Mean Paleomagnetic Direction Research Articles

First Late Cambrian Paleomagnetic Results From the Oulongbuluke Terrane, Northern Tibetan Plateau: Implications for the Paleogeography of the Proto‐Tethys Ocean

AbstractThe Early Paleozoic paleolatitudinal position of the terranes in the northern Tibetan Plateau is the key to unraveling the evolution of the Proto‐Tethys Ocean. We present the first Late Cambrian paleomagnetic results from the Oulongbuluke terrane (OT), in the northern Tibetan Plateau. The mean paleomagnetic direction for 16 sites is Ds = 196.2°, Is = −36.9° with κs = 31.5, α95 = 6.7°, corresponding to a paleopole at 68.2°S/51.9°E with dp/dm = 4.6°/7.8°. Rock magnetic and petrologic analyses demonstrate the primary origin of the magnetic mineralogy. This paleomagnetic result defines the Late Cambrian paleolatitude of the OT as 20.6 ± 4.6°S (reference point: 37.2°N/96.6°E). Combined paleomagnetic and geological evidence suggests that the terranes in the northern Tibetan Plateau were located to the northwest of the Indian plate of Gondwana during the Late Cambrian.

PMTOOLS: New Application for Paleomagnetic Data Analysis

Abstract—This paper introduces PMTools (https://pmtools.ru), a novel cross-platform open-source web application designed for the analysis of paleomagnetic data. Our software offers a user-friendly interface and supports the most commonly used data formats in paleomagnetism, including .pmd, .dir, .pmm, .rs3, .squid, .vgp, and .gpml. It encompasses all the necessary functionalities for conducting principal component analysis of natural remanent magnetization and computing mean paleomagnetic directions along with corresponding virtual geomagnetic (paleomagnetic) poles. PMTools facilitates the application of various paleomagnetic tests to user data, such as the fold test, conglomerate test, and reversals test. Notably, the application allows for seamless import and export of paleomagnetic data (tables) in formats compatible with .csv and MS Excel. Additionally, all generated figures can be exported as high-quality vector graphics in .svg format, specifically designed for direct integration into publications and presentations. Furthermore, PMTools enables the export of sets of paleomagnetic poles in GMap and GPlates software formats, enabling researchers to promptly use their paleomagnetic data for paleotectonic reconstructions. PMTools has an intuitive interface, customizable hotkeys, and an extensive array of graph elements, all of which contribute to making PMTools an appealing, cutting-edge tool for processing and analyzing the results of paleomagnetic studies.

A Bootstrap Common Mean Direction Test

AbstractPaleomagnetic statistical inference is underpinned by a family of parametric null hypothesis tests. In many cases, however, paleomagnetic data do not meet the distributional assumptions of these tests, which can lead to spurious inferences. Earlier studies have proposed the bootstrap as a nonparametric alternative for paleomagnetic analysis, which can be applied even when the distributional form of the data is unknown. Key among these approaches is the bootstrap test for a common mean direction, which relies on assessment of the overlap of estimated confidence regions. In its current form, the bootstrap test for a common mean paleomagnetic direction does not consider a null hypothesis and can yield outcomes that cannot be interpreted in terms of a statistical significance level. To resolve these issues, we use recent advances to place such bootstrap tests within a null hypothesis significance testing framework, and unify them with the existing family of paleomagnetic statistical tests. Furthermore, using numerical experiments we demonstrate the applicability of such a nonparametric approach to moderately sized paleomagnetic data sets typical of modern and legacy studies. Finally, we demonstrate how a confidence region can be estimated for the common mean of two sets of directions and how known directions, such as the expected field produced by a geocentric axial dipole, can be compared to that mean.

Paleomagnetic constraint on the formation of the Eastern Himalayan Syntaxis: A new late Eocene result from the Mangkang area of the eastern Tibetan Plateau

The Eastern Himalayan Syntaxis (EHS) is a major structural and geomorphological unit in Eastern Tibet; however, its mechanism of formation and the extent of its northward effect of compression remain unclear. To address these questions, we conducted a paleomagnetic and geochronologic study of the late Eocene trachyte intrusive mass in the Mangkang area of Tibet. The U-Pb SHRIMP II age estimate of the trachyte intrusive mass is ∼40 Ma, and the estimated mean paleomagnetic direction we obtained is D=27.0°, I=36.8°, k=42.9, α95=1.3°, and n=282 specimens. These results indicate that the Mangkang area was located at a paleolatitude of 20.5 ± 0.9° N during the late Eocene. A comparison of paleolatitudes of the Qiangtang block with those inferred from the apparent polar wander paths (APWP) of India and Eurasia suggests that the Eastern Himalayan Syntaxis was formed after 35 Ma when Eurasia was penetrated by ∼600 km by northward-moving India. This is consistent with geologic evidence of intra-continental shortening in the foreland of the Eastern Himalayan syntaxis. The northward counterclockwise penetration of the corner of India also resulted in the large right-lateral strike-slip system of the Sagaing fault (SGF), with ∼400-600 km of displacement between the EHS and the Shan-Thai block. Thus, the northward wedging of the EHS indicates minimal southward displacement of the Shan-Thai block. Our results link the tectonic activity of the EHS to the lateral extrusion of the southeastern margin of the Tibetan Plateau, providing a new perspective on the evolution of this region since the late Eocene.

Paleomagnetic Study of the Late Miocene Volcanic Sequence in Sonora, Mexico: Clockwise Vertical Axis Rotation Related to the Oblique Rifting in the Pacific‐North American Plate Boundary

AbstractThe evolution of oblique divergence along the Pacific‐North American plate boundary during the late Miocene is well‐preserved along the eastern margin of the Gulf of California. The early phase of this deformation occurred between ∼12.5 and ∼6 Ma in Sonora, NW Mexico, related to oblique rifting of the Gulf of California. The magmatic sequence for this period is exposed in the Cerro El Vigía, Guaymas region. This sequence is characterized by intrusive rocks, lava flows, and pyroclastic flows ranging in age from 12 to 10 Ma. The Cerro El Vigia has a predominant semicircular geomorphology traditionally ascribed to a volcanic caldera structure, such as large ring faults. Rock magnetic analyses of these magmatic products identify the magnetic minerals and their reliability for tectonic interpretations. The mean paleomagnetic direction from 14 sites from the Cerro El Vigia reveals up to 90° clockwise rotation about an inclined axis (65–85° axis plunge) with respect to stable North America. When interpreted in the context of the semicircular structure, these observations suggest that the structure has a tectonic, not volcanic origin. Paleomagnetic results and tectonics during the late Miocene suggest that this crustal block rotation corresponds to the development of large tension gashes deformed within a dextral transtensional system associated with continental deformation of the proto‐Gulf of California rift. Based on the paleomagnetic data, reported ages, and magmatic stratigraphy, we conclude that the magmatism and associated deformation at Cerro El Vigia was synchronous with transtensional tectonics between ∼11 Ma to ∼8 Ma.

Paleomagnetic dating of prehistoric lava flows from the urban district of Catania (Etna volcano, Italy)

Abstract Determining the ages of past eruptions of active volcanoes whose slopes were historically inhabited is vitally important for investigating the relationships between eruptive phenomena and human settlements. During its almost three-millennia-long history, Catania—the biggest city lying at the toe of Etna volcano—was directly impacted only once by the huge lava flow emplaced during the A.D. 1669 Etna flank eruption. However, other lava flows reached the present-day Catania urban district in prehistoric ages before the founding of the city in Greek times (729/728 B.C., i.e., 2679/2678 yr B.P.). In this work, the Holocene lava flows of Barriera del Bosco, Larmisi, and San Giovanni Galermo, which are exposed in the Catania urban district, were paleomagnetically investigated at 12 sites (120 oriented cores). Paleomagnetic dating was obtained by comparing flow-mean paleomagnetic directions to updated geomagnetic reference models for the Holocene. The Barriera del Bosco flow turns out to represent the oldest eruptive event and is paleomagnetically dated to the 11,234–10,941 yr B.P. and 8395–8236 yr B.P. age intervals. The mean paleomagnetic directions from the San Giovanni Galermo and Larmisi flows overlap when statistical uncertainties are considered. This datum, along with geologic, geochemical, and petrologic evidence, implies that the two lava flows can be considered as parts of a single lava field that erupted in a narrow time window between 5494 yr B.P. and 5387 yr B.P. The emplacement of such a huge lava flow field may have buried several Neolithic settlements, which would thus explain the scarce occurrence of archaeological sites of that age found below the town of Catania.

Paleomagnetism of Permian-Triassic volcanic sequences from Son La province, northwest Vietnam

Nineteen sites with 198 oriented-core samples have been collected from the Upper Permian-Lower Triassic volcanic rocks of Vien Nam Formation at Quynh Nhai locality, Son La Province, northwestern Vietnam. The characteristic remanent magnetization components carried by magnetite and hematite were successfully isolated from secondary components reveal a mean paleomagnetic direction Ds = 48.3°, Is = -10.0°, a95 = 8.0°, corresponding to a virtual geomagnetic pole located at l = 35.7°N, f = 217.4°E and a paleo-latitude of study area situated at 5.1°S during the Permian time. Compared with the Late Permian-Early Triassic pole of the South China Block (SCB), the data show that crustal elements of NW Vietnam have been close to, but not unequivocally a coherent part of the SCB, since the Late Permian. Development of the parallel NW-SE striking Song Ma and Song Chay orogenic belts did not involve the closure of wide (> 500 km) ocean basins.

Paleomagnetism of Permian-Triassic volcanic sequences from Son La province, northwest Vietnam

Nineteen sites with 198 oriented-core samples have been collected from the Upper Permian-Lower Triassic volcanic rocks of Vien Nam Formation at Quynh Nhai locality, Son La Province, northwestern Vietnam. The characteristic remanent magnetization components carried by magnetite and hematite were successfully isolated from secondary components reveal a mean paleomagnetic direction Ds = 48.3°, Is = -10.0°, a95 = 8.0°, corresponding to a virtual geomagnetic pole located at l = 35.7°N, f = 217.4°E and a paleo-latitude of study area situated at 5.1°S during the Permian time. Compared with the Late Permian-Early Triassic pole of the South China Block (SCB), the data show that crustal elements of NW Vietnam have been close to, but not unequivocally a coherent part of the SCB, since the Late Permian. Development of the parallel NW-SE striking Song Ma and Song Chay orogenic belts did not involve the closure of wide (> 500 km) ocean basins.

Semicontinuous paleomagnetic record of the last 1 Ma from radiometrically dated igneous rocks (Trans-Mexican Volcanic Belt and surrounding areas)

The radiometric data available in the Trans-Mexican Volcanic Belt enables a quasi-continuous record of the Earth's magnetic field over the last million years to be obtained. Here, we analyze 149 well-defined paleomagnetic directions supported by an equal number of isotope ages. Eleven new paleomagnetic directions from the Tacambaro-Puruaran area (the Michoacán Guanajuato Volcanic Field) are incorporated into the database. Global mean paleomagnetic directions are calculated for one Ma after discarding transitional lavas. The mean direction agrees with the expected paleomagnetic directions obtained from the Pliocene-Quaternary reference poles within the range of uncertainty. The secular variation rate estimated through the scattering of the calculated virtual geomagnetic poles, and the inclination anomaly, both agree with recently reported latitudinal distribution models. An interesting feature of the paleomagnetic direction compilation is the possible evidence of relatively short geomagnetic events or excursions within Matuyama and Brunhes chrons, which may help to refine the geomagnetic instability time scale (GITS).

Reassessment of the eruptive chronology of El Metate shield volcano (central-western Mexico) based on a comprehensive rock-magnetic, paleomagnetic and multi-approach paleointensity survey

Abstract We study seven out of the thirteen lava flows associated to El Metate shield volcano using paleomagnetic, absolute intensity and rock-magnetic experiments. Characteristic paleomagnetic directions were successfully determined from most of the flows. A combined mean direction of D = 346.4° and I = 32.1° (N = 34, R = 33.73, k = 121.74, α95 = 2.2) was obtained for the early flows (Flow 1 − Flow 3). Flow 6a (a flow from the late emplacement stage) yielded a statistically distinguishable mean paleomagnetic direction of D = 359.2° and I = 29.6° (N = 9, R = 8.97, k = 294.6, α95 = 3.0). Furthermore, the significant between-flow variation of absolute paleointensity values obtained using two different methodologies, which resembles the trend followed by that of the global model SHA. DIF 14k intensity curve for the period 1000 AD 1600 AD, disagrees with previous hypothesis supporting a short emplacement period between 50 and 100 years for the whole sequence. Additionally, paleodirectional and paleointensity dating of the studied flows allowed to refine the eruptive chronology indicating that early flows were produced over a short time period, whereas at least two of the later flows were erupted 200 years later; the activity ending just prior to the arrival of the Spaniards in ∼1520. Under these circumstances, and in light of the presented high quality paleomagnetic and paleointensity data, the assumed monogenetic nature of El Metate volcano seems to be questionable.

Paleomagnetism and rock magnetic properties of Late Pleistocene volcanism from El Pinacate Volcanic Field, northwest Mexico

The Pinacate Volcanic Field is a Neogene volcanic sequence at the northwestern part of Mexico. This paleomagnetic study (directional and paleointensity) will continue to improve the global paleomagnetic databases. It is the first full vector paleomagnetic analysis within the more than 400 volcanic structures of the Pinacate Volcanic Field, e.g. scoria, cinder cones, large ‘pahoehoe’ and ‘aa’ lava flows. A total of 140 oriented standard paleomagnetic cores from 16 sites of lava flows were collected. Rock magnetic experiments such as thermomagnetic analyses show that Ti-poor titanomagnetite is the main magnetic carrier of the remanence. Microscopy of oxide minerals supports this observation. Parameters from hysteresis and FORC diagrams suggest pseudo-single domain grain sizes or a mixture of single and multidomain particles. The directional analysis of 11/12 volcanic structures provides a mean paleomagnetic direction and corresponding mean paleomagnetic pole (calculated from the site VGPs): Dec = 0.1°, Inc = 54.5°, n = 11, α95 = 6.7°, k = 48, Plat = 86.5°, Plong = 247.8°, A95 = 7.5°, respectively. The mean paleointensity, obtained by using the Thellier-Coe protocol, and estimated from seven volcanic structures is 42.2 ± 3.6 μT, and VDM is 8.1 × 1022Am2. The obtained directions are compatible with the Late Pleistocene paleodirection as expected from the geocentric axial dipole (GAD) field. The virtual geomagnetic pole scatter (Sb) was calculated at 13.5° which is consistent with the paleosecular variation value expected from El Pinacate latitude.

Paleomagnetism of Holocene lava flows from Los Humeros caldera, eastern Mexico: Discrimination of volcanic eruptions and their age dating

Los Humeros caldera in eastern Mexico has been formed since about 164 ka, with important effusive activity during the upper Holocene, which covered more than 100 km2 with lava. Two lava flows were 14C dated at 2692-1950 cal BCE and 1117-930 cal BCE, but the detailed succession of eruptions is yet unknown even though the interest for volcanic risk assessment. All Holocene lava flows were studied paleomagnetically in 22 sites to determine which of them have been contemporaneous and to apply the paleomagnetic dating method. For most sites well defined mean directions could be defined, resulting in an overall paleodirection of D = 7.7° E, I = 16.9°, α95 = 6.7°. This direction is discordant compared to the geocentric axial dipole field and indicates that secular variation has not averaged out. Paleointensities were obtained using the Thellier IZZI protocol, with only a small success rate of 27% due to thermally induced alterations, the presence of multidomain magnetic minerals, or unstable paleomagnetic directions as indicated by the checks during the experiments. Accepted paleointensities of 37 samples varied between 21 and 63 μT, with site mean paleointensity of 27–54 μT. The F-test was applied to mean directions of sites potentially belonging to the same lava flow, and if these were indistinguishable they were combined in flow mean paleomagnetic directions. Otherwise, these sites were assumed to represent independent eruptions. This way, thirteen independent flow mean directions were obtained and used for paleomagnetic dating by means of the SHA.DIF.14k reference model and the archaeo_dating software. The dating often provided several possible age ranges, and erosion state, soil cover, lava flow contacts, and the available 14C ages were used all together for selecting the most probable age range. Two well defined eruptive periods bracketed between 2500 and 1900 BCE, and 1300-600 BCE were defined, suggesting that all studied lava flows were emplaced during two main eruptive phases, which may indeed have been much shorter than 600 years. Paleomagnetic data support those eruptive periods and time among the different lava flow units can be inferred from those data. Early settlers occupied this region since about 2000 BCE and thus were certainly affected by these voluminous eruptions. At least another 12 nearby Holocene monogenetic volcanoes are known in the surrounding area, and all together these eruptions indicate that this is a highly active volcanic region, which might be affected by eruptions in the near future.

Extensional vs. compressional deformation in the Central High Atlas salt province: A paleomagnetic approach

In this paper we address the problem of the distinction between diapiric, salt-driven and compressional structures, using the outstanding example of the Central High Atlas (Morocco). A remagnetized component carried by magnetite has been isolated in 32 new paleomagnetic sites. It is characterized by: maximum unblocking temperatures around 450 °C, syn-folding behavior and normal polarity.These 33 mean paleomagnetic directions were analyzed together with other 68 from published works around the study area to construct a robust paleomagnetic dataset along a cross-section perpendicular to the main structures. The remagnetization direction (n: 100, Dec: 332.2°, Inc: 34.5°, η: 6.2°, ξ: 2.0°, A/n: 6.427°) and the paleo-dip of beds (the attitude of the beds at the remagnetization occurrence) were calculated through small circle methods. The remagnetization can be dated as ca. 100 Ma. Because of its occurrence between the extensional and compressional periods, this remagnetization offers the possibility of restore the basin to its pre-inversion geometry.Comparison between present-day and pre-inversion structure allows discriminating three different evolutionary patterns: (i) thrusted and welded salt-walls mainly structured during the extensional stage (Ikkou ridge) with steep limbs close to the salt-wall core. (ii) Jurassic salt-walls with weaker deformation, restricted to the areas adjacent to the structure (Tadaghmamt and Timedouine); in this case, Cenozoic compression is limited to welding of the salt-walls and buttressing of the sedimentary sequences against faults. (iii) salt-rollers gently initiated during the Jurassic (Toumliline diapir), thrusted during the Cenozoic compression. Results show the importance of salt tectonics both during extension and compression, as well as the control of the compressional features by the inherited extensional structures. The performed restorations prove that paleomagnetism is a useful, independent tool to obtain palinspastic restorations and to separate, and quantify, the imprint generated during the basinal stage from the inversional features.

Paleomagnetically inferred ages of a cluster of Holocene monogenetic eruptions in the Tacámbaro-Puruarán area (Michoacán, México): Implications for volcanic hazards

The paleomagnetic dating procedure was applied to a cluster of four partly overlapping monogenetic Holocene volcanoes and associated lava flows, namely La Tinaja, La Palma, Mesa La Muerta, and Malpaís de Cutzaróndiro, located in the Tacámbaro-Puruarán area, at the southeastern margin of the Michoacán-Guanajuato volcanic field. For this purpose, 21 sites distributed as far apart as possible from each other were sampled to obtain a well-averaged mean paleomagnetic direction for each single lava flow. For intensity determinations, double-heating Thellier experiments using the IZZI protocol were conducted on 55 selected samples. La Tinaja is the oldest of these flows and was dated by the 14C method at ~5115±130years BP (cal 4184–3655BCE). It is stratigraphically underneath the other three flows with Malpaís de Cutzaróndiro lava flow being the youngest. The paleomagnetic dating procedure was applied using the Matlab archaeo-dating tool in couple with the geomagnetic field model SHA.DIF.14k. Accordingly, for La Tinaja several possible age ranges were obtained, of which the range 3650–3480BCE is closest to the 14C age. Paleomagnetic dating on La Palma produced a unique age range of 3220–2880BCE. Two ages ranges of 2240–2070BCE and 760–630BCE were obtained for Mesa La Muerta and a well-constrained age of 420–320BCE for Malpaís de Cutzaróndiro. Although systematic archaeological excavations have so far not been carried out in this area, it is possible that the younger eruptions were contemporary to local human occupation. Our paleomagnetic dates indicate that all four eruptions, although closely clustered in space, occurred separately in time with varying recurrence intervals ranging between ~300 and ~2300years. This finding should be considered when constraining the nature of the magmatic plumbing system and developing a strategy aimed at reducing risk in the volcanically active Michoacán-Guanajuato volcanic field, where several young monogenetic volcano clusters have been identified recently. These enigmatic small “flare-ups” (outbursts of small pods of magma in geologically short periods of time within a small area) have also been encountered in other subduction-related volcanic fields around the globe (e.g. Cascades arc in the western U.S.A.) and still require to be investigated by geophysical and petrological means in order to understand their origin.

Paleomagnetic study of 1765 Ma dyke swarm from the Singhbhum Craton: Implications to the paleogeography of India

We report the first key paleopole as a result of paleomagnetic study on a precisely dated 1765.3±1.0Ma WNW-ESE trending dyke swarm from Singhbhum Craton. This pole has been used in this study to propose the paleogeographic reconstruction of India with Baltica Craton and North China Craton. Incremental alternating field (AF) and thermal demagnetization, isolated high coercivity components with north to north-westerly declination and shallow negative inclination from 9 sampling sites which are representing different individual dykes. The primary origin of the ChRM is supported by the positive baked contact test. The WNW-ESE trending dykes yield a mean paleomagnetic direction with a declination=329.2° and an inclination=−22.8° (k=31.6; α95=9.3°). The positive bake contact test proves the primary nature of remanence. The pole position of Singhbhum Craton at 1765Ma is 45°N, 311°E (dp=5.2 and dm=9.9). Paleogeographic reconstruction at ca. 1770Ma, supported by geological, tectonic and metallogenic evidences indicate that the Baltica Craton and India linkage can be stable for at least ∼370Ma (∼1770–1400Ma). There is also reasonable evidence in support of India-North China Craton spatial proximity at ∼1770Ma.

Paleomagnetism and magnetic fabrics of Mio-Pliocene hypabyssal rocks of the Combia event, Colombia: tectonic implications

Mio-Pliocene hypabyssal rocks of the Combia event in the Amaga basin (NW Andes-Colombia), contain a deformational record of the activity of the Cauca-Romeral fault system, and the interaction of terranes within the Choco and northern Andean blocks. Previous paleomagnetic studies interpreted coherent counterclockwise rotations and noncoherent modes of rotation about horizontal axes for the Combia intrusives. However, rotations were determined from in-situ paleomagnetic directions and the existing data set is small. In order to better understand the deformational features of these rocks, we collected new paleomagnetic, structural, petrographic and magnetic fabric data from well exposed hypabyssal rocks of the Combia event. The magnetizations of these rocks are controlled by a low-coercivity ferromagnetic phase. Samples respond well to alternatingfield demagnetization isolating a magnetization component of moderate coercivity. These rocks do not have ductile deformation features. Anisotropy of magnetic susceptibility and morphotectonic analysis indicate that rotation about horizontal axes is consistently to the south-east, suggesting the need to apply a structural correction to the paleomagnetic data. The relationships between magnetic foliations and host-rock bedding planes indicate tectonic activity initiated before ~10 Ma. We present a mean paleomagnetic direction (declination D = 342.8°, inclination I = 12.1°, 95% confidence interval α95 = 12.5°, precision parameter k = 8.6, number of specimens n = 18) that incorporates structural corrections. The dispersion S = 27° of site means cannot be explained by secular variation alone, but it indicates a counterclockwise rotation of 14.8° ± 12.7° relative to stable South America. Paleomagnetic data within a block bounded by the Sabanalarga and Cascajosa faults forms a more coherent data set (D = 336.5°, I = 17.4°, α95 = 11.7°, k = 12.5, n = 14), which differs from sites west of the Sabanalarga fault and shows a rotation about a vertical axis of 20.2° ± 10.7°. Deformation in the Amaga basin may be tentatively explained by the obduction of the Canas Gordas terrane over the northwestern margin of the northern Andean block. However, it can also be related to the local effects of the Cauca-Romeral fault system.

Paleomagnetic study of El Metate shield volcano (Michoacán, Mexico) confirms its monogenetic nature and young age (~ 1250 CE)

In a recent study, Chevrel et al. (2016a, b) radiocarbon-dated the oldest lava flow of the voluminous (~9.2km3) El Metate shield volcano (Michoacán, Mexico) at cal 1250–1260 CE and proposed that its eruption was monogenetic in origin, with twelve younger lava flows emplaced during a short period of only ~35years, but certainly <275years. In order to test this hypothesis, we undertook a detailed paleomagnetic study of five lava flows from El Metate to check the consistency of their paleomagnetic directions. Additionally, a group of representative specimens was treated with the double-heating Thellier experiment using the IZZI protocol for paleointensity determination. Flow mean paleomagnetic directions obtained for four of the flows are indistinguishable, and discordant directions were obtained from the site of the 5th flow measured, probably due to the tilting of the sampled block after remanence acquisition. Mean paleodirections and intensities were used for paleomagnetic dating applying the global paleosecular variation model SHA.DIF.14k. The resulting age range for the eruption is 1150–1290 CE, which overlaps with the range previously determined by the 14C method by Chevrel et al. (2016a). Accepting the 14C age of the oldest flow as the maximum age, the age range would be reduced to 1250–1290 CE, which strongly supports the hypothesis of a monogenetic nature of the El Metate eruption.

Large southward motion and clockwise rotation of Indochina throughout the Mesozoic: Paleomagnetic and detrital zircon U–Pb geochronological constraints

We report a combined paleomagnetic and U–Pb geochronologic study of sedimentary rocks from the Huai Hin Lat and Nam Phong formations of Mesozoic age in NE Thailand in order to provide independent constraints on the tectonic movement of the Indochina Block during convergence of the major blocks now comprising East Asia. The maximum allowable depositional age of the two formations is estimated to be 227 Ma and 215 Ma, respectively, from detrital zircon U–Pb geochronologic analysis which also indicates a sediment source transition in the Khorat Plateau Basin during the Middle–Late Jurassic. A formation mean paleomagnetic direction of Dg/Ig=21.4°/38.1° (kg=19.5, α95=9.6°) before and Ds/Is=43.0°/48.0° (ks=47.4, α95=6.1°, N=13) after tilt correction is derived from samples with different lithologies, bedding attitudes, magnetic carriers and polarities and yields a positive fold test. Hence, the magnetization is likely primary. The revised Mesozoic APWP of the Indochina Block yields paleolatitudes (for a reference site of 22°N, 102°E) of 33.4 ± 7.2°N during the Norian Late Triassic, 25.9 ± 5.9°N during the Late Triassic to Early Jurassic, 23.9 ± 8°N during the Late Jurassic to Early Cretaceous, 27.5 ± 3.2°N during the Early Cretaceous and 24.5 ± 4.9°N by the Late Cretaceous; corresponding declinations are 45.2 ± 8.6°, 38.0 ± 6.6°, 36.3 ± 8.8°, 29.6 ± 3.6° and 24.9 ± 5.4° respectively. These data indicate a significantly southward displacement accompanied by clockwise rotation during the Mesozoic. A reconstruction of the Indochina Block within the now well-studied merging process of South China and North China indicates that the Indochina Block was located at a higher latitude than the South China Block during the Norian stage of Late Triassic times whilst no significant relative poleward displacement apparently occurred during the Early Jurassic to Early Cretaceous interval. Our study supports a post-Cretaceous tectonic extrusion model with a southeastward displacement of Indochina with respect to the South China Block estimated to be 1000 ± 850 km since the Late Cretaceous.

Magnetostratigraphy of the Xiaolongtan Formation bearing Lufengpithecus keiyuanensis in Yunnan, southwestern China: Constraint on the initiation time of the southern segment of the Xianshuihe–Xiaojiang fault

The late Cenozoic extensional basins in Yunnan Province (southwestern China), which are kinematically linked with the regional strike-slip faults, can provide meaningful constraints on the fault activity history and tectonic evolution of the southeast margin of the Tibetan Plateau (SEMTP), and further on the geodynamic evolution of the Tibetan Plateau. However, this has been severely impeded by the lack of precise age constraints on the timing of fault activity. To better constrain the timing of fault activity and the tectonic rotation of SEMTP, we undertook a high-resolution magnetostratigraphic study on the Xiaolongtan Formation in the Xiaolongtan Basin, which is located at the southern tip of the Xianshuihe–Xiaojiang fault and is well-known by the presence of hominoid Lufengpithecus keiyuanensis. Rock magnetic experiments indicate that magnetite is the main remanence carrier. Correlation to the geomagnetic polarity timescale was achieved by combining magnetostratigraphic and biostratigraphic data. Our correlation suggests that the Xiaolongtan Formation sedimentary sequence spans from Chron C5Ar.1r to Chron C5n.2n, which indicates that the age of the Xiaolongtan Formation ranges from ~10Ma to 12.7Ma, and that the ages of the two sedimentary layers possibly bearing the hominoid L. keiyuanensis are ~11.6Ma or ~12.5Ma. The basal age of the sediments is 12.7Ma, which indicates that the activation of the southern Xianshuihe–Xiaojiang fault was initiated at this time. The overall mean paleomagnetic direction (D=353.2°, I=34.2°, α95=2.1°, n=166) documents a counter-clockwise vertical axis rotation of −8±3° with respect to Eurasia, which is the response to the activity of the left-lateral Xianshuihe–Xiaojiang Fault.

New paleomagnetic results from the Upper Cretaceous red marls of the Pieniny Klippen Belt, Western Carpathians: Evidence for general CCW rotation and implications for the origin of the structural arc formation

The Pieniny Klippen Belt (PKB) is a narrow arcuate structure separating the Central and Outer Western Carpathians formed during several Cenozoic deformational stages. The primary aim of this study was to obtain paleomagnetic constraints for the mechanism of formation of the arc. We investigate Albian–Santonian red pelagic marls from 14 localities, distributed along a strike length of ca. 400km. AMS measurements reveal a pattern characteristic of weakly deformed sedimentary rocks and magnetic lineations do not correlate with the general strike of the PKB. Paleomagnetic analysis revealed well defined hematite-based ancient magnetization components at 13 localities, which are dated using fold- and inclination tests. A within-locality fold test is negative for two localities exhibiting large CCW rotations of similar magnitude situated at the two ends of the PKB. Remanences of pre-folding age were documented for 11 localities, with an overall mean paleomagnetic direction of D=311°, I=53°, and α95=11°. The indicated general CCW rotation most probably took place during the Miocene, together with Western Central and Outer Carpathians. Paleolatitudes for the PKB indicate a considerable separation from the southern margin of stable Europe leaving space for coordinated rotation. A paleomagnetic oroclinal test involving all localities with primary magnetizations was negative. When localities with monoclinal steep dips are omitted due to possible declination bias, the overall mean paleomagentic direction does not change significantly, but correlation is observed between the general trend of the PKB and the paleomagnetic declinations. Thus, we conclude that the present shape of the arc can be partly due to oroclinal bending. This must have happened before Oligocene since paleomagnetic declinations for neighboring Paleogene basins in the Central and Outer Western Carpathians reveal a uniform CCW rotation of ca. 50° magnitude, irrespective of the position of the localities in relation to the Carpathian arc.