Characteristic Remanent Magnetization Component Research Articles

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

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

Magnetostratigraphy and magnetic properties of the Jurassic to Lower Cretaceous Girón Group (northern Andes, Colombia)

AbstractWe report paleomagnetic results from the Jurassic to Lower Cretaceous continental sedimentary succession exposed in the eastern limb of the Los Yariguíes anticlinorium, Eastern Cordillera, Colombia. About 820 m of a stratigraphic section of the upper part of the Girón Group (Angostura del Río Lebrija and Los Santos Formations) was sampled to construct a magnetic polarity stratigraphy. A total of 199 independent samples that yield interpretable and acceptable data have a characteristic remanent magnetization component (ChRM) isolated between 400 °C and 680 °C in progressive thermal demagnetization. Demagnetization behavior and rock magnetic properties are interpreted to indicate that hematite is the principal magnetization carrier with a possible contribution by magnetite in some parts of the section. After tilt correction, 123 samples are of normal polarity (declination [D] = 44.9°, inclination [I] = +9.7°, R = 110.64, k = 9.87, and α95 = 4.3°), and the other 76 accepted samples are of reverse polarity (D = 216.4°, I = −6.1°, R = 68.29, k = 9.72, and α95 = 5.5°). The statistical reversal test conducted on virtual geomagnetic poles is positive (class B). Based on paleontologic age estimates for the Cumbre and Rosablanca Formations, we assume a Berriasian age for the Los Santos Formation. The magnetostratigraphic data from the Girón Group strata are interpreted to suggest an age for the sampled part of the section between early Kimmeridgian and early Valanginian (ca. 157–139 Ma). The age of the Angostura del Río Lebrija Formation is estimated as between early Kimmeridgian and early Tithonian (ca. 157–146.5 Ma). The age of the Los Santos Formation is estimated between early Tithonian and early Valanginian (146.5–139.3 Ma). With our proposed, but nonunique, correlation with the Geomagnetic Polarity Time Scale, the Jurassic-Cretaceous boundary is interpreted to be located within the Los Santos Formation. The Girón Group is characterized by two periods of high (>8 cm/k.y.) and two periods of low (< 2 cm/k.y.) sedimentation rates. An inferred clockwise rotation of ∼44°, based on paleomagnetic declination data from the Girón Group, is similar to rotation estimates reported in some previous studies in the general area, and this facet of deformation could be related to local and regional response to displacement along regional-scale strike-slip faults.

Paleomagnetism of the Guanyang Devonian sedimentary successions in Guangxi province, South China

The Devonian position of the South China Block (SCB) remains debated because apparently contradictory Devonian paleopoles have been documented. Here we report two characteristic remanent magnetization components from the mildly deformed Guanyang Devonian successions (Lochkovian-Famennian) within the cratonic interior of the SCB. Both components pass reversal tests and occur in different samples across the successions. Component A and associated paleopole DA (228.9°E, 37.7°N, A95 = 3.7°) are comparable with published Devonian data and were likely acquired before Permian-Triassic folding and likely reflect syn-depositional magnetization. Component B and its associated paleopole DB (204.6°E, 4.5°N, A95 = 5.5°) are distinct from published results and were probably acquired during minor tilting associated with the Late Devonian Liujiang orogeny. If paleopole DA was acquired prior to the Late Devonian Liujiang orogeny, the paleolatitude of the Guanyang successions varied little throughout the Devonian, but the azimuthal orientation of recorded declinations exhibits a 40° difference. The distinct positions of the two paleopoles are broadly consistent with existing enigmatic Devonian field records. Alternatively, given the uncertainty in the acquisition ages of both components in the Guanyang successions, a case can be made for component A being a secondary magnetization acquired during the Permian-Triassic Indosinian orogeny. Nonetheless, the similar distribution of Devonian paleopoles from Gondwana to those of cratonic interior of the SCB suggests a Devonian connection between the two continents, and adds further support for a non-uniformitarian geomagnetic field during this enigmatic time interval.

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

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

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

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

On the validity of archeomagnetic dating method in Korea: a case study

An archeomagnetic study has been performed for six archeological relics of the Proto-Three Kingdoms period excavated in Jungdo Island, South Korea. Among a total of 90 samples from hearths remains of six dwelling sites, the characteristic remanent magnetization components were isolated from 70 samples using the progressive alternating field demagnetizations and considered to record faithfully a thermoremanent magnetization at the timing of the last cooling of the hearths. Two different approaches were made to determine the archeomagnetic ages: One is the conventional method using the relocated paleosecular variation (PSV) curve obtained from southwestern Japan. The other method adopts a Matlab archaeo_dating tool using a Bayesian statistics with global geomagnetic field prediction model (ARCH3K.1). Both archeomagnetic ages using two individual dating methods generally overlapped with the radiocarbon age ranges for each sites. However, no systematic relationship between archeomagnetic ages and radiocarbon ages was observed, implying that neither the relocated Japanese PSV data nor the global model accurately represent the Korean secular variation during the archeological period of this study. At present, archeomagnetic dating in Korea should be carefully applied in conjunction with other dating methods on the consideration of archeological context.

An analysis of apparent polar wander path for southwest Japan suggests no relative movement with respect to Eurasia during the Cretaceous

To test the hypothesis that southwest Japan was involved in large-scale tectonic movement with southward translation as far as 2000km with respect to Eurasia during the Cretaceous, we examined Cretaceous paleomagnetic poles from southwest Japan to compare with those from Eurasia. Red and gray sandstone samples from the Upper Cretaceous Onogawa Group were collected from twelve sites in the Onogawa Basin in the western part of southwest Japan for paleomagnetic analysis. This group formed over the time span in which the proposed tectonic event is hypothesized to have occurred. A characteristic remanent magnetization component was isolated from red sandstone at ten sites; it is interpreted to be of primary Late Cretaceous origin. The primary directions combined with previously reported data provide a mean direction (D=76.8°, I=44.6°, α95=11.1°, N=15) and a paleomagnetic pole (24.4°N, 202.6°E, A95=11.0°) for the Onogawa area. This pole is consistent with other Late Cretaceous poles from a wide area of southwest Japan, and a mean Late Cretaceous pole (28.4°N, 202.5°E, A95=7.5°, N=6) is calculated and regarded as representative of this region. The Late Cretaceous pole, together with mid- and Early Cretaceous poles, constitutes an apparent polar wander path (APWP) for southwest Japan during the Cretaceous. After restoration of post-Cretaceous tectonic rotation, each Cretaceous pole for southwest Japan shows agreement with the coeval poles for Eurasia; therefore, it is unlikely that the previously proposed tectonic model that includes southward translation of southwest Japan occurred in the Late Cretaceous. Southwest Japan is considered to have behaved as a stable part of the Eurasian continental margin during the Cretaceous.

Virtual Directions in Paleomagnetism: A Global and Rapid Approach to Evaluate the NRM Components

We introduce a method and software to process demagnetization data for a rapid and integrative estimation of characteristic remanent magnetization (ChRM) components. The virtual directions (VIDI) of a paleomagnetic site are “all” possible directions that can be calculated from a given demagnetization routine of “n” steps (being m the number of specimens in the site). If the ChRM can be defined for a site, it will be represented in the VIDI set. Directions can be calculated for successive steps using principal component analysis, both anchored to the origin (resultant virtual directions RVD; m * (n2+n)/2) and not anchored (difference virtual directions DVD; m * (n2-n)/2). The number of directions per specimen (n2) is very large and will enhance all ChRM components with noisy regions where two components were fitted together (mixing their unblocking intervals). In the same way, resultant and difference virtual circles (RVC, DVC) are calculated. Virtual directions and circles are a global and objective approach to unravel different natural remanent magnetization (NRM) components for a paleomagnetic site without any assumption. To better constrain the stable components, some filters can be applied, such as establishing an upper boundary to the MAD, removing samples with anomalous intensities, or stating a minimum number of demagnetization steps (objective filters) or selecting a given unblocking interval (subjective but based on the expertise). On the other hand, the VPD program also allows the application of standard approaches (classic PCA fitting of directions a circles) and other ancillary methods (stacking routine, linearity spectrum analysis) giving an objective, global and robust idea of the demagnetization structure with minimal assumptions. Application of the VIDI method to natural cases (outcrops in the Pyrenees and u-channel data from a Roman dam infill in northern Spain) and their comparison to other approaches (classic end-point, demagnetization circle analysis, stacking routine and linearity spectrum analysis) allows validation of this technique. The VIDI is a global approach and it is especially useful for large data sets and rapid estimation of the NRM components.

Fracture network, fluid pathways and paleostress at the Tolhuaca geothermal field

In this study, we examine the fracture network of the Tolhuaca geothermal system located in the Southern Andean volcanic zone that may have acted as a pathway for migration and ascent of deep-seated fluids under the far/local stress field conditions of the area. We collected the orientation, slip-data and mineralogical content of faults and veins recovered on a ca. 1000 m deep borehole (Tol-1) located in the NW-flank of the Tolhuaca volcano. Tol-1 is a non-oriented, vertical borehole that recovered relatively young (<1 Ma) basaltic/andesitic volcanic rocks with subordinate pyroclastic/volcanoclastic interbedded units of Pleistocene age. Here, we examined and measured the inclination, geometry, texture, mineralogy, and relative sense of displacement of veins and faults. To determine the actual azimuthal orientation of fault and veins we reoriented 66 segments (89 standard mini-cores) of Tol-1 using stable Characteristic remanent magnetization component (ChRM) obtained by thermal demagnetization methodology. Paleo-declination of ChRM vectors was used to re-orient the borehole pieces, as well as fault and veins, to a common anchor orientation value consistent with the Geocentric Axial Dipole approximation (GAD). Inversion of RM-corrected fault-slip data reveals a local tensional stress field with a vertically oriented σ1 axis (083/74) and a subhorizontal, NS-trending σ3 axis (184/03). Within the topmost 400 m of the borehole, faults and veins are randomly oriented, whereas below 400 m depth, faults and veins show preferential NE-to EW-strikes and steep (>50°) dips. The EW-striking veins are compatible with the calculated local stress field whereas NE-striking veins are compatible with the regional stress field, the morphological elongation of volcanic centers, alignments of flank vents and dikes orientation. Our results demonstrate that the paleomagnetic methodology proved to be reliable and it is useful to re-orient vertical boreholes such as Tol-1. Furthermore, our data show that the bulk transpressional regional stress field has local variations to a tensional stress field within the NE-striking fault zone belonging to the Liquiñe-Ofqui Fault System, favoring the activation of both NW- and NE-striking pre-existent discontinuities, especially the latter which are favorably oriented to open under the prevailing stress field. The vertical σ1 and NS-trending subhorizontal σ3 calculated in the TGS promote the activation of EW-striking extensional veins and both NE and NW-striking hybrid faults, constituting a complex fluid pathway geometry of at least one kilometer depth.

Paleomagnetic study of Late Paleozoic rocks in the Tacheng Basin of West Junggar (NW China): Implications for the tectonic evolution of the western Altaids

To better understand the tectonic evolution of West Junggar in the western Altaids, we have conducted a combined paleomagnetic and geochronological study on Late Devonian (D3), Late Carboniferous (C2) and Early Permian (P1) successions from the Tacheng Basin, northwest of China. Both primary and secondary characteristic remanent magnetization components have been successfully isolated and yield three reliable paleomagnetic poles of C2 and Late Permian (P2) ages. The C2 and P2 poles reveal a significant clockwise rotation of 35.6°±10.4° which is comparable to the rotation deduced in the western part of the Chingiz Range during the time interval C2 to P2. Similar rotation patterns for the two units suggest that the Chingiz–West Junggar has remained as a quasi-rigid block by the P2 time and constituted the northeast limb of the Kazakhstan Orocline. The differential rotation of ca. 25° to 30° deduced between the northeast and middle limbs of the Kazakhstan Orocline further suggests that at least a part of the curvature of the Balkhash Arc was acquired during the C2 to P2 interval and was a continuation of the middle Paleozoic oroclinal bending. Moreover, the C2 poles for West Junggar and the Junggar Block are significantly different while the P2 ones almost overlap supporting the view that final closure of the Balkhash–Junggar Ocean occurred during the C2 to P2 interval. On the other hand, a large-scale counterclockwise rotation of ca. 80° to 90° is observed in West Junggar and Junggar relative to the western part of the Chingiz Range since the P2 time, and this rotation probably represents an important intra-continental deformation within the western part of the Altaids. A reconstruction of the Paleo-Asian Ocean has been proposed in which the Kazakhstan Orocline was emphasized by using the paleomagnetic results and geological evidence.

Phanerozoic Paleomagnetism Characteristics of the Qomolangma Area in Tibet

Abstract:This paper conducts systematic test research on the 2920 paleomagnetic directional samples taken from Ordovician‐Paleogene sedimentary formation in the north slope of Qomolangma in south of Tibet and obtains the primary remanent magnetization component and counts the new data of paleomagnetism the times. Based on the characteristic remanent magnetization component, it calculates the geomagnetic pole position and latitude value of Himalaya block in Ordovician‐Paleogene. According to the new data of paleomagnetism, it draws the palaeomagnetic polar wander curve and palaeolatitude change curve of the north slope of Qomolangma in Ordovician‐Paleogene. It also makes a preliminary discussion to the structure evolution history and relative movement of Himalaya bloc. The research results show that many clockwise rotation movements had occurred to the Himalaya block in northern slope of Qomolangmain the process of northward drifting in the phanerozoic eon. In Ordovician‐late Cretaceous, there the movement of about 20.0° clockwise rotation occurred in the process of northward drifting. However, 0.4° counterclockwise rotation occurred from the end of late Devonian epoch to the beginning of early carboniferous epoch; 6.0° and 8.0° counterclockwise rotation occurred in carboniferous period and early Triassic epoch respectively, which might be related with the tension crack of continental rift valley from late Devonian period to the beginning of early carboniferous epoch, carboniferous period and early Triassic epoch. From the Eocene epoch to Pliocene epoch, the Himalaya block generated about 28.0° clockwise while drifting northward with a relatively rapid speed. This was the result that since the Eocene epoch, due to the continuous expansion of mid‐ocean ridge of the India Ocean, the neo‐Tethys with the Yarlung Zangbo River as the main ocean basin closed to form orogenic movement and the strong continent‐continent collision orogenic movement of the east and west Himalayas generated clockwise movement in the mid‐ Himalaya area. According to the calculation of palaeolatitude data, the Himalaya continent‐continent collusion orogenic movement since the Eocene epoch caused the crustal structure in Indian Plate‐ Himalaya folded structural belt‐ Lhasa block to shorten by at least 1000 km. The systematic research on the paleomagnetism of Qomolangma area in the phanerozoic eon provides a scientific basis to further research the evolution of Gondwanaland, formation and extinction history of paleo‐Tethys Ocean and uplift mechanism of the Qinghai‐Tibet Plateau.

Magnetostratigraphy and Anisotropy of Magnetic Susceptibility of the Lulehe Formation in the Northeastern Qaidam Basin

Abstract:The timing of onset of deposition of the Lulehe Formation is a significant factor in understanding the genesis of the Qaidam basin and the evolution of the Tibetan Plateau. Here, we describe a detailed magnetostratigraphic and magnetic fabric study of the middle and lower parts of the Lulehe Formation. A total of 234 samples were collected from 117 sites throughout a thickness of almost 460 m of fluvial and lacustrine deposits at the Xitieshan section in the northeastern Qaidam basin. Out of these sites, 94 sites yielded well‐defined characteristic remanent magnetization components by stepwise thermal demagnetization and were used to establish the magnetostratigraphy of the studied section. Based on correlation with the geomagnetic polarity timescale, the studied section spans the period from 53.8 Ma to 50.7 Ma. Our results show a three‐fold decrease in sedimentation rates as well as marked change in facies from braided river to delta and shore–shallow lake around 52.6 Ma, which suggests tectonic uplift of the northeastern Qaidam basin margin ridge was rapid at the onset of formation of the Qaidam basin and subsequently weakened after 52.6 Ma. The anisotropy of magnetic susceptibility results indicate that tectonic compression stress had reached the northeastern Tibetan Plateau by the early stages of Indo–Eurasian plate collision and that the direction of stress in the study area was NE–SW. Furthermore, a weakening of tectonic compression stress around 52.6 Ma is consistent with sedimentary records. The age of initial deposition of the Qaidam basin (around 53.8 Ma) was almost synchronous with that of the Qiangtang, Hoh Xil, Xining, and Lanzhou basins, which implies that stress was transferred rapidly through the Tibetan Plateau during or immediately after the onset of Indo–Eurasian collision.

Remagnetization of Upper Jurassic limestones from the Danubian Unit (Southern Carpathians, Romania): tectonic implications

Abstract We present a pioneering paleomagnetic study on Upper Jurassic limestones from the Danubian Unit (Southern Carpathians, Romania). Thermal and alternating field demagnetizations were applied to define the characteristic remanent magnetization component in all six localities (81 samples). All samples have a normal polarity characteristic remanent magnetization. Negative regional and local fold tests suggest that this remanent magnetization is in fact a remagnetization produced by late diagenetic processes. The studied limestones were probably remagnetized during the collision of the Getic Unit and Danubian Unit which took place during the long normal polarity Chron C34 (82-118 Ma). The area mean direction (D = 75.5°, I = 50.0°, α95 = 10.2°, k = 44) implies about 75° clockwise rotation post remagnetization. Our paleomagnetic results further indicate the absence of significant relative rotation between the Getic Unit and the Danubian Unit during the Cenozoic.

Paleomagnetism of Jurassic carbonate rocks from Sardinia: No indication of post-Jurassic internal block rotations

[1] Several paleomagnetic studies on Carboniferous and Permian sedimentary and volcanic rocks from Sardinia and Corsica have recently demonstrated (1) the tectonic coherence between southern Corsica and northern Sardinia and (2) significant rotations between individual crustal blocks within Sardinia itself. The geodynamic significance of these rotations, however, is not clearly understood mainly because of uncertainties in defining their timing and causes. In order to contribute to these issues, a pioneering paleomagnetic study on Jurassic carbonates from the Baronie-Supramonte region of eastern-central Sardinia has been extended regionally and stratigraphically. A total of 280 oriented drill cores were taken from 44 sites of Middle and Late Jurassic age in the Nurra, Baronie-Supramonte, Barbagia-Sarcidano, and Sulcis regions. Despite generally weak remanent magnetization intensities, on the order of less than 1 mA/m, thermal and alternating field demagnetizations were successfully applied to define a characteristic remanent magnetization component in about 60% of the samples. Site mean directions show rather good agreement after correction for bedding tilt and yield Middle and Late Jurassic overall mean directions of D = 269.7° and I = 45.0° (α95 = 8.0°, k = 14, and n = 25 sites) and D = 275.5° and I = 50.7° (α95 = 7.2°, k = 45.3, and n = 10 sites). Positive regional and local fold and reversal tests demonstrate the primary character of the natural magnetic remanence, which is carried by magnetite. These results indicate only insignificant amounts (±10°) of post-Jurassic rotations within the island of Sardinia. The resulting Middle and Late Jurassic paleopoles (latitude (Lat) = 16.5°, longitude (Long) = 299.1°, dp = 6.4°, and dm = 10.1° and Lat = 23.4°, Long = 301.2°, dp = 6.5°, and dm = 9.7°), corrected for the opening of (1) the Liguro-Provençal Basin and (2) the Bay of Biscay using rotation parameters from the literature, fall near the coeval segment of the European apparent polar wander path. These results constrain the timing of large differential block rotations found in Late Carboniferous-Permian rocks to a pre–Middle Jurassic age and lead us to exclude tectonics related to the Alpine orogeny for such rotations.

Paleomagnetism and rock magnetism of the Neoproterozoic Itajaí Basin of the Rio de la Plata craton (Brazil): Cambrian to Cretaceous widespread remagnetizations of South America

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 550 Ma. 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.

Palaeomagnetism of the Mahatta Humaid Group (Cambrian – Early Ordovician, Oman), including a re-interpretation of previous Neoproterozoic palaeomagnetic data

ABSTRACTWe carried out a palaeomagnetic study in the Al-Huqf region (Sultanate of Oman) on rocks that belong to different units of the Cambrian – Ordovician Mahatta Humaid Group. Thirty-nine samples were systematically collected from a succession ca. 520–495 Ma old. Seventeen samples showed characteristic remanent magnetization components with two antipodal polarities carried by hematite. Evidence suggests that these components have a primary origin. A detailed petrographic analysis revealed syntaxial overgrowths parallel to the easy plane of magnetization of the magnetic carriers that has probably enhanced and reinforced the primary magnetization. A palaeopole computed with the mean direction of the 17 characteristic remanent magnetization components was considered alongside previously published Neoproterozoic – early Palaeozoic palaeomagnetic data, which we placed in an updated chronostratigraphic framework for the Neoproterozoic – Cambrian Huqf Supergroup. Two interpretations were considered: (1) Oman was detached from the Arabian-Nubian craton until ca. 660 Ma, and it became attached (or was nearby) to it by ca. 630 Ma. In this interpretation, an apparent polar wander (APW) path of Arabia is proposed between ca. 630 and 500 Ma. The palaeomagnetic directions of Mirbat obtained by Killner et al. (2005) in rocks 720–660 Ma old are therefore assumed as primary, and taking into account that Oman was an independent block of the Arabian-Nubian craton, the corresponding palaeopole is not considered in the proposed segment of the Arabian APW path. (2) The Neoproterozoic data belong to two different tectonic blocks within the Arabian-Nubian craton and were involved in left-lateral, strike-slip movements along NW-trending faults. One block included the localities of Al Jabal al-Akhdar and Al-Huqf and may have rotated counter-clockwise c. 45° about a vertical axis between ca. 600 and 500 Ma. The other block included the locality of Mirbat and rotated counter-clockwise c. 25° about a vertical axis between ca. 600 and 550 Ma. These suggested block rotations may have played a role in generating the underlying fabrics for some of the sedimentary basins in Oman. In the second model, the rocks sampled by Killner et al. (2005) in Mirbat were re-magnetized during the intrusion of dike swarms at ca. 615–600 Ma.

Paleomagnetic and rock magnetic study of the Vredefort impact structure and the Johannesburg Dome, Kaapvaal Craton, South Africa—Implications for the apparent polar wander path of the Kaapvaal Craton during the Mesoproterozoic

New paleomagnetic and rock magnetic results are presented for various pre-, syn- and post-impact lithologies from the region of the 2023 ± 4 Ma, originally ca. 250 km diameter, Vredefort impact structure, on the Kaapvaal Craton of South Africa. After removal of a viscous remanent magnetization (VRM) component, the characteristic remanent magnetization (ChRM) component from 16 Vredefort Granophyre and pseudotachylitic (PT) breccia samples ( D = 18.3°, I = 54.8°, α 95 = 8.1°) was isolated and a paleomagnetic pole at 25.1°N and 43.5°E ( A 95 = 10.6°) obtained. Since, such a ChRM component was isolated also from the Archean Basement rocks of the Vredefort Dome, we consider its nature as a primary component of the impactites proven. The pole falls onto the Paleoproterozoic part of the apparent polar wander path of the Kaapvaal Craton. Rock magnetic analysis and scanning electron microscopy (SEM) revealed that the pseudotachylitic breccia and Granophyre samples contain two distinct magnetite phases: one ultra-small (micrometer-size), and one altered and larger (>50 μm) phase. The ultra-small magnetite is interpreted as the carrier of the ChRM and the larger grains as the carriers of the VRM component. Additionally, rock magnetic and petrophysical data reveal unusually high Koenigsberger ratios ( Q values) in all pre-impact lithologies, in some Vredefort impactite samples, and in the much younger (1.1 Ga) Anna's Rust Sheet (ARS) gabbro samples. As the high Q values, which had also been reported by previous studies of Vredefort lithologies, are now also seen in samples from the Johannesburg Dome, a direct link to the Vredefort impact can be ruled out. This is also supported by the hysteresis data of this study. As the observed magnetization is rather hard and shows multiple components of remanent magnetization, we exclude lightning as a cause for all observed high Q values (except in case of ARS gabbros). It is instead suggested that the cause of the high Q values could be related to the high temperatures of the rocks that were uplifted by the impact event from a mid-crustal original setting, and to fluid circulation within the two domes that made the rocks vulnerable to acquire high thermochemical remanence. Moreover, paleomagnetic analysis of the rocks around the ARS gabbro intrusion in the northern part of the Vredefort structure revealed the presence of either a shallow north or a shallow south direction, which is tentatively related to emplacement of the Umkondo large igneous province. Analysis of all rocks, including the Vredefort impactites, yields occasionally distinct great circle paths towards these shallow directions. A likely explanation for this overprint direction is the heating caused by now eroded ARS-type gabbro in the area, or by regional, ca. 1.1–1.0 Ga orogenic effects. No evidence of Karoo-type (0.18 Ga) overprint is seen.

Tectonic implications of paleomagnetic data from upper Cretaceous sediments in the Oyubari area, central Hokkaido, Japan

Abstract Paleomagnetic studies provide constraints on the geometric configuration of the eastern Eurasian margin on geological time scales. Characteristic remanent magnetization components were isolated from eight sites by progressive demagnetization executed on samples from 25 sites in the Oyubari area, central Hokkaido where the Late Cretaceous Yezo Group is distributed. After tilt‐correction, all sites show normal polarity site‐mean directions, and well‐clustered directions pass a positive fold test and a correlation test. Planktonic foraminifera indicate an age range of Cenomanian to Turonian, and the studied section is correlated to the geomagnetic polarity chron C34n. Reliable formation‐mean directions that have been corrected for post‐depositional shallowing (D = 7.5°, I = 65.9°, α95 = 6.6°) are characterized by inclination data indicative of no significant latitudinal translation since the Late Cretaceous. Central Hokkaido has, therefore, been situated adjacent to easternmost Mongolia including Sikhote Alin around the present latitude since the Late Cretaceous. Declination data require significant differential rotation between Hokkaido and the eastern Asian margin, which may be indicative of rearrangement of crustal blocks along the continental margin.

Tectonic elements with South China affinity in the Korean Peninsula: new Early Jurassic palaeomagnetic data

SUMMARY Early Jurassic sandstones and shales were collected at 14 sites from the Kimpo area in the western part of the Korean Peninsula for a palaeomagnetic study. A characteristic remanent magnetization component, which passes the fold test, was isolated from seven sites. This characteristic component is interpreted to be of primary Early Jurassic origin because the folding age is Middle–Late Jurassic. The primary directions, together with previously reported ones, provide a mean direction (D= 38.1°, I= 39.7°, α95= 3.8°, 11 sites) and a palaeomagnetic pole (54.1°N, 230.2°E, A95= 4.1°) for the Kimpo area. This palaeomagnetic pole shows good agreement with the coeval poles for the South China Block and is significantly removed from that for the North China Block, suggesting that the Kimpo area has been part of the South China Block since at least Early Jurassic times. The Kimpo area constitutes a part of the Imjingang Belt which is palaeontologically allocated to the South China Block in its origin, while all Precambrian tectonic blocks that compose the basement of the Korean Peninsula are interpreted to be of North China origin. We conclude that the Imjingang Belt, including the Kimpo area, is a tectonic extension of the South China Block on the Korean Peninsula. The South China elements were obducted onto the North China Block at the site of the present Korean Peninsula during the collision event of the North and South China Blocks, and hence the Korean Peninsula is composed of areas with both North and South China affinities.

New chronostratigraphy of the Cretaceous–Paleogene boundary interval at Bidart (France)

The Cretaceous–Paleogene boundary section at Bidart (French basque coast) was prevented for a long time to be considered as a reference section for the K–T boundary due to a suspected hiatus comprising most of Chron C29r.In our new magneto- and biostratigraphic study of this section we discovered that the underlying primary magnetic polarity is not revealed in the vector directions but in a small, but consistent, intensity excursion during laboratory demagnetization trends. Magnetite and maghemite are the main carriers of remanence, with contribution of hematite to the magnetic mineralogy of the red-brown lowermost Danian and Maastrichtian marly limestones. Magnetite is the main magnetic mineral in the white to pink Danian limestones. The NRM of marls and marly limestones is characterized by: (1) a present-day field overprint, (2) a medium-temperature (300–400 °C) reversed component interpreted as a relic of the characteristic remanent magnetization (ChRM) carried by magnetite grains which are not completely oxidized into maghemite, and (3) a high temperature (up to 590 °C) normal component due to late diagenesis hematite growth. The NRM of Danian limestones is characterized by (1) a present-day field overprint and (2) a ChRM component of normal or reversed polarity.The ChRM directions allow a new correlation of the magnetic polarity sequence with the Geomagnetic Polarity Time Scale (GPTS): Chron C29r corresponds to the Maastrichtian unit and extends up to 3.5 m above the K–T boundary, and Chron C29n occurs at the top of the section. There is no evidence for a hiatus at or below the K–T boundary, and the section does not extend up to Chron C28r as previously suggested. These new chronostratigraphic assignments are in good agreement with the previous biostratigraphic data and from the new biostratigraphic analyses based on calcareous nannofossils presented in this paper.Our study demonstrates that the section of Bidart can be considered as a key section to study the K–T boundary event(s).