Paleomagnetic evidence for non-apenninic origin of the sila nappes (Calabria)

The magnetization of Lower Permian rocks from Sila has a mean direction D = 56.5°, I= +20.4° with ₉₅ = 9.1° after correction for Upper Neogene tilting. A further correction for the attitude of the nappes after their Middle Miocene emplacement establishes paleolatitudes consistent with those from the Lower Permian Tethys. The remarkable internal consistency of the data has not supported the distinction of units with opposite vergences within the Sila crystalline nappes. The declination indicates that the Sila massif has rotated counter-clockwise by about 90° relative to the Apennines, Sardinia and the Southern Alps and therefore the well-known Apenninic rotation alone does not account for the total change of direction in tectonic transport. Accordingly, the structural trends of tectonic phases older than the emplacement time of the Calabrian nappes should no longer be referred to present-day geographic coordinates. The post-Late Cretaceous motion relative to the north Calabrian Apennines enhances the geotectonic role of the northern boundary of the Calabrian—Peloritan arc, since its sinistral-shear character permits both tectonic transport from the west and counter-clockwise motion during tectonic transport.     

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.     

Emplacement at granulite facies conditions of the Sesia–Lanzo metagabbros: an early record of Permian rifting?

The evolution of the pre-Alpine Corio and Monastero metagabbros points to strong chemical and mineralogical similarities with that of other Permian gabbro bodies of the Alps, which are concentrated in the Southalpine and Austroalpine domains. The structural and metamorphic pre-Alpine evolution of these gabbros records a re-equilibration following the emplacement in the deep crust (P=0.6–0.9 GPa and T=850±70 °C), exhumation through amphibolite facies conditions (P=0.5–0.35 GPa and T=570–670 °C), followed by a greenschist facies imprint (0.25≤P≤0.35 GPa and T<550 °C). This retrograde P–T evolution suggests that the exhumation occurred in a high thermal gradient regime, such as that induced by upwelling of an asthenospheric plume during continental rifting. This would be consistent with the crustal thinning known to have occurred in both the Southalpine and Austroalpine domains during Permian times. The gabbros and their country acid granulites are spatially associated with the serpentinised subcontinental mantle of the Lanzo Massif. This lithologic association and the metamorphic evolution is similar to that of the Fedoz gabbro (Austroalpine Domain of the Central Alps) and completely different from that observed in passive margins, where no remnants of the lower crust occur and the upper granitic crust directly overlies the serpentinized lherzolites. The location of Permian gabbro bodies in the Austroalpine and Southalpine domains and their absence in the Helvetic domain is evidence for asymmetric rifting.     

Potassium-argon ages from the Ceneri Zone,Southern Swiss Alps

The Ceneri Zone is a unit of the crystalline basement of the Southern Alps. Its northern boundary is the Tonale Line segment of the Periadriatic Line, an important tectonic lineament separating the Oligocene and younger features of the Central Alps from the older metamorphic and structural trends of the Southern Alps. Unmetamorphosed Permian and younger sedimentary units lap onto the Southern Alpine basement from the south.Potassium-argon results from the Ceneri Zone define a Hercynian age pattern typical for the basement of continental Europe. This pattern extends to within at least 100 meters of the Tonale Line. Thus, amphibolite facies metamorphism in this region occurred around 325 m.y. ago. The geochronologic similarity of the Southern Alps to many other European regions must be taken into account in megatectonic theories.In detail, the Hercynian age pattern of the Ceneri Zone is complicated. Some hornblendes have apparent ages between the Hercynian and a Caledonian value (430 m.y.). They probably retained some radiogenic argon during the Hercynian upper amphibolite facies metamorphism. In addition, mica results between 200 and 300 m.y. have a strong geographic correlation. Apparently, the northwestern portion of the Ceneri Zone was reheated or mildly metamorphosed during the Upper Triassic to Lower Jurassic. A relationship between these ages and 170–180 m.y. ages from the neighboring Ivrea-Verbano Zone seems likely. No geologic evidence for any post-Hercynian event has been noted as yet in the Ceneri Zone.     

Tectonic implications of paleomagnetic research into Upper Cretaceous magmatic rocks in the Apuseni Mountains, Romania

The paleomagnetism of Upper Cretaceous magmatic rocks from 47 collecting sites (172 samples, 692 specimens) in the Apuseni Mountains was studied. After AF cleaning, characteristic magnetizations were identified for various collecting areas in the study zone, which defined a few spatial and temporal units for which paleomagnetic poles could be derived statistically. At 21 sampling sites the paleomagnetic directions showed a high level of intrasite and intersite consistency, with a mean direction of I_f = −38° and D_f = −100°, with ₉₅ = 6°. The paleomagnetic results show that to reach their present-day position the Apuseni Mountains moved to the north, around 14° with respect to Europe, or around 25° with respect to the geographic poles, between the Campanian and, probably, Late Miocene, while a clockwise rotation, of around 80°, was taking place.     

Paleomagnetic results from the Permian Rodez basin implications: the Late Variscan tectonics in the southern French Massif Central

A paleomagnetic study has been carried out on three sedimentary formations of the Permian Rodez basin in the southern France. Two of them yield paleomagnetic poles of Saxonian and Thuringian age showing counterclockwise rotation of moderate amplitude, during or after the Thuringian deposition. For the French Massif Central, contrary to its stable southern (Lodève basin) and eastern (Largentière basin) borders, on its southwestern border, in a large area including the Rodez, Saint-Affrique and perhaps Brive basins suffered rotations due to the extensional tectonics during the Late Variscan period.     

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.     

准噶尔地块二叠纪岩脉的古地磁研究

作者通过对准噶尔地块的岩脉采样的古地磁研究以及大量地质资料,确认了准噶尔地块在早二叠世位于北纬35°,与塔里木地块、哈萨克斯坦地块及西伯利亚基本上在同一纬度带中,作为劳亚大陆的一个组成部分,作者并对该区自早二叠世以来的构造变动作了分析.     

Paleomagnetic results from the Permian Rodez basin implications: the Late Variscan tectonics in the southern French Massif Central

Abstract

A paleomagnetic study has been carried out on three sedimentary formations of the Permian Rodez basin in the southern France. Two of them yield paleomagnetic poles of Saxonian and Thuringian age showing counterclockwise rotation of moderate amplitude, during or after the Thuringian deposition. For the French Massif Central, contrary to its stable southern (Lodève basin) and eastern (Largentière basin) borders, on its southwestern border, in a large area including the Rodez, Saint-Affrique and perhaps Brive basins suffered rotations due to the extensional tectonics during the Late Variscan period. © 2002 Editions scientifiques et médicales Elsevier SAS. All rights reserved.     

Paleomagnetism of the triassic chugwater redbeds revisited (Wyoming, U.S.A.)

Paleomagnetic results from 107 samples of the Chugwater Group near Lander, Wyoming, show a regular progression in pole positions from bottom to top of the sequence. This pole position trend of about 25° matches very well the North American apparent polar wander path between Early Permian and Early Triassic. It could be argued that this “agreement” results in a conflict between the apparent magnetic age (Permian) and the Early to Late Triassic age generally assigned to the Chugwater Group. However, similar progressions of paleomagnetic pole positions have been reported for the Early Triassic Moenkopi Formation in Colorado; thus it appears that long-term variations and swings characterized the geomagnetic field at that time. With detailed paleomagnetic sampling, these features can be utilized for stratigraphic correlation in addition to magnetic-reversal stratigraphy. This will eliminate, to some degree, part of the non-uniqueness inherently present in correlations based on reversal stratigraphy only.     

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。     

Oroclinal bending of the Middle and Late Paleozoic volcanic belts in Kazakhstan: Paleomagnetic evidence and geological implications

Paleomagnetic data on Middle- and Late-Paleozoic rocks from the central part of the Ural-Mongolian Belt in Kazakhstan are considered. The primary remanences in the Permian rocks and secondary magnetization components of the same age in pre-Permian rocks of central and northern Kazakhstan are not rotated relative to the East European Platform. In southern Kazakhstan adjoining the Tien Shan almost all data point to large, up to 90°, counterclockwise rotation of blocks. These rotations, related to the regional wrench fault zone, must be subtracted from older paleomagnetic data to ensure their correct interpretation. The paleomagnetic declinations in Upper Carboniferous rocks coincide more or less over all of Kazakhstan, whereas the Silurian and Early Devonian declinations in the north and south of Kazakhstan differ approximately by 180°. It can be suggested that the Devonian volcanic belt, having a horseshoe outline, was initially an almost rectilinear NW-trending feature. Its oroclinal bending took place in the Devonian and Early Carboniferous and completed by the Late Carboniferous. We compared the model of the Kazakh Orocline based on paleomagnetic data with the geological events in this territory. It turned out that a slow bending of an initially rectilinear subduction zone is consistent with lateral migration of active volcanism and folding inside a developing loop, whereas extension outside the loop was accompanied by subsidence and rifting. In general, the proposed model connects the main tectonic events in Kazakhstan with the movements established from paleomagnetic data.     

In the wake of a counterclockwise rotating Adriatic microplate: Neogene paleomagnetic results from northern Croatia

In northern Croatia, the Neogene sediments cover complicated basement rocks which consist of Alpine and Dinaridic elements in the Zagorje area, and Variscan - Alpine formations of the Tisia (Tisza) megatectonic unit in the Slavonian Mountains (Mts.). The Neogene sediments were deposited in two separate basins before the Karpatian, but sedimentation became uniform from the Karpatian onwards (~17.5 Ma). Of the 24 localities that we have studied so far paleomagnetically from northern Croatia, 16 localities are of Karpatian or younger age, while the rest are pre-Karpatian. As a result of laboratory analysis, 16 localities yielded tectonically interpretable results. Our data suggests that northern Croatia shifted northwards, while rotating moderately counterclockwise, probably before the Karpatian. A second counterclockwise rotation occurred at the present latitude in post-Pontian times. There is no significant difference between localities situated above different basements, though Tisia is pictured as rotating clockwise in the Neogene. The paleomagnetic pattern of northern Croatia resembles that of areas situated north of the Periadriatic-Balaton line. Therefore, we conclude that northern Croatia is part of a larger block, dissected by several important tectonic lines, driven by the counterclockwise rotated Adriatic microplate.     

New paleomagnetic data from Late Neoproterozoic sedimentary successions in Southern Urals,Russia: implications for the Late Neoproterozoic paleogeography of the Iapetan realm

We present the results of paleomagnetic study of Ediacaran sedimentary successions from the Southern Urals. The analysis of the sedimentary rocks of the Krivaya Luka, Kurgashlya and Bakeevo Formations reveal stable mid-temperature and high-temperature remanence components. Mid-temperature components were acquired during Devonian (Bakeevo Formation) and Late Carboniferous–Early Permian remagnetization events. The high-temperature components in Kurgashlya and Bakeevo Formations are interpreted to be primary, because they are supported by a positive conglomerate test (Bakeevo Formation) and magnetostratigraphic pattern (Kurgashlya Formation). The high-temperature component in the Krivaya Luka Formation is interpreted to be a Late Ediacaran overprint. Our new paleomagnetic poles together with some previously published Ediacaran poles from Baltica and Laurentia are used herein to produce a series of paleogeographic reconstructions of the opening of the Iapetus Ocean.     

Internal deformation of Sikhote Alin volcanic belt, far eastern Russia: Paleocene paleomagnetic results

We present paleomagnetic results of Paleocene welded tuffs of the 53–50 Ma Bogopol Group from the northern region (46°N, 137°E) of the Sikhote Alin volcanic belt. Characteristic paleomagnetic directions with high unblocking temperature components above 560 °C were isolated from all the sites. A tilt-corrected mean paleomagnetic direction from the northern region is D=345.8°, I=49.9°, α₉₅=14.6° (N=9). The reliability of the magnetization is ascertained through the presence of normal and reversed polarities. The mean paleomagnetic direction from the northern region of the Sikhote Alin volcanic belt reflects a counterclockwise rotation of 29° from the Paleocene mean paleomagnetic direction expected from its southern region. The counterclockwise rotation of 25° is suggested from the paleomagnetic data of the Kisin Group that underlies the Bogopol Group. These results establish that internal tectonic deformation occurred within the Sikhote Alin volcanic belt over the past 50 Ma. The northern region from 44.6° to 46.0°N in the Sikhote Alin volcanic belt was subjected to counterclockwise rotational motion through 29±17° with respect to the southern region. The tectonic rotation of the northern region is ascribable to relative motion between the Zhuravlevka terrane and the Olginsk–Taukhinsk terranes that compose the basements of the Sikhote Alin volcanic belt.     

Late Cenozoic to recent transtensional deformation across the Southern part of the Gaoligong shear zone between the Indian plate and SE margin of the Tibetan plateau and its tectonic origin

Chuquicamata, in northern Chile, is one of the largest porphyry copper deposits in the world; the western side of its orebody is bounded by a major longitudinal fault, the West fault. We report paleomagnetic results from surface sites and drill cores from different geological units at Chuquicamata, especially within the late Eocene Fiesta granodiorite of the western block of the West fault. Characteristic remanent magnetizations (ChRM) were determined after detailed thermal or alternating field demagnetization. Soft components carried by multidomain magnetite crystals in the Fiesta granodiorite were removed by AF demagnetization at 10–20 mT. The ChRMs, not demagnetized by alternating fields up to 100 mT, have unblocking temperatures above 580 °C with ~ 75% of the magnetization removed in the temperature range of 580–590 °C. Optical and SEM mineralogical observations, and microprobe data indicate the occurrence of multidomain magnetite formed during a late magmatic stage of alteration coeval with strong oxidation of primary titanomagnetite and formation of ilmenite, hematite, pseudobrookite, and rutile. The characteristic directions have negative inclinations and declinations (330° to 230°); strongly deflected from the expected Eocene direction. Anisotropy of magnetic susceptibility (AMS), with degree up to 1.4, is carried by multidomain magnetite. AMS ellipsoids have subvertical foliations with azimuth varying strongly from N280° to N20°. We show that both the ChRMs and the AMS fabrics record the same apparent relative rotations between sites. Although the AMS anisotropy is high, there is no evidence for a solid-state deformation and the apparent rotation of the magnetic fabric is interpreted to be the consequence of small-block rotation. The apparent large (> 100°) counterclockwise rotations of small blocks within the Fiesta granodiorite suggest a wide damaged zone related to sinistral displacement along the West fault. This interpretation is consistent with previous models indicating that the Fiesta granodiorite was sinistrally translated and brought in front of the early Oligocene porphyry copper deposit during the Oligocene–early Miocene. This study shows that paleomagnetic markers are useful for improving the quantification and understanding of small-scale deformation within plutons adjacent to major fault zones.     

Paleomagnetism of Permo-Triassic Redbeds from the Asturias and Cantabric Chain (northern Spain): evidence for strong lower Tertiary remagnetizations

The paleomagnetic analysis of the Permo-Triassic redbeds outcropping in the western part of the Cantabric Chain and the small Mesozoic basin from the Asturias shows that these formations have a history of complex magnetization. Only a few sites did not experience the remagnetization processes and retained original directions. The most reliable results yield a paleomagnetic pole located at: lat. 49° N, long. 217° E (n = 11, ₉₅ = 3.7°), which is suggested as reliable Permo-Triassic data for the Iberian plate. Two remagnetization phases are recognized: a moderate phase predating the folding gave rise to a first overprinting. It is connected with the distension which occurred in the Pyreneo-Cantabrian region during the upper Jurassic-lower Cretaceous. The main remagnetization phase which occurred after the folding is dated from the lower Tertiary, and can be related to the compression induced on the northern boundary of Iberia from upper Cretaceous onwards. In some cases this phase led to a complete replacement of the primary magnetization.

Previously published data, which were at the time interpreted as being European-like in direction, are attributed to this phase. Hence, our results do not support the hypothesis of a micro-plate called “le Danois block”, which was suggested in order to explain these results. We believe that there is no paleomagnetic evidence supporting the existence of a complicated boundary between Europe and Iberia during the mid-Cretaceous opening of the Bay of Biscay.     

A palaeomagnetic study of some basaltoids and ores from the Pontic Ranges, Northern Turkey: Palaeogeographic implications

The palaeomagnetism of basic rocks and sulphide ores has been studied in the Küre area, Pontic Ranges, Turkey. Progressive alternating-field demagnetization revealed a characteristic remanent magnetization in all investigated rock types except a dacite. The following virtual geomagnetic poles were obtained:

Basalt and quartz diabase (oldest): D = 59°, I = +66°, ₉₅ = 4.8, pole 49°N, 93°E. Diabase: D = 210°, I = −15°, ₉₅ = 15.0, pole 47°N, 167°E. Massive sulphide ores: D = 107°, I = +63°, ₉₅ = 8.7, pole 18°N, 80°E. Peridotite: D = 131°, I = +54°, ₉₅ = 10.9, pole 2°S, 72°E. Amphibolitized diabase (youngest): D = 293°, I = +59°, ₉₅ = 12.6, pole 40°S, 145°E.

The longidutinal difference in pole positions between the oldest and the youngest rocks is interpreted as being due to a post-Permian counterclockwise rotation of the studied region in relation to the African continent. In addition, there are indications of local rotational movements within the Küre area.     

New Cretaceous paleomagnetic results from the foreland of the Southern Alps and the refined apparent polar wander path for stable Adria

The central-western and the eastern Southern Alps are separated by the triangular shaped Adige embayment, which belongs to stable Adria and was the site of pelagic sedimentation from the Tithonian through Maastrichtian. The first part of this study presents paleomagnetic results from the Tithonian–Cenomanian Biancone and Turonian–Maastrichtian Scaglia Rossa formations sampled at 33 geographically distributed and biostratigraphically dated localities.The new and high quality paleomagnetic results from the Adige embayment are then combined with coeval paleomagnetic directions from autochthonous Istria (Márton et al., 2008), which also belongs to stable Adria. The combined data set (which for the Late Albian–Maastrichtian time period is constructed similarly to the synthetic African curve by Besse and Courtillot, 2002, 2003) reveals an important tectonic event (Late Aptian–Early Albian) characterized by 20° CCW rotation and sedimentary hiatus.Comparison between paleomagnetic declinations/inclinations expected in an African framework (i.e. with the assumption that Adria is still an African promontory) leads to the following conclusions. The time-distributed Tithonian and Berriasian (150–135 Ma) paleomagnetic directions exhibit the “African hairpin” with an inclination minimum and a sudden change from CW to CCW rotation at 145 Ma. Concerning the younger ages, the declinations for Adria continue to follow the African trend of CCW rotation till the end of Cretaceous. However, the Tithonian–Maastrichtian declination curve for stable Adria is displaced by 10° from the “African” curve as a result of two rotations. The first, an about 20° CW rotation of Adria with respect to Africa took place between the Maastrichtian and the mid-Eocene. During this time the orientation of Adria remained the same, while Africa continued its CCW rotation. The younger rotation (30°CCW) changed the orientation of Adria relative to Africa as well as to the present North.     

阿尔金断裂走滑作用对青藏高原东北缘山脉形成的古地磁证据

通过对青藏高原北部阿尔金断裂东缘早白垩世-第三纪红层与玄武岩38个采点的系统古地磁测定,获得了研究区早白垩世-第三纪高温特征剩磁分量。结果表明,昌马乡早白垩世红层与玄武岩剖面层面坐标下高温特征剩磁平均方向(Ds=32.8°,Is=59.4°,κs=36.2,α95=8.1°)和北大窖早白垩世玄武岩剖面层面坐标下高温特征剩磁平均方向(Ds=335.4°,Is=55.1°,κs=34,α95=9.6°)均通过了褶皱检验,可能代表岩石形成时的原生剩磁。旱峡地区早白垩世地层层面坐标下高温特征剩磁平均方向(Ds=26.1°,Is=49.5°,κs=28.6,α95=7.3°)和红柳峡早第三纪地层层面坐标下高温特征剩磁平均方向(Ds=355.4°,Is=48.3°,κs=135.8,α95=7.9°),这两组高温特征剩磁方向在地理坐标下均远离现代地磁场方向,且具有正、反双极性特征,说明其也可能代表了岩石形成时的原生剩磁方向。结合已有阿尔金断裂及周边早白垩世-第三纪古地磁结果,提出柴达木块体在新生代印度/欧亚大陆碰撞挤压下并没有发生明显的整体顺时针旋转作用,青藏高原东北地区的块体旋转作用是阿尔金断裂左旋走滑作用在青藏高原东北缘转换的重要表现形式。