Paleomagnetic study of Cretaceous rocks of Peru, South America: Evidence for rotation of the Andes

Paleomagnetic data for the Cretaceous volcanic and sedimentary rocks in the Andean region of Peru are given. Reliable paleomagnetic field directions were obtained for three Cretaceous (Albian to Cenomanian) formations from calcareous sediments in northern Peru. Stable remanent magnetization directions were also derived from twelve Cretaceous lava flows and dikes in coastal Peru. Paleomagnetic data of the same age from the stable areas of South America such as Brazil demonstrate that the paleomagnetic poles are nearly coincident with the present pole, but Peruvian paleomagnetic directions studied here showed several tens of degrees of counterclockwise declination shifts. This suggests counterclockwise tectonic rotation of an extensive block which includes the whole of Andean Peru.     

Hydrothermal fluid-controlled remagnetization of sedimentary rocks in Korea: Tectonic importance of pervasive Tertiary remagnetization

In order to better understand the tectonic environment of the Korean Peninsula since Cretaceous, a paleomagnetic study was carried out on the Jinan Basin located in southwestern Korea. From the demagnetization of 597 samples, we found that remanence is carried by pyrrhotite and/or magnetite (black siltstone) and hematite (red siltstone), which is a common magnetic mineral assemblage of the rocks remagnetized in Early Tertiary in Korea. All the rock units recorded identical ancient geomagnetic field directions. In particular, paleodirections were better clustered in geographic coordinates, indicating a secondary origin. The presence of abundant silicates as inclusions within Fe-oxides and Fe-sulfides confirms the authigenic origin of the magnetic carriers, precipitated by fluid mediated chemical processes. Compilation of the Korean Cretaceous and Tertiary poles indicates that the Jinan Basin was remagnetized in the Early Tertiary age. In addition, comparison of the Korean Cretaceous and Tertiary poles with those from adjacent major terrains including China and Siberia reveals that a dominant feature of tectonic motions in Korea was a vertical-axis rotation. Sense of vertical-axis rotation was time-dependent, with clear distinction between clockwise rotation in Cretaceous and counter-clockwise rotation in Early Tertiary. Such differences in the mode of vertical axis rotation might be caused by the activities of basin-bounding strike–slip faults in Korea, possibly driven by the changes of subduction regime of the Kula-Pacific plate in Early Tertiary.     

江西信江盆地上白垩统风成红层的古地磁研究

红层是古地磁学的重要研究对象之一。以往对河湖相红层的古地磁研究较多,而对于风成红层的研究较少。因 此,对于风成红层剩磁记录是否可靠等基本问题仍然缺乏清晰的认识。文章对江西信江盆地上白垩统圭峰群塘边组风成 红层和河口组河流相红层开展了古地磁研究,并通过对比风成红层与河流相红层的古地磁结果,探究风成红层剩磁记录 的可靠性及不同沉积过程对古地磁记录的影响。逐步热退磁实验结果显示仅有19% 的塘边组风成红层分离出稳定的特征 剩磁,而且其强度衰减曲线为凸形,表明特征剩磁为碎屑赤铁矿携带的原生剩磁。其平均方向为Ds=15.6 °, Is=28.9 °, n= 25, κ=13.0, α95=8.3 °;对应的古地磁极为Latitude=70.7 °, Longitude=245.6 °, A95=6.8 °。该古地磁极与赣州地区河湖相红层 的古地磁极及华南晚白垩世的古地磁极位置一致,表明风成红层的剩磁记录是可靠的。河口组河流相红层绝大部分样品 未能分离出稳定的特征剩磁。磁化率各向异性结果显示塘边组和河口组为沉积组构。岩石磁学结果表明,载磁矿物为赤 铁矿和磁铁矿。通过对塘边组风成红层的薄片观察和红度值比较等进一步研究表明,颗粒粒度和胶结程度可能对红层剩 磁记录的稳定性有一定影响。     

江西信江盆地上白垩统风成红层的古地磁研究

红层是古地磁学的重要研究对象之一。以往对河湖相红层的古地磁研究较多，而对于风成红层的研究较少。因 此，对于风成红层剩磁记录是否可靠等基本问题仍然缺乏清晰的认识。文章对江西信江盆地上白垩统圭峰群塘边组风成 红层和河口组河流相红层开展了古地磁研究，并通过对比风成红层与河流相红层的古地磁结果，探究风成红层剩磁记录 的可靠性及不同沉积过程对古地磁记录的影响。逐步热退磁实验结果显示仅有19% 的塘边组风成红层分离出稳定的特征 剩磁，而且其强度衰减曲线为凸形，表明特征剩磁为碎屑赤铁矿携带的原生剩磁。其平均方向为Ds=15.6 °, Is=28.9 °, n= 25, κ=13.0, α95=8.3 °；对应的古地磁极为Latitude=70.7 °, Longitude=245.6 °, A95=6.8 °。该古地磁极与赣州地区河湖相红层 的古地磁极及华南晚白垩世的古地磁极位置一致，表明风成红层的剩磁记录是可靠的。河口组河流相红层绝大部分样品 未能分离出稳定的特征剩磁。磁化率各向异性结果显示塘边组和河口组为沉积组构。岩石磁学结果表明，载磁矿物为赤 铁矿和磁铁矿。通过对塘边组风成红层的薄片观察和红度值比较等进一步研究表明，颗粒粒度和胶结程度可能对红层剩 磁记录的稳定性有一定影响。     

Paleomagnetism of the Istria peninsula, Yugoslavia

280 core samples were collected from Upper Jurassic, Cretaceous and Eocene sediments outcropping in the Istria peninsula (Yugoslavia). Due to the very low intensities of the initial natural remanent magnetizations, more than 50% of the collection, consisting mainly of rock samples of Jurassic and Eocene sediments, was not suitable for paleomagnetic studies.The Cretaceous samples yield a mean paleomagnetic pole (lat. 53°, long. 275° and α₉₅ = 4.8°), which is significantly different from the African and European paleomagnetic poles of the same age. The position of the Istria peninsula on the autochthonous Adriatic platform allows the result to be interpreted as applicable to all the autochthonous Periadriatic region. This new paleomagnetic result indicates that the autochthonous Adriatic platform rotated counterclockwise over an angle of about 30° with respect to Africa in post-Mesozoic times.     

郯庐断裂带东侧华南地块部分逆时针转动——古地磁新证据

本文对郯庐断裂带东西两侧的莱阳、六安、庐枞和怀宁等中新生代盆地内的中三叠世至早白垩世沉积构造变形进行了古地磁研究。采样区大多数特征磁化方向通过了褶皱检验或反极性检验,从上述地区共获得了10个可靠的中三叠世-早白垩世的古地磁极。该结果与华南地块的视极移曲线对比,可以看出,从中侏罗世以来,郯庐断裂带东西两侧不存在大规模地水平相对位移。但是,断裂带东边的华南地块部分存在15°-25°的逆时针转动。这种转动主要应发生在晚侏罗世,主要与华南、华北地块的碰撞以及太平洋板块的挤压有关。      

Paleomagnetism and geochronology of an Early Proterozoic quartz diorite in the southern Rind River Range, Wyoming, USA

We present geochronologic and paleomagnetic data from a north-trending quartz diorite intrusion that cuts Archean metasedimentary and metaigneous rocks of the South Pass Greenstone Belt of the Wyoming craton. The quartz diorite was previously thought to be either Archean or Early Proterozoic (?) in age and is cut by north and northeast-trending Proterozoic diabase dikes of uncertain age, for which we also report paleomagnetic data. New U–Pb analyses of baddeleyite and zircon from the quartz diorite yield a concordia upper intercept age of 2170±8 Ma (95% confidence). An ⁴⁰Ar/³⁹Ar amphibole date from the same sample yields a similar apparent age of about 2124±30 Ma (2σ), thus confirming that the intrusion is Early Proterozoic in age and that it has probably not been thermally disturbed since emplacement. A magmatic event at ca. 2.17 Ga has not previously been documented in the Wyoming craton. The quartz diorite and one of the crosscutting diabase dikes yield essentially identical, well-defined characteristic remanent magnetizations. Results from eight sites in the quartz diorite yield an in situ mean direction of north declination and moderate to steep positive inclination (Dec.=355°, Inc.=65°, k=145, α₉₅=5°) with a paleomagnetic pole at 84°N, 215°E (δm=6°, δp=7°). Data from other diabase dike sites are inconsistent with the quartz diorite results, but the importance of these results is uncertain because the age of the dikes is not well known. Interpretation of the quartz diorite remanent magnetization is problematic. The in situ direction is similar to expected directions for magnetizations of Late Cretaceous/early Tertiary age. However, there is no compelling evidence to suggest that these rocks were remagnetized during the late Mesozoic or Cenozoic. Assuming this magnetization to be primary, then the in situ paleomagnetic pole is strongly discordant with poles of 2167, 2214, and 2217 Ma from the Canadian Shield, and is consistent with proposed separation of the Wyoming Craton and Laurentia prior to about 1.8 Ga. Correcting the quartz diorite pole for the possible effects of Laramide-age tilting of the Wind River Range, based on the attitude of nearby overlying Cambrian Flathead Sandstone (dip=20°, N20°E), gives a tilt corrected pole of 75°N, 58°E (δm=4°, δp=6°), which is also discordant with respect to time-equivalent poles from the Superior Province. Reconstruction of the Superior and Wyoming Province using a rotation similar to that proposed by Roscoe and Card [Can. J. Earth Sci. 46(1993)2475] is problematic, but reconstruction of the Superior and Wyoming Provinces based on restoring them to their correct paleolatitude and orientation using a closest approach fit indicates that the two cratons could have been adjacent at about 2.17 Ga prior to rifting at about 2.15 Ga. The paleomagnetic data presented are consistent with the hypothesis that the Huronian and Snowy Pass Supergroups could have evolved as part of a single epicratonic sedimentary basin during the Early Proterozoic.     

龙门山南段二叠系—下三叠统的古地磁研究

为了约束龙门山南段的构造运动特征,文章对龙门山南段大川镇附近的下三叠统飞仙关组淡紫灰色泥岩、粉砂岩和宝兴地区的二叠系灰岩开展了古地磁研究。古地磁样品取自10个采样点,其中3个采点为二叠系灰岩;7个采点为飞仙关组淡紫灰色泥岩、粉砂岩。对样品开展了逐步热退磁、岩石磁学（等温剩磁获得曲线和三轴等温剩磁热退磁）及扫描电镜实验。80个样品进行的逐步热退磁实验结果显示,二叠系灰岩样品未分离出稳定的特征剩磁;飞仙关组样品分离出了稳定的特征剩磁,并通过了广义褶皱检验,其特征剩磁的平均方向为：Ds=36.9°,Is=16.5°,α95=5.9°,K=33.8,N=18,对应的古地磁极投在了华南视极移曲线的早三叠世段附近。岩石磁学实验结果表明飞仙关组样品的载磁矿物为磁铁矿,扫描电镜观察展示其为碎屑状的铁氧化物,且无明显成岩后自生特征。结合退磁曲线特征,扫描电镜微观特征,特征剩磁的古地磁极位置和岩石磁学结果,飞仙关组样品的特征剩磁很可能为原生剩磁。该结果表明龙门山褶皱冲断带与四川盆地的没有明显地相对构造旋转运动,自晚三叠世以来,其与龙门山北段以及四川盆地在动力学上是统一的构造单元。     

New paleomagnetic data of the Middle Jurassic igneous complex in the Bodrak River valley, Mountainous Crimea

In this paper, the results of paleomagnetic studies for the Middle Jurassic subvolcanic bodies and volcanogenic-sedimentary rocks that are exposed in the Bodrak River valley within the limits of the second range of the Crimean Mountains are presented. Detailed magnetic cleanings showed the applicability of most of the sampled objects for paleomagnetic studies. The natural remanent magnetization of the examined sample is usually the sum of two components; the most stable of them possesses a bipolar distribution, indicating its primary character. The similarity between the paleomagnetic directions of subvolcanic bodies and nearly coeval volcanogenic-sedimentary rocks, which occur at angles of about 60°, suggests the disturbed occurrence of the igneous bodies. These results can be used for further detailed paleomagnetic studies of the Middle Jurassic igneous complexes in Mountainous Crimea for paleomagnetic reconstruction and the solution of local geological and structural geological problems.     

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

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

Evidence for the Mesozoic endogenous activity in the northeastern part of the Fennoscandian Shield

Paleomagnetic study of dykes and intrusions remanent in the central part of the Kola Peninsula has been carried out; the Devonian age of these objects has been confirmed by isotopic-geochronological studies. The component analysis of the magnetization vector in the samples has shown that there are two magnetization components in most samples. The paleomagnetic pole corresponding to the direction of a more stable component is located in the close vicinity of the Middle Devonian segment of the apparent polar wander path (APWP) for the East European Craton, so this enables us to estimate its age to be as old as the Devonian. The second magnetization component was found in Devonian dykes of both northern and southern parts of the Kola Peninsula; the paleomagnetic pole corresponding to this component is located close to the Mesozoic (Early Jurassic) part of the APWP for the East European Craton. It is suggested that the extensive remagnetization of Devonian intrusions in the Kola Peninsula was caused by the thermal effect of the Barents-Amerasian superplume and by the appearance of an extensive area with trap magmatism within the modern Arctic Basin region. Discovery of a significant thermal event that covered the Fennoscandian northeast allows us to explain the geochronological problem concerning the Mesozoic ages of particular singular zircon grains from Precambrian rocks of the shield derived via the SHRIMP method.     

The red bed controversy revisited: shape analysis of Colorado Plateau units suggests long magnetization times

Several Triassic and earliest Jurassic sedimentary units from the Colorado Plateau region have distributions of virtual geomagnetic poles (VGPs) that are highly elongate along the path of apparent polar wander (APW). This suggests that the remanent magnetizations measured in these units were acquired over an extended period of time, possibly approaching 35 m.y., and are not precisely coeval with the stratigraphic age of the rock. Comparison with other paleomagnetic studies shows that the observed elongation is not a general attribute of the age of the rock, nor is it related to paleolatitude. The rocks that yield elongate VGP distributions are dominantly red to brown mudstones, and it is possible that their remanence is dominated by a slowly acquired chemical remanent magnetization, as suggested by Larson et al. [J. Geophys. Res. 87 (1982) 1081] and other authors. However, several superficially similar units from the Colorado Plateau have nearly circular VGP distributions. The process by which remanence is acquired in clastic sedimentary rocks merits further study.     

Paleomagnetism of late Cretaceous dykes in the Gangdese belt: New constraints on the position and structure of the southern margin of Asia prior to the India-Asia collision

This paper report paleomagnetic data from late Cretaceous diorite dykes that sub-vertically intrude granodiorites in the eastern Gangdese belt near the city of Lhasa. Our research goals are to provide further constraints on pre-collisional structure of the southern margin of Asia and the onset of the India-Asia collision. Magnetite is identified as the main magnetic carrier in our study. The magnetite shows no evidence of metamorphism or alteration as determined from optical and scanning electron microscope observations. A strong mineral orientation is revealed by anisotropy of magnetic susceptibility analysis both for the intruded dykes and the country rocks. The authors interpret this AMS fabric to have formed during intrusion rather than deformation. Fifteen of 23 sites yield acceptable site mean characteristic remanences with dual polarities. A scatter analysis of the virtual geomagnetic poles suggests that the mean result adequately averaged paleosecular variation. The paleomagnetic pole from the Gangdese dykes yields a paleolatitude of 14.3°N±5.8°N for the southern margin of Asia near Lhasa. The paleolatitude corresponds to an in-between position of the Lhasa terrane during about 130–60 Ma. Furthermore, the mean declination of the characteristic remanent magnetization reveals a significant counterclockwise rotation of 18°±9° for the sampling location since about 83 Ma. In the light of tectonic setting of the dykes, the strike of the southern margin of Asia near Lhasa is restored to trend approximately about 310°, which is compatible with the hypothesis that the southern margin of Eurasia had a quasi-linear structure prior to its collision with India.©2023 China Geology Editorial Office.     

四川盆地前白垩系岩石物理性质的研究

<正> 作者通过对四川盆地及其附近地区自前震旦系至侏罗系各地层岩石的密度、纵波速度、横波速度、孔隙度、磁化率、剩余磁化强度和电阻率的测定,研究本区各地质时代地层岩石的物性参数分布特征,初步探讨影响这些物性参数变化的主要因素,为本区进一步寻找和勘探石油、天然气以及区域地质的研究,提供基础资料。     

Paleomagnetism of ultramafics from the Konder Massif and its age assessment

The petro- and paleomagnetic studies of ultramafic rocks (dunites, clinopyroxenites, kosvites) from the Konder Massif revealed the primary thermal remanance nature of the defined characteristic magnetization components. The calculated coordinates of the paleomagnetic poles are as follows: Plat = −4°, Plong = 178°, d_p = 5°, and d_m = 8° for the dunites; Plat = −2°, Plong = 181°, d_p= 6°, and d_m = 10° for the clinopyroxenites; and Plat = 71°, Plong = 206°, d_p = 5°, and d_m = 6° for the kosvites. Based on paleomagnetic and petromagnetic data, the age is estimated to be the Early Neoproterozoic for the dunites and clinopyroxenites and the Early Cretaceous for the kosvites. The massif as a whole is dated back to the Early Neoproterozoic (1000–950 Ma).     

Paleomagnetism of the Cretaceous Morelos and Mezcala Formations, southern Mexico

A paleomagnetic study of platform-facies carbonate rocks of the mid-Cretaceous Morelos Formation and deep-water carbonate rocks of the overlying Upper Cretaceous Mezcala Formation, sampled at Zopilote canyon, in Guerrero State, southern Mexico, indicates that their characteristic magnetization was acquired contemporaneously with folding of these rocks during the Late Cretaceous Laramide orogeny. The remanence carrier is interpreted to be magnetite, although other mineral phases of high coercivity carry recent secondary overprints. The overall mean is of Dec=323.1° and Inc=36.5° (k=162.7; α₉₅=2.7°; N=18 sites; 64% unfolding). Comparison with the North America reference direction indicates that this area has experienced a small, yet statistically significant, counterclockwise direction of 19.2±4.0°. Similar rotations are documented in other localities from southern Mexico; rotations are linked to mid-Tertiary deformation associated with the left-lateral strike-slip fault system that accommodated motion of the Chortis and Xolapa blocks.     

New Early Permian paleomagnetic results from the Brive basin (French Massif Central) and their implications for Late Variscan tectonics

In order to assess the structural evolution of the Brive basin and the Paleozoic activity of surrounding major faults in the French Massif Central, we carried out a paleomagnetic study on Early Permian rocks from this basin. Positive-fold tests and solely reversed polarities indicate that the characteristic remanent magnetization is likely to be primary. Early Permian tilt-corrected site mean declinations vary from 207°–167° indicating that the Brive basin experienced internal vertical-axis rotations. On the contrary, Late Permian paleomagnetic site means exhibit a circular Fisherian distribution showing no relative rotations. Detailed analyses of Permian paleomagnetic data from five contemporaneous basins of the French Massif Central reveal that these basins share the same equatorial paleolatitude with stable Europe throughout the Permian. However, in Early Permian, three of the five basins experienced differential rotations. The Saint-Affrique basin not only suffered internal deformation during the Early Permian, but the basin as a whole underwent a full-scale counterclockwise vertical-axis block rotation with respect to stable Europe. As a consequence, paleomagnetic data from similar late orogenic basins have to be thus carefully considered for establishment of Apparent Polar Wander paths.     

Paleomagnetism in the Neoarchean polyphase Panozero intrusion in the Fennoscandian Shield

Paleomagnetic research of the Neoarchean polyphase Panozero sanukitoid massif of the Fennoscandian Shield was performed. Paleomagnetic studies of three rock associations of the massif was used to obtain the paleomagnetic pole Φ = −10.2°C Λ = 226.1°C, dp = 4.9°, dm = 3.5° ϕm = − 36.1°. Positive tests of the contact zone between rocks of the sanukitoid massif and the country Mesoarchean metavolcanics, as well as overlying Jatulian amygdaloid basalts and diabases of the Segozero structural feature testify to the primary origin of the high-temperature component of rock magnetization obtained. The paleomagnetic pole obtained indicates that during the period from 2.74 to 2.73 Ga the Karelian Craton was located in tropical moderate latitudes of the South Hemisphere and it possibly moved to the tropical latitudes during the Neoarchean.     

Hydrothermal alteration and magnetic properties of rocks in the Carolina de Michilla stratabound copper district,northern Chile

In the Carolina de Michilla district, northern Chile, stratabound copper mineralization is hosted by Jurassic volcanic rocks along the trace of the Atacama fault system. In this study, we present the overall effects of hydrothermal alteration on the magnetic properties of rocks in this district. Two types of metasomatic alteration associations occur, one of regional extent and the other of local hydrothermal alteration associated with copper mineralization (e.g., Lince–Estefanía–Susana). Regional alteration is interpreted as a low-grade “propylitic association” characterized by an epidote–chlorite–smectite–titanite–albite–quartz–calcite association. The local hydrothermal alteration is characterized broadly by a quartz–albite–epidote–chlorite–calcite mineral assemblage. The most pervasive alteration mineral is albite, followed by epidote and, locally, actinolite. These minerals contrast sharply against host rock minerals such as chlorite, calcite, zeolite, prehnite, and pumpellyite, but alteration is constrained to mineralized bodies as narrow and low contrast alteration halos that go outwards from actinolite–albite to epidote–albite, to epidote–chlorite, and finally to chlorite. Hydrothermal alteration minerals, compared to regional alteration minerals, show iron-rich epidotes, a lower chlorite content of the chlorite–smectite series, and a nearly total albite replacement of plagioclase in the mineralized zones. Opaque minerals associated with regional alteration are magnetite and maghemite, and those associated to hydrothermal alteration are magnetite, hematite, and copper sulphides. We present paleomagnetic results from nine sites in the Michilla district and from drill cores from two mines. Local effects of hydrothermal alteration on the original magnetic mineralogy indicate similar characteristics and mineralogy, except for an increase of hematite that is spatially associated with the Cu–sulphide breccias with low magnetic susceptibilities. Results indicate that it is impossible to magnetically differentiate mineralized bodies from unmineralized lavas, except for pyrite-rich hydrothermal breccias. In conclusion, for stratabound copper deposits of the Michilla type, the overall effect of hydrothermal alteration on the paleomagnetic properties of rocks is of low contrast, not clearly discernable even at a small scale. From an exploration point of view, magnetic exploration surveys should not discern mineralized bodies of Cu–sulphide breccias except in detailed ground surveys due to the small size of contrasting bodies. Unoriented drill cores with primary ore mineralization record a characteristic remanent magnetization of reverse polarity. Taking into account the azimuth and dip of the drill cores, we were able to compare the magnetization of the mineralized bodies with the characteristic directions from sites drilled in situ from Late Jurassic–Early Cretaceous intrusives mostly. The characteristic direction recorded by the Pluton Viera is similar to the magnetization of the ore bodies of the Estefania mine. If copper mineralization mostly postdates the tilt of the volcanic flows, the low paleomagnetic inclinations suggest an age for the mineralization near 145 Ma, the time of the lowest paleolatitude for the South American plate during the Mesozoic.