首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 109 毫秒
1.
Results of a systematic paleomagnetic study are reported based on Late Carboniferous to Early Permian sedimentary rocks on the north slope of the Tanggula Mountains,in the northern Qiangtang terrane(NQT),Tibet,China.Data revealed that magnetic minerals in limestone samples from the Zarigen Formation(CP^z)are primarily composed of magnetite,while those in sandstone samples from the Nuoribagaribao Formation(Pnr)are dominated by hematite alone,or hematite and magnetite in combination.Progressive thermal,or alternating field,demagnetization allowed us to isolate a stable high temperature component(HTC)in 127 specimens from 16 sites which successfully passed the conglomerate test,consistent with primary remnance.The tilt-corrected mean direction for Late Carboniferous to Early Permian rocks in the northern Qiangtang terrane is D_s=30.2°,I_s=-40.9°,k_s=269.0,a_(95)=2.3°,N=16,which yields a corresponding paleomagnetic pole at 25.7°N,241.5°E(dp/dm=2.8°/1.7°),and a paleolatitude of 23.4°S.Our results,together with previously reported paleomagnetic data,indicate that:(1)the NQT in Tibet,China,was located at a low latitude in the southern hemisphere,and may have belonged to the northern margin of Gondwana during the Late Carboniferous to Early Permian;(2)the Paleo-Tethys Ocean was large during the Late Carboniferous to Early Permian,and(3)the NQT subsequently moved rapidly northwards,perhaps related to the fact that the Paleo-Tethys Ocean was rapidly contracting from the Late Permian to Late Triassic while the Bangong Lake-Nujiang Ocean,the northern branch of the Neo-Tethys Ocean,expanded rapidly during this time.  相似文献   

2.
We have obtained additional evidence for the Early Carboniferous paleomagnetic field for cratonic North America from study of the Barnett Formation of central Texas. A characteristic magnetization of this unit was isolated after thermal demagnetization at four sites (36 samples) out of eight sites (65 samples) collected. The mean direction of declination = 156.3°, inclination = 5.8° (N = 4 ,k = 905 , α95 = 3.0°), corresponds to a paleomagnetic pole position at lat. = 49.1°N,long. = 119.3°E (dp = 1.5° , dm = 3.0°). Field evidence suggests that characteristic magnetization was acquired very early in the history of the rock unit whereas the rejected sites are comprised of weakly magnetized limestones dominated by secondary components near the present-day field direction. Comparison of the Barnett pole with other Early Carboniferous (Mississippian) paleopoles from North America shows that it lies close to the apparent polar wander path for stable North America and that the divergence of paleopoles from the Northern Appalachians noted previously for the Devonian persisted into the Early Carboniferous. We interpret this difference in paleopoles as further evidence for the Northern Appalachian displaced terrain which we refer to here as Acadia, and the apparent coherence of Late Carboniferous paleopoles as indicating a large (～1500 km) motion of Acadia with respect to stable North America over a rather short time interval in the Carboniferous.  相似文献   

3.
The characteristic magnetization of redbed samples from the upper part of the Série d'Abadla (probably Early Permian 31°N, 2.7°W) has a mean direction derived from 13 sites of D=129°, I=11°, k=59, α95=6° and a corresponding south paleopole at 29°S, 60°E, A95=5°. All directions have reversed polarity. The paleolatitude of the northern fringe of the Saharan craton was 6°±3°S, which is in excellent agreement with that for the Moroccan Meseta. Therefore, in all probability, there has been no paleolatitudinal displacement greater than about 500 km of the Moroccan Meseta relative to Africa since Permian time. Comparison of results from sedimentary rocks shows no evidence for relative rotation of the Moroccan Meseta since Permian time. Small apparent rotations are indicated by evidence from massive trachyandesite lavas from Morocco, but we argue that these could have arisen from the incomplete averaging of secular variation and uncertainties in estimates of paleohorizontal, rather than from true tectonic rotations. The combined latest Carboniferous/Early Permian paleopole for the Saharan craton and the Meseta differs form the path of apparent polar wandering for North America when the continents are assembled in Wegener's Pangea (Pangea A, in which northwest Africa is opposite North America). It is in reasonable agreement when the continents are assembled in the Pangea B configuration (northwest Africa opposite Europe).  相似文献   

4.
The paleomagnetism of the Late Cretaceous Poços de Caldas alkaline complex (46.6°W, 21.9°S) was investigated through 42 oriented cores from seven sites. Six sites, reversed relative to the present magnetic field of the Earth, yield a pole at 127°W, 82°S (dp = 8°,dm = 13°). This pole is located close to other Late Cretaceous poles for South America obtained by Creer [1] from untreated paleomagnetic samples. The results are significantly different from those for the nearby Early Cretaceous Serra Geral basalt but close to the Triassic pole for South America. The polar wandering path for South America for the Mesozoic seems to be more complicated than anticipated. The available paleomagnetic information may not yet be precise enough to determine the time of opening of the Atlantic.  相似文献   

5.
We report paleomagnetic results from oriented drill core samples collected at 10 sites (80 samples) from the Covey Hill and 19 sites (96 samples) from the overlying, fossiliferous Cha?teauguay Formations of the gently dipping Late Cambrian Potsdam Group sandstones exposed in the St. Lawrence Lowlands of Quebec. Stepwise thermal demagnetization analyses ave revealed the presence of two predominant groups of coherent magnetizations C-1 and C-2, after simple correction for bedding tilt. The C-1 group magnetization is a stable direction (D=332°, I=+18°) with unblocking temperatures (TUB) between 550 and 650°C, present in the older Covey Hill Formation; this direction is probably a chemical remanence acquired during the Covey Hill diagenesis and carried predominantly by hematite. The C-2 group magnetization (D=322°, I=+9°) is present at 13 sites of the younger Cha?teauguay Formation; this is probably carried by magnetite and represents a penecontemporaneous, depositional DRM, characterized by TUB spectra 400–550°C. We believe that C-2 is relatively younger than C-1 based on a combination of arguments such as the presence of opposite polarities, internal consistency, similarity and common occurrence of C-1 and C-2 respectively in the Covey Hill and Cha?teauguay members. The corresponding paleomagnetic poles C-1 (46°N, 149°E; dp, dm=3°, 5°) and C-2 (37°N, 156°E; dp, dm=2°, 5°) are not significantly different from most of the other Late Cambrian (Dresbachian-Franconian) poles derived from sediments exposed in the southern region (Texas) of the North American craton which are also believed to have been deposited during Croixian Sauk sea transgression similar to the Potsdam sandstones. Although adequate faunal control is lacking (in particular for the Covey Hill Formation), this comparison with the Cratonic poles suggests a Late Cambrian age to the Potsdam poles. The agreement between the results also gives the evidence for internal consistency of cratonic poles at least for Late Cambrian.The incoherent C-3 group remanence (D=250°, I=?15°) is commonly present at 7 sites in both the formations; this may not correspond to a reliable paleomagnetic signal. The other remanence C-4 (D=180°, I=+10°) is found only at 3 sites located in the uppermost stratigraphic levels of the Cha?teauguay Formation; the corresponding paleomagnetic pole (40°N, 107°E) does not differ significantly from the Ordovician and some Late Cambrian poles. The present data are insufficient to resolve a problem in apparent polar wander for Middle and Late Cambrian time posed by the existence of high-latitude poles for some strata of Middle Cambrian age and low-latitude poles for some strata of Late Cambrian age.  相似文献   

6.
A paleomagnetic and potassium-argon dating investigation has been carried out on a 530-km-long dike system which transects the western Iberian Peninsula in a northeasterly direction. The K-Ar age determinations were made on mineral separates exclusively. They range between 160 and 200 Ma and the authors suppose that this reflects the actual time interval of the intrusion, in accord with previous results. The paleomagnetic pole derived from 12 sites regularly distributed along the dike (71°N, 236°E) coincides well with other Mesozoic paleomagnetic poles from the western Africa. A contemporaneous pole from stable Europe is tentatively deduced from African and North American Late Triassic/Early Jurassic poles using different reconstruction models around the North Atlantic Ocean. The divergence between this pole and the Iberian pole corresponds to the result obtained for Permian poles.  相似文献   

7.
用热退磁辅以交变退磁方法对采自塔里木盆地阿克苏地区四石厂剖面47个采样点518块标本进行了逐步磁清洗和测试。由本征剩磁方向统计得到塔里木地台晚古生代的古地磁极位置(晚泥盆世φ=10.5°S、λ=151.2°E;晚石炭世φ=52.2°N、λ=179.5°E;早二叠世φ=56.5°N,λ=190.1°E)。古地磁结果表明:塔里木地台在晚古生代是北方大陆的块体之一。从晚石炭世至早二叠世塔里木地台已和北方的哈萨克斯坦板块、西伯利亚地台、俄罗斯地台等连成一片,并且从中生代以来它们之间的相对位置没有发生过大规模的变动  相似文献   

8.
Early Carboniferous (Viséan to possibly earliest Namurian) sedimentary rocks of the Deer Lake Group of western Newfoundland rest unconformably on Grenvillian basement rocks of the Canadian Shield which form the western border of the Early Palaeozoic Appalachian orogen. In addition to magnetically soft magnetizations directed along the present field, three families of magnetization directions are found. Two of them (referred to as N (north) and S (south)) are antiparallel and prefolding, and were probably acquired during the depositional process. N and S are roughly of equal frequency. They have a mean direction irrespective of sign of 0.7°, ?35.2°, k = 40, α95 = 8.9°, and a palaeopole 21.5°N, 121.8°E (10.3°, 6.0°) corresponding to a palaeolatitude of 20 ± 6°S. This agrees with the palaeolatitude (17 ± 5°S) determined from the somewhat older Early Carboniferous (Tournaisian) Terenceville Formation of the Avalon Platform on the eastern side of the Appalachian orogen in Newfoundland. The third magnetization, referred to as H (roughly horizontal), has a mean direction 156.8°, ?13.3°, k = 37, α95 = 10.1°, and a palaeopole 45.4°N, 140.3°E (10.3°, 5.3°) corresponding to a palaeolatitude of 7 ± 4°N; we interpret this to be an early Kiaman (latest Carboniferous to early Permian) overprint probably acquired chemically. The palaeolatitude determined from the H overprint agrees with that determined from Early Carboniferous rocks of cratonic North America west of the Appalachians. Therefore, we argue, Early Carboniferous palaeofield determinations for cratonic North America have been strongly biased by unremoved Kiaman overprints. Because of this, and because of the good agreement between Early Carboniferous palaeolatitudes obtained from opposite margins of the Appalachian orogen, we suggest that there is, at present, no palaeomagnetic evidence for the previously proposed 1500 km displacement from the south of an eastern portion of the Appalachians (“Acadia”) relative to cratonic North America during the Carboniferous.  相似文献   

9.
  相似文献   

10.
Paleomagnetic samples from the Nolichucky Formation (Late Cambrian), sampled at two sites in the Valley and Ridge Province of east Tennessee, yield a possibly penecontemporaneous characteristic magnetization that appears to reside in detrital magnetite. The paleomagnetic pole positions are “Paleozoic”, but differ: site I, lat. 41°N, long. 109°E,dp = 1°, dm = 2°; site II, lat. 39°N, long. 131°E,dp = 4°, dm = 7°. The difference in poles reflects a significant difference in declination between the site-mean directions, and this declination difference probably reflects relative tectonic rotation as the sites are in different thrust sheets. The paleontologic age of both sections is exceptionally well-constrained as they are sampled across an abrupt “biomere boundary” between contrasting trilobite faunas. Comparison of these results with paleomagnetic data from coeval strata elsewhere in North America reveals gross discrepancies, so that at least some of the published data must reflect remagnetization and/or tectonic rotation.  相似文献   

11.
The paleomagnetic study of the Lower Ordovician and Cambrian sedimentary rocks exposed on the Narva River’s right bank revealed a multicomponent composition of natural remanent magnetization. Among four distinguished medium- and high-temperature magnetization components, the bipolar component, which carries the reversal test, is probably the primary component and reflects the geomagnetic field direction and variations during the Late Cambrian and Early Ordovician. The pole positions corresponding to this component have coordinates 22°N, 87°E (dp/dm = 5°/6°) for the Late Cambrian, and 18°N, 55°E (dp/dm = 5°/7°) for the Early Ordovician (Tremadocian and Arenigian). Together with the recently published paleomagnetic poles for the sections of the Early Ordovician in the Leningrad Region and the series of poles obtained when the Ordovician limestones were studied in Sweden, these poles form new key frameworks for the Upper Cambrian-Middle Ordovician segment of the apparent polar-wander path (APWP) for the Baltica. Based on these data, we propose a renewed version of the APWP segment: the model of the Baltica motion as its clockwise turn by 68° around the remote Euler pole. This motion around the great circle describes (with an error of A95 = 10°) both variations in the Baltic position from 500 to 456 Ma ago in paleolatitude and its turn relative to paleomeridians. According to the monopolar components of natural remanent magnetization detected in the Narva rocks, the South Pole positions are 2°S, 351°E (dp/dm = 5°/9°), 39°S, 327°E, (dp/dm = 4°/7°), and 42°S and 311°E (dp/dm = 9°/13°). It is assumed that these components reflect regional remagnetization events in the Silurian, Late Permian, and Triassic.  相似文献   

12.
Paleomagnetic characteristics of Carboniferous-Permian and Early Mesozoic geological complexes in Mongolia are studied. The studied rocks are shown to possess a multicomponent magnetization. Lowtemperature overprinting components of normal polarity discovered in nearly all of the studied strata were acquired after main deformation stages of the rocks, apparently in the Cenozoic. High-temperature overprinting components of reversed polarity identified in rocks of an active continental margin (ACM) were acquired when bimodal magma melts moved through ACM volcanic sequences. Late Carboniferous and Early Permian paleomagnetic poles of Mongolia calculated from directions of primary magnetization components are, respectively (Λ = 154.6, Φ = 32.2, A = 7.8) and (Λ = 95, Φ = 71, A = 8.7). Apparently, the territory of Mongolia in the Early Permian was a margin of the Siberian craton and was separated from the Northern China block by a basin extending for no less than 2000 km in the E-W direction. The strike of a marginal-continental volcanic belt was submeridional and a plate subducted under the continent from the east. Late Carboniferous-Permian intraplate magmatic complexes of Mongolia formed at various latitudes from various mantle sources during the northward movement of the Mongolian part of the Siberian continent. The oldest bimodal sequences of the Gobi-Tien Shan zone (318–314 Ma) formed at more southern latitudes (40°–47°–54°N) as compared with the 275-Ma complexes of the Gobi-Altai zone (51°–58°–67°N). Thus, sources of the Carboniferous-Permian intraplate magmatism in Central Asia either occupied a vast mantle region (up to 1000 km in the latitude direction) or moved together with the Asian continent.  相似文献   

13.
Of 16 sites collected in the Taru grits (Permian) and Maji ya Chumvi beds (Permo-Triassic) of East Africa only 6 sites from the Maji ya Chumvi sediments gave meaningful palaeomagnetic results. After thermal cleaning the 6 sites (32 samples) give an Early Triassic pole at 67°N, 269°E with A95 = 17° in excellent agreement with other African Mesozoic poles. There are now 26 Mesozoic palaeomagnetic poles for Africa from widely diverse localities ranging in present latitude from 35°N to 30°S. The poles subdivide into Triassic (17 poles) and Cretaceous (9 poles) groups whose means are not significantly different. The palaeomagnetic pole for Africa thus remained in much the same position for 170 m.y. from Early Triassic to Late Cretaceous. The data form an especially good set for estimating the palaeoradius using Ward's method. Values of 1.08 ± 0.15 and 1.03 ± 0.19 times the present radius are deduced for the Triassic and Cretaceous respectively with a mean value of 1.08 ± 0.13 for all the Mesozoic data combined. The analysis demonstrates that hypotheses of earth expansion are very unattractive.  相似文献   

14.
Paleomagnetic study of China and its constraints on Asia tectonics has been a hot spot. Some new paleomagnetic data from three major blocks of China. North China Block (NCB), Yangtze Block (YZB) and Tarim Block (TRM) are first reported, and then available published Phanerozoic paleomagnetic poles from these blocks with the goal of placing constraints on the drift history and paleocontinental reconstruction are critically reviewed. It was found that all three major blocks were located at the mid-low latitude in the Southern Hemisphere during the Early Paleozoic. The NCB was probably independent in terms of dynamics. its drift history was dominant by latitudinal placement accompanying rotation in the Early Paleozoic. The YZB was close to Gondwanaland in Cambrian, and separated from Gondwanaland during the Late-Middle Ordovician. The TRM was part of Gondwanaland, and might be close to the YZB and Australia in the Early Paleozoic. Paleomagnetic data show that the TRM was separated from Gondwanaland during the Late-Middle Ordovician, and then drifted northward. The TRM was sutured to Siberia and Kazakstan blocks during the Permian, however, the composite Mongolia-NCB block did not collide with Siberia till Late Jurassic. During Late Permian to Late Triassic, the NCB and YZB were characterized by northern latitudinal placement and rotation on the pivot in the Dabie area. The NCB and YZB collided first in the eastern part where they were located at northern latitude of about 6°—8°, and a triangular oceanic basin remained in the Late Permian. The suturing zone was located at northern latitude of 25° where the two blocks collided at the western part in the Late Triassic. The collision between the two blocks propagated westward after the YZB rotated about 70° relative to the NCB during the Late Permian to Middle Jurassic. Then two blocks were northward drifting (about 5°) together with relative rotating and crust shortening. It was such scissors-like collision procedure that produced intensive compression in the eastern part of suturing zone between the NCB and YZB, in which continental crust subducted into the upper mantle in the Late Permian, and then the ultrahigh-pressure rocks extruded in the Late Triassic. Paleomagnetic data also indicate that three major blocks have been together clockwise rotating about 20° relative to present-day rotation axis since the Late Jurassic. It was proposed that Lahsa Block and India subcontinent successively northward subducted and collided with Eurasia or collision between Pacific/Philippines plates and Eurasia might be responsible for this clockwise rotating of Chinese continent.  相似文献   

15.
石炭纪末古地理图   总被引：8，自引：0，他引：8       下载免费PDF全文
过去发表的石炭纪古地理重建图存在着不少问题,特别是对亚洲各板块位置的认识上。例如,过去的重建图中华北和华南在石炭纪末都处于北半球40°—50°纬度带,但是,地层古生物资料清楚地表明,它们当时处于热带和亚热带环境。这是因为在编制上述古地理图时(70年代末和80年代初),华南和华北等东亚和东南亚地块还没有可靠的古地磁数据,因而这一地区的地块的位置是由距它们最近的西伯利亚地台的地极位置推算出来的。但是,由于这些地块和西伯利亚地台自石炭纪以来曾发生过相对运动,因此,上述作法是不合理的  相似文献   

16.
An Upper Permian paleomagnetic pole has been determined for the Cribas Formation in eastern Timor. The co-ordinates for the mean pole are 159.8°E and 56.6°S,α95 = 9.0. The reliability of the pole is ascertained through thermal demagnetization, a fold test, comparison between red beds and a lava flow, and the presence of normal and reversed polarities. The Timor pole is in excellent agreement with the Australian Upper Permian and Triassic poles. From this it is inferred that autochthonous Timor formed part of the Australian continental margin at least since the Upper Permian.  相似文献   

17.
The South American palaeomagnetic poles published after the Upper Mantle Conference on Solid Earth Problems held at Buenos Aires in 1970, are summarized.The Late Palaeozoic-Cretaceous section of the South American polar wandering curve is now defined on the basis of twenty palaeomagnetic poles; these poles define five “age groups” at Late Carboniferous, Permo-Carboniferous, Middle Permian, Triassic and Cretaceous times.The comparison of the Late Palaeozoic-Mesozoic sections of the polar wandering curves of South America, Australia and Africa suggests that the former fragmentation of the Gondwana occurred in Late Carboniferous or Permo-Carboniferous times and that the origin of the South Atlantic Ocean took place after the Middle Jurassic (160 m.y.) but before the Early Cretaceous (120 m.y.).  相似文献   

18.
The palaeomagnetism of Middle Triassic (224 ± 5 m.y.) igneous rocks from the Ischigualasto-Ischichuca Basin (67°40′W, 30°20′S) was investigated through 86 oriented hand samples from 11 sites. At least one reversal of the geomagnetic field has been found in these rocks. Nine sites yield a palaeomagnetic pole at 239°E, 79°S (α95 = 15°, k = 13).The K-Ar age determinations of five igneous units of the Puesto Viejo Formation give a mean age of 232 ± 4 m.y. (Early Triassic). The palaeomagnetism of six igneous units of the Puesto Viejo Formation (68°W, 35°S) was investigated through 60 oriented samples. These units, two reversed relative to the present magnetic field of the Earth and four normal, yield a pole at 236°E, 76°S (α95 = 18°, k = 14).Data from the Puesto Viejo Formation indicate, for the first time on the basis of palaeomagnetic and radiometric data, that the Illawarra Zone, which defines the end of the Kiaman Magnetic Interval, extends at least down to 232 ± 4 m.y. within the Early Triassic. The palaeomagnetic poles for the igneous rocks of the Ischigualasto-Ischichuca Basin and Puesto Viejo Formation form an “age group” with the South American Triassic palaeomagnetic poles (mean pole position: 239°E, 77°S; α95 = 6.6°, k = 190). The Middle and Upper Permian, Triassic and Middle Jurassic palaeomagnetic poles for South America would define a “time group” reflecting a quasi-static interval (mean pole position: 232°E, 81°S; α95 = 4°, k = 131).  相似文献   

19.
For long time the western-central Mexico has been affected by oblique subduction caused by Farallon plate beneath North America. As result, smaller plates (e.g. Cocos Plate), several fault systems outlining crustal blocks (e.g. Michoacán block) and magmatic arcs (e.g. Paleocene-Early Oligocene magmatism and the Trans-Mexican Volcanic Belt) were developed. Still, no paleomagnetic data are available for Oligocene and Miocene. The principal aim of this study is to evaluate whether the tectonic rotations and relative motions of these blocks occurred before the Miocene. Here, we report a detailed rock-magnetic and paleomagnetic results from Tecalitlan area, located in the Michoacán block. Sixteen sites (about 150 oriented samples) were collected including one radiometrically dated diabase dike (35.0 ± 1.8 Ma). Rock-magnetic experiments permitted identification of magnetic carriers and assessment of the paleomagnetic stability. Continuous susceptibility measurements vs temperature in most cases yield reasonably reversible curves with Curie points close to that of magnetite. Reliable paleomagnetic directions were obtained for 12 sites. Inclination I and declination D of the mean paleomagnetic direction obtained in this study are I = 33.1°, D = 345.0°, and Fisherian statistical parameters are k = 25, α95 = 8.9°. The corresponding mean paleomagnetic pole position is Plat = 75.7°, Plong = 166.6°, K = 31, A95 = 8.0°. The mean inclination is in reasonably good agreement with the expected value, as derived from reference poles for the stable North America. Magnetic declination is not significantly different from that expected which is in disagreement with a counterclockwise tectonic rotation of about 20° previously reported for the studied area. Based on paleomagnetic results obtained in this study compiled with those currently available from the Michoacán Block, we propose a simple model suggesting that sometime in Eocene epoch the convergence vector of the Farallon plate relative to North America plate was normal to the trench before reaching an actual oblique convergence.  相似文献   

20.
Paleomagnetic results are reported from 13 sites of red beds of Early Devonian age from Central Iran. Detailed paleomagnetic analyses were carried out. Two types of partial progressive demagnetization were applied, one using alternating magnetic fields, the other heating. These procedures resulted in the detection of the characteristic remanences with a mean direction with D = 24.2°, I = 1.3° (α95 = 10.1°). The paleomagnetic pole is located at 51.3°N, 163.7°W. If one shifts the Iranian landmass to its most likely position in the Gondwana configuration, then the position of the paleomagnetic pole coincides with the alternative polar wander path [14,15] which crossed South America in early Middle Paleozoic times.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号