Remanent Magnetization Directions Research Articles

The importance of multidisciplinary dating in rescue excavations: The case of Santhià, Northern Italy

We present the results of a combined archaeomagnetic and thermoluminescence (TL) investigation on a baked clay kiln discovered near Santhià, Northern Italy. The site, brought to light during a rescue excavation for the installation of methane tubes, provides important evidence for the presence of a brick production workshop in the area. Standard thermal demagnetization was applied to isolate the direction of the Characteristic Remanent Magnetization (ChRM). The mean ChRM direction was used for the archaeomagnetic dating of the kiln, suggesting that it was abandoned around the 16th century CE. TL analysis carried out on the same kiln further confirms this dating. The comparison between the archaeomagnetic and TL results shows excellent agreement, underlining the great potential of these two techniques in dating baked clays. Moreover, thanks to the independent TL dating, the new archaeomagnetic data can be used as a reference point for the reconstruction of the past secular variation path in Italy. Such combined multidisciplinary dating is extremely important in the case of rescue excavations, particularly in urban centers where the rapid expansion of new constructions inevitably causes the destruction of our cultural heritage and the loss of important information about our past.

Review of magmatic iron-ore mineralization in central-western Mexico: Rock-magnetism and magnetic anomaly modelling of Las Truchas, case study

Las Truchas deposits formed by contact metasomatism in a Mesozoic volcano-sedimentary sequence intruded by a batholith. Later, post-mineralization hydrothermal alteration affected minerals and magnetic properties. For this study, we sampled the host rocks and iron ores. The mineralogy and magnetic domain states of oxide minerals were characterized by reflected light and scanning electron microscopy together with rock magnetic experiments such as thermomagnetic measurements, frequency dependence of magnetic susceptibility, hysteresis, first-order-reversal-curves and UnMix curves.Qualitative and quantitative forward modelling of the subsurface structure was inferred by processing of magnetic anomaly maps. Induced and remanent magnetization (intensity and direction) and the Königsberger ratio were used as input parameters for the quantitative modelling process to adjust our calculated anomaly with the observed magnetic anomalies. We attempt to understand the geological factors that control the magnetic signatures, which can be used to improve the geological interpretation of magnetic studies.

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

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

Multistep Parametric Inversion of Scanning Magnetic Microscopy Data for Modeling Magnetization of Multidomain Magnetite

AbstractDevelopment in instrumentation and technology now allows for mapping of magnetic anomalies, caused by spatial variations in magnetization in the source material, from the mineral to the crustal anomalies scale. High‐resolution magnetic mapping techniques allow for accurate investigation of the magnetization in natural rock samples and particularly of their remanence carriers, which can record geologically meaningful information. Multidomain magnetic grains are expected to retain a remanence that is susceptible to change by exposure to magnetic fields or by changes in temperature. Although this makes multidomain grains less reliable remanence carriers for paleomagnetic studies, their magnetization contributes to rocks bulk magnetization and to induced anomalies in the Earth crust. Here, we investigate the fine‐scale magnetization of a sample that exhibits a multidomain behavior. We used a scanning magnetic microscope, equipped with a room temperature magnetic tunnel junction sensor, to map the magnetic field over a petrographic thin section of the sample containing large magnetite grains (>100 μm) surrounded by serpentine and carbonate. We modeled the fine‐scale remanent magnetization of the magnetite by inverting the magnetic scans data acquired in near‐field‐free conditions. We applied a multistep inversion, a priori tested on a synthetic model, with a controlled range on the intensity of the magnetization. Modeling results on the study sample suggest homogenously magnetized regions within the magnetite grains with variable remanent magnetization intensities and directions coherent with the multidomain behavior inferred from bulk measurements.

Eruption age and duration of the ∼9 km3 Burney Mountain dacite dome complex, northern California, USA

Abstract At ∼9 km3, the six dacite domes (db1–db6) of Burney Mountain, northern California, USA, constitute the most voluminous Quaternary dome complex in the Cascades volcanic arc. Whole-rock geochemistry, electron microprobe, and petrographic data indicate that the domes are magmatically related, which when integrated with geomorphology and stratigraphic superposition, indicate early (db1, db2, and db3) and late (db4, db5, and db6) erupted groups. We present 40Ar/39Ar ages of 271.9 ± 4.6 ka (db1), 280.8 ± 8.2 and 281.7 ± 6.8 ka (db2), and 290.2 ± 6.0 ka (db3) along with a previous age of 280 ± 12 ka (db1). These ages scatter over 20 k.y., whereas remanent magnetic directions are similar between 53.3–59.0° inclination and 352.7–355.9° declination. The latter data set indicates that the dacite domes were emplaced over a geologically brief time interval, not thousands of years. Crystal-size distribution patterns of plagioclase were used to calculate residence times, which we use to infer the duration over which the eruptions likely occurred. Three slopes represent three populations of plagioclase crystals (fine-grained groundmass, coarse-grained groundmass, and phenocrysts). A commonly used growth rate for plagioclase in dacitic magmas (10−10 mm/s) yields 9–10 yr of growth for the coarse-grained groundmass (early erupted domes of db1, db2, and db3), whereas plagioclase in the fine-grained groundmass (late erupted domes of db4, db5, and db6) grew over 4–5 yr. All plagioclase phenocrysts have apparent residence times of 26–36 yr; however, they contain high anorthite (An)>70 resorbed cores with sieve textures, which have euhedral, lower An<65 overgrowth rims. Similarities in chemistry between groundmass plagioclase and phenocryst overgrowth rims indicate that they grew concurrently, and we therefore propose that both have similar residence times. Thus, the Burney Mountain dacite dome complex was emplaced during a single eruptive episode over the course of years to decades at 281.1 ± 4.8 ka (weighted mean age).

New archaeomagnetic secular variation data from Central Europe, I: Directions

Summary Archaeomagnetic directions of one hundred and forty-one archaeological structures have been studied from 21 sites in Austria, 31 sites in Germany and one site in Switzerland. Characteristic remanent magnetisation directions obtained from alternating field and thermal demagnetisations provided 82 and 78 new or updated (12 and 10 per cent) directions of Austria and Germany, respectively. Nine of the directions are not reliable for certain reasons (e.g. displacement) while three of the features are not well dated. Apart from this some updated age information for the published databases is provided. Rock magnetic experiments revealed magnetite as main magnetic carrier of the remanences. The new data agree well with existing secular variation reference curves. The extended data set covers now the past 3500 years and a lot of progress were made to cover times BC with data. Here enhanced secular variation is observed manifested in declinations with values up to 70°. The new data will allow for recalculation of archaeomagnetic calibration curves for Central Europe from mid Bronze Age until today.

Age of the N7/N8 (M4/M5) planktonic foraminifera zone boundary: constraints from the zircon geochronology and magnetostratigraphy of early Miocene sediments in Ichishi, Japan

The age assigned to the Miocene N7/N8 (M4/M5) planktonic foraminifera zone boundary is markedly different from the astronomically tuned Neogene time scales published in 2004 (ATNTS2004) and 2012 (ATNTS2012). This difference is primarily because of the lack of reliable assignment of the zone boundary to magnetostratigraphy, which needs to be resolved for the better refinement of the geologic time scale. Our previous biostratigraphic results from an onland sedimentary sequence of the upper Ichishi Group in central Japan allow location of the stratigraphic position of the lowest occurrence datum (LOD) of Praeorbulina sicana, a key planktonic foraminifera species for defining the N7/N8 (M4/M5) zone boundary. In this study, we performed laser ablation-multiple collector-inductively coupled plasma mass spectrometry U–Pb dating on zircons from two felsic tuff beds in the Ichishi sequence. The age of the zone boundary is approximated by a U–Pb age of 17.03±0.11 Ma for a tuff bed ∼2m above the boundary. We also conducted a magnetostratigraphic analysis in which site-mean characteristic remanent magnetization directions were determined for 19 sites and showed a reverse–normal–reverse (R–N–R) stratigraphic change in polarity. The N7/N8 (M4/M5) zone boundary is within the upper reverse polarity zone. The R–N–R sequence can be correlated with a stratigraphic portion corresponding to chrons C5Dr–C5Dn–C5Cr; hence, the age of the zone boundary is within Chron C5Cr. Our results are thus compatible with the age (16.97 Ma) assigned to the boundary in ATNTS2004 rather than ATNTS2012 and should be considered when the Neogene time scale is updated. A normal polarity subzone found in the upper reverse polarity zone can also be dated at ∼17.0 Ma and is probably correlated with a short normal polarity episode in Chron C5Cr. In terms of tectonic rotation, the site-mean magnetization directions determined in this study suggest an ∼35° clockwise rotation in Ichishi with respect to the North China Block in the Asian continent since ∼17.2 Ma (the upper boundary age of Chron C5Dn). Clockwise rotation of Southwest Japan, which occurred in association with backarc opening of the Japan Sea, was probably in progress during deposition of the upper Ichishi Group sediments.

Rock magnetic and palaeomagnetic studies on the alkaline complexes of western Rajasthan, India

Forty-five oriented block samples were collected from 12 sites of Mundwara and Sarnu-Dandali alkaline complexes of western Rajasthan for palaeomagnetic and rock magnetic investigations to constrain the timing of the emplacement of alkaline suites in relation to the time-frame of main Deccan volcanism. The rock magnetic studies indicated (titano) magnetite as the main magnetic carrier of stable remanence. The Lowrie–Fuller (L–F) test revealed SD and PSD type magnetic carrier in samples. AF and thermal demagnetizations were piloted on the samples to isolate Characteristic Remanent Magnetisation (ChRM) directions. The samples exhibited stable remanence between 5 and 35 mT during AFD. Thermal demagnetizations unblocked remanence between 350° and 500°C. The mean ChRM directions with Declm = 342, Inclm = –35 (α95 = 4.39, K = 121, N = 9) corresponds to paleopole position at 42°N and 274°E with a palaeo-latitude of 24.5°S is coincident with the Deccan Super pole position. The comparison of ChRM in alkaline complexes with those documented in DVP implies coeval emplacement of alkaline suites with Deccan eruption. Further, the ChRM marked largely by normal polarity suggests that alkaline intrusions were accomplished within the magnetic Chron C30N, during the onset of Deccan volcanism. The results also indicate that Deccan volcanism extended far beyond the present day boundaries of the traps, especially in the north.

Ultralow-field nuclear magnetic resonance of liquids confined in ferromagnetic and paramagnetic materials

A nuclear magnetic resonance (NMR) procedure is used to measure weak magnetic fields in the vicinity of dilute ferromagnetic and/or paramagnetic materials. By detecting 1H Larmor precession in common solvents at extremely low frequencies (<50 Hz), the magnetic field produced by remanent magnetization of the material is measured by NMR to a precision of <1 nT. In one example, the technique is used to quantify the magnitude and direction of remanent magnetization in a common aluminum alloy. In another example, a 1H NMR linewidth <1 Hz is demonstrated for liquid decane (n-C10H22) embedded inside a mesoporous silica matrix, despite the high concentration of paramagnetic cobalt sites that produce magnetic susceptibility gradients in the system. Application to systems of industrial relevance is discussed.

Palaeomagnetism for chronologies of recent alpine lake sediments: successes and limits

Chronologies of lake-sediment records covering the last centuries to millennia are usually based on both short-lived radionuclides and radiocarbon dating. However, beyond the range of short-lived radionuclides, age model accuracy often suffers from large radiocarbon uncertainties. For high-altitude records, this issue is even more prominent as terrestrial plant fragments for radiocarbon dating are often lacking due to the sparse vegetation in such environments. In this study, we evaluate the potential of the geomagnetic field secular variations as a complementary tool to establish more robust age–depth relationships. Our palaeomagnetic study, applied to five high-altitude lakes from the western European Alps, first shows that recent unconsolidated sediments can carry stable remanent magnetization. The analysis of the magnetic parameters indicates that low-coercivity pseudo-single domain magnetite grains carry the natural magnetization. Nevertheless, the quality of palaeomagnetic secular variation records varies from one lake to another. This quality can be illustrated through the calculation of the declination/inclination maximum angular variations and their comparison to the expected value. Compared with available models, the declination variations are usually too large and the inclination too high. We discuss the validity of palaeosecular variation (PSV) of the Earth’s magnetic field regarding rock magnetism, magnetization processes and possible deformation during coring. From a magnetic point of view, the quality of data is variable, but the characteristic remanent magnetization direction is consistent at site level between neighbouring lakes and with the reference curve, suggesting that geomagnetic field secular variations are approximately recorded. Finally, we attempt to correlate the declination/inclination variations of the characteristic remanent magnetization measured in the five records to the reference geomagnetic model to provide additional chronological markers for age–depth modelling. These stratigraphic chrono-markers appear in systematic agreement with our previous chronological data and enable a reduction of dating uncertainties up to 30% when including these chrono-markers in the age–depth modelling. This agreement supports the interpretation that PSV may have been recorded more or less accurately depending on the studied lake. Therefore, coupled with a comprehensive understanding through other analysis (sedimentology, dating, geochemistry), PSV can be used to improve the age models in the more favourable cases.

Unidirectional-propagating surface magnetoplasmon based on remanence and its application for subwavelength isolators

Ferrimagnetic material with remanence holds the potential to realize unidirectional propagation of the electromagnetic field by taking advantage of magnetoplasmon in the subwavelength regime. Here, we theoretically investigate magnetoplasmons in a layered structure consisting of a dielectric sandwiched by two magnetic materials with anti-parallel remanent magnetization directions, which shows a complete unidirectional propagating region for both even and odd symmetry modes when the thickness of the dielectric is smaller than a certain value. Additionally, the even symmetry mode supported by such a one-way waveguide can be effectively, with low insertion loss, excited by the fundamental transverse-electric mode of a traditional metal slab waveguide. Relying on low insertion loss and a one-way propagation feature, we propose a broadband and subwavelength isolator working at the microwave region. Our results demonstrate that remanence based magnetoplasmons provide a promising way to realize devices below the diffraction limit with new functionalities.

Paleomagnetic determination of the age and properties of the 1780–1800 AD dome effusion/collapse episode of Mt. Taranaki, New Zealand

Understanding the timing and nature of effusion of lava in the summit regions of stratovolcanoes is important, because these volcanoes generate deadly and destructive block-and-ash flows (BAFs). The most recent eruption of Mt. Taranaki in New Zealand left a portion of a lava dome which now forms its 2518 m summit. In this study we demonstrate the value of applying paleomagnetic techniques to understand recent effusive episodes. By matching the direction of remanent magnetization of the dome lava with the regional paleomagnetic secular variation curve NZPSV1k, we estimate that this eruption occurred between AD1780 and 1800. The paleomagnetic signal and geochemistry show that the summit dome was part of a large lava coulee that spilled from a NW-breach in the crater, with other parts of this structure still remaining down slope (the “Turtle”). Paleomagnetic analyses were also used to reconstruct the thermal history of deposits formed by the dome/coulee collapse. Emplacement temperature estimates and groundmass textural data from the BAF deposit clasts show that the lava dome collapsed in multiple stages. A small, cooler portion of lower dome coulee collapsed first, before the progressively thicker and hotter parts of the central dome. The collapses generated BAFs which travelled up to 5 km from the source and maintained temperatures > 500 °C more than 2 km from the source.

Paleomagnetic and Geochronological Results From the Zhela and Weimei Formations Lava Flows of the Eastern Tethyan Himalaya: New Insights Into the Breakup of Eastern Gondwana

AbstractThe breakup of eastern Gondwana is among the hottest topics in the Earth sciences because of its effect on global climate during the Jurassic‐Cretaceous, its influence on the evolution of life, and its importance to paleogeographic reconstruction. To better constrain the Jurassic and Cretaceous paleogeographic position of the Tethyan Himalaya and the breakup of eastern Gondwana, a combined paleomagnetic and geochronological study was performed on the Zhela and Weimei Formations lava flows, dated at ~138–135 Ma, in the Luozha area of the eastern Tethyan Himalaya. Both positive fold and reversal tests together with a maximum grouping at 100% unfolding indicate that the characteristic remanent magnetization directions are primary magnetizations acquired before folding. The tilt‐corrected directions yielded a paleopole at 0.9°N, 293.4°E with A95 = 7.0° and a corresponding paleolatitude of 53.5°S ± 7.0°S for the Luozha sampling area (28.9°N, 91.3°E), validating that the original erupted position of the Zhela and Weimei Formations lava flows was located in the center of the Kerguelen mantle plume. Our new results, together with the published paleomagnetic, geochronological, and geochemical results, demonstrate that the Comei‐Bunbury large igneous province originated from the Kerguelen mantle plume. The temporal and spatial relationships between the Comei‐Bunbury large igneous province and the Kerguelen mantle plume indicate that eastern Gondwana initially rifted at ~147 Ma and that the Indian Plate fully separated from the Australian‐Antarctic Plate before ~124 Ma.

Insights on the geomagnetic secular variation in the Eastern Mediterranean: First directional data from Cyprus

The first directional archaeomagnetic results from Cyprus are presented, based on the study of seven baked clay structures excavated at the copper smelting site of Agia Varvara-Almyras, situated some 20 km south of Nicosia. This ancient copper working site represents a unique example of complete primary production of the copper metal in Cyprus. In total five furnaces, one ore roasting pit and an oven were sampled in situ for an archaeomagnetic investigation. All the studied structures are well dated, based on pottery finds and radiocarbon dates, with ages ranging from 600 BCE to 50 BCE. Several rock magnetic experiments were performed to determine the main magnetic carrier and to investigate the thermal stability of the studied material. The direction of the Characteristic Remanent Magnetization (ChRM) for each structure has been determined by demagnetization of 7–15 specimens in alternating magnetic fields up to 100–120 mT. Mean directions were calculated assuming a Fisherian distribution and are well defined for five out of the seven structures studied. The new directions are compared with the directional data and the secular variation curves available from the surrounding area, and with the predictions of global geomagnetic field models. They are an important contribution to improving our knowledge of the geomagnetic field variations in the Eastern Mediterranean and offer new insights about fast geomagnetic directional changes during the first millennium BCE that were not previously identified by the available geomagnetic field models’ predictions in this area.

Mapping magnetic sources at the millimeter to micrometer scale in dunite and serpentinite by high-resolution magnetic microscopy

Rock samples can have wide range of magnetic properties depending on composition, amount of ferromagnetic minerals, grain sizes and microstructures. Here, we used scanning magnetic microscopy, a highly sensitive and high-resolution magnetometric technique to map remanent magnetic fields over a planar surface of a rock sample. The technique allows for the investigation of discrete magnetic mineral grains, or magnetic textures and structures with submillimeter scale resolution. Here, we present a case-study of magnetic scans of pristine and serpentinized dunite thin sections from the Reinfjord Ultramafic Complex, in northern Norway. The magnetic mineralogy is characterized by electron microprobe, scanning electron- and optical-microscopy, and with rock magnetic methods. In serpentinized samples the magnetic carrier is end-member magnetite occurring as large discrete grains and small grains in micron scale veins. By contrast, the pristine dunite sample contains large Cr-spinel grains with very fine equant exsolutions ranging in composition from ferrichromite to end-member magnetite. Forward and inverse modeling of the magnetic anomalies is used to determine the remanent magnetization directions and intensities of discrete magnetic sources observed in the scanning magnetic microscopy. The fine-scale magnetization of the rock sample is used to investigate the magnetic carriers and the effect of serpentinization on the magnetic properties of the dunite. Modeling shows that the dipolar magnetic anomalies that are mapped by scanning magnetic microscopy are caused by grains with heterogeneous magnetic sources. The intensity of the magnetization and the amount of magnetic minerals are higher in the serpentinized sample than the pristine dunite sample, consistent with the measured bulk magnetic properties. Furthermore, the serpentinized samples show a larger variability in the direction of the magnetization and a stronger heterogeneity with respect to the pristine sample. The ability to rigorously associate components of the bulk magnetic properties to individual mineral phases creates new possibilities for rock magnetic, paleomagnetic, and exploration applications.

Equatorial paleolatitude for Northeast Africa in the Late Triassic: paleomagnetic study on the Gezira and Bir-Umhebal [229–223 Ma] ring complexes, Southeastern Desert, Egypt

Two Late Triassic alkaline ring complexes in the Southeastern Desert of Egypt: the Gezira [229 ± 5 Ma, K/Ar biotite] and Bir-Umhebal [223 ± 9 Ma, Rb/Sr], were paleomagnetically studied to determine the paleo-latitude position of Northeast Africa during the Late Triassic just before the Jurassic dispersion of Pangaea. The initial isothermal remanent magnetization [IRM] study was followed by the progressive thermal demagnetization of all samples to verify the characteristic natural remanent magnetic directions of both complexes. The analysis revealed that: (i) the magnetite-dominated sites generally, had dual-polarity, bi-vectorial with shallow sub-equatorial N–S to NNW–SSE anchored remanent magnetic component, while (ii) the hematite/goethite sites were characterized by normal polarity, bi-vectorial decay with a northward anchored component fluctuating around zero inclination. Based on their thermal demagnetization behavior, only the magnetite-dominated sites were considered as representing the average Late Triassic paleomagnetic field in the study areas during cooling of the studied complexes. The mean paleomagnetic north poles of the isolated characteristic component using the principal component analysis [PCA] for each complex were: (i) the north poles of Gezira Complex [22.3°N/33.6°E] was at (54.6°N/226.2°E, Dp/Dm= 5°/9°); (ii) the north poles of Bir-Umhebal Complex [23.75°N/28 33.3°E] was at (56.4°N/236°E, Dp/Dm= 3°/6°), and (iii) the mean north pole of both complexes [23°N/33.5°E] was at (56°N/231°E, Dp/Dm = 3°/6°). The obtained Late Triassic [Late Carnian-Early Norian] paleomagnetic poles for the two ring complexes of Egypt show reasonable consistency with their synchronous African poles as well as poles rotated from the main tectonic units to the Northeast Africa using appropriate Euler pole parameters. These poles reveal that in the Late Triassic:The obtained Late Triassic [Late Carnian-Early Norian] paleomagnetic poles for the two ring complexes of Egypt show reasonable consistency with their synchronous African poles as well as poles rotated fromthe main tectonic units to the Northeast Africa using appropriate Euler pole parameters. These poles reveal that in the Late Triassic: (i) the northern part of Africa was in Equatorial paleo-latitudes with Northeast Africa lying just south the Equator. Cairo [Now at 30°N] was at −4.4° to −1° paleo-latitude; (ii) the Azimuth of the African Plate was between N and 37 NNE about 9° to14° clockwise to its present-day azimuth.

Paleomagnetism and Geochemistry of ~1144‐Ma Lamprophyre Dikes, Northwestern Ontario: Implications for the North American Polar Wander and Plate Velocities

AbstractWe present new paleomagnetic and geochemical data from a suite of the ~1144‐Ma ultramafic lamprophyre dikes that outcrop in the Canadian Shield northeast of Lake Superior (Ontario, Canada). Nineteen of 22 sampled dikes yielded consistent characteristic remanent magnetization directions of normal (n = 5) and reversed (n = 14) polarity. The primary origin of characteristic remanent magnetization is bolstered by positive baked contact tests and a reversal test. The group mean direction (D = 306.4°, I = 72.1°, α95 = 5.5°, N = 19) obtained from the lamprophyre dikes is statistically indistinguishable from the group mean direction (D = 297.4°, I = 65.5°, α95 = 8.3°, N = 8) previously reported for the nearly coeval ~1142‐Ma Abitibi dikes. The geochemistry of the lamprophyre dikes suggests strong affinity with magmas derived from ocean island basalt‐type mantle sources, consistent with the mantle plume hypothesis for the formation of the ~1.1‐Ga North American Midcontinent Rift. The similarity in age, trend, paleomagnetism, and geochemistry indicates that the lamprophyre and Abitibi dike suites represent the earliest magmatic event associated with the commencement of rifting. The combined mean direction (D = 303.1°, I = 70.2°, α95 = 4.5°, N = 27) corresponds to a paleomagnetic pole at Plat = 55.8°N, Plong = 220.0°E (A95 = 7.3°). The new pole merits the highest classification on the Q‐scale of paleomagnetic reliability and represents a key pole defining the North American apparent polar wander path during the late Mesoproterozoic. Combined with high‐quality data from the ~1108‐Ma Coldwell Complex, our data indicate an equatorward motion of Laurentia at 3.8 ± 1.4 cm/year, comparable with the present‐day velocities of continental plates, before switching to extremely rapid motion between ~1108 and ~1099 Ma.

Topography and landslides in weathered granitic rock areas—Hai Van landslide in central Vietnam

Many landslides occur every year during heavy rains at the Hai Van Pass and surrounding area in central Vietnam, where granitic rocks are distributed. As is common in granite areas, these landslides often occur as small-scale flow-type and slump-type landslides. However, several horseshoe-shaped loose slopes of widths and lengths of 500 to 800 m, which incorporate these landslides, are observed on slopes across the area. These topographies resemble those formed by past and present large-scale landslides. The presence of such a topography and the repeated occurrences of landslides within this topography are rare in granite areas, where shallow flow-type landslides are generally frequent. To understand the mechanism causing the landslides in the Hai Van region, and as a support for future risk assessment, the factors and processes leading to the formation of such a topography and their relationship with these landslides must be identified and assessed. This study investigated the history of past landslide movement in the Hai Van Pass and surrounding area through observations of drill cores and outcrops, and analysis of the direction of remanent magnetism in the granitic rocks. Mineral compositions, cracks, degrees of weathering, and topographic shapes of the granitic rocks and their relationship to the landslides occurring today were also investigated. The results of the study reveal no variation in the direction of remanent magnetism in the granitic rocks in the region that would indicate disturbance of the ground due to a past large-scale landslide. No evidence of such an event could also be found both in the drill cores and the rock outcrops. Further, results of the analysis of cracks and weathering pattern confirm that the topography of the region is affected by the weathering of the granitic rocks that progresses in concentric circles of various sizes. Thus, it can be concluded that these topographies were not formed by a singular large-scale landslide of the past, but rather by a composite of relatively shallow landslides occurring on the slope of dome structures unique to granite areas, which are formed by differential weathering and denudation regulated by cracks.

Paleomagnetic evidence for the emplacement mechanism of an enigmatic boulder accumulation on a coastal cliff top in New South Wales: implications for the Australian Megatsunami Hypothesis

ABSTRACTPaleomagnetic sampling and measurement of a boulder accumulation on Little Beecroft Head on the Illawarra coastline of New South Wales was undertaken to evaluate potential emplacement mechanisms. This deposit is of central importance in the Australian Megatsunami Hypothesis (AMH) debate, but to date, there has been no unequivocal determination of its provenance. The most likely emplacement mechanisms are by slow collapse during denudation of overlying strata, storm wave overwash or a combination of these. Characteristic Remanent Magnetisation (ChRM) directions were obtained from 15 individual boulders and the in situ bedrock platform on which they currently rest. The in situ Permian bedrock has a normal polarity mean ChRM direction of D/I = 1.6°/–66.7° (α95 = 5.2°; k = 33.9) that is statistically indistinguishable from the Present Earth Field direction at the site. The magnetisation is most likely due to Cenozoic/recent weathering, which is common in surficial rocks throughout the Sydney Basin. ChRM directions for the boulders are stable but scattered, although not random, and the mean boulder direction is indistinguishable in geographic (i.e. current in situ) coordinates, at the 5% significance level, from the mean direction of the in situ bedrock. Further statistical tests confirm that the scatter in the mean directions of the boulders and the in situ bedrock is different, at the 5% significance level, with the boulder mean being more scattered. At an individual boulder level, some blocks have mean ChRM directions that are statistically indistinguishable from the mean in situ rock ChRM direction, whereas others are distinguishable at the 5% significance level.These results indicate that the boulders were magnetised prior to emplacement but were not moved far from their original positions during emplacement. The emplacement age is constrained to the last ca 780 000 years. These observations strongly support the hypothesis that the Little Beecroft Head boulder deposit was emplaced by a non-catastrophic mechanism, namely slow collapse during denudation of pre-existing cliff material or overtopping from severe storms, which occur regularly on the east coast of New South Wales. Even if a catastrophic wave were responsible, the results constrain the age of that event to be older than 780 000 years. Therefore, the results presented here are not supportive of the AMH as it currently stands. Further paleomagnetic work, on similar deposits along the Illawarra coastline and from elsewhere in Australia, is needed to evaluate the interpretations presented here.

53–43 Ma Deformation of Eastern Tibet Revealed by Three Stages of Tectonic Rotation in the Gongjue Basin

AbstractThe Gongjue basin from the eastern Qiangtang terrane is located in the transition region where the regional structural lineation curves from east‐west‐oriented in Tibet to north‐south‐oriented in Yunnan. In this study, we sampled the red beds in the basin from the lower Gongjue to upper Ranmugou formations for the first time covering the entire stratigraphic profile. The stratigraphic ages are bracketed within 53–43 Ma by new detrital zircon U‐Pb ages constraining the maximum deposition age to 52.5 ± 1.5 Ma. Rock magnetic and petrographic studies indicate that detrital magnetite and hematite are the magnetic carriers. Positive reversals and fold tests demonstrate that the characteristic remanent magnetization has a primary origin. The Gongjue and Ranmugou formations yield mean characteristic remanent magnetization directions of Ds/Is = 31.0°/21.3° and Ds/Is = 15.9°/22.0°, respectively. The magnetic inclination of these characteristic remanent magnetizations is significantly shallowed compared to the expected inclination for the locality. However, the elongation/inclination correction method does not provide a meaningful correction, likely because of syn‐depositional rotation. Rotations relative to the Eurasian apparent polar wander path occurred in three stages: Stage I, 33.3 ± 3.4° clockwise rotation during the deposition of the Gongjue and lower Ranmugou formations; Stage II, 26.9 ± 3.7° counterclockwise rotation during deposition of the lower and middle Ranmugou formation; and Stage III, 17.7 ± 3.3° clockwise rotation after 43 Ma. The complex rotation history recorded in the basin is possibly linked to sinistral shear along the Qiangtang block during India indentation into Asia and the early stage of the extrusion of the northwestern Indochina blocks away from eastern Tibet.