Paleomagnetic study on the Early Triassic red beds from Jiaocheng, Shanxi Province--Local rotation and tectonic significance     

SHI Ruiping  Huang Baochun  ZHU Rixiang  Ren Shoumai 《中国科学D辑(英文版)》2004,47(2)

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   

New paleomagnetic pole and magnetostratigraphy of Faroe Islands flood volcanics, North Atlantic igneous province     

Peter Riisager  Janna Riisager  Niels AbrahamsenRegin Waagstein 《Earth and Planetary Science Letters》2002,201(2):261-276

A paleomagnetic sampling was carried out along four sections (altogether 86 lava flows, 548 samples) in the North Atlantic Igneous Province outcropping in Faroe Islands, Denmark. The four polarity zones in the 700-m-thick exposed part of the Faroes lower formation can be correlated with the geomagnetic polarity time scale as C26n-C25r-C25n-C24r. The seven lava flows erupted during C25n indicate a very low eruption rate in the upper part of the Faroes lower formation of ∼1/70 kyr. The Faroes middle and upper formations (composite thickness ∼2300 m) are all reversely magnetized corresponding to C24r. The eruption rate at the onset of middle formation volcanism was very high as evidenced by several thick lava sequences recording essentially spot readings of the paleomagnetic field. The shift in eruption rate between the Faroes lower and middle formations and evidence that onset of the Faroes middle formation volcanism took place in C24r are of particular importance, placing onset of middle formation volcanism in close temporal relation to North Atlantic continental break-up and the late Paleocene thermal maximum. After grouping flows recording the same field directions, we obtained 43 independent readings of the paleomagnetic field, yielding a paleomagnetic pole with coordinates 71.4°N, 154.7°E (A₉₅=6.0°, K=14, N=43); age 55-58 Ma. The pole is supported by a positive reversal test. Paleosecular variation, estimated as the angular standard deviation of the virtual geomagnetic pole distribution 21.7°+3.9°/−2.8°, is close to expected for the given age and paleolatitude. Our new Faroes paleomagnetic pole is statistically different from the majority of previously published poles from the British and Faroes igneous provinces, and we suggest that these older data should be used with care.  相似文献   

Relative hotspot motions versus True Polar Wander     

T.H TorsvikAuthor Vitae  R Van der VooAuthor VitaeT.F RedfieldAuthor Vitae 《Earth and Planetary Science Letters》2002,202(2):185-200

The fixity of hotspots and mantle plume locations has long been axiomatic. If the assumption of fixed hotspots is granted, ‘absolute’ plate motions and movements of the spin axis with respect to the hotspot framework, defined by some as True Polar Wander (TPW), can be determined. However, this assumption can be tested by paleomagnetic data, and such tests are gradually raising some doubts about the fixity of hotspots. The result is that discrepancies between Cretaceous and Tertiary hotspot and paleomagnetic reference frames are now beginning to be interpreted as the result of plume drift within a convective mantle. In the Indo-Atlantic, hotspots have remained relatively stationary with respect to the spin axis for the last 95 million yr. However, the Pacific hotspots, notably Hawaii, appear to have undergone large-scale southward drift with respect to the spin axis during the Early Tertiary. Global paleomagnetic data do not indicate that any TPW occurred during the Late Cretaceous or Tertiary. Although the Early Cretaceous paleomagnetic and hotspot frames for the Indo-Atlantic realm can be interpreted as slow TPW, direct estimates of paleolatitude and hotspot motion, in particular the Kerguelen hotspot, challenge TPW as a global phenomenon. At present, we consider that the large Early Cretaceous discrepancy between hotspot and paleomagnetic data is best explained by southward drift of the Atlantic hotspots prior to ∼95 Ma.  相似文献   

Ordovician palaeogeography with new palaeomagnetic data from the Montagne Noire (Southern France)   总被引：1，自引：0，他引：1  

E. Nysæther  T.H. Torsvik  R. FeistH.J. Walderhaug  E.A. Eide 《Earth and Planetary Science Letters》2002,203(1):329-341

A joint palaeomagnetic and ⁴⁰Ar/³⁹Ar study has been performed on two olistolithic blocks from the Cabrières Wildflysch in the Montagne Noire region of the Massif Central in France. There, andesitic volcanic and volcaniclastic rocks of Llanvirn-Early Caradoc age (ca 470-458 Ma) occur. Despite extensive secondary alteration, destruction of the dominant magnetic mineral phase and ⁴⁰Ar/³⁹Ar whole rock experiments that demonstrate that the volcanic rocks suffered significant argon loss, a positive fold test and the presence of dual polarities suggest that a primary, Ordovician magnetisation has mostly survived. This is one of the few documented cases where the argon system was substantially reset whilst a subordinate set of small, relatively unaltered magnetite grains, probably hosted in silicates, still carry the original, in this case Ordovician, remanence.The new data show that the Montagne Noire region was located at high southerly latitudes (68° ⁺¹⁷/_-15) during the Mid-Ordovician. This latitude represents the location for NW Gondwana of which the Massif Central was a part. Palaeomagnetic data from all the Central European massifs and terranes demonstrate a close link to the Gondwana Margin during the Lower and Middle Ordovician.  相似文献   

Quaternary tectonic rotation in central Japan: Paleomagnetism of the Eboshidake volcanic rocks     

Yasuto  Itoh  Toshiyasu  Miyazaki  Seiji  Nishizaki 《Island Arc》2007,16(3):457-464

Abstract   Neotectonic crustal deformation in central Japan near a triple-junction of plates is investigated on the basis of paleomagnetic data. The progressive thermal demagnetization test isolated characteristic remanent magnetization from 18 sites of the early Quaternary Eboshidake volcanic rocks erupted around the termination of active strike-slip faults. The site-mean directions show considerably large scatter in declinations, and easterly deflection in average (D_m = −161.7°). On the basis of inclination statistics, measured inclinations (I_m = −48.9°, δI = 6.6°) are concordant with an expected value from latitude of the study area. Because the sampling was planned to cover a wide stratigraphic range and eliminate the effect of geomagnetic secular variation, an easterly deflection is attributed to clockwise rotation around vertical axis. Together with previous paleomagnetic data, the present study indicates that clockwise-rotated areas in central Japan are aligned on a northeast–southwest recent shear zone delineated through geodetic survey. Deflection and scatter of paleomagnetic declinations of the Eboshidake volcanic rocks are much greater than those extrapolated from a recent strain rate, and might be explained by complicated motion anticipated at fault terminations and/or enhanced crustal rotation under elevated temperatures around a Quaternary volcanic province.  相似文献   

Paleomagnetic constraints on the Plio–Pleistocene geodynamic evolution of the external central–northern Apennines (Italy)     

Leonardo Sagnotti  Aldo Winkler  Laura Alfonsi  Fabio Florindo  Fabrizio Marra 《Earth and Planetary Science Letters》2000,180(3-4):243-257

We report on new paleomagnetic results obtained from 27 sites sampled in the Plio–Pleistocene sequences at the external front of the central–northern Apennines. Previous analyses of Miocene (Messinian) sediments indicated that the present shape of the northern Apenninic arc is due to the oroclinal bending of an originally straight belt oriented around N320° and that vertical axis rotations accompanied the migration of the thrust fronts toward the Adriatic foreland [F. Speranza et al., J. Geophys. Res. 102 (1997) 3153–3166]. We tried to provide new paleomagnetic constraints for the timing and rates of the oroclinal bending process during the Pliocene and the Pleistocene. The results suggest that CCW rotations observed in the northern part of the studied area are possibly younger than 3 Ma. No regional rotation is recorded in the Pliocene and Pleistocene sediments from the southern part of the study area, analogously to the Messinian sediments of the ‘Acquasanta’ domain of Speranza et al. [F. Speranza et al., J. Geophys. Res. 102 (1997) 3153–3166]. A local significant CCW rotation (23°±10°) is identified in the Early Pleistocene sediments that crop out along the Adriatic coast between Ascoli and Pescara, indicating differential motion of the thrust sheets. This rotation must be younger than 1.43 Ma.  相似文献   

PALEOMAGNETIC ESTIMATE OF THE MESOZOIC—CENOZOIC LATITUDINAL DISPLACEMENT OF TERRENES IN THE QINGHAI—TIBET PLATEAU AND ITS SIGNIFICANCE     

Li Pengwu  Cui Junwen  Li Li 《地学前缘》2000,(Z1)

PALEOMAGNETIC ESTIMATE OF THE MESOZOIC—CENOZOIC LATITUDINAL DISPLACEMENT OF TERRENES IN THE QINGHAI—TIBET PLATEAU AND ITS SIGNIFICANCE1　QianFang ,PreliminarystudyonthehorizontalmovementofNgariarea ,Tibet,sincePliocene[A].AbstractfromInterna tionalSymposiumonHimalayaGeologySciences[C].1984,2 49～ 2 5 0 . 2　JiangChunfa .OpeningandclosingstructuresofKunlun[M ].Beijing :GeologicalPublishingHouse ,1992 ,15 4～ 2 17. 3　XuZhiqin ,…  相似文献   

钻井岩芯裂缝重定向方法研究     

韦乐乐  程鑫  刘秀婷 《江苏地质》2015,39(2):318-321

为了研究特定区域储层裂缝的展布,选取垂直钻取的岩芯上任意角度裂缝,在古地磁岩芯定向及岩芯垂直裂缝定向方法的基础上,推导出岩芯坐标系中用样品磁倾角和磁偏角表达的裂缝走向公式,从而得到任意角度岩芯裂缝的走向,恢复每条裂缝在井下的原始方位,进而获得某区块岩芯裂缝整体展布趋势。  相似文献   

Chinese Continental Blocks in Global Paleocontinental Reconstruction during Paleozoic and Mesozoic     

WAN Tianfeng  ZHU Hong 《《地质学报》英文版》2011,85(3):581-597

The Cambrian to Cretaceous paleomagnetic data from Chinese continental and adjacent blocks were collected using principles to obtain reliable and high-precision paleomagnetic data and to pay attention to the similarity of paleobiogeography and the coordination of tectonic evolution. The Chinese continental blocks were laid up on the reconstruction of proposed global paleocontinents with almost the same scale. Thus, it can be clearly recognized that the global continents, including Chinese continental blocks, range along latitudes on the southern side of the equator during the Early Paleozoic. In the Paleozoic, Chinese continental blocks were still located among the Laurentia, Siberia and Gondwana plates, following the fast moving of the Siberia Plate northwards, the amalgamation in a north-south direction at the western parts of the Laurentia and Gondwana plates, and the Iapetus and Rheic Oceans were subducted, eventually to form a uniform Pangea in the Late Paleozoic. The Australian and Indian plates of Eastern Gondwana moved and dispersed gradually southwards, continued to extend the Paleo-Tethys Ocean. The Chinese continental and adjacent blocks were still located in the Paleo-Tethys Ocean, preserved the status of dispersion, gradually moving northwards, showing characteristics of ranging along a north–south orientation until the Permian. In addition, a series of local collisions happened during the Triassic, and consequently most of the Chinese continental blocks were amalgamated into the Pangea, except for the Gangdise and Himalayan blocks. There was a counter-clockwise rotation of the Eastern Asian continent in the Jurassic and northwards migration of the Chinese continent in varying degrees during the Cretaceous, but the Himalayan and Indian plates did not collide into the Chinese continent during this period.  相似文献   

Paleomagnetic evidence for the emplacement mechanism of an enigmatic boulder accumulation on a coastal cliff top in New South Wales: implications for the Australian Megatsunami Hypothesis     

D. N. Thomas  P. W. Schmidt 《Australian Journal of Earth Sciences》2018,65(4):503-515

Paleomagnetic sampling and measurement of a boulder accumulation on Little Beecroft Head on the Illawarra coastline of New South Wales was undertaken to evaluate potential emplacement mechanisms. This deposit is of central importance in the Australian Megatsunami Hypothesis (AMH) debate, but to date, there has been no unequivocal determination of its provenance. The most likely emplacement mechanisms are by slow collapse during denudation of overlying strata, storm wave overwash or a combination of these. Characteristic Remanent Magnetisation (ChRM) directions were obtained from 15 individual boulders and the in situ bedrock platform on which they currently rest. The in situ Permian bedrock has a normal polarity mean ChRM direction of D/I = 1.6°/–66.7° (α₉₅ = 5.2°; k = 33.9) that is statistically indistinguishable from the Present Earth Field direction at the site. The magnetisation is most likely due to Cenozoic/recent weathering, which is common in surficial rocks throughout the Sydney Basin. ChRM directions for the boulders are stable but scattered, although not random, and the mean boulder direction is indistinguishable in geographic (i.e. current in situ) coordinates, at the 5% significance level, from the mean direction of the in situ bedrock. Further statistical tests confirm that the scatter in the mean directions of the boulders and the in situ bedrock is different, at the 5% significance level, with the boulder mean being more scattered. At an individual boulder level, some blocks have mean ChRM directions that are statistically indistinguishable from the mean in situ rock ChRM direction, whereas others are distinguishable at the 5% significance level.

These results indicate that the boulders were magnetised prior to emplacement but were not moved far from their original positions during emplacement. The emplacement age is constrained to the last ca 780 000 years. These observations strongly support the hypothesis that the Little Beecroft Head boulder deposit was emplaced by a non-catastrophic mechanism, namely slow collapse during denudation of pre-existing cliff material or overtopping from severe storms, which occur regularly on the east coast of New South Wales. Even if a catastrophic wave were responsible, the results constrain the age of that event to be older than 780 000 years. Therefore, the results presented here are not supportive of the AMH as it currently stands. Further paleomagnetic work, on similar deposits along the Illawarra coastline and from elsewhere in Australia, is needed to evaluate the interpretations presented here.  相似文献