Continental drift during the Palaeozoic

Rapid polar shifts relative to Gondwanaland are identified in the Late Ordovician and Carboniferous. These shifts form part of the “Common polar wander path” interpretation of the palaeomagnetic poles for the Gondwana continents during the Palaeozoic. For western Europe a transition occurs between the Ordovician and Silurian poles, but is of smaller magnitude than the Late Ordovician Gondwana shift. Similarly Carboniferous shifts with respect to Europe and North America are smaller than the Gondwana shifts. A third shift in Europe is dated as mid-Devonian, but could be as old as Late Silurian, and has no counterpart in Gondwanaland. The differences in timing and magnitudes of these shifts provide evidence of the predominant role of continental drift rather than polar wandering. Attempts to explain the data exclusively in terms of polar wandering lead to geologically and geometrically untenable conclusions. Whilst there were one or perhaps two supercontinents in most of the upper Palaeozoic, it seems Laurasia was itself a set of separate fragments.     

A new palaeomagnetic result from East Africa and estimates of the Mesozoic palaeoradius

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.     

Palaeomagnetism and KAr age of Mesozoic and Cenozoic igneous rocks from Antarctica

A new analysis of palaeomagnetic data for igneous rocks from Deception Island, 25 de Mayo Island (King George Island) and Cape Spring, are given.K-Ar age determinations indicate that most of the igneous samples from 25 de Mayo Island included in the palaeomagnetic study are of Late Mesozoic/Early Tertiary age. The significance of these palaeomagnetic-radiometric data on the hypothesis of oroclinal bending of the Antarctic Peninsula and on the apparent polar movement of Antarctica is discussed.The positions of palaeomagnetic poles for the Andean igneous complex indicate that there has not been any apparent post-Late Cretaceous/Early Tertiary oroclinal bending in the Antarctic Peninsula from 74°S to 62°S.A comparison of the positions of palaeomagnetic poles for Antarctica and Australia suggests that the direction of apparent polar movement relative to Antarctica reversed after the Miocene.     

Palaeomagnetism and K-Ar ages of triassic igneous rocks from the Ischigualasto-Ischichuca Basin and Puesto Viejo Formation,Argentina

The palaeomagnetism of Middle Triassic (224 ± 5 m.y.) igneous rocks from the Ischigualasto-Ischichuca Basin (67°40′W, 30°20′S) was investigated through 86 oriented hand samples from 11 sites. At least one reversal of the geomagnetic field has been found in these rocks. Nine sites yield a palaeomagnetic pole at 239°E, 79°S (α₉₅ = 15°, k = 13).The K-Ar age determinations of five igneous units of the Puesto Viejo Formation give a mean age of 232 ± 4 m.y. (Early Triassic). The palaeomagnetism of six igneous units of the Puesto Viejo Formation (68°W, 35°S) was investigated through 60 oriented samples. These units, two reversed relative to the present magnetic field of the Earth and four normal, yield a pole at 236°E, 76°S (α₉₅ = 18°, k = 14).Data from the Puesto Viejo Formation indicate, for the first time on the basis of palaeomagnetic and radiometric data, that the Illawarra Zone, which defines the end of the Kiaman Magnetic Interval, extends at least down to 232 ± 4 m.y. within the Early Triassic. The palaeomagnetic poles for the igneous rocks of the Ischigualasto-Ischichuca Basin and Puesto Viejo Formation form an “age group” with the South American Triassic palaeomagnetic poles (mean pole position: 239°E, 77°S; α₉₅ = 6.6°, k = 190). The Middle and Upper Permian, Triassic and Middle Jurassic palaeomagnetic poles for South America would define a “time group” reflecting a quasi-static interval (mean pole position: 232°E, 81°S; α₉₅ = 4°, k = 131).     

Paleomagnetism of Late Cretaceous Poços de Caldas alkaline complex,Southern Brazil

The paleomagnetism of the Late Cretaceous Poços de Caldas alkaline complex (46.6°W, 21.9°S) was investigated through 42 oriented cores from seven sites. Six sites, reversed relative to the present magnetic field of the Earth, yield a pole at 127°W, 82°S (dp = 8°,dm = 13°). This pole is located close to other Late Cretaceous poles for South America obtained by Creer [1] from untreated paleomagnetic samples. The results are significantly different from those for the nearby Early Cretaceous Serra Geral basalt but close to the Triassic pole for South America. The polar wandering path for South America for the Mesozoic seems to be more complicated than anticipated. The available paleomagnetic information may not yet be precise enough to determine the time of opening of the Atlantic.     

华北蓟县中、上元古界古地磁研究

本文论述了华北蓟县中、上元古界地层标准剖面古地磁采样及实验室技术,采样地层的磁性特征,多磁成分的分析和测试结果.主要讨论了蓟县中、上元古界地层所代表的古地磁极性、极移路径和古纬度,并与北美大陆及华南(扬子)地块该时期的极移路径进行对比.     

Aspects of Caledonian palaeomagnetism and their tectonic implications

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.     

Paleomagnetic results from the Late Carboniferous/Early Permian Casper Formation: implications for northern Appalachian tectonics

Paleomagnetic samples were collected from 190 m of the Late Carboniferous/Early Permian Casper Formation in southeastern Wyoming. A total of 549 samples was drilled near the vicinity of Horse Creek Station at an average stratigraphic interval of 33 cm. All samples were reversely magnetized. Rock magnetic analyses indicate that the primary carrier of remanence in the formation is hematite. A selection criterion applied to the partial demagnetized data restricted the sample population to 233, resulting in a paleomagnetic North Pole located at 47.4°N, 127.4°E (δ_p=0.7;δ_m=1.4). The Casper pole agrees well with other Late Carboniferous/Early Permian poles for cratonic North America. The tight clustering of these paleomagnetic poles suggests that little apparent polar motion with respect to North America occurred during this time. Comparing the stable North American poles with paleomagnetic poles from Late Carboniferous/Early Permian strata of the New England-Canadian Maritime region (Acadia) indicates that this region did not reach its present position relative to North America until at least the Early Permian.     

Definition of pre-2000 m.y. apparent polar movements

In this paper palaeomagnetic poles known to be older than 2000 m.y. in age are assessed in the context of a continental reconstruction derived from younger Precambrian palaeomagnetic results. It is found that the combined data from North America and Africa define a single apparent polar wander path during the intervals 2700-2500 m.y. and 2160-2000 m.y. using the same continental reconstruction as that derived from younger poles. A rapid polar shift is identified at ca. 2150 m.y. and a closed loop is present in the curve between 2160 and 2000 m.y. Palaeomagnetic results from the Rhodesia/Kaapvaal, Kasai, West Africa and North America (Slave and Superior) cratons define segments of this loop which are statistically identical within errors of the pole positions and their assigned ages.These results in common with younger Proterozoic data (2000-800 m.y.) confirm that the crust behaved as an integral unit during these times, although undergoing internal deformation along mobile zones which has not yet proved detectable by the palaeomagnetic method. The 2700-2000 m.y. interval includes the Archaean-Proterozoic transition during which major structural anisotropy began to be imprinted on the sialic crust. Tectonic straight belts and deformed anorthosites lie within a single great-circle belt on the continental reconstruction incorporating Gondwanaland and North America. This same belt later developed into an arc of major tectonic and magmatic activity (<2250 m.y.) including massive anorthosites, rapakivi granites, acidic volcanism and mobile belts.     

Implications of a combined biostratigraphic and palaeomagnetic study of the Umbrian Maiolica Formation

The study of a 275.5 m thick section of white, pelagic limestones occupying the valley of the Fonte del Giordano river on the southern slope of Mt. Montagnola has yielded a biostratigraphically controlled clear magnetic reversal pattern after thermal cleaning. The magnetic stratigraphy of the lower 131 m of the section (Calpionellid zones) is correlatable with the M-sequence of oceanic magnetic anomalies between M-19 and M-14. The reversal stratigraphy of the upper 81.5 m of the section (Radiolaria zone) has also been tied to the oceanic polarity time scale by making linear interpolation for a missing 63 m thickness underneath it.Besides the Fonte del Giordano section two Berriasian outcrops each with a different bedding attitude were studied at Gubbio and near Cagli for tectonic tilt test giving positive results. The mean palaeomagnetic pole position for the Late Jurassic/Early Cretaceous after bedding correction is: Φ = 19.1°, Λ = 288.2°, k = 148.7, α₉₅ = 10.2° (N = 3), confirming the presence of a large swing in the polar path, a common behaviour of apparent polar wandering for the peri-Adriatic region during this time.     

Paleomagnetic poles and polarity zonation from Cambrian and Devonian strata of Arizona

Basal Paleozoic Tapeats Sandstone (Early and Middle Cambrian) in northern and central Arizona exhibits mixed polarity and a low-latitude paleomagnetic pole. Carbonates of Middle and early Late Cambrian age, and directly superposed carbonate and carbonate-cemented strata of latest Middle(?) and early Late Devonian age, are characterized by reversed polarity and high-latitude poles. The high-latitude Middle Cambrian pole, which appears to record a large but brief excursion of the polar wandering path, is considered provisional pending additional work. The Devonian data from Arizona indicate that a shift of the pole to a “late Paleozoic” position had occurred by Middle Devonian time.     

Palaeomagnetism and palaeogeography of Mongolia from the Carboniferous to the Cretaceous—final report

The present study summarizes the results of palaeomagnetic investigations of Carboniferous to Cretaceous rocks with respect to global-tectonic interpretations of Eurasia. Normal and reverse directions of stable remanence components were found for the Carboniferous to Cretaceous rocks within single outcrops, sometimes even within individual samples. Triassic, Permian and Carboniferous pole positions for Mongolia are strikingly different from those of the Siberian platform. The conclusion about the different palaeogeographical development of Mongolia with respect to the Siberian platform is well seen from differences in palaeolatitudes (up to 50°) and palaeorotations (up to 150°). The apparent polar wander paths for Mongolia and the north China block are almost identical since the Carboniferous. This way, palaeomagnetic and palaeogeographical data are contributing to the global-tectonics model of the regions under study. During the Carboniferous, Permian and Triassic, the regions of Mongolia investigated were not part of Eurasia. Together with the north China block, they show a palaeogeographical affinity with the Pacific Plate.     

Cenozoic paleomagnetic results and phanerozoic apparent polar wandering path of Tarim Block*

New paleornagnetic data from Cenozoic rocks in Tarim enable people to revise the Phanerozoic apparent polar wandering path (APWP) of this block. This modified Tarim APWA is supported by data from other Chinese blocks. On the basis of the APWA, it is concluded that Tarim rode on a plate subducting under the Kazakhstan plate between Carboniferous and Permian time. By the Late Permian, subduction had finished. The APWP also revealed that tectonic evolution of the Tarim was characterized by northern latitudinal displacement during the Paleozoic time, while Tarim remained at relative low latitude (about 20°) until1 Cretaceous.     

Palaeomagnetism of the Msissi norite (Morocco) and the Palaeozoic reconstruction of Gondwanaland

Palaeomagnetic results are reported from eight sites in an Upper Devonian basic intrusion (the Msissi norite) in southeast Morocco. Specimens from one site are suspected of having been affected by lightning, but results from the other seven sites indicate the presence of a less-stable component, probably of viscous origin. The pole position corresponding to the stable component(0.5°S, 25°E, A₉₅ = 16.5) is interposed between the Middle Cambrian/Ordovician pole and the Lower Carboniferous pole on the African polar wander curve. When the southern continents are reassembled on the Smith/Hallam reconstruction of Gondwanaland the new Moroccan Devoniån pole is in excellent agreement with the corresponding portion of the main Australian polar-wander curve. This places additional constraints on the possible date of fusion of the separate Southeast Australian plate with the rest of Gondwanaland, postulated recently on palaeomagnetic grounds by M.W. McElhinny and B.J.J. Embleton (1974). The combined African/Australian polar-wander curve is compared with the South American curve, and two possible interpretations of available data are discussed, one involving possible relative tectonic motion between South America and the rest of Gondwanaland during the Lower and Middle Palaeozoic, and the other, favoured here, requiring a reassessment of the ages of several South American pole determinations.     

Paleomagnetism of the western Cape Fold belt, South Africa, and its bearing on the Paleozoic apparent polar wander path for Gondwana

In order to test two different proposals for the poorly defined African Paleozoic apparent polar wander path (APWP), a paleomagnetic study was carried out on Ordovician through Carboniferous clastic sediments from the Cape Fold belt, west of the 22nd meridian. One proposal involves a relatively simple APWP connecting the Ordovician Gondwana poles in North Africa with the Late Paleozoic poles to the east of South Africa in a more or less straight line crossing the present equator in the Devonian. The other proposal adds a loop to this path, connecting Ordovician poles in North Africa with poles to the southwest of South Africa and then returning to central Africa. This loop would occur mainly in Silurian time. New results reported herein yield paleopoles in northern and central Africa for Ordovician to lowermost Silurian and Lower to Middle Devonian formations. The best determined paleopole of our study is for the Early Ordovician Graafwater Formation and falls at 28°N, 14°E (k = 25, α₉₅ = 8.8°, N = 28 samples). The other paleopoles are not based on sufficient numbers of samples, but can help to constrain the apparent polar wander path for Gondwana. Our results give only paleopoles well to the north of South Africa and we observe no directions within the proposed loop. Hence, if the loop is real, it must have been of relatively short duration (60–70 Ma) and be essentially of Silurian/Early Devonian age, implying very high drift velocities for Gondwana (with respect to the pole) during that interval.     

Episodic accretion and plutonism in southwestern Alaska

Paleontologic and radiometric dating of the accretionary prism and magmatic arc of southwestern Alaska reveal an history of episodic accretion and plutonism. Possible accretion events in the Triassic (220-195 m.y.) and Early Jurassic (184-176 m.y.) were followed by Middle Cretaceous (108-83 m.y.), earliest Paleogene (65-60 m.y.), Middle Paleogene (50-40 m.y.), and Neogene (25-0 m.y.) accretion episodes. Plutonic events, which alternate with the accretion events, occurred in the Early Jurassic (193-184 m.y.), Middle/Late Jurassic (176-145 m.y.), Late Cretaceous/Early Paleogene (83-50 m.y.), and Late Paleogene (38-26 m.y.). Episodicity of accretion events is an apparent cause of incomplete stratigraphic records in the accretionary prism and forearc basin.     

A palaeomagnetic study of the coast-parallel Jurassic dyke swarm in southern Greenland

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.     

Palaeomagnetic study of late precambrian rocks of the midland craton of England and Wales

The Precambrian basement of the British region south of the Caledonian orogenic belt is only observed in a few small inliers; this paper reports a detailed paleomagnetic study of four of these inliers. The Stanner-Hanter amphibolitised gabbro-dolerite complex of uncertain age yields a mean direction of magnetisation D = 282°, I = 51° (15 sites,α₉₅ = 11.4°) after AF and thermal cleaning. Uriconian lavas and tuffs (～700-600 m.y.) of the Pontesford and Wrekin inliers require both thermal and AF cleaning for complete analysis of NRM. The former region (Western Uriconian) yields a mean of D = 136°, I = ?25° (6 sites,α₉₅ = 15.3°) and the latter region (Eastern Uriconian) a mean of D = 78°, I = 17° (9 sites, α₉₅ = 12.8°); the Eastern Uriconian shows a marked improvement in precision after a two-stage fold test, and the palaeomagnetic data suggest that some apparent polar movement took place between eruption of the two sequences. The Uriconian rocks in both areas were intruded by dolerites which yield a mean direction of magnetisation D = 72°, I = 54° (11 sites,α₉₅ = 13.2°).The collective data give palaeomagnetic poles related to Upper Proterozoic metamorphic episodes (Stanner-Hanter Complex and Rushton Schist) which are in close agreement with earlier studies of the Malvernian metamorphic rocks, and to the late Precambrian Uriconian volcanic/hypabyssal igneous episode. All of these magnetisations are probably confined to the interval 700-600 m.y., and are indicative of appreciable polar movement during this interval. The palaeomagnetic poles define an apparent polar wander path for this crustal block between Late Precambrian and Lower Cambrian times and show that cratonic Britain south of the Caledonian suture is unrelated to the Baltic Shield.     

Late Mesozoic palaeomagnetism from Spitsbergen; implications for continental drift in the Arctic

Thermal and alternating-current demagnetization combined with ore microscopy and measurements of the temperature dependence of saturation magnetization have been carried out on some Mesozoic, probably Cretaceous, basaltic lavas from two areas (Seidfjell and Sørlifjell) at Spitsbergen. The experimental studies suggest that the Seidfjell locality has undergone extensive oxidations, which resulted in remagnetization. The estimated palaeomagnetic pole for this area is 77°N 107°E, which suggests a remagnetization, probably some time in the Late Tertiary. On the other hand the experimental data from the Sørlifjell locality suggest that the magnetization is primarily of deuteric origin. The mean palaeomagnetic pole position for this latter formation is at 75°N 235°E, which is significantly different from previously published European Mesozoic data. However, closing the Neo-Arctic basin by rotating Spitsbergen towards the Lomonosov Ridge, makes the suggested Cretaceous pole coincide with poles of similar age from North-America. This suggests that the estimated pole from Sørlifjell is a good approximation for a Late Mesozoic palaeomagnetic pole for Europe and it also confirms that the process of continental separation in the Arctic has taken place in Tertiary time.     

Geodynamic evolution of Korea: A view

Abstract Evidence for South Korean Palaeozoic geodynamic evolution is restricted to the Ogcheon Belt, which is a complex polycyclic domain forming the boundary between the Precambrian Gyeonggi Block to the northwest and the Ryeongnam Block to the southeast. Two independent sub-zones can be distinguished: the Taebaeksan Zone to the northeast and the Ogcheon Zone sensu stricto. The Taebaeksan Zone and Ryeongnam Block display characteristic features of the North China palaeocontinent. This domain remained relatively stable during the Palaeozoic. In contrast, the Ogcheon Belt s. s. is a highly mobile zone that belongs to the South China palaeocontinent and corresponds to a rift that opened during the Early Palaeozoic. In lowermost Devonian times, the rift basin was closed and the Ogcheon Belt was structured in a pile of nappes. From the lack of suture in the Ogcheon Belt it can be inferred that the Gyeonggi Block belongs to the South China palaeocontinent. Thus, the boundary between the North China and South China blocks should be located to the north of Gyeonggi Block, that is, in the Palaeozoic Imjingang Belt. From the Middle Carboniferous, sedimentation started again on a weakly subsiding paralic platform located in the hinterland of the Late Palaeozoic orogen of southwest Japan. In the Late Carboniferous, increasing subsidence recorded extensional tectonics related to the opening of the Yakuno Oceanic Basin (southwest Japan). In the Middle Permian, the end of marine influences in the platform and emplacement of terrestrial coal measures, may be correlated with the closure of the oceanic area and subsequent ophiolite obduction. In Late Permian to Early Triassic times, the Honshu Block (the eastern palaeomargin of the Yakuno Basin) collided with Sino-Korea. Post-collisional intracontinental tectonics reached the Ogcheon Belt in the Middle Triassic (Songnim tectonism). Ductile dextral shear zones associated with synkinematic granitoids were emplaced in the southwest of the belt. In the Upper Triassic, the late stages of the intracontinental transcurrent tectonics generated narrow intramontane troughs (Daedong Supergroup). The Daedong basins were deformed during two tectonic events, in the Middle (?) and Late Jurassic. The Upper Jurassic to Lower Cretaceous basins (Gyeongsang Supergroup), that are controlled by left-lateral faults, may have resulted from the same tectonic event.