Cambro-Ordovician palaeomagnetic and geochronologic data from southern Victoria Land, Antarctica: revision of the Gondwana apparent polar wander path

We present new palaeomagnetic and isotopic data from the southern Victoria Land region of the Transantarctic Mountains in East Antarctica that constrain the palaeogeographic position of this region during the Late Cambrian and Early Ordovician. A new pole has been determined from a dioritic intrusion at Killer Ridge (⁴⁰Ar/³⁹Ar biotite age of 499 ± 3 Ma) and hornblende diorite dykes at Mt. Loke (21°E, 7°S, A 95 = 8°, N = 6 VGPs). The new Killer Ridge/Mt. Loke pole is indistinguishable from Gondwana Late Cambrian and Early Ordovician poles. Previously reported palaeomagnetic poles from southern Victoria Land have new isotopic age constraints that place them in the Late Cambrian rather than the Early Ordovician. Based upon the new palaeomagnetic and isotopic data, new Gondwana Late Cambrian and Early Ordovician mean poles have been calculated.     

New palaeomagnetic result from Vendian red sediments in Cisbaikalia and the problem of the relationship of Siberia and Laurentia in the Vendian

A palaeomagnetic study of Vendian red sediments from the Lena River section on the western margin of Lake Baikal in the region of Cisbaikalia (54°N, 108°E) has isolated a stable remanence with direction D = 296.3°, I = −27.7° (high-temperature component) and a corresponding pole of 2.7°S, 168.2°E. The primary nature of this remanence is confirmed from a positive fold test, dual polarities and the presence of detrital haematite. This result, together with all late Precambrian–Early Cambrian palaeomagnetic data from Siberia, indicates that Siberia occupied low latitudes during that time. It has been proposed on the basis of palaeomagnetic data that Laurentia occupied high latitudes during the Vendian, so it would appear that there cannot have been any Laurentia–Siberia connection at that time. A review of Vendian to Cambrian Laurentian palaeomagnetic data shows that such an interpretation is ambiguous. An alternative interpretation places Laurentia in low latitudes and confirms the Laurentia–Siberia fit of Hoffman (1991 ) and Pelechaty (1996 ). However, the lack of Late Vendian palaeomagnetic data for Siberia still allows the possibility that it could have occupied high latitudes during that time.     

An objective method for smoothing palaeomagnetic data

Summary. This paper describes a new method of smoothing noisy data, such as palaeomagnetic directions, in which the optimum degree of smoothing is determined objectively from the internal evidence of the data alone. As well as providing a best-fitting smooth curve, the method indicates, by means of confidence limits, which oscillations or fluctuations in the fitted curve are real. The procedure, which is illustrated by an analysis of palaeomagnetic decimation directions from Lake Windermere, has potential applications throughout the Earth Sciences. It may be used in any investigation requiring the estimation of a smooth function from noisy data, provided certain basic assumptions are reasonably satisfied.     

An investigation of the magnetic variation within two recent lava flows

Samples collected in vertical sections from two recent (1169 AD and 1971 AD) lava flows show significant variations in both the palaeomagnetic vector and the rock magnetic characteristics. The age and the short cooling time of the flows (both are less than 3 m thick) are such that the observed range of palaeomagnetic variation is more than can reasonably be explained by secular variation. The variations in palaeomagnetic and rock magnetic characteristics appear in part to be a response to the relative proportions of two coexisting ferrimagnetic phases, one that displays a primary coarsegrained and Ti-rich composition and a second that has finer-grained, Ti-poor characteristics. The use of AF demagnetization in this study means that the latter phase will have a disproportionate influence on the palaeomagnetic results. It has been suggested elsewhere that in such a magnetic system the Ti-poor phase may represent a CRM formed at some temperature below its final Curie temperature; if so, the results of this study imply that not only does it affect the palaeointensity estimate (which requires a pure TRM) but it appears also to influence the palaeomagnetic direction to an extent that far exceeds the normal secular change that might be expected to occur during delayed acquisition. Such a result has important implications for palaeomagnetic studies that use lavas, both in the selection of a suitable sampling strategy and in the geomagnetic interpretation of palaeomagnetic variation within single flow units.     

A sea-level test for inertial interchange true polar wander events

The possibility of inertial interchange true polar wander (IITPW) events, in which the rotation pole moves 90° with respect to the solid Earth in a matter of ∼10  Myr, has been discussed in the geophysical literature for more than three decades. Recent evidence for an IITPW event in Early Cambrian time has renewed interest in the issue; however, the veracity of supporting palaeomagnetic evidence remains a matter of significant debate. We propose that sea-level variations driven by polar wander provide an important independent test for the occurrence of IITPW events. Our numerical simulations of the response of a viscoelastic planet to an IITPW-induced forcing predict sea-level changes of up to 200  m, depending on the details of the earth model, the location of the site relative to the rotation path and the elapsed time for the reorientation of the pole. A preliminary comparison of our predictions to Early–Middle Cambrian sea-level records for Australia, Laurentia and Baltica shows qualitative agreement. This comparison suggests that a definitive test for the Cambrian IITPW hypothesis is possible given a sufficiently accurate, and globally distributed, database of sea-level histories.     

Palaeomagnetism of the Lower Ordovician Orthoceras Limestone, St. Petersburg, and a revised drift history for Baltica in the early Palaeozoic

Palaeomagnetic investigation of Lower Ordovician limestone in the vicinity of St. Petersburg yields a pole position at latitude 34.7°N, longitude 59.1°E ( dp / dm =5.7°/6.4°). A probable primary remanence origin is supported by the presence of a field reversal. The limestone carries one other remanent magnetization component associated with a Mesozoic remagnetization event.
An apparent polar wander path is compiled for Baltica including the new result, ranging in age from Vendian to Cretaceous. Ages of the published Lower to mid-Palaeozoic palaeomagnetic pole positions are adjusted in accordance with the timescale of Tucker & McKerrow (1995). The new Arenig result is the oldest of a series of Ordovician and Silurian palaeomagnetic pole positions from limestones in the Baltic region. There are no data to constrain apparent polar wander for the Tremadoc, Cambrian and latest Vendian. If the Fen Complex results, previously taken to be Vendian in age ( c . 565 Ma), are reinterpreted as Permian remagnetizations, an Early Ordovician–Cambrian–Vendian cusp in the polar wander path for Baltica is eliminated. The apparent polar wander curve might then traverse directly from poles for Vendian dykes on the Kola peninsula ( c . 580 Ma) towards our new Arenig pole ( c . 480 Ma). The consequence of this change in terms of the motion of Baltica in Cambrian times is to reduce significantly a rotational component of movement.
The new Arenig pole extends knowledge of Ordovician apparent polar wander an increment back in time and confirms the palaeolatitude and orientation of Baltica in some published palaeogeographies. Exclusion of the Fen Complex result places Baltica in mid- to high southerly latitudes at the dawn of the Palaeozoic, consistent with faunal and sedimentological evidence but at variance with some earlier palaeomagnetic reconstructions.     

Clockwise rotations recorded in Early Cretaceous rocks of South Korea: implications for tectonic affinity between the Korean Peninsula and North China

Recent interest has focused on whether South Korea may have undergone variable tectonic rotations since the Cretaceous. In an effort to contribute to the answer to this question, we have completed a palaeomagnetic reconnaissance study of Early Cretaceous sedimentary and igneous rocks from the Kyongsang basin in southeast Korea. Stepwise thermal demagnetization isolated well-defined characteristic magnetization in all samples. The palaeomagnetic directions reveal patterns of increasing amounts of clockwise (CW) rotation with increasing age for Aptian rock units. Palaeomagnetic declinations indicate clockwise vertical-axis rotations of R = 34.3° ± 6.9° for the early Aptian rock unit, R = 24.9° ± 10.6° for the middle Aptian, and R = −0.9° ± 11.8° for the late Aptian relative to eastern Asia. The new Cretaceous palaeomagnetic data from this study are consistent with the hypothesis that Korea and other major parts of eastern Asia occupied the same relative positions in terms of palaeolatitudes in the Cretaceous. An analysis of and comparison with previously reported palaeomagnetic data corroborates this hypothesis and suggests that much of Korea may have been connected to the North China Block since the early Palaeozoic. A plausible cause of the rotation is the westward subduction of the Kula plate underneath the Asian continent, which is inferred to have occurred during the Cretaceous according to several geological and tectonic analyses.     

3D seismic imaging of a Tertiary Dyke Swarm in the Southern North Sea,UK

We use three‐dimensional (3D) seismic reflection and magnetic data to interpret and describe the 3D geometry of igneous dykes in the southern North Sea. The dykes were emplaced into Paleozoic and Mesozoic sediments and have a common upper termination in Early Tertiary sediments. We interpret the dykes to be part of the British Tertiary volcanic province and estimate the age of the dykes to be 58 Ma. The dykes are characterized by a narrow 0.5–2 km wide vertical disturbance of seismic reflections that have linear plan view geometry. Negative magnetic anomalies directly align with the vertical seismic disturbance zones and indicate the presence of igneous material. Linear coalesced collapse craters are found above the dykes. The collapse craters have been defined and visualized in 3D. Collapse craters have formed above the dyke due to the release of volatiles at the dyke tip and resulting volume loss. Larger craters have potentially formed due to explosive phreatomagmatic interaction between magma and pore water. The collapse craters are a new Earth analogue to Martian pit chain craters.     

Palaeomagnetic study of Permocarboniferous glacial varves from the Itarare Subgroup, southern Brazil

Summary. A sequence of 167 glacial varves from Itu, State of São Paulo (Brazil), has been sampled for palaeomagnetic analysis. Since the varve thicknesses permit individual measurements, an attempt was made to compare annual palaeomagnetic results with recent observatory data. Inclination and declination plots show larger variations than those displayed by the present geomagnetic field. This could, however, be due to sedimentation factors since it may be assumed that the pattern of the present geomagnetic field variations is not greatly different from that of the Permocarboniferous field. In the next paper (Ernesto & Pacca) the experimental data are submitted to spectral analysis and the results support some of the hypotheses raised in this work.     

The Slate Islands meteorite impact site: a study of Shock Remanent Magnetization

Summary. A detailed palaeomagnetic study of the Slate Islands meteorite impact site, northern Lake Superior, has successfully isolated a secondary Component of remanence which on the basis of field relationships, models of impact crater formation and laboratory investigation appears to be a Shock Remanent Magnetization (SRM).
Evidence for this hitherto rare type of naturally formed remanence is that:
(a) the component is confined to rocks having experienced intense shock (50 to more than 100 kb) on the basis of shatter-cone development and the presence of planar features in quartz and felspar
(b) the component appears to have been acquired virtually instantaneously in terms of normal geological processes as a combined palaeomagnetic—shatter cone analysis suggests that it was formed within a time interval (perhaps several minutes in duration) between the moment of impact and the formation of the central uplift
(c) the extent of magnetic resetting decreases with increasing distance from the centre of the impact site, and
(d) the degree of resetting increases with the abundance of low coercivity magnetic grains as given by H_cr , a relationship found by other investigators for experimentally-produced SRM.
Intrusive breccias found on the islands yield a secondary component of TRM or TRCM origin which was frozen in after central uplift formation. Increased convergence of shatter cone axes to a central focus after rotation of host rock blocks according to palaeomagnetic data, suggests that the SRM had the same direction as that found in the breccias and hence was aligned with the ambient field at the time of impact.     

Palaeomagnetism of some late Mesozoic dolerite sills of East Central Spitsbergen, Svalbard Archipelago

Summary. Two late Mesozoic dolerite sills, situated near Agardhbukta on the east coast of Vestspitsbergen and dated radiométrically at 100 ± 4 Myr BP, have been sampled in five localities and subjected to detailed mineralogical and rock magnetic studies to determine the direction and origin of their magnetization. Although the sills lie outside the Tertiary orogenic belt, one locality (no. 4) has undergone strong hydrothermal alteration and a small part of another locality (no. 3) has also been affected. A conventional procedure based on examination of Zijderveld diagrams, applied to specimens demagnetized by alternating fields and thermally, yielded similar remanence directions at all five localities, except at the altered part of locality 3. Using a least squares computer méthod of analysis of step demagnetization data, comparable directions were isolated from all localities, including the altered part of locality 3. Except in this last case, all directions were reversed. The adjusted mean direction obtained from this analysis is D = 159.0°, I = 62.2°, α₉₅= 9.0° yielding a palaeomagnetic pole situated at 225.0°, 54.3°N comparable with pole positions obtained from other late Mesozoic igneous rocks on Spitsbergen and distinct from palaeopoles derived from Mesozoic rocks in North America and Eurasia. This suggests that during the late Mesozoic Svalbard existed as a semi-independent microplate.     

The geomagnetic field over the past 5 million years

The modern geomagnetic field is usually expressed as a spherical harmonic expansion. Although the palaeomagnetic record is very incomplete in both space and time, sufficient data are available from a span of ages to generate time-averaged spherical harmonic field models with many degrees of freedom. Here three data sets are considered: directional measurements from lavas, inclination measurements from ocean sediments, and intensity measurements from lavas. Individual data are analysed, as well as site-averages, using the same methods that have been developed for the modern field, to give models for the past 5 Myr. The normal-polarity field model has an axial-dipole intensity similar to that of the modern-day field, whilst the equatorial-dipole component is very much smaller. The field is not axisymmetric, but shows flux concentrations at the core's surface under Canada and Siberia similar to those observed in the field over historical timescales. Tests on synthetic data show that it is unlikely that these similarities result from the overprinting of the palaeomagnetic field due to inadequate cleaning of the samples. The reverse-polarity field model does not show such obvious features, but this may be due to the sparsity of the data.
The patterns observed in the normal-polarity field, with persistent features in the northern hemisphere and a smooth southern hemisphere, could be explained if the present pattern of secular variation is typical of the past several million years. This would reveal itself as large variations over time in the direction of the magnetic vector in regions of high secular variation, with relatively little change over quieter regions. However, we have been unable to find any evidence for a geographical pattern of secular variation in the data.     

Mesozoic palaeomagnetism in Vestfjella, Dronning Maud Land, East Antarctica

Summary. Thermal and alternating field demagnetization of oriented samples of Mesozoic lava flows (200–230 Myr) and dykes (154–172Myr) collected from seven nunataks within the Vestfjella region, Dronning Maud Land, revealed stable directions of magnetization of normal and reverse polarity. Directional distributions of both polarities define tight groups along the same palaeomagnetic axis. Depending on whether the regional westward dip of the lava flows originated prior to, or after the intrusive igneous phase, two or one, respectively, palaeomagnetic pole(s) can be estimated. Both poles, however, are in general accord with previous Mesozoic poles from East Antarctica.     

The British Tertiary igneous province: palaeomagnetism of the dyke swarm along the Sleat coast of Skye

Summary. A palaeomagnetic investigation has been made of a swarm of more than 400 dykes along the south coast of Skye, Scotland, by the Sound of Sleat.
Seven red lamprophyre dykes have palaeomagnetic directions inconsistent with Tertiary age, and not inconsistent with their previously held Caledonian age. The remaining 409 dykes have palaeomagnetic directions that are consistent with a Lower Tertiary age. We present evidence suggesting that the Tertiary dykes might have been emplaced during a short time, over which the geomagnetic polarity occupied as few as three polarity intervals (NRN or RNR).
Certain 'intermediate' directions of magnetization have also been found, and are presented here.     

Palaeomagnetism of a sequence of red beds of the Middle and Upper Sections of Pagnazo Group (Argentina) and the correlation of Upper Palaeozoic-Lower Mesozoic rocks

Palaeomagnetic data from 182 hand samples collected in a rock sequence of about 620-m of red beds of Late Palaeozoic to Early Triassic age exposed in north-western Argentina (30.3° S 67.7° W), are given.
After cleaning, the majority of the Upper Palaeozoic samples (Middle Section of Paganzo Group) show reversed polarity and yield a palaeomagnetic pole at 78° S 249° E (α₉₅= 3°). They also record a polarity transition which we have correlated with the Middle Permian Quebrada del Pimiento Normal Event. The position of the palaeomagnetic pole and the K-Ar age of a basalatic sill at the base of the sequence support this correlation.
Stable remanent magnetization has been isolated in the majority of samples from the Upper Section of the Paganzo Group; it is predominantly reversed and reveals three normal events and also three geomagnetic excursions suggesting an Illawarra Zone age (post Kiaman, Late Tatarian-Early Scythian). The palaeomagnetic pole of the reversely magnetized samples is located at 75° S 285° E(α₉₅= 13°).
The red beds involved in this study are correlated with red beds from the Corumbataí Formation (State of Paraná, Brazil) and with igneous rocks from the Quebrada del Pimiento Formation (Province of Mendoza, Argentina).
The South American Middle and Upper Permian, Upper Permian—Lower Triassic, Lower, Middle and Upper Triassic and Middle Jurassic palaeomagnetic poles reflect a quasistatic period with mean pole at 82° S 244° E, (α₉₅= 4°) which followed the South American Late Palaeozoic polar shift.     

Palaeomagnetism of the Cretaceous Pirgua Subgroup (Argentina) and the age of the opening of the South Atlantic

Palaeomagnetic data for the Cretaceous Pirgua Subgroup from 14 different time units of basalts and red beds exposed in the north-western part of Argentina (25° 45' S 65° 50' W) are given.
After cleaning all the units show normally polarized magnetic remanence and yield a palaeomagnetic pole at 222° E 85° S ( d Φ= 7°, d χ= 10°).
The palaeomagnetic poles for the Pirgua Subgroup (Early to Late Cretaceous, 114–77 Myr), for the Vulcanitas Cerro Rumipalla Formation (Early Cretaceous,<118 Myr, Valencio & Vilas) and for the Poços de Caldas Alkaline Complex (Late Cretaceous, 75 Myr, Opdyke & McDonald) form a 'time-group' reflecting a quasi-static interval (mean pole position, 220° E 85° S, α₉₅= 6°) and define a westward polar wander in Early Cretaceous time for South America.
Comparison of the positions of the Cretaceous palaeomagnetic poles for South America with those for Africa suggests that the separation of South America and Africa occurred in late Early Cretaceous time, after the effusion of the Serra Geral basalts.
The K-Ar ages of basalts of the Pirgua Subgroup (114 ± 5; 98 ± 1 and 77 ± 1 Myr) fix points of reference for three periods of normal polarity within the Cretaceous palaeomagnetic polarity column.     

Subsidence analyses from the Betic Cordillera, southeast Spain

Fifty‐four Mesozoic–Cenozoic stratigraphic sections from the Betic Cordillera of southeast Spain have been analysed in order to estimate the timing and amount of lithospheric stretching that occurred at the western end of the Tethyan Ocean since the Hercynian Orogeny. The standard backstripping technique has been used in order to calculate the water‐loaded subsidence of basement for each section. Water‐loaded subsidence curves were then inverted in order to determine the variation of lithospheric strain rate as a function of time, which yields estimates of timing, magnitude and intensity of stretching. Rifting commenced during the Late Permian/Early Triassic times and continued intermittently throughout the Mesozoic in response to the opening of the Tethyan Ocean to the east and the opening of the Atlantic Ocean to the west. Two major events in the Permo‐Triassic/Early Jurassic and the Late Jurassic/Early Cretaceous can be clearly identified. Stretching factors are generally small (1.1–1.25) probably because the Betic Cordillera was located at the westernmost end of the Tethys. Peak strain rates of ～10^?15 s^?1 were obtained for Mesozoic rift events and these values are in broad agreement with those obtained throughout the Tethyan Realm. We have also analysed the Neogene extensional event, which played an important role in forming the existing Mediterranean Sea. A combination of well‐log information and calibrated seismic reflection data was modelled. Peak strain rates in these younger basins are almost one order of magnitude faster than those estimated for the Mesozoic basins. These higher values appear to be typical of back‐arc extensional basins elsewhere. To the west and north of the Betic Cordillera, the Guadalquivir foreland basin developed as extension took place further east. Backstripped sections from this basin clearly record the northward migration of foreland basin subsidence through time.     

Palaeomagnetism and K—Ar ages of Lower Ordovician and Upper Silurian—Lower Devonian rocks from north-west Argentina

Summary. In this paper we present palaeomagnetic data from 87 hand samples collected in a sequence of tuffs and shales (Surf Formation) of Llanvirnian age, exposed in north-western Argentina (27° 47' S, 68° 06' W). After cleaning, the majority of samples showed reversed polarity and yielded a palaeomagnetic pole at 5.9° E, 8.5° S (α₉₅= 5.9°). They also showed reversals of declination and inclination at the top of the sequence, which we have associated with geomagnetic excursions. Whole rock K—Ar age de-terminations suggest an age older than 416 ± 25 Myr for the Suri rocks. The predominant reversed stable remanence of these rocks is consistent with the reversed polarity reported for Early Llanvirnian rocks from USSR. The palaeomagnetic pole for the Suri Formation is consistent with the interpretation that Gondwana was a single unit in Early Palaeozoic times.
Palaeomagnetic data from 27 hand samples collected from 10 igneous units of Late Silurian—Early Devonian age (Ñuñorco Formation), exposed in the same area, are also given. The majority of the igneous units showed reversed polarity after cleaning. The positions of VGP's for the Ñuñorco igneous units are scattered and they are not used for geodynamic interpretations. Whole rock K—Ar age determinations suggest ages of 416 ± 25 and 360 ± 10 Myr for two igneous units of the Ñuñorco Formation.     

Early Triassic palaeomagnetic results from the Ryeongnam Block, Korean Peninsula: the eastern extension of the North China Block

Greenish sandstones in the Early Triassic Nogam Formation of the Ryeongnam Block, Korean Peninsula were collected at 23 sites for palaeomagnetic study. A high-temperature magnetization component with unblocking temperatures of 670–690 °C was isolated from seven sites and yielded a positive fold test at the 95 per cent confidence level. The high-temperature component is interpreted to be of primary origin because the folding age is Middle Triassic. The Early Triassic palaeomagnetic direction for the Ryeongnam Block after tilt correction is D =347.1°, I =23.8° ( α ₉₅=5.5°). The palaeomagnetic pole (62.5°N, 336.8°E, A ₉₅ = 4.7°) shows good agreement with the coeval pole for the North China Block, suggesting that the Ryeongnam Block has been part of the North China Block at least since Early Triassic times. A tectonic history of the Korean Peninsula includes obduction of the eastern part of the South China Block onto the central part of the Korean Peninsula in the Permian, with the Ryeongnam Block geographically isolated from the main part of the North China Block. Collision of the North and South China blocks commenced initially at the Korean Peninsula, and suturing of the two blocks progressed westwards.     

Palaeomagnetic record in Late Pleistocene and Holocene dry lake deposits at Tlapacoya, Mexico

Summary. A palaeomagnetic investigation of Carbon-14-dated marsh and near-shore lacustrine sediments deposited between about 25000–5000 yr bp at Tlapacoya, Mexico, reveals normal polarity of the geomagnetic field in all samples measured. At one site, anomalous palaeomagnetic directions in a mud unit dated about 14500 yr bp raised the possibility of a geomagnetic excursion, but subsequent work at six additional sites in the unit revealed no abnormal directions. Thus the anomalous directions are most likely not a true reflection of geomagnetic field behaviour, although no specific alternative explanation is entirely convincing. The preliminary Tlapacoya data of anomalous directions have been cited by others as positive evidence for an excursion. We strongly recommend it no longer be considered as such.