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1.
Early Ordovician (Late Arenig) limestones from the SW margin of Baltica (Scania–Bornholm) have multicomponent magnetic signatures, but high unblocking components predating folding, and the corresponding palaeomagnetic pole (latitude=19°N, LONGITUDE=051°E) compares well with Arenig reference poles from Baltica. Collectively, the Arenig poles demonstrate a midsoutherly latitudinal position for Baltica, then separated from Avalonia by the Tornquist Sea.Tornquist Sea closure and the Baltica–Avalonia convergence history are evidenced from faunal mixing and increased resemblance in palaeomagnetically determined palaeolatitudes for Avalonia and Baltica during the Mid-Late Ordovician. By the Caradoc, Avalonia had drifted to palaeolatitudes compatible with those of SW Baltica, and subduction beneath Eastern Avalonia was taking place. We propose that explosive vents associated with this subduction and related to Andean-type magmatism in Avalonia were the source for the gigantic Mid-Caradoc (c. 455 Ma) ash fall in Baltica (i.e. the Kinnekulle bentonite). Avalonia was located south of the subtropical high during most of the Ordovician, and this would have provided an optimum palaeoposition to supply Baltica with large ash falls governed by westerly winds.In Scania, we observe a persistent palaeomagnetic overprint of Late Ordovician (Ashgill) age (pole: LATITUDE=4°S, LONGITUDE=012°E). The remagnetisation was probably spurred by tectonic-derived fluids since burial alone is inadequate to explain this remagnetisation event. This is the first record of a Late Ordovician event in Scania, but it is comparable with the Shelveian event in Avalonia, low-grade metamorphism in the North Sea basement of NE Germany (440–450 Ma), and sheds new light on the Baltica–Avalonia docking. 相似文献
2.
Cuyania, an Exotic Block to Gondwana: Review of a Historical Success and the Present Problems 总被引：3，自引：0，他引：3
A review of the early history of the Cuyania terrane and the numerous pioneering works of the past century provides the present robust framework of evidence supporting a derivation from Laurentia, travel towards Gondwana as an isolated microcontinent, and final amalgamation to the protomargin of western Gondwana in Middle to Late Ordovician times. The major remaining uncertainties and inconsistencies, such as the time of deformation and collision with Gondwana, the lack of evidence of Famatinian-derived zircons, the effects of strike-slip displacements proposed along the suture, as well as the potential sutures defined by ophiolite assemblages, are discussed. The precise boundary along the northern and southern limits is not yet well defined.
The most suitable hypothesis based on present data is that Cuyania originated as a conjugate margin of the Ouachita embayment, south of the Appalachian platform during Early Cambrian times. The subsequent travel toward the Gondwana protomargin is clearly depicted by the changing faunal assemblages in the carbonate platform. New geochemical and age data on K-bentonites presented by several authors reinforce the strong connection between Cuyania ash-fall tuffs and Famatina volcanics by 468–470 Ma, indicating Cuyania and Gondwana were in close proximity at that time.
Extension related to flexural subsidence, preceded by the drowning of the carbonate platform in early Llanvirnian times, is recorded by abrupt facies changes in the sedimentary cover during late Llanvirnian and early Caradocian times. This episode marked the beginning of contact between Cuyania and Gondwana. The subsequent evolution of the foreland basin indicates that deformation lasted until latest Silurian-Early Devonian times.
The time of collision is tracked by the cessation of arc-related magmatic activity in the upper plate (Gondwana protomargin), at about 465 Ma in western Sierras Pampeanas, and ages around 454 Ma corresponding to syncollisional and postcollisional magmatism. The age of the collision is also preserved in the lower plate (Cuyania), where both angular unconformities in the sedimentary cover and the ages of peak of regional metamorphism in the basement rocks point to 460 Ma as the most probable age for the beginning of the collision. Evidence from the upper plate is essentially identical with an age of 463 Ma. Thermal gradients along this suture vary from 13°C/km in the lower plate, to 18°C/km in the fore arc upper plate, reaching more than 30°C/km along the Famatinian arc. Based on these data, a Llandelian-Caradocian age for the collision can be postulated on firm grounds. Deformation continued through most of the early Paleozoic until amalgamation of the Chilenia terrane by the Late Devonian. 相似文献
3.
R.A. Facer 《Tectonophysics》1981,74(3-4)
A section 300 m thick across the Permian—Triassic boundary has been sampled in the Southern Coalfield of the Sydney Basin, New South Wales. 55 samples, mainly grey to drab sandstones, were collected from 9 diamond drill holes which penetrated the entire Narrabeen Group and the upper part of the conformably underlying Illawarra Coal Measures, as well as a sill emplaced into the coal measures. The samples included fully oriented cores. Additional reconnaissance samples from two further drill holes were also studied.Partial alternating field demagnetization and petrography indicate the magnetic remanence to be a stable DRM. Partial thermal demagnetization above 300°C or 400°C caused large increases in magnetic susceptibility. Partial chemical demagnetization did not cause significant changes in remanence directions.For the Coal Cliff Sandstone (basal Narrabeen Group, Triassic) the palaeomagnetic pole position (Normal) was calculated to be at 59°N 322°E (dp = 27°, dm = 29°), which agrees with previously published data. For the uppermost coal measures (Permian) the pole position was calculated as 58°N 340°E (dp = 09°, dm = 10°). Data for samples from the lower to middle coal measures yield a pole position which is between the new Permian—Triassic pole position and that for the underlying Middle Permian igneous rocks. The top of the Reversed “Kiaman Magnetic Interval” (Permian) may be near the Tongarra coal and Appin Formation boundary — (early) Late Permian. 相似文献
4.
A palaeomagnetic study of Vendian and Early Cambrian sediments from the Angara block of the Siberian platform: Shaman (52.08°N, 108.83°E) and Minya (58.0°N, 110.0°E) Formations, and the Tuva-Mongolian block: Tsagan-Olom and Bayan-Gol Formations (46.76°N, 96.37°E) isolated three different components of magnetization through thermal demagnetization. The stable high-temperature characteristic remanence directions show both normal and reverse polarities. The mean palaeopoles computed after these high-temperature components are: 32.0°S/71.1°E (dp/dm=6.9°/13.8°) for the Vendian Shaman Formation (10 sites, 80 samples), 33.7°S/37.2°E (dp/dm=8.6°/14.7°) for the Vendian Minya Formation (12 samples), 22.8°S/28.4°E (dp/dm=10.8°/21.6°) for the Vendian Tsagan-Olom Formation (4 sites, 25 samples) and 21.4°S/167.1°E (dp/dm=9.6°/19.1°) for the Early Cambrian Bayan-Gol Formation (6 sites, 49 samples). From a compilation of Vendian and Early Cambrian palaeopoles from the Anabar, Angara and Aldan blocks of the Siberian platform and Tuva-Mongolia block, we propose a model where these blocks were situated in an equatorial to low south palaeolatitude position, with their present-day southern boundaries facing the north pole. From the analysis of the scatter of these poles, we conclude that the Siberian platform might not have fully amalgamated by this time, and that significant rotations occurred after the Early Cambrian. Our new palaeopoles for the Tuva-Mongolia block, together with previously published ones, show that this block was already adjacent to Siberia by the Vendian and Early Cambrian. We propose that the large counterclockwise rotation of the Tuva-Mongolia block with respect to Angara block could mark the end of the closure of the part of the Palaeo-Asian ocean separating these two blocks, and could account for the occurrence of Vendian-Early Cambrian ophiolites in the region. 相似文献
5.
We present a compilation of reliable paleomagnetic pole positions from five continental plates (North America, Europe, the Iberian Peninsula, Africa, and South America) for ten time intervals ranging from Late Carboniferous to Eocene. Only well-dated results obtained by demagnetization techniques have been used. Paleomagnetic poles are plotted with respect to the paleo-positions of the continents, as reconstructed from correlations of marine magnetic anomalies in the Atlantic Ocean by Pitman and Talwani and from the fit by Bullard et al. The poles from North America, Europe and the younger poles from Africa show a very good grouping for most of the ten intervals considered, and a continuous apparent polar wandering path is obtained. These data have been used to construct paleolatitude maps for most intervals; thus the relative positions of the continents were established from sea-floor spreading data and their absolute positions on the globe were determined from paleomagnetic data. The older data from South America and the other Gondwana continents show a systematic deviation from those of the northern continents for Late Paleozoic and Early Triassic time periods. An explanation is offered in a different continental reconstruction between Laurasia and Gondwanaland before Middle Triassic times. 相似文献
6.
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。 相似文献
7.
The island of Madagascar experienced widespread magmatism at ca. 90 Ma due to its interaction with the Marion hotspot. Previous paleomagnetic data from igneous rocks in the southwestern and northwestern regions of the island indicated that the Marion hotspot has remained fixed for the past 90 Ma. We report paleomagnetic data from northeastern Madagascar (d'Analava Complex). Samples were collected from basalts, rhyolites, gabbros and a dolerite dyke. Sixty samples from 5 sites yield a paleomagnetic pole at 66.7°S, 43.5°E (A95 = 10.7°) and a grand mean pole (GMP) calculated from 10 different studies covering the entire island of Madagascar falls at 68.9°S, 49.0°E (A95 = 4.4°). This pole translates to a paleolatitude for the Volcan de l'Androy (focal point of the hotspot) at 45.2° + 6°/− 5°S compared to the current location of the Marion hotspot at 46°S. Our results confirm, and expand upon, previous studies that argue for the fixity of the Marion hotspot for the past 90 Ma. 相似文献
8.
This article reports the first joint paleomagnetic and U-Pb geochronologic study of Precambrian diabase dikes in the Anabar Shield and adjacent Riphean cover of Siberia. It was undertaken to allow comparison with similar published studies in Laurentia and to test Proterozoic reconstructions of Siberia and Laurentia. An east-trending Kuonamka dike yielded a provisional U-Pb baddeleyite emplacement age of 1503+/-5 Ma and a virtual geomagnetic pole at 16 degrees S, 221 degrees E (dm=17&j0;, dp=10&j0;). A paleomagnetic pole at 6 degrees N, 234 degrees E (dm=28&j0;, dp=14&j0;) was obtained from five Kuonamka dikes. An east-southeast-trending Chieress dike yielded a U-Pb baddeleyite emplacement age of 1384+/-2 Ma and a virtual geomagnetic pole at 4 degrees N, 258 degrees E (dm=9&j0;, dp=5&j0;). Kuonamka and Chieress poles are interpreted to be primary but do not average out secular variation. Assuming that the Siberian Plate has remained intact since the Mesoproterozoic, except for mid-Paleozoic opening of the Viljuy Rift, then the above results indicate that the Siberian Plate was in low latitudes at ca. 1503 and 1384 Ma, broadly similar to low latitudes determined for Laurentia from well-dated paleopoles at 1460-1420, 1320-1290, and 1267 Ma. This would allow Laurentia and Siberia to have been attached in the Mesoproterozoic, as suggested in several recent studies based on geological criteria. However, because paleomagnetic results from the Anabar Shield region do not average out secular variation and the ages of poles from Siberia and Laurentia are not well matched, it is not yet possible to distinguish between these reconstructions or to rule out other configurations that also maintain the two cratons at low paleolatitudes. 相似文献
9.
The only Iberian lower Jurassic paleomagnetic pole come from the “Central Atlantic Magmatic Province”-related Messejana Plasencia dyke, but the age and origin of its remanence have been a matter of discussion. With the aim of solving this uncertainty, and to go further into a better understanding of its emplacement and other possible tectonic features, a systematic paleomagnetic investigation of 40 sites (625 specimens) distributed all along the 530 km of the Messejana Plasencia dyke has been carried out. Rock magnetic experiments indicate PSD low Ti-titanomagnetite and magnetite as the minerals carrying the NRM. The samples were mostly thermally demagnetized. Most sites exhibit a characteristic remanent component of normal polarity with the exception of two sites, where samples with reversed polarities have been observed. The paleomagnetic pole derived from a total of 35 valid sites is representative of the whole structure of the dyke, and statistically well defined, with values of PLa = 70.4°N, PLo = 237.6°E, K = 47.9 and A95 = 3.5°. Paleomagnetic data indicates that: (i) there is no evidence of a Cretaceous remagnetization in the dyke, as it was suggested; (ii) most of the dyke had a brief emplacement time; furthermore, two dyke intrusion events separated in time from it by at least 10,000 y have been detected; (iii) the high grouping of the VGPs directions suggests no important tectonic perturbations of the whole structure of the dyke since its intrusion time; (iv) the pole derived from this study is a good quality lower Jurassic paleopole for the Iberian plate; and (v) the Messejana Plasencia dyke paleopole for the Iberian plate is also in agreement with quality-selected European and North American lower Jurassic paleopoles and the magnetic anomalies data sets that are available for rotate them to Iberia. 相似文献
10.
Critical assessment of Paleozoic paleomagnetic results from Australia shows that paleopoles from locations on the main craton and in the various terranes of the Tasman Fold Belt of eastern Australia follow the same path since 400 Ma for the Lachlan and Thomson superterranes, but not until 250 Ma or younger for the New England superterrane. Most of the paleopoles from the Tasman Fold Belt are derived from the Lolworth-Ravenswood terrane of the Thomson superterrane and the Molong-Monaro terrane of the Lachlan superterrane. Consideration of the paleomagnetic data and geological constraints suggests that these terranes were amalgamated with cratonic Australia by the late Early Devonian. The Lolworth-Ravenswood terrane is interpreted to have undergone a 90° clockwise rotation between 425 and 380 Ma. Although the Tamworth terrane of the western New England superterrane is thought to have amalgamated with the Lachlan superterrane by the Late Carboniferous, geological syntheses suggest that movements between these regions may have persisted until the Middle Triassic. This view is supported by the available paleomagnetic data. With these constraints, an apparent polar wander path for Gondwana during the Paleozoic has been constructed after review of the Gondwana paleomagnetic data. The drift history of Gondwana with respect to Laurentia and Baltica during the Paleozoic is shown in a series of paleogeographic maps. 相似文献
11.
Yan Chen Bernard Henry Michel Faure Jean-François Becq-Giraudon Jean-Yves Talbot Lucien Daly Maxime Le Goff 《International Journal of Earth Sciences》2006,95(2):306-317
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. 相似文献
12.
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 总被引：1，自引：0，他引：1
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。 相似文献
13.
A paleomagnetic study of the late Middle to possibly early Late Cambrian Liberty Hills Formation in the Ellsworth Mountains, Antarctica, reveals a stable magnetization with positive fold and reversal tests. The paleopole is based on 16 sites from volcanic and sedimentary rocks and lies at lat 7.3 degrees N and long 326.3 degrees E (A95=6.0&j0;). The new paleomagnetic data support the view that the Ellsworth Mountains are part of a microplate-the Ellsworth-Whitmore Mountains crustal block-that rotated independently of the main Gondwana continental blocks during breakup. The Liberty Hills pole differs from both previous poles recovered from Cambrian rocks in the Ellsworth Mountains and from the available Gondwana reference pole data. Our pole indicates a more northerly prebreakup position for the Ellsworth Mountains than previously suggested, contradicting the overwhelming geologic evidence for a prebreakup position close to southern Africa. The reasons for this are uncertain, but we suggest that problems with the Gondwana apparent polar wander path may be important. More well constrained, early Paleozoic paleomagnetic data are required from the Ellsworth Mountains and the Gondwana continents if the data are to constrain further the Middle-Late Cambrian location of the Ellsworth-Whitmore Mountains block. 相似文献
14.
Paleomagnetic data from lavas and dikes of the Unkar igneous suite (16 sites) and sedimentary rocks of the Nankoweap Formation (7 sites), Grand Canyon Supergroup (GCSG), Arizona, provide two primary paleomagnetic poles for Laurentia for the latest Middle Proterozoic (ca. 1090 Ma) at 32°N, 185°E (dp=6.8°, DM=9.3°) and early Late Proterozoic (ca. 850–900 Ma) at 10°S, 163°E (dp=3.5°, DM=7.0°). A new 40Ar/39Ar age determination from an Unkar dike gives an interpreted intrusion age of about 1090 Ma, similar to previously reported geochronologic data for the Cardenas Basalts and associated intrusions. The paleomagnetic data show no evidence of any younger, middle Late Proterozoic tectonothermal event such as has been revealed in previous geochronologic studies of the Unkar igneous suite. The pole position for the Unkar Group Cardenas Basalts and related intrusions is in good agreement with other ca. 1100 Ma paleomagnetic poles from the Keweenawan midcontinent rift deposits and other SW Laurentia diabase intrusions. The close agreement in age and position of the Unkar intrusion (UI) pole with poles derived from rift related rocks from elsewhere in Laurentia indicates that mafic magmatism was essentially synchronous and widespread throughout Laurentia at ca. 1100 Ma, suggesting a large-scale continental magmatic event. The pole position for the Nankoweap Formation, which plots south of the Unkar mafic rocks, is consistent with a younger age of deposition, at about 900 to 850 Ma, than had previously been proposed. Consequently, the inferred 200 Ma difference in age between the Cardenas Basalts and overlying Nankoweap Formation provides evidence for a third major unconformity within the Grand Canyon sequence. 相似文献
15.
Ana María Sato Hugo Tickyj Eduardo Jorge Llambías Miguel Angelo Stipp Basei Pablo Diego Gonzlez 《Gondwana Research》2004,7(4):1077-1087
The Las Matras Block in Central Argentina constitutes the southernmost part of the Cuyania terrane, which was accreted to the southwestern margin of Gondwana during the Early to Mid Ordovician Famatinian orogeny. The Grenville-aged rocks of the Las Matras Block are represented by the tonalitic to trondhjemitic Las Matras pluton. A new U-Pb conventional zircon age of 1244±42 Ma confirms previous Sm-Nd and Rb-Sr isochron ages of this pluton. Mineral composition data are consistent with the tonalitic-trondhjemitic character of the pluton, and constrain its emplacement level to 1.9 to 2.6 kb. This shallow level of emplacement and the undeformed character of the pluton are distinctive features of this southernmost basement. A regional comparison indicates that the igneous-metamorphic evolution of the Grenville-aged basement rocks of the Cuyania terrane occurred over a period of more than 200 million years, with ages older than 1200 Ma up to those close to 1000 Ma. The shallowest crustal level is found in Las Matras, suggesting a southward shallowing of the exposed level of basement. The deformation and metamorphism associated with the collisional Famatinian orogeny affect both the Cuyania terrane and the adjacent western margin of Gondwana, and the Gondwana margin was also the locus of the related arc magmatism, but the compressive effects of the collision decrease in intensity toward the south. The Famatinian metamorphism and magmatism continue even further south into the Patagonia region, but the southern continuity of the Cuyania terrane into this region remains uncertain. 相似文献
16.
Natalia V. Lubnina Sergei A. Pisarevsky Victor N. Puchkov Vjacheslav I. Kozlov Nina D. Sergeeva 《International Journal of Earth Sciences》2014,103(5):1317-1334
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. 相似文献
17.
Progressive thermal demagnetization of samples from the Tan y Grisiau granite defines a coherent easterly positive characteristic remanence (D/I = 124.9/60.3°;, 42 samples, R = 40–51, a95 = 4.8°;) residing in magnetite. An ancient reversal of magnetization is recovered in the highest blocking temperature spectrum of a few samples and suggests that a cooling-related dipolar axis is recorded by this pluton. Only facies of the granite which have been reddened, probably by submagmatic streaming, have recorded a stable remanence. Adjustment for tilt yields a very steep remanence (D/I = 193/88°;) incompatible with any known Early Palaeozoic and younger field direction from Britain. The in situ remanence has a similar declination to the primary magnetization in Late Ordovician dolerites from the Welsh Borderlands and yields a comparable palaeolatitude (41.5°;S). It is concluded that the Tan y Grisiau pluton was magnetized in Late Ordovician times after deformation. Folding in this region is therefore interpreted to be substantially of Taconic (Late Ordovician) origin and not Acadian in age. As both in situ and tilt-adjusted remanence directions are incompatible with Silurian and younger palaeofield directions from Britain, the pluton is interpreted as a subvolcanic component of the North Wales igneous province. Large anticlockwise rotation of Avalonia is identified between Late Ordovician and Late Silurian times. 相似文献
18.
《Gondwana Research》2014,25(1):159-169
The Ediacaran–Early Ordovician interval is of great interest to paleogeographer's due to the vast evolutionary changes that occurred during this interval as well as other global changes in the marine, atmospheric and terrestrial systems. It is; however, precisely this time period where there are often wildly contradictory paleomagnetic results from similar-age rocks. These contradictions are often explained with a variety of innovative (and non-uniformitarian) scenarios such as intertial interchange true polar wander, true polar wander and/or non-dipolar magnetic fields. While these novel explanations may be the cause of the seemingly contradictory data, it is important to examine the paleomagnetic database for other potential issues.This review takes a careful and critical look at the paleomagnetic database from Baltica. Based on some new data and a re-evaluation of older data, the relationships between Baltica and Laurentia are examined for ~ 600–500 Ma interval. The new data from the Hedmark Group (Norway) confirms suspicions about possible remagnetization of the Fen Complex pole. For other Baltica results, data from sedimentary units were evaluated for the effects of inclination shallowing. In this review, a small correction was applied to sedimentary paleomagnetic data from Baltica. The filtered dataset does not demand extreme rates of latitudinal drift or apparent polar wander, but it does require complex gyrations of Baltica over the pole. In particular, average rates of APW range from 1.5° to 2.0°/Myr. This range of APW rates is consistent with ‘normal’ plate motion although the total path length (and its oscillatory nature) may indicate a component of true polar wander. In the TPW scenario, the motion of Baltica results in a back and forth path over the south pole between 600 and 550 Ma and again between 550 and 500 Ma. The rapid motion of Baltica over the pole is consistent with the extant database, but other explanations are possible given the relative paucity of high-quality paleomagnetic data during the Ediacaran–Cambrian interval from Baltica and other continental blocks.A sequence of three paleogeographic maps for Laurentia and Baltica is presented. Given the caveats involved in these reconstructions (polarity ambiguity, longitudinal uncertainty and errors), the data are consistent with geological models that posit the opening of the Iapetus Ocean around 600 Ma and subsequent evolution of the Baltica–Laurentia margin in the Late Ediacaran to Early Ordovician, but the complexity of the motion implied by the APWP remains enigmatic. 相似文献
19.
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. 相似文献
20.
During the Triassic, the Thakkhola region of the Nepal Himalaya was part of the broad continental shelf of Gondwana facing a wide Eastern Tethys ocean. This margin was continuous from Arabia to Northwest Australia and spanned tropical and temperate latitudes.A compilation of Permian, Triassic and early Jurassic paleomagnetic data from the reconstructed Gondwana blocks indicates that the margin was progressively shifting northward into more tropical latitudes. The Thakkhola region was approximately 55° S during Late Permian, 40° S during Early Triassic, 30° S during Middle Triassic and 25° S during Late Triassic. This paleolatitude change produced a general increase in the relative importance of carbonate deposition through the Triassic on the Himalaya and Australian margins. Regional tectonics were important in governing local subsidence rates and influx of terrigenous clastics to these Gondwana margins; but eustatic sea-level changes provide a regional and global correlation of major marine transgressions, prograding margin deposits and shallowing-upward successions. A general mega-cycle characterizes the Triassic beginning with a major transgression at the base of the Triassic, followed by a general shallowing-upward of facies during Middle and Late Triassic, and climaxing with a regression in the latest Triassic. 相似文献