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.     

Palaeomagnetic and rock-magnetic study of a Pliocene volcanic section in southern Georgia (Caucasus)

Twenty-six basaltic flows were sampled for a paleomagnetic and rock-magnetic study of the Tchuntchka section in the Akhalkalaki volcanic region in southern Georgia (Caucasus). Three to five samples from each flow were subjected to thermal or alternating-field demagnetization. The upper part of the section yields normal (D = 355.5°, I = 54.2°, N = 17, ₉₅ = 2.5°, k = 226), and the lower part reversed polarity directions (D = 180.3°, I = – 59.4,° N = 8, ₉₅ = 4.8°, k = 135). An anomalous direction was found in one flow in the upper part (D = 118.5°, I = – 77.3°). Rock-magnetic experiments show that the remanence is carried in most cases by magnetite or low-Ti titanomagnetite. The fraction of grains with a multidomain magnetic structure does not seem to be important. The remanence carried by such grains is removed only partly by low-temperature demagnetization. A tentative magnetostratigraphic correlation between the 3.8-Ma-old Thoki and Tchuntchka sites is proposed.     

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.     

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。     

Middle Paleozoic paleomagnetism of east Kazakhstan: post-Middle Devonian rotations in a large-scale orocline in the central Ural–Mongol belt

In order to test different hypotheses concerning the Paleozoic evolution of the Ural–Mongol belt (UMB) and the amalgamation of Eurasia, we studied Middle Devonian basalts from two localities (11 sites) and Lower Silurian volcanics, redbeds, and intra-formational conglomerates from three localities (20 sites) in the Chingiz Range of East Kazakhstan. The Devonian rocks prove to be heavily overprinted in the late Paleozoic, and a high-temperature, presumably primary, southerly, and down component is isolated at only four sites from a homoclinal section. Most Silurian redbeds are found to be remagnetized in the late Paleozoic; in contrast, a bipolar near-horizontal remanence, isolated from Silurian volcanics, is most probably primary as indicated by positive tilt and conglomerate tests. Analysis of paleomagnetic data from the Chingiz Range shows that southward-pointing directions in Ordovician, Silurian, and Devonian rocks are of normal polarity and hence indicate large-scale rotations after the Middle Devonian. The Chingiz paleomagnetic directions can be compared with Paleozoic data from the North Tien Shan and with the horseshoe-shaped distribution of subduction-related volcanic complexes in Kazakhstan. Both paleomagnetic and geological data support the idea that today's strongly curved volcanic belts of Kazakhstan are an orocline, deformed mostly before mid-Permian time. Despite the determination of nearly a dozen new Paleozoic paleopoles in this study and other recent publications by our team, significant temporal and spatial gaps remain in our knowledge of the paleomagnetic directions during the middle and late Paleozoic. However, the paleomagnetic results from the Chingiz Range and the North Tien Shan indicate that these areas show generally coherent motions with Siberia and Baltica, respectively.     

Primary curvature in the Mid-Continent Rift: Paleomagnetism of the Portage Lake Volcanics (northern Michigan, USA)

Rocks of the North American Mid-Continent Rift (MCR) in the Keweenaw Peninsula of northern Michigan display a change in structural trend from east to west, varying in strike from 100° in the east to 35° farther west, before returning to a more east–west trend of 80° near Wisconsin. In general, curvature can be described either as of primary origin, meaning that the arc developed in its present curved state, or as of secondary origin, when the arc formed from an initially straighter geometry. A powerful tool in evaluating the origin and degree of curvature in any deformed belt is paleomagnetism, where coincident change in declination and strike is evidence for secondary curvature. Several paleomagnetic studies have been completed on rift-related sedimentary rocks, as well as lava flows, from the MCR in the Lake Superior region. Paleomagnetic directions for the sedimentary rocks vary much more in declination than in inclination, possibly reflecting a vertical axis rotation. If secondary rotation has affected the sedimentary rocks, then underlying volcanic rocks should also demonstrate a similar rotation.Thirty-one sites were collected from the Portage Lake Volcanics in the most highly curved part of the Keweenaw Peninsula in the Upper Peninsula of Michigan. Sites were chosen to maximize the variation in structural trend. Thermal demagnetization results showed two components in samples from most sites, a lower temperature A component (< 580 °C) as well as a higher temperature B component (> 580 °C). Both components were tested for primary remanence using a conglomerate test. The A component, carried by magnetite, passes the conglomerate test at a 95% confidence level, and is therefore considered a primary remanence. The B component, carried by hematite, fails at the 95% level and is considered a secondary magnetization. Most importantly, declination of the primary (A) magnetization between sites shows no correlation with strike, demonstrating that vertical axis rotations cannot explain the curvature of the Mid-Continent Rift. We conclude that curvature in the Lake Superior Region is a primary feature, likely reflecting a pre-existing zone of weakness that was exploited during rifting.     

Paleomagnetism of the Society Cliffs dolostone and the age of the Nanisivik zinc deposits, Baffin Island, Canada

Dolostones of the ∼1200 Ma Society Cliffs Formation within the hydrothermal zone surrounding the Nanisivik zinc deposits retain a stable characteristic remanent magnetization (ChRM) on alternating field and thermal step demagnetization. Based on the thermal data and saturation isothermal remanence analyses, the ChRM resides in pseudosingle domain magnetite and hematite. A paleomagnetic fold test favours a post-folding ChRM, and a paleomagnetic contact test, using a Franklin gabbro dike, indicates that the ChRM predates ∼720 Ma. The pole position calculated from the ChRM direction is at 168.2°E, 42.8°N (δ_p=4.9°, δ_m=6.8°), giving an age of 1095 ± 10 Ma on the well-defined “Logan Loop” portion of the North American apparent polar wander path. This age is considered to date recrystallization of the dolostone host rocks in the halo around the hydrothermal sulfide deposits. No evidence is found for a postulated Cretaceous remagnetization event in the region. Received: 9 January 1999 / Accepted: 3 March 2000     

Low paleosecular variation at the equator: a paleomagnetic pilgrimage from Galapagos to Esterel with Allan Cox and Hans Zijderveld

McFadden and Merrill (1995) suggested that the paleosecular variation (PSV) measured by the angular scatter of the virtual geomagnetic pole is minimal at the equator and should be smaller during a superchron than during the last 5 Myr. We revisited a key site of the 0–5 Ma database, the Galapagos archipelago, studied by Allan Cox in the early sixties. We obtained 79 sites with reliable mean directions on four islands (San Cristobal, Floreana, Santa Cruz and Pinzon), showing a larger proportion of transitional data than Cox (16 instead of 6%), because the sampling was concentrated on the Brunhes-Matuyama transition as delimited by Cox. This dataset allowed us to test the statistical method of Vandamme (1994) to separate PSV from transitional data. We obtained an angular scatter value of 11.2° (9.9–12.9°), instead of 16.8° for an a-priori rejection angle of 40°, compared with the 12.7° predicted from the global compilation (McFadden et al. 1991). Studies of sequences of lava flows are quite scarce in the Permian Kiaman Superchron, and the Esterel volcanics with their subequatorial paleolatitude are a good candidate to test the above prediction. We confirm the quality of the original data of Zijderveld (1975) and we improved the mean direction from one site. We also used new geological and geochronological data: Ar/Ar ages point to the period 264–278 Ma for a totally reversed volcanic sequence, in agreement with an ending of the Kiaman Superchron at 262–268 Ma. The extremely low angular scatter obtained (4 to 8°, depending on data selection) confirms the prediction, but an alternative interpretation invoking a post-volcanic Permian remagnetization is discussed.     

How far between Iran and Eurasia was the Turan plate during Triassic–Jurassic times?

Calc-alkaline volcanic deposits from the south-west of the Turan plate, near the city of Turkmenbasi (40°00N, 52°58&\prime;E) in Turkmenistan, were studied paleomagnetically. These rocks have been affected by a greenschist-facies metamorphism, possibly of regional extent, that has been K/Ar-dated as 200 to 227 Ma old. A low-blocking-temperature component (D = 349°, I = 64°), close to the present field direction and probably of viscous or recent chemical origin, was isolated by a negative fold test at three sites. The mean direction of a high-blocking-temperature component isolated at 15 sites, mainly carried by magnetite, is scattered before and after tectonic correction and is therefore difficult to interpret. A group of seven sites with low inclinations before and after tectonic correction was isolated. The mean inclination of these sites (31 ± 8°), syn-folding or post-folding, corresponds to a paleolatitude of 17 ± 8° which is lower than the conventional Eurasian paleolatitudes for post-Permian times. The paleomagnetic data from the Turan and Iran plates constrain this low paleolatitude to the Late Triassic and Jurassic period. This requires a shortening of at least 7° between the Turan plate and Eurasia during this time.     

Paleomagnetism and U–Pb geochronology of the Black Range dykes,Pilbara Craton,Western Australia: a Neoarchean crossing of the polar circle

We report a new paleomagnetic pole for the Black Range Dolerite Suite of dykes, Pilbara craton, Western Australia. We replicate previous paleomagnetic results from the Black Range Dyke itself, but find that its magnetic remanence direction lies at the margin of a distribution of nine dyke mean directions. We also report two new minimum ID-TIMS ²⁰⁷Pb/²⁰⁶Pb baddeleyite ages from the swarm, one from the Black Range Dyke itself (>2769 ± 1 Ma) and another from a parallel dyke whose remanence direction lies near the centre of the dataset (>2764 ± 3 Ma). Both ages are slightly younger than a previous combined SHRIMP ²⁰⁷Pb/²⁰⁶Pb baddeleyite weighted mean date from the same swarm, with slight discordance interpreted as being caused by thin metamorphic zircon overgrowths. The updated Black Range suite mean remanence direction (D = 031.5°, I = 78.7°, k = 40, α₉₅ = 8.3°) corresponds to a paleomagnetic pole calculated from the mean of nine virtual geomagnetic poles at 03.8°S, 130.4°E, K = 13 and A₉₅ = 15.0°. The pole's reliability is bolstered by a positive inverse baked-contact test on a younger Round Hummock dyke, a tentatively positive phreatomagmatic conglomerate test, and dissimilarity to all younger paleomagnetic poles from the Pilbara region and contiguous portions of Australia. The Black Range pole is distinct from that of the Mt Roe Basalt (or so-called ‘Package 1’ of the Fortescue Group), which had previously been correlated with the Black Range dykes based on regional stratigraphy and imprecise SHRIMP U–Pb ages. We suggest that the Mt Roe Basalt is penecontemporaneous to the Black Range dykes, but with a slight age difference resolvable by paleomagnetic directions through a time of rapid drift of the Pilbara craton across the Neoarchean polar circle.     

Palaeomagnetic data of late Cretaceous rocks from Sumba,Indonesia; the rotation of the Sumba continental fragment and its relation with eastern Sundaland

Sumba island forms part of a continental fragment, located near the transition of the Sunda Arc to the Banda Arc. It lies within the forearc region, between the active volcanic arc to the north and the Java Trench to the south. Palaeomagnetic studies of Cretaceous (late Albian-early Campanian) Lasipu sediments revealed a mean characteristic remanence (ChRM) direction with D = 42.5°, I = –23.0° and _{_95} = 6.1°, indicating a palaeolatitude of 12° S. This ChRM is, most likely, a secondary magnetization, possibly caused by the intrusion of the 65-Ma-old Tanadaro granodiorite. This granodiorite gave a mean ChRM direction with D = 44.7°, I = –16.3°, and ₉₅ = 12.2°, pointing to a palaeolatitude of 8.3° S. Eastern Sundaland with Borneo, west and south Sulawesi, and Sumba formed one continental unit in the late Mesozoic, most likely attached to the southeast Asian mainland. Borneo and west and south Sulawesi underwent large counterclockwise (CCW) rotations since the Jurassic with 45° during the Cretaceous, and 45° during the Palaeogene. The Sumba microcontinent, most likely, became detached from eastern Sundaland soon after deposition of the Lasipu sediments. Palaeomagnetic data show that Sumba underwent subsequent clockwise (CW) rotations of up to 96°: 53° between 82 and 65 Ma, and 38° between 65 and 37 Ma. Since the late Eocene, only small rotations occurred. The data indicate that eastern Sundaland, including Sumba, remained close to the equator since the Jurassic. CW rotations occurred in Sundaland both in the north (Indochina) and in the west (Sibumasu) as a consequence of the India – Eurasia collision. The same sense of rotation is seen further east in Sulawesis East Arm and the Philippine Sea plate. Eastern Sundaland (Borneo and west Sulawesi) with CCW rotations is being trapped between these CW rotating plates.     

内蒙古中部晚石炭世—早二叠世古地磁新数据及其地质意义

对采自内蒙古苏尼特左旗白音乌拉乡晚石炭世—早二叠世宝力高庙组和敖汉旗安庆沟乡晚石炭世酒局子组的凝灰岩和凝灰质砂岩开展了古地磁学研究。宝力高庙组和酒局子组样品退磁特征表明,高温剩磁分量均以赤铁矿为主要载磁矿物,可能携带了岩石形成时期的特征剩磁分量。层面校正后2个组平均特征剩磁分量分别为:D=159.3°,I=-33.1°,α95=8.7°;D=328.8°,I=31.3°,α95=5.7°。这反映了内蒙古中部晚石炭世—早二叠世位于北纬17°—18°N。将所获得的古地磁结果与已发表的相邻地区古地磁数据进行比较,并结合古生物、古气候等地质数据,推测晚石炭世—早二叠世时内蒙古中部与华北地块关系密切,距离较近,而远离西伯利亚地块,内蒙古中部晚石炭世—早二叠世时可能是华北地块的组成部分。     

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 African Lower Carboniferous paleomagnetic pole from intrusive rocks of the Tin Serririne basin (Southern border of the Hoggar, Algeria)

A paleomagnetic study has been conducted on intrusive doleritic rocks cropping out within Devonian horizontal tabular formations of the Saharan craton (Tin Serririne basin, South of Hoggar shield). The ⁴⁰K/⁴⁰Ar dating of the dolerites gave an age of 347.6 ± 8.1 Ma, i.e. Tournaisian. The paleomagnetic data present three different directions. The first has a paleomagnetic pole close to the previous African poles of Permian age. This direction is therefore interpreted as a Permian remagnetization. The second direction, which is defined by both linear regression and remagnetization circles analysis, is considered as the primary magnetization. It yields a new African Tournaisian paleomagnetic pole (λ = 18.8° S,  = 31.2° E, K = 29, A₉₅ = 7.5°) very close to the Ben Zireg Tounaisian pole [Aifa, T., Feinberg, H., Pozzi, J.P., 1990. Devonian/Carboniferous paleopoles for Africa. Consequences for Hercynian geodynamics. Tectonophysics, 179, 288–304]. The third direction has intermediate orientation between those of the first or second directions and that of the Upper Cenozoic field. It is interpreted as related to a composite magnetization. This new Tin Serririne pole improves the APWP of Gondwana, for this key period of the evolution of the Pangea. This APWP confirms the previous paleogeographic reconstruction which shows that the pre-Hercynian ocean between Gondwana and Laurussia is still not close during the beginning of the Carboniferous.     

Late Paleozoic remagnetization of the Trenton Formation in Ordovician petroleum reservoirs of southwestern Ontario

A paleomagnetic study of subsurface core samples from dolomitized carbonates of two producing reservoirs in the Upper Ordovician Trenton Formation, collected from four wells in southwestern Ontario yielded a paleomagnetic direction of D = 152.3°, I = − 12.3° (N = 49, α₉₅ = 8.7). This characteristic remanent magnetization (ChRM) direction was azimuth-corrected by aligning the viscous remanence magnetization (VRM) with the present Earth's magnetic field direction. A drilling-induced magnetization (VRMdi) was present in less than half the specimens sampled in this study. In addition, where the VRM correction could not be made, a paleolatitudinal arc calculated from the inclination-only mean of I = − 9.0° (N = 34, α₉₅ = 3.0°) intersected the apparent polar wander path in the Late Permian–Early Triassic. These paleodirections are similar to the paleomagnetic directions observed in Ordovician Trenton carbonates from the Michigan Basin and New York State, U.S.A., suggesting a related regional late Paleozoic remagnetization.     

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.     

Middle Devonian Paleomagnetism of Geological Complexes of Central Tuva

New paleomagnetic data are obtained for Middle Devonian rocks of Central Tuva. The rocks contain one-, two-, or three-component magnetization. The low-temperature (LT) components of magnetization are close to the directions of the present-day or Cenozoic magnetic field in Tuva. Based on the directions of the high-temperature (HT) components of magnetization, which were distinguished in the magnetite spectrum of blocking temperatures of up to 580оС, we revealed a prefolding magnetization of different polarity. The time when Middle Devonian rocks acquired the prefolding HT component of magnetization almost does not differ from the time of rock formation. Middle Devonian sequences were formed at low latitudes (19°–25° N). We calculated the Middle Devonian paleomagnetic pole (Φ =–13°, Λ = 106°, A₉₅ = 7), which can be used to describe the movement of the Caledonian block in Central Asia, and probably Siberia, if these blocks had been tectonically coupled by the Devonian.     

Shear in the tethys and the Permian paleomagnetism in the Southern Alps, including new results

To verify paleomagnetic proof for megatectonic translation in the Tethys a large collection of samples from a key area, the Bolzano Quartz Porphyry Plateau in the Southern Alps, was examined. Their natural remanent magnetization was analyzed with thermal, and mainly alternating field demagnetization. The result is a well-established paleomagnetic direction of D: 150° and I: −19.5° (₉₅ = 4.9), obtained from 152 samples from 39 sites distributed over 12 volcanic units. It is argued that the inclination of this result is not significantly different from that which can be extrapolated for the Southern Alps from Early Permian paleomagnetic directions of the stable European shield. Consequently it is concluded that a paleomagnetic indication for megatectonic translation of the Southern Alps is virtually absent. But a large counterclockwise deviation of the declination is evident, and is easily explained by a counterclockwise rotation of 50° of the Southern Alps with respect to stable Europe. Since the paleomagnetic direction of the Early Permian volcanics of the Southern Alps fits in reasonably well with the (poorly known) Early Permian paleomagnetic pattern of Africa, a coherence between both regions is presumed.     

帕米尔-西昆仑地区新生代古地磁结果及其构造意义

通过对帕米尔-西昆仑地区新生代地层51个采点古地磁样品系统的古地磁测试,获得了研究区新生代较可靠的古地磁数据。尽管上述研究剖面因为单斜地层无法对所获得的古地磁结果进行褶皱检验,但从实验结果可以看出,其地理坐标下平均的高温特征剩磁方向远离现代地磁场方向,且和田朗如乡古近纪、策勒恰恰古近纪、叶城柯克亚乡新近纪剖面所获得的古地磁结果具有正、反2种极性,由此,我们认为以上剖面的高温特征剩磁很可能代表了岩石形成时的原生剩磁方向。结合研究区已有的古地磁数据,认为在新生代印度板块向北挤压作用下,塔里木地块西缘地区(帕米尔高原东北缘)早白垩世-晚白垩世始相对欧亚大陆在古地磁误差范围内并没有发生明显的构造旋转作用(1°~1.6°),而始新世以来相对欧亚大陆则发生了明显的逆时针旋转(22°~38°),该地区的逆时针旋转作用可能与塔拉斯-费尔干纳断裂新生代以来的右旋走滑作用有关,而在帕米尔高原以东则主要以沿大型走滑断裂的走滑作用为主,并没有发生明显的旋转作用。