共查询到20条相似文献，搜索用时 671 毫秒
1.
Summary. The Gubbio section, in the Umbrian Apennines, yielded a sequence of Upper Cretaceous polarity zones precisely corresponding to the sea-floor magnetic anomalies of that age; the Gubbio polarity zones have been dated with planktonic foraminifera. As a test of the validity of the Gubbio reversal sequence, we have studied the section at Moria, 16 km to the north. The polarity sequence at Moria closely agrees with the sequence at Gubbio; only the two thinnest polarity subzones were not found. Comparison of polarity zone thicknesses in the two sections gives an excellent correlation. Declinations in the upper part of the Moria section are somewhat erratic, suggesting the possibility of detachment surfaces and relative structural rotations at these levels. Remanent magnetic intensity varies in a cyclical pattern that is unmistakably the same in the Gubbio and Moria sections. The results from the Moria section provide strong confirmation of the validity of the magnetic polarity sequence established at Gubbio. 相似文献
2.
Roberto S. Molina Garza John W. Geissman & Spencer G. Lucas 《Geophysical Journal International》2000,141(3):778-786
Continental red sandstone and siltstone rocks of the Dewey Lake (Quartermaster) Formation at Maroon Cliffs, near Carlsbad, New Mexico, are characterized by two components of magnetization with partially overlapping laboratory unblocking temperature spectra. Both magnetizations display high coercivities (>100 mT), probably residing in haematite. A north-directed magnetization with steep positive inclination unblocks between 100 and 650 °C, isolating a predominantly northwest-directed magnetization, with shallow inclination, of near uniform normal polarity and maximum unblocking temperatures of 680 °C.
We collected samples from 24 palaeomagnetic sites (i.e. individual beds) from a ~60 m thick section of flat-lying strata disconformably overlying carbonate and evaporite rocks of the Rustler Formation. The upper member of the Rustler Formation contains a Late Permian (early Changxingian) marine invertebrate and conodont fauna. Of the sampled sites, four yield only steep magnetizations, interpreted to be recent overprints. Eight sites did not yield well-grouped site means and were excluded from the final calculations. The formation mean (dec = 337.7°, inc = 9.2°; k = 31.6, α95 = 7.8°, N = 12 sites) defines a palaeomagnetic pole located at 55.2°N, 117.5°E, in good agreement with other Late Permian North American cratonic poles.
Correlation of the short polarity sequence of this section of Dewey Lake strata is unambiguous. Compared with the polarity stratigraphy of marine sections in Asia, and supported by isotopic age determinations on a widespread bentonite bed in Dewey Lake strata in west Texas (approximately 251 Ma) and fossil data for the underlying Rustler Formation, the magnetostratigraphy is consistent with deposition of the Dewey Lake Formation during the latest Changxingian (Late Permian) stage. 相似文献
We collected samples from 24 palaeomagnetic sites (i.e. individual beds) from a ~60 m thick section of flat-lying strata disconformably overlying carbonate and evaporite rocks of the Rustler Formation. The upper member of the Rustler Formation contains a Late Permian (early Changxingian) marine invertebrate and conodont fauna. Of the sampled sites, four yield only steep magnetizations, interpreted to be recent overprints. Eight sites did not yield well-grouped site means and were excluded from the final calculations. The formation mean (dec = 337.7°, inc = 9.2°; k = 31.6, α
Correlation of the short polarity sequence of this section of Dewey Lake strata is unambiguous. Compared with the polarity stratigraphy of marine sections in Asia, and supported by isotopic age determinations on a widespread bentonite bed in Dewey Lake strata in west Texas (approximately 251 Ma) and fossil data for the underlying Rustler Formation, the magnetostratigraphy is consistent with deposition of the Dewey Lake Formation during the latest Changxingian (Late Permian) stage. 相似文献
3.
The magnetostratigraphy of five new sections through the Neogene Siwalik Group of Nepal is presented. Rock magnetic experiments and detailed thermal demagnetization experiments prove that haematite is the carrier of a primary DRM or a PDRM. After stepwise thermal demagnetization of specimens from all sections, directions of characteristic remanent magnetization were obtained. The results show a positive reversal test and exhibit inclination errors of about 20. Due to gaps in exposures and extremely variable demagnetization behaviour of the haematite-bearing sediments, the resulting polarity sequences are poorly defined for some parts of the sections. However, double-sampled parts of some sections yield similar results. A correlation with the Surai Khola section (Appel, Rösler & Corvinus 1991) is proposed, based on rock magnetic parameters. AMS results from three of the sections prove the existence of primary sedimentary magnetic fabrics in these sections with systematic orientations of minimum and maximum susceptibility axes. Correlation with a standard polarity timescale indicates that the ages of all new sections lie within the age limits of the Surai Khola section. 相似文献
4.
A palaeomagnetic study has been carried out on late Palaeozoic rocks exposed in the Sierras Australes thrust and fold belt of Buenos Aires province (Argentina), in the early Permian red sandstones and clay siltstones of the Tunas Formation. The sections sampled are exposed in the eastern parts of the belt, in Sierra de las Tunas (north) and Sierra de Pillahuincó (south). More than 300 specimens were collected from 25 sites, in three localities with different structural attitudes. Demagnetization at high temperatures isolated a characteristic remanence at 20 sites. All the localities have a reverse characteristic remanence, suggesting that the magnetization was acquired during the Kiaman interval. Stepwise tectonic tilt correction suggests that the Tunas Formation in these localities acquired its magnetization during folding in early Permian times. Palaeomagnetic poles were computed for each locality based on partial tilt-corrected remanence directions. Taking into account the fact that these localities are close to one another and that the rocks are all of reverse polarity, a group syntectonic palaeomagnetic pole called Tunas was calculated: longitude: 13.9°E, latitude: 63.0°S; A 95 = 5.4°, K = 39.7, N = 19. This pole is consistent with previously calculated poles from South America assigned to the early Permian. In age it corresponds to the early Permian San Rafaelic tectonic phase of the Sierras Australes. Independent geological evidence indicates that the Tunas Formation underwent syndepositional deformation. We conclude that the Tunas Formation was deposited, deformed and remagnetized, all during the early Permian. 相似文献
5.
Helmut Mayer 《Geophysical Journal International》1997,131(2):387-400
The bottom part of the Cretaceous Cismon section in the Southern Alps was sampled for high-resolution magnetostratigraphy. Although the almost pure pelagic nannofossil limestones ( c. 90 per cent CaCO3 ) of the Maiolica/Biancone Formation are extremely weakly magnetized, stepwise thermal and alternating-field demagnetization removed overprints and isolated a characteristic remanent magnetization which is interpreted as a primary magnetization. The dominant magnetic carrier mineral is magnetite; a small fraction of haematite may be present. A clear reversal pattern can be correlated unambiguously with Mesozoic polarity chrons CM10N to CM8. A less well-constrained magnetostratigraphy from the Pra da Stua section could not be directly correlated with the global polarity scale, but biostratigraphic information allows its assignment to the interval CM10-CM5. A counterclockwise rotation of 56 and a northward translation of 28 latitude for the Cismon locality since the Early Cretaceous are derived from the palaeomagnetic data, consistent with previous results from the Southern Alps. The high-resolution magnetostratigraphy of the Cismon section is used in an effort to refine the Cretaceous timescale by the combination of magnetostratigraphic and cyclostratigraphic results from the same section. The cyclostratigraphic duration estimates of chrons CM10N to CM8 are compared to their equivalents in a number of traditional timescales and found to be shorter by a factor of 1.26-2.58. 相似文献
6.
The early Miocene Dumri Formation and middle Miocene–Pliocene Siwalik Group were deposited in the Himalayan foreland basin in response to uplift and erosion in the Himalayan fold‐thrust belt. We report magnetostratigraphic data from four sections of these rocks in Nepal. Three of these sections are in the Siwalik Group in the hanging wall of the Main Frontal thrust, and one section is from the Dumri Formation in the hanging wall of the Main Boundary thrust (MBT). Thermal demagnetization experiments demonstrate that laminated siltstones yield palaeomagnetic data useful for tectonic and magnetostratigraphic studies whereas other lithofacies yield data of questionable reliability. Magnetostratigraphic data have been acquired from 297 sites within a 4200‐m‐thick section of Siwalik deposits at Surai Khola. The observed sequence of polarity zones correlates with the geomagnetic polarity time scale (GPTS) from chron C5Ar.1n to chron C2r.2n, spanning the time frame ca. 12.5–2.0 Ma. At Muksar Khola (eastern Nepal), 111 palaeomagnetic sites from a 2600‐m‐thick section of the Siwalik Group define a polarity zonation that correlates with the GPTS from chron C4Ar.2n to chron C2Br.1r, indicating an age range of ca. 10.0–3.5 Ma. At Tinau Khola, 121 sites from a 1824‐m‐thick section of the Siwalik Group are correlated to chrons C5An.1n through C4r.1n, equivalent to the time span ca. 11.8–8.1 Ma. At Swat Khola, 68 sites within a 1200‐m‐thick section of lower Miocene Dumri Formation are correlated with chrons C6n through C5Bn.2n, covering the time span ca. 19.9–15.1 Ma. Together with previous results from Khutia Khola and Bakiya Khola, these data provide the first magnetostratigraphic correlation along nearly the entire NW–SE length of Nepal. The correlation demonstrates that major lithostratigraphic boundaries in the Siwalik Group are highly diachronous, with roughly 2 Myr of variability. In turn, this suggests that the major sedimentological changes commonly inferred to reflect strengthening of the Asian monsoon are not isochronous. Sediment accumulation curves exhibit a 30–50% increase in accumulation rate in four of the five sections of the Siwalik Group, but the timing of this increase ranges systematically from ～11.1 Ma in western Nepal to ～5.3 Ma in eastern Nepal. If this increase in sediment accumulation rate is interpreted as a result of more rapid subsidence owing to thrust loading in the Himalaya, then the diachroneity of this increase suggests lateral propagation of a major thrust system, perhaps the MBT, at a rate of ca. 103 mm year?1 across the length of Nepal. 相似文献
7.
Kenneth P. Kodama 《Geophysical Journal International》1984,76(2):273-287
Summary. The Precambrian basement under east-central Kansas was drilled at two circular aeromagnetic positives, one at Osawattamie and one at Big Springs. The core retrieved from these sites is a coarse to medium grained granite which has been dated by U-Pb to be 1350 Ma old. The palaeomagnetism of these azimuthally unoriented cores was studied to see if a technique which uses low-coercivity, low-temperature magnetization components to orient the cores would allow an independent confirmation of the core's mid-Proterozoic age. Orthogonal projection plots of the alternating field (af) and thermal demagnetization data show that the magnetization of these cores is relatively simple, having only two components: a low-temperature, low-coercivity magnetization with steep positive inclinations and a shallow, negative inclination characteristic magnetization for the Osawattamie core or a positive, moderate inclination characteristic magnetization for the Big Springs core. If the declination of the low-temperature, low-coercivity component is aligned parallel to the present field declination, the characteristic directions may be azimuthally oriented. This allows the calculation of palaeomagnetic poles for the Big Springs core (lat. = 4.5°S, long. = 29.9°E) and the Osawattamie core (lat.= 20.2°N, long. = 39.3°E) which are consistent with Irving's apparent polar wander path for Laurentia at about 1300–1400 Ma. Comparison of anhysteretic remanent magnetization (ARM), viscous remanent magnetization (VRM), and isothermal remanent magnetization af demagnetization curves with a natural remanent magnetization (NRM) demagnetization curve suggests that the Osawattamie core probably acquired a piezoremanent magnetization (PRM) parallel to the core axis during drilling. 相似文献
8.
J.N. Prasad S.K. Patil M. Venkateshwarlu P.D. Saraf S.C. Tripathi & D.R.K. Rao 《Geophysical Journal International》1998,133(3):519-528
A palaeomagnetic study of 115 samples (328 specimens) from 22 sites of the Mid- to Upper Cretaceous Bagh Group underlying the Deccan Traps in the Man valley (22° 20'N, 75° 5'E) of the Narmada Basin is reported. A characteristic magnetization of dominantly reverse polarity has been isolated from the entire rock succession, whose depositional age is constrained within the Cretaceous Normal Superchron. Only a few samples in the uppermost strata have yielded either normal or mixed polarity directions. The overall mean of reverse magnetization is D m =144°, I m =47° ( α 95 =2.8°, k =152, N =18 sites) with the corresponding S-pole position 28.7°S, 111.2°E ( A 95 =3.1°) and a palaeolatitude of 28°S±3°. The characteristic remanence is carried dominantly by magnetite. Similar magnetizations of reverse polarity are also exhibited by Deccan basalt samples and a mafic dyke in the study area. This pole position falls near the Late Cretaceous segment of the Indian APWP and is concordant with poles reported from the Deccan basalt flows and dated DSDP cores (75–65 Ma) of the Indian Ocean. It is therefore concluded that the Bagh Group in the eastern part of the Narmada Basin has been pervasively remagnetized by the igneous activity of Deccan basalt effusion. This overprinted palaeomagnetic signature in the Bagh Group indicates a counter-clockwise rotation by 13°±3° and a latitudinal drift northwards by 3°±3° of the Indian subcontinent during Deccan volcanism. 相似文献
9.
Maria Irene Bartolomeu Raposo 《Geophysical Journal International》1997,131(1):159-170
Magnetic fabric was determined by applying the anisotropy from the low-field magnetic susceptibility (AMS) technique in 62 mafic dykes from the Mesozoic Florianópolis (Santa Catarina Island) dyke swarm, southern Brazil. These dykes cut the crystalline basement rocks, which are mainly Proterozoic. They are vertical or subvertical in dip and trend mainly NE, although NW-trending dykes are also found. Dykes are tholeiitic in composition and are geochemically similar to those from the Ponta Grossa swarm. Thicknesses vary from 0.3 to 60 m. Polished sections show that titanomagnetites carry the AMS in these dykes. Hysteresis parameters show that the magnetic minerals fall in the PSD range. Two types of magnetic fabric are recognized. Type I is characterized by K 1 - K 2 parallel to the dyke wall, representing magma flow within the dykes; type II, with K 1 - K 3 parallel to the dyke wall, was found in four dykes. Type I is found in 94 per cent of the dykes, and approximately 20 per cent of these have K 1 inclinations of less than 30°, suggesting horizontal or subhorizontal flow. About 80 per cent have K 1 inclinations of greater than 30°, due to inclined to vertical flow. The comparison of AMS studies from both the Florianópolis and the Ponta Grossa dykes suggests a source position closer to Santa Catarina Island than the Ponta Grossa arch. 相似文献
10.
C. Seiler M.C. Quigley J.M. Fletcher D. Phillips A.J.W. Gleadow B.P. Kohn 《Basin Research》2013,25(4):388-418
The Santa Rosa basin of northeastern Baja California is one of several transtensional basins that formed during Neogene oblique opening of the Gulf of California. The basin comprises Late Miocene to Pleistocene sedimentary and volcanic strata that define an asymmetric half‐graben above the Santa Rosa detachment, a low‐angle normal fault with ca. 4–5 km of SE‐directed displacement. Stratigraphic analysis reveals systematic basin‐scale facies variations both parallel and across the basin. The basin‐fill exhibits an overall fining‐upward cycle, from conglomerate and breccia at the base to alternating sandstone‐mudstone in the depocentre, which interfingers with the fault‐scarp facies of the detachment. Sediment dispersal was transverse‐dominated and occurred through coalescing alluvial fans from the immediate hanging wall and/or footwall of the detachment. Different stratigraphic sections reveal important lateral facies variations that correlate with major corrugations of the detachment fault. The latter represent extension‐parallel folds that formed largely in response to the ca. N‐S constrictional strain regime of the transtensional plate boundary. The upward vertical deflection associated with antiformal folding dampened subsidence in the northeastern Santa Rosa basin, and resulted in steep topographic gradients with a high influx of coarse conglomerate here. By contrast, the downward motion in the synform hinge resulted in increased subsidence, and led to a southwestward migration of the depocentre with time. Thus, the Santa Rosa basin represents a new type of transtensional rift basin in which oblique extension is partitioned between diffuse constriction and discrete normal faulting. 40Ar/39Ar geochronology of intercalated volcanic rocks suggests that transtensional deformation began during the Late Miocene, between 9.36 ± 0.14 Ma and 6.78 ± 0.12 Ma, and confirms previous results from low‐temperature thermochronology (Seiler et al., 2011). Two other volcanic units that appear to be part of a conformable syn‐rift sequence are, in fact, duplicates of pre‐rift volcanics and represent allochthonous, gravity‐driven slide blocks that originated from the hanging wall. 相似文献
11.
Michel Westphal Evgueni L. Gurevitch Boris V. Samsonov Hugues Feinberg & Jean Pierre Pozzi 《Geophysical Journal International》1998,134(1):254-266
The deep drill hole SG6 in western Siberia (66°N, 78.5°E) penetrated 1.1 km of lower Triassic basalts, which are possibly an extension of the central Siberian Permo– Triassic flood basalt province. About 300 samples of these basalts were progressively demagnetized and measured. Principal component analysis often shows multiple magnetizations carried by haematite and magnetite. The corrected mean inclinations are +77° and −77° for the haematite component. A magnetostratigraphic scale was derived and showed a N–R–N–R–N succession. This is quite different from the Noril'sk and Taimyr typical polarity scale, R–N.
The basalts found in the SG6 deep drill hole are slightly younger than those of central Siberia and Taimyr. They correspond to middle–upper Induan age, whereas the Noril'sk and Taimyr sections correspond to an uppermost Permian and lower Induan age. Altogether they indicate that, after a high output rate of volcanic material near the Permo–Triassic boundary, this activity slowed down drastically on the Siberian platform and Taimyr, but persisted for several million years in western Siberia. 相似文献
The basalts found in the SG6 deep drill hole are slightly younger than those of central Siberia and Taimyr. They correspond to middle–upper Induan age, whereas the Noril'sk and Taimyr sections correspond to an uppermost Permian and lower Induan age. Altogether they indicate that, after a high output rate of volcanic material near the Permo–Triassic boundary, this activity slowed down drastically on the Siberian platform and Taimyr, but persisted for several million years in western Siberia. 相似文献
12.
We report on a detailed palaeomagnetic study of the Miocene Farellones volcanic formation in the Chilean Andes near Santiago (two sections, 37 sites, about 400 orientated cores). Petrological observations show evidence of low-grade metamorphism increasing downwards through the volcanic sequence. Optical observations of opaque minerals and magnetic experiments suggest that in many cases maghemitization is associated with hydrothermal alteration. However, thermal demagnetization data indicate that the low-grade metamorphism did not significantly modify the direction of the primary remanent magnetization recorded at the time of emplacement of the volcanic lava flows. Four intervals of polarity with two intermediate palaeodirections were observed in the ~650-m-thick composite section. According to the dispersion of flow average directions, palaeosecular variation was slightly larger than that observed in general during the Upper Cenozoic. The site mean directions obtained in this study differ significantly from the expected Miocene direction. Clockwise rotations of up to 20° of small blocks are probably associated with the deformation of the Andean Cordillera since middle Miocene times. Geomagnetic palaeointensity data were obtained, using the Thellier method, on 24 samples from eight distinct lava flows. The flow mean VDM varies from 1.4 to 4.0 × 1022 A m−2 . Altogether, our data seem to suggest the existence of a relatively low geomagnetic field undergoing large fluctuations. Although a linear relationship was observed between the natural remanent magnetization and the thermal remanent magnetization acquired during the Thellier–Thellier experiments, undetected chemical alteration of the magnetic minerals during hydrothermalism may also explain the unusually low palaeointensity obtained. 相似文献
13.
Magnetostratigraphic sampling of the Middle Triassic Badong Formation in South China was conducted at three sections. A dual-polarity characteristic remanent magnetization (ChRM) resolved from most samples by thermal demagnetization is shown to have been acquired prior to folding. The primary nature of the ChRM is corroborated by the discovery of the same magnetic polarity at equivalent stratigraphic levels in more than one section. The relative sample VGP (virtual geomagnetic pole) latitudes define nine magnetozones for the three major constituent members of the formation. Comparison with the Mid-Triassic magnetic polarity sequence observed from the western Tethyan region appears to indicate that the bulk of the Badong Formation is Anisian in age and that Ladinian sediments are largely missing. This agrees with palaeontological and stratigraphic evidence in the region and supports the view that Ladinian regression is a major event in the geological evolution of South China, which may signal the onset of amalgamation of the Yangtze Block (YB) with the North China Block (NCB). 相似文献
14.
15.
Magnetostratigraphy of Palaeocene basalts from the Vaigat Formation of West Greenland 总被引：1，自引：0，他引：1
A palaeomagnetic study comprising the directional results from 289 individual lava flows, sampled along eight sections in the Palaeocene basalts of West Greenland, is reported. The eight individual sections are correlated using lithostratigraphical marker horizons to form a single composite profile. Generally, the lithological correlation is in good agreement with the record of geomagnetic secular variation.
The total composite palaeomagnetic profile represents a stratigraphic thickness of 1.6 km through the Vaigat Formation, which is the lowermost of the two volcanic formations formed during the main stage of plateau volcanism. Only two polarity zones are found in the composite profile, suggesting a very short duration for the West Greenland main plateau-building volcanism.40 Ar/39 Ar dates support a high extrusion rate and also indicate that the lower normal polarity zone is Chron C27n and that the upper reverse polarity zone is Chron C26r.
The C27n–C26r transition is fully recorded along one of the sections (Nuusap Qaqqarsua), with intermediate directions covering a 200 m thick succession of lavas. A combined palaeomagnetic, field and geochemical study along this profile showed good agreement; that is, geochemically and geologically derived single magmatic events show groupings of the palaeomagnetic directions. Supposing a duration for the geomagnetic transition of 5000 years, the eruption frequency during this period was as high as one flow every 80 years. 相似文献
The total composite palaeomagnetic profile represents a stratigraphic thickness of 1.6 km through the Vaigat Formation, which is the lowermost of the two volcanic formations formed during the main stage of plateau volcanism. Only two polarity zones are found in the composite profile, suggesting a very short duration for the West Greenland main plateau-building volcanism.
The C27n–C26r transition is fully recorded along one of the sections (Nuusap Qaqqarsua), with intermediate directions covering a 200 m thick succession of lavas. A combined palaeomagnetic, field and geochemical study along this profile showed good agreement; that is, geochemically and geologically derived single magmatic events show groupings of the palaeomagnetic directions. Supposing a duration for the geomagnetic transition of 5000 years, the eruption frequency during this period was as high as one flow every 80 years. 相似文献
16.
William Lowrie Walter Alvarez Isabella Premoli Silva Simonetta Monechi 《Geophysical Journal International》1980,60(2):263-281
Summary. A record of geomagnetic field polarity for the Barremian, Aptian and Albian stages of the Early Cretaceous has been derived in three over-lapping sections of pelagic carbonate rocks in the Umbrian Apennines of northern Italy. The remanence carrier in the greyish-white Majolica limestone and Fucoid Marls is magnetite, with haematite also an important constituent in a zone of 'couches rouges' within the Fucoid Marls. The weak remanent magnetizations were measured with a cryogenic magnetometer. Alternating field or thermal demagnetization was used to isolate the characteristic remanent magnetization (ChRM) in 655 specimens from 248 stratigraphic levels. The samples respond positively to a tectonic fold test, indicating that the ChRM predates the Late Tertiary folding of the Umbrian sequence. The magnetic stratigraphy derived from variations of virtual geomagnetic pole latitude clearly defines the recognizable reversal pattern associated with Mesozoic marine magnetic anomalies M0 to M4. The sections have been zones palaeontologically on the basis of planktonic foraminifera and calcareous nannofossil assemblages. The ages of magnetic anomalies M0 to M4 determined in this way are somewhat older than those in the reversal time scale of Larson & Hilde (1975). Anomaly M0 is located in the Early Aptian, close to the Aptian/Barremian boundary. A long period of normal polarity in the Aptian and Albian corresponds to the early part of the Cretaceous magnetic quiet zone. It is interrupted in the Late Aptian by a reversal which we find in only one of the Fucoid Marl sections, and which has not been reported in oceanic magnetic anomaly investigations. 相似文献
17.
Tianshui Yang Masayuki Hyodo Zhenyu Yang Jianli Fu 《Geophysical Journal International》2007,171(1):104-114
Detailed palaeomagnetic integrated with rock magnetic studies have been carried out on a loess-palaeosol sequence in Baoji, Shaanxi province, southern Chinese Loess Plateau. For most samples stepwise thermal demagnetization revealed two well-defined magnetization components. A low-temperature component (LTC), which was isolated between 100 and 200 °C, is consistent with the present geomagnetic field direction. A high-temperature component (HTC), which was isolated between 250 and 620–680 °C, shows normal, reversed or transitional polarities. Our new magnetostatigraphy revealed two distinct geomagnetic excursions recorded in loess unit of L5 and palaeosol unit of S7, respectively, and the Matuyama-Brunhes (M-B) polarity boundary in loess unit of L8. Rock magnetic experiments demonstrated that the specimens from the excursion zones have the same magnetic properties as those from the Brunhes normal or Matuyama reversed polarity zones. Measurements of anisotropy of magnetic susceptibility (AMS) showed that the sediments have primary sedimentary fabrics. Based on the palaeoclimatological and magnetostratigraphic age models, the middle Brunhes excursion in loess L5 is dated at 413–433 ka, and the early Brunhes excursion is estimated to occur 23–33 ka after the M-B reversal. Comparing with previously reported geomagnetic excursions in the Brunhes chron, the middle Brunhes excursion (L5) is likely global. For the early Brunhes excursion (S7), we need further studies to examine its global occurrence. 相似文献
18.
J. Grabowski L. Krzemi&#;ski P. Nescieruk E. Starnawska 《Geophysical Journal International》2006,166(3):1077-1094
Results of palaeomagnetic investigations of the Lower Cretaceous teschenitic rocks in the Silesian unit of the Outer Western Carpathians in Poland bring evidence for pre-folding magnetization of these rocks. The mixed-polarity component reveals inclinations, between 56° and 69°, which might be either of Cretaceous or Tertiary age. Apparently positive results of fold and contact tests in some localities and presence of pyrhotite in the contact aureole suggest that magnetization is primary, although a Neogene or earlier remagnetization cannot be totally excluded since inclination-only test between localities gives 'syn-folding' results. Higher palaeoinclinations (66°–69°) correlate with a younger variety of teschenitic rocks dated for 122–120 Ma, while lower inclinations (56°–60°) with an older variety (138–133 Ma). This would support relatively high palaeolatitudes for the southern margin of the Eurasian plate in the late part of the Early Cretaceous and relatively quick northward drift of the plate in this epoch, together with the Silesian basin at its southern margin. Declinations are similar to the Cretaceous–Tertiary palaeodeclinations of stable Europe in the eastern part of the studied area but rotated ca. 14°–70° counter-clockwise in the western part. This indicates, together with older results from Czech and Slovakian sectors of the Silesian unit, a change in the rotation pattern from counter-clockwise to clockwise at the meridian of 19°E. The rotations took place before the final collision of the Outer Carpathians nappe stack with the European foreland. 相似文献
19.
The Surai Khola section in southwest Nepal, a 5000 m continuously exposed record of fluvial sedimentation since Middle Miocene, was revisited for high-resolution magnetostratigraphy in sequences with expected cryptochrons and reversals of the geomagnetic field. Polarity intervals with durations of a few tens of thousands of years are recorded as zones of stable palaeomagnetic directions. Polarity transitions are recorded as zones with complex demagnetization behaviour of specimens in the sedimentary column. Almost antiparallel palaeoremanence directions, residing in different haematite phases in the same specimens, could generally not be separated properly by thermal demagnetization. Differing demagnetization paths for neighbouring specimens during a reversal suggest that measured transitional directions are not true geomagnetic field directions, but rather are generated by the superposition of variable amounts of at least two almost antiparallel components of magnetization. Accompanying studies of recent river sand deposits demonstrate that these sediments acquire a true depositional remanent magnetization (DRM) with considerable inclination errors and scattered directions for individual specimens. 相似文献
20.
Summary. Palaeomagnetic investigations are reported from 24 sites in the Proterozoic Zig-Zag Dal Basalt Formation and 12 sites in the Midsominersø Dolerites of eastern North Greenland. The Zig-Zag Dal Basalt is a typical tholeiitic flood basalt sequence, and dolerite intrusions in the underlying sandstones are thought to be genetically related to the basalts.
After a detailed AF demagnetization programme 19 sites in the basalts and 10 sites in the dolerites reveal one stable component of magnetization, probably of TRM and/or CRM origin residing in small single domain titano-magnetite grains. The degree of anisotropy has not affected the direction of the remanent magnetization. The maximum axis of the anisotropy ellipsoid is parallel to the flow direction of the magma, whereas the minimum axis is perpendicular to the flow plane.
Only one polarity of the geomagnetic field was found. The mean palaeomagnetic pole positions for the two rock types are not significantly different (basalt: 12.2°S, 62.8°E with A95 = 3.8°; dolerites: 6.9°S, 62.0°E with A 95 = 5.1°). After correction for Phanerozoic drift of Greenland the two mean poles compare closely to a relevant North American APW-curve for 1250–1350 Ma, in good agreement with Rb-Sr isochron ages of 1250 Ma obtained for the intrusives. The palaeogeographical position of Greenland was near equator with the major geographical axis orientated E-W. 相似文献
After a detailed AF demagnetization programme 19 sites in the basalts and 10 sites in the dolerites reveal one stable component of magnetization, probably of TRM and/or CRM origin residing in small single domain titano-magnetite grains. The degree of anisotropy has not affected the direction of the remanent magnetization. The maximum axis of the anisotropy ellipsoid is parallel to the flow direction of the magma, whereas the minimum axis is perpendicular to the flow plane.
Only one polarity of the geomagnetic field was found. The mean palaeomagnetic pole positions for the two rock types are not significantly different (basalt: 12.2°S, 62.8°E with A