Natural Remanent Magnetization Directions Research Articles

Remanent magnetization, cooling history, and paleomagnetic record of the Marys Peak Sill, Oregon

A paleomagnetic investigation of the middle Oligocene (29.6 m.y.) Marys Peak sill (44°30′N, 123°35′W) demonstrates a relationship between the intrusion's cooling history and its remanent magnetization. The 390-meter-thick sill is vertically differentiated, varying in compositon from a gabbro to a diorite with associated pegmatite and aplite facies. From 7 to 10 cores per sampling site were taken at 26 sites separated vertically by 7 to 10 meters along a traverse through the sill. Directions and intensities of remanent magnetization were measured for all specimens at the original magnetization level and after alternating-field demagnetization at 100−, 200−, and 400-oe peak demagnetizing fields. Demagnetization behavior. Curie temperatures, and polished section examinations indicate that the remanence is stable. The directions of natural remanent magnetization (NRM) at individual sites are well grouped and significantly different from the present field. The site mean direction pattern changes systematically from D ≅ 20°, I ≅ 50°, in the lower portion to D ≅ 10°, I ≅ 50°, in the central region to D ≅ 25°, I ≅ 50°, at the top. A mathematical approximation of the cooling history permits the transformation from the geometric record of the sill to a time record of the earth's magnetic field that existed when the sill cooled. Several models were evaluated and the results indicate that the range of the paleomagnetic record is approximately 2000 to 6000 years. It is concluded that the change in remanent magnetization direction could have been caused by a large nondipole field component or an excursion of the main dipole field.

Paleomagnetism of Jurassic and Cretaceous plutonic rocks in the Sierra Nevada, California, and its significance for polar wandering and continental drift

Directions of natural remanent magnetization (NRM) have been determined at thirteen sites in two intrusive igneous bodies in the western Sierra Nevada, California. Late Jurassic to Early Cretaceous potassium-argon ages of these rocks are: Guadalupe Mountains igneous complex, 136 m.y., and diorite of the Bucks batholith, 142 to 129 m.y. Normal and reversed NRM directions are found in both plutons. The mean paleomagnetic pole position corresponding to mean NRM directions at all sites after ac demagnetization is at 54°N 174°W, and the radius of the 95% confidence circle is 9.5°. This pole position is not significantly different from five previously published poles from widely separated North American rocks that range in age from 124 to 84 m.y. Such concordance strongly favors the geocentric dipolar configuration of the earth's magnetic field and further indicates that from about 138 to about 84 m.y ago polar wandering relative to North America did not occur. Comparison with similar paleomagnetic data from Australia and Africa suggests that apparent polar wandering during Mesozoic time was entirely due to continental drift and not to true polar wandering as an independent process.

A New Palaeomagnetic Result from the Lower Cretaceous of East-Central Africa

Summary The mean direction of natural remanent magnetization (NRM) in eight samples of syenite of Lower Cretaceous age from the Mlanje Massif, Malawi, has declination 333 and inclination - 54 (upward) after magnetic cleaning. The stability of remanence in alternating magnetic fields suggests that the NRM is primary. On the assumption of a geocentric dipole field, the palaeomagnetic pole is calculated at 60 N, 98 W. This is in agreement with previous results from volcanics of the Lupata Series nearby in Mozambique, which are of similar age. Combination of results from the Mlanje Massif and the Lupata volcanics yields an estimate of the palaeomagnetic pole for the lower Cretaceous of this part of Africa at 609 N, 993 W, with circular standard deviation (related to palaeosecular variation) of 10.0. Directions of natural remanent magnetization in eight samples from the Lauderdale erosion crater (16 1' S, 35 329 E) on the southern side of the Mlanje Massif in south-eastern Malawi are reported. The massif is a syenite-granite complex and has been mapped by Garson & Walshaw (in preparation). The samples were collected from the outer syenite ring of the complex, along about 150 metres of a stream running at right-angles to its contact with the Palaeozoic basement. The boundary of the complex is transitional, the adjacent basement having been syenitized. Only the intrusive body has been sampled. It is several kilometres in diameter, and it is likely that the section which has been sampled represents a long period in the cooling history of the body. Therefore, in spite of the small collection there is reason to believe that palaeosecular variation may have been effectively averaged out in calculating the mean direction, and that the dispersion may be a fair estimate of the palaeosecular variation. Two recent K-Ar age determinations by Snelling (1966) indicate the Cretaceous age of the complex : biotite from perthosite (sample WllOS) from the outer ring at Chambe indicated 1 16T 6 m.y., and biotite-hornblende mixture from quartz-syenite (sample W1112) from the Nanchidwa Hills, some 19 km east of the Lauderdale Crater gave 128 T 6 m.y. Gough & Opdyke (1963) have reported the palaeomagnetism of the Lupata alkaline volcanics and also of one site in redbeds of the Lupata Series, some 160 km south-west of Mlanje, in Mozambique. McDougall made two determinations on anorthoclases from the Lupata volcanics at 110.5 and 106.7m.y. (Gough et al. 1964). Because Gough & Opdyke sampled seven sites in single lava flows, and hence sampled no more

Paleomagnetism of Upper Cretaceous volcanic rocks of southwestern Montana

Paleomagnetic studies of folded and flat-lying volcanic rocks in southwestern Montana indicate that most of the natural remanent magnetization (NRM) is primary and that secondary magnetization, where found, has been produced mainly by lightning. NRM directions of most rock units are grouped in either a normal or reversed sense after partial demagnetization, although directions of a few units of probable mudflow origin are highly dispersed. No evidence of self-reversed magnetization has been found; consequently, a field polarity sequence has been tentatively established in connection with Late Cretaceous and early Tertiary orogenic events. All folded rocks investigated belong to the Upper Cretaceous Elkhorn Mountains Volcanics; stratigraphic correlation between widely separated, structurally dissimilar outcrops of petrologically similar rocks is supported by NRM directions and polarities. Mean NRM directions of structurally restored, age-equivalent sites do not differ significantly from one another, which suggests that fold axes are nearly horizontal; however, they differ significantly from directions of nearby flat-lying volcanic rocks of probable Tertiary age. The virtual geomagnetic pole calculated for the Elkhorn Mountains Volcanics (69°N, 171°W) agrees generally with Late Cretaceous poles previously calculated by others for North America, Madagascar, and the U.S.S.R., but it differs significantly from Early Cretaceous poles of Japan and Australia.

IN‐SITU DETERMINATION OF THE REMANENT MAGNETIC VECTOR OF TWO‐DIMENSIONAL TABULAR BODIES

Recent studies have shown that many rocks of the earth’s crust have a substantial component of remanent magnetization. Extensive sampling is required to determine adequately the remanent vector from small samples. A field technique has been developed (and tested on model data) for the in‐situ determination of the resultant (induced+remanent) magnetic vector of bulk volumes of rock, using a combined analysis of the gravity and magnetic fields of a disturbing body (Poisson’s Theorem). The potential fields are sampled adequately at a limited expenditure of time and effort in the field by utilizing the geometry of two‐dimensional bodies. The major limitation to the analysis is the removal of regional gradients and the estimation of the base levels of anomalies. Combined gravity and magnetic surveys were conducted over six diabase bodies in the Triassic Basin of Pennsylvania. The results of these surveys indicate a resultant direction of magnetization given approximately by: declination 2° W, inclination 41 degrees below the horizon. The corresponding direction of natural remanent magnetization has a declination of 1° W and an inclination of 28 degrees. The ratio of remanent to induced magnetization for the diabase is approximately two. These results have been used to provide a better interpretation of magnetic survey data over a magnetite deposit in the Triassic Basin.

Analysis of magnetic inclination in borecores

In palaeomagnetism it is usually assumed that the distribution of directions of natural remanent magnetization at a site is Fisherian. This assumption is used here for an analysis of dispersion on a sphere when only inclination (and not declination) of NRM directions is known. By this method, in spite of the lack of declination data, the mean inclination and precision of the parent Fisherian population are estimated, together with the probable errors in these estimates. The method is tested against known Fisherian populations and is then used for dealing with data from borecores, where the way-up of cores and dip of the hole are known, but the azimuth of cores is not. Uses of the information derived from this analysis include estimations of age of NRM, support for palaeomagnetic data from surface studies, and comparison of dispersion of the palaeomagnetic field with that of the present geomagnetic field.

Palaeomagnetic Study on a Granitic Rock Mass with Normal and Reverse Natural Remanent Magnetization

The author collected intrusive rocks in Nose district in Kinki Province and measured their natural remanent magnetization. Results of measurements of rocks collected from one rock body are as follows: the natural remanent magnetization of the intrusive rocks collected at the contact zone with country rocks was normal, while that of the intrusive rocks collected at places a little far from the contact zone was reverse. The directions of remanent magnetization were neither typical normal nor reverse. The normal direction deviated easterly relative to the present field direction, and the reverse one westerly.A fact that the rocks from a single mass possess both the normal and reverse magnetization can be explained, considering that the rock mass had been formed at the times when a transition of the geomagnetic field occurred, and the intermediate direction of magnetization seems to indicate the transition of the dipole of the geomagnetic field. The pole positions obtained from the intermediate direction of natural remanent magnetization lie in the same zone as those obtained from the Mio-Pliocene rocks in southwestern Japan.

Palæomagnetic Results from Northern Canada suggesting a Tropical Proterozoic Climate

THE Coppermine and September Mountains, in the north-central District of Mackenzie, North-west Territories of Canada, are composed of a thick succession of tholeiite basalt flows. Directions of natural remanent magnetization were measured on 51 oriented samples from 24 sites (Fig. 1). The sites represent 24 of the more than 60 separate flows occurring in a vertical thickness of more than 5,000 ft. The flows dip from 0° to 5° north in the area sampled. Two potassium/argon whole rock age determinations on flow rocks, made in the Isotope and Nuclear Geology Section of the Geological Survey of Canada (Wanless, private communication), gave an average age of 1,150 m.y. with a probable error of ± 100 (calculated as two standard deviations from the mean). This places the age of extrusion of the flows in upper middle Proterozoic time. The lavas conformably overlie dolomite of the Hornby Bay Group, and in places thin sandstone bands are intercalated between the flows. A sample from a red sandstone intercalation gives a direction of D = 236, I = +24, which is similar to that of the flows.

The paleomagnetism of the Shawa Ijolite

Oriented cores have been drilled at seven sites in ijolite exposures of the Shawa carbonatite complex, Southern Rhodesia. The ijolite has been dated at 209±16 m.y. from Rb → Sr decay. The directions of natural remanent magnetization are not consistent between sites. The ijolite proved difficult to treat by partial demagnetization in alternating fields, as instability of the direction of remanent magnetization tended to develop with demagnetizing fields only slightly larger than those required to remove isothermal magnetizations. Nevertheless, consistent directions of remanent magnetization were secured from five of the seven sites, after alternating-field treatment. On the assumption of a geocentric dipole field, the mean south magnetic pole position is 64.2°S, 85.6°E, with radius of 95 per cent confidence 14.1°. This is in excellent agreement with other Mesozoic pole positions from Africa.

Reverse and Normal Remanent Magnetization in Hydrothermal Veins

AN attempt has been made to apply the measurements of the direction of natural remanent magnetization to the age determinations of epigenetic hydrothermal mineralization. The investigation of materials from Czechoslovakian hydrothermal deposits has brought positive results which have changed some of the previous opinions concerning metallogenetic problems in Czechoslovakia1. The work was based on several scores of hydrothermal deposits from four metallogenic areas in the West Carpathians and in the Czech Massif. As a comparative geochronological scale for age-determinations the polar wandering curve computed in the way published by Creer et al.2 was used. In the hydrothermal mineral deposits of the Carpathian System the parameters introduced by Blackett et al.3, particularly the so-called divergence and inclination difference of remanent magnetization, were also used.

The magnetization of Red Hill Dike, Tasmania

A detailed laboratory study has been made of the magnetization of Red Hill dike, which is a single rock body within the Tasmanian dolerite intrusive complex. A potassium-argon age of 170 m.y. has been measured on this dike using samples from some of the same sites at which the magnetization was measured. The results show that the direction of natural remanent magnetization has an insignificant amount of secondary magnetization and that there is no evidence of its having any systematic error due to stress or shape effects. The mean direction is Dm = 294°, Im = −75°, which is regarded as a good estimate of the geomagnetic field 170 m.y. ago; the steep inclination is similar to that found in other Jurassic rocks from Australia.

A statistical enquiry into the partial remagnetization of folded old red sandstone rocks

The directions of natural remanent magnetization of Old Red Sandstone rocks from five sites in south Wales where the strata are steeply inclined to the horizontal are significantly different from the average direction of magnetization of Old Red Sandstone rocks from flat-lying sites. The hypothesis, that the observed directions of magnetization are resultants of a primary component acquired in the Devonian and a secondary component acquired after folding, either in the Permian or Triassic, is shown to be tenable. A statistical technique, whereby the significance of the normal to the best-fitting plane through the observed and the supposed primary and secondary mean directions, is described. Ovals of confidence surrounding these normals are also calculated. Partial remagnetization of these rocks is shown to be slightly more probable in the Triassic than in the Permian at four out of five sites considered.

Further Palaeomagnetic Results from South Victoria Land, Antarctica

A palaeomagnetic study has been made of rock samples taken from a 5000 ft sequence in South Victoria Land. The sequence consists of granites, sediments and hypabyssal intrusives of Mesozoic, Palaeozoic and possibly Pre-Cambrian ages. The directions of natural remanent magnetization are approximately uniform throughout, but small and apparently real differences do occur between some of the units within the sequence. Laboratory tests using alternating magnetic fields suggest that these differences are due to the presence of unstable components of small but variable magnitude and that the mean directions of the stable components are parallel in all the units studied. This uniformity in direction could have resulted from the geomagnetic field in the region being constant in direction for a long period of time in the Palaeozoic and Mesozoic, or from the reheating of the whole area during the last phase of intrusion (that of the Ferrar dolerites) in Mesozoic times. The latter interpretation is favoured. The variations in magnetic properties through the Ferrar dolerite sheets are described and provide information relevant to theories of their emplacement and differentiation.

Some Preliminary Measurements of Anisotropy of Magnetic Susceptibility of Rocks

summary Measurements have been made on four sets of rocks, two of which showed anisotropy of magnetic susceptibility and two were isotropic. The maximum and intermediate principal susceptibilities lie in the cleavage plane for some Welsh slates and in the horizontal plane for some igneous rocks of the Skaergaard layered intrusion. In both cases the directions of natural remanent magnetization have been affected. The two sets of isotropic rocks are from the northern Pyrenees and Deccan of India respectively. Both may have been affected by stress environments, the nature of which did not cause measurable anisotropy. It is unlikely that the directions of natural remanent magnetization have been affected for these rocks.

Anisotropy of Susceptibility and the Natural Remanent Magnetization of Some Welsh Slates

IN order to investigate the effect of the deformation associated with mountain building upon the aniso-tropy of susceptibility and the natural remanent magnetization of rocks, approximately a hundred specimens were prepared for measurement from some fifty samples of Lower Palaeozoic Welsh slates cleaved during the Caledonian orogeny. The anisotropy in fields of less than two œrsted was determined with a transformer bridge1 and the natural remanent magnetization with an astatic magnetometer. The results showed that the cleavage is a plane of high susceptibility, and that the remanent vector is parallel to it. In some of the slates it appears that the direction of natural remanent magnetization has been controlled by the anisotropic susceptibility, and in others that the pattern of anisotropy and the natural remanent magnetization have been aligned with the cleavage independently. I am indebted to Dr. J. C. Belshe and Dr. R. W. Girdler, with whose help the susceptibility work was begun.

A Palaeomagnetic Study of some Lower Jurassic Rocks of N.W. Europe

A study has been made of the directions of natural remanent magnetization of the Upper Lias Sands of England (Section 1) and reasons are given for considering the magnetization of some of these to be stable. The results show that the direction of the Earth's magnetic field in Lower Jurassic times was significantly different from that of the present Earth's field, having an easterly declination and inclination 64° to 70°. The work was extended (Section 2) to include some Lower Jurassic volcanic rocks from Southern France and these gave results in general agreement with those for England. It is concluded (Section 3) that the ancient geomagnetic pole position for the Lower Jurassic period lies in the region of the Caspian Basin.

A.C. Demagnetization of Unstable Triassic Keuper Marls from S.W. England

Summary In 1953, 329 specimen disks taken from 35 rock samples covering a stratigraphical thickness of 170 ft of Keuper Marl (Triassic) were collected from Sidmouth, Devonshire, and the results were described by Creer (1955, 1957a) in which an analysis of the resultant directions of natural remanent magnetization indicated the presence of three separate components, there called ‘?rimary”, “secondary” and “temporary”. Re-measurement of 65 of these rock disks from 26 of the 35 rock samples after storage in field free space for 3 years indicates that the temporary component of magnetization has decayed away. In further experiments, the same 65 rock disks were demagnetized in alternating magnetic fields of peak value up to I 200 oersteds. The directions of magnetization which were originally strung out along the great circle between the primary and secondary directions progressively coalesce into two opposed groups of magnetization as the intensity of the demagnetizing field is increased. It appears that in many cases the secondary magnetization has been almost completely destroyed and in the remainder of cases considerably reduced in comparison with the primary. The method of a.c. demagnetization is described.