Primary Component Of Magnetization Research Articles

The influence of terrestrial processes on meteorite magnetic records

In early solar system history there are several electromagnetic processes (electric discharges, pressure shock waves, electric discharges and currents) capable of magnetizing the primitive solid particles condensating from the solar nebula. The record of these magnetic events is the main objective of rock magnetic laboratory studies of meteorites found on the Earth. However, terrestrial environment can affect the magneto-mineralogy, can cause changes in magnetic parameters and can overprint the primary magnetic record. The entry of a meteorite into the terrestrial atmosphere causes surface heating and pressure effects due to large initial velocity. The effect of surface heating was the subject of the study with the CM2 Murchison meteorite. Results show the remagnetised zone to be at least 6 mm thick. On CM3, Allende meteorite we studied an effect of pressure during the atmospheric entry. According to our results the pressure does not seem to be a source responsible for meteorite remagnetization. Some meteorites are found several days after the fall, some are deposited in a desert or on the Antarctic ice for thousands of years. Most of them contain visible traces of terrestrial oxidation and weathering. We used the sample of an L6 chondrite DaG 979 found in the Libya desert, sample of the iron meteorite Campo del Cielo (found in Argentina 5000 years after the fall), and sample of the H5 Zebrak meteorite (found several days after the fall) for weathering simulations. Weathering plays an important role in the meteorite terrestrial history and is capable of complete remagnetization of the meteoritic material. To document the terrestrial processes that influence meteorite magnetism we monitored changes in magnetic remanence and magnetic hysteresis parameters. Our results indicate that the terrestrial processes are capable of changing magnetic properties and can overprint the primary magnetic record. Therefore, an extreme care is required when selecting the meteorite samples for primary magnetic component study.

Magnetic constraints on the thermal evolution of a collapsing orogen

SUMMARY The thermal evolution of an orogen undergoing late-stage extension was investigated using rock magnetic properties of a suite of mafic dyke rocks affected by greenschist facies metamorphism in the internal zones of the Betic Cordillera, southern Spain. The natural remanent magnetization (NRM) in the dykes intruded into the lowest geological unit is made up of up to three components. The lowest temperature component (LT) is in the direction of the present day magnetic field and is believed to be a chemical remanent magnetization (CRM) or viscous remanent magnetization (VRM) acquired in the recent magnetic field. The intermediate temperature (IT) component unblocked between 200 ◦ C and 450 ◦ C is thought to be largely a thermoviscous overprint acquired during metamorphism. This component is carried by either primary or authogenic sulfides and low-unblocking temperature magnetite. The component of magnetization with the highest blocking temperature (HT) is isolated above 450 ◦ C and is interpreted as the primary component of remanent magnetization. It is most likely that this component is carried by magnetite that resides in the plagioclase and has been shielded from the metamorphism, which transformed most of the original magnetite to metamorphic amphibole, chlorite and biotite. Thermal demagnetization of these dykes separates the IT overprint from the HT primary remanence at a sharp junction occurring at 450 ◦ C. For single domain grains this translates to a peak palaeotemperature in the natural sample of approximately 370 ◦ C, which is close to the estimated temperature experienced by the greenschist facies country rocks (400 ◦ C). In contrast, results obtained from a dyke that is intruded into an overlying weakly metamorphosed geological unit, indicates that temperatures only reached about 175 ◦ C in this unit. These results are consistent with temperatures deduced from geological constraints and they imply that between 4 and 6 km of section has been removed between the two units during late orogenic extension. These new data are then combined with previously published Ar‐Ar wholerock geochronology and information on vertical axis rotations to put constraints on the thermotectonic evolution of this collapsing orogen.

Multiphase reddening and weathering events in Upper Carboniferous red beds from the English West Midlands

Sediments of the Halesowen and Salop Formations (Westphalian D-Stephanian) in the West Midlands were deposited on the southern margin of the Pennine Basin during a period of relative tectonic quiescence during the Variscan Orogeny. The clastic framework of the predominantly red sandstones and mudstones in the Salop Formation includes large detrital hematite grains; these yield a primary component of magnetization indicating that sediments were deposited close to the Carboniferous palaeo-equator. Variscan uplift, erosion and subsequent deep oxidation in an arid climate during the Permian resulted in the precipitation of finer hematite in rocks of both formations (which are now found at depths of over 150 m below the present-day ground level). These grains carry secondary magnetizations which indicate formation at palaeolatitudes of between 5° and 15°N, the position that Britain occupied during the Permian. The pattern of Permian remagnetization (particularly in borehole samples) suggests that the secondary fine-grained hematite was precipitated as a result of lateral flow of meteoric water through connected, permeable sandstone units, rather than downward percolation through the entire succession. Present-day surface weathering has resulted in the dissolution of the finer grains of hematite and, in some cases, where permeabilities are relatively high, it has also affected the coarser fraction. Consequently, the record of late Carboniferous and Permian magnetization events is commonly incomplete in strata close to the present day ground level.

Paleomagnetic evidence for no tectonic rotation of the central Italy Tyrrhenian Margin since Upper Pliocene

We present paleomagnetic results from 141 samples collected at 12 sites from Plio‐Pleistocene clayey units in the extensional Central Italy Tyrrhenian margin. All samples were subjected to stepwise alternating field and/or thermal demagnetization. Positive fold and reversal tests indicate that the results refer to a primary component of magnetization. Paleomagnetic mean direction (Declination = 358.0°; Inclination = 55.2°; α95 = 4.1°) reflects the expected geocentric axial dipole field for the locality, thus indicating that this sector of the Tyrrhenian margin did not undergo any tectonic rotation during the past 4 million years.

Implications of the Born approximation for the magnetotelluric problem in three‐dimensional environments

A first‐order Born approximation is obtained for the integral equations governing the surface magnetotelluric response over a three‐dimensional earth. Although accurate only in cases of low resistivity contrasts, the resulting expressions: (1) exhibit a linear relationship between a spatial perturbation in subsurface resistivity and the ensuing perturbation on the surface field response, and, more importantly, (2) allow arbitrary degrees of complexity in the geometrical characteristics of the subsurface. The linear system solutions derived from the Born approximation are studied by examining the properties of their associated kernels. These kernels may be thought of as a suite of horizontal magnetotelluric “wavelets” weighting the subsurface resistivity distribution at different depth levels. Analytical expressions for the wavelets are obtained in the wavenumber domain, thus generating a suite of magnetotelluric “transfer functions.” Expressions for the latter are particularized to the cases of one‐ and two‐dimensional geolectric media yielding results consistent with the characteristics of the magnetotelluric fields known to hold in these low‐order environments. Inspection of the electric transfer functions reveals severe sensitivity to near‐surface lateral variations of resistivity, which persists even at deep sensing frequencies. This near‐surface sensitivity is the result of an additive term in the electric field transfer functions, the static component, acting as a spatial highpass filter of the lateral variations of surface resistivity. A second additive component in the electric transfer functions, the induction component, functions as a spatial lowpass filter of the lateral variations in subsurface resistivity, and is primarily responsible for the inductive part of the surface electric field response. A common problem in magnetotelluric interpretation, the electric static effect can be reduced by inverting the role of the static component, i.e., by spatially low‐pass filtering the surface electric field. The suggested low‐pass filter for such an operation is one for which the cutoff wavenumber increases with frequency and is therefore insensitive to the response from the induction component. Low‐pass filtering of the surface electric field is best implemented in the field if the electric dipoles are deployed end‐to‐end continuously along a survey path. The magnetic field transfer functions, on the other hand, exhibit a single induction term with band‐pass filter properties which may actually lead to some amount of local distortion on the measured surface magnetic field. We propose to reduce this distortion by referring all electric field measurements to the primary magnetic field within the survey area. The primary magnetic field components, in turn, can be estimated by the spatial average of the magnetic measurements acquired at an array of magnetic stations. The suggested procedures for both the acquisition and processing of natural electric and magnetic field data encompass altogether a novel adaptation of the magnetotelluric method.

Paleomagnetism and fission-track geochronology on the Goto and Tsushima Islands in the Tsushima Strait area Implications for the opening mode of the Japan Sea.

Paleomagnetism of Neogene rocks in the Goto Islands and fission-track (FT) geochronology of Miocene igneous rocks in the Goto and Tsushima Islands were investigated in order to reveal Miocene tectonics in the Tsushima Strait area. Untilted directions of primary magnetic components from early to middle Miocene sedimentary rocks in the Goto Islands dominantly showed counter-clockwise (CCW) deflections to the expected direction of the geocentric axial dipole field, while paleomagnetic directions from Miocene igneous rocks and Quaternary basalts were concordant with the expected direction. Zircon FT ages determined on sixteen Miocene igneous rocks show good agreement at about 15Ma. These results, in conjunction with previously-reported paleomagnetic data in the Tsushima Islands, suggest that the Goto Islands were rotated at the early to middle Miocene before about 15Ma and that the CCW of the Tsushima islands occurred after about 15Ma. The CCW rotations of these islands imply that the Tsushima Strait area did not belong to the Southwest Japan block in terms of clock-wise (CW) rotation at about 15Ma, constraining the western margin of the CW-rotated block. The FT ages confine the time of the compressive deformation of pre-middle Miocene sediments in the area to be at the early to middle Miocene before about 15Ma. The CCW rotations of these islands probably took place in response to the movement of the fault system in the area associated with the compressive regime, especially a sinistral motion of the Tsushima-Goto tectonic line. The early to middle Miocene compressive deformation in the area occurred almost coeval with the opening of the Japan Sea. The compressive tectonic regime implies the convergence of the western margin of the Southwest Japan block to the Korean Peninsula during the CW rotation of the block at about 15Ma, which gives a new constraint on the position of the rotation pivot of the block.

No relative rotation detected between Corsica and Sardinia

Forty dikes of Upper Paleozoic age have been investigated paleomagnetically on both sides of the Straits of Bonifacio to detect a possible rotation of Sardinia relative to Corsica. A primary component of magnetization has been isolated after thermal and alternating field cleaning. Eleven sites in Corsica yield a paleomagnetic mean of D = 134.7, I = −11.3 ( α 95 = 6.8), while 11 sites from Sardinia showed a paleomagnetic mean of D = 132.7, I = −1.6 ( α 95 = 7.3). The two directions are not significantly different at the 95% confidence level. No relative rotation between Corsica and Sardinia occurred across the Bonifacio Straits and the two islands maintained the present spatial relationship since the Upper Paleozoic. A mean paleomagnetic direction of D = 133.7, I = −6.5 ( α 95 = 5.1) for the Sardo-Corso Massif yield a corresponding virtual mean pole position at 249.4°E, 33.7°N coincident with Upper Paleozoic data of the Mediterranean region. This data implies about 70° of counterclockwise rotation of the block respect stable Europe.

Magnetic stratigraphy of the Siwalik group, Bhittani, Marwat and Khasor ranges, northwestern Pakistan and the timing of neocene tectonics of the Trans Indus

Nine stratigraphic sections in the Siwalik Group, three from the Bhittani Range, five from the Marwat Range, and one from the Khasor Range, were measured and sampled for magnetostratigraphic studies. Progressive alternating field and partial thermal demagnetization of samples indicated the presence of two components of magnetization. These are a secondary component of low coercivity, which can be removed by alternating field demagnetization in fields up to 40 mT and/or temperatures to 400°6, and a stable primary component of magnetization which has blocking temperatures above 450°C. Therefore all samples were subjected to blanket demagnetization at 450°C and 500°C, and some to temperatures up to 600°C. Positive fold tests from the Bhittani and Marwat ranges indicate the successful isolation of a prefolding characteristic direction of magnetization. A magnetic polarity reversal sequence for each of the sampled sections is determined and correlated to the magnetic polarity time scale utilizing a fission track date and fauna from the sequences which have previously been shown correlated to the magnetic polarity time scale. This correlation shows that the Siwalik Group in the area of study ranges in age from the late Gilbert chron to the early Brunhes chron (about 0.5 m.y. B.P.). Deposition of the thick sandstone units of the Siwalik Group in the Marwat Range began at least by Gauss chron time. The correlation of observed magnetic polarity stratigraphy, of the Khassor Range, with the standard magnetic polarity time scale shows that the Siwalik Group of this area ranges in age from the early Gauss chron (about 3.2 Ma) to the Brunhes chron. The region of this study began to subside prior to 4 Ma and subsided at a rate of from 60 to 100 cm per 1000 years. The tectonism of the region began within the last 1 m.y. and may well be synchronous with folding in the Pabi Hill anticline on the other limb of the syntaxis near Jelum. This orogeny is therefore one of the world's youngest and is undoubtedly still in progress.

Differential rotation of the eastern part of southwest Japan inferred from paleomagnetism of Cretaceous and Neogene rocks

Paleomagnetic directions have been determined on Late Cretaceous and Miocene rocks in the Nohi area, in the eastern part of southwest Japan, in an effort to clarify within‐block deformation of the rotated part of southwest Japan. Twenty‐two sites in the area are found to have a reliable primary magnetic component through progressive demagnetization tests using thermal and alternating field methods. Tiltcorrected paleomagnetic data in the Nohi area are divided into three stages based on geologic age: Late Cretaceous, early Miocene (23–15 Ma), and middle Miocene (12–10 Ma). The formation‐mean direction of 10 sites obtained from Late Cretaceous Nohi rhyolite is D=17.0°, I=49.8°, and α95=6.1°. Neogene rock units underlain by Nohi rhyolite show two distinct formation‐mean directions: D=13.0°, I=52.7°, and α95=7.2° for eight sites of early Miocene, and D=−2.5°, I=55.6°, and α95=6.6° for four sites of middle Miocene. Differential rotation is proposed between the eastern part of southwest Japan including the Nohi area and the central part of southwest Japan, which is known to have rotated clockwise through about 50° around 15 Ma. The spatial distribution of contemporaneous paleomagnetic data in southwest Japan suggests that two differentially rotated parts are connected in the western neighborhood of the Nohi area. It is concluded that the Nohi area had rapidly rotated counterclockwise through 48° relative to the central part of southwest Japan between 15 and 12 Ma. Together with previous paleomagnetic data from early Miocene rocks in the Kanto area, which is situated to the east of the Nohi area, the present results suggest that differential rotation of the eastern part of southwest Japan is attributed to the formation of northward cuspate shapes of pre‐Neogene terranes which constitute the framework of the Japan arc. One probable explanation of the formation of cusps is a crustal indentation caused by the collision of landmasses on the Philippine Sea plate against the Japan arc in the middle Miocene.

Biogenic magnetite as a primary remanence carrier in limestone deposits

Studies on the microbial communities and magnetic phases of samples collected from carbonate oozes at Sugarloaf Key, FL, U.S.A. and calcareous laminated sediments from Laguna Figueroa, Baja California, Mexico have revealed the existence of magnetotactic bacteria and ultrafine-grained single domain magnetite in both environments. Magnetotactic bacteria were identified by light and electron microscopy. The single domain magnetite was detected by coercivity spectra analysis with a SQUID magnetometer and examined under the transmission electron microscope. The similarity, in terms of size and shape, between the single domain magnetite found in these sediments and the magnetite observed in the bacterial magnetosome from enriched cultures indicates the ultrafine-grained magnetite in these two marine environments was biologically formed. These results, combined with the common occurrences of ultrafine-grained magnetite in limestone deposits detected rock magnetically, suggest biogenic magnetite may be present and contribute to the magnetic remanence in these rocks. Several Cambrian limestone samples, separately collected from Siberia, China, and Kazakhstan, were examined for the presence of bacterial magnetite. Samples from the Lower Cambrian Sinskian Formation at Siberia Platform were found to contain both a large amount of apparently bacterial magnetite particles and a very stable primary magnetic component. Post-Cambrian diagenesis does not seem to affect the microgranulometry of these apparently bacterial magnetite crystals or the magnetic remanence carried by them. Assessing the potential role of biogenic magnetite as a primary remanence carrier in other Phanerozoic limestone deposits ought to be further pursued.

Paleomagnetic Results from the Sadlerochit and Shublik Mountains, Arctic National Wildlife Range (ANWR), and Other North Slope Sites, Alaska: ABSTRACT

Carboniferous through Triassic sedimentary units exposed in the Shublik and Sadlerochit Mountains were sampled in an attempt to obtain reliable primary magnetic components. Reliable pre-Cretaceous paleomagnetic poles from this area would greatly advance the understanding of the rotation and latitudinal displacement history of the North Slope. Carbonate rocks of the Carboniferous Lisburne Group were drilled in south-dipping units of Katakturuk Canyon, Sadlerochit Mountains, and in the north-dipping Fire Creek section, Shublik Mountains. Magnetic cleaning involved stepwise thermal demagnetization to 550°C. Principal component analysis of the demagnetization results defines two major components of magnetization. The secondary component is steep and down (inc = 87°), but the characteristic component (325°C-500°C) is reversed. The secondary magnetization postdates Cretaceous and younger folding, whereas the characteristic component was acquired before folding. The components may have recorded two phases of overprinting: a Late Cretaceous into Cenozoic normal overprint and a predeformation remagnetization epi ode during a time of reverse polarity. However, the reverse component more likely is primary remanence. If so, it would suggest little latitudinal displacement but 40° of clockwise rotation with respect to North America. The Devonian Nanook Limestone, sampled in the Shublik Mountains, also reveals two major components of magnetization; however, the characteristic component is isolated at blocking temperatures greater than 500°C and is shallower in inclination than expected from the Devonian reference pole for North America. The recovery of the reversed characteristic component in this study is a significant result by itself. It is good evidence that at least part of the northeast Brooks Range has escaped the thorough Cretaceous normal-polarity overprinting that has been observed in the north-central Brooks Range. We hope that analyses of additional samples from the Katakturuk Dolomite, Nanook Limestone, Lisburne Group, Sadlerochit Group, and Shublik Formation in ANWR and from the Triassic and Jurassic Otuk Formation in the east-central foothills will also discriminate pre-Cretaceous magnetizations and will provide constraints on the time they were set. End_of_Article - Last_Page 676------------

Palaeomagnetism of Sukinda chromites and their geological implications

Thirty-eight oriented samples were collected from seven sites over the Precambrian Sukinda chromite ore (between 85°43′E; 21°01′N and 85°50′E; 21°03′N), which is in the form of a plunging syncline. On isolating the primary component of magnetization and affecting the necessary tectonic correction, four sites (A, B, C, F) were found to possess a reverse magnetic polarity (D=210°, I=+22°, with K=122 and α95=6° and one site (E) was found to bear a normal polarity (D=49°, I=-16° with K=474 and α95=2°). The remaining two sites D and G were rejected, the former because of scatter in directions and the latter due to overprinting by a dominant secondary magnetization, induced by lateritization. On comparison of VGPs evaluated for these chromite deposits, with those of other Precambrian rocks almost of a similar age, it is seen that these chromites are probably older than the Gwalior Traps whose age is 1830 ± 200 Ma. From the rock magnetic and microscopic examination it is deduced that the massive chromites of the Sukinda valley have not been affected magnetically to any great extent by serpentinization, though their host rocks are serpentinized ultramafics. Consistent directions of remanent magnetism point to the fact that these deposits crystallized in fairly stable conditions, characteristic of the stratiform type of chromite deposits, therefore, leading us to infer the Sukinda chromites to be of that type. These palaeomagnetic data, when subjected to the fold test, show the primary remanence to pre-date the tectonic disturbance and hence prove these chromites to have consolidated prior to the regional tectonic activity of the area.

A palaeomagnetic reconnaissance of Kashmir, northwestern Himalaya, India

Thermal demagnetization studies of 800 Palaeozoic and Lower Mesozoic samples, mainly carbonates, from 14 localities in Kashmir, showed four magnetic components: (A)A recent field component of normal polarity and large intensity. (B)A Middle to Late Tertiary secondary component of large intensity and of reversed and normal polarity, which indicates clockwise rotation of central and eastern Kashmir over approximately 45° with respect to peninsular Indo-Pakistan. (C)Primary magnetic components of Triassic, Late Permian, Early Carboniferous, Siluro-Ordovician and Middle to Early Ordovician age. The Triassic and Late Permian components show a 20–30° cumulative counterclockwise offset with respect to the Indo-Pakistan apparent polar wander path (APWP). This indicates a post-Middle to -Late Triassic counterclockwise rotational movement of Kashmir over 65–75° with respect to peninsular Indo-Pakistan. (D)A component of low inclination and predominantly east-west declination, which is observed only in rocks of pre-Panjal Traps age. A secondary origin associated with extrusion of the Panjal Traps flows is surmised, but could not be established beyond doubt. Alternatively, this component may be related to the regional foliation. The Triassic and Late Permian primary component directions are predominantly of normal polarity. This suggests a pre-Punjabian (Late Permian) upper age limit for the Permo-Carboniferous (Kiaman) reversed polarity interval, which is earlier than the Early to Late Tatarian limit observed in the U.S.S.R.

The extent of greater India, III. Palaeomagnetic data from the Tibetan Sedimentary series, Thakkhola region, Nepal Himalaya

The palaeomagnetism of 750 carbonate and sandstone samples from the Tibetan Sedimentary Series (TSS) of the Thakkhola region (north central Nepal) has been studied. This region forms part of India's former leading edge, underthrust by India along the Main Central Thrust (MCT). Samples of Devono-Carboniferous to Early Cretaceous age were collected from 14 localities. Five magnetic components have been distinguished: (1) a predominant component of recent origin, (2) a secondary component probably acquired during early Tertiary collision of India with Asia or offshore island arcs, (3) primary magnetic components of Middle? Permian to Early Cretaceous age, and another two secondary components either tentatively related with the predominant b-axes pattern of the TSS (4), or attributed to MCT-related metamorphism of the lower TSS (5). Declinations of the secondary collision component and the primary components mismatch declination data from the Indian subcontinent by 10–15°. A testable model is proposed, which related this mismatch to rotational underthrusting of India along the MCT beneath its former leading edge and beneath the Tibetan Plateau. The model accounts for underthrusting of continental lithosphere over 200–350 km at the longitude of the sampled region in central Nepal. A minimum estimate for the northern extent of India (post-Neotethys formation) is proposed, based on this magnitude of rotational underthrusting and on generally accepted values for crustal shortening across the Himalaya. Within the modified Smith and Hallam reconstruction of Gondwanaland, this estimate satisfies constraints imposed by seafloor spreading data from the eastern Indian Ocean and facies analyses of the western Australian continental margin. The palaeomagnetic results herein presented as well as those obtained elsewhere in the Himalayan region can be interpreted in support of Molnar and Tapponnier's model for deformation of southern Asia.

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).

Early Palaeozoic Palaeomagnetism in South East Australia

The palaeomagnetism of 22 formations from the Cowra Trough and Molong High of South East Australia have been studied. Samples of Ordovician through Middle Devonian age were collected in three major areas, covering a time period crucial to the understanding of the Palaeozoic apparent polar wander path of Australia. Five magnetic components have been identified: 1) A recent field overprint, 2) a secondary component related to the break up of Australia and Antarctica, 3) a predominent Devonian secondary overprint associated with the major orogenic episode, 4) a weak Silurian overprint associated with an earlier orogenic episode, and 5) primary magnetic components ranging in age from the Ordovician to the Devonian. The directions obtained enable construction of a well defined polar wander path for South East Australia and Australia as a whole.

The Precambrian-Cambrian boundary problem: paleomagnetic directions from the Amadeus Basin, Central Australia

Late Precambrian to early Cambrian sediments of the Amadeus Basin, Central Australia, contain two stable directions of magnetization. Lithologies are diverse and include red beds, green beds, and carbonates, all of which respond to thermal cleaning. Positive fold and unconformity tests, and a well-defined polarity zonation reveal the primary component and direction of magnetization. This direction lies between those reported for the Proterozoic lower Pound Quartzite and the lower Cambrian Antrim Plateau Volcanics of Australia. Poles calculated from the primary direction of magnetization for three stratigraphic units in the sequence overlap at the 95% confidence level, indicating that little apparent polar wander occurred in the Australian part of Gondwana at this time.

Palaeomagnetism of Permian Redbeds from the South-western United States

Samples were collected from Permian redbeds of Guadalupian to Ochoan age at 61 sites in the south-western U.S.A. Upon thermal demagnetization, a stable primary component of magnetization could be isolated in most of the samples. Because of the elimination of most of the secondary component of magnetization, the grouping of the virtual pole positions is better than previous results have suggested. There is no apparent chronological migration of the pole position through Permian time. Measurement has shown that although most of the Permian rocks are reversely magnetized, the rocks from four formations appear to have stable primary components approximately anti-parallel to the directions of magnetization of the reversed rocks. These rocks are of middle Leonardian, late Guadalupian, and middle and late Ochoan ages and it is suggested that they represent three, and possibly four, normal polarity events during the Permian.

The Paleomagnetism of Diabase Dikes from the Grenville Province

A paleomagnetic study was attempted of the diabase dike swarm intruding the Grenville structural province of the Canadian Shield. Both the alternating field and thermal demagnetization studies indicated that the dikes have varying degrees of stability of magnetization. Some dikes were shown to have stable primary component of magnetization. Microscopic observations indicated that, in most cases, the primary magnetic mineral titanomagnetite was altered to titanomaghemite, probably due to low temperature oxidation. A good correlation is obtained between magnetic stability and relative abundance of titanomagnetite to titanomaghemite in that dike. The mean paleomagnetic pole for the Grenville dike swarm, which is probably of late Precambrian age, was computed to lie at 3.0° N, 29.0° W with A25 = 11.0°. Comparison of this result with other Precambrian pole positions of similar age relative to North America suggests that the dikes were intruded after the Grenville orogeny.

Magneto-thermo-elastic plane waves

AbstractThe combined effects of uniform thermal and magnetic fields on the propagation of plane waves in a homogeneous, initially unstressed, electrically conducting elastic medium have been investigated.When the magnetic field is parallel to the direction of wave propagation, the compression wave is purely thermo-elastic and the shear wave is purely magneto-elastic in nature. For a transverse magnetic field, the shear waves remain elastic whereas the compression wave assumes magneto-thermo-elastic character due to the coupling of all the three fields—mechanical, magnetic and thermal. In the general case, the waves polarized in the plane of the direction of wave propagation and the magnetic field are not only coupled but are also influenced by the thermal field, once again exhibiting the coupling of the three fields. The shear wave polarized transverse to the plane retains its magneto-elastic character.Notation.Hi = primary magnetic field components,ht = induced magnetic field components,To = initial thermal field,θ = induced thermal field,C = compression wave velocity.S = shear wave velocity,ui = displacement components,cv = specific heat at constant volume,k = thermal conductivity,η = magnetic diffusivity,μe = magnetic permeability,λ, μ = Lamé's constants,β = ratio of coefficient of volume expansion to isothermal compressibility.