerate tests suggest that the remagnetization occurred in the Early Cretaceous. In the Central Pontides, no reliable palaeomagnetic results can be obtained from Triassic–Upper Jurassic rocks, however, a negative fold test in Upper Jurassic–Lower Cretaceous rocks from the Western Pontides shows that the palaeolatitude agrees with Lower Cretaceous data from Crimea. Our new palaeomagnetic results indicate a pervasive remagnetization in Crimea and the Western Pontides that could be attributed to the rifting phase of the Black Sea Basin during Lower Cretaceous. [ABSTRACT FROM AUTHOR]

This study revisits the palaeomagnetism of the Sines massif (∼76 Ma) in the southwestern Iberian Margin (Portugal). The palaeomagnetic analysis was complemented by a comprehensive study of the magnetic mineralogy by means of rock magnetic measurements and petrographic observations. The overall dispersion of palaeomagnetic directions (declination ranging between ∼N0° and ∼N50°) and their migration observed during stepwise demagnetizations have revealed the superposition of remanence components. We interpret this complex palaeomagnetic behaviour as related to the regional hydrothermalism associated with the last stages of Late Cretaceous magmatic activity. This environment favoured mineralogical alteration and a partial chemical remagnetization, giving in most samples a composite magnetization, which has been erroneously interpreted as the primary one in a previous study, then leading to a questionable model for Cretaceous Iberia rotation. Nonetheless, for some samples a single component has been isolated. Interesting rock magnetic properties and microscopic observations point to a well-preserved magnetic mineralogy for these samples, with magnetite clearly of primary origin. The associated ChRM mean direction (D/I = 3.9°/46.5°, α95 = 1.7°, N = 31 samples) then represents the true primary magnetization of the Sines massif. This new palaeomagnetic direction and the corresponding palaeomagnetic pole (long = 332.0°, lat = −79.5°, A95 = 1.7°) agrees with those from the other palaeomagnetic works for the same period and region (e.g. the Sintra and Monchique massifs), yielding a lack of significant rotation of Iberia relative to stable Europe since the uppermost Late Cretaceous (Campanian–Maastrichtian). [ABSTRACT FROM AUTHOR]

New reliable palaeomagnetic data from the Siberian platform help in deciphering its palaeogeography during ∼450–400 Ma. Geochronology of late Devonian mafic sills provides time constraints for tectonic deformation along the southern margin of the Siberian Platform and thus a minimum age for the regional magnetic overprint. From a late Ordovician–Silurian sedimentary section of the Nyuya syncline in the southern part of the Siberian platform with the Devonian sills hosted nearby in early Palaeozoic sediments, pre-folding presumably primary magnetization was isolated from the sediments during stepwise thermal cleaning. High unblocking temperatures imply that haematite is the main carrier of magnetization. The sample-mean direction for 37 Ordovician samples from nine sites in stratigraphic coordinates is Ds = 168.5, Is = −5, ks = 22.3 and α95 = 5.1 and for 77 Silurian samples from six sites is Ds = 193.9, Is = 20.9, ks = 16 and α95 = 4.2. Another component recorded in both the Silurian and Ordovician samples is pre-folding, with a sample-mean direction of Ds = 204.4, Is = 37.9, ks = 27.2 and α95 = 2.7 for 104 samples from eight sites. This component was probably formed during a regional remagnetization event, which took place in post-early Silurian time. Putting this secondary component into a framework with available Palaeozoic data and geochronology further constrains its age to be early Devonian.The isolated components yield new late Ordovician, early Silurian and early-middle Devonian palaeomagnetic poles. The revised middle Palaeozoic segment of the apparent polar wander path (APWP) for the Siberian platform provides new palaeogeographic constraints. Our data suggest that in late Ordovician the platform was situated in equatorial latitudes and was rotated 180° with respect to its present position. During middle-late Ordovician time, the platform did not experience any notable latitudinal drift. It started drifting to the north in the late Ordovician, and by the late Silurian it had travelled ∼1500 km northwards and had rotated ∼30° counter-clockwise (CCW). During late Silurian time, the platform continued northward drift and CCW rotation, and by the early Devonian it had drifted ∼1100 km northwards and rotated 10° CCW relative to its Silurian position. After that, the rotation of the platform changed to clockwise (CW), and by the late Devonian had drifted another 1500 km to the north and had rotated ∼60° CW.We evaluated palaeomagnetically viable positions from 450–400 Ma of the three largest Laurasian cratons, Siberia, Baltica and Laurentia, based on the new data and previously published APWPs. Contrary to several published reconstructions, the Siberian platform could not have been situated to the north of the Caledonian suture in mid-Silurian time, but was probably located either at the eastern or the western side of Laurussia. The new data are compatible with an early Devonian position of Siberia similar to the modern Eurasian configuration. They also support the post- early or middle Devonian relative rotation between the Aldan and Angara blocks of the Siberian platform. [ABSTRACT FROM AUTHOR]

The new paleomagnetic data on forty dikes and two intrusive plutons of Devonian age located in different parts of the Kola Peninsula, which have not been previously covered by systematic paleomagnetic studies, are reported. We describe the results of the rock magnetic, petrographic, and microprobe investigations of the Devonian dikes and present their isotopic ages (Ar/Ar, stepwise heating). Within the studied area, almost all the Devonian dikes, metamorphic Archaean-Proterozoic complexes of the Fennoscandian Shield, and Proterozoic dikes have undergone low-temperature hydrothermal-metasomatic alteration, which resulted in the formation of new magnetic minerals with a secondary (chemical) component of magnetization. The comparison of the paleomagnetic poles indicates the Early Jurassic age of the secondary component. We suggest that regional remagnetization event was caused by endogenic activity genetically related to the formation of the Barents Sea trap province 200-170 Ma ago. On the basis of the obtained data, the preliminary Devonian paleomagnetic pole of the East European Platform is determined. [ABSTRACT FROM AUTHOR]

Abstract: The Late Ediacaran to Cambrian Sierras Bayas Group (Villa Mónica, Cerro Largo, Olavarría and Loma Negra Formations) and the Cerro Negro Formation, exposed along the Tandilia system in the province of Buenos Aires (Argentina) were revisited and studied paleomagnetically. Our results supersede those of Valencio et al. (1980) for the La Tinta Formation (old stratigraphic name of these units). Three hundred and twenty-eight samples were collected from forty-four sites in gently folded to subhorizontal strata distributed along the whole stratigraphic succession. Detailed paleomagnetic study comprised systematic stepwise demagnetization by both AF and thermal methods, the latter being generally the most effective in isolating the characteristic remanence. Different magnetic components were defined from different units of the succession. Besides a recent, probably viscous, secondary component (component A), the most widespread magnetic remanence (component B) is a dual-polarity post-tectonic secondary remanence. This component, carried by both hematite and magnetite, corresponds to that originally determined by Valencio et al. (1980) and previously interpreted as primary. This component found in all carbonatic rocks of Villa Mónica and Loma Negra Formations as well as in several claystones and siltstones of the Olavarría Formation do not pass conglomerate and regional tilt tests. The mean in situ direction of component B is Dec: 359.8°, Inc: −63.3°, n: 85 samples, k: 24, α95: 3.2° and yields a paleomagnetic pole virtually identical to the previous one of Valencio and colleagues. It also matches those recently determined from secondary magnetizations in carbonatic and clastic Ediacaran units exposed in Uruguay. The pole positions suggest a Late Permian–Triassic age as the more likely for the acquisition of component B and reveal the presence of a widespread remagnetization event that affected very large areas of the Rio de la Plata craton. Despite this widespread event, some clastic units (claystones, marls) apparently escaped remagnetization. A pre-tectonic, dual polarity, mean remanence (Dec: 28.7°, Inc: 56.1°, n: 17 samples, k: 15, α95: 9.5°) was isolated from the latest Ediacaran–Early Cambrian Cerro Negro Formation (component C). In addition, the Ediacaran Olavarria Formation recorded another apparently ancient remanence, although no field test is available. Its direction (component D) is at Dec: 350.9°, Inc: 47.3°, n: 13 samples, k: 37, α95: 7.0°. Siltstones and claystones of the Ediacaran Cerro Largo Formation were carriers of a characteristic remanence (component E) that shows a better directional grouping after bedding correction, although the field test is not statistically significant, and yield a mean corrected direction at: Dec: 73.7°, Inc: −36.6°, n: 11 samples, k: 15, α95: 12.1°. Finally, a purple horizon of marls on top of the Villa Mónica Formation associated with weathering processes before deposition of the Colombo diamictite, was carrier of a characteristic remanence that attained a better grouping after bedding correction, but again with no statistical significance. This direction (component F) was at Dec: 43.4°, Inc: −36.3°, n: 7 samples, k: 45, α95: 9.1°. Components C–F are interpreted as ancient magnetizations associated either to post-depositional or early to late diagenesis. Mean geomagnetic poles computed from these components fall on the apparent polar wander path for the Rio de la Plata craton from around 600 to 520Ma, in a correct stratigraphic order and with ages consistent with the most likely ages (or slightly younger) of the different sampled units. These results confirm the already proposed Ediacaran to Cambrian APWP for the Rio de la Plata craton, indicating that it remained at intermediate to low latitudes during most of the Ediacaran. Comparison with coeval paleomagnetic poles from other cratons indicate that by 575Ma the Rio de la Plata and Congo-Sao Francisco cratons were likely a single plate. It also strongly argues against the generally accepted model that the Rio de la Plata craton was part of the conjugate margin of Eastern Laurentia during the final stages of Rodinia break-up at around 580Ma. [Copyright &y& Elsevier]

Paleomagnetic records show very often that the directions of normal and reverse polarity are not quite antipodal, as one would expect for an axial geocentric dipole model. Specifically, in magnetostratigraphic studies of Tertiary sediments, which include large data sets of paleomagnetic directions of both polarities, it is commonly observed that reverse polarity inclinations are shallower than those of normal polarity. Such an inclination anomaly has important implications for both paleomagnetism and paleogeographic reconstructions. We investigate the conventional explanation for such asymmetry, which involves the presence of a persistent, partially unremoved present day field magnetization in the studied rocks. Both plate motion history and inclination shallowing, which we evaluate in detail, can play an important role in the paleomagnetic record. Our analysis shows that alternatively, it is plausible that the inclination anomaly is due to a contribution of an axial octupole, in accordance with observations of the recent (0–5 Ma) geomagnetic field recorded in lava flows. [ABSTRACT FROM PUBLISHER]

SUMMARY The Howards Pass district is located in the Selwyn Basin, Yukon. The district consists of >15 laminated clastic-dominated (CD) sphalerite-galena (Zn-Pb) deposits in the 'zinc corridor' that trends northwest-southeast and extends for ∼35 km. The stratiform mineralized zone, the Active Member, is hosted in carbonaceous cherts and black shales of the Early Silurian Road River Group. Using mostly thermal and then alternating field step demagnetization isolated a stable characteristic remanent magnetization (ChRM) by the core-magnetization-angle method from 18 sites (339 specimens) in 18 variably oriented exploration drill hole cores from 6 Zn-Pb mineralized panels. Rock magnetic analyses show that the main remanence carriers are single- or pseudosingle-domain pyrrhotite and titanomagnetite. The deposits' mean ChRM direction yields a pole position of either ∼170 Ma on the North American apparent polar wander path or ∼162 Ma on a corrected path for the Intermontane Belt (IMB) terranes. A negative palaeomagnetic fold test indicates that the mineralization's ChRM is post-folding, setting a minimum age for regional metamorphic deformation of ∼170 ± 20 Ma and supporting an Early Jurassic arrival for the IMB's collision and accretion to North America. An autochthonous or para-autochthonous North American tectonic model is favoured for the ChRM of the metamorphosed Selwyn Basin strata rather than an allochthonous IMB model. Further, the palaeomagnetic age indicates that the coarse-grained Zn-Pb mineralization in fine fractures that cut the laminated fine-grained Zn-Pb mineralization at Howards Pass at least was formed by remobilization during Middle Jurassic metamorphism. [ABSTRACT FROM AUTHOR]

SUMMARY Middle Triassic (Anisian) red bedded cherts were collected from the Ajiro area in Kyushu Island, southwest Japan, for a palaeomagnetic study. Thermal demagnetization showed five distinct remanent magnetization components from the cherts. The lowest temperature component below 200 °C (component A) is a present-day viscous overprint. The second- (up to 420 °C, component B) and third-demagnetized (up to 580 °C, component C) components are secondary magnetizations of thermoviscous and chemical origin, respectively, both of which reside in magnetite. The fourth-removed component (component D) was demagnetized at 630 °C and is a secondary magnetization carried by haematite. Components B and C are considered to be a result of widespread remagnetization in southwest Japan during the Cretaceous; the basis of this consideration is that red chert remagnetizations from an area about 600 km away from the present study area, are of the same origins with unblocking spectra similar to those observed in this study. The last-removed component during thermal demagnetization (up to 695 °C, component E) is a primary magnetization carried by haematite. The directions of this component, after correction for the inclination flattening, yielded a mean palaeolatitude of 2°S at the time of deposition in the Middle Triassic. The Ajiro cherts accreted as part of an accretionary complex to the eastern margin of the South China Block during the Middle Jurassic and were then translated northwards from the continental margin to their present position through a margin-parallel translation process owing to the oblique convergence of the oceanic plate. [ABSTRACT FROM AUTHOR]

Abstract: A detailed rock magnetic and paleomagnetic study was performed on samples from the Neoproterozoic Itajaí Basin in the state of Santa Catarina, Brazil, in order to better constrain the paleogeographic evolution of the Rio de la Plata craton between 600 and 550Ma. However, rock magnetic properties typical of remagnetized rocks and negative response in the fold test indicated that these rocks carried a secondary chemical remanent magnetization. After detailed AF and thermal cleaning, almost all samples showed a normal polarity characteristic remanent magnetization component close to the present geomagnetic field. The main magnetic carriers are magnetite and hematite, probably of authigenic origin. The mean paleomagnetic pole of the Itajaí Basin is located at Plat=−84°, Plong=97.5° (A95=2°) and overlaps the lower Cretaceous segment of the apparent polar wander path of South America, suggesting a cause and effect with the opening of the South Atlantic Ocean. A compilation of remagnetized paleomagnetic poles from South America is presented that highlights the superposition of several large-scale remagnetization events between the Cambrian and the Cretaceous. It is suggested that some paleomagnetic poles used to calibrate the APWP of Gondwana at Precambrian times need to be revised; the indication of remagnetized areas in southern South America may offer some help in the selection of sites for future paleomagnetic investigations in Precambrian rocks. [Copyright &y& Elsevier]

Abstract: The origin of the Triassic–Jurassic (Tr–J) mass extinction is still a matter of debate: proponents of the idea that continental flood basalts of the Central Atlantic Magmatic Province (CAMP) are responsible for the crisis are opposed by those who favor an extraterrestrial origin linked to the impact of meteorite. Principal limitations reside in the difficulty to date and correlate CAMP lavas with the marine realm turnover. One argument widely used to suggest that CAMP lavas pre-dated the Tr–J boundary in Morocco is based on the presence of two brief magnetic reversals in the intermediate units of the Tiourjdal and Oued Lahr sections (Morocco) that were correlated to the E23r chron from the Newark basin and to the SA5n.2r/3r and SA5r chrons of the Saint Audrie Bay [Knight, K.B., Nomade, S., Renne, P.R., Marzoli, A., Betrand, H., Youbi, N., 2004. The Central Atlantic Magmatic Province at the Triassic–Jurassic boundary: paleomagnetic and 40Ar/30Ar evidence from Morocco for brief, episodic volcanism. Earth and Planetary Science Letters 228, 143–160]. However the primary origin for these negative (reverse) magnetic components is questionable since no field or reversal test was provided to constrain the primary character of the remanence as well as because the small number of samples. Here we have conducted a detailed paleomagnetic and magnetic mineralogy study of the interbedded limestones of the Tiourjdal section and of other CAMP lavas sections where the intermediate unit is complete, namely the Tizi El Hajaj, Jbel Imzar and Aït Ourir sections, to better constrain the origin and stratigraphic location of these negative magnetic components. We show that the interbedded limestones of the Tiourjdal section were entirely remagnetized by chemical processes via acid and oxidizing hydrothermal fluids generated by eruptions of CAMP lavas. In addition, magnetostratigraphic data of the Tizi El Hajaj, Jbel Imzar and Aït Ourir sections show that the entire intermediate unit encompassed a positive (normal) magnetic interval. A good quality paleomagnetic pole for the CAMP lava in Morocco is then provided (Plat=60.0°; Plong=241.6°; A95=2.6; N=99) that is now in better agreement with its trans-Atlantic counterpart. [Copyright &y& Elsevier]

Abstract: Chinese loess during the Quaternary is important for both paleomagnetic and paleoclimatic studies. Although magnetostratigraphy has been used for the initial loess timescale, paleomagnetic records in loess still remain in debates, such as magnetic polarity anomalies in L9, which is a thick and coarse sandy horizon as a typical marker bed. To better understand the magnetization mechanism of these anomalies, a new age model for L9 was first constructed based on a new correlation between loess and marine δ18O, which proposed that L9 should be correlated to marine oxygen stage (MIS) 20–24, not 22–24 as previous suggested. Using combined grain size and environmental magnetism parameters, an extended climatic optimum corresponding to MIS 21 can be identified in middle of L9. A remagnetization mechanism of viscous remanent magnetization overprinting on characteristic remanent magnetization (ChRM) carriers (mainly large pseudo single domain and multidomain magnetite particles), associated with the chaotic detrital remanent magnetization caused by low efficiency alignment of ChRM carriers along with the ambient field during deposition, is proposed to interpret the magnetic anomalies. This study provides new insights into the magnetization mechanism of the Chinese loess, and the new age model for L9 helps to accurately refine the finer scale correlation of paleoclimatic records between the Chinese loess and marine sediments. [Copyright &y& Elsevier]

Abstract: The Vrica section in Calabria, southern Italy, was the global stratotype for the Pliocene–Pleistocene boundary until this boundary was redefined in 2009. Several paleomagnetic investigations have been carried out at Vrica to determine the age of the formerly defined Pliocene–Pleistocene boundary, which was a key calibration point for the astronomical polarity timescale (APTS). Each study has documented a complex polarity pattern at and above the top of the Olduvai subchron and in relation to the existence of the so-called Vrica subchron. When constructing the APTS, two alternative interpretations for the Vrica section were proposed, neither of which could be conclusively supported. Authigenic growth of magnetic iron sulphide minerals was proposed to explain the complex magnetic polarity record. Availability of a fresh 50-m sediment core enabled us to test this possibility. Our magnetostratigraphic record is similar to that of previous studies, but it is also complex above the Olduvai subchron. We confirm abundant occurrences of authigenic greigite and pyrrhotite, along with detrital titanomagnetite. Authigenic monoclinic pyrrhotite indicates growth significantly later than deposition, and greigite can grow at any time during diagenesis, depending on the availability of dissolved iron and sulphide. The spatially variable magnetic polarity pattern at Vrica is therefore interpreted to have resulted from post-depositional magnetic iron sulphide formation at variable times. Tectonism along the Calabrian arc provides a plausible mechanism for forcing reducing fluids through the sediments, thereby supplying the dissolved ions needed to produce late diagenetic sulphide growth and remagnetization. The complex magnetostratigraphic record at Vrica was taken into account when the APTS was developed, and alternative interpretations result in a maximum age difference of 50kyr for the upper Olduvai reversal. Our results therefore do not undermine the APTS. Rather, they explain the complex magnetic polarity pattern at this globally important location and highlight the importance of remagnetization processes in such sediments. [Copyright &y& Elsevier]