Palaeomagnetic Pole Position Research Articles

Palaeomagnetism of the Proterozoic Zig-Zag Dal Basalt and the Midsommers� Dolerites, eastern North Greenland

Palaeomagnetic investigations are reported from 24 sites in the Proterozoic Zig-Zag Dal Basalt Formation and 12 sites in the Midsominersϕ 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 A95= 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.

40Ar-39Ar incremental heating studies on the Tudor Gabbro, Grenville Province, Ontario: its bearing on the North American apparent polar wander path in late Proterozoic times

Summary. 40 Ar- 39 Ar incremental heating studies have been carried out on samples taken from the Tudor Gabbro, Grenville Province, Ontario. In an earlier K-Ar study, these rocks have yielded an isochron age of -700Ma together with very high initial argon ratios. Age spectrum plots on whole­ rock samples, in general, display a saddle-shaped character, with two of them exhibiting minima close to 700 Ma. No clear plateaus are observed for these rocks. A hornblende separate records the time - 1110 Ma at which the stock finally cooled through the - 590°C isotherm. The Tudor Gabbro was prob­ ably intruded into an area undergoing middle-amphibolite facies meta­ morphism about 1180 Ma ago. The age spectra of two whole-rock samples together with that of their plagioclase separates, suggest that the stock cooled to - 200-250°C at about 720 Ma. Slow cooling, averaging about 1 o C Ma - 1 is indicated for this section of the Grenville Province for the period 1100-700 Ma. If the age of the Tudor Gabbro's palaeomagnetic pole position is taken to be 720 Ma, the Hadrynian Track Hypothesis leads to very high polar wander rates of > 20 em a - 1 for the period 820-720 Ma. If this hypo­ thesis is rejected, the average drift rate for this period would be - 4 em a - 1 , in much better agreement with published values of - 5 em a -I for the period 1400-820Ma.

Palaeomagnetic study of some charnockites of the Palni Hills

The intensity of natural remanence magnetisation (NRM) is measured along the three mutually-perpendicular directions using an astatic magnetometer. The intensity of induced magnetisation is measured using an apparatus fabricated in the laboratory. The Koenigsberger ratio has been calculated for all the samples and the ratio has been used to test the stability of NRM in the samples. The mean direction of magnetisation is determined from the three components of the NRM intensity. The palaeomagnetic pole position of the samples is determined using the direction of magnetisation and the site location. An attempt has been made to fix the geological age of the charnockites using palaeomagnetic methods.

A robust least-squares Gondwanan apparent polar wander path and the question of palaeomagnetic assessment of Gondwanan reconstruction

Two hundred and nineteen palaeomagnetic results are combined in calculating Gondwanan apparent polar wander paths, their confidence intervals, and the residual mean square errors about the paths, for six reconstructions. A quantitative assessment of the six reconstructions of Gondwanaland is made on the basis of the consistency of palaeomagnetic pole positions using a new statistical technique. Smith and Hallam's [1] Gondwanan reconstruction is the most effective in accounting for the distribution of the palaeomagnetic data and is used in calculating our preferred apparent polar wander path.

Palaeomagnetic data for Permian and Triassic rocks from drill holes in the Southern Sydney Basin, New South Wales

A section 300 m thick across the Permian—Triassic boundary has been sampled in the Southern Coalfield of the Sydney Basin, New South Wales. 55 samples, mainly grey to drab sandstones, were collected from 9 diamond drill holes which penetrated the entire Narrabeen Group and the upper part of the conformably underlying Illawarra Coal Measures, as well as a sill emplaced into the coal measures. The samples included fully oriented cores. Additional reconnaissance samples from two further drill holes were also studied.Partial alternating field demagnetization and petrography indicate the magnetic remanence to be a stable DRM. Partial thermal demagnetization above 300°C or 400°C caused large increases in magnetic susceptibility. Partial chemical demagnetization did not cause significant changes in remanence directions.For the Coal Cliff Sandstone (basal Narrabeen Group, Triassic) the palaeomagnetic pole position (Normal) was calculated to be at 59°N 322°E (dp = 27°, dm = 29°), which agrees with previously published data. For the uppermost coal measures (Permian) the pole position was calculated as 58°N 340°E (dp = 09°, dm = 10°). Data for samples from the lower to middle coal measures yield a pole position which is between the new Permian—Triassic pole position and that for the underlying Middle Permian igneous rocks. The top of the Reversed “Kiaman Magnetic Interval” (Permian) may be near the Tongarra coal and Appin Formation boundary — (early) Late Permian.

Early palaeozoic palaeomagnetism in Australia I. Cambrian results from the Flinders Ranges, South Australia II. Late Early Cambrian results from Kangaroo Island, South Australia III. Middle to early-Late Cambrian results from the Amadeus Basin, Northern Territory

I. Cambrian results from the Flinders Ranges, South Australia A total of 460 samples from six sequences spanning the Cambrian succession of the Flinders Ranges (Adelaide “Geosyncline”, South Australia) has been analyzed through thermal demagnetization studies. All samples showed a recent field component, generally constituting more than 50% of the initial intensity, which in most cases was removed by 200–400°C. Two characteristic magnetic components have been identified: 1. (A) A secondary magnetic component of Cambro-Ordovician age (S-pole at 75.3°E 26.0°N, α 95 = 7.4°, N = 5 localities) interpreted as having been induced by thermochemical activity during a period of enhanced heat flux prior to the Late Cambrian-Early Ordovician diastrophistic phases of the Delamarian Orogeny. 2. (B) A primary magnetic component, which indicates rapid polar motion during the Early Cambrian and a much reduced polar motion during the Middle Cambrian. Representative palaeomagnetic pole positions for the primary component are: 1. (1) Basal Hawker Group (earliest Cambrian): S-pole at 2.3°E 26.7°S, d p = 8.1°, d m = 14.3°, N = 10 (sites). 2. (2) Billy Creek Formation— Wirrealpa Limestone— Aroona Creek Limestone (late Early Cambrian to early Middle Cambrian): S-pole at 20.1°E 37.4°S, d p = 7.2°, dm = 14.4°, N = 11(sites). 3. (3) Basal Lake Frame Group (Middle Cambrian): S-pole at 26.1°E 29.3°S, d p = 6.6°, d m = 13.1°, N =10 (sites). 4. (4) Pantapinna Formation (late Middle Cambrian?): S-pole at 29.2°E 36.4°S, d p = 8.4°, dm = 16.7°, N = 4 (sites). Available data suggest that deposition of the Lake Frome Group beds probably did not continue into the Late Cambrian. II. Late Early Cambrian results from Kangaroo Island, South Australia A total of 108 block samples from a late Early Cambrian red-bed sequence on Kangaroo Island (Adelaide “Geosyncline”, South Australia) has been analysed through thermal demagnetization studies. All samples contained a recent field component of considerable intensity. Two characteristic magnetic components have been identified: 1. (A) A secondary magnetic component of Late Cambrian—Early Ordovician age (S-pole at: 75.8°E 17.4°N, d p = 4.2°, d m = 1.9°, N = 54 specimens), attributed to thermochemical activity predating the main folding phases of the Delamarian Orogeny. 2. (B) A primary magnetic component corresponding to a S-pole position at 15.1°E 33.8°S (d p = 6.2°, d m = 12.3°, N = 16 sites). Both the primary and the secondary magnetic component are in very good directional agreement with the magnetization pattern from the correlated Billy Creek Formation of the Flinders Ranges (I). Consequently, noticeable rotational movement since late-Early Cambrian times between Kangaroo Island and the northwestern part of the Adelaide “Geosyncline” can be ruled out. III. Middle to early-Late Cambrian results from the Amadeus Basin (Northern Territory) A total of 328 samples from a Middle Cambrian red-bed succession and a Middle to early-Late Cambrian carbonate succession in the Amadeus Basin (Central Australia) have been analyzed through thermal demagnetization studies. All samples contained a recent field component of considerable intensity which became broken down, respectively below 200°C in the carbonate samples and between 300°C and 500°C in the red-bed samples. Another recent field component, broken down between 600°C and 675°C, was noted in some of the red-bed samples. Three characteristic magnetic components have been identified: 1. (A) A secondary magnetic component of Late Devonian—Early Carboniferous age (S-pole at 110.5°E 46.9°S, N = 2 localities) which predates the main folding phase of the Early Carboniferous Alice Springs Orogeny. 2. (B) Another secondary magnetic component (S-pole at 60.8°E 33.8°N, N = 2 localities) which is very similar to a thermo-chemically induced Cambro-Ordovician magnetic component, noted in rocks from the Adelaide “Geosyncline”. 3. (C) A primary magnetic component which suggests reduced polar motion during the Middle Cambrian (S-pole at 16.6°E 35.0°S, α 95 = 7.4°, N = 5 sites) and rapid polar motion during the early-Late Cambrian (S-pole at 34.9°E 14.9°N, d p = 6.3°, d m = 11.6°, N = 9 specimens).

The palaeomagnetism of Cambro-Ordovician redbeds, the Erquy Spilite Series and the Tregastel-Ploumanac'h granite complex, Armorican Massif (France and the Channel Islands)

Summary. In addition to a component (A) of recent origin, two NRM components are distinguished in the Cambro-Ordovician redbeds of the Armorican Massif. In most sites other than those from northern Brittany the oldest (C) is probably Silurian or early Devonian, and is mainly carried by specularite with high blocking temperatures. This component was variably overprinted by a Devonian or early Carboniferous component (B3) which was probably acquired as a viscous PTRM on uplift after burial, and is carried by hematite pigment with intermediate to high blocking temperatures. In the red succession of Plourivo-Brehec (northern Brittany) declination scatter of two intermediate to high blocking temperature components (B1 and B2) is consistent with clockwise rotation of the bulk of Europe during the late Carboniferous, implied independently by published European Carboniferous palaeomagnetic data. Stable NRM in the Erquy Spilite Series yields a palaeomagnetic pole at 344 E, 35 N (dp = 21, dm = 22), and was probably acquired during remagnetization following Late Precambrian or early Cambrian folding. This is consistent with a middle to late Cambrian age of remagnetization estimated by comparison with other poles of known age. A palaeomagnetic pole position at 332 E, 34 S (dp = 4, drn = 7) determined for the Hercynian Trbgastel-Ploumanac'h complex is consistent with other middle to late Carboniferous poles from elsewhere in Europe.

An analysis of the palaeomagnetic field

The paper is devoted to a critical evaluation of the hypothesis of the prevalent character of the palaeomagnetic field in the course of the whole Phanerozoic. The study ties in with the synthetic processing of all so far published palaeomagnetic data for Eurasia and Africa respecting some of the models ensuing from the concept of plate tectonics. Initial material for testing the dipolar hypothesis was obtained by statistical processing of the palaeomagnetic pole positions and by analyzing the values of the so-called palaeorotation, the palaeoinclination difference and of palaeodivergence. It was shown that, by defining tectonically stable blocks (plates, palaeoplates) within the scope of the Eurasian continent and within the scope of the African continent as a whole, data were obtained bringing evidence of the predominating dipolar character of the mean geomagnetic field for the period from the Quaternary to the Cambrian, with the exception of the Silurian which has not been studied sufficiently. The deviations of the palaeomagnetic directions from the direction of the dipolar field within the scope of the stable blocks have the character of a Gaussian distribution and the standard deviations only amount to a few degrees.

The origin of lunar palaeomagnetism

THE new determination of magnetic field anomalies over part of the Moon's surface from Apollo 15 and 16 subsatellite magnetometer data has recently been interpreted1 in terms of magnetised areas of the lunar surface. I show here that, from this analysis, palaeomagnetic pole positions can be calculated and that these are so clustered on the lunar surface that there is evidence against meteoritic or cometary processes as explanations of the remanent magnetisation of the Apollo rocks. I conclude, therefore, that the Moon had a magnetic field in its early history produced by dynamo processes in a fluid electrically conducting core.

Palaeomagnetism of alkaline lava flows from El Salto-Almafuerte, C�rdoba Province, Argentina

Stable components of magnetization have been isolated in 15 lava flows (mean K-Ar age 123 ± 4 Myr) from the alkaline sequence outcropping at El Salto-Almafuerte, Province of Córdoba, Argentina. Magnetic and geologic stratigraphy, as well as K-Ar ages indicate that this sequence was probably extruded in the Lower Cretaceous during the first volcanic cycle of the Sierra de los Cóndores Group (Vulcanitas Cerro Colorado Formation). The palaeomagnetic pole-position for El Salto-Almafuerte lava flows, computed from the mean of 15 virtual geomagnetic poles and denoted SAK7, is: 25° E, 72° S (k= 35, α95= 6.5°); it is fairly close to other Lower Cretaceous palaeomagnetic poles for South America. The elongated distribution of Cretaceous palaeomagnetic poles suggest recurrent drift for South America in early Cretaceous time. The palaeomagnetic and radiometric data for the igneous rocks from El Salto-Almafuerte support the magnetic reversal time-scale for the early Cretaceous suggested by oceanic magnetic lineations.

Tectonic implications of a new palaeomagnetic result from the Jurassic of the Transdanubian Central Mountains

The analysis of the natural remanent magnetization (N.R.M.) of the Jurassic limestones from the Transdanubian Central Mountains (T.C.M's) (6 sites, 137 samples) resulted in the following palaeomagnetic pole position: ф = 54°, λ = 289° with α 95 = 11°. This result is explicable from two entirely different points of view as follows: 1. (1) The T.C.M's may always have formed part of stable Europe but rotated 73° counterclockwise about a vertical axis relative to it. 2. (2) The T.C.M's may have formed part of the African plate in the Adriatic region, shared the movements of Africa relative to stable Europe until when rotated to some degree counterclockwise about a vertical axis.

Palaeomagnetism of limestones from the Gargano Peninsula (Italy), and the implication of these data

The platform limestones of Apulia are usually too weakly magnetic for precise measurement. The East Gargano basin is an autochthonous extension of Apulia and incorporates deeper water limestones which, though weakly magnetic (J NRM ≃ 50 nG), can be reliably measured using a cryogenic magnetometer. The magnetization is attributed to the presence of detrital magnetite and the pelagic limestones yield a mean magnetic direction for the Late Cretaceous (Dec. = 335°, Inc. = 38°, α95= 6.5°). The circle of confidence associated with this direction overlaps with those associated with Late Cretaceous magnetic directions from Iblei (Sicily) and from the Vicentinian (Southern) Alps. Palaeomagnetic pole positions for Iblei, Gargano/Apulia and the Southern Alps indicate that these three autochthons need not have rotated significantly relative to each other since the Late Cretaceous. An inferred Late Cretaceous pole position for Africa coincides with the pole positions obtained from these Italian data.

Palaeomagnetism of 4 dolerite dikes around Listed, Bornholm (Denmark)

Four dolerite dikes from the island of Bomholm have been investigated with the purpose of determining the palaeomagnetic pole positions and the magnetic ages, and to contribute to the general knowledge of the geological development of Bomholm. A total of 137 specimens were cored from 67 oriented hand samples from the dikes. After a.f. demagnetizations up to 1800 oe, three significantly different stable mean directions were isolated. The poles and Fisher statistics are for the Vaseaa dike l6°S, 128°E, N = 11, cns = 9°; for the Listed dike 14°N, 250°E, N= 32, "'" = 4°; and for the Vigehavn dike 15°N, 159°E, N = 7, "'" = 6°, while the Bolshavn dike gives two directions, a remagnetized direction A with a pole position of 13° N, 153° E, N = 9, aos = 9°, and a 'ghost' (supposed original) direction B with pole 21°N, 224°E. A comparison with known palaeomagnetic poles from Northwestern Europe and North America suggests that the palaeomagnetic data may be interpreted as follows: After cooling of the Svaneke granite the Vaseaa dike was probably injected around 1200 Myr. Some 200 Myr later the fault system was reactivated and injection of the Listed and Bolshavn dikes took place. Finally, the same fault system was reactivated once again in lower to mid-Palaeozoic time and the Vigehavn dike was intruded; at the same time the Bolshavn dike was partially remagnetized leaving only weak traces of the original direc­tion of magnetization. During all the time covered, Bomholm was at fairly low latitudes, between 7° and 26°. By a reconstruction of the apparent polar wander curve it is argued that the polarity should be reversed for poles older than 1200 Myr to give a better fit of known data.

Palaeomagnetism of the giant dykes of Tugtutoq and Narssaq Gabbro, Gardar· Igneous Province, South Greenland

The earliest phases of the last major episode of magmatism within the Precarnbrian Gardar Igneous Province of South Greenland are NE-SW trending giant dykes. Palaeomagnetic samples from seven localities within the Hviddal syenite giant dyke (Rb-Sr whole rock isochron 1175±9 m.y.) yield a palaeomagnetic pole at 215°E, 33°N (k = 38, A,s = 9.9"J. Thirteen sites from gabbro giant dykes (<1175 m.y., >1168m.y.) give a palaeomagnetic pole at 226°E, 42°N (k = 30, A,s = 9.5°), and four sites from the Narssaq gabbro of comparable age give a pole at 225°E, 32°N (k= 97, A,s = 9.9"). All sites have the same polarity although eight additional sites suggest that a secondary intermediate direction was occupied by the magnetic field at intervals with comparable intensity to the main field. The palaeomagnetic pole positions define an easterly apparent polar movement and are consistent with poles of similar age from the Superior craton of North America on the pre-drift reconstruction.

Palaeomagnetic results from the Borrowdale Volcanic Group, English Lake District

Summary Palaeomagnetic results are reported from 47 sites within the Borrowdale Volcanic Group of late Llanvirnian–Llandeilian age. The mean stable remanence direction is D= 329°, I= - 49° (α95= 7°) after alternating field demagnetization and correction for tectonic dip. The corresponding palaeomagnetic pole position is 0°, 203° E. This pole differs from both older and younger Lake District Ordovician poles but the differences are more likely to arise from local geological complexity, sampling problems and the limitations of available statistical methods than from true polar wander. It is suggested that no systematic polar movement relative to the British Isles can be inferred during Ordovician time. An overall Ordovician pole position for the British Isles is calculated from the present result and eight others to be in the vicinity of 7°s N, 190°E (α95= 9°).

The non-uniqueness of the Australian Mesozoic palaeomagnetic pole position

Summary Palaeomagnetic results for the Jurassic obtained from the Tasmanian dolerites, the Gingenbullen dolerite, the Prospect dolerite, the Gibraltar syenite (all previously examined over a decade ago) and the Garrawilla volcanics, the Nombi extrusives and the Glenrowan intrusives (representing a new study) are presented. It appears that for some rock formations early experimental techniques failed to remove completely secondary components. Evidence suggests that a particularly harsh period of weathering (probably during the Late Cretaceous/Early Tertiary) may be partly responsible for secondary components. Rotational remanent magnetization (RRM) has also been detected and eliminated during alternating field (AF) demagnetization procedures. The results from the Tasmanian doleraites are in close agreement with the previously published results, while those from the other reinvestigations are seen to be markedly different from the early results. In the light of these and other recent data, a new interpretation of the Australian Mesozoic palaeomagnetic pole position is given. The mean Triassic (52° S, 153° E), Jurassic (47° S, 176° E) and Cretaceous (53° S, 152° E) pole positions are now considered as separate. On the basis of the palaeomagnetic data, a spread in age for the Tasmanian dolerite is suggested and the age of the breakup of eastern Gondwanaland is constrained between 100 and 160 My ago. The Australian Jurassic pole position is compared to the Late Triassic–Middle Jurassic palaeomagnetic poles from other southern lands and India in their predrift configuration as dictated by geomorphological reconstructions. It is shown that these poles form a tight group around their mean pole position at 63° S, 71° E (A95= 3°) with respect to present day Africa.

Precambrian palaeomagnetic results compatible with contemporary operation of the Wilson cycle

Sutures marking zones along which continental blocks have collided can be recognized on geological evidence in rocks with ages from Archaean onward. The existence of these sutures leads to the conclusion that the Wilson cycle of ocean opening and closing has been in operation since early Precambrian times. We have used published palaeomagnetic pole positions to test the validity of this conclusion by plotting APW (apparent polar wander) paths for suture-bounded blocks. We find that APW paths produced in this way fit the palaeomagnetic data better than APW paths for present-day continents or Pangea. Previous suggestions based on the interpretation of palaeomagnetic data that plate tectonics did not operate between about 2500 and 600 m.y. ago therefore appear invalid.

Palaeomagnetism of Cretaceous and Jurassic volcanic rocks in west Sicily

The mean palaeomagnetic pole position obtained from Upper Cretaceous rocks in west Sicily is at 21°N, 100°E ( A 95 = 15°), and at 38°N, 67°E ( A 95 = 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.

Palaeomagnetism of the Svecofennian Loftahammar gabbro and some Jotnian dolerites in the Swedish part of the Baltic Shield

The Svecofennian Loftahammar gabbro (RbSr isochron age 1,694 · 10 6 year) and Jotnian dolerites (KAr isochron age 1,245 · 10 6 year) in Sweden were found to have palaeomagnetic pole positions in close agreement with poles from other Svecofennian rocks in Sweden and Jotnian dolerites in Finland, respectively. These results support Neuvonen's apparent polar wandering path during the period from 1,900 · 10 6 to 1,300 · 10 6 year. A reconstruction of the position of the Baltic Shield during the (1,900-1,200) · 10 6-year interval, based on available pole positions, indicates that the Baltic shield was close to Greenland and the North American Shield and reveals a contiguity of North Atlantic geologic-geochronological zones until 1,200 · 10 6 year ago.

The Palaeomagnetism of Late Cenozoic Volcanic Rocks from Kenya and Tanzania

Summary Palaeomagnetic results from 246 sites in the East African Cenozoic volcanics are reported. Palaeomagnetic pole positions are obtained for different age ranges as follows: 0-1-8 My, 104 E, 89 N, A,, = 3 (54 sites); 1-8-7.0 My, 148 E, 87 N, A,, = 2 (102 sites); around 12 My, 187 E, 87 N, A,, = 6 (22 sites); around 17 My, 163 E, 85 N, A,5 = 2 (62 sites). Apart from the youngest these poles show a far-sided and righthanded tendency which is ascribed to a late Tertiary offset dipole field rather than to polar wander. For at least the past 7 My and possibly for the past 13 My, secular variation in East Africa has been large and in accord with model predictions for near equatorial sites. This contrasts with low secular variation observed in Iavas younger than 5 My from the Hawaiian Islands.