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1.
Generally, the lack of bedding criteria in basement units hampers the interpretation of paleomagnetic results in terms of geotectonics. Nevertheless, this work demonstrates that successive remagnetizations recorded in Early Carboniferous metamorphic and plutonic units, without clear bedding criteria, can be used to constrain a polyphased tectonic evolution consisting of a regional clockwise rotation, followed by a folding phase, a tilting phase and a second regional clockwise rotation.Metamorphic, ultrabasic, tonalitic and granitic rocks from different parts of Limousin (western French Massif central; 45.5°N/1.25°E), which underwent metamorphism during Devonian–Early Carboniferous or were intruded in the Early–Middle Carboniferous, were sampled in order (a) to identify the magnetic overprinting phases and the related tectono-magmatic events and (b) to constrain the regional and plate tectonic evolution of Limousin. Paleomagnetic results from 32 new and 26 sites investigated previously show that at least 90% of the magnetization isolated in rocks older than 330 Ma are overprints. In agreement with results from adjacent areas of the Variscan belt, the major overprinting phases occurred: (a) in the last stages of the major exhumation phase [332–328 Ma; mean Virtual Geomagnetic Pole (VGP) “Cp”: 37°N/70.5°E], (b) during the post-collisional syn-orogenic extension (325–315 Ma; VGP “B”: 11°N/114°E), (c) in the Latest Carboniferous and Early Permian (VGP “A1”: 27°N/149°E) and (d) in the Late Permian (VGP “A”: 48°N/146°E). The Middle–Late Carboniferous overprints “Cp” and “B” are contemporaneous with emplacement of leucogranitic, crustal derived plutons, and probably result from the hydro-thermal activity related to the magmatism. The drift from “Cp” directions to “B” directions implies that after 330 Ma, Limousin underwent a clockwise rotation by 65°, together with the Central Europe Variscides. The “Bt” components, the VGPs of which deviate from the mean apparent polar wander path (APWP) of the belt, are interpreted as “B” overprints tilted during Late Variscan tectonics, that is, in the time range 325–315 Ma. The first and most important generation of “Bt” overprints was tilted during NW–SE folding associated with NE–SW shortening, updoming and emplacement of leucogranitic plutons. The second generation reveals southeastward tilting due to NE-striking normal faulting. The drift from “B” to “A1” directions implies that Limousin has participated to the second clockwise rotation by 40° of the whole belt in Westphalian times. 相似文献
2.
Manoel Souza D'Agrella-Filho Maria Irene Bartolomeu Raposo Marcos Egydio-Silva 《Gondwana Research》2004,7(1):103-113
The Juiz de Fora Complex is mainly composed of granulites, and granodioritic-migmatite gneisses and is a cratonic basement of the Ribeira belt. Paleomagnetic analysis on samples from 64 sites widely distributed along the Além Paraíba dextral shear zone (SE Brazil, Rio de Janeiro State) yielded a northeastern, steep downward inclination direction (Dm=40.4°, Im=75.4, a95=6.0°, K=20.1) for 30 sites. The corresponding paleomagnetic pole (RB) is situated at 335.2°E; 0.6°S (a95=10.0°; K=7.9). Rock magnetism indicates that both (titano)magnetite and titanohematite are the main magnetic minerals responsible for this direction. Anisotropy of low-field magnetic susceptibility (AMS) measurements were used to correct the ChRM directions and consequently its corresponding paleomagnetic pole. This correction yielded a new mean ChRM (Dm = 2.9°, Im = 75.4°, a95 = 6.4°, K = 17.9) whose paleomagnetic pole RBc is located at 320.1°E, 4.2° N (a95=10.3°, K=7.5). Both mean ChRM and paleomagnetic pole obtained from uncorrected and corrected data are statistically different at the 95% confidence circle. Geological and geochronological data suggest that the age of the Juiz de Fora Complex pole is probably between 535–500 Ma, and paleomagnetic results permit further constraint on these ages to the interval 520–500 Ma by comparison with high quality paleomagnetic poles in the 560–500 Ma Gondwana APW path. 相似文献
3.
The Southern Uplands terrane is an Ordovician–Silurian back-arc/foreland basin emplaced at the northern margin of the Iapetus Ocean and intruded by granite complexes including Loch Doon (408.3 ± 1.5 Ma) during Early Devonian times. Protracted cooling of this 130 km3 intrusion recorded magnetic remanence comprising a predominant (‘A’) magnetisation linked to initial cooling with dual polarity and mean direction D / I = 237 / 64° (α95 = 4°, palaeopole at 316°E, 21°N). Subsidiary magnetisations include Mesozoic remanence correlating with extensional tectonism in the adjoining Irish Sea Basin (‘B’, D / I = 234/− 59°) and minority populations (‘C’, D / I = 106/− 2° and ‘D’, D / I = 199/1°) recording emplacement of younger ( 395 Ma) granites in adjoining terranes and the Variscan orogenic event. The ‘A’ directions have an arcuate distribution identifying anticlockwise rotation during cooling. A comparable rotation is identified in the Orthotectonic Caledonides to the north and the Paratectonic Caledonides to the south following closure of Iapetus. Continental motion from midsoutherly latitudes ( 40°S) at 408 Ma to equatorial palaeolatitudes by 395 Ma is identified and implies minimum rates of continental movement between 430 and 390 Ma of 30–70 cm/year, more than double maximum rates induced by plate forces and interpreted as a signature of true polar wander. Silurian–Devonian palaeomagnetic data from the British–Scandinavian Caledonides define a 430–385 Ma closed loop comparable to the distributed contemporaneous palaeomagnetic poles from Gondwana. They reconcile pre-430 Ma and post-380 Ma APW from this supercontinent and show that Laurentia–Baltica–Avalonia lay to the west of South America with a relict Rheic Ocean opening to the north which closed to produce Variscan orogeny by a combination of pivotal closure and right lateral transpression. 相似文献
4.
S.J. Malone J.G. Meert D.M. Banerjee M.K. Pandit E. Tamrat G.D. Kamenov V.R. Pradhan L.E. Sohl 《Precambrian Research》2008,164(3-4):137-159
The utility of paleomagnetic data gleaned from the Bhander and Rewa Groups of the “Purana-aged” Vindhyanchal Basin has been hampered by the poor age control associated with these units. Ages assigned to the Upper Vindhyan sequence range from Cambrian to the Mesoproterozoic and are derived from a variety of sources, including 87Sr/86Sr and δ 13C correlations with the global curves and Ediacara-like fossil finds in the Lakheri–Bhander limestone. New analyses of the available paleomagnetic data collected from this study and previous work on the 1073 Ma Majhgawan kimberlite, as well as detrital zircon geochronology of the Upper Bhander sandstone and sandstones from the Marwar SuperGroup suggest that the Upper Vindhyan sequence may be up to 500 Ma older than is commonly thought. Paleomagnetic analysis generated from the Bhander and Rewa Groups yields a paleomagnetic pole at 44°N, 214.0°E (A95 = 4.3°). This paleomagnetic pole closely resembles the VGP from the well-dated Majhgawan intrusion (36.8°N, 212.5°E, α95 = 15.3°).Detrital zircon analysis of the Upper Bhander sandstone identifies a youngest age population at 1020 Ma. A comparison between the previously correlated Upper Bhander sandstone and the Marwar sandstone detrital suites shows virtually no similarities in the youngest detrital suite sampled. The main 840–920 Ma peak is absent in the Upper Bhander. This supports our assertion that the Upper Bhander is older than the 750–771 Ma Malani sequence, and is likely close to the age of the 1073 Ma Majhgawan kimberlite on the basis of the paleomagnetic similarities. By setting the age of the Upper Vindhyan at 1000–1070 Ma, several intriguing possibilities arise. The Bhander–Rewa paleomagnetic pole allows for a reconstruction of India at 1000–1070 Ma that overlaps with the 1073 ± 13.7 Majhgawan kimberlite VGP. Comparisons between the composite Upper Vindhyan pole (43.9°N, 210.2°E, α95 = 12.2°) and the Australian 1071 ± 8 Ma Bangamall Basin sills and the 1070 Ma Alcurra dykes suggest that Australia and India were not adjacent at this time period. 相似文献
5.
Yo-ichiro Otofuji Takaaki Matsuda Ryo Enami Koji Uno Katsuhiko Nishihama Li Su Ruslan G. Kulinich Petr S. Zimin Anatoly P. Matunin Vladimir G. Sakhno 《Tectonophysics》2002,350(3)
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. 相似文献
6.
We present geochronologic and paleomagnetic data from a north-trending quartz diorite intrusion that cuts Archean metasedimentary and metaigneous rocks of the South Pass Greenstone Belt of the Wyoming craton. The quartz diorite was previously thought to be either Archean or Early Proterozoic (?) in age and is cut by north and northeast-trending Proterozoic diabase dikes of uncertain age, for which we also report paleomagnetic data. New U–Pb analyses of baddeleyite and zircon from the quartz diorite yield a concordia upper intercept age of 2170±8 Ma (95% confidence). An 40Ar/39Ar amphibole date from the same sample yields a similar apparent age of about 2124±30 Ma (2σ), thus confirming that the intrusion is Early Proterozoic in age and that it has probably not been thermally disturbed since emplacement. A magmatic event at ca. 2.17 Ga has not previously been documented in the Wyoming craton. The quartz diorite and one of the crosscutting diabase dikes yield essentially identical, well-defined characteristic remanent magnetizations. Results from eight sites in the quartz diorite yield an in situ mean direction of north declination and moderate to steep positive inclination (Dec.=355°, Inc.=65°, k=145, α95=5°) with a paleomagnetic pole at 84°N, 215°E (δm=6°, δp=7°). Data from other diabase dike sites are inconsistent with the quartz diorite results, but the importance of these results is uncertain because the age of the dikes is not well known. Interpretation of the quartz diorite remanent magnetization is problematic. The in situ direction is similar to expected directions for magnetizations of Late Cretaceous/early Tertiary age. However, there is no compelling evidence to suggest that these rocks were remagnetized during the late Mesozoic or Cenozoic. Assuming this magnetization to be primary, then the in situ paleomagnetic pole is strongly discordant with poles of 2167, 2214, and 2217 Ma from the Canadian Shield, and is consistent with proposed separation of the Wyoming Craton and Laurentia prior to about 1.8 Ga. Correcting the quartz diorite pole for the possible effects of Laramide-age tilting of the Wind River Range, based on the attitude of nearby overlying Cambrian Flathead Sandstone (dip=20°, N20°E), gives a tilt corrected pole of 75°N, 58°E (δm=4°, δp=6°), which is also discordant with respect to time-equivalent poles from the Superior Province. Reconstruction of the Superior and Wyoming Province using a rotation similar to that proposed by Roscoe and Card [Can. J. Earth Sci. 46(1993)2475] is problematic, but reconstruction of the Superior and Wyoming Provinces based on restoring them to their correct paleolatitude and orientation using a closest approach fit indicates that the two cratons could have been adjacent at about 2.17 Ga prior to rifting at about 2.15 Ga. The paleomagnetic data presented are consistent with the hypothesis that the Huronian and Snowy Pass Supergroups could have evolved as part of a single epicratonic sedimentary basin during the Early Proterozoic. 相似文献
7.
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
8.
The Banded Hematite Jasper Formation within the Iron Ore Supergroup of the Singhbhum Craton in eastern India comprises fine alternating layers of jasper and specularite. It was deposited at 3000 Ma and deformed by a mobile episode at 2700 Ma. Hematite pigment (<1 μm) mixed with cryptocrystalline silica and specularite (> 10 μm) is chiefly responsible for red to brown rhythmic bands in the hematite jasper facies although thermomagnetic study also shows that minor amounts (1–2%) of magnetite are present. Palaeomagnetic study identifies a dual polarity remanence resident in hematite (D/I = 283/60°, α95 = 12°) which predates deformation. Studies of the fabric of magnetic susceptibility and rock magnetic results suggest a diagenetic origin for this magnetisation with the hematite formed from oxidation of primary magnetite. The palaeopole (32°E, 24°N, dp/dm = 14/18°) records the earliest post-metamorphic magnetisation event in the Orissa Craton. A minimum apparent polar wander motion of the Orissa-Singhbhum craton of through 80° is identified during Late Archaean times (2900-2600 Ma). 相似文献
9.
The recent tectonics of the Arctic Basin and northeastern Asia are considered as a result of interaction between three lithospheric plates: North-America, Eurasia and Spitsbergen. Seismic zones (coinciding in the Norway-Greenland basin with the Kolbeinsey, Mohns and Knipovich ridges, and in the Arctic Ocean with the Gakkel Ridge) clearly mark the boundaries between them. In southernmost Svalbard (Spitsbergen), the secondary seismic belt deviates from the major seismic zone. This belt continues into the seismic zone of the Franz Josef Land and then merges into the seismic zone of the Gakkel Ridge at 70°–90°E. The smaller Spitsbergen plate is located between the major seismic zone and its secondary branch.Within northeastern Asia, earthquake epicenters with magnitude over 4.5 are concentrated within a 300-km wide belt crossing the Eurasian continent over a distance of 3000 km from the Lena estuary to the Komandorskye Islands. A single seismic belt crosses the northern sections of the Verkhoyansky Ridge and runs along the Chersky Ridge to the Kolymo-Okhotsk Divide.To compute the poles of relative rotation of the Eurasian, North-American and Spitsbergen plates we use 23 new determinations of focal-mechanism solutions for earthquakes, and 38 azimuths of slip vectors obtained by matching of symmetric mountain pairs on both sides of the Knipovich and Gakkel ridges; we also use 14 azimuths of strike-slip faults within the Chersky Ridge determined by satellite images. The following parameters of plate displacement were obtained: Eurasia/North America: 62.2°N, 140.2°E (from the Knipovich Ridge section south of the triple junction); 61.9°N, 143.1°E (from fault strikes in the Chersky Ridge); 60.42°N, 141.56°C (from the Knipovich section and from fault strikes in the Chersky Ridge); 59.48°N, 140.83°E, α = 1.89 · 10−7 deg/year (from the Knipovich section, from fault strikes in the Chersky Ridge and from the Gakkel Ridge section east of the triple junction). The rate was calculated by fitting the 2′ magnetic lineations within the Gakkel Ridge).North-America/Spitsbergen: 70.96°N, 121.18°E, α = −2.7 · 10−7 deg/year from the Knipovich Ridge section north of the triple junction, from earthquakes in the Spitsbergen fracture zone and from the Gakkel Ridge section west of the triple junction). Eurasia/Spitsbergen: 70.7°N, 25.49°E, α = −0.99 · 10−7 deg/year (from closure of vector triangles). 相似文献
10.
R.L. Carlson 《Tectonophysics》1981,77(3-4)
In response to at least one change in the direction of sea-floor spreading, the Juan de Fuca Ridge and Gorda Rise have rotated approximately 20° clockwise with respect to geographic North during the last 10 million years. The rotation histories of these ridge segments have been determined from the ages and azimuths of linear magnetic anomalies within the corresponding “zed” patterns. In each case the rotations were systematic and occurred between about 9 and 3 Ma B.P. Significantly, the rotations occurred in a number of discrete stages during each of which the rates of rotation were approximately constant; rotation rates range from 1.3 to 8.6°/Ma.Though the rotation histories of these spreading centers are generally similar, some changes in the rotation rates are not synchronous, and until 3 Ma B.P., the Juan de Fuca Ridge had a 5–10° more easterly trend than the Gorda Rise. For the last 3 million years both ridge segments have had stable trends near 19°E of North.On a time scale of millions of years, ridge reorientation may be regarded as a continuous process wherein the rotation of the spreading center results from asymmetric spreading. Discontinuous changes in the degree of asymmetric spreading are required to account for observed changes in rotation rate. If the orthogonal arrangement of spreading centers and transform faults represents a least-work condition in which the resistance to plate motions is minimized by minimizing the lengths of ridge segments, as suggested previously, and if the rate at which the system seeks to reduce the total resistance after a change in spreading direction is maximum, it follows that the degree of asymmetric spreading, and hence the rate of rotation, are inversely proportional to the resistance to motion on transform faults. Thus, the various stages of rotation of the Juan de Fuca Ridge and Gorda Rise probably reflect different stress conditions on the Blanco Fracture Zone.It is difficult to account for the different trends of the Juan de Fuca Ridge and Gorda Rise largely because the Gorda Block is not behaving as a rigid plate and because the Mendocino Fracture Zone is not a transform fault. However, the fact that the Gorda Rise has had a stable trend for 3 million years, in spite of the deformation of an adjacent plate, suggests that the motion of the Gorda Block is not controlled by the motions of the vast Pacific and North American Plates, and that the Driving mechanism is “felt” directly at the ridge. 相似文献
11.
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
12.
Freddy Corredor 《Tectonophysics》2003,372(3-4):147-166
Remote sensing and field studies of several extensional basins along the northern margin of the Gulf of Aden in Yemen show that Oligocene–Miocene syn-rift extension trends N20°E on average, in agreement with the E–W to N120°E strike of main rift-related normal faults, but oblique to the main trend of the Gulf (N70°E). These faults show a systematic reactivation under a 160°E extensional stress that we interpret also as syn-rift. The occurrence of these two successive phases of extension over more than 1000 km along the continental margin suggests a common origin linked to the rifting process. After discussing other possible mechanisms such as a change in plate motion, far-field effects of Arabia–Eurasia collision, and stress rotations in transfer zones, we present a working hypothesis that relates the 160°E extension to the westward propagation since about 20 Ma of the N70°E-trending, obliquely spreading, Gulf of Aden oceanic rift. The late 160°E extension, perpendicular to the direction of rift propagation, could result from crack-induced extension associated with the strain localization that characterises the rift-to-drift transition. 相似文献
13.
Arijit Ray S.K. Patil D.K. Paul S.K. Biswas Brindaban Das N.C. Pant 《Journal of Asian Earth Sciences》2006,27(6):907-921
Mafic volcanic rocks of the Mesozoic Kutch basin represent the earliest phase of Deccan volcanic activity. An olivine-clinopyroxene-plagioclase-phyric undersaturated basalt occurs as a sill near Sadara in the Pachham upland, Northern Kutch. The Sadara sill is deformed and emplaced along faults. The sill is alkaline in character and is transitional between basalt and basanite. Compared to primitive mantle, the Sadara sill is enriched in Sr, Ba, Pb and LREE but depleted in Nb, Cr, Y, Cs and Lu. Fractional crystallization of olivine and clinopyroxene from an alkaline mafic melt generated by low degree partial melting of mantle peridotite can explain the observed chemical variation in the sill.IRM and L-F test experiments and mineral analyses show titano-magnetite as the major remanence carrying magnetic mineral. AF and thermal demagnetizations of the Sadara sill yielded a mean ChRM direction as D=315.6°, I=−43.0° (α95=9.78; k=25.38) and the corresponding VGP at 25°S; 114.6°E (dp/dm=6.58°/11.6°). The Sadara sill pole is significantly different from those of the Deccan (65 Ma) and the Rajmahal Traps (118 Ma) but is close to the Cretaceous poles of 85–91 Ma rock units from southern India. This suggests a pre-Deccan age for the sill. 相似文献
14.
William Harbert Sergei Sokolov Maxim Alexutin Kirill Krylov Vladimir Grigoriev Alexander Heiphetz 《Tectonophysics》2003,361(3-4):215-227
The northernmost Kamchatka Peninsula is located along the northwestern margin of the Bering Sea and consists of complexly deformed accreted terranes. Progressing inland from the northwestern Bering Sea, the Olyutorskiy, Ukelayat and Koryak superterranes (OLY, UKL and KOR) are crossed. These terranes were accreted to the backstop Okhotsk-Chukotsk volcanic-plutonic belt (OChVB) in northernmost Kamchatka. A sedimentary sequence of Albian to Maastrichtian age overlaps the terranes and units of the Koryak superterrane, and constrains their accretion time. A paleomagnetic study of blocks within the Kuyul (KUY) terrane of the Koryak superterrane was completed at two localities (Camp 2: λ=61.83°N, φ=165.83°E and Camp 3: λ=61.67°N, φ=164.75°E). At both localities, paleomagnetic samples were collected from Late Triassic (225–208 Ma) limestone blocks (2–10 m in outcrop height) within a melange zone. Although weak in remanent magnetization, two components of remanent magnetization were observed during stepwise thermal demagnetization at 32 sites. The A component of magnetization was observed between room temperature and approximately 250 °C. This magnetic component is always of downward directed inclination and shows the best grouping at relatively low degrees of unfolding. Using McFadden–Reid inclination-only statistics and averaging all site means, the resulting A component mean is Iopt=60.3°, I95=5.0° and n=36 (sites). The B magnetic component is observed up to 565 °C, at which temperature, most samples have no measurable remanent magnetization, or growth of magnetic minerals has disrupted the thermal demagnetization process. Combining sites with Fisher estimates of kappa (k-value)≥13 and n (sites)≥3, where bedding orientation differs within a block, most of these sites show the best grouping of B component directions at 100% unfolding, and two of the blocks display remanent magnetizations of both upward and downward directed magnetic inclination. Combining sites with Fisher estimates of kappa (k-value)≥13 and n (sites)≥3, the resulting overall B component paleolatitude and associated uncertainty are λobs=30.4°N or S, λ95=8.9° and n=19 (sites). When compared with the expected North America paleolatitude of λAPWP expected=57.9°N, our data support a model in which blocks within the Koryak superterrane are allochthonous and far travelled. 相似文献
15.
The flood basalt province in Siberia is one of the largest in the world but the number of reliable paleomagnetic data on these volcanics is still limited. We studied lava flows and trap-related intrusions from two areas in the north and west of the Siberian platform. A dual-polarity characteristic component was isolated from most samples with the aid of stepwise thermal and alternating field demagnetization. We then compiled all published paleomagnetic data on the Siberian traps that have been obtained according to modern standards; also included are presumably trap-related overprint directions in Paleozoic rocks. Although these overprints and trap results may locally differ, the corresponding mean poles based on remagnetized sediments and volcanics show excellent overall agreement and justify pooling of both data types. Several ways of data grouping were attempted; the trap mean pole proved to be rather insensitive to statistical treatment. Irrespective of the averaging procedure used, the overall mean poles for the Siberian traps (NSP2: 55.1°N, 147.0°E; N = 8, K = 123, A95 = 5.0° or NSP4: 57.2°N, 151.1°E; N = 8, K = 192, A95 = 4.0°) differ slightly, but significantly from the coeval mean poles of Baltica [Torsvik, 2001; Van der Voo, R., and Torsvik, T.H., The quality of the European Permo-Triassic paleopoles and its impact on Pangea reconstructions, in: Timescales of the Paleomagnetic Field, J. E. T. Channell, D.V. Kent, W. Lowrie, and J.G. Meert, eds., AGU Geophys. Monogr., 2004, 135, 29–42]. We consider possible causes for this difference and conclude that it could be explained either by persistent non-dipole terms in the Permo-Triassic geomagnetic field or widespread inclination shallowing in the European data. 相似文献
16.
From alternating-field and thermal demagnetization studies on two dolerite “Traps” in the Gwalior Series (Central India), dated at 1830 ±200 m.y., three different palaeomagnetic directions could be distinguished. The characteristic magnetization component, which is considered as the primary magnetization, has a mean direction: D=78°, I=+34.5°, α95=5°, k=369, N=4 (Pole): 155.5°E19°N, dp=3°, dm=5.5°.A comparison of the presented data with other Precambrian and Phanerozoic data from the Indian subcontinent might suggest that the Indian subcontinent underwent a continuous anticlockwise rotational movement during the last 1800 m.y. 相似文献
17.
The Mascot–Jefferson City (M-JC) Mississippi Valley-type (MVT) deposits are in the Valley and Ridge province of the Appalachian orogen in East Tennessee. They have been a major source of zinc for the USA but their age is uncertain and thus their genesis controversial. About 10 specimens from each of 37 sites have been analysed paleomagnetically using alternating field and thermal step demagnetisation methods and saturation isothermal remanence methods. The sites sample limestones, dolostones, breccia clasts and sphalerite–dolomite MVT mineralisation from mines in the Lower Ordovician Kingsport and Mascot formations of the Knox Group. The characteristic remanent magnetisation (ChRM) is carried by magnetite in the limestones, by both magnetite and pyrrhotite in the dolostones and by pyrrhotite preferentially to magnetite in the mineralisation. Mineralized sites have a more intense ChRM than non-mineralised, indicating that the mineralising and magnetisation event are coeval. Paleomagnetic breccia tests on clasts at the three sites are negative, indicating that their ChRM is post-depositional remagnetisation, and a paleomagnetic fold test is negative, indicating that the ChRM is a remagnetisation, and a post-dates peak Alleghanian deformation. The unit mean ChRM direction for the: (a) limestones gives a paleopole at 129°E, 12°N (dp=18°, dm=26°, N=3), indicating diagenesis formed a secondary chemical remanent magnetisation during the Late Ordovician–Early Silurian; (b) dolomitic limestones and dolostone host rocks gives a paleopole at 125.3°E, 31.9°N (dp=5.3°, dm=9.4°, N=7), recording regional dolomitisation at 334±14 Ma (1σ); and (c) MVT mineralisation gives a paleopole at 128.7°E, 34.0°N (dp=2.4°, dm=4.4°, N=25), showing that it acquired its primary chemical remanence at 316±8 Ma (1σ). The mineralisation is interpreted to have formed from hydrothermal fluid flow, either gravity or tectonically driven, after peak Alleghanian deformation in eastern Tennessee with regional dolomitisation of the host rocks occurring as part of a continuum during the 20 Ma prior to and during peak deformation. 相似文献
18.
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
19.
A re-evaluation of the existence of true polar wander (TPW) since the Late Cretaceous and a comparison among the various approaches are made using updated paleomagnetic, hotspot and relative motion datasets. Previous attempts to determine the existence of TPW had resulted in different conclusions: comparison of hotspot locations and paleomagnetic poles required significant pole motion, although lithospheric plate displacement analysis yielded insignificant motion. However, these earlier determinations cannot be directly compared to find the reason for the discrepancies, because each used different datasets. For this study the different approaches are applied to a single updated model with three alternative relative motions of East and West Antarctica. Although the results are model-dependent, in general there was not significant motion of the pole relative to the lithosphere (1–5°) since the early Tertiary, but a large motion (10–12°) relative to the hotspot framework. It is unlikely that errors in the determinations could account for this disagreement: the A95 of the plate reconstruction is about 3°, the uncertainty in Antarctica motion is estimated to no larger than 3°, and cumulative errors in the relative plate motions may also amount to 3°. Only if all these errors are present in the maximum estimated amount, and in the same direction, could they account for the 10–12° gap between the two approaches. This conclusion of pole motion relative to the hotspots, but not the lithosphere, may indicate an independent shift of the mesosphere relative to the lithosphere (or “mantle roll” of Hargraves and Duncan). 相似文献
20.
Subduction zones with deep seismicity are believed to be associated with the descending branches of convective flows in the mantle and are subordinated to them. Therefore, the position of subduction zones can be considered as relatively fixed with respect to the steady-state system of convective flows. The lithospheric plate overhanging a subduction zone (as a rule of continental type) may:
- 1. (1) either move away from the subduction zone; or
- 2. (2) move onto it. In the first case extensional conditions originate behind the subduction zone and the new oceanic crust of back-arc basins forms. In the second case active Andean-type continental margins with thickening of the crust and lithosphere are observed.