The origin of Neogene tectonic rotations in the Galatean volcanic massif, central Anatolia

A paleomagnetic study was carried out on Neogene volcanic rocks at 30 sites within the Galatean massif (40.4°N, 31.5°E) to determine possible block rotations due to stress variations. Two phases of rotation could be characterized as the result of Neogene volcanic activity. We suggest that the first stage of rotation was isolated in Early Middle Miocene calc-alkali rocks, with a relative counterclockwise rotation of R ± ΔR = −20.2 ± 9.3° with respect to Eurasia. This accommodates the south-westward rotational collapse of the Western Anatolia peninsula across a pole on the Bitlis suture. In the neotectonic period, on other hand, a relative clockwise rotation of R ± ΔR = 27.3 ± 6.4° with respect to Eurasia is predicted. In contrast to the uniform clockwise rotations, extremely large clockwise rotations up to 264° are restricted in a narrow zone between two dextral faults. We believe that the second stage rotations support the idea of individual microblock rotations due to deformation along the North Anatolian Fault zone.     

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。     

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.     

Paleomagnetic constraints on neotectonic deformation in the Kashi depression of the western Tarim Basin,NW China

A paleomagnetic study is reported of Eocene to Pliocene formations from the Kashi depression, which aims to constrain the pattern of neotectonic deformation within the western sector of the Tarim Basin in northwest China. With the exception of Pliocene specimens from one locality (East Kulukeqiati) which show large within site-mean variations in declination, most sites from five sampled formations yield well-grouped characteristic remanent magnetizations and positive fold tests and are of probable post-depositional detrital origin. First-order consistency of paleomagnetic results from a range of rock ages and localities demonstrates that only small inter-locational vertical-axis rotation has occurred here and indicates that the Kashi depression is decoupled from the remainder of Tarim to the east and has behaved as a quasi-rigid block which has rotated by 20–30° counterclockwise relative to Eurasia and North China since the late Pliocene. The crustal-scale Talas-Ferghana Fault cuts the Tian Shan and meets the Kashi depression in the region immediately to the northwest of the study region and we find no paleomagnetic evidence for differential rotations to suggest that this fault zone extends southwards across the Kashi depression to link with the North Pamir Thrust Fault (NPTF). Instead, we argue that the southern extension of this zone is a transform-orogen junction with southward motion of the eastern wall accommodated by southward thrusting at the margins of the south Tian Shan and the Tarim Basin. We propose that dextral transpression around the margins of the crustal block incorporating the Kashi depression was responsible for the contrasting amounts of thrusting on the NPTF in the southwest and the South Tian Shan Thrust Fault in the north. Extensive evidence for neotectonism in the bordering zones of this block, as well as some paleomagnetic evidence from low unblocking temperature components, indicates that the deformation produced by block rotation is ongoing.     

Reinterpretation of the Ordovician rotations in NW Argentina and Northern Chile: a consequence of the Precordillera collision?

Early Paleozoic paleomagnetic data from NW Argentina and Northern Chile have shown large systematic rotations within two domains: one composed of the Western Puna that yields very large (up to 80°) counter-clockwise rotations, and the other formed by the Famatina Ranges and the Eastern Puna that shows (~40°) clockwise rotations around vertical axes. In several locations, lack of significant rotations in younger rocks constrains this kinematic pattern to have occurred during the Paleozoic. Previous tectonic models have explained these rotations as indicative of rigid-body rotations of large para-autochthonous crustal blocks or terranes. A different but simple tectonic model that accounts for this pattern is presented in which rotations are associated to crustal shortening and tectonic escape due to the collision of the allochthonous terrane of Precordillera in the Late Ordovician. This collision should have generated dextral shear zones in the back arc region of the convergent SW Gondwana margin, where systematic domino-like clockwise rotations of small crustal blocks accommodate crustal shortening. The Western Puna block, bordering the Precordillera terrane to the north, might have rotated counterclockwise as an independent microplate due to tectonic escape processes, in a fashion similar to the present-day relationship between the Anatolia block and the Arabian microplate.     

Paleo—Latitude Variation of Guizhou Terrain from Devonian to Cretaceous

Over 800 paleomagnetic samples were collected from 79 sample localities, ranging in age from Devonian, Carboniferous, Permian to Jurassic for paleo-latitude research on the Guizhou terrain. The area sampled covers 13 counties with an area of about 50000 km². The paleomagnetic results obtained indicate that the Guizhou terrain was at 11.4°S in Devonian, 4.5°-9.3°S in Carboniferous, 2.6° − 4.5°S in Permian, 14.8°N in Triassic and 24.5° − 26.0°N in Jurassic. In the Cretaceous period, the paleo-latitude of the area was at 22.4 − 23.6°N. Therefore, a variation curve of paleo-latitude is established in this paper for the Guizhou terrain from Late Devonian to Late Cretaceous time.     

Triassic paleomagnetism from the Western Pyrenees revisited: implications for the Iberian–Eurasian Mesozoic plate boundary

Since the pioneering studies of Van der Voo [Tectonophysics 7 (1969) 5] and Van der Voo and Boessenkool [J. Geophys. Res. 78 (1973) 5118], paleomagnetism of Permo-Triassic redbeds and volcanics from the Western Pyrenees has furnished important contributions for delineating the Mesozoic boundary between the Iberian and Eurasian plates. In this paper, we present a new paleomagnetic study focussed on Triassic red beds (23 sites) of the Paleozoic Basque Massifs (PBM). The aim of this study is to complement previous studies done in those massifs to better constrain the complex kinematics of the Western Pyrenees. Two stable magnetic components have been isolated: (1) a dual polarity, pre-folding magnetisation carried by specular hematite; and (2) a secondary, normal polarity component also carried by hematite. Our data confirm both the origin and the rotation pattern of the primary remanence described in previous works. Nevertheless, field tests performed on the secondary component do not confirm the earlier interpretations by Schott and Peres [Tectonophysics 156 (1988) 75] as they indicate a synfolding nature of the remagnetisation instead of a post-folding origin. We consider that the secondary component is better explained if a Cretaceous age is considered. The presence of such remagnetisation in the western Pyrenees strengthens the widespread occurrence of similar remagnetisation events reported in northern Iberia in connection with the extensional tectonic events that occurred during Cretaceous times. A comparison of the rotations recorded by the Triassic component and by the remagnetisation indicate that the Paleozoic units underwent variable tectonic rotations before the remagnetisation was acquired, most likely in connection with the counterclockwise rotation of Iberia with respect to Eurasia. These results favour that the Mesozoic plate boundary between the Iberian and Eurasian plates was a wide domain of distributed deformation and therefore contradict previous interpretations claiming for a discrete plate boundary.     

中国东北及邻区白垩纪古地磁分析与块体旋转运动动力学背景

内容提要：为了更好地理解中国东北及邻区白垩纪各块体的旋转运动模式及其运动量,本文根据最新的地层学进展分早白垩世中期、早白垩世晚期与晚白垩世三个阶段对该区古地磁数据进行了精细分析。通过与华北-华南地块（不包括朝鲜半岛、辽东及胶东半岛）相应时期的古地磁极对比,说明朝鲜半岛及辽东早白垩世中、晚期相对华北-华南地块分别顺时针旋转约11º,晚白垩世以来没有明显的旋转运动。辽西地区则出现了小幅度的逆时针旋转运动。以上旋转运动与太平洋板块俯冲引起的中国东北及邻区强烈岩石圈减薄和伸展作用具有密切联系。     

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).     

Genesis and evolution of a curved mountain front: paleomagnetic and geological evidence from the Gran Sasso range (central Apennines, Italy)

The Gran Sasso range is a striking salient formed by two roughly rectilinear E–W and N–S limbs. In the past 90° counterclockwise (CCW) rotations from the eastern Gran Sasso were reported [Tectonophysics 215 (1992) 335], suggesting west–east increase of rotation-related northward shortening along the E–W limb. In this paper, we report on paleomagnetic data from Meso-Cenozoic sedimentary dykes and strata cropping out at Corno Grande (central part of the E–W Gran Sasso limb), the highest summit of the Apennine belt. Predominant northwestward paleomagnetic declinations (in the normal polarity state) from both sedimentary dykes and strata are observed. When compared to the expected declination values for the Adriatic foreland, our data document no thrusting-related rotation at Corno Grande. The overall paleomagnetic data set coupled with the available geological information shows that the Gran Sasso arc is in fact a composite structure, formed by an unrotated-low shortening western (E–W trending) limb and a strongly CCW rotated eastern salient. Late Messinian and post-early Pliocene shortening episodes documented along the Gran Sasso front indicate that belt building and arc formation occurred during two distinct episodes. We suggest that the southern part of a late Messinian N–S front was reactivated during early–middle Pliocene time, forming a tight range salient due to CCW rotations and differential along-front shortening rates. The formation of a northward displacing bulge in an overall NW–SE chain is likely a consequence of the collision between the Latium-Abruzzi and Apulian carbonate platforms during northeastward propagation of the Apennine wedge, inducing lateral northward extrusion of Latium-Abruzzi carbonates towards ductile basinal sediment areas.     

青藏高原东北缘柴达木盆地路乐河地区新生代构造变形的古地磁证据

青藏高原东北缘是高原由西南向东北方向扩展的前缘位置，其新生代构造变形对揭示青藏高原隆升、扩展的过程与动力学机制具有重要的意义。柴达木盆地是青藏高原东北缘最大的新生代沉积盆地，发育巨厚的新生代地层，这些地层所记录的古地磁极旋转信息是定量约束柴达木盆地新生代以来构造变形发生的时间、方式与幅度的载体。本文以柴达木盆地北缘新生代地层出露良好、具有精确地层年代控制的路乐河剖面为研究对象，开展了古地磁极旋转研究，统计分析路乐河剖面24. 6~5. 2 Ma之间1477个可靠古地磁样品的特征剩磁方向(ChRM)，发现柴达木盆地北缘路乐河地区在24. 6~16. 4 Ma发生小幅度(不显著)的逆时针旋转，旋转角度约为8. 4°±6. 1°；16. 4~13. 9 Ma路乐河地区发生显著的顺时针旋转，旋转角度可达36. 1°±6. 0°；13. 9~5. 2 Ma 该地区未发生明显的构造旋转；5. 2 Ma以后路乐河地区逆时针旋转了~6°。结合柴达木盆地北缘区域构造变形的分析，我们提出柴达木盆地北缘路乐河地区在16. 4~13. 9 Ma 之间发生强烈的顺时针旋转构造变形(~36°)可能代表了盆地北缘中中新世遭受强烈的地壳差异缩短变形，从而成为高原最新形成的部分。     

Paleomagnetism of the late Cryogenian Nantuo Formation and paleogeographic implications for the South China Block

A new paleomagnetic pole position is obtained from the well-dated (636.3 ± 4.9 Ma) Nantuo Formation in the Guzhang section, western Hunan Province, and the correlative Long’e section in eastern Guizhou Province, South China. Remagnetization of the recent geomagnetic field was identified and removed for both sections. The hard dual-polarity, interpreted as primary, component of the Nantuo Formation, directs east–westward with medium inclinations, yielding an average pole of 9.3°N, 165°E, A₉₅ = 4.3° that, for the first time, passed a strata-bound reversals test. The new data are consistent with previously published paleomagnetic data of the Nantuo Formation from Malong county, central Yunnan Province, which passed a positive syn-sedimentary fold test. Together, these sites represent shallow- to deep-water sections across a shelf-to-basin transect centered at ∼33° paleolatitude. The sedimentary basin may have faced an expansive ocean toward the paleo-East. In the ∼750 Ma and ∼635 Ma global reconstructions, the South China Block (SCB) was best fitted in the northern hemisphere close to northwestern Australia. However, a direct SCB-northwestern Australia connection, inferred to have existed during the Early Cambrian–Early Devonian, had not formed by the time of ∼635 Ma.     

Precambrian microcontinents of the Ural-Mongolian Belt: New paleomagnetic and geochronological data

The knowledge on the early stages of evolution of the Ural-Mongolian Belt (UMB) (Late Neoproterozoic-Cambrian) is a key for understanding of its evolution in the Paleozoic. Unfortunately, this stage remains poorly studied. The tectonic reconstructions of the UMB for this time primarily depend on the views on the kinematics and tectonic evolution of numerous sialic massifs with Precambrian basement in the structure of the Tien Shan, Kazakhstan, Altai, and Mongolia. At present, the concept of the origin of these massifs is largely based on the lithostratigraphic similarity of the Neoproterozoic and Lower Paleozoic sections of the Tarim, South China, and Siberian platforms with coeval sections of Precambrian massifs within the UMB. New paleomagnetic and geochronological data can serve as additional sources of information on the origin and paleotectonic position of the microcontinents. In this paper, we present new isotopic datings and a new paleomagnetic determination for the Neoproterozoic volcanic rocks of the Zabhan Formation from the Baydrag microcontinent in central Mongolia. It is established that 805−770 Ma ago (U-Pb LA-MC-ICP-MS age of zircon) the Baydrag microcontinent was situated at a latitude of 47 ± 14° in the Northern or Southern hemisphere. These data provide new insights into the possible origin of the Precambrian micro-continents in the UMB. Analysis of paleomagnetic data and comparison of the age of the basement beneath various plates allow us to state rather confidently that ∼800 Ma ago the micro-continents of the UMB belonged to one of the North Rodinian plates: Indian, Tarim, or South China; their Australian origin is less probable.     

Equatorial electrojet in the south Atlantic anomaly region

Features of the equatorial electrojet are studied at Sao Luiz (2.6°S, 44.2°W, inclination −0.25°) in eastern Brazil and Sikasso (11.3°N, 5.7°W, inclination 0.1°) in the western African sector. The stations are situated on either side of the lowest magnetic field intensity in the region of rapid changes in the declination. The daily variations of ΔX at the two stations are almost similar with the peak around noon with maximum values during equinoxes and minimum values during J-solstices. Daily variations of ΔY differ with the maximum deviation of about −35 nT around noon at Sao Luiz and much smaller value of about −10 nT around 14 h LT for Sikasso. The direction of the H vector varies from 15°W of north at 08 h to more than 30°W of north at 17 h for Sao Luiz and from 14°E of north to 25°W of north at 18 h for Sikasso. The plot of the deviations in ΔX and ΔY at different hours for the two stations shows the points along narrow ellipses with major axis aligned along 22°W of north for Sao Luiz and along 3°W of north for Sikasso as compared to declination of 20°W for Sao Luiz and 6°W for Sikasso. The deviations in ΔX at the two stations are fairly well correlated.     

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.     

Oroclinal bending in the Alborz Mountains (Northern Iran): New constraints on the age of South Caspian subduction and extrusion tectonics

In this study, we report an extensive paleomagnetic study (76 sites) carried out in the Alborz Mts. (northern Iran), with the aim of reconstructing the rotation history and the origin of curvature of this orogenic chain. The analyzed deposits are the sedimentary successions of the Upper Red Formation (Miocene), Lower Red Formation (Oligocene) and Eocene clastic units. Paleomagnetic results indicate that the Alborz Mts. can be considered a secondary arc that originated as a linear mountain belt that progressively acquired its present day curvature through opposite vertical axis rotations along its strike. The curvature of the arc was entirely acquired after the middle-late Miocene, which is the age of the youngest investigated sediments (Upper Red Formation). Overall, our paleomagnetic data indicate that the Alborz Mts. can be considered an orocline.Our results define, for the first time, the rotational history of the entire Alborz curved mountain belt, and enable us to reconstruct the paleogeographic and tectonic evolution of northern Iran in the framework of Arabia-Eurasia continental deformation. The kinematics inferred by the pattern of paleomagnetic rotations is at odds with the present day kinematics of northern Iran, characterized by the westward extrusion of the South Caspian block, and by a left lateral shear between Central Iran and the central and western sectors of the Alborz Mts. By integrating paleomagnetic data with stratigraphic, thermochronological, structural and GPS information, we propose that the initiation of South Caspian subduction and the activation of westward extrusion of South Caspian block occurred diachronously and that the initiation of the present-day kinematics of northern Iran was quite recent (Lower Pleistocene, < 2 Ma).     

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.     

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。