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1.
Paleomagnetic and rock magnetic study has been conducted on the Early Triassicred beds of Liujiagou Formation from Jiaocheng, Shanxi Province. Hematite was shown as themain magnetic mineral. After eradicating an initial viscous component at room temperature to～100℃-200℃, thermal demagnetization shows that most samples contain two remanencecomponents, intermediate-temperature remanence component at 250℃-500℃ and high-tem-perature component at 500℃-680℃. The intermediate-temperature component has a negativefold test at the 95% confidence level. And the pole position of the intermediate-temperaturecomponent in geographic coordinates is correlated with the Middle Jurassic reference pole of theNorth China Block (NCB) within the 95% confidence, suggesting that it might be a remagnetiza-tion component acquired during the Yanshanian period. The high-temperature component con-tains both reversal and normal polarities with positive fold test and C-level positive reversal test atthe 95% confidence level, which suggests that this high-temperature component can be regardedas primary magnetization. Comparison of this newly obtained Early Triassic paleopole with thecoeval mean pole of the Ordos Basin suggests that a locally relative rotation may have happenedbetween the Ordos and the Jiaocheng area of Shanxi Province. This rotation may be related withtwo faults: one is Lishi big fault separating Ordos from Shanxi and the other is Jiaocheng big fault,which is situated in the southeast of sampling locality and was still in motion during the Cenozoic. 相似文献
2.
Paleomagnetic study on orogenic belt: An example from Early Cretaceous volcanic rocks,Inner Mongolia,China 总被引：3，自引：0，他引：3
REN Shoumai ZHU Rixiang HUANG Baochun ZHANG Fuqin & WANG Hongqiang .Institute of Geology Geophysics Chinese Academy of Sciences Beijing China .Strategic Research Center for Oil Natural Gas The Ministry of Land Resources of China Beijing China 《中国科学D辑(英文版)》2004,47(12):1127-1133
Since the 1990s, a large number of paleomagneticstudies have been carried out in the North China block(NCB) and Tarim block[1-8], and more and more geo-physicists recently believe that the last collision andconvergence between Siberia and the Mongolia-NorthChina plate happened in the Late Jurassic, which wascontributed to a paleomagnetic study on these areas byZhao and his colleagues[2]. However, we lack paleo-magnetic results obtained directly from the orogenicbelt between Siberia and th… 相似文献
3.
Lower Cretaceous red sedimentary rocks from the depositional basin of East Qilian fold belt have been collected for a paleomagnetic study. Stepwise thermal demagnetization reveals two or three components of magnetization from dark red sandstones. Low-temperature magnetic component is consistent with the present Earth Field direction in geographic coordinates. High-temperature magnetic components are mainly carried by hematite. The mean pole of 19 sites for high-temperature magnetic components after tilt-correction is λ=62.2°N, φ=193.4°E, A95=3.2°, and it passes fold tests at 99% confidence level and reversal tests at 95% confidence level. The paleopole is insignificantly different from that of Halim et al. (1998) from the same sampling area at the 95% confidence level. Compared with paleomagnetic results for North China, South China, and Eurasia, our results suggest that no significant relative latitudinal displacement has taken place between Lanzhou region and these blocks since Cretaceous time. Remarkably, the pole of Lanzhou shows a 20° clockwise rotation with respect to those of North China, South China, and Eurasia. Geological information indicates that the crustal shortening in the western part of Qilian is greater than that in eastern part. In this case, the clockwise rotation of sampling area was related to India/Eurasia collision, and this collision resulted in a left-lateral strike-slip motion of the Altun fault in north Tibetan Plateau after the Cretaceous. 相似文献
4.
蛇绿岩中枕状玄武岩的古地磁学研究可为古海洋的恢复与演化提供定量化依据.黑龙江省饶河地区中侏罗世枕状玄武岩的岩石学、岩石磁学研究表明,该岩石具备水下喷出特点,发育辉长结构,载磁矿物为磁铁矿.17个采点181块样品的热退磁实验表明,中侏罗世枕状玄武岩记录了高温分量和中温分量,前者为熔岩喷发记录的原生剩磁方向,平均方向D/I=59.4°/46.3°,α95=6.8°,对应的极位置为40.3°N,224.6°E,A95=7°;后者可能为晚侏罗世—早白垩世岩浆热事件的叠加,平均方向D/I=55.4°/60.6°,α95=3.9°,对应的极位置为50.8°N,210.6°E,A95=5.2°.综合考虑区域地质背景,将这一结果与邻区同时代的古地磁数据对比,推测在中侏罗世之前,在饶河杂岩与佳木斯地体之间存在一定规模的海域,与现今日本海相似;早白垩世时期,该海域封闭,饶河杂岩与华北、西伯利亚板块在动力学上已成为整体. 相似文献
5.
6.
A paleomagnetic study of about 95 samples from 16 sites sampled in the Early Cretaceous in Luanping basin in Hebei Province was reported. Stepwise thermal demagnetization was used to isolate magnetic components. Most samples have a characteristic direction with a high temperature component above 500°C. The tectonic-corrected data areD = 347.8°,I = 50.4°, α95 = 7.l°, and the corresponding pole position is at 76.1°N, 346.3°E,with dp =6.4°,dm = 3.8°, paleolatitude λ = 31.1°N. This result indicates a counterclockwise post-Cretaceous rotation of 30.7° ±9.8° with respect to the stable Ordos basin in the west of North China Block, and a non-significant northward motion. This rotation could be related to local fault action or structural detachment, or regional NNW-NWWward motion and collision of Kula-Pacific plate with eastern China since the Early Cretaceous.
相似文献7.
Daniel A. Valencio JoséE. Mendía Juan F. Vilas 《Earth and Planetary Science Letters》1975,26(3):319-330
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
8.
A paleomagnetic study of about 95 samples from 16 sites sampled in the Early Cretaceous in Luanping basin in Hebei Province was reported. Stepwise thermal demagnetization was used to isolate magnetic components. Most samples have a characteristic direction with a high temperature component above 500°C. The tectonic-corrected data areD = 347.8°,I = 50.4°, α95 = 7.l°, and the corresponding pole position is at 76.1°N, 346.3°E,with dp =6.4°,dm = 3.8°, paleolatitude λ = 31.1°N. This result indicates a counterclockwise post-Cretaceous rotation of 30.7° ±9.8° with respect to the stable Ordos basin in the west of North China Block, and a non-significant northward motion. This rotation could be related to local fault action or structural detachment, or regional NNW-NWWward motion and collision of Kula-Pacific plate with eastern China since the Early Cretaceous. 相似文献
9.
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
10.
Miguel Cervantes Solano Avto Goguitchaichvili Leda Sánchez Bettucci Ruben Cejudo Ruiz Manuel Calvo-Rathert Vicente Carlos Ruiz-Martinez Ruth Soto Luis M. Alva-Valdivia 《Studia Geophysica et Geodaetica》2010,54(4):533-546
We report detailed rock-magnetic and paleomagnetic directional data from 35 lava flows (302 standard paleomagnetic cores)
sampled in the Central-Northern region of Uruguay in order to contribute to the study of the paleosecular variation of the
Earth’s magnetic field during early Cretaceous and to obtain precise Cretaceous paleomagnetic pole positions for stable South
America. The average unit direction is rather precisely determined from 29 out of 35 sites. All A95 confidence angles are less than 8°, which points to small within-site dispersion and high directional stability. Normal polarity
magnetizations are revealed for 19 sites and 10 are reversely magnetized. Two other sites yield well defined intermediate
polarities. The mean direction, supported by a positive reversal test is in reasonably good agreement with the expected paleodirection
for Early Cretaceous stable South America and in disagreement with a 10° clockwise rotation found in the previous studies.
On the other hand, paleomagnetic poles are significantly different from the pole position suggested by hotspot reconstructions,
which may be due to true polar wander or the hotspot motion. Our data suggest a different style of secular variation during
(and just before) the Cretaceous Normal Superchron and the last 5 Ma, supporting a link between paleosecular variation and
reversal frequency. 相似文献
11.
Three components of magnetization have been observed in ninety-six samples (twelve sites) of amygdaloidal basalts and “sedimentary greenstones” of the Unicoi Formation in the Blue Ridge Province of northeast Tennessee and southwest Virginia. These components could be isolated by alternating field as well as thermal demagnetization. One component, with a direction close to that of the present-day geomagnetic field is ascribed to recent viscous remanent magnetizations; another component, with intermediate blocking temperatures and coercivities, gives a mean direction of D = 132°, I = +43°,α95 = 9° for N = 10 sites before correction for tilt of the strata. This direction and the corresponding pole position are close to Ordovician/Silurian data from the North American craton and we infer this magnetization to be due to a thermal(?) remagnetization during or after the Taconic orogeny. This magnetization is of post-folding origin, which indicates that the Blue Ridge in our area was structurally affected by the Taconic deformation. The third component, with the highest blocking temperatures and coercivities, appears to reside in hematite. Its mean direction, D = 276°, I = ?17°,α95 = 13.8° for N = 6 sites (after tilt correction) corresponds to a pole close to Latest Precambrian and Cambrian poles for North America. The fold test is inconclusive for this magnetization at the 95% confidence level because of the near-coincidence of the strike and the declinations. We infer this direction to be due to early high-temperature oxidation of the basalts, and argue that its magnetization may have survived the later thermal events because of its intrinsic high blocking temperatures. A detailed examination of the paleomagnetic directions from this study reveals that the Blue Ridge in this area may have undergone a small counterclockwise rotation of about 15°. 相似文献
12.
I. Evans S.A. Hall M.F. Carman M. Senalp S. Coskun 《Earth and Planetary Science Letters》1982,61(1):199-208
Paleomagnetic analyses of samples collected from a 500 m thick Jurassic section in the Pontides reveal the presence of two components of remanent magnetization: an unstable, low-temperature component which is removed during thermal demagnetization through 220°C and a dominant component which displays consistent directions through 650°. Curie point and IRM studies indicate that goethite is responsible for the low-temperature component whereas both magnetite and hematite contribute to the more stable component. The pole position determined from the stable magnetization is located at 18.8°N, 91.8°E (α95=7.7°, N=134) indicating that the section has undergone more than 90° clockwise rotation since the Late Jurassic. Ancillary geologic evidence, particularly the orientation of Jurassic facies belts is also consistent with a 90° clockwise rotation in this region of northwest Anatolia. The pole suggests that the section may also have migrated slightly northward. Although the age of these movements is currently unknow, it is proposed that they are principally related to the closure of the Neo-Tethys during the Late Cretaceous/Early Tertiary. Some of the rotation may be related to the right lateral movement along the North Anatolian Transform Fault which was initiated in the Miocene. 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.
Anna Caccavari Manuel Calvo-Rathert Avto Goguitchaichvili Vicente Soler Bertha Aguilar Reyes 《Studia Geophysica et Geodaetica》2010,54(4):547-560
We present rock-magnetic and paleomagnetic results obtained on samples belonging to a Neogene sequence of 11 successive lava
flows and a dyke from La Gomera (Canary Islands, Spain). Analysis of thermomagnetic curves allows to distinguish three types
of samples: (i) Type H samples with low-Ti titanomagnetite as the only carrier of remanence; (ii) type M samples with a main
intermediate Curie-temperature phase (TC = 450°C) and low-Ti titanomagnetite; (iii) type L curves with a low Curie-temperature phase (TC = 120 to 200°C) and an intermediate Curie-temperature phase (TC = 400°C). Analysis of hysteresis parameters suggests that the grain size of most studied samples corresponds to pseudo single-domain
particles, which can be also interpreted as a mixture of single-domain and multi-domain particles. Paleomagnetic experiments
reveal only a single paleomagnetic component. Characteristic remanence of all studied lava flows and the dyke shows reverse
polarity. The mean direction of the whole sequence is D = 188.2°, I =−35.4° (k = 46.9; α95 = 6.4°) and the calculated paleomagnetic pole yields a longitude λ= 150.7° and a latitude ϕ= 78.8° (k = 59.4; A95 = 5.7°). Secular variation is analysed through the scatter of virtual geomagnetic poles (VGP). A VGP angular scatter SB = 5.9 with an upper confidence limit Sup = 8.0 and a lower confidence limit Slow = 4.6 are obtained. This scatter is clearly smaller than the average for this latitude obtained for the last 5 Ma. The studied
lava flows were probably emitted in a relatively short time interval. 相似文献
15.
SHEN Zhongyue CHEN Hanlin FANG Dajun DING Jinghai ZHANG Shiben HUANG Zhibin & LI Meng . Department of Earth Sciences Zhejiang University Hangzhou China . Tarim Oil Field Company PetroChina Company Limited Korla China 《中国科学D辑(英文版)》2005,48(3):406-416
Southwest Tarim (hereafter SW Tarim) is one of afew areas that well developed Cretaceous marinesedimentary rocks in China [1]. The Cretaceous marinesediments are stretched in front area along the Tian-shan and Kunlun Mountains. Toward the center ofTarim Basin, the Cretaceous sediments are buried bygreat thickness of Tertiary and Quaternary sedimentswith little exposure. Compared with the Cretaceousterrestrial strata of north Tarim, the Cretaceous marinestrata of SW Tarim continue and d… 相似文献
16.
17.
F.H. Chamalaun 《Earth and Planetary Science Letters》1977,34(1):107-112
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
18.
本文报道青藏高原羌北地区中-晚侏罗世雁石坪群古地磁新结果.对采自青海省格尔木市唐古拉山乡雁石坪剖面(33.6°N, 92.1°E)11个灰岩采点(118块)和10个碎屑岩采点(99块)定向样品系统古地磁学研究表明,大部分样品的退磁曲线具有双分量特征.低温分量方向在地理坐标系下较为集中,应该为地层褶皱之后的黏滞剩磁.高温特征剩磁分量方向可分为两类:(1)索瓦组(J3s)和布曲组(J2b)灰岩,以磁铁矿为主要载磁矿物,高温特征剩磁分量(Ds=355.7°,Is=42.1°,k=58.2,α95=6°)可通过99%置信度的褶皱检验.(2)雪山组(J2x)和雀莫错组(J2q)碎屑岩,以赤铁矿、磁铁矿为主要载磁矿物,高温特征剩磁分量(Ds=3.3°,Is=28.9°,k=30.7,α95=8.9°)可通过95%置信度的倒转检验和99%置信度的褶皱检验.两组分量都应该是岩石形成时的原生剩磁信息.碎屑岩组的磁倾角比灰岩组偏低13°左右,其剩磁方向很可能存在着与压实作用相关的剩磁倾角变浅的状况.本文取灰岩组平均磁化方向作为雁石坪群的原生剩磁分量,获得羌北地区雁石坪群古磁极位置:80.0°N,295.2°E(dp/dm=7.4/4.5).古地磁结果表明,羌北-昌都地区晚石炭-晚二叠世期间位于南纬中低纬度地区,早三叠世以后开始大规模北向漂移,至中-晚侏罗世已到达24.3°N.其快速北向运动主要发生在早三叠至早侏罗世期间(3500 km左右),与现今位置相比中晚侏罗世之后的北向迁移总量为900 km左右. 相似文献
19.
在鄂尔多斯盆地的韩城、铜川等7条剖面144个采样点上,采集了下二叠统至下白垩统的样品约1500个.分别在中国、英国、法国的4个古地磁实验室中进行测试和实验研究.样品均经系统热退磁或交变退磁处理.数据经主向量分析、部分线性谱分析,以分离剩磁成分和选取特征剩磁方向.全部特征剩磁方向通过了倒转检验,晚二叠世和早、中三叠世的结果还通过了广义褶皱检验.并做了大量磁化率、等湿剩磁、薄片岩矿鉴定和少量居里温度测定的实验研究. 所得数据以世（统）为单位计算了古地磁极位置和采样地区古纬度,绘制了鄂尔多斯盆地晚古生代以来视极移曲线和地块古方位变化图,提出了华北地块运动模式,并通过与现有的华南地块资料的综合对比分析,提出华北地块与华南地块的碰撞在东部始于晚三叠世之前,全部拼合完成于中侏罗世末. 相似文献
20.
J.D.A. Piper 《Earth and Planetary Science Letters》1975,25(3):355-360
Detailed alternating field demagnetisation of Upper Llandovery volcanics of the Mendip Hills and Gloucestershire has isolated remanence directions interpreted as primary from each of five sites. Well-defined high-coercivity secondary magnetisation is present in six samples of one site and low-coercivity secondary remanence is present in all samples from another site; the former component was apparently acquired in Permo-Triassic times. Primary directions of magnetisation show marked improvement in precision after correction for penecontemporaneous folding, and show a late Llandovery reversal in the sense R → N.The group mean directions of magnetisation isD = 243.5°,I = 47.5° (precision parameterk = 29). Petrographic examination confirms observations from magnetic properties that relict titanomagnetite (oxidation classes 3 to 5) is the remanence carrier in most samples. Hematite, probably mostly late magmatic in origin, is widely developed in all samples, but only the principal remanence carrier where it has thoroughly replaced the titanomagnetite. Low-coercivity remanence is apparently caused by weathering effects but there is no clear visible cause for secondary high-coercivity remanence carried by some samples.The mean virtual geomagnetic pole position is close to Upper Silurian/Lower Devonian pole positions from other parts of Britain and defines a minimum apparent polar shift of 60° between late Ordovician and Upper Llandovery times. Reference to absolute age dates suggests that this shift took place between ca. 447 and 434 m.y. followed by slight polar movement between ca. 434 and 394 m.y. 相似文献