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1.
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   

2.
The Ordovician and Silurian Setul Limestone of the Langkawi Islands, northwest Peninsular Malaysia, has a mean magnetic vector ofD = 338°,I = 62° after cleaning and correction for tilt. This is equivalent to a palaeolatitude of 43°, and a palaeomagnetic pole at 46°N, 76°E. The Silurian part of the Setul limestone also shows a similar direction. The Ordovician results are equivalent to a palaeolatitude of 43°, N or S. Recent reconstructions, based on palaeontology, place Indochina and China in the northern hemisphere in the Ordovician; if this is correct, a palaeolatitide of 43° for Langkawi would imply that Malaya-Indochina was the most northerly continental fragment at that time.  相似文献   

3.
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   

4.
From Middle-Upper Jurassic volcanics at the western margin of the Maranha?o Basin (6.4°S, 47.4°W) 15 sites (121 samples) have a mean magnetization directionD = 3.9°,I = ?17.9° withα95 = 9.3°,k = 17.9 after AF cleaning (all sites have normal polarity). This yields a pole (named SAJ2) at 85.3°N, 82.5°E (A95 = 6.9°) which is near to the other known Middle Jurassic South American pole. For 21 sites (190 samples) from Lower Cretaceous basalt intrusions from the eastern part of the Maranha?o Basin (6.5°S, 42°W) the mean direction isD = 174.7°,I = +6.0° withα95 = 2.8°,k = 122 (all sites have reversed polarity) yielding a pole (SAK9) at 83.6°N, 261°E (A95 = 1.9°) in agreement with other Lower Cretaceous pole positions for South America. Comparing Mesozoic pole positions for South America and Africa in the pre-drift configuration after Bullard et al. [13] one finds a significant difference (with more than 95% probability) for the Lower Cretaceous and Middle Jurassic poles and also a probable difference for the mean Triassic poles indicating a small but probably stationary separation of the two continents from the predrift position in the Mesozoic until Lower Cretaceous time which may be due to an early rifting event.  相似文献   

5.
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   

6.
Tongaporutuan Stage (Late Miocene, 11-6 Myr) sedimentary rocks at two localities within the axial tectonic belt of New Zealand have a direction of magnetisation in which the declination is 020 ± 11° and 030 ± 8° (D ± Δ95). The directions of magnetisation in rocks of similar age outside the belt have a declination like that of the earth's present axial dipole, 360°. The clockwise deviation of the declination is inferred to indicate bending of the rocks within the zone of deformation between the Pacific and Indian plates due to the oblique convergence of these plates.  相似文献   

7.
One hundred samples from nine sites in Upper Cretaceous volcanics (K/Ar age 85–99 m.y.) of the magmatic province of Cabo de Santo Agostinho, Pernambuco (8.4°S, 35.0°W) yield a mean direction of magnetizationD = 0.4°, I = ?20.6°withα95 = 4.8°, k = 114 after AF cleaning. All sites have normal polarity with a mean pole, named SAK10, at 87.6°N, 135°E withA95 = 4.5° which is close to other Upper Cretaceous poles for South America. These poles are compared with Upper Cretaceous poles of Africa for various reconstructions of the two continents.  相似文献   

8.
Palaeomagnetic results from the Lower Palaeozoic inliers of northern England cover the upper part of the (Middle Ordovician) Borrowdale Volcanic Series (palaeomagnetic pole 208°E, 18°S, A95 = 9.4°), minor extrusive units relating to the Caradoc and Ashgill stages of Ordovician times, intrusive episodes of Middle Ordovician and Middle Silurian to Late Devonian age, and the Shap Granite of Devonian (393 m.y.) age (palaeomagnetic pole 313°E, 33°S, A95 = 5.6°).A complete assessment of Ordovician to Devonian palaeomagnetic data for the British region shows that the pole was nearly static relative to this region for long intervals which were separated by shifts occupying no more than a few millions of years. The mean palaeomagnetic poles are: Ordovician (6°E, 16°S), Lower Silurian (58°E, 16°N), Middle Silurian/Lower Devonian (318°E, 6°N) and Middle/Upper Devonian (338°E, 26°S); the first two shifts separating these mean poles can be explained predominantly in terms of rotational movements of the crustal plate but the last involved appreciable movement in palaeolatitude.Comparison of Lower Palaeozoic palaeomagnetic data from the British region with contemporaneous data from continental Europe/North America on the Pangaean reconstruction reveals a systematic discrepancy in palaeolatitude between the two regions prior to Middle Devonian times. This discrepancy was eliminated during a few millions of years of Lower/Middle Devonian times (ca. 395 m.y.) and can be explained in terms of ca. 3500 km of sinistral strike-slip movement close to the line of the orthotectonic Caledonides. This motion is linked both in time and place to the impingement of the Gondwanaland and Laurentian supercontinents during the Acadian orogeny; this appears to have displaced the British sub-plate until it became effectively locked between the Baltic and Laurentian regions. Although movement of the dipole field relative to the British region in Lower Palaeozoic times is now well defined, nearly one fifth of the total data show that the geomagnetic field was more complex than dipolar during this interval. Until the significance of these anomalies is fully resolved, the tectonic model derived from the palaeomagnetic data cannot be regarded as unambiguous.  相似文献   

9.
Results are reported from palaeomagnetic samples collected in two traverses across the coast-parallel dyke swarm of southern Greenland. This swarm probably resulted as the consequence of initial rifting between Greenland and Labrador, and a reversal of magnetisation has been found which is correlated on the basis of KAr age determinations (～168 m.y.) with the Mateke event of the Middle Jurassic (Bajocian). All of fifteen sites show significant grouping of directions after a.f. cleaning; three have anomalous directions of magnetisation while the remainder (nine normal, three reversed) give a combined mean direction of D = 336°, I = 66° (α95 = 4.6°) with a palaeomagnetic pole at 191°E, 72°N. The dykes exhibit the same corelation between polarity and deuteric oxidation state as that found in Tertiary volcanics. There is a systematic change in magnetisation across the dyke swarm in south Greenland from normal to anomalous to reversed directions; this is interpreted as due to lateral migration of the response to the regional stress field with time. The pole position lies in the vicinity of Jurassic poles from North America after closing the Labrador Sea according to the reconstruction of Bullard, Everett and Smith, but the scatter of these latter poles precludes a confirmation of this reconstruction for Middle Jurassic and earlier times.  相似文献   

10.
The Middle Ordovician Volginsky and Kirensky fossil zones were sampled in the Krivaya Luka section (Krivolutsky suite) that outcrops along the Lena river in Siberia. The Volginsky and Kirensky zones are coeval to the Llandeilo in the global geologic time scale. The Krivaya Luka section consists of siltstones, clays, sandstones, and limestones, and displays a remarkably distinct sedimentary cyclicity, especially in its reddish middle part.Stepwise thermal demagnetization yields three NRM components. Component A, isolated in the 100—250°C interval can be either normal or reversed. The normal A-component has a direction close to recent local magnetic field. The reversed A-component directions are scattered around a direction close to that of the lower Triassic Siberian traps. Component B has unblocking temperatures that range from 400 to 500°C and is represented mainly by normal polarity directions. The B-component, isolated from rocks of the middle part of the section is of a normal polarity with D = 176.5°, I = 30.0° and a North pole position at 16.2°S, 111.3°E. The other parts of the section are characterized by intermediate B-directions, which resulted possibly by partially overlapping A- and C-components. The highest temperature dual-polarity component C was isolated in the 550—670°C interval, resulting in the detection of two complete polarity zones and three magnetic reversals. The C-component is characterized by the following mean directions: for the reversed component D = 335.7°, I = 6.9°, and for the normal component D = 188.6°, I = 28.0°, which is very close to the normal polarity directions of the B-component. The corresponding paleomagnetic North pole for reversed polarity rocks is 32.6°S, 137°E, which is typical of Middle Ordovician rocks from Siberia – the mean pole for Llanvirn-Llandeilo is 30°S, 136°E (cf. Smethurst et al., 1998) – whereas for normal polarity rocks the pole position 17.2°S, 99.1°E is markedly different. Nevertheless, we assume that the C-component records the ancient geomagnetic field of Ordovician times, even though it does not pass the reversals test. This could be explained by overlapping NRM unblocking temperature spectra for the B and C components. In this case, the paleomagnetic pole positions should be interpreted with some caution.In addition, the section was logged and sampled in detail for cyclostratigraphic purposes. Spectral analysis in the depth domain using the high-field susceptibility as input parameter showed that the observed cyclicity is most likely orbitally forced. Detected spectral peaks (significant at the 95% confidence level) were close to the expected positions of the periodicities of precession, obliquity and eccentricity for the Ordovician. Consequently, the average sediment accumulation rate is estimated at 3.5 cm/kyr. Extrapolating this sedimentation rate yields a total duration of at least 1 Myr for the Volginsky fossil zone and 1.2 Myr for the entire Krivaya Luka section.  相似文献   

11.
Detailed alternating field demagnetisation of Upper Llandovery volcanics of the Mendip Hills and Gloucestershire has isolated remanence directions interpreted as primary from each of five sites. Well-defined high-coercivity secondary magnetisation is present in six samples of one site and low-coercivity secondary remanence is present in all samples from another site; the former component was apparently acquired in Permo-Triassic times. Primary directions of magnetisation show marked improvement in precision after correction for penecontemporaneous folding, and show a late Llandovery reversal in the sense R → N.The group mean directions of magnetisation isD = 243.5°,I = 47.5° (precision parameterk = 29). Petrographic examination confirms observations from magnetic properties that relict titanomagnetite (oxidation classes 3 to 5) is the remanence carrier in most samples. Hematite, probably mostly late magmatic in origin, is widely developed in all samples, but only the principal remanence carrier where it has thoroughly replaced the titanomagnetite. Low-coercivity remanence is apparently caused by weathering effects but there is no clear visible cause for secondary high-coercivity remanence carried by some samples.The mean virtual geomagnetic pole position is close to Upper Silurian/Lower Devonian pole positions from other parts of Britain and defines a minimum apparent polar shift of 60° between late Ordovician and Upper Llandovery times. Reference to absolute age dates suggests that this shift took place between ca. 447 and 434 m.y. followed by slight polar movement between ca. 434 and 394 m.y.  相似文献   

12.
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   

13.
Of 16 sites collected in the Taru grits (Permian) and Maji ya Chumvi beds (Permo-Triassic) of East Africa only 6 sites from the Maji ya Chumvi sediments gave meaningful palaeomagnetic results. After thermal cleaning the 6 sites (32 samples) give an Early Triassic pole at 67°N, 269°E with A95 = 17° in excellent agreement with other African Mesozoic poles. There are now 26 Mesozoic palaeomagnetic poles for Africa from widely diverse localities ranging in present latitude from 35°N to 30°S. The poles subdivide into Triassic (17 poles) and Cretaceous (9 poles) groups whose means are not significantly different. The palaeomagnetic pole for Africa thus remained in much the same position for 170 m.y. from Early Triassic to Late Cretaceous. The data form an especially good set for estimating the palaeoradius using Ward's method. Values of 1.08 ± 0.15 and 1.03 ± 0.19 times the present radius are deduced for the Triassic and Cretaceous respectively with a mean value of 1.08 ± 0.13 for all the Mesozoic data combined. The analysis demonstrates that hypotheses of earth expansion are very unattractive.  相似文献   

14.
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   

15.
A palaeomagnetic pole is established at 25.1°N 273.9°E (dp = 10.6°, dm = 14.3°) from the norite-charnockite complex at Angmagssalik, emplaced at 1800 Ma. A somewhat older palaeomagnetic pole at 4.2°S 246.7°E (dp = 4.2°, dm = 8.3°) is obtained from Archaean gneisses close to the northern boundary of the Nagssugtoqidian mobile belt; reversals of magnetization are present here. Both magnetizations were imposed during slow cooling following the (late) Nagssugtoqidian metamorphism.In general the gneisses, dyke amphibolites and granite of the Nagssugtoqidian mobile belt are unstably magnetized; their magnetization is attributable to the Earth's present field, and is often extremely weak.A pseudotachylyte within the Archaean gneisses has had a long cooling history. A fragment of the remanence reflects the magnetization characteristic of the Archaean gneisses, whereas most of the magnetization corresponds to a palaeomagnetic pole near that of the Angmagssalik complex. The pseudotachylyte is much older than its magnetizations.An apparent polar wander path is presented for Greenland at ca. 1750 Ma based on the above results and data from west Greenland.  相似文献   

16.
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   

17.
Benioff's suggestion that the 58-min period sinusoidal oscillation found on a Pasadena strain seismogram after the Kamchatka earthquake of November 4, 1952 may represent the earth's gravest normal mode is re-examined in terms of a slow large-scale post-seismic deformation. The mechanism and the seismic moment of the main shock of the Kamchatka earthquake are determined by using the amplitude and the initial phase of G2 and R2 recorded at Pasadena and R6 recorded at Palisades. By constraining the dip angle and the strike of the fault at 30° (towards NW) and N34°E, respectively, on the basis of the geometry of the Benioff zone, the slip angle is determined as 110° which represents 74% thrust and 26% right-lateral faulting. The direction of the slip angle agrees with the slip direction of the Pacific plate. A seismic moment of 3.5 · 1029 dyn cm is obtained. If a fault area of 650 · 200 km2 is assumed, an average dislocation of 5 m is obtained. Spectral analyses of the Pasadena strain records show that the 58-min sinusoidal oscillation in fact consists of a spectral peak near 54 min which is very close to the 0S2 mode and other high-frequency peaks which can be correlated to the earth's normal modes. The records from two independent recording galvanometers correlate with each other very well, indicating that the recorded oscillation represents a real strain and not instrumental noise. The phase relation between the NS and EW components is consistent with the strain field associated with 0S2 mode. Although these results provide positive evidence for a slow post-seismic deformation, the cause of the abrupt termination of the oscillation and the excitation mechanism remain unresolved.  相似文献   

18.
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   

19.
Four mafic and two dioritic igneous bodies intruding along the flanks of the Charlotte belt, within the King's Mountain belt and near the Slate-Kiokee-Charlotte belt boundaries in the South Carolina Piedmont, were studied paleomagnetically. The results suggest that these mafic rocks with a single characteristic magnetization are broadly contemporaneous. A mean paleomagnetic pole position of 38.9°N 120.8°E has been calculated for the six bodies. This pole position falls near the 300 Ma old point of Irving's apparent polar wander path (APWP) for North America. The α95 circle of confidence (10.2°) includes points on the APWP between 250–360 Ma. Anomalously old KAr apparent ages, greater than one billion years, are suggestive of excess 40Ar contamination for the mafic Buffalo pluton, whereas apparent ages in the range of 360–395 Ma old are interpreted as a maximum age for the other bodies, due to the possibility of small amounts of excess 40Ar being present. A 10° westerly tilt correction suggested by Dooley and Smith for early Mesozoic diabase brings the mafic pole position of this study to more nearly coincide with the 350 Ma virtual geomagnetic position of Irving's curve, but the test is inconclusive, awaiting better definition of radiometric ages. The simplest interpretation of the data is that the mafic pole position reflects the direction of the geomagnetic field in late Devonian Carboniferous times. The similarity of this pole position with the points on the APWP for North America provides little evidence for displaced terrains and, with the precision of this pole position, horizontal displacements on the order of that suggested by the Consortium for Continental Reflection Profile (COCORP) results, or by subsequent studies, are not detectable. This and other pole positions from granitic rocks in the southern Appalachian orogen suggest that if displaced terrains exist, the evidence must be found in older rocks, or in other geologic belts.  相似文献   

20.
Palaeomagnetic study of Middle Liassic siltstones shows a stable magnetization with a mean direction of D = 12.3°, I = 64.6° (N = 60, k = 26, α95 = 3.9°) corresponding to a palaeomagnetic pole at 79.8°N, 125.6°E, similar to that for southern Germany and confirming predictions based on palaeogeographic reconstructions using North American data. Sideritic concretions of Lower Liassic age show a higher magnetic stability with a mean direction of D = 12.6°, I = 61.4° (N = 125, k = 50, α95 = 1.8°) which is not significantly different from the siltstones. This confirms the sedimentological evidence that suggests that such concretions grew very shortly after deposition, i.e. within the Liassic, and suggests that similar concretions of other ages could thus be used for palaeomagnetic studies. Although the Liassic palaeomagnetic pole (76.9°N, 134.7°E), based on this work, appears valid it is still not possible to evaluate a sensible Mesozoic polar wandering curve for the North Atlantic bordering continents.  相似文献   

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