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1.
Summary. Palaeolatitudes estimated from DSDP sediments provide important constraints on the apparent polar wander path (APWP) of Africa during the Cenozoic. A revised APWP is suggested based on new information about palaeomagnetic poles from the African continent and predicted palaeolatitudes are compared with those determined from Leg 73 sediments. Other published paths are discussed. 相似文献
2.
J. Nawrocki M. Fanning A. Lewandowska O. Polecho&#;ska T. Werner 《Geophysical Journal International》2008,174(2):475-488
Permian rhyodacites, melaphyres and tuffs from the Cracow area (South Poland) were sampled for the palaeomagnetic and isotope studies. Single-grain U-Pb dating of most zircon grains separated from the rhyodacites gave mean age of magma emplacement of 294.2 ± 2.1 Ma. Some zircons, however, displayed younger ages (268.7 ± 3.4 Ma), probably related to the metasomatic alterations of these rocks. Two Permian components of magnetizations related to these processes were isolated and together with previously defined Late Carboniferous–Permian palaeomagnetic poles from South Poland were used for construction of the regional apparent polar wander path (APWP). The Early Permian segment of this APWP shows a certain departure from the coeval part of the Fennoscandian APWP due to anticlockwise rotations of studied rocks most probably caused by mid-Permian sinistral tectonic movements along reactivated prominent Variscan faults of Central Europe. This sense of tectonic mobility does not support the hypothesis about transformation from Pangea 'B' to Pangea 'A' along an intra-Pangea dextral megashear during the Permian. Older than previously assumed ages of the post-Variscan igneous rocks of Central Europe reduce overlap of Gondwana's and Laurussia's parts of the Early Permian Pangea 'A'. 相似文献
3.
Late Mesozoic and Cenozoic palaeomagnetism of Australia–I. A redetermined apparent polar wander path
Mart Idnurm 《Geophysical Journal International》1985,83(2):399-418
Summary. Palaeomagnetic measurements have been carried out on one Cretaceous and five Cenozoic sedimentary sequences of Australia; of those, one is in the Carnarvon Basin of north-western Australia, and the others are in the Otway Basin of south-eastern Australia. The new results are used together with those published previously to define a revised late Mesozoic–Cenozoic apparent polar wander path for Australia. This path differs from earlier, basalt-derived paths by the absence of a large westward excursion and zig-zag irregularities. It is characterized instead by a substantially straight Cenozoic trajectory, a sharp bend in the Late Cretaceous, and a non-uniform rate of apparent polar wander.
The early Tertiary segment of the new path lies north of the original paths, thus eliminating a discrepancy that has been noted previously between the Indian and Australian palaeomagnetic data (Luyendyk & Rennick, Peirce and Klootwyk & Peirce). A re-examination of the original data suggests that this discrepancy, as well as two others in the Australian results, may have been caused principally by incomplete time-averaging of remanence directions, because the volcanics on which the results were based had been extruded episodically. 相似文献
The early Tertiary segment of the new path lies north of the original paths, thus eliminating a discrepancy that has been noted previously between the Indian and Australian palaeomagnetic data (Luyendyk & Rennick, Peirce and Klootwyk & Peirce). A re-examination of the original data suggests that this discrepancy, as well as two others in the Australian results, may have been caused principally by incomplete time-averaging of remanence directions, because the volcanics on which the results were based had been extruded episodically. 相似文献
4.
Mart Idnurm 《Geophysical Journal International》1985,83(2):419-433
Summary. New palaeomagnetic results from Australia indicate that throughout the Cenozoic era the continent lay further south than suggested by hot-spot data. Moreover, while hot spots give a uniform rate of drift during most of the Cenozoic, the drift rate obtained from apparent polar wander varies considerably.
The discrepancies between the palaeomagnetic and hot-spot results are analysed by comparing the Australian data with those of Europe and the central Pacific. The analysis suggests that the discrepancies are due to: (1) departures of the Earth's magnetic field from the geocentric axial dipole model, and (2), either true polar wander or a non-axial inclined dipole component. It is found that since the mid-Tertiary the dominant non-dipole component has been a quadrupole, and that during this period both the quadrupolar field and the true polar displacement/non-axial dipole component decreased progressively. During the Quaternary, and also at the earliest Tertiary, the non-dipole components appear to have been moderate or small.
The comparison of data sets demonstrates that considerable errors may be incurred when Cenozoic, and presumably earlier, poles from one geographic region are used to derive those of another, widely separated, region. The results also imply that absolute plate velocities estimated from palaeomagnetic data can contain substantial errors, and that hot-spot data may need significant adjustments for true polar wander to yield correct palaeolatitudes.
Finally, the new early Tertiary pole for Australia is used in conjunction with updated early Tertiary poles from other lithospheric plates to reapply the McKenzie test for true polar wander. The results indicate a small true polar displacement since the beginning of the Tertiary. The amount and direction of the displacement, however, differ from those generally obtained from hot-spot data. 相似文献
The discrepancies between the palaeomagnetic and hot-spot results are analysed by comparing the Australian data with those of Europe and the central Pacific. The analysis suggests that the discrepancies are due to: (1) departures of the Earth's magnetic field from the geocentric axial dipole model, and (2), either true polar wander or a non-axial inclined dipole component. It is found that since the mid-Tertiary the dominant non-dipole component has been a quadrupole, and that during this period both the quadrupolar field and the true polar displacement/non-axial dipole component decreased progressively. During the Quaternary, and also at the earliest Tertiary, the non-dipole components appear to have been moderate or small.
The comparison of data sets demonstrates that considerable errors may be incurred when Cenozoic, and presumably earlier, poles from one geographic region are used to derive those of another, widely separated, region. The results also imply that absolute plate velocities estimated from palaeomagnetic data can contain substantial errors, and that hot-spot data may need significant adjustments for true polar wander to yield correct palaeolatitudes.
Finally, the new early Tertiary pole for Australia is used in conjunction with updated early Tertiary poles from other lithospheric plates to reapply the McKenzie test for true polar wander. The results indicate a small true polar displacement since the beginning of the Tertiary. The amount and direction of the displacement, however, differ from those generally obtained from hot-spot data. 相似文献
5.
Mart Idnurm 《Geophysical Journal International》1986,86(2):277-287
Summary. Palaeomagnetic results from Part I of this study and their analysis in Part II are combined to eliminate bias from the Cenozoic apparent polar wander path for Australia – a bias due to non-dipole components in past geomagnetic fields or, for poles calculated from hot-spot data, due to the motion of hot spots relative to the Earth's rotational axis. This path is extended in approximately bias-free form to the late Mesozoic, and indicates a significant change in the drift direction of the continent between 26 and about 60 Ma.
The bias-corrected Australian path is used, first, with seafloor spreading data for the Southern Ocean to derive a corresponding late Mesozoic–Cenozoic pole path for Antarctica. The latter shows that the Antarctic drift direction reversed in the early Tertiary. It is suggested that the early Tertiary directional changes of both Australia and Antarctica are part of a global reorganization of plates during the Eocene, postulated by Rona & Richardson, Cande & Mutter and Patriat & Achache.
Next, the Australian path is compared with hot-spot data from the African and Australian plates, indicating a movement of the hot spots relative the Earth's rotational axis during the Cenozoic. The direction of this movement is found to be consistent with previous results from other parts of the world.
Finally, the Australian path is used together with non-dipole components in the geomagnetic field to explain a prominent westward displacement of the mid- and late Cenozoic poles of India relative to those of Australia.
Because of uncertainties in the original poles and in the analysis, the present results are likely to contain appreciable errors. Nevertheless, their consistency with independent findings supports the dipole-quadrupole model of Part II for mid- and late Cenozoic geomagnetic fields. 相似文献
The bias-corrected Australian path is used, first, with seafloor spreading data for the Southern Ocean to derive a corresponding late Mesozoic–Cenozoic pole path for Antarctica. The latter shows that the Antarctic drift direction reversed in the early Tertiary. It is suggested that the early Tertiary directional changes of both Australia and Antarctica are part of a global reorganization of plates during the Eocene, postulated by Rona & Richardson, Cande & Mutter and Patriat & Achache.
Next, the Australian path is compared with hot-spot data from the African and Australian plates, indicating a movement of the hot spots relative the Earth's rotational axis during the Cenozoic. The direction of this movement is found to be consistent with previous results from other parts of the world.
Finally, the Australian path is used together with non-dipole components in the geomagnetic field to explain a prominent westward displacement of the mid- and late Cenozoic poles of India relative to those of Australia.
Because of uncertainties in the original poles and in the analysis, the present results are likely to contain appreciable errors. Nevertheless, their consistency with independent findings supports the dipole-quadrupole model of Part II for mid- and late Cenozoic geomagnetic fields. 相似文献
6.
Palaeomagnetism of the Lower Ordovician Orthoceras Limestone, St. Petersburg, and a revised drift history for Baltica in the early Palaeozoic 总被引：2，自引：0，他引：2
MarkA.Smethurst AlexeyN.Khramov Sergei Pisarevsky 《Geophysical Journal International》1998,133(1):44-56
Palaeomagnetic investigation of Lower Ordovician limestone in the vicinity of St. Petersburg yields a pole position at latitude 34.7°N, longitude 59.1°E ( dp / dm =5.7°/6.4°). A probable primary remanence origin is supported by the presence of a field reversal. The limestone carries one other remanent magnetization component associated with a Mesozoic remagnetization event.
An apparent polar wander path is compiled for Baltica including the new result, ranging in age from Vendian to Cretaceous. Ages of the published Lower to mid-Palaeozoic palaeomagnetic pole positions are adjusted in accordance with the timescale of Tucker & McKerrow (1995). The new Arenig result is the oldest of a series of Ordovician and Silurian palaeomagnetic pole positions from limestones in the Baltic region. There are no data to constrain apparent polar wander for the Tremadoc, Cambrian and latest Vendian. If the Fen Complex results, previously taken to be Vendian in age ( c . 565 Ma), are reinterpreted as Permian remagnetizations, an Early Ordovician–Cambrian–Vendian cusp in the polar wander path for Baltica is eliminated. The apparent polar wander curve might then traverse directly from poles for Vendian dykes on the Kola peninsula ( c . 580 Ma) towards our new Arenig pole ( c . 480 Ma). The consequence of this change in terms of the motion of Baltica in Cambrian times is to reduce significantly a rotational component of movement.
The new Arenig pole extends knowledge of Ordovician apparent polar wander an increment back in time and confirms the palaeolatitude and orientation of Baltica in some published palaeogeographies. Exclusion of the Fen Complex result places Baltica in mid- to high southerly latitudes at the dawn of the Palaeozoic, consistent with faunal and sedimentological evidence but at variance with some earlier palaeomagnetic reconstructions. 相似文献
An apparent polar wander path is compiled for Baltica including the new result, ranging in age from Vendian to Cretaceous. Ages of the published Lower to mid-Palaeozoic palaeomagnetic pole positions are adjusted in accordance with the timescale of Tucker & McKerrow (1995). The new Arenig result is the oldest of a series of Ordovician and Silurian palaeomagnetic pole positions from limestones in the Baltic region. There are no data to constrain apparent polar wander for the Tremadoc, Cambrian and latest Vendian. If the Fen Complex results, previously taken to be Vendian in age ( c . 565 Ma), are reinterpreted as Permian remagnetizations, an Early Ordovician–Cambrian–Vendian cusp in the polar wander path for Baltica is eliminated. The apparent polar wander curve might then traverse directly from poles for Vendian dykes on the Kola peninsula ( c . 580 Ma) towards our new Arenig pole ( c . 480 Ma). The consequence of this change in terms of the motion of Baltica in Cambrian times is to reduce significantly a rotational component of movement.
The new Arenig pole extends knowledge of Ordovician apparent polar wander an increment back in time and confirms the palaeolatitude and orientation of Baltica in some published palaeogeographies. Exclusion of the Fen Complex result places Baltica in mid- to high southerly latitudes at the dawn of the Palaeozoic, consistent with faunal and sedimentological evidence but at variance with some earlier palaeomagnetic reconstructions. 相似文献
7.
B. A. Duff 《Geophysical Journal International》1980,60(3):355-375
Summary. Stable natural remanent magnetization (NRM) in the Jersey Volcanics and in a single rhyolite dyke was probably acquired during the Cambrian before folding of the volcanics in the Cadomian Orogeny. After dip correction, the volcanics yield a palaeomagnetic pole at 323° E, 52° N ( dp = 33°, dm = 35°). In Jersey dolerite dykes three groups of stable NRM directions are recognized, with palaeomagnetic poles at 248° E, 26° N ( dp = 10°, dm = 20°), 339° E, 1° S ( dp = 9°, dm = 12°), and 336° E, 31° S ( dp = 5°, dm = 9°). Comparison with the European apparent polar wander path implies that stable NRM in these groups was acquired respectively during Late Precambrian or early Cambrian, Siluro-Devonian and middle Carboniferous time. The stable NRM of the Jersey lamprophyre dykes yields a palaeomagnetic pole at 322° E, 16° N ( dp = 31°, dm = 38°) and is probably of Silurian or Devonian age.
These palaeomagnetic poles and other new data determined by the author for the Armorican Massif can be fitted to a common apparent polar wander path for Europe, and this implies that the basement of Lower Palaeozoic Europe extended from the Baltic Shield at least as far south as the Armorican Massif. The Hercynian Orogeny in these parts of Europe was therefore probably intracratonic. This polar wander path implies that in early Cambrian time the pole did not move significantly relative to Europe, but that this was followed by a large middle to late Cambrian polar shift which corresponded to rapid drift of Europe across the South Pole. 相似文献
These palaeomagnetic poles and other new data determined by the author for the Armorican Massif can be fitted to a common apparent polar wander path for Europe, and this implies that the basement of Lower Palaeozoic Europe extended from the Baltic Shield at least as far south as the Armorican Massif. The Hercynian Orogeny in these parts of Europe was therefore probably intracratonic. This polar wander path implies that in early Cambrian time the pole did not move significantly relative to Europe, but that this was followed by a large middle to late Cambrian polar shift which corresponded to rapid drift of Europe across the South Pole. 相似文献
8.
Marek Lewandowski 《Geophysical Journal International》1999,137(3):783-792
Calcite and sedimentary fills in fractures cutting the Upper Devonian carbonates in the Holy Cross Mountains (HCM) were dated palaeomagnetically by comparison with the apparent polar wander path (APWP). Haematite-bearing calcite possessed well-defined components of natural remanent magnetization (NRM), which were preserved under thermal demagnetization to temperatures of approximately 500 °C, when specimens disintegrated. Although not completely demagnetized, some specimens revealed a stable NRM component before destruction, thus making a component analysis possible. Five components were determined using density point distribution and cluster analysis. One has a mean that is similar to the present-day local geomagnetic vector. The remaining four components yielded palaeomagnetic poles located at: A (70.3°S, 5.5°E), B (71.3°S, 31.2°E), C (48.7°S, 351.0°E, virtual geomagnetic pole), and D (11.6°S, 312.3°E). Antipodal polarities found in the fracture fills, together with dissimilarities in magnetization found in calcite and hosting carbonates, indicate the lack of simultaneous remagnetization, and different times of remanence acquisition for the rocks under comparison. Taking both palaeomagnetically inferred palaeolatitudes and regional tectonics into consideration, a Mesozoic (Cretaceous?) age is estimated for palaeopoles A and B, a Permian age for pole C, and a Carboniferous age for pole D. These age determinations are in line with the calcite ages estimated from isotopic studies. A comparative palaeomagnetic study performed on a well-dated Upper Devonian neptunian dyke of limestone and a Lower Triassic clastic vein yielded virtual geomagnetic poles (VGPs) close to the APWP for Baltica. Generally, the remanence from fracture fills may be useful for dating related tectonics, karst phenomena and mineralization processes. 相似文献
9.
Apparent polar wander in the mean-lithosphere (= no-net-rotation = no-net-torque uniform drag) reference frame is compared with apparent polar wander in the hotspot reference frame over the past 100 Myr. Palaeo-magnetic poles and plate rotations previously used to determine an apparent polar wander path for the hotspot reference frame are here used to determine an apparent polar wander path in the mean-lithosphere reference frame. We find that the two paths are similar, especially for Late Cretaceous time, when a 10°–20° shift of the pole occurred. To first-order the hotspots and lithosphere (as a whole) moved in unison relative to the palaeomagnetic axis during Late Cretaceous time. A non-dipole field explanation for the apparent shift can probably be excluded. However, either motion of the time-averaged geomagnetic axis relative to the spin axis or polar wandering could have caused this shift, the latter being the more likely explanation. 相似文献
10.
Joseph G. Meert Elizabeth A. Eide Trond H. Torsvik 《Geophysical Journal International》1997,129(3):637-650
The Nama Group of southern Namibia is a candidate for the Terminal Proterozoic Global Stratotype Section and Point (GSSP). Desirable characteristics of a GSSP include a well-preserved index-fossil assemblage, little deformation or metamorphism. well-constrained isotopic ages, stable-isotope records and magnetostratigraphic control. The age of the Nama Group sediments is now constrained to between 570 and 510 Ma. Assuming the Gondwana assembly was nearly complete at this same time, there is a discrepancy between the previously published Nama poles, a revised 550-510 Ma apparent polar wander path for Gondwana and the preceding supercontinental assemblages of Rodinia and Panottia. For these reasons, the Nama Group sediments were resampled in an effort to evaluate the potential of detailing the magnetostratigraphy of the Nama Group and resolving the discrepancy between the Nama poles and the APWP of Gondwana. Collectively, both the previous studies of the Nama Group and this one show a complex series of overprints and no easily discernible primary direction of magnetization. We therefore urge caution in using the Nama Group poles in any tectonic models of the Neoproterozoic-Early Palaeozoic. Specifically, the N1 component of magnetization, previously identified as a primary magnetization, was discovered in a younger suite of samples. Therefore, previous tectonic models that used the N1 magnetization direction as representative of the time of Nama deposition should be revised in light of these recent findings. 相似文献
11.
Palaeomagnetism of 935 Ma mafic dykes in southern Sweden and implications for the Sveconorwegian Loop 总被引：1，自引：0，他引：1
New palaeomagnetic results for the 935 Ma Göteborg-Slussen mafic dykes in southern Sweden provide a well-dated high-quality palaeomagnetic pole for Early Neoproterozoic Baltica. New U-Pb geochronological data for several palaeomagnetically studied mafic intrusions yield three additional well-dated palaeopoles and one virtual geomagnetic pole. This set of dated poles suggests minimal drift of Baltica in moderate latitudes between ∼965 and 915 Ma. They also support the hypothesis of a post-900 Ma regional remagnetization event in SW Sweden and SW Norway. The positions of three distinct clusters of ∼1100 to 850 Ma palaeopoles suggest a clockwise time progression of the Baltica apparent polar wander path (the Sveconorwegian Loop) during this time interval. New well-dated palaeomagnetic poles for ∼970 to 900 Ma from Laurentia are required to verify the palaeogeographic reconstructions of Baltica and Laurentia. 相似文献
12.
H. B. Turnell 《Geophysical Journal International》1985,83(2):363-378
Summary. Study of the palaeomagnetism of two complexes from the Newer Granite Suite in Scotland, at Ratagan (NW Highlands) and Comrie (central Highlands), reveals the variable nature of the natural remanence encountered in granodioritic intrusions and the surrounding metamorphic country rock. Forty-eight specimens from Ratagan, dated at 415 ± 5 Ma, gave a mean direction: D = 8°, I =−32°, and a palaeomagnetic south pole: 15°S, 346°E (δ p = 5°, δ m = 9°). Twenty-eight specimens from Comrie, dated at 408±5 Ma, gave a mean direction: D = 75°, I =−30°, and a palaeomagnetic south pole: 6°S, 287°E (δ p = 4°, δ m = 7°). These results have been compared with the established apparent polar wander path (APWP) for Britain. The Ratagan pole improves the reliability of the APWP but doubt remains as to whether the primary magnetization from Comrie represents a true late Silurian direction or whether it has been affected by post-cooling rotation, possibly associated with the nearby Highland Boundary Fault. 相似文献
13.
Summary. The remanent magnetization of intrusive/extrusive rocks of the 'basement' complex of East Maio constitutes four components that define two different axes of magnetization, at around dec. 328, inc. 12 and dec. 007, inc. 14 respectively. In general, two or more components co-exist in separate specimens or sites but both axes are present most frequently in the normal sense. The NNW-striking axis, the B-axis, fits very well with the Upper Cretaceous polar wander path for Africa. It is consequently inferred that the major phase of sheet intrusions in Maio dates from this time, probably from the interval 90–70 Myr bp. Comparisons of the directional dispersions in the folded and unfolded states suggest that this injection phase post-dates the uplift of the Central Igneous Complex of the island. The second axis of magnetization, the A -axis, agrees very well with late Teritary—Quaternary palaeomagnetic data for Africa and the Canary Islands. The A -axis is therefore regarded as of secondary origin, being the consequence of a thermal/ chemical overprint during the Miocene—Pliocene volcanism on the island. The occurrence of a 50–70 Myr long period of volcanic quiescence and erosion, between the termination of the early igneous activity (Upper Cretaceous) and the rejuvenated magmatism in Miocene/Pliocene time, is compatible with similar observations in the Canary Islands. In contrast to the palaeomagnetic conclusions, the K/Ar data only give ages around 10 Myr. The unusually young isotope dates are regarded as being due to an almost complete age resetting and are seen in conjuction with the overprinted magnetization. This explanation is further supported by the fact that K/Ar results of pillow lavas underlying Upper Jurassic limestones only give Tertiary ages. 相似文献
14.
15.
PALEOMAGNETISM OF THE LATE CRETACEOUS AND EARLY TERTIARY ROCKS FROM FILDES PENINSULA,WEST ANTARCTICA, AND ITS GEOTECTONIC SIGNIFICANCE 
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下载免费PDF全文 《极地研究》1992,3(1):40-49
The paleomagnetis m of 109 oriented samples collected from drill cores through 5 rock units of Late Cretaceous and Early Tertiary on Fildes Peninsula were systematically studied. According to the study, the paleomagnetic pole position of this area is different from the position of Australia during the 55-45 Ma period. This means that when the break-up of the Gondwanaland at 55-45 Ma ago, Australia was separated from the Gondwanaland, drifting southward 20°-30°Lat. and rotating 70°-80°westward and then gradually arriving at recent position. The paleolatitudinal data indicate that it is not impossible that the area studied was covered with land glacier at that time. The apparent polar wander path of Antarctica through the geological time are also roughly worked out. 相似文献
16.
We use three‐dimensional (3D) seismic reflection and magnetic data to interpret and describe the 3D geometry of igneous dykes in the southern North Sea. The dykes were emplaced into Paleozoic and Mesozoic sediments and have a common upper termination in Early Tertiary sediments. We interpret the dykes to be part of the British Tertiary volcanic province and estimate the age of the dykes to be 58 Ma. The dykes are characterized by a narrow 0.5–2 km wide vertical disturbance of seismic reflections that have linear plan view geometry. Negative magnetic anomalies directly align with the vertical seismic disturbance zones and indicate the presence of igneous material. Linear coalesced collapse craters are found above the dykes. The collapse craters have been defined and visualized in 3D. Collapse craters have formed above the dyke due to the release of volatiles at the dyke tip and resulting volume loss. Larger craters have potentially formed due to explosive phreatomagmatic interaction between magma and pore water. The collapse craters are a new Earth analogue to Martian pit chain craters. 相似文献
17.
Masao Nakada 《Geophysical Journal International》2000,143(1):230-238
Previous studies of the wander of the rotation pole associated with the Late Pleistocene glacial cycles indicate that the predicted polar wander speed is sensitive to the density jump at the 670 km discontinuity, the thickness of the elastic lithosphere, and the lower mantle viscosity. In particular, the M1 mode related to the density jump at 670 km depth has been shown to contribute a dominant portion of predicted polar wander speed for sufficiently small lower mantle viscosities. In this study, we examine the sensitivity of polar wander to variations in the viscosity of the viscoelastic lithosphere using simplified compressible Maxwell viscoelastic earth models. Model calculations for earth models with a viscoelastic lithosphere of finite viscosity indicate that the contribution of the M1 mode is similar to those associated with the density discontinuity at the core–mantle boundary (C0 mode) and the lithosphere (L0 mode). We speculate that this is due to the interaction between the M1 mode and the transient mode associated with the viscoelastic lithosphere, which reduces the magnitude of polar wander rates. Therefore, the M1 mode does not contribute a dominant portion of the predicted polar wander speed for earth models with a viscoelastic lithosphere of finite viscosity. In this case, predictions of polar wander speed as a function of lower mantle viscosity exhibit the qualitative form of an 'inverted parabola', as predicted for the J ˙2 curve. We caution, however, that these results are obtained for simplified earth models, and the results for seismological earth models such as PREM may be complicated by the interaction between the M1 mode and the large set of transient modes. 相似文献
18.
Continental drift and true polar wandering 总被引：1，自引：0，他引：1
S. R. Dickman 《Geophysical Journal International》1979,57(1):41-50
Summary . Evidence in the form of 75 yr of ILS data is accumulating which suggests that true polar wander may be currently taking place. It seems likely that true wander of some magnitude must always accompany plate motions, but the extrapolated ILS rate is an order of magnitude larger than the rate of true polar wander deduced from palaeomagnetic data over the past 55 Myr. The conflict between palaeomagnetic and latitude data provides the motivation for investigating one possible excitation of polar wander, the mass redistribution which accompanies continental drift.
The mass redistribution arises mainly because of the contrasting density structure of oceanic and continental regions. The change in the inertia tensor resulting from 106 yr of plate motions is found to be negligibly small; even consideration of episodic plate movements, anelasticity, or a decoupled lithosphere cannot boost the effect to the ILS rate of polar wander. These conclusions are strengthened by the fact that any one of several absolute plate velocity models, based on extremely diverse assumptions, yields the same results.
In contrast, preliminary findings regarding the effect of Pleistocene deglaciation activities on the inertia tensor reveal that such non-isostatic phenomena may have a large influence on polar wander. 相似文献
The mass redistribution arises mainly because of the contrasting density structure of oceanic and continental regions. The change in the inertia tensor resulting from 10
In contrast, preliminary findings regarding the effect of Pleistocene deglaciation activities on the inertia tensor reveal that such non-isostatic phenomena may have a large influence on polar wander. 相似文献
19.
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
20.
Summary. In this paper we show that: (1) The positions of the Cretaceous palaeomagnetic poles (PP) for South America and Africa exhibit elongated distributions that are due to rapid movement of these continents from the south pole.
(2) The positions of the Middle—late Jurassic virtual geomagnetic poles for South America exhibit an elongated distribution along the meridians 20–200° E; it is suggested that this is due to a rapid shift of South America in Middle—late Jurassic time.
(3) The late early—early late Cretaceous sections of the apparent polar wandering paths for South America and Africa are consistent with South Atlantic seafloor spreading data.
On the basis of the comparison of the reliable late Palaeozoic—late Cretaceous PPs for South America and Africa, taking into account the restrictions established by geological, palaeontological and seafloor spreading data, it is suggested that minor movements could have occurred within Western Gondwana in middle—late Jurassic time along a narrow zone which later became the South Atlantic divergent boundary.
Four 'hairpins' are defined in the late Palaeozoic—late Cretaceous section of the apparent polar wandering path for South America; the two youngest of these can be correlated with the origin of the South Atlantic Ocean basin and the onset of the Andean Orogeny, respectively.
The magnetostratigraphy for the Serra Geral lava flow sequence suggests that some of these flows were poured out rapidly without significant interruption. 相似文献
(2) The positions of the Middle—late Jurassic virtual geomagnetic poles for South America exhibit an elongated distribution along the meridians 20–200° E; it is suggested that this is due to a rapid shift of South America in Middle—late Jurassic time.
(3) The late early—early late Cretaceous sections of the apparent polar wandering paths for South America and Africa are consistent with South Atlantic seafloor spreading data.
On the basis of the comparison of the reliable late Palaeozoic—late Cretaceous PPs for South America and Africa, taking into account the restrictions established by geological, palaeontological and seafloor spreading data, it is suggested that minor movements could have occurred within Western Gondwana in middle—late Jurassic time along a narrow zone which later became the South Atlantic divergent boundary.
Four 'hairpins' are defined in the late Palaeozoic—late Cretaceous section of the apparent polar wandering path for South America; the two youngest of these can be correlated with the origin of the South Atlantic Ocean basin and the onset of the Andean Orogeny, respectively.
The magnetostratigraphy for the Serra Geral lava flow sequence suggests that some of these flows were poured out rapidly without significant interruption. 相似文献