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1.
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   

2.
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   

3.
Some 50 oriented samples (120 specimens) have been collected on eight sites of volcanic rocks from the Lower Devonian Dalhousie Group of northern New Brunswick and Devonian andesitic to basic dykes from central New Brunswick. Univectorial and occasional multivectorial components were extracted from the various samples. Results after AF and thermal demagnetization compare relatively well. In the volcanics and tuffs, two components of magnetization have been isolated: A (D = 33°, I = ?58°, α95 = 7.3°, K = 236) for four sites and B (D = 66°, I = +53°) for three sites. The grouping of component A is improved after tilt correction but the fold test is not significantly positive at the 95% confidence level. Component A is interpreted as being primary while component B is unresolved and appears to be the resultant magnetization of a Late Paleozoic and a recent component. The pole position obtained for tilt corrected component A is 268°E, 1°S, dp = 6.5°, dm = 8.8°. The paleolatitude calculated for component A is 39°S. The paleopole of in situ component A is located close to those of the Early-Middle Devonian formations from Quebec, New Brunswick and New England states while the paleopole of tilt-corrected component A is similar to Lower Devonian poles of rock units from the Canadian Arctic Archipelago. If component A is primary (as we believe it to be), then the western half of the northern Appalachians had already docked onto the North American Craton by Early Devonian time. Alternatively, if component A is secondary the same conclusion applies but the juxtaposition took place in Middle Devonian time.  相似文献   

4.
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   

5.
In the western part of the Gardar Igneous Province of southern Greenland, lamprophyre dykes intruded at ca. 1276-1254 m.y. RbSr biotite ages yield a palaeomagnetic pole at 206.5°E,3°N (nine sites, dψ = 5.1°, dχ = 10.1°) Slightly younger dolerite dykes with RbSr biotite ages in the range 1278-1263 m.y. give a pole at 201.5°E,8.5°N (24 sites, dψ = 4.7°, dχ = 9.4°), and the syeno-gabbro ring dyke of the Kûngnât complex (RbSr isochron age 1245 ± 17 m.y.) cutting both of these dykes swarms, gives a pole at 198.5°E, 3.5°N (four sites, dψ = 2.3°,dχ = 4.4°). All these rock units have the same polarity and the poles are identical to those from Mackenzie and related igneous rocks of North America (1280-1220 m.y.) after closure of the Davis Strait; they confirm that this part of the Gardar Province is a lateral extension of the Mackenzie igneous episode within the Laurentian craton.In the Tugtutôq region of the eastern part of the Gardar Province 47 NNE-trending dykes of various petrologic types, and intruded between 1175 ± 9 and 1168 ± 37 m.y. (RbSr isochron ages) yield a palaeomagnetic pole at 223.9° E, 36.4°N (dψ = 4.1°, dχ = 6.1°). Fifteen other dykes in this swarm were intruded during a transitional phase of the magnetic field which, however, does not appear to have achieved a complete reversal over a period of several millions of years. The majority of dykes studied are highly stable to AF and thermal demagnetisation and contain single high blocking temperature components with single Curie points in the range 380–560°C.Palaeomagnetic poles from the Gardar Province between ca. 1330 and 1160 m.y. in age define the earlier part of the Great Logan apparent polar-wander loop; they correlate closely with contemporaneous North American results and confirm the coherence of the Laurentian craton in Upper Proterozoic times.  相似文献   

6.
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   

7.
Paleomagnetic analyses of samples collected from a 500 m thick Jurassic section in the Pontides reveal the presence of two components of remanent magnetization: an unstable, low-temperature component which is removed during thermal demagnetization through 220°C and a dominant component which displays consistent directions through 650°. Curie point and IRM studies indicate that goethite is responsible for the low-temperature component whereas both magnetite and hematite contribute to the more stable component. The pole position determined from the stable magnetization is located at 18.8°N, 91.8°E (α95=7.7°, N=134) indicating that the section has undergone more than 90° clockwise rotation since the Late Jurassic. Ancillary geologic evidence, particularly the orientation of Jurassic facies belts is also consistent with a 90° clockwise rotation in this region of northwest Anatolia. The pole suggests that the section may also have migrated slightly northward. Although the age of these movements is currently unknow, it is proposed that they are principally related to the closure of the Neo-Tethys during the Late Cretaceous/Early Tertiary. Some of the rotation may be related to the right lateral movement along the North Anatolian Transform Fault which was initiated in the Miocene.  相似文献   

8.
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   

9.
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   

10.
The Mt. Stuart Batholith is a composite pluton of Late Cretaceous age that intrudes the crystalline North Cascades terrane of northwestern Washington. Its paleomagnetic direction (D=10.0°, I=45.5°; α95=4.9°) is markedly different from the direction expected for Cretaceous rocks from northwestern Washington (D=330.5°, I=73.0°), which means that the Mt. Stuart Batholith either has moved relative to the North American interior, or has been tilted through a substantial angle, or both, since it acquired its magnetism. Either tilt or translation are possible, geometrically, but translation is more likely, because: (1) local geology apparently does not support tilt in the required direction; (2) it probably is not possible for a panel of rock the size of the Mt. Stuart Batholith to tilt through the necessary angle (ca. 35°) after its deeper parts have cooled sufficiently to retain remanent magnetization; (3) the sense of paleomagnetic discordance found in the Mt. Stuart Batholith (clockwise-rotation of declination, flattened inclination) is identical to that found in nearly every other allochthonous terrane in the western North American Cordillera, including every other Cretaceous batholith studied except the Sierra Nevada.  相似文献   

11.
Paleomagnetic studies have been made of certain constituents of the Bay St. George sub-basin. Specifically, results are reported from the Spout Falls Formation (Tournaisian), the Jeffreys Village Member of the Robinsons River Formation (Visean), and the Searston Formation (Namurian-Westphalian). The following magnetizations have been isolated: Spout Falls A (Tournaisian) with D = 343.5°, I = ?22.7°, k = 61.2, α95 = 7.1° and the corresponding pole at 28.6°N, 139.5°E (4.5°, 8.5°); Spout Falls B (Kiaman) with D = 166.7°, I = 12.2°, k = 51.7, α95 = 10.7° and the corresponding pole at 34.5°S, 42.7°W (5.5°, 10.9°); Jeffreys Village A (Visean) with D = 351.2°, I = ?27.3°, k = 54.0, α95 = 7.6° and the corresponding pole at 26.5°N, 130.7°E (4.5°, 8.3°); Searston A (Namurian) with D = 161.7°, I = 11.7°, k = 107, α95 = 7.4° and the corresponding pole at 33.9°S, 37.2°W (3.8°, 7.5°); and Searston C with D = 111.6°, I = ?13.8°, k = 28.8, α95 = 14.5° and the corresponding pole at 19.6°S, 19.0°E (7.6°, 14.8°). After comparison with paleopoles of similar ages derived from eastern and western Newfoundland rocks, from constituents of the east coast basin and for interior North America, it is concluded that: (1) it is unlikely that any large scale relative motion took place since the Early Carboniferous between eastern and western Newfoundland; (2) it is unlikely that any north-south relative motion took place between the east coast basin and the Bay St. George sub-basin; and (3) the Bay St. George sub-basin results do not support the earlier proposed displaced terrane hypothesis of the northern Appalachians in as much as the motions during the Carboniferous are not supported. There is evidence of the northward motion of the Appalachians and North America as a whole during the Carboniferous. The magnetostratigraphic horizon marker in the Carboniferous separating a dominant normal and reversed magnetization on the older side and an entirely reversed (Kiaman) magnetization on the younger side may be placed in the Bay St. George sub-basin at the base of the Searston Formation.  相似文献   

12.
A paleomagnetic study was carried out on the mid-Cretaceous sedimentary strata in west-central Kyushu Island, southwest Japan, to elucidate the origin of sedimentary basins along the Asian continental margin in the Cretaceous. We collected paleomagnetic samples from a total of 34 sites of the mid-Cretaceous Goshonoura Group, shallow-marine clastic deposits in west-central Kyushu, and characteristic remanent magnetizations were recognized from 18 horizons of red beds. Thermal demagnetization has revealed that the red beds contain three magnetization components, with low (<240°C), intermediate (240-480°C), and high (480-680°C) unblocking temperatures. The low unblocking temperature component is present-field viscous magnetization, and the intermediate one is interpreted as chemical remanent magnetization carried by maghemite that was presumably formed by post-folding, partial oxidation of detrital magnetite. Rock magnetic and petrographic studies suggest that the high unblocking temperature component resides largely in hematite (martite and pigmentary hematite) and partly in maghemite. Because of the positive fold test, this high temperature component can be regarded as primary, detrital remanent magnetization. The tilt-corrected mean direction of the high temperature component is Dec=65°, Inc=63° with α95=5°, which yields a paleomagnetic pole at 39°N, 186°E and A95=8°. A combination of this pole with those of the Late Cretaceous rocks in southwest Japan defines an apparent polar wander path (APWP), which is featured by a cusp between the Late Cretaceous and the Paleogene. A comparison of this APWP with the coeval paleomagnetic pole from northeast Asia suggests an approximately 50° post-Cretaceous clockwise rotation and 18±8° southward drift with respect to northeast Asia. The southward transport of the Cretaceous basin suggests that the proto-Japanese arc originated north of its present position. We propose that the coast-parallel translation of this landmass was caused by dextral motion of strike-slip faults, which previous geodynamic models interpreted to be sinistral through the Mesozoic. The change in strike-slip motion may have resulted from Mesozoic collision and penetration of exotic terranes, such as the Okhotsk microcontinent, with the northeastern part of Asia.  相似文献   

13.
Two components of magnetization have been observed in fourty-four samples (five sites) of the anorthosites in the Arden Pluton. One component, withD = 325°,I = ?75°,k = 32, α95 = 13.6°, was isolated in many samples by progressive alternating field demagnetization and in the remainder of the collection by the use of intersecting great circles of remagnetization. The corresponding pole is located at 16°N, 303°E,dp = 22.7°,dm = 24.9°. Assuming the age of the last metamorphism (Taconic, ca. 440 Ma) of the Cambrian Arden Pluton to be the age of the magnetization, this pole deviates significantly from coeval poles thus far obtained from the North American craton. The preferred explanation for this deviation is that the Arden Pluton and the surrounding Piedmont rocks belonged to a different Early Paleozoic plate on the south or east side of the Iapetus Ocean, most likely the African (Gondwana) plate, and that it was transferred to the North American plate during a subsequent continental collision.  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.
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   

16.
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17.
A paleomagnetic study of about 95 samples from 16 sites sampled in the Early Cretaceous in Luanping basin in Hebei Province was reported. Stepwise thermal demagnetization was used to isolate magnetic components. Most samples have a characteristic direction with a high temperature component above 500°C. The tectonic-corrected data areD = 347.8°,I = 50.4°, α95 = 7.l°, and the corresponding pole position is at 76.1°N, 346.3°E,with dp =6.4°,dm = 3.8°, paleolatitude λ = 31.1°N. This result indicates a counterclockwise post-Cretaceous rotation of 30.7° ±9.8° with respect to the stable Ordos basin in the west of North China Block, and a non-significant northward motion. This rotation could be related to local fault action or structural detachment, or regional NNW-NWWward motion and collision of Kula-Pacific plate with eastern China since the Early Cretaceous.  相似文献   

18.
The mean palaeomagnetic pole position obtained from Upper Cretaceous rocks in west Sicily is at 21°N, 100°E (A95 = 15°), and at 38°N, 67°E (A95 = 31°) obtained from Middle Jurassic rocks. These pole positions are completely different from comparable pole positions for southeast Sicily and Africa and imply a clockwise rotation of west Sicily since the Upper Cretaceous of about 90° relative to southeast Sicily and Africa and also a clockwise rotation of about 60° relative to “stable” Europe. The sense of rotation of west Sicily is opposite to any known rotation of other crustal blocks in the central Mediterranean.  相似文献   

19.
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   

20.
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   

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