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1.
An intensive paleomagnetic investigation has been conducted on the Middle Triassic Leikoupo Formation on the Wangcang section (32.14°N, 103. 17°E). The results indicate the magnetic minerals are dominant by multidomain magnetite or maghemite, and the characteristic remnant magnetization revealed by stepwise thermal/alternating field demagnetization is close to the present-day geomagnetic direction of the sampling site. This suggests that dolomitization/thermal viscous magnetization is responsible for the remagnetization of this kind of rocks.  相似文献   

2.
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   

3.
A paleomagnetic study was made of the granitic rock farsundite, exposed in southern Norway. An objective was to test the contemporaneity of this body with the neighbouring Egersund anorthosite of presumed age about 900 m.y. Two of the nine sites sampled were rejected, as the magnetization was dominantly unstable. At the seven other sites, this unstable component was either absent or it could be equally well removed by AF or thermal demagnetization: after AF treatment, all samples from these sites were left with a very stable remanence, directed steeply upwards. This magnetization was probably acquired at the time of either emplacement or recrystallization of the farsundite. A magnetic test for anisotropy indicated that the stable remanence is misaligned with the ancient Earth's field direction by about 3°, apparently due to layering of the rock fabric. After correction for this anisotropy, the mean direction from the seven sites is D = 341°, I = 82.2°, k = 142, α = 5.0°, corresponding to a paleomagnetic north pole at 43.3°S, 166.0°W, dp = 9.3°, dm = 9.7°, which lies on Spall's European polar wandering curve. The farsundite pole is not significantly different from a pole position based on the Egersund anorthosite, which supports the supposition that the two rock formations are cogenetic.  相似文献   

4.
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   

5.
本文对"鲁科一井"(CCSD-LK-Ⅰ)768.9~1112.3m之间的上白垩统沉积岩样品进行了岩石磁学、磁化率各向异性(AMS)以及天然剩磁组分的研究.在此基础上,分析了利用特征剩磁(ChRM)和黏滞剩磁(VRM)方向恢复岩芯原始方位的可行性.三轴等温剩磁热退磁曲线、磁滞回线、反向场退磁曲线、一阶反转曲线等岩石磁学测量结果表明,沉积岩的主要载磁矿物为磁铁矿和赤铁矿.335块样品的AMS测量结果表明磁化率椭球主轴的最大轴K1和中间轴K2与水平面夹角较小,最小轴K3接近垂直于水平面分布,说明沉积岩保留了原始沉积磁组构特征.系统热退磁实验表明,多数样品在25~350℃和500~690℃温度段分别获得VRM和ChRM分量.利用ChRM偏角方向,并考虑构造旋转量校正,对VRM偏角方向进行恢复,Fisher统计得到DVRM=-1.3°,IVRM=59.6°,与当地现代地磁场方向(D=-6.7°,I=53.9°)基本一致.用ChRM偏角方向对磁化率主轴K1偏角方向进行校正,校正的结果为:D_(ch_K1)=349.2°,I_(ch_K1)=-0.7°.本文研究结果对于地质勘探中利用古地磁学方法恢复钻孔岩芯原始方位具有一定参考意义.  相似文献   

6.
This palaeomagnetic study is centered on agglomerates and volcanic rocks from the western margin of the Appalachian belt in the Drummondville-Actonvale-Granby area, Quebec (long.: 72°30′W, lat.: 46°00′N). It involves a total of 36 oriented samples (111 speciments) distributed over eleven sites. Both thermal and AF cleaning techniques were used to isolate residual remanent components. The dispersion of the directions is slightly reduced after AF cleaning and thermal treatment.The palaeopole position obtained is 191°E, 6°N (dm = 14°, dp = 7°) after thermal treatment and 164°E, 19°N (dm = 11°, dp = 6°) after AF cleaning. The polarity of most of the sites (two exceptions) are reversed. The thermal-treated data appear to be relatively stable and an approximate value of the primary magnetization is extracted from them. The palaeopole obtained does not lie close to the tentatively defined position of the Cambrian and Ordovician poles from rocks of the North American plate; it is located near the Upper Cambrian and Lower Ordovician poles from eastern Newfoundland and the Lower Ordovician pole from the Caledonides in Europe.  相似文献   

7.
The natural remanent magnetization of 22 out of a total of 31 oriented cores from the layered series of the Skaergaard gabbroic intrusion (age: 55 m.y.) in East Greenland shows good stability in thermal and AF testing. The average direction of 22 AF and 9 thermally treated specimens isD = 170°,I = ?59°,α95 = 4.2 before correction for tilt. The mean directions after rotation around strike to horizontal and after rotation to original attitudes suggested by others yields poorer population statistics. It is therefore concluded that flexuring took place between solidification and acquisition of remanent magnetization, a range in temperature of about 500°C which may represent an interval of somewhat less than 250,000 years. No evidence for secular variation is observed which may also suggest slow cooling through the blocking temperature range. The polarity is reversed and the pole position without “tilt correction” is 165°E, 61°N,dm = 6.2,dp = 4.6, which is similar to pole positions reported by others for the overlying slightly older basalt.  相似文献   

8.
The Wackerfield Dyke is exposed in northern England and its field relationships suggest it could be related to the Eocene Mull Dyke Swarm. Wackerfield Dyke specimens taken from below the zone of surface weathering have a natural remanent magnetization that is extremely stable to thermal and alternating field demagnetization. Stability is expected because the Fe-Ti oxides are highly oxidized. The stable magnetization is identical with a primary remanence that can be isolated from certain samples of the Whin Sill of northern England, and gives a mean pole at 175°E, 22°N (dp = 2°, dm = 3°). An Upper Carboniferous age for the Wackerfield Dyke is confirmed by a K-Ar whole rock age of 303 my.  相似文献   

9.
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10.
Two coeval sections of red to white ammonite-rich pelagic limestones spanning the complete Kimmeridgian and most of the Tithonian were sampled in detail. All samples were treated by progressive thermal demagnetization to remove a present field overprint. Characteristic magnetization is carried primarily by magnetite. Polarity intervals are easily identified and correlate well between the two sections. The Tithonian polarity sequence can also be correlated to sections in northern Italy. The similarity between the polarity sequence and the M-sequence of marine magnetic anomalies, coupled with the precise biostratigraphic control, allows assignment of the following ages to the M-sequence: the Late/Early Tithonian boundary is correlated to the end of M-20, the Tithonian/Kimmeridgian boundary to the end of M-23, the Late/Early Kimmeridgian boundary to the latter part of M-24, and the Kimmeridgian/Oxfordian boundary within or slightly after M-25.The mean directions of characteristic magnetization have α95's less than 3° and demonstrate extensive differential block rotation within the Subbetic province. Paleolatitudes during the Kimmeridgian/Tithonian are in the range of 16–24°N.  相似文献   

11.
Magnetic properties of samples from Bell Island sedimentary rocks have been studied. X-ray analysis indicates that the main magnetic mineral is hematite in all samples. The other iron-bearing minerals identified are siderite and chamosite. Microscope observations of thin sections suggest that the rocks consist of oolitic hematite in a matrix of siderite or calcite. The intensity of natural remanent magnetization (NRM) varies in the range of (0.03–0.4 A m?1), depending on the percentage of hematite. The thermal demagnetization curves of NRM show in some cases a sharp increase in magnetization at temperatures in the range 500–600°C. The peaks that occur in these demagnetization curves are due to a chemical change of siderite during repeated laboratory heating. X-ray analysis confirmed that the newly formed material is magnetite. Since the original NRM has been masked by the new intergrown material, this would result in a serious error in the determination of paleomagnetic pole positions. The samples showing this behaviour were not considered for paleomagnetic study. The samples containing oolitic hematite in a calcite matrix exhibit very high stability of NRM, including directional stability until almost 670°C. For these samples, a virtual pole position based on N = 6 samples (32 specimens) demagnetized to 665°C is 34°N, 114°E, not far from published Ordovician poles for the North American craton.  相似文献   

12.
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   

13.
A total of 120 samples from 12 sites were collected from two flanks of a fold. Stepwise thermal demagnetization has successfully revealed characteristic magnetization components from the rocks in each case. A well-defined component determined from red fine-grained sandstone is clustered in the northeasterly direction with shallow upward inclination (D = 29.3°,I= -19.2°,k = 283.7, α95 = 7.3°. tilt-corrected). The pole position (39.5°N, 247.3°E,dp = 4.0°,dm = 7.6°) derived from this component is close to the Permian pole for the Yangtze Block, indicating that the red fine-grained sandstone has been overprinted. The red mudstone reveals two characteristic components Component A with lower unblocking temperature, characterized by northerly declination and moderate to steep inclination corresponds to a pole position overlay with the present North Pole. Component B (D = 129.1°,I=-23.6°,k = 44.6, α95 = 7.8°, tilt-corrected) with higher unblocking temperature, passes fold test, and yields a pole position (39.5°S, 185.l°E,dp = 4.4°,dm = 8.3°) different from the other poles for the Yangtze Block. It is therefore suggested that component B was probably a primary magnetization and the Yangtze Block was situated at low latitudes in the Southern Hemisphere in the Middle Cambrian.  相似文献   

14.
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   

15.
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   

16.
Some 50 oriented samples (120 specimens) have been collected on eight sites of volcanic rocks from the Lower Devonian Dalhousie Group of northern New Brunswick and Devonian andesitic to basic dykes from central New Brunswick. Univectorial and occasional multivectorial components were extracted from the various samples. Results after AF and thermal demagnetization compare relatively well. In the volcanics and tuffs, two components of magnetization have been isolated: A (D = 33°, I = ?58°, α95 = 7.3°, K = 236) for four sites and B (D = 66°, I = +53°) for three sites. The grouping of component A is improved after tilt correction but the fold test is not significantly positive at the 95% confidence level. Component A is interpreted as being primary while component B is unresolved and appears to be the resultant magnetization of a Late Paleozoic and a recent component. The pole position obtained for tilt corrected component A is 268°E, 1°S, dp = 6.5°, dm = 8.8°. The paleolatitude calculated for component A is 39°S. The paleopole of in situ component A is located close to those of the Early-Middle Devonian formations from Quebec, New Brunswick and New England states while the paleopole of tilt-corrected component A is similar to Lower Devonian poles of rock units from the Canadian Arctic Archipelago. If component A is primary (as we believe it to be), then the western half of the northern Appalachians had already docked onto the North American Craton by Early Devonian time. Alternatively, if component A is secondary the same conclusion applies but the juxtaposition took place in Middle Devonian time.  相似文献   

17.
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   

18.
Palaeomagnetic investigation of basic intrusives in the Proterozoic Mount Isa Province yields three groups of directions of stable components of NRM after magnetic cleaning in fields up to 50 mT (1 mT= 10 Oe). The youngest group (IA) includes results from the Lakeview Dolerite, and yields a palaeomagnetic pole at 12°S, 124°E (A95 = 11°). The second group (IB) has a palaeomagnetic pole 53°S, 102°E (A95 = 11°). The third group (IC) is derived from the Lunch Creek Gabbro and contains normal and reversed polarities of magnetization with a palaeomagnetic pole at 63°S, 201°E (A95 = 9°). Some samples from the gabbro have anomalously low intensities of remanent magnetization in obscure directions attributed to the relative enhancement of the non-dipole component of the palaeomagnetic field during polarity reversal. The present attitude of the igneous lamination is probably of primary, not tectonic origin.  相似文献   

19.
Archaeomagnetism in Greece has continuously developed during the last decades. Numerous studies have provided high quality data and accurate secular variation curves for the direction and intensity of the geomagnetic field have been constructed. The Greek Secular Variation Curves (SVCs) cover the last 8 millennia for intensity and 6 millennia for direction. The coverage of the archaeological periods remains uneven, with several gaps, mostly in the directional dataset, with only two results for periods older than 2500 B.C. In the present contribution, the first archaeomagnetic results from Neolithic settlements in northern Greece are presented. For the present study, samples were collected from three different archaeological sites: burnt structures in Avgi (Kastoria) and Vasili (Farsala) and one oven from Sosandra (Aridaia). The natural remanent magnetization (NRM) grouping of all specimens indicated that the majority of the samples were burnt in situ, providing thus a reliable direction of the ancient field. Magnetic cleaning (both alternating-field and thermal) revealed the presence of one stable component of magnetisation. Rock magnetic experiments (acquisition of isothermal remanent magnetization (IRM), thermal demagnetisation of the IRM, thermomagnetic curves) have been performed on pilot samples indicating that low coercivity magnetic minerals such as magnetite or Timagnetite are prevailing. The mean directions (declination D, inclination I and parameters of the Fisherian statistics), which arose from the three sites are as follows: Sosandra: D = 343°, I = 55.6°, ??95 = 4.8°; Avgi: D = 10.1°, I = 53.4°, ??95 = 4.2° and Vasili: D = 357.5°, I = 43.1°, ??95 = 4.1°. The obtained data are in a very good agreement with results from Neolithic Bulgaria. This study represents the beginning of an effort to fill the gaps of the Greek secular variation curves and their extension to the Neolithic period.  相似文献   

20.

Paleomagnetic samples were collected from four localities located in the southern rim of the Tarim basin. The samples were taken from volcanic rocks erupted between Jurassic and Quaternary. Detailed analysis of all samples has been carried out with progressive thermal demagnetization. A characteristic remanence (ChRM) with higher unblocking temperature has been isolated from all samples. The pole position from the middle Jurassic is at 52.5°N, 187.9°E(dp = 3.7°,dm =6.5°); the directions of the ChRM of Cretaceous correspond to a paleopole at 69.7°N, 211.6°E (dp = 9.8°,dm = 15.9°); the Quaternary pole from the Pulu site is at 79. 9°N, 183.1°E(dp = 1.6°.dm =2.4°). On the basis of these new paleomagnetic data, tectonic evolution of Tarim block is presented.

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